TYPE Original Research
PUBLISHED 03 October 2022
DOI 10.3389/feart.2022.982694

OPEN ACCESSEDITED BY

Manuel Will,
University of Tübingen, Germany
REVIEWED BY

Emma Loftus,
Ludwig Maximilian University of Munich,Germany
Esteban Alvarez Fernández,
University of Salamanca, Spain

Human-ecodynamics and theintertidal zones of the Zanzibar
Archipelago
Patrick Faulkner 1,2*, Akshay Sarathi 3, Alison Crowther 4,2,
Tam Smith 4, Matthew Harris 5, Abdallah K. Ali 6, Othman Haji 6,
Adria LaViolette 7, Neil L. Norman 8, Mark Horton 9,2 andNicole Boivin 2,4,10,11**CORRESPONDENCE

Patrick Faulkner,
patrick.faulkner@sydney.edu.au
Nicole Boivin,boivin@shh.mpg.de
SPECIALTY SECTION

This article was submitted to Quaternary
Science, Geomorphology andPaleoenvironment,a section of the journal
Frontiers in Earth Science1

Archaeology, School of Humanities, The University of Sydney, Camperdown, NSW, Australia, 2Max
Planck Institute for the Science of Human History, Jena, Germany, 3Department of Anthropology,
Texas A&M University, College Station, TX, United States, 4School of Social Science, The University of
Queensland, Brisbane, QLD, Australia, 5Niche Environment and Heritage, Parramatta, QLD, Australia,
6
Department of Museums and Antiquities, Zanzibar, Tanzania, 7Department of Anthropology,
University of Virginia, Charlottesville, VA, United States, 8Department of Anthropology, College ofWilliam & Mary, Williamsburg, VA, United States, 9Cultural Heritage Institute, Royal Agricultural
University, Cirencester, United Kingdom, 10Department of Anthropology, Smithsonian National
Museum of Natural History, Washington, DC, United States, 11Department of Anthropology andArchaeology, University of Calgary, Calgary, AB, Canada

RECEIVED 30 June 2022
ACCEPTED 12 September 2022
PUBLISHED 03 October 2022
CITATION

Faulkner P, Sarathi A, Crowther A,
Smith T, Harris M, Ali AK, Haji O,
LaViolette A, Norman NL, Horton M and
Boivin N (2022), Human-ecodynamics
and the intertidal zones of theZanzibar Archipelago.
Front. Earth Sci. 10:982694.
doi: 10.3389/feart.2022.982694
COPYRIGHT© 2022 Faulkner, Sarathi, Crowther,
Smith, Harris, Ali, Haji, LaViolette,
Norman, Horton and Boivin. This is an
open-access article distributed under
the terms of the Creative Commons
Attribution License (CC BY). The use,
distribution or reproduction in otherforums is permitted, provided theoriginal author(s) and the copyrightowner(s) are credited and that theoriginal publication in this journal iscited, in accordance with accepted
academic practice. No use, distribution
or reproduction is permitted which does
not comply with these terms.

The intertidal zone, covering the nearshore fringe of coasts and islands andextending from the high-water mark to areas that remain fully submerged,
encompasses a range of habitats containing resources that are as important tomodern populations as they were to humans in prehistory. Effectively bridging
land and sea, intertidal environments are extremely dynamic, requiring
complexity and variability in how people engaged with them in the past,
much as they do in the present. Here we review and reconsider
environmental, archaeological, and modern socio-ecological evidence fromthe Zanzibar Archipelago on eastern Africa’s Swahili coast, focusing on marine
molluscs to gain insight into the trajectories of human engagement withnearshore habitats and resources. We highlight the potential drivers ofchange and/or stability in human-intertidal interactions through time andspace, set against a backdrop of the signiﬁcant socio-economic and socioecological changes apparent in the archipelago, and along the Swahili coast,
during the late Holocene.
KEYWORDS

archaeomalacology, intertidal foraging, marine subsistence, human-environment
interactions, eastern Africa

1 Introduction
Coastal areas, and by extension the intertidal zone, are of high environmental
signiﬁcance. From an ecological perspective, the coastal zone represents atransitionary area between terrestrial and marine environments, providing thephysical connection between land and sea. The intertidal zone represents dynamic

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terrestrial fauna and ﬁsh evidence, they indicate the diversityof economic strategies apparent on the coast and islands,
including mollusc gathering, ﬁshing, hunting and animalhusbandry.
Greater emphasis has been placed on the archaeologicalanalyses of terrestrial vertebrate fauna and marine ﬁsh sincethe 1980s, with research undertaken in various areas between the
Lamu Archipelago on the north Kenyan coast to southwest
Madagascar (e.g., Mudida and Horton, 1996; Quintana
Morales and Horton, 2014; Prendergast et al., 2016;
Prendergast et al., 2017; Douglass et al., 2018; Quintana
Morales and Prendergast, 2018; Quintana Morales et al.,2022). Molluscs were little mentioned in the ethnographic andethnohistoric records on foraging on the eastern African islands
as reviewed by Walsh (2007), where emphasis is on terrestrial
fauna. To date, systematic assessment of molluscan assemblages
is restricted to the ethnoarchaeological work of Msemwa (1994)
and Ichumbaki (2014) on the mainland Tanzanian coast, and ahandful of studies on Unguja (Zanzibar) Island (Faulkner et al.,2018, 2019a; Sarathi, 2018, 2020), Maﬁa Island (Christie, 2013;
Faulkner et al., 2019b), and southwest Madagascar (Douglass
2017).
Perspectives on the role of molluscs in past economies
vary, from a dietary staple to supplementary or starvation food
(Erlandson, 2001). Similar interpretations of the relative
importance of these resources have occurred in eastern
Africa, where they have also been viewed as fallback or
starvation food (e.g., Msemwa, 1994; Fleisher, 2003).
Although broader syntheses of the molluscan evidence havebeen hampered by different analytical standards andincompatible data presentation (Fleisher, 2003), recent
analyses of speciﬁc site assemblages in the Zanzibar andMaﬁa Archipelagos indicate that the economic importanceof molluscs varied greatly through time and space(i.e., Faulkner et al., 2018; Faulkner et al., 2019b; Sarathi,
2020; Rødland, 2021). This suggests that how people engaged
with the intertidal zone also changed considerably, and notnecessarily unidirectionally, with long-term transitions fromhunting and gathering to urban economies.
Extending from this point of view, here we present the ﬁrst
detailed, comparative analyses of molluscan assemblages fromthe Zanzibar Archipelago. By broadening the spatial andchronological scope within a deﬁned region, we aim toinvestigate the nature of human interactions with nearshore
environments, to identify change or stability in how people
foraged and made decisions when engaging with the intertidal
zone. We take an ecologically oriented approach tounderstanding human ecodynamics (following Fitzhugh
et al., 2019), considering the nature of palaeoenvironmental
shifts and contextualised within patterns of socio-economic/
cultural changes, to provide a baseline understanding ofintertidal foraging and decision-making through time.

and complex systems, largely due to long- and short-term
variability in tidal range and amplitude, wave energy,
ﬂuctuating salinity levels and temperature, and the effects
of high-energy storm activity. Although nearshore coastal
habitats present a unique set of challenging conditions forhuman exploitation as a result of these factors, importantly,
the intertidal zone often comprises highly productive
ecosystems. This is particularly the case for eastern Africa,
which is characterised as containing highly productive coastal
habitats and rich biodiversity (Richmond, 2011; Nordlund
et al., 2014). Often forming one of the most prevalent groupsof taxa inhabiting the intertidal zone, molluscs ﬁll crucial roles
within marine ecosystems, being carnivores/predators andherbivores/detritivores operating at all trophic levels. They
serve as primary consumers, increase habitat complexity, andimprove water quality through ﬁltration.
Human engagement with intertidal habitats extends backminimally 160,000 years in southern Africa. Intertidal
foraging and mollusc harvesting are pivotal elements inongoing debates about human evolution, adaptation to
dynamic marine environments, and behavioural modernity(Klein and Bird, 2016; Marean, 2016; Will et al., 2019).
Although there is ongoing debate over the timing andnature of initial coastal resource use, when people became
coastally adapted, and transitioned to fully maritime societies(Marean, 2014; Fleisher et al., 2015; Jerardino, 2016), of key
importance when considering the nature of humanengagement with marine environments is that the intertidal
zone connects the land, and therefore people, to the sea.
Intertidal foraging is thus an important component ofmarine resource use through time, with intertidal resources
providing critical evidence in understanding past humanecodynamics in coastal settings. Molluscs therefore play apotentially important role in disentangling key elements ofhuman interactions with nearshore environments. This isparticularly true on eastern African coasts and islands,
where our understanding of intertidal foraging andmolluscan resource use is less well developed.
Msemwa(1994)
notedin
his
foundational
ethnoarchaeological study of mollusc harvesting on the
Tanzanian coast that the literature on shellﬁsh collection
across the eastern Africa intertidal zone was particularly
limited. While archaeological research has grown
substantially since then, archaeomalacology has not
signiﬁcantly progressed in the region, a point echoed inseveral recent studies. In their review of zooarchaeologicalevidence from the coast and islands of eastern Africa, forexample, Quintana Morales and Prendergast (2018) commenton the richness and productivity of the intertidal zone, butnote that their consideration of molluscan evidence is limited
due to the lack of regional systematic archaeomalacologicalanalyses. Although their focus is on wild and domesticated

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FIGURE 1
The Zanzibar Archipelago study area, showing locations mentioned in the text on Unguja Island (A) and Pemba Island (B) with the distribution of
the dominant benthic habitats (Spalding et al., 2010; Khamis et al., 2017 [with permission from Elsevier, order no. 5338180474815]; UNEP-WCMC,WorldFish Centre, World Resources Institute, The Nature Conservancy, 2021; UNEP-WCMC and Short, 2021). Map created using ArcGIS Pro 2.9.1.
Bathymetric data from GEBCO Compilation Group. (2021), administrative boundaries from GADM (2022).

2 Backgroundwith deeper soils in these areas being more favourable for
agriculture compared with the coral rag and shallow soils of
the eastern coasts (Arthurton et al., 1999; Khamis et al., 2017).
The vegetation of the coastal lowlands and islands of eastern
Africa forms the Zanzibar-Inhambane regional mosaic,
containing grassland, shrubland, and a mix of dry and moist
forests (Burgess et al., 1998). At present, natural forest areas arelargely restricted to protected areas, such as Jozani-Chwaka on
Unguja and Ngezi-Vumawimbi on Pemba (Khamis et al., 2017).
The Unguja and Pemba coastlines are characterised byintertidal fringing platforms, commonly extending more than
1 km (in some areas up to 3 km) from the shore, with associatedmuddy ﬂats, sand ﬂats and beaches, and coral reefs. The
northwest and eastern coasts of Pemba, and south andnorthwest coasts of Unguja, have narrow intertidal zones, with

2.1 Unguja and Pemba Islands, Zanzibar
Archipelago
The Zanzibar Archipelago, adjacent to mainland Tanzania(Figure 1), consists of c.50 islets and two large islands: Unguja at
1,670 km2 and 119 m above sea level, and Pemba at 990 km2 and96 m above sea level. Unguja is separated from continental Africa
by 25–30 km of shallow ocean (Zanzibar Channel, c.35–65 mdeep), whereas Pemba sits c.55 km from the mainland
surrounded by deeper waters, being separated from thecontinent by the c.400–800 m deep trench of the Pemba
Channel (Agnarsson and Kuntner, 2012; Khamis et al., 2017).
Higher precipitation occurs on the western sides of both islands,

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merchant elite that controlled trade along the coast and between
coast and interior. Some stonetowns, such as the UNESCOWorld Heritage site of Kilwa off the central Tanzanian coast,gained sufﬁcient wealth and power to function as autonomous
city-states (Kusimba, 1999).
The 1500s saw the beginning of the colonial period, when
foreign powers sought to dominate the Swahili coast. The
Portuguese arrival in the Indian Ocean in the late 15th
century was matched by Ottoman expansion into the Red Sea
in the 16th century. While the Ottomans claimed theoreticalsovereignty over much of the Swahili coast, the Portuguese set up
factories and fortresses in some major ports to project power inreal terms. On Unguja, the Portuguese period is represented bysites such as one located beneath the Old Fort in Stone Town, andMvuleni in the northwest. Portuguese-period sites suggest a shift
in trade networks as new coastal settlements were established andolder ones were abandoned. The great Swahili city of Kilwa, forexample, was abandoned after it was attacked by the Portuguese.
The decline of Portuguese power in the 18th century coincidedwith the rise of Omani dominance over much of the Western
Indian Ocean. Ultimately, the Omani Empire was partitioned,
leading to the establishment of the Sultanate of Zanzibar (18561964) which itself became a British protectorate in 1890
(Glassman, 2011).
The year 1964 brought revolution to the archipelago andeventual merger with the Tanganyikan mainland to form modern
Tanzania. Since its independence from the Omanis, Zanzibar has
modernised in a variety of ways. As a result, modernity and its
social, cultural, and economic contexts represent a rupture fromthe past in many respects. The tourist industry, with its enormousinternational investments, has severely disrupted traditionalmodes of subsistence—more so on Unguja than Pemba—with
the rise of industrial ﬁshing and increasing availability of
motorised boats affecting how ﬁshing and shellﬁsh gatheringtake place. Traditional modes of subsistence persist, however, inplaces such as Mifupani on Unguja.

sandy-muddy shores dominating the west coasts. Much of theintertidal platform supports a cover of unconsolidated sedimentsthat form the substrate of seagrass meadows. The platforms andfringing beaches on both islands’ eastern coasts are ﬂanked bysandy beach-ridge plains, the seaward edges of which are marked
by prominent storm ridges (Arthurton et al., 1999; Arthurton,
2003; Khamis et al., 2017). In sheltered, shallow embayments,
such as those on the south, west and northern coasts of Pemba,
and north and southwest coasts and in Chwaka Bay of Unguja,
extensive mangrove and seagrass habitats with deeper sand/mud
substrates are common. Some of these areas have developed more
recently due to ongoing sedimentation and processes ofprogradation (Arthurton et al., 1999). Unguja’s reefs support alarge proportion of regional and global reef-coral diversity(Zvuloni et al., 2010). Living corals cover c.90 km2 across thearchipelago, although corals are primarily distributed around thetwo main islands. Corals form continuous reefs on the eastern
coasts, with a patchier distribution on the west, particularly on
Unguja (Khamis et al., 2017).

2.2 Archaeological and historical overview
The Zanzibar Archipelago evidences human occupation
dating to 20,000 BP at Kuumbi Cave on Unguja. Kuumbi’s
pre-Holocene occupation corresponds with a land bridge thatconnected Unguja to the mainland. Occupation at Kuumbi
disappears with the submergence of that connection at thestart of the Holocene. Kuumbi was not reoccupied until the
mid-ﬁrst millennium CE (Chami, 2009; Kourampas et al., 2015;
Shipton et al., 2016), when ironworking, farming communities
expanded from the mainland onto the Zanzibar Archipelago.
These communities are associated with a regional ceramic
tradition known as Early Tana Tradition/Triangular Incised
Ware (ETT/TIW) that marks the Middle Iron Age (MIA, c.
600–1,000 CE) (Fleisher and Wynne-Jones, 2011).
Contemporaneous with Kuumbi’s reoccupation included theappearance of small villages across the archipelago such as
Fukuchani in north Unguja, and trading ports such as Unguja
Ukuu in the south (Juma, 2004). This period was characterised by
the rise of Indian Ocean trade on the Swahili coast, linking it with
China, South Asia, and the Middle East (e.g., Juma, 2004;
LaViolette, 2008; Wood et al., 2016), as well as theintroduction of Islam (Crowther et al., 2015; Fitton and
Wynne-Jones, 2017; Sarathi et al., 2022).
The abandonment of Unguja Ukuu in the early second
millennium CE (Juma, 2004) coincided with new economic
and political growth on the coast. The period from c.1000 to1500 CE saw the emergence of urban centres at localities such as
Msuka Mjini, Ras Mkumbuu, and Mvuleni (Horton andMiddleton, 2000). These sites, often referred to as“stonetowns”, were marked architecturally by the constructionof stone and coral houses, mosques and tombs and housed a

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2.3 Study site locations, descriptions andchronology
2.3.1 Northern Pemba: Pango la Watoro andMsuka Mjini
North Pemba supports coral reefs, mangroves, intertidal andsubtidal ﬂats with seagrass beds and algal growth. Some areas
contain rocky cliffs and intertidal rock ﬂats while others havelong sandy beaches. On the western side, numerous embayments
are characterised by shallow water, sand banks, seagrass, andmangrove forests. The northern tip (Ras Kigomasha) has a wide
intertidal zone with vast seagrass meadows. Fringing coral reefs
surround Ras Kigomasha with some interruptions due to input
from freshwater channels. Reefs on the western side are similar tothat described below for southern Pemba, with reefs on the east04

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Southern Pemba supports coral reefs, mangroves, intertidal
and subtidal ﬂats with seagrass beds and algal growth. The
continental shelf is narrow and deep waters lie close to theshore. This combination of characteristics has resulted in highcoastal species diversity. Around Ras Mkumbuu, the peninsula issurrounded by expansive sand and mudﬂats fringed by coral reefand containing areas of patch reef. Mangroves are common
around the mouths of rivers and creeks and in the shelteredbays of this area. There are rocky outcrops and headlands withsmall isolated sandy beaches in small bays and rocky inlets. The
expansive intertidal sand and mudﬂats backed by mangrove
forests support high taxonomic diversity (Richmond, 2014).

having a steep slope and low coral cover due to strong wave
action (Richmond, 2014). To Ras Kigomasha’s southeast is arelatively large area of mangroves within Ngezi Forest and at themouth of a creek and estuary.
Pango la Watoro, the northernmost site in our study, is asmall limestone cave that fronts directly onto a sandy beach on
the western side of Ras Kigomasha. Excavations in the cave (1 ×
2.5 m) by the Sealinks Project in 2012 uncovered low densities ofcultural remains throughout the deposits, including c.17th-18th
century pottery, fragments of iron implements, charred millet
grains, and terrestrial vertebrate faunal remains, as well as a large
molluscan assemblage. Although analyses of the non-molluscan
excavated components are ongoing, the available evidence
suggests that the cave was used only intermittently over thelast few hundred years.
Msuka Mjini is a large settlement located at thesoutheastern end of Ras Kigomasha, c.300 m west of its
eastern shoreline. The site covers c.2ha and is known
primarily for the ruins of a large, well-preserved 15th
century mosque situated at its centre. The mosque famously
bears an inscription of the date AH 816 (1413–4 CE) on theinside of its mihrab (Horton, 2017). East of the site is a long
sandy beach with a large and expansive sand/mud and rocky
intertidal zone fringed by an uninterrupted coral reef. A single
1 × 1 m test trench was excavated at the site in 2016 by the
Sealinks Project, uncovering a series of rapidly formed midden
deposits. These contained c.13th–15th-century pottery
including Middle Eastern and Chinese trade wares, terrestrial
vertebrate faunal remains, and ﬁsh bone, with analyses of thesematerials in progress.

2.3.3 Northern Unguja: Fukuchani and Mvuleni
The northern coast of Unguja is characterised by expansive
sand ﬂats and rocky platforms, seagrass beds, patch, and fringing
reefs, with little representation of mangrove communities(Richmond, 2014).
The site of Fukuchani is situated on the northwest coast of
Unguja, opposite Tumbatu Island. It comprises ten mounded
midden deposits (c.2 m × 10 m) running parallel to the coastline
and the remains of daub structures (Horton and Middleton,
2000; Horton and Chami, 2018). Three trenches excavated by the
Sealinks Project in 2011 (FK10, FK11, FK12) revealed Middle
Iron Age occupation evidence, with radiocarbon dates anddiagnostic ceramics indicating site use in the 7th–8th
centuries CE (c.1,350–1,450 BP). In addition to substantial
midden deposits, wild terrestrial vertebrate fauna, near-shore
marine ﬁsh (primarily reefs and estuarine), Middle Eastern trade
wares, Early Tana Tradition/Triangular Incised Ware (ETT/TIW) ceramics, shell and glass beads, and charred millet
grains were recovered (Crowther et al., 2016; Prendergast
et al., 2017). Fukuchani’s molluscan assemblages were
presented in Faulkner et al. (2018), with the initial analyses
suggesting a primary focus on rocky habitat taxa akin to reef
sweeping. The Fukuchani assemblages are reanalysed here,combining data from the three trenches and reassessing
taxonomic composition and habitat at a higher resolution.
Mvuleni is a late 16th–17th-century Portuguese site locatedat the foot of an escarpment c.750 m inland from Fukuchani withevidence for commercial farming (e.g., tobacco). The site wasinvestigated through detailed mapping and photogrammetry,
shovel-test pit (STP) survey, and test and block excavations to
identify activities and implications for Portuguese-Swahili
interactions in a rural setting (LaViolette and Norman, 2023).
A large central house/warehouse dominates, built by Swahili
craftsmen of mortared coral-rag, surrounded by a loopholed
coral-rag wall. The walling extends north around space where thecoralline substrate is at ground surface and a staircase with accessto a shallow limestone cave with brackish water; there is local
memory of fresh water there. The walling extends south around a
2ha plot of farmland, evidence for earth-and-thatch structures,and latrines and washing spaces. Inside, a small bastion juts out

2.3.2 Southern Pemba: Ras Mkumbuu
The site of Ras Mkumbuu is located at the end of a long,narrow and low-lying peninsula on the western side of Pemba.
Covering 10ha at its zenith, the site boasts one of the mostextensive and best-preserved set of ruins on Pemba, and has
attracted archaeological attention since the 1930s. Survey andexcavations by Kirkman (1959) and Horton (2017)
established the presence of two main settlement phases:
10th–12th centuries CE (c.1,050–750 BP) focused on thesite’s eastern ridge, and 14th–16th centuries CE(c.550–350 BP) at the site’s western end. Ras Mkumbuu isrich in cultural materials, including diverse imported pottery,glass and stone beads, and metal objects, attesting to its
important role as a Swahili trading port and stone town. In
2012, the Sealinks Project excavated four small (1 × 1 m, 2 ×
1 m) test trenches at the eastern end of the site. Three of thesetargeted an area adjacent to the beach at the base of the easternridge, where midden material was located. The fourth wasplaced into the side of a mound on the top of the ridge, which
cut through a shallow sequence of compacted house ﬂoors. A
range of cultural materials, c.10th–12th century in date, wererecovered.

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have been no more than 7–8 km from the shore due to the steep
continental shelf off eastern Unguja (Prendergast et al., 2016).
The excavations, stratigraphy, and chronology from the2012 Sealinks Project excavations have been previously
detailed (e.g., Kourampas et al., 2015; Prendergast et al., 2016;
Shipton et al., 2016; Faulkner et al., 2019a). The terrestrial andmarine molluscan assemblages reported in Faulkner et al.
(2019a) focused on the role of these resources in long-termcoastal economic adaptations. Highlighted was the use ofdifferent intertidal habitats through time relative to postglacial
sea level rise, with people using a generalised, sweeping strategyof habitat foraging. Here we restructure and re-analyse theKuumbi marine molluscan assemblages, with these data fallingwithin four main chronological phases: a late Pleistocene
occupation (combining the previously identiﬁed Phases 3 and4) that spans c.20,000–17,000 BP; terminal Pleistocene-early
Holocene (Phase 2: c.13,000–11,000 BP); and separating thePhase 1 layers reported in Faulkner et al. (2019a) into theMiddle Iron Age (as 1B: c.1,350–1,000 BP) and Late Iron Age(as 1A: c.1,000 to 500 BP) following Prendergast et al. (2016).
On the southeastern coast of Unguja, northeast of Kuumbi
Cave, research undertaken in Mifupani village provides a robust
sample of modern mollusc harvesting. The people of Mifupani
gain their livelihood participating in the tourist industry, ﬁshing,mollusc gathering, horticulture, raising livestock, and somehunting. Sarathi (2020) conducted a 12-months subsistence
study of ﬁsh and invertebrates gathered there for comparison
with archaeofaunal datasets; the marine vertebrate data are notconsidered in the analyses below as our focus is on intertidal
foraging and molluscan assemblages. People can forage along the
sandy beach, across the intertidal platform comprised of rock,sandy substrate and seagrass, and onto subtidal reef fringing theintertidal zone c.2–3 km from shore. Although people forage forbivalves (Donax incarnatus) by digging through sand, mostmollusc gathering occurs on the intertidal reef ﬂat or justbeyond it (Sarathi, 2020). The Mifupani study complementssocio-ecological research in Nungwi, Chwaka Bay and Menai
Bay (Nordlund et al., 2010; Yahya, 2013; Fröcklin et al., 2014), but
is more longitudinal and comprehensive, providing a modern
baseline that can be used alongside archaeomalacological datasets
on Pemba and Unguja.

from the seaward wall. STPs and excavations around the bastion
revealed the highest density of midden materials at the site,
including locally made and European ceramics, a game token andmarble, iron knife, crossbow bolt, animal bone (analysis of which
is ongoing), and many kgs of burned mussel and crab shell. On
the escarpment are 170 m of unmortared coral-rag walls which
provided an extraordinary view of activity in Tumbatu andZanzibar Channels.

2.3.4 Southern Unguja: Unguja Ukuu, Kuumbi
Cave and Mifupani
Unguja Ukuu is located on Menai Bay, on the southwest
coast of Unguja. Menai Bay is characterised by extensive algal
ﬂats and seagrass beds, with patch reefs in deeper parts of the bay,and fringing reefs on the seaward edge of the rocky platforms andsmall islets. The eastern shore of Menai Bay is a back-reef
shoreline, or intertidal shelf, containing incised tidal channels,
and extending up to 2 km offshore to the fringing reef system.
Mangroves occur in the northern, western, and eastern marginsof the bay, with dense forests associated with Uzi Creek andChannel (Berkström et al., 2013; Punwong et al., 2013b; Yahya,
2013; Kotarba-Morley et al., 2022).
Unguja Ukuu is a large proto-urban centre and signiﬁcanttrading port. Based on numerous radiocarbon dates and presenceof diagnostic ceramics, occupation there dates to the Middle Iron
Age, 7th–10th centuries CE (c.1,350–1,050 BP) (Crowther et al.,2016). Earth-and-thatch structures are associated with signiﬁcantvertebrate assemblages and middens, as well as evidence for a mix
of industrial (lime burning, iron slag) and domestic activities.
Similar to Fukuchani, ETT/TIW ceramics and shell and glass
beads were recovered, in addition to bead grinders, local andimported trade wares, and African and Asian crops. The
vertebrate evidence indicates caprine herding, wild animal
exploitation, and the remains of fast-swimming ﬁsh (tuna,potentially shark) from deeper waters alongside the reef andestuarine taxa (Juma, 2004; Crowther et al., 2016; Prendergast
et al., 2017; Culley et al., 2021). Geoarchaeological evidence fromtwo trenches (UU11, UU14) provide a c.200–350-years sequenceof geomorphological change and coastal progradation, combinedwith increasing human occupation, deposition of anthropogenic
debris and potentially land clearance for agricultural activity(Kotarba-Morley et al., 2022; see also Fitton, 2017). Molluscan
remains from four of ﬁve trenches excavated by the Sealinks
Project in 2011–2012 (UU11, UU13, UU14, UU15) were
published by Faulkner et al. (2018), focusing on establishingbasic assemblage structure and an initial understanding of themollusc exploitation there. As with Fukuchani, data from thefour trenches are combined and reanalysed here to enable robust
comparisons with the other assemblages.
Kuumbi Cave is in the Jambiani district of southwest Unguja,
c.2.5 km from the present coastline. One of a series of solutional
limestone caves within marine limestone terraces of Pleistocene
age (Kourampas et al., 2015), from c.20,000 BP the site would

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2.4 Palaeoenvironmental context
Assessment of relative sea levels in the southwestern Indian
Ocean has indicated a high degree of regional complexity in thetiming and nature of post-glacial sea levels. Variability in relative
sea levels across this broad area is likely due to glacio-isostatic
processes, tectonic movement, and changing oceanographic
conditions (e.g., Pirazzoli, 1991; Camoin et al., 1997;
Woodroffe et al., 2015; Prendergast et al., 2016). Regionally,
there appear to be two broad types of relative sea level patterns

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FIGURE 2
Bathymetry of the Tanzanian coast and Zanzibar Archipelago, including the location of the marine sediment cores for sea level andpalaeoenvironmental reconstruction (A). Reconstructed sea levels at c. 20,000, 12,000 and 9000 BP (B) after Prendergast et al. (2016). Map createdusing ArcGIS Pro 2.9.1. Bathymetric data from GEBCO Compilation Group. (2021), administrative boundaries from GADM (2022).

(Camoin et al., 2004; Woodroffe and Horton, 2005). The ﬁrst
represents a constant rate of sea level rise to 3,000–2,000 cal BP
without evidence for a mid-to late Holocene highstand. Thispattern appears prevalent on offshore volcanic and coral islands,e.g., Réunion, Mauritius, Mayotte, and the Seychelles (Colonna
et al., 1996; Camoin et al., 2004; Woodroffe et al., 2015). The
second pattern, following a rapid rise during the terminal
Pleistocene-early Holocene, indicates a highstand in the orderof 2.5–3 m above present c.6,000 cal BP, with a subsequent slow
regression to present levels in the late Holocene. This second
pattern has been identiﬁed in Madagascar, and on the Kenyan
and Mozambican coasts (Camoin et al., 2004; Woodroffe et al.,2015; Accordi and Carbone, 2016).
The recent reconstruction of relative sea levels presented byPrendergast et al. (2016) provides a localised sea level curve for
the Zanzibar Archipelago over the last 20,000 years. As Pemba

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Island effectively lies off the continental shelf, it was isolated
during the Last Glacial Maximum (LGM) (and since the early
Pliocene; Rowson et al., 2010) due to lower sea levels and the
800 m deep Pemba Channel (Stagna et al., 2022). The
Prendergast et al. (2016) model indicates that Unguja wasconnected to the mainland until c.9,000 BP, with a phase of
more rapid sea level rise between c.12,000–9,000 BP (Figure 2;
Figure 3A). This accords well with two pulses of rapid sea level
rise recorded in Mayotte in the Comoros, dated between
12,800 and 12,400 BP and between 10,100 and 9,600 BP,
followed by a gradual increase to roughly present levels fromc.7,000 to 1,500 BP (Colonna et al., 1996). The sea level model isalso largely supported by mangrove observational data from the
Ruﬁji Delta (Punwong et al., 2013a) on the southern mainland
Tanzanian coast, as well as in Unguja Ukuu and Makoba Bay(Punwong et al., 2013b, 2013c; Woodroffe et al., 2015) in the07

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FIGURE 3
Composite sea level curve spanning 20,000 BP to present (A) adapted from Colonna et al. (1996) with permission from John Wiley & Sons (orderno. 5334670340538); Prendergast et al. (2016); high resolution SST record, Pemba/Zanzibar Channel (B) adapted from Kuhnert et al. (2014) withpermission from Cambridge University Press (order no. 5334730499127); combined site occupation chronology for Unguja and Pemba (see Table 1
for further site-speciﬁc chronological and assemblage details) and major climatic phases since the LGM in eastern Africa (C) based on
information in Liu et al. (2018).

1,550–1,450 cal BP, whereas at Unguja Ukuu, the mangrove
observational and archaeological data suggest sea level was
0.5–1 m below present levels between c.1,300 and 1,000 cal
BP, followed by a more recent transgressive episode (Mörner,
1992; Punwong et al., 2013b, 2013c; Prendergast et al., 2016;
Punwong et al., 2018). Although mangrove communities havebeen a feature of the Unguja and Pemba coastlines fromc.8,000–7,000 years ago, there is evidence for a reduction in
the extent of mangroves (e.g., around Unguja Ukuu) sincec.530 cal BP. This reduction has been linked to increasing useof mangrove wood as construction material and fuel (Punwong
et al., 2013b).
Modern development of the expansive reef systems fringing
Unguja and Pemba was likely initiated in the early Holocene
based on sea levels, with reef accretion tracking sea level rise(albeit with a time lag between sea level rise and reef growth) until
the reefs reached their near-surface position by the midHolocene (Camoin et al., 1997; Camoin et al., 2004). By thattime, rising sea levels would have breached the ocean-facing
fringing reefs to cover the limestone platforms noted above. The
ﬂooding of these platforms would have signiﬁcantly increased
reef system development and intertidal/shallow subtidal habitats
around the islands (Arthurton et al., 1999; Arthurton, 2003). In

southwest and northwest of Unguja Island. Sea surface
temperature (SST) varies through time, with cooler periods
from 9,700 to 7,800 cal BP and 5,600–4,200 cal BP,
interspersed with warm (7,800–5,600 cal BP) and moderate(4,200–1,350 cal BP) temperatures (Figure 3B). Within theseoverall trends are high-frequency SST variations at centennial
scales (Kuhnert et al., 2014).
Importantly, these mangrove data also indicate a phase ofearly to mid-Holocene sea level rise and late Holocene sea levelﬂuctuation, with sea levels potentially higher than present at
c.4,700–4,600 cal BP. Although variable in the rate and timing ofsea level rise, it appears that sea levels possibly increased to
0.5–0.6 m above present at Makoba Bay and Unguja Ukuu. This
minor increase above present levels is partly supported by theobservations of maximum erosional notches on undercut
limestone cliffs on Unguja (Arthurton et al., 1999), estimatedto approximate that of contemporary higher water levels at
spring tides. Mangrove development continued after the mid
Holocene, with lower sea levels around c.4,000 cal BP and
c.2,000 cal BP, albeit with variability in the nature of
ﬂuctuations between locations attributed to local mangrove
response to sedimentation and/or erosion (Punwong et al.,2018). For example, at Makoba Bay, sea levels rose after

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phases of occupation in the archipelago, even accounting for
ﬂuctuating sea levels and associated habitat reconﬁguration, thenearshore environments around the islands were largely
established by c.3,000–2,000 years ago.

combination with these processes, relatively low sedimentation
rates identiﬁed for Unguja Ukuu and Makoba Bay(0.3–6.6 mm cal yr−1) on Unguja indicate minor progradation
effects in the archipelago (Punwong et al., 2018), at least until thelatter half of the ﬁrst millennium CE with increased
anthropogenic impacts associated with coastal settlements(e.g., Fitton, 2017; Kotarba-Morley et al., 2022).
Palaeoclimatic evidence from eastern Africa indicates
temporal and spatial complexity since the LGM (Figure 3C).
As summarised by Liu et al. (2018) in comparing broader
climatic records with a marine sediment core extracted 40 km
offshore, from the mouth of the Ruﬁji River (GeoB12624-1), theLGM is broadly characterised by arid conditions, with two
distinctive dry-wet periods (at 15,000–14,500 BP and
11,500–11,000 BP). The latter mark the transition into theAfrican Humid Period of the late Pleistocene-early Holocene.
Severe drought would have been experienced in the area north of
8–10°S during the Heinrich Stadial 1 (c.18,000-14,600 BP) andthe Younger Dryas (c.12,800–11,500 BP), which includes theZanzibar Archipelago. The African Humid Period represents asigniﬁcant phase of climate change across the continent during
the Holocene. Based on analyses of marine sediments andbenthic foraminifera extracted from an offshore core locatedc.20 km from the Pangani River mouth, at the western transition
between the Pemba and Zanzibar channels (GeoB12605-3), theAfrican Humid Period is represented by two regional phases. The
ﬁrst is an intense humid period from c.12,000 to 8,000 cal BP(with an abrupt drought event around c. 8,200 cal BP), with asecond, moderate humid phase from 8,000 to 5,500 cal BP. Thesehumid phases were followed by a transitionary period between
5,500 and 3,500 cal BP and a broad late Holocene arid phase(Romahn et al., 2015; Liu et al., 2017). Evidence of sedimentation
rates from this core supports these broad climatic phases, with
continuous sedimentation between 9,700 and 1,350 BP, although
at a decreasing rate from the core base to the top. The higher
sedimentation rates between 9,700 and 8,000 cal BP are
attributed to lower sea levels and increased precipitation in
the Tanzanian hinterland, followed by a decrease in
sedimentary input due to increasing aridity and higher sea
levels in the mid to late Holocene (Kuhnert et al., 2014; Liuet al., 2016). More recent climatic ﬂuctuations occur acrosseastern Africa, with increased aridity from c.950-680 BP,
wetter conditions from c.680 to 100 BP, and persistent
drought conditions spanning the late 18th and early 19th
centuries (Lane and Breen, 2018).
There are gaps in the known occupation sequences for
Unguja and Pemba, and critically between c.12,000 and
1,500 BP which represents a period of signiﬁcantenvironmental change connected to sea-level rise, separationof Unguja from the mainland and broad climatic ﬂuctuations.
Nonetheless, reviewing the baseline paleoenvironmental data
provides a clear frame of reference for the available
archaeological evidence. Importantly for the late Holocene

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3 Materials and methods
3.1 Identiﬁcation and abundance
All archaeological molluscan assemblages were analysed at
the House of Wonders Museum (Beit-el-Ajaib) or the Peace
Memorial Museum (Beit-el-Amani) in Stone Town, Zanzibar,
between 2013 and 2019. Identiﬁcations were made to the lowest
taxonomic level possible (family, genus, or species) based on
published keys, descriptions, specimen photographs (e.g., Spry,
1964, 1968; Abbott and Dance, 1998; Carpenter and Niem, 1998;
Robin, 2008, 2011; Richmond, 2011), and in comparison with thephysical reference collections housed at the University of Sydney.
Shell modiﬁcation was also noted, including burning, damage by
marine sponges and polychaete worms, drilling and boring,presence of vermetids (sessile gastropods which may attach toother shells), hermitting by crabs, root etching, and dissolution of
the shell (chalkiness). Where they do occur, evidence of burning(2.6%–7.2%), hermit crab alteration (0.1%–0.2%) and predatory
modiﬁcation/epibiont adhesions (0.1%–1.7%) are minimallyrepresented across the assemblages. Evidence of water rollingis also minimal, ranging between 0.1 and 3.0% of assemblages,
although there is the possibility of these kinds of taphonomic
indicators being obscured by the effects of shell dissolution inthese sites. No anthropic modiﬁcation of the shell (i.e., forornament production or tool use) was recorded for any of theassemblages. Quantiﬁcation is based on the calculation of the
Minimum Number of Individuals following Harris et al. (2015)
and Giovas (2009), using a range of taxon-speciﬁc non-repetitiveelements (outlined in Faulkner et al., 2018; 2019a). Previously
published molluscan data from Unguja Ukuu and Fukuchani(Faulkner et al., 2018) and Kuumbi Cave (Faulkner et al., 2019a)
have been reviewed, the taxonomic attributions revised whererequired, and additional habitat data compiled. Similarly,
modern molluscan assemblage data has been compiled for thevillage of Mifupani in the Jambiani district (Sarathi, 2020). The
taxonomic attributions for all archaeological and modern
assemblages were checked using the World Register of Marine
Species (WoRMS Editorial Board 2022) to ensure that the mostup to date nomenclature was recorded.
The following methods are applied to the data at genus andspecies levels separately to investigate variability in foraging atdifferent taxonomic levels. We do not differentiate between taxa
identiﬁed as economic or incidental taxa. Economic taxa include
those species incorporated into the diet and for the production oftools and ornaments, with incidental taxa deﬁned as those likely
to represent harvesting by-products (e.g., juveniles, small-bodied

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contextual information to test whether a series of assemblages
distributed through time and space may be derived from similar
palaeoecological communities (Peacock et al., 2012; Wolverton
et al., 2016).
Nestedness analyses are based on species presence-absence
data derived from a site or sampling location. The presenceabsence matrix is ordered by the most common species (to theupper rows) and locations with the greatest richness (to the left
columns), with nestedness indicated by the concentration of
species present in the upper left matrix corner (Ulrich andAlmeida-Neto, 2012). The most typically used method ofquantifying nestedness is the matrix temperature measure (T),with values ranging between 100° (no nestedness) and 0° (perfectnestedness) (Atmar and Patterson, 1993; Ulrich et al., 2009). As it
appears unclear whether the temperature metric should beapplied to networks of interacting species (Bascompte et al.,2003), as is effectively the case in investigating archaeologicalassemblages derived from the interaction of humans with otherspecies of interest, the NODF metric is also calculated (AlmeidaNeto et al., 2008; Ulrich et al., 2009). This measure is based on thestandardised differences in column and row ﬁlls, combined withthe overlap of presences in adjacent columns. The NODF values
range from 0 (no nestedness) to 100 (perfect nestedness).
Nestedness matrices, metrics and signiﬁcance (Z-scores andp-values) are calculated using the NeD (http://purl.oclc.org/ned) online interface (Strona et al., 2014).
Correspondence analysis and chord distance analysis areused here to explore similarities and differences in thetaxonomic composition of the assemblages (e.g., Faith andLyman, 2019; Harris et al., 2016; Semken and Graham,
1996). Correspondence analysis is a method of ordinationthat displays the rows and columns of a data matrix as
points in corresponding low-dimensional space, such as abivariate scatterplot. As such, assemblages will plot closer
together when similar in taxonomic composition, with taxa
plotting closer to those assemblages to which they are morestrongly associated (Faith and Lyman, 2019). Chord distance isa metric used for abundance data (as a measure of Euclidean
distance between normalised vectors), useful for assessing
dissimilarity between samples or assemblages (Legendre andGallagher, 2001). Chord distance provides a useful measure ofdissimilarity as taxa are not heavily weighted when represented
by single individuals (Harris et al., 2016). The values derived
from chord distance analyses range between 0 (no difference in√relative abundance) and 2 (no taxa in common) (Faith andLyman, 2019). Chord distance is calculated as:taxa) or natural incorporations into the deposit. As our focus ison investigating the nature of intertidal foraging strategies, weview all taxa as representing the intertidal habitats from which
they originated regardless of whether they were harvested
incidentally, and as such are considered to provide important
information linked to foraging across the intertidal zone. The
exceptions to this are the small Batillariidae (mudcreepers) and
Vermetidae (worm shells) gastropods, as these taxa largely reﬂect
storm surge deposition or are cemented to other mollusc shells(such as Pinctada spp.). These taxa have the potential to skew
results of the habitat analyses as a result and are therefore
excluded. The Polyplacophora (chiton) are included as
separate categories within each of the analyses as these taxa
have been attributed to class level only within each dataset (andare therefore independent categories).

3.2 Richness, nestedness and taxonomic
composition
Taxonomic richness is deﬁned as the number of species orother taxonomic category (family, genus) in a sample, andrepresented as NTAXA (Grayson, 1984; Lyman, 2008). This isa simple and effective measure of assemblage richness, provided
that the taxonomic categories identiﬁed for analysis are
independent, although it can be adversely affected by sample
size and may conceal variability in assemblage evenness/dominance. As such, it is important to combine species
richness with the measures of heterogeneity focused on
dominance and/or evenness discussed further below(Magurran, 2004; Santoro et al., 2017).
In ecology and biogeography, nestedness analyses have beenused to investigate species composition, distribution, andinteractions across continental and island ecosystems (Ulrich
and Almeida-Neto, 2012). First described for insular faunas, suchas those found on many islands, nestedness has been related topatterns of colonisation and extinction. Therefore, the general
expectation is for an ordered pattern of species presence andrichness, where the species composition of a small assemblage is anested subset of a larger assemblage. Similar trends have been
identiﬁed at the meta-community level, with strong nestedness
reﬂecting species traits and environmental conditions(Bascompte et al., 2003; Ulrich et al., 2009; Ulrich andAlmeida-Neto, 2012). In archaeology, nestedness has beenused to complement assessment of the relationship between
taxonomic composition and sample size. In these analyses,
smaller assemblages with low richness should nest withinlarger assemblages if they have been derived from the samecommunity (e.g., Wolverton et al., 2015; Faulkner et al., 2021).
This also has the potential to shed signiﬁcant light on past humanbehaviours, such as foraging strategies, selection, andtransportation. This is especially true when consideredalongside the available environmental and archaeological

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
xji xki
CDjk  2 − 2 
x2ji x2ki
where xji is the abundance of taxon i in assemblage xki and i isthe abundance of taxon i in assemblage k.

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variations in richness and sample size effects, has high efﬁciency,
has the greatest discriminatory power for uneven communities,
and appears to be less sensitive to inter-site differences than the
Shannon index (Magurran, 2004; Faith and Du, 2018).
As with the correspondence and chord distance analyses, thediversity analyses were undertaken using the PAST
Paleontological Statistics Package Version 4.10 (Hammer
et al., 2001). Conﬁdence intervals at 95% for the Shannon andSimpson indices were generated via bootstrapping(9,999 iterations), with diversity permutation tests(9,999 random permutations) used to assess signiﬁcantdifferences in each measure (Hammer and Harper, 2007;
Harris et al., 2016; Santoro et al., 2017).

The correspondence analyses and chord distance analyses
were performed using the PAST Paleontological Statistics
Package Version 4.10 (Hammer et al., 2001).

3.3 Diversity indices
Heterogeneity measures can be divided into Type 1 indicesthat are affected by the occurrence of rare species (richness), andType 2 indices sensitive to the abundance of the most common
species (dominance). Evenness and dominance refer to the
distribution of specimens across taxonomic categories. Aneven assemblage is one where all species are similarly
abundant, and an uneven assemblage one where one or few
taxa dominate in combination with a high number of rare species(Faith and Du, 2018). The best-known and widely used examplesof Type 1 and Type 2 measures in archaeological research are theShannon (H′) and Simpson (1-D) indices respectively (Lyman,
2008; Magurran, 2004).
The Shannon index (H′) measures diversity based on therelationship between species richness (NTAXA) and the relative
abundance (MNI) of each taxonomic category. Using taxonomic
richness and abundance, this index calculates the likelihood ofpredicting the taxonomic afﬁliation of the next individual drawn
from a population, assuming that individuals are sampled
randomly from an inﬁnitely large population (Magurran,
2004; Gifford Gonzalez, 2018). The Shannon index is
calculated from the equation:

3.4 Molluscan zonation and benthic
habitat attribution
Recent investigations of molluscan habitats linked to humanforaging undertaken by Harris and Weisler (2017) and Rogers andWeisler (2022) in the eastern Paciﬁc Islands form themethodological basis of the analyses of zonation and benthic
habitats on Unguja and Pemba. These authors appliedhierarchical classiﬁcation schemes for marine benthic habitats,with mollusc taxa assigned to discrete environmental/geographic
zones and habitats. Environmental zone refers to the location of thebenthic community (e.g., shoreline intertidal, reef), with habitat
referring to the geomorphological structure (e.g., coral, rock/debris,sand). As a similar, detailed classiﬁcation scheme has not yet beendeveloped for eastern African coastal and island areas, a modiﬁed
and simpliﬁed scheme is applied to the assemblages from Unguja
and Pemba. Simpliﬁcation of the hierarchical classiﬁcation scheme
also suits the geographical and chronological scale of these analyses,
providing an opportunity to develop baseline comparative data
across the two islands. It also suits the resolution of theinformation available for molluscan taxa of the Western Indian
Ocean, which demonstrates an extremely high degree of species
diversity. For example, there are numerous genera of
Polyplacophora with c.40 species from more than three families,c.400 gastropod families with at least 2,500 species, and c.50 bivalvefamilies comprised of more than 800 species (Richmond, 2011).
While not all these taxa occur within the Zanzibar Archipelago, ithighlights the taxonomic complexity of the region.
We use the major tidal areas of the intertidal zone, moving along
the gradient from the shoreline to sublittoral as the initialenvironmental zone designation. Three broad intertidal zones aredeﬁned following Richmond (2011): supralittoral (or supratidal),eulittoral and sublittoral (or subtidal). The supralittoral zone islargely dry, being inundated for only a few days a year. The
littoral fringe at the seaward edge of the supralittoral zone is only
covered by the sea during spring high tides. The eulittoral zone,
falling between mean high and low water neap levels, is exposed on
every tide. Finally, the sublittoral zone is permanently submerged,

H′  −Σpi ln pi
where pi is the proportional abundance of taxon i.
Shannon index values usually fall between 1.5 and 3.5(reﬂecting lower to higher diversity), rarely increasing beyond
4.0 (Magurran, 2004), and as a measure of heterogeneity, thisindex is sensitive to both richness and evenness (Faith and Du,2018).
The Simpson index (1-D) represents the probability that two
randomly samples individuals will belong to the same taxon. For
zooarchaeological assemblages, which are classiﬁed as ﬁnite
samples (Magurran, 2004; Lyman, 2008; Faith and Du, 2018),
the unbiased option of the Simpson index is calculated as:D′ 

Σini(ni − 1)
N(N − 1)

where ni is the abundance of taxon i and N is the total number ofindividuals.
Simpson index values fall between a minimum of 1/NTAXA(where all taxa are equally abundant) to approximately equal to
one where an assemblage dominated by a single taxon. The
Simpson index is usually expressed as 1-D′ so that larger values
are associated with greater evenness (Magurran, 2004; Faith andLyman, 2019). Importantly, the Simpson index is less sensitive to

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TABLE 1 Assemblage structure per site, showing richness (NTAXA) and relative abundance (MNI) values. Note: Polyplacophora are included in eachtaxonomic level as an individual category.

PhaseaApprox. Age
(BP)

Site

Code

Family level

Genus level

Species level

NTAXA

MNI

NTAXA

MNI

NTAXA

MNI

LSA

19,400

Kuumbi Cave 3/4

KC3/4

21

392

18

378

17

340

LSA

12,500

Kuumbi Cave 2

KC2

17

434

17

424

18

364

MIA

1,400

Fukuchani

FK

41

1,363

47

1,044

45

467

MIA

1,200

Unguja Ukuu

UU

41

5,174

62

4,901

64

3,730

MIA

1,200

Kuumbi Cave 1B

KC1B

26

1,200

27

1,167

26

1,110

LIA

750

Kuumbi Cave 1A

KC1A

17

96

15

87

16

78

LIA

700

Ras Mkumbuu

RM

28

1894

33

1873

31

1798

LIA

650

Msuka Mjini

MMJ

24

217

26

206

24

113

Colonial

350

Pango la Watoro

PLW

43

997

64

892

67

726

Colonial

350

Mvuleni

MVU

46

4,167

62

4,059

71

3,512

Modern0

Mifupani

MFP

47

70,49793

70,497

124

70,15676

86,431

151

85,528208

82,394Total
aLSA, later stone age; MIA-Middle Iron Age; LIA, later iron age.

distinct chronological phases noted above, with all other sites
recorded as individual assemblages (i.e., aggregating data into asingle site-based assemblage). The assemblages are listed inTable 1 in approximate chronological order, providing details
on the number of taxa and relative abundance (MNI) at family,
genus, and species levels. The number of taxa and MNI varyconsiderably across the assemblages, with the highest MNI and/or NTAXA values irrespective of taxonomic level consistently
derived from the modern Mifupani dataset, and lowest values
from Kuumbi Cave 1A.
To investigate potential sample size effects on assemblage
structure and richness, NTAXA values are plotted against MNI
at genus (Figure 4A) and species (Figure 4B) levels. At bothgenus and species levels, the positive relationship indicated by
the linear trendline appears to be driven by the large sample sizeand high richness at Mifupani, although the strength of these
relationships is moderate (genus and species R2=0.499). Thereis little change when removing Mifupani, with a similarly
moderate strength of relationship between NTAXA and MNI(genus R2=0.506; species R2=0.519). That said, although
individual assessment of the Msuka Mjini, Ras Mkumbuu,
Pango la Watoro and Mvuleni assemblages is not currently
possible, previous assessment of species area curves for Unguja
Ukuu and Fukuchani (Faulkner et al., 2018), and for eachchronological phase at Kuumbi Cave (Faulkner et al., 2019a),
indicates these assemblages were sampled to redundancy. These
results may be seen to indicate that approximately 50% of thevariance in NTAXA is explained by sample size, however the
degree of data dispersion combined with the differences in site
context and chronology (see section 2.3) indicates that a rangeof human behavioural and environmental factors are likely
strong contributors.

with the sublittoral fringe bordering the eulittoral on the landward
edge only exposed during or near spring low tides. Geomorphic
structures, or substrate characteristics, are designated based on thedeﬁnitions provided in Harris and Weisler (2017). Taxa are also
categorised according to their benthic organism type, either as
infaunal (those that live and burrow within the substrate) or
epifaunal (those that live on or attached to the outer surface of
their environment or other organisms).
The compilation of habitat data was undertaken with reference
to Abbott (1959, 1960, 1961), Abbott and Dance (1998), Carpenterand Niem (1998), Cernohorsky (1984), Dekker (2018), Harasewych
and Moretzsohn (2017), Harris and Weisler (2017), Healy et al.
(2011), Lamprell and Healy (1998), Lamprell and Whitehead (1992),
Richmond (2011), Shefer et al. (2012), Willan et al. (2015), andWilson (1993). Data collection was restricted to taxa at genus andspecies levels, excluding family-only level taxonomic attributions(11 bivalve, 17 gastropod families) in line with the range of analyses
outlined above. Reliable habitat information could not be
determined for several taxa, with one bivalve genus (8 MNI),
three gastropod genera (4 MNI), and 11 gastropod species(combined 378 MNI). The habitat information and coding per
taxon are provided in Supplementary Material S1.

4 Results
4.1 Assemblage characteristics
The characteristics of the molluscan assemblages from theeight sites outlined above are presented in Table 1, with the fulllist of species per site assemblage provided in Supplementary
Material S1. The data from Kuumbi Cave are separated into the

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FIGURE 4
NTAXA: MNI plots at genus level (A) and species level (B), with linear regression lines and standard error (grey shading). KC 3/4, Kuumbi Cave 3/4;
KC 2, Kuumbi Cave 2; FK, Fukuchani; UU, Unguja Ukuu; KC 1B, Kuumbi Cave 1B; KC 1A, Kuumbi Cave 1A; RM, Ras Mkumbuu; MMJ, Msuka Mjini; PLW,
Pango la Watoro; MVU, Mvuleni.

4.2 Richness and nestedness

all assemblages; 27 genera and 25 species occur in 50% or more of
the assemblages. The overall distribution of taxa, with what canbe described as unexpected absence and presence above andbelow the diagonal reference lines respectively, indicates
imperfect nestedness. This is a result of 124 genera and183 species being represented in less than 50% of theassemblages. The matrices provide a relative indication ofassemblage grouping, with Mifupani separated from Pango la
Watoro, Mvuleni and Unguja Ukuu. Fukuchani, Ras Mkumbuu,
Kuumbi Cave 1B (MIA) and Msuka Mjini cluster together, with
the LSA (KC 3/4 and KC 2) and the LIA (KC 1A) assemblages
from Kuumbi Cave falling together.

The nestedness results at genus and species levels are relatively
similar. The nestedness metrics and signiﬁcance statistics for theassemblages at both levels are presented in Supplementary Material
S2, with the Z-scores and p-values (all at p < 0.001) for each measureindicating that the assemblages with lower NTAXA values nest
within those with higher NTAXA. The nestedness temperature
values are low for genus (16.935°) and species (15.532°),
indicating a high degree of nestedness for all assemblages. In
comparison, the combined NODF values of 57.359 (genus) and
49.611 (species) indicate a moderate rather than high degree ofnestedness. These overall NODF values are conﬁrmed by theseparate row and column values.
Visual inspection of the genus and species nestedness
matrices (Figure 5; with matrix data provided in full in
Supplementary Material S3 at genus level and Supplementary
Material S4 at species level) provides support for the moderate
NODF nestedness results. There is a clear concentration oftaxonomic representation in the upper left of each matrix,
representing a core group of genera and species common to

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4.3 Taxonomic composition
Consideration of taxonomic composition provides anindication of similarity and difference between assemblages,
effectively building on the nestedness results. The contributionof key taxa (at ≥5% by MNI to one or more of the assemblages) at
genus level per assemblage is presented in Figure 6.

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FIGURE 5
Nestedness matrices for the assemblages at genus level (A) and species level (B). Red diagonal lines used for visual reference. Example matrices(C) showing high, moderate, and low nestedness with associated T and NODF values (adapted from Johnson et al., 2013). MFP, Mifupani; PLW, Pango
la Watoro; MVU, Mvuleni; UU, Unguja Ukuu; FK, Fukuchani; RM, Ras Mkumbuu; KC 1B, Kuumbi Cave 1B; MMJ, Msuka Mjini, KC 3/4, Kuumbi Cave 3/4;
KC 2, Kuumbi Cave two; KC 1A, Kuumbi Cave 1A.

but more evenly represented in comparison with the otherassemblages.
The trends at genus level are largely replicated by species leveltaxonomic representation (Figure 7), although exclusion of thegenus level data forces some reordering depending on level ofidentiﬁability. For example, while Pinctada was the dominantgenus at Fukuchani, species level attributions for this genus were
unattainable for this assemblage. As a result, the contribution ofseveral bivalves (Anadara antiquata, Arca ventricosa, Atactodea
striata, Saccostrea cuccullata) and gastropods (Pleuroploca
trapezium, Nerita balteata, Lunella coronata) becomes more
apparent.
The chord distance results at both genus and species levels
highlight similarities and differences in taxonomic composition
between these assemblages (Supplementary Material S2). At
genus level, there is minimal dissimilarity noted for some site
pairs; between all Kuumbi Cave assemblages, between Pango la
Watoro and Kuumbi Cave 3/4, 1B and 1A, and between

The four Kuumbi Cave assemblages are dominated by
gastropods, primarily by Nerita with minor contributions ofTurbo, Lunella, the Polyplacophora and Monodonta.
Fukuchani and Msuka Mjini have a greater proportional
representation of the bivalve Pinctada, with variable but
comparatively minor contribution to each assemblage by both
bivalve and gastropod taxa. Unguja Ukuu has a similar
composition, albeit with an increased proportional
representation of the bivalves Anadara and Atactodea, and alower contribution of a range of other bivalves and gastropods.
Pango la Watoro and Mvuleni are dominated by Nerita, withminor contributions from Gibberulus, Vasum and Saccostrea atPango la Watoro. At Mvuleni, a mix of bivalves and gastropods,
including Vasum, Pinctada, Arca, Cerithium and Tucetona, arealso strongly represented. In contrast, Ras Mkumbuu is heavily
dominated by Anadara (c.80%), and Mifupani exhibiting a moreeven distribution for Anadara and Gibberulus, followed by
Anodontia, Volema and Gafrarium. All other taxa are minor,

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FIGURE 6
Contribution of key taxa (%MNI) at genus level to each assemblage. Each taxon listed contributes ≥5% by MNI to one or more assemblage.

the genus level data, most assemblage pairs exhibit moderate tohigher levels of dissimilarity.
Correspondence analyses exploring the relationship between
taxonomic composition and assemblage variability at genus andspecies levels reinforce the data presented above (Figure 8). For
the genus level correspondence analysis (Figure 8A), axes 1 and
2 account for 37.2 and 25.3% of the variance in taxonomic
abundance respectively. Mifupani falls close to the origin,indicating that it is less differentiated or distinct relative tothe other assemblages based on taxonomic composition. Thisis highlighted by the clustering of taxa close to the origin,including Anadara, Barbatia, Anodontia, Gafrarium,
Gibberulus and Vasum, which occur across several
assemblages but are less discriminating. The Kuumbi Cave

Fukuchani and Msuka Mjini. There is moderate dissimilarity
between Mvuleni and all Kuumbi assemblages, as well as Mvuleni
and Pango la Watoro. Kuumbi Cave 2 exhibits moderatedissimilarity to Pango la Watoro, as does Unguja Ukuu withboth Ras Mkumbuu and Msuka Mjini. Moderate dissimilarity isalso exhibited between Kuumbi Cave 2 and Pango la Watoro, andbetween Ras Mkumbuu and Mifupani. All other assemblage pairs
indicate moderate to high dissimilarity at genus level. At species
level, only the comparisons between the Kuumbi Caveassemblages indicate minimal dissimilarity. Moderatedissimilarity is exhibited between Pango la Watoro and the
Kuumbi Cave assemblages, between Unguja Ukuu and bothRas Mkumbuu and Msuka Mjini assemblages, and between
Ras Mkumbuu and both Msuka Mjini and Mifupani. As with

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FIGURE 7
Contribution of key taxa (%MNI) at species level to each assemblage. Each taxon listed contributes ≥5% by MNI to one or more assemblage.

associated with Pinctada, Atactodea, Arca and Monetaria,
with other dominant taxa such as Anadara being lessdiscriminating for these assemblages.
For the species level correspondence analysis (Figure 8B),
axes 1 and 2 account for 28.9 and 24.9% of the variance intaxonomic abundance respectively. Mifupani retains its position
close to the origin, with both Ras Mkumbuu and Msuka Mjini
now shifting to fall closer to the origin. This indicates greater
similarity in assemblage structure and less discrimination bytaxonomic composition at species level. The Kuumbi Caveassemblages retain their discrete cluster and association with
Turbo setosus, Monodonta labio and several Nerita species, with
Pango la Watoro also falling within a relatively similar position toassemblages fall positively on axes 1 and 2, cluster closely
together, and are more strongly associated with Monodonta,
Turbo and Nerita. Similarly, Pango la Watoro and Mvuleni
exhibit positive axis 1 and 2 scores, but with Mvuleni falling
closer to 0 on axis 2. These assemblages are largely discriminatedby Nerita, Lunella and Tucetona, although to a lesser degree than
Kuumbi Cave. Somewhat similarly, Ras Mkumbuu falls at 0 along
axis 1 and is negatively scored on axis 2. This site sits in closer
proximity to Mifupani as it is dominated by Anadara andBarbatia. Msuka Mjini, Unguja Ukuu and Fukuchani all score
positively on axis 1 and negatively on axis 2, and sit within
relative proximity indicating greater similarity (particularly
between Msuka Mjini and Unguja Ukuu). These sites are

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FIGURE 8
Correspondence analysis of taxonomic abundance at genus level (A) and species level (B). Key taxa for the combined and individual
assemblages (contributing ≥5% by MNI) are labelled. KC 3/4, Kuumbi Cave 3/4; KC 2, Kuumbi Cave 2; FK, Fukuchani; UU, Unguja Ukuu; KC 1B, Kuumbi
Cave 1B; KC 1A, Kuumbi Cave 1A; RM, Ras Mkumbuu; MMJ, Msuka Mjini; PLW, Pango la Watoro; MVU, Mvuleni; MFP, Mifupani.

Watoro, Mvuleni and Mifupani, with moderate to high diversity
recorded for the four Kuumbi Cave assemblages. Ras Mkumbuu
shows the lowest values for both measures of diversity. The
results of the diversity permutation tests at genus level(Supplementary Material S2) indicate signiﬁcant differences
for both H′ and 1-D between the majority of assemblages.
Kuumbi Cave 1A, 1B and 2 are not signiﬁcantly different,with Kuumbi Cave 3/4 not differing signiﬁcantly from 1A
alone. For H′, Fukuchani and Unguja Ukuu are signiﬁcantly
different from each other, but neither are signiﬁcantly different
from Msuka Mjini. For 1-D, only Fukuchani is not signiﬁcantly
different from Msuka Mjini. Across both measures of diversity,
Unguja Ukuu, Ras Mkumbuu, Pango la Watoro, Mvuleni and
Mifupani are signiﬁcantly different from all other assemblages.
Both diversity index values are consistently high at species
level (Figure 9B), although the H′ values show greater variability
than those calculated for 1-D. With these data, Fukuchani, Pango
la Watoro, Mvuleni and Mifupani demonstrate higher diversity,
followed by Kuumbi Cave and Msuka Mjini. For both measures,
as with the genus level data, Ras Mkumbuu exhibits substantiallylower values in comparison with all other assemblages. The
diversity permutation test results at species level(Supplementary Material S2) again indicate signiﬁcantthat observed at genus level. The three assemblages that score
positively on axis 1 and negatively on axis 2, Unguja Ukuu,
Fukuchani and Mvuleni, are associated with different taxa, with
Unguja Ukuu falling closer to Msuka Mjini and Ras Mkumbuu(and therefore closer to the origin). Both Fukuchani and Mvuleni
are highly differentiated from each other and the otherassemblages, with Fukuchani associated with Lunella coronata,
Saccostrea cucullata and Atactodea striata, and Mvuleni
associated with (and discriminated by) Vasum ceramicum,
Tucetona pectunculus, Cerithium caeruleum and Nerita undata.

4.4 Assemblage diversity
The Shannon H′ and Simpson 1-D values are presented inTable 2, along with richness (NTAXA) and MNI values for each
assemblage. The NTAXA, H′ and 1-D values are graphed in
Figure 9 for visual comparison, with the assemblages inapproximate chronological order from the oldest to mostrecent (left to right).
At genus level (Figure 9A), the H′ and 1-D values parallel
each other to a large degree. These data indicate a higher degreeof diversity for Fukuchani, Unguja Ukuu, Msuka Mjini, Pango la

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TABLE 2 Genus and species level richness and diversity values by assemblage for Shannon H9 (upper) and Simpson 1-D (lower) indices.

KC 3/4

KC 2

FK

UU

KC 1B

KC 1A

RM

MMJ

PLW

MVU

MFP

NTAXA

18

17

47

62

27

15

33

26

64

62

93

MNI

378

424

1,044

4,901

1,167

87

1873

206

892

4,059

70,497

H′

1.69

1.42

2.39

2.50

1.44

1.65

0.96

2.37

2.90

2.77

3.06

1-D

0.69

0.63

0.77

0.86

0.58

0.62

0.35

0.83

0.87

0.90

0.92

NTAXA

17

18

45

64

26

16

31

24

67

71

124

MNI

340

364

467

3,730

1,110

78

1798

113

726

3,512

70,156

H′

2.34

2.12

2.95

2.22

2.27

2.44

0.78

2.57

3.16

2.89

3.14

1-D

0.89

0.84

0.91

0.80

0.86

0.89

0.29

0.88

0.92

0.91

0.93

Genus

Species

FIGURE 9
Plots of richness (NTAXA) and diversity indices (H′ and 1-D) for each assemblage at genus level (A) and species level (B). Conﬁdence intervals foreach measure are the percentiles bootstrapped at 9,999 iterations.

Mifupani are consistently signiﬁcantly different from the otherassemblages.
The diversity index values and results of the permutation
tests at both genus and species level indicate, for the most part,lack of a clear, directional chronological trend in richness anddiversity. Irrespective of assemblage richness, these data largely
indicate a moderate to high or high level of diversity, with adegree of variability apparent across assemblages. Regardless ofdifferences for both H′ and 1-D between most assemblages,
although these data are more complex than indicated at genus
level. For both measures, Kuumbi Cave 3/4 is not signiﬁcantly
different from Kuumbi Cave 1A and Msuka Mjini, and both
Fukuchani and Msuka Mjini are not signiﬁcantly different fromMvuleni. Kuumbi Cave 1A and 1B do not differ signiﬁcantly
from each other, and Msuka Mjini is not signiﬁcantly different
from Kuumbi Cave 1A. At species level, Ras Mkumbuu andFrontiers in Earth Science18

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FIGURE 10
Contribution of dominant habitat zones (%MNI) at genus level to each assemblage. Each habitat listed contributes ≥10% by MNI to one or more
assemblage. See Supplementary Material S1 for further information on habitat code deﬁnitions.

Msuka Mjini (37% MNI) are both dominated by taxa fromcoral and rock substrates in intertidal ﬂat-reef habitats, withmolluscs from a range of habitats and soft substrates
comprising 21 and 17% MNI at these sites respectively.
Pango la Watoro and Mvuleni exhibit similar distributions,with both containing proportionally higher intertidal ﬂat-reef
coral and rubble taxa (14 and 16% MNI respectively) andshoreline intertidal rock/mangrove taxa (32 and 20% MNI
respectively). Unguja Ukuu contains a higher proportion ofshoreline intertidal-intertidal ﬂat sand and/or mud taxa (26%
MNI), intertidal ﬂat-reef coral, rock, rubble and sand taxa (23%
MNI), and intertidal ﬂat-reef coral and rock taxa (12% MNI).
Ras Mkumbuu and Mifupani exhibit very different patterns ofhabitat representation. Mifupani contains a combined 40%
MNI of taxa from shoreline intertidal-intertidal ﬂat sand/mud habitats, with a lower proportional representation
across all other habitat categories. Ras Mkumbuu contains
81% MNI of taxa from shoreline intertidal-intertidal ﬂat
sand and/or mud substrates, being the assemblage mostskewed towards a single habitat zone.
As with the results of taxonomic representation above, thetrends in proportional habitat designations at genus level arelargely reproduced at species level (Figure 11). The exclusion of
the genus only level data shifts the proportions of some habitat
zones, as well as expanding on the range of identiﬁed habitat

taxonomic level and diversity measure, Mifupani and Ras
Mkumbuu are consistently separated from all otherassemblages; Mifupani for its high richness and diversity, Ras
Mkumbuu for its low richness and signiﬁcantly lower levels ofdiversity.

4.5 Zonation and benthic habitat analysis
Habitat categories identiﬁed for taxa at genus and species
levels are presented in Figures 10, 11. In both cases, the dominant
habitat areas that represent ≥10% by MNI of one or more
assemblage are included. At genus level, 10 habitat categories
represent between 47% (Mifupani) and 88% (Kuumbi Cave 2) of
the total assemblages, and at species level 16 habitat categories
represent between 65% (Mifupani) and 99% (Kuumbi Cave 3/4)of the total assemblages.
At genus level (Figure 10), Kuumbi Cave shows aconsistently high representation across all occupation phasesof taxa from mangrove, rock and coral substrates acrossshoreline intertidal to intertidal ﬂat habitats (76%–88%
MNI), with minimal representation of taxa from sand and/or
mud substrates from shoreline intertidal, intertidal ﬂat and reef
habitats (2%–4% MNI). Kuumbi Cave 2 only contains taxa fromconsolidated/hard substrates. Fukuchani (46% MNI) and

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FIGURE 11
Contribution of dominant habitat zones (%MNI) at species level to each assemblage. Each habitat listed contributes ≥10% by MNI to one or more
assemblage. See Supplementary Material S1 for further information on habitat code deﬁnitions.

between all occupation phases regardless of whether habitat
representation is viewed at genus or species level. Similarly,
there is moderate dissimilarity when comparing Unguja Ukuu
with Fukuchani, Ras Mkumbuu and Msuka Mjini. Moderatedissimilarity is noted between Pango la Watoro and Mvuleni,
and between both sites relative to all Kuumbi Cave phases.
Mifupani exhibits moderate dissimilarity when comparedwith Unguja Ukuu, Ras Mkumbuu and Pango la Watoro.
The main differences in chord distance are slightly
increasing values from genus to species when comparing
Mvuleni with both Fukuchani and Unguja Ukuu, withslightly decreasing values exhibited from genus to species
when comparing Msuka Mjini with both Ras Mkumbuu
and Mifupani. The major difference in habitat
representation occurs between Fukuchani and Msuka Mjini,increasing from minimal to moderate-high dissimilarity fromgenus to species. All other comparisons present moderate tohigher levels of dissimilarity.

categories, moving from more inclusive genus to more exclusive
or explicit species categories. That said, the distributional
patterns are similar across the taxonomic levels. This isparticularly the case with the degree of skew in the data fromthe four Kuumbi Cave occupation phases, Ras Mkumbuu, andMifupani. The habitat distributions from Fukuchani, Msuka
Mjini, Pango la Watoro, and Mvuleni are still concentrated
within particular habitat areas, but there is now a slightly
ﬂatter distribution, with the peaks observable in the genus
level data being slightly reduced or redistributed. In contrast,
the species level habitat distribution for Unguja Ukuu now showsa greater proportional representation in the shoreline intertidalintertidal ﬂat sand and/or mud habitat (33% MNI) and theintertidal ﬂat-reef coral/rock habitat (30% MNI), with allother habitat zones being substantially reduced.
The chord distance results for habitat representation
highlights several key similarities (Supplementary Material
S2). Kuumbi Cave demonstrates minimal dissimilarity

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5 Discussion

comparison with other Unguja/Pemba assemblages reinforce theinitial interpretation of Faulkner et al. (2019b), that theseassemblages primarily reﬂect a combination of foraging andtransportation, with the latter being the key driver explaining
the high similarity in assemblage structure and composition
through time. Similar issues were ﬂagged in South Africa (e.g.,
Plug, 2006; Steele and Klein, 2013) and at Panga ya Saidi in Kenya(Faulkner et al., 2021). The Kuumbi Cave assemblages therefore
reﬂect selectivity for transportation, with the movement of key
taxa from the coast in the order of 7–8 km during the Pleistocene,
and c.2.5 km over the last 2,000 years.
Ethnographic data collection at Mifupani (Sarathi 2020) took
place between June 2018 and June 2019 and involved the direct
observation of shellﬁsh and ﬁsh harvested by c.30 male andfemale informants on individual gathering trips. Mifupani
represents a true generalised foraging strategy (Szabó, 2009;
Harris et al., 2016; Harris and Weisler, 2017), with theassemblage characterised by high richness and high diversity,and although there is a proportionally greater representation of
species from a variety of nearshore unconsolidated substrate
habitats, people access all available intertidal habitat areas fromthe shoreline to fringing reef. Canoe use combined with wadable
shallow waters created by the fossil reef system allows for the
exploitation of multiple habitats. Signiﬁcant factors absent in thearchaeological past inﬂuence marine faunal exploitation at
Mifupani, including the major impact of the tourist industry
on maritime subsistence activities. The participation ofMifupani’s residents in a market economy also greatly affected
their subsistence decisions, e.g., the availability of meat forpurchase having impact on how ﬁshing and shellﬁshgathering took place in the village.
On Unguja, recent socio-ecological studies in Nungwi(Nordlund et al., 2010), Chwaka Bay (Fröcklin et al., 2014),
and Menai Bay (Yahya, 2013) have focused on understanding
critical elements of intertidal foraging primarily by women, andthe increasing impacts of human predation pressure,anthropogenic habitat disturbance and climate change. Thesestudies highlight differences in species abundance, richness, anddiversity between areas under intensive collection and thosesubjected to minimal harvesting. Interestingly, 224 harvested
species were identiﬁed by respondents at Nungwi in thequalitative interviews, however only 10 taxa were recorded
during the biological ﬁeld sampling (Nordlund et al., 2010).
This is similar to data from Menai Bay, albeit to a lower
degree, with 26 taxa recorded during the biological surveys
and 34 taxa from the catch assessments (Yahya, 2013). In
contrast, at Chwaka Bay the higher numbers of taxa wererecorded during the biological surveys (39 taxa) compared to
21 taxa from the catch assessments (Fröcklin et al., 2014).
On the mainland, Msemwa (1994) focused on areas
encompassing protected embayments with fringing reefs orcoral islands, with areas of straight coastline excluded basedon stronger wave action. Ethnographic observation was restricted

In their review of the Swahili coast zooarchaeological record,
Quintana Morales and Prendergast (2018) highlight achronological trajectory in the exploitation of terrestrial andmarine fauna. The earliest coastal records indicate a focus onwild terrestrial game, ﬁshing and mollusc harvesting, withmarine resource use continuing with the incorporation offood production while wild terrestrial hunting reduced
through time. Between the ﬁrst and second millennium CE, associo-economic structures shifted concomitant with increasing
urbanism, social changes and long-distance trade, people focusedmore intensively on domesticated animals and offshore marine
environments. Interestingly, this perspective connects toterrestrial vertebrates and marine ﬁsh resources more thanmollusc foraging (Quintana Morales et al., 2022), as any shift
away from the shoreline and broader intertidal zone is not
reﬂected in the archaeological or modern socio-ecological
data. Although available data indicate the maintenance ofintertidal foraging as an economic activity in the Zanzibar
Archipelago, as people engaged with the intertidal zone incomplex ways, key differences in nearshore marine foraging
occur through time and space.
The Kuumbi Cave data indicate different behaviours to thoseseen in the other assemblages from Unguja and Pemba. Acrossthe four occupation phases there is a pattern of low richness andmoderate to high diversity in assemblage structure, representinga small number of key taxa harvested from across the intertidal
gradient, but with an emphasis on mangrove, rock, and reef hardsubstrate gastropod species (e.g., Nerita spp, Turbo spp. And
Lunella coronata). Interestingly, these characteristics are
consistent across all occupation phases irrespective ofchronology, changing sea levels or climatic ﬂuctuation, being
evident in the two LSA phases at c.19,400 (KC 3/4) and 12,500 BP(KC 2), and in the MIA and LIA phases at c.1200 (KC 1B) and
750 BP (KC 1A). This indicates accessibility of shoreline,
intertidal ﬂat and reef habitats on Unguja’s eastern coast
before separation of the island from the mainland c.9,000 BP,
and into the late Holocene. Due largely to the steep bathymetry
off the eastern Unguja coast, in a broad sense there has been
comparatively minimal change in the structure of the dominant
coastal habitats through time. Environmental composition andavailability of taxa, however, are not the primary determinants of
the Kuumbi Cave assemblage characteristics, as we would expect
a range of intertidal ﬂat habitats to have been accessible
throughout the last 20,000 years. As in situ processing of
harvested molluscs on the coast is likely, followed bydifferential transportation (sensu Bird et al., 2002; Meehan1982), the nature of intertidal foraging during the LSA andprior to island formation cannot be easily identiﬁed, although
the MIA/LIA assemblages give some indication of likely foraging
behaviours that connect to the two late Holocene Kuumbi Caveoccupation phases. The re-analyses of the Kuumbi Cave data andFrontiers in Earth Science

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Unguja Ukuu and Msuka Mjini, and grouping the colonial
period Pango la Watoro and Mvuleni assemblages. The
Fukuchani and Unguja Ukuu assemblages were previouslyinterpreted as reﬂecting generalised foraging inﬂuenced by
local habitat structure linked to species richness and diversity
within the environment (Faulkner et al., 2018). The higher
resolution analyses undertaken here reinforce thisinterpretation, with Msuka Mjini demonstrating similar
trends. All three sites therefore reﬂect a general foraging
pattern as one component of a broad-based economic
structure that encompasses hunting and trapping, ﬁshing, andfood production to greater or lesser degrees. Pango la Watoro
and Mvuleni are somewhat similar, but indicate a higher degreeof selectivity and focus on hard substrate habitats across theintertidal gradient. This is unlikely to be purely determined byenvironmental factors and habitat distribution, particularly given
the proximity of Mvuleni to Fukuchani and Pango la Watoro to
Msuka Mjini, and the nearshore characteristics of both locations.
In these cases, foraging strategies may be inﬂuenced by broader
socio-economic transitions on the coast associated with thecolonial period. As such, all sites exhibit somewhat generalised
foraging strategies with variability mediated by the nature of their
physical and social environmental contexts.
Extending from this last group of assemblages, Ras
Mkumbuu provides evidence for a different set of foraging
behaviours, deviating substantially from the other MIA-LIA
assemblages in the archipelago, as well as the modern
Mifupani data. Located on a long narrow peninsula, the site
represents an urban complex bordered to the north and south by
expansive intertidal sand and mud ﬂats, patch and fringing reefs.
These habitat zones are interspersed with patches of sandy beachand rocky headlands, indicating a mosaic of environments thatwould have supported a rich and diverse fauna. The reef systems
in this area are particularly productive, supporting hightaxonomic diversity. The molluscan assemblage, however, ischaracterised by a moderate degree of richness and very low
diversity, being overwhelmingly dominated by the shoreline
intertidal-intertidal ﬂat sand/mud infaunal bivalve Anadara
antiquata (84% MNI). Although the intertidal ﬂats areprominent, the focused gathering of A. antiquata to the
exclusion of all other taxa and available habitats indicates that
foraging strategies and species selection were not determined byenvironmental structure. In addition, the ﬁsh assemblages,
although small, indicate a focus on coral, rocky and outer reeftaxa, with less exploitation of estuarine and inshore zones, and no
sandy/muddy ﬂoor species (Quintana Morales and Horton,
2014). The Ras Mkumbuu evidence appears to conform more
to Msemwa’s (1994) modern urban foraging data, as well as thepredictions proposed by Fleisher (2003) for the nature of mollusc
foraging in complex urban sites. At Ras Mkumbuu, intertidal
foraging and mollusc harvesting appears to reﬂect a strategy ofhighly focused gathering of an accessible and abundant species asa supplementary resource.

to September-October, combining informal and formalinterviews with ﬁrst-hand observation. Emphasis was placedon collection during periods of spring tides, which correlatewith productive ﬁshing. The nature of harvesting and the taxa
being collected were largely determined by shorelinecharacteristics and habitat structure, which were associatedwith species diversity and availability. Based on these data, it
is suggested that mollusc collection around Dar es Salaam is asecondary activity used to cushion periods of ﬁsh scarcity. In thiscontext, Msemwa (1994) recorded only 17 commonly harvested
taxa, including six bivalves and 11 gastropods, with collection
driven by factors including subsistence, use as bait, andcommercial sale. Although most taxa could be collected yearround, Anadara antiquata was the dominant taxon harvested
due to meat yield and accessibility, with individuals
concentrated/clustering within the intertidal zone.
The full Mifupani study revealed the exploitation of manyﬁsh and invertebrate taxa totaling 281 individual species andincorporating 126 mollusc taxa. Most ﬁshing and foraging took
place in the shallows created by Mifupani’s fossil reef, withminimal inﬂuence from tool use or modes of transportation.
Collection was for subsistence, with no clear seasonal pattern ofharvest for most taxa, although future analyses may reveal
seasonal variation in exploitation patterns. Unlike Msemwa(1994), Sarathi (2020) concludes that ﬁshing and mollusc
gathering served as important sources of meat rather than as“crisis” foods. The most important factor affecting what was
harvested is the availability of taxa, although this is now
inﬂuenced by increased touristic activity in the intertidal area,
climate change, and levels of human predation.
Fukuchani, Unguja Ukuu, Msuka Mjini, Pango la Watoro
and Mvuleni exhibit similarities and differences across the rangeof comparative analyses. Fukuchani, Unguja Ukuu, Pango la
Watoro and Mvuleni are all characterised by moderate species
richness and moderate to high diversity, with Msuka Mjini also
demonstrating moderate to high diversity but low species
richness. There is variability in the diversity metrics, andwhich assemblages are signiﬁcantly different from each other,
depending on the taxonomic level being assessed. Evaluation oftaxonomic and habitat representation using chord distanceindicates a generally moderate to high distance value incomparing each of these ﬁve assemblages, although again
these results vary depending on whether the assessment is at
genus or species level. Correspondence analysis indicates greater
similarity between Msuka Mjini, Unguja Ukuu and Fukuchani at
species level, with Mvuleni and Pango la Watoro falling
separately, but in closer proximity to these three assemblages
than to Kuumbi Cave, Ras Mkumbuu or Mifupani. In bothtaxonomic and habitat representation there are broad
similarities, but in many respects these are moderate. Whatdraws these assemblages together is the proportional
distribution of taxa and habitats, with broadly similar patterns
loosely grouping the MIA-LIA assemblages from Fukuchani,

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6 Conclusion

Author contributions

The longitudinal, multi-site comparative analyses of theZanzibar Archipelago molluscan assemblages clearly highlightthat the environment, habitat conﬁguration, taxonomic
distribution and density all provide the parameters forforaging across the intertidal zone. Complex relationships
between these factors vary spatially and chronologically,
dependant on broader environmental processes and how thesemanifest locally. This does not mean, however, that there is a
simple, deterministic relationship between environment andhuman foraging behaviour, with the nature of intertidal
foraging mediated by human choice embedded within speciﬁcsocio-economic contexts. Given the complexities in the ways
people engage with the dynamic conditions of the intertidal zone,
it is not surprising that there is no single pattern of foraging
behaviour across Unguja and Pemba.
The available data for the Zanzibar Archipelago have
limitations, principally the lack of evidence for occupation
between the terminal Pleistocene and mid to late Holocene, as
well as our ability to incorporate broader economic data for eachof the sites discussed here. For example, while high-resolution
terrestrial and marine vertebrate data and archaeobotanicalevidence are currently available for Kuumbi Cave, Unguja
Ukuu and Fukuchani, further detailed analyses are required(or are ongoing) for the other archaeological sites
incorporated into the analyses above. That said, inapproaching the available data in a systematic manner,incorporating archaeomalacological and ecologically oriented
modes of analysis across multiple archaeological contexts, weare able to move beyond interpretations constrained to individualsites. The analyses and results presented here provide robust
baseline data for future investigations of human-ecodynamics on
the coast and islands of eastern Africa, highlighting combinationsof stability and variability in foraging behaviours that effectively
argue against a single trajectory of foraging behaviour within theintertidal zone.

PF, AS, and AC contributed to conception and design of thestudy. PF, AS, TS, MH, AA, and OH collected the assemblage
data. PF performed the analyses and wrote the ﬁrst draft of themanuscript. AS, AL, NN, and AC wrote sections of themanuscript. MH produced Figures 1, 2. PF, AS, AC, TS, MH,
AA, OH, AL, NN, MH, and NB contributed to manuscript
revision, read, and approved the submitted version.

Funding
PF was funded by a Faculty of Arts and Social Sciences
Research Support Scheme grant and Tom Austen Brown
Research funding (University of Sydney), and by the Max
Planck Society. PF is an Australian Research Council Future
Fellow (FT200100464). AS was funded by grants from theFulbright-Hays Program (award#P022A170037), the National
Geographic Society (award#EC-227-18), the Department of
Anthropology at the University of Wisconsin-Madison, andthe University of Wisconsin-Madison Graduate School. AC isan Australian Research Council Future Fellow (FT200100179)
and was funded by postdoctoral fellowships from the British
Academy and The University of Queensland. AL and NN were
supported by Springer/Nature and the British Institute in Eastern
Africa. NB was funded by a European Research Council grant for
the SEALINKS Project (grant no. 206148). Radiocarbon dates for
the Sealinks Project were provided by a UK Natural Environment
Research Council grant (NF/2012/2/4).

Acknowledgments
We are grateful to the Zanzibar Department of Museums andAntiquities and the Ofﬁce of Chief Government Statistician(Zanzibar Research Committee) for supporting our archaeologicalﬁeldwork and providing permission to conduct the research. PF, AC,
MH, and NB extend their appreciation to the local communities on
Pemba and Unguja, particularly in Makangale, Fukuchani, Unguja
Ukuu, and Jambiani. AL and NN thank PF for his invitation tocontribute to this paper. They are deeply grateful to AA for his
magnanimity in all aspects of this research; the student crews fortheir many contributions, and the communities in Nungwi,
Fukuchani and Mvuleni for their gracious assistance. AS isespecially grateful to the people of Mifupani for sharing theirsubsistence activities with researchers daily for more than a year.
Special thanks are due to Muhidini Mgana and Ameir Jecha for
facilitating research at Mifupani. Finally, we thank the guest editorial
team of Manuel Will, Andrew Green, Jon Erlandson, Ximena Suarez
Villagran and Antonieta Jerardino for their invitation to contributeto this special issue.

Data availability statement
The original contributions presented in the study areincluded in the article/Supplementary Material, further
inquiries can be directed to the corresponding authors.

Ethics statement
The studies involving human participants were reviewed andapproved by the University of Madison-Wisconsin Institutional
Review Board (2016-1353). The patients/participants provided
their written informed consent to participate in this study.

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Conﬂict of interest

organizations, or those of the publisher, the editors and thereviewers. Any product that may be evaluated in this article, or
claim that may be made by its manufacturer, is not guaranteed or
endorsed by the publisher.

The authors declare that the research was conducted in theabsence of any commercial or ﬁnancial relationships that could
be construed as a potential conﬂict of interest.

Supplementary material
Publisher’s note
The Supplementary Material for this article can be foundonline at: https://www.frontiersin.org/articles/10.3389/feart.2022.
982694/full#supplementary-material

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