Cancellus makrothrix Stebbing 1924
Creators: name: Landschoff, Jannes
name: Lemaitre, Rafael
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Diogenidae
Cancellus
Cancellus makrothrix

description: Cancellus makrothrix Stebbing, 1924
(Figs. 1–7)
Cancellus makrothrix Stebbing, 1924: 240, pl. 3, figs. as, car, m, mx1, mx2, mxp1, mxp2, mxp3, prp1, prp3, prp4, prp5, T (Crustacea pl. 118) (type locality: Algoa Bay, South Africa); Barnard 1950: 447, fig. 82; Day et al. 1970: 56; Kensley 1974: 65; Kensley 1981: 32; Morgan 1987: 536; Forest & McLaughlin 2000: 88.
Cancellus macrothrix. — Gordan 1956: 305; Mayo 1973: 3, 11 (key), tbl. 1, fig. 22c, d; McLaughlin et al. 2010: 19; Wittmann & Griffiths 2017: 15; Landschoff & Gouws 2018: 783 (tree); Landschoff et al. 2018a: 2, fig. 1 E, F. [Misspelling]
Type material (see Taxonomic remarks). Lectotype, herein selected: male 16.5 mm, Bird Island Passage, Algoa Bay, SS Pieter Faure, 18 m, 33°51'00.0"S 26°16'58.8"E, SAMC A1541; Paratype: female, body missing except for right antennule, left cheliped, left pereopods 3–5, and right maxillipeds 1–3, same station data as lectotype, NHM 1928.12.286 (Stebbing Collection).
Other material examined. Historical material. 1 female 8.1 mm, Beacon East of East London, SS Pieter Faure, dredge, no depth, Stn. No. 13533, “NW 0.5 W 2 min” (adapted to: 33°00'57.6"S, 27°59'06.0"E), 15 Aug 1901, SAMC A1542; 1 “brooding” female 9.5 mm (with 8 juveniles in housing), Cape Morgan, SS Pieter Faure, dredge, 31 m, Stn. No. 13495, “W 0.5 S, 4 min” (adapted to: 33°00’56”S, 27°59‘06”E), 15 Aug 1901, SAMC A3265; 1 male 5.5 mm, False Bay, UCT Ecological Survey, dredge, 29 m, Sta. No. FAL 4689, 34°06'S, 18°42'E, 25 Oct 1961, SAMC–A19626 (in poor condition; dried, rehydrated back in alcohol).
New material. All from False Bay, South Africa, coll. J. Landschoff: 1 male 9.0 mm (intersex, with unpaired left female gonopore), in buccinid gastropod shell, Roman Rocks (Simons Town), SCUBA, 20 m, 34°10'40.8"S, 18°27'28.8"E, 13 May 2015, SAMC A066204; 1 male 12.2 mm, Muizenberg Beach (wash-up), 34°06'21.6"S 18°28'40.8"E, 9 June 2015, SAMC A066216; 1 ovig. female 10.4 mm (with 46 eggs, ~ 2.3 mm mean diameter), Simons Town (Long Beach), SCUBA, 5 m, 34°11'09.6"S 18°25'37.2"E, 6 Sep 2016, USNM 1292095; 1 male (intersex, with paired female gonopores) 9.5 mm, Millers Point, freediving, 8 m, 34°13'51.6"S 18°28'40.8"E, 22 Mar 2016, in Argobuccinum pustulosum, USNM 1292096; 1 male 8 mm, molt of same specimen as USNM 1292096 (kept in tank), SAMC A066602; 1 male (molt) 14.3 mm, off Millers Point, SCUBA, 25 m, 34°13'40.8"S 18°29'13.2"E, 29 Sep 2017, SAMC A066603. 1 female 10.8 mm, south of A-Frame, freediving, 5 m, 34°12'40.9"S 18°27'49.2"E, 21 Mar 2023, SAMC A096852; 1 male 12.1 mm, Millers Point towards Castle Rock, freediving, 8 m, 34°14'01.2"S 18°28'37.3"E, 22 Mar 2023, SAMC A A096853.
Redescription. Cephalothoracic shield (Fig. 1A) width subequal to shield length. Rostrum obtusely triangular, terminating in small spine, distinctly overreaching lateral projections. Frontal rim consisting of flattened w-shaped ridge, weakly interrupted by fissure at each inferior angle of W-shaped ridge. Anterior margin between rostrum and lateral projections concave; lateral projections broadly rounded, armed with row of small spines and short setae; anterolateral margins sloping; lateral margins broadly rounded, moderately setose; posterior margin rounded, together with cervical grooves forming broad triangle. Dorsal surface slightly convex, uneven; with scattered tufts of setae, low rugae, and numerous small tubercles or blunt spines on each side near lateral margins; anteromedian surface with broad m-shaped flat depression directly behind rostrum and lateral projections; linea-d weakly marked. Branchiostegites (Fig. 1B) each with anterior margin rounded, setose; membranous except for well calcified dorsal portion. Presence of unnamed plate (Fig. 1A–B) between calcified dorsal portion of branchiostegite and adjacent carapace lateral lobe. Carapace lateral lobes (Fig. 1A–B) distinct, well calcified. Posterior carapace (Fig. 1A) with posteromedian plate calcified, sparsely setose; posterolateral plates dorsally calcified near posteromedian plate, with remaining portion membranous, densely setose.
Ocular peduncle (including cornea) approximately 0.68–0.84 length of cephalothoracic shield, slender, slightly curved outward, slightly broadened proximally; with moderately long setae dorsally; basal surface of ventromesial face with distinct pore (function unknown). Cornea subspherical, not dilated (subequal in diameter to width of adjacent peduncle). Ocular acicles subtriangular, mesial margins adjacent, dorsally with few setae; each terminating in strong multifid projection with 2–5 distal spines.
Antennular peduncle reaching to proximal margin of cornea when fully extended; ultimate and penultimate segments glabrous, ultimate twice as long as penultimate; basal segment with ventrodistal angle blunt, and laterodistal lobe armed distally with minute spines.
Antennal peduncle with fifth segment extending slightly beyond mid-length of ocular peduncle. Fifth segment approximately twice as long as fourth segment, unarmed except for dorsodistal setae. Fourth segment with bifid dorsodistal spine. Third segment with strong ventrodistal spine. Second segment with bristle-like setae, dorsolateral distal angle produced into strong spine-like process, dorsomesial angle with strong spine. First (basal) segment lateral face with small blunt distal spine; ventromesial angle rounded, with bristle-like setae. Acicle variably extending to approximately mid-length of ocular peduncle to slightly beyond ocular peduncle, terminating in strong bifid spine, usually with 1 dorsomesial spine proximally (at least on one side). Flagellum approximately as long as cephalothoracic shield, reaching to approximately distal margin of cheliped palm; articles bearing short setae less than 1 flagellar article in length.
Maxilliped 3 endopod (Fig. 1D) ischium with well-developed, coarsely toothed crista dentata, appearing serrate, lacking accessory tooth; basis armed with weak blunt spine on mesial angle proximally. Mandible with robust calcareous molar process.
Chelipeds (Figs. 2, 5A–F, 6) symmetrical, similar in strength, armature and setation. Carpi and chelae, together with merus and dactyl of second pereopod forming operculum sealing housing when retracted; chelae positioned more or less at right angle with carpi when retracted; opercular faces of chelipeds and second pereopods forming rounded surface, with face of each second pereopod slightly flaring laterally. Fingers stout, each terminating in distinct, darkly pigmented corneous spoon-like claw; cutting edges each with 2 large, subequal blunt calcareous teeth. Dactyl gaping widely from fixed finger, dorsal (opposable) margin with row of subacute tubercles terminating in corneous tips; outer (opercular face) face with blunt tubercles, and tufts of long bristle-like setae; mesial face with 2 longitudinal rows of corneous-tipped tubercles, and scattered tufts of short setae. Fixed finger with lateral and mesial faces unarmed except for few tufts of bristle-like setae. Palm with dorsal (opposable) margin armed with strong subconiform calcareous simple or bifid spines terminating in darkly pigmented corneous tip; outer (opercular) face with scattered small tubercles and tufts of short bristle-like setae more numerous on upper half; mesial face mostly smooth, with scattered tufts of short setae; ventral margin with few low tubercles. Carpus short; dorsal (opposable) margin crest-like, with distal angle produced into lobe armed with small spines; outer (opercular) face unarmed except for scattered setae and small spines on distal margin, with distinct longitudinal groove on upper half. Merus subtriangular in cross-section, unarmed except for setose dorsal margin and minutely spinulose distolateral margin; lateral and mesial surfaces mostly smooth; lateral face with subdistal transverse groove extending nearly to ventral margin.
Pereopod 2 (Fig. 3A–C) with opercular face formed by mesial faces of dactyl, merus, dorsal face of carpus, and dorsodistal portion of merus; opercular face densely covered with bristle-like setae. Dactyl approximately as long as merus, terminating in sharp corneous claw curving ventromesially; dorsal margin uneven, with low setose rugae; lateral face smooth, naked; mesial (opercular) face with few tufts of setae; lateral face more or less smooth, with distal tuft of setae; ventromesial margin armed with 4 small corneous spines on distal half. Propodus with dorsal margin consisting of distinct, multidenticulate setose tubercles; lateral and mesial faces mostly smooth, with scattered setae; ventral margin unarmed, at most with few setae. Carpus thick, heavy, length about 1.5 times height; operculate (dorsal) face more or less flat, delimited by 2 distally diverging rows, one lateral and one mesial, of multispinose setose tubercles, distal tubercle of mesial row with distinct terminal spine; lateral and mesial faces mostly smooth, with scattered setae and tufts of setae; ventral margin unarmed. Merus dorsal margin with short setae on proximal two-thirds; opercular (dorsal) surface more or less flat, delimited my mesial and lateral ridges of multidenticulate setose tubercles; lateral and mesial faces mostly smooth, with scattered setae or bristle-like tufts of setae; ventral margin with row of minute spines. Ischium unarmed except for setae on dorsal and ventral margins.
Pereopod 3 (Fig. 3D–E) similar to pereopod 2 except for weakly marked opercular surface on segments.
Pereopod 4 (Fig. 4C) semichelate, segments robust. Dactylus subcylindrical, opposed to dorsodistal margin of propodal rasp, with ventrolateral row of 4 small blunt corneous spines, terminating in subacute corneous tip. Propodus with well-developed rasp oval in shape and covering entire distal half of segment; rasp consisting of densely packed ovate corneous scales.
Pereopod 5 (Fig. 4D) chelate; fingers with opposable surfaces spoon-like, cutting edges consisting of minute, fused corneous spinules. Dactyl slightly shorter than propodus; dorsal margin setose, with 1–3 dorsolateral rows of small ovate scales proximally. Propodus with well-developed rasp on dorsal face extending posteriorly to near margin with carpus; with mesial fringe of long setae. Coxae with ventral face convex, smooth, appearing semiglobular.
Thoracic sternum with anterior lobe of sternite XI (of third pereopods; Fig. 4A) weakly elevated, broadly rounded.
Pleon robustly subcylindrical, distinctly to weakly coiled; soft integument overall covered dorsally by fine dense pubescence (Figs. 5A–B, D, G–H), surface of left side with longitudinal fold extending full length of soft integument just below level of pleopods; dorsally with 4 distinct, separated, translucent, lightly sclerotized transverse tergites. Sixth pleonal tergite (Figs. 4E–F, 5G–H) strongly calcified, subdivided by transverse furrow into subsemicircular anterior portion and subrectangular posterior portion, dorsal face with numerous tufts of setae set on minute tubercles; anterior portion with broadly rounded distal margin armed with few subdistal spines or tubercles; posterior portion divided medially by shallow furrow, distal margin subdivided into 4 multispinose lobes (2 laterally, 2 median) armed with strong spines.
Uropods (Figs. 4F, 5G–H) weakly asymmetrical; protopod superior surface slightly excavate posterior to articulation of exopod, projection posterior to endopod bearing 2 small blunt spines; with rasp on dorsodistal face of endopod and exopod. Telson (Figs. 4F, 5G–H) symmetrical to slightly asymmetrical, ovoid, slightly broader than long; dorsal surface uneven, with many tufts of setae; posterior margin subdivided by shallow sinus into equal lobes with marginal setae.
Males without pleopods, often with female gonopores present (intersex). Females with biramous left pleopods 2–5 (all carrying eggs, Fig. 5B, D).
Genetic data. Male 9.5 mm, USNM1292096, GenBank: MH481976, BOLD: SEAKY1184-17.
Three-dimensional microCT data. Male 9.0 mm, SAMC A066204; see Landschoff et al. (2018b).
Colour. General colour in life (Figs. 5, 6F–G): cream to orange brown and darker patches, sprinkled with small brown-red dots; strong setae appear orange-yellow at base, white on distal half, plumose setae cream. Shield cream to orange near midline, with usually two dark brown patches on anterior one-third and darker areas laterally; evenly to irregularly covered with small brown-red dots.Anterior half or calcified part of cardiac region dark brown, remainder of carapace and pleon somewhat uniformly cream to orange-brown and with the usual red dots. Ocular peduncles cream with irregularly spaced brown-red dots, often in groups of 3–5, becoming smaller distally, less intense in colour, and less dense; corneas light blue; acicles as proximal part of peduncles, with white calcareous spines. Antennular and antennal peduncles like proximal ocular peduncles, but with spines and tubercles cream to white; antennular peduncles with white dorsal flagellum, ventral flagellum with light and small brown-orange yellowish dots; antennal peduncles without dots, semi-transparent to cream. Chelipeds and pereopods primary color as carapace, but usually each segment with proximal broad dark brown patch on dorsal inner and outer face, sometimes as ring around entire segment; brown patches less pronounced on chelipeds, and obscured by setation, most pronounced on second pereopod. Ventral surfaces, uropods and telson as primary color. Eggs of later developmental stage orange.
Distribution and habitat. South coast of South Africa from East London to False Bay, Cape Town. Depth range: 5–31 m; commonly found at diving depths of 5–25 m on various habitats from sandy substrate to rocky reefs in kelp forest. Typically inhabits oversized gastropods shells such as those of the genus Murex as reported by Barnard (1950) or large buccinids in False Bay.
Proposed common name. “Hercules hermit crab”.
Taxonomic remarks. As previously mentioned, Stebbing’s (1924) description of Cancellus makrothrix is brief and lacks morphological details, a situation exacerbated by the confusion on the number, fate and condition of his original material. A first problem during this study was to locate the type material of Stebbing’s taxon. In his description Stebbing (1924: 241) stated that “The specimen is a female, with the sexual openings conspicuous at the bases of the third paraeopods”, and he did not mention any other specimens. Stebbing indicated the locality of his female specimen to be Algoa Bay, and stated that the specimen was deposited in the South African Museum with registration number “A 1541” (currently SAMC A1541). However, examination of the specimen deposited in that museum under that registration number showed that it is actually a male (not intersex with both male and female gonopores, which could have been mistaken for a female by Stebbing in his description). In view of this discrepancy in the sex of the specimen, we searched the handwritten record in the Iziko South African Museum pertaining to the SS Pieter Faure collections used by Stebbing (1924), and confirmed Barnard’s (1950) discovery that lot “A1541” originally contained two specimens, of which only a male was found during our study in that SAMC lot. On the suspicion that the missing female type might have been transported to London to be worked on by the Rev. T.R.R. Stebbing (1835–1926; Mills 1976), we requested a search at the Natural History Museum, London, where indeed their “Stebbing Collection” was found to include a lot (NHM 1928.12.286) from Algoa Bay with the appendages of a specimen that unquestionably is of C. makrothrix, the only species of Cancellus known to occur in South African waters. Although the body of that specimen is missing and thus the sex cannot be ascertained, the appendages left in the jar (a right antennule, a left cheliped, left pereopods 3–5, and right maxillipeds 1–3, see “Type material”) must belong to Stebbing’s (1924) female mentioned in his description. So it is clear that Stebbing worked with two specimens, a female and a male that must be considered as syntypes. From these syntypes, the male (sl = 16.5 mm, SAMC A1541) is herein selected as lectotype for Stebbing’s taxon.
A second problem during this study was to determine which specimen was illustrated by Stebbing (1924, pl. 3, [Crustacea pl. 118]). Stebbing’s illustrations are few and crudely executed. The illustration (Stebbing 1924: pl. 3 fig. T) of the posterior part of the pleon shows a nearly symmetrical telson, what appears to be an inaccurately drawn posterior part of the sixth pleonal somite, and incomplete or mutilated uropods; the illustrations (Stebbing 1924: pl. 3 figs. prp 1, prp 3, prp 4) of a cheliped and pereopods 3 and 4 appear distorted and do not show any surface ornamentation. Those illustrations could possibly belong to either the female or male syntypes, except for the mouthparts, which should belong to the female syntype as the male syntype (SAM-C 1541) is still intact and its mouthparts clearly were not dissected. The illustration (Stebbing 1924: pl. 3, fig. car) of the anterior portion of the cephalic shield and ocular peduncles initially appeared to be erroneous as it shows ocular acicles that are distally bifid (left) or simple (right), whereas all specimens we studied and believed to represent Stebbing’s C. makrothrix have ocular acicles that are distally multifid. However, examination of the male syntype specimen catalogued as SAMC-1541 showed exactly the same spinulation on the ocular acicles as depicted by Stebbing (1924: pl. 3, fig. car). We thus consider that Stebbing’s illustration (pl. 3, fig. car) is of the male catalogued as SAMC-1541. Given the unusually large size of this male (cephalothoracic shield length 16.5 mm), the largest known of C. makrothrix, we attribute the morphology of its ocular acicles to size variation, a condition frequently seen (pers. observation) in many other paguroids.
In summary, we conclude that Stebbing’s (1924: pl. 3) illustrations of Cancellus makrothrix represent a composite of both the female syntype now at the Natural History Museum, London (NHM 1928.12.286) and the male syntype herein selected as lectotype at Iziko South African Museum, Cape Town (SAMC-1541).
Since Stebbing’s (1924) description of Cancellus makrothrix, this species was not reported again until Barnard (1950) remarked on the lack of details in the original description. Based on three specimens of C. makrothrix found in SAMC, i.e., the male syntype (now lectotype) from Algoa Bay and two more specimens from East London, Barnard (1950) provided details about the morphology, habitat, and presumed development of Stebbing’s species. As previously mentioned, Barnard (1950: 449) considered C. makrothrix to be a “very aberrant” species of the genus Cancellus. He observed that in contrast to other congenerics C. makrothrix lives in gastropod shells (Murex) instead of excavated burrows in corals, soft rocks, and sponges; has a coiled rather than straight pleon; the chelipeds as well as pereopods 2 and 3 (instead of just the chelipeds and pereopods 2) together tightly form a closure for the aperture of the gastropod shell used as housing; and the upper surface of the carpus of the chelipeds do not form a strong knuckle-like projection. Furthermore, Barnard’s (1950, fig. 82) illustration shows slightly asymmetrical uropods and a distinctly asymmetrical telson. This unusual morphology for a species of Cancellus led Mayo (1973) to consider C. makrothrix to be more primitive than other symmetrical congenerics, and consequently intermediate between the more typical pagurids and species of Cancellus. Presumably, according to Mayo, the slightly asymmetrical uropods, flattened opercular surfaces of the chelipeds, and the spirally coiled pleon in C. makrothrix, reflected incomplete evolution from the use of asymmetrical to tubular housing.
It appears that Gordan (1956) was the first to change the spelling of Stebbing’s (1924) species epithet to “ macrothrix ” rather than “ makrothrix ”, most likely in order to Latinize the Greek letter “k”. Since then, the spelling “ macrothrix ” was used by several carcinologists (Mayo 1973; McLaughlin et al. 2010; Wittmann & Griffiths 2017; Landschoff & Gouws 2018; Landschoff et al. 2018a, b). Forest & McLaughlin (2000) correctly reverted to Stebbing’s original spelling.
Mayo (1973) provided an in-depth review of the taxonomic literature, morphological features, and life history information available for the nine species of Cancellus known at that time. However, Mayo did not have any material of C. makrothrix available for examination and thus did not add any new information on the morphology or biology of this species, although she pointed out that Stebbing’s (1924: pl. 3, fig. car) rough illustrations of the anterior part of the cephalothoracic shield and eyestalks did not agree with the expanded description of this species provided by Barnard (1950) and in fact appeared more like that in species of other genera of pagurids.
Kensley (1974: 65) provided more precise information about the type locality of Cancellus makrothrix in Algoa Bay as “Bird Island passage, Cape Province, 10 fathoms”, and also added that the original sample was collected by the “S.S. Pieter Faure ”, a Cape Government trawler that investigated the coast of what is now South Africa (Barnard, 1964). Subsequently, Kensley (1981) indicated that C. makrothrix occurred from False Bay to East London, in a depth range of 34– 80 m. While the distribution remains correct, the depth information is an error as this information provided by Kensley (1974, 1981) was based on the few previously reported specimens, all listed above and all from shallower waters. Except for one additional specimen listed in Day et al (1970), no new material of C. makrothrix has been taxonomically reported since Barnard’s report until the present study. In an apparent oversight, Emmerson’s (2016) did not include Cancellus makrothrix in his recent guide and checklist to the decapods from the south African coasts of Namibia to Mozambique.
Morphological and biological oddities. As previously mentioned under “Taxonomic Remarks”, C. makrothrix stands out morphologically among species of Cancellus in that the outer surfaces of the chelipeds (palm and carpi) that form part of the opercular surface are flattened or slightly convex rather than concave; both pereopods 1 and 2, instead of only pereopod 1, together with the chelipeds, form the opercular surface; the coiled pleon; and the slightly asymmetrical uropods. In addition, C. makrothrix differs from other species of Cancellus in the use of gastropod shells for housing rather than straight burrows in a variety of petriculous substrates, a condition which presumably has caused the slight asymmetry present on pleon and uropods.
The study of specimens of Cancellus makrothrix revealed the puzzling presence of what appears to be a pore on the basal surface of the ventromesial face of each ocular peduncle (Fig. 1C). Such pore, if indeed our anatomical interpretation is correct, has not been reported on the ocular peduncles in any species of Cancellus, or to our knowledge in any other paguroid. The function of this pore, if any, can only be speculated on at this point.
Among the specimens examined of Cancellus makrothrix, we encountered two specimens with both male and female gonopores. In one (USNM 1292096) these were both paired, and in the other (SAMC A066204) the female gonopore was observed only on the left. In these specimens the pores are not as wide or as well-developed as in true females and they have been sexed as males here also, given the lack of pleopods as in strictly functional males. To our knowledge, intersex individuals have not been previously reported for species of Cancellus. However, the occurrence of intersex individuals in paguroids, often considered “aberrant”, has been known since the late 19 th century although only in the diogenid Dardanus deformis (H. Milne Edwards, 1836) has this phenomenon regularly been observed (Hilgendorf 1879; Fize & Serène 1955; Lewinsohn 1982; McLaughlin & Lemaitre 1993). It has been hypothesized that this condition might indicate a functional hermaphroditism or perhaps latent sex reversal (McLaughlin & Lemaitre 1993), although only in the last 25 years have authors focused on studying intersexuality in paguroids (Turra & Leite 2000, 2001; Turra 2004, 2007; Fantucci et al. 2007; Gusev & Sabotin 2007; Sant’Anna et al., 2010; Obuid-Allah et al., 2019). Sant’Anna et al. (2010) have shown that at least in one species of the diogenid genus Clibanarius Dana, 1852, a portion of the population consists of intersex individuals that have both male and female functional reproductive systems, meaning they can reproduce as males and females, and thus supporting the existence of sequential hermaphroditism. Whether the same is true in C. makrothrix, or possibly other species of Cancellus, as well as the evolutionary factors that explain hermaphroditism in these paguroids, remains to be investigated.
Cancelllus makrothrix is one of only four species of paguroid in which abbreviated development has been reported, the others being C. typus H. Milne Edwards, 1836, Areopaguristes abbreviatus (Dechancé, 1963a), and Paguristes frontalis (H. Milne Edwards, 1836) (Hale 1927, 1941; Barnard 1950; Dechancé 1963b as Paguristes abbreviatus; Morgan 1987), although we have observed abbreviated development, and indeed brooding, of several other diogenid hermit crabs in the genus Paguristes and Areopaguristes from South African waters (J. Landschoff, unpubl. data). Glaucothoë larvae as well as juveniles have been reported in the former four species from inside the housing of the hermit crab or attached to the pleopods, an indication of parental care. Barnard (1950) observed “juveniles” already hatched as glaucothoë larvae or still inside the egg adhering to the pleon and sides of the carapace in one female (SAMC A3265), and concluded that in this species the free-swimming zoeal larval stage was suppressed and the young developed inside the housing occupied by the female. A similar conclusion indicating direct development was reported earlier by Hale (1941) in the Australian species C. typus H. Milne Edwards, 1836.
The ovigerous female specimens we collected contained 46 large sized eggs with an ~ 2.3 mm average in diameter. Although no more juveniles were found inside the shell or the back of a female adult, these large eggs add to Barnard’s observation of glaucothoë stage post-larvae living inside the gastropod shell used as housing. The specimens we found carried shells that seemed enormous for such a small crab, and the free space in the spire of the gastropod shell provides a safe environment for the post-larvae to grow to juvenile stages (Figs. 6F–H).
Ecological and behavioral notes. The preference of Cancellus makrothrix for heavy, thick and usually greatly oversized shells is noteworthy, and this behaviour is indeed the best way to locate adult specimens in the field. Smaller individuals are probably overlooked among other hermit crab species. In the waters around Cape Town C. makrothrix co-occurs with several other paguroids, of which Paguristes gamianus (H. Milne Edwards, 1836) is highly abundant but commonly occupies smaller and lighter Bunupena, Turbo or Bullia shells despite a similar adult body size. From our experience in the field, inhabited shells larger than 100 mm are nearly always occupied by C. makrothrix. The thick and strongly developed pereopods allow them to even climb highest or vertical reef structures. The habit of using heavy and difficult-to-carry petricolous dwellings might explain the evolutionary adaptation of C. makrothrix to easily carry these large and thicker gastropod housings, a specialization that is likely to provide better protection from predation.
The shells Cancellus makrothrix occupies in False Bay are generally overgrown by and/or pervaded by boring invertebrates and a detailed study of the associated fauna would make an intriguing study on its own. Additionally, the preference of C. makrothrix of large homes (Fig. 6F, G) quite literally creates, as previously noted, room for its developing young inside the shell after hatching, and the cavity provides shelter and habitat for smaller crustaceans. In fact, the collection of samples that led to this present study already led to the discovery and description of Heteromysis cancelli Wittmann & Griffiths, 2017, an obligatory commensal shrimp that exclusively lives inside the shells utilized by C. makrothrix.
By keeping Cancellus makrothrix for several months in the aquaria (Fig. 6H), it was repeatedly observed how C. makrothrix climbed onto the back of a sea urchin, holding on to the urchin test with its strong legs, and one-byone clipped off and fed on the spines, tube feet, and pedicellaria. Initially we were inclined to think that the spines were only cut away to expose the tube feet for consumption, but over a period of>2 h C. makrothrix slowly clipped away and ingested all parts. The specimen molted a short time later but it is speculative if this food preference (the animal was fed with mussel meat as well) was related to perhaps increase calcium intake. Furthermore, it remains to be discovered if this predation occurs in the wild. Given the low abundance of C. makrothrix it seems unlikely that this species would have any noteworthy impact on wild urchin populations. Cancellus makrothrix might be an opportunistic generalist feeder as the paratype male specimen still has the remains of a polychaete worm held between its mouthparts.
description: Published as part of Landschoff, Jannes & Lemaitre, Rafael, 2024, Rediscovering the unusual Cancellus makrothrix Stebbing, 1924 (Crustacea: Decapoda: Diogenidae) in the " Great African Seaforest ", pp. 530-544 in Zootaxa 5492 (4) on pages 531-542, DOI: 10.11646/zootaxa.5492.4.3, http://zenodo.org/record/13268862