Bull. Natl. Mus. Nat. Sci., Ser. B, 38(4), pp. 147–152, November 22, 2012

First Record of Discosporangium mesarthrocarpum (Meneghini) Hauck(Phaeophyceae, Ochrophyta) from the Ogasawara Islands, Japan
Taiju Kitayama
Department of Botany, National Museum of Nature and Science,
Amakubo 4–1–1, Tsukuba, Ibaraki 305–0005, Japan
E-mail: kitayama@kahaku.go.jp(Received 5 September 2012; accepted 26 September 2012)

Abstract A brown alga, Discosporangium mesarthrocarpum (Meneghini) Hauck (Discosporangiales, Phaeophyceae, Ochrophyta) was collected from the deep water off Chichi-jima Island,Ogasawara Islands, Japan. This is the ﬁrst record of the species and order from East Asia or Northwestern Paciﬁc Ocean. This alga differs from all other brown algae in having uniseriate ﬁlaments
formed by apical growth and unique disc-shaped plurilocular sporangia without a stalk cell. Thesemorphological features characterize the order Discosporangiales, which is considered as the mostprimitive in the all brown algal orders.
Key words : brown algae, Discosporangiales, Discosporangium mesarthrocarpum, Japan, Ogasawara Islands, Phaeophyceae.

Introduction
Discosporangium mesarthrocarpum (Meneghini)
Hauck (Discosporangiales, Phaeophyceae,Ochrophyta) is a small brown alga having uniquemorphological features: uniseriate ﬁlamentous
thalli with apical growth cells and disc-shaped
plurilocular sporangia without a stalk cell, whichcharacterize the previously monotypic genus
Discosporangium.
The systematic position of the species had
been controversial for a long time since the original description because of its primitive andstrange morphology. Kjellman (1891) treated
Discosporangium as a member of Choristocarpaceae with also monotypic genus Choristocarpus because of their similarity in vegetative morphology of the ﬁlaments with apical cells. In
contrast, Schmidt (1937) established the new
family Discosporangiaceae and the new order
Discosporangiales for the species in regarding
the unique reproductive organs as the importantcharacter, while Fritsch (1945) incorporated theChoristocarpaceae in Sphacelariales without Discosporangium. Nevertheless, many authors also
placed both Discosporangium and Choristocarpus under Sphacelariales because they could not
admit Discosporangiales in the previous classiﬁcation system of brown algae (e.g., Womersley,1987; Prudʼhomme van Reine, 1993; Abbott andHuisman, 2004). However, using molecular analysis in a recent study, Kawai et al. (2007) clariﬁed the position of D. mesarthrocarpum andreinstated the family Discosporangiaceae and theorder Discosporangiales, which is considered tobe phylogenetically sister to all other brownalgae.
Discosporangium mesarthrocarpum has very
few records outside of the Mediterranean Seaand Womersley (1987) only recorded this species
in the Paciﬁc in the last century. Thus there was ahypothesis that Australian algae are derived fromthe North Atlantic by recent introduction(Prudʼhomme van Reine, 1993). However, arecent molecular phylogenetic analysis of D.
mesarthrocarpum by Kawai et al. (2007) sug-148

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gested that the alga of Discosporangium in Australia can be a distinct species, which has a reasonable difference in its rbcL sequence from theD. mesarthrocarpum. Moreover, the second species of Discosporangium, D. pulvillocarpum wasdescribed by Kraft (2009) based on the molecular implications of Kawai et al. (2007) and morphological differences from the type species, D.
mesarthrocarpum in plastids and reproductiveorgans, for the algae from Lord Howe Island,Australia. On the other hand, Abbott and Huisman (2003) reported D. mesarthrocarpum fromHawaiian Islands.
In this study, to conﬁrm the identity of thepresent brown alga from the Ogasawara Islands,
I made anatomical observations on the materials
using a microscope.
Materials and Methods
The brown algal material referable to thegenus Discosporangium was collected from thedeep water (46.2–50.8 m in depth) off Chichijima
Island in the Ogasawara archipelago (＝BoninIslands), Japan by dredge using the research vessel, the Koyo (87 tonnage), operated by the Ogasawara Fisheries Center, Tokyo Metropolitan.
For preservation, the material was dried on
sheets of paper or ﬁxed in 10% Formalin-seawater. Anatomical observations were made on thematerial using a microscope. Voucher specimens
were deposited in the algal herbarium of theNational Museum of Nature and Science (TNS).
Descriptions
Discosporangiales O. C. Schmidt, 1937
Discosporangiaceae O. C. Schmidt, 1937
Discosporangium mesarthrocarpum (Meneghini)
Hauckin L. Rabenhorst, Kryptogamen-ﬂora, ed. 2, vol.
2, 525, f. 236 (1885).
Basionym: Callithamnion mesarthrocarpum
Meneghini, Giornale Botanico Italiano 1: 288(1844) [Type locality: Dalmatia, Adriatic Sea,Mediterranean (Guiry and Guiry, 2012)].
[Figs. 1–18]
Plants light to golden brown in color, epilithic,
attached to rocks or entangled with grains ofdead coral by basal cell with rhizoidal appendages (Figs. 1, 2). Thalli erect and densely tufted,
composed of branched ﬁlaments, up to 1.5 cm inheight. Erect ﬁlaments uniseriate, irregularly,
radially and laterally branched (Figs. 3, 11). Vegetative cells of erect ﬁlaments cylindrical,
18–30 μm in diameter, 80–195 μm in length.
Growth by cell divisions in a prominent apicalcell on each ﬁlament (Fig. 4, arrowhead). Apicalcells cylindrical, 14–28 μm in diameter, 47–138 μm
in length, with rounded ends (Fig. 4). Lateralbranches usually initiated singly per cell at various angles, though laterals occasionally initiatedoppositely from terminal cell of wounded ﬁlaments (Fig. 5). Haptera digitate (Figs. 6, 17, 18).
Plastids numerous per cell in both erect ﬁlaments
and haptera, disc-shaped, circle to ellipsoidal,
3–5 μm in diameter.
Plurilocular sporangia sessile, monolayered,various in form with maturation (Figs. 7–10,12–16). Young plurilocular sporangia sphericalto rounded (Figs. 7, 12). Mature or empty plurilocular sporangia disk-shaped, quadrate or squarein surface view (Fig. 8), up to 40 μm in length, up
to 35 μm in width, up to 16 μm in thickness,bending outward convexly on the center (Figs. 9,10), usually composed of eight locules in length,
eight locules in width (Fig. 16).
Specimens examined: Subtidal from 46.2 m to50.8 m in depth, off Chichijima Island, Ogasawara
Islands, Japan (27°07′08–15″N, 142°10′42–44″E),
8 July 2010, leg. T. Kitayama (TNS-AL 176344,176345).
Japanese name: Tama-kushige (nom. nov.).
Distribution: Mediterranean Sea: Croatia (typelocality), Italy, Morocco, Spain (Ribera et al.,1992), France (Coppejans, 1979; Ribera et al.,1992), Greece (Tsirika and Haritonidis, 2005;
Kawai et al., 2007), Malta (Cormaci et al.,1997); Atlantic Ocean: Bermuda (Schneider andSearles, 1998), Canary Islands (Haroun et al.,1993), Madeira (Neto et al., 2001); Paciﬁc

Discosporangium mesarthrocarpum new to Japan

149

Figs. 1–10. Discosporangium mesarthrocarpum from Ogasawara Islands, Japan. 1. Habit showing a thallus entangledwith coral sands (dried specimen, TNS-AL 176344). 2. Habit showing tufts of erect ﬁlaments. 3. Erect ﬁlaments withapical cells and laterals. 4. Apical cell showing a newly formed cell wall (arrowhead). 5. Opposite pair of laterals juston the terminal of a broken erect ﬁlament. 6. Holdfast with a digitate hapteron. 7. Immature plurilocular sporangium
with eight cells. 8. Surface view of mature plurilocular sporangium with mostly 32 cells. 9, 10. Side views of identical
empty plurilocular sporangium with mostly 64 locules, showing outward bending of this sporangium in the different
focuses.

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Figs. 11–18. Discosporangium mesarthrocarpum from Ogasawara Islands, Japan. 11. Erect ﬁlaments. 12–16. Various
stages of plurilocular sporangia. 12. Initial stage of plurilocular sporangium with a single cell. 13. Side view of a primordial plurilocular sporangium with eight cells. 14. Surface view of a primordial plurilocular sporangium with sixteen cells. 15. Surface view of a mostly mature plurilocular sporangium with 36 cells. 16. Surface view of a mature
plurilocular sporangium with 64 cells. 17, 18. Haptera showing digitate ends containing many chloroplasts (c).

Discosporangium mesarthrocarpum new to Japan

Ocean: South Australia (Womersley, 1987),
Hawaii (Abbott and Huisman, 2003, 2004),
Japan (Ogasawara Islands, the present study).
Discussion
The present brown algal materials from theOgasawara Islands, Japan are in morphologicalagreement with descriptions or ﬁgures of Mediterranean algae of Discosporangium mesarthrocarpum by Hauck (1885), Coppejans (1983),Kawai et al. (2007) in both vegetative and reproductive morphology. This is the ﬁrst record of thespecies from East Asia and the northwestern
Paciﬁc Ocean.
Finding this alga from the northwestern Paciﬁcsuggests that this species has a wide distribution
in both the Atlantic and Paciﬁc, though
Prudʼhomme van Reine (1993) suggested that
this species is endemic to the Mediterranean andintroduced recently from the North Atlantic
Ocean because of rare records outside of theMediterranean Sea. Kawai et al. (2007) denied
the hypothesis inferring from the genetic divergence between Greek algae and Australian algae.
In general, it seems that artiﬁcial transportation is
not easy for deep marine algae like this species,
whose habitat is probably 20–50 m in depth.
When Kraft (2009) described D. pulvillocarpum, he considered the South Australian plantsof D. mesarthrocarpum illustrated by Womersley(1987) as the possibly identical taxon with hisnew species. According to Kraft (2009), vegetative cells of D. pulvillocarpum has many bacilliform plastids, while the plastids of Mediterranean plants of D. mesarthrocarpum cultured by
Kawai et al. (2007) and illustrated by Coppejans(1983) are discoid or ellipsoidal in shape. On theother hand, the plants of “D. mesarthrocarpum”
from the Hawaiian Islands recorded by Abbottand Huisman have “disc-shaped to elongate”
plastids, which are bacilliform judging from thephotograph (Abbott and Huisman, 2003, f. 20;
Abbott and Huisman, 2004, f. 71). In the present
Ogasawara plants, plastids are discoid and similar to the ones observed so far in Mediterranean

151

plants.
As for reproductive structures, Kraft (2009)
showed morphology of plurilocular sporangia in
his new species, which was described as different
from ones of type species, though the present
author could not ﬁnd any deﬁnite difference inthe organs between the Japanese plants and D.
pulvillocarpum. Only outward bend of mature
plurilocular sporangia (Figs. 9, 10) is possible tobe a morphological difference, which is known
in D. mesarthrocarpum (Hauck, 1885, f. 236, c),but the feature is unclear in the original description of D. pulvillocarpum (Kraft, 2009). In thispaper, therefore, the present alga is treated tentatively as D. mesarthrocarpum inferring fromdifferences in shape of plastids. To clarify therelationship among the populations of Discosporangium inside the Paciﬁc, a molecular analysis of the plants from the Ogasawara Islands isrequired.
Acknowledgements
I am grateful to Mr. Yuji Aoki, director of theOgasawara Fisheries Center, Tokyo Metropolitan, and the crew of the research vessel, Koyo,
for providing me the facilities to processing thespecimens. I thank Drs. Toshihiko Fujita, Kazunori Hasegawa, Hironori Komatsu, Departmentof Zoology, National Museum of Nature and Science for their kind help in the ﬁeld study in theisland. This study was undertaken as a part of thesurveys of the ﬂora of Izu and Ogasawara archipelagoes, Japan during 2006–2010, entitled“Study on Environmental Changes in the Sagami
Sea and Adjacent Area with Time Serial Comparison of Fauna and Flora.” I also thank Mr. AlexBarnes, North Carolina, U.S.A., for correcting
the English of the manuscript.
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