Bull. Natl. Mus. Nat. Sci., Ser. A, Suppl. 5, pp. 119–126, February 21, 2011

A New Genus and Species of Euxanthine Crab (Crustacea:
Decapoda: Xanthidae) from Submarine Banks
off the Izu Islands, Central Japan
Masatsune Takeda1 and Hironori Komatsu2
1Faculty of Modern Life, Teikyo Heisei University,
2–51–4 Higashi-Ikebukuro, Toshima-ku, Tokyo, 170–8445 Japan
E-mail: takeda-m@thu.ac.jp
2
Department of Zoology, National Museum of Nature and Science,
3–23–1 Hyakunincho, Shinjuku-ku, Tokyo, 169–0073 Japan
E-mail: h-komatu@kahaku.go.jp

Abstract A new genus and species of euxanthine crab, Pilomedaeus okutanii gen. et sp. nov., isdescribed from submarine banks off the Izu Islands, central Japan. The monotypic new genus Pilomedaeus can be distinguished from its congeners by the tomentose carapace and the combinationof some morphological characters.
Key words : Decapoda, Brachyura, Xanthidae, Pilomedaeus okutanii, new genus, new species,
Japan.

During the years of 1960–1974, Dr. Takashi
Okutani of the Tokai Regional Fisheries Research Laboratory, Fisheries Agency, surveyed
the benthic fauna of the submarine banks off the
Izu Islands in central Japan by using the research
vessel Soyo Maru. Of the extensive collections
made, many mollusks of new species and newrecords for Japanese waters have been published(see Okutani, 1972, 1975). As for his decapod
crustacean specimens, now preserved in the National Museum of Nature and Science, Tokyo(NSMT), some of the material has already beenreported (Takeda and Galil, 1980, 2005; Ohe andTakeda, 1986; Takeda and Ng, 1997; Komatsu
and Takeda, 2004; Takeda and Watabe, 2004).
The Showa Memorial Institute, the National Museum of Nature and Science, Tsukuba, also conducted a zoological survey at Sagami Sea and adjacent seas, including the Izu Islands. This wasdone to elucidate the faunal change over the past
60 years since the field work and publications of
the late Showa Emperor (see Takeda et al.,2006a–c).
While sorting the crab specimens in the two
above-mentioned collections, we found somespecimens of the Xanthidae previously unknown
from Japanese waters, which were collected fromsome offshore banks in the Izu Islands. In thispaper, they are described as a new genus andspecies, Pilomedaeus okutanii gen. et sp. nov.
All the specimens examined are preserved in
the National Museum of Nature and Science,
Tokyo, and the Showa Memorial Institute, National Museum of Nature and Science, Tsukuba,
under the registration form of NSMT-Cr and
NSMT-Cr S, respectively. Abbreviations used in
the description, cb and cl, and G1 and G2, referto the breadth and length of the carapace, and thefirst and second male gonopods, respectively.
Taxonomy
Family Xanthidae MacLeay, 1838
Subfamily Euxanthinae Alcock, 1898
Genus Pilomedaeus gen. nov.
Type species. Pilomedaeus okutanii gen. et sp.
nov., by monotypy.
Diagnosis. Carapace transversely ovoid; dorsal
surface covered with tomentum, with regions

120

Masatsune Takeda and Hironori Komatsu

well defined, 4 M indistinct; endostome withoutridge; cheliped crested on dorsal surfaces of carpus and palm; major chela with molariform toothon base of dactylus; dorsal margin of ambulatory
merus crested; abdominal somites 3–5 fused but
sutures separating somites discernible; G1 tapering, apex rounded without long setae; G2 short,
distal segment sickle-shaped.
Etymology. The generic name is derived fromthe Latin pilus (hair), in combination with thegenus name Medaeus (generic name of an euxanthine crab), alluding to the tomentose carapace.
Gender masculine.
Remarks. Pilomedaeus gen. nov. is similar to
Crosnierius Serène and Vadon, 1981, andLadomedaeus Števčić, 2005 (Euxanthinae Alcock, 1898), as well as Glyptocarcinus Takeda,
1973, Antrocarcinus Ng and Chia, 1994, andCrytocarcinus Ng and Chia, 1994 (Antrocarcininae Ng and Chia 1994) (see Ng et al., 2008, forcurrent xanthid classification), in the shape of
their gonopods, i.e. the G1 is relatively stout,with a rounded apex which lacks long setae; anda G2 which is relatively short and has a sickleshaped distal segment.
Pilomedaeus is similar to Crosnierus in the indistinct 4 M and the absence of endostomial
ridges in addition to the features of gonopods,
but can be easily distinguished from Crosnieriusby the tomentose carapace (vs. glabrous) and thediscernible sutures between abdominal somites
3–5 (vs. completely fused with sutures only visible laterally) (cf. Serène and Vadon, 1981; Ng
and Chen, 2005).
Pilomedaeus is also similar to Ladomedaeus in
the visible sutures between abdominal somites
3–5, but can be distinguished by 1) the carapace
is convex and tomentose (vs. flattened andglabrous), 2) the 4 M is indistinct (vs. distinct),3) the endostome has no ridges (vs. with distinct
oblique ridges), and 4) the anteroexternal angleof third maxilliped merus is not auriculiform (vs.
distinctly auriculiform) (cf. Manuel-Santos andNg, 2007).
Glyptocarcinus, Antrocarcinus and Crytocarcinus (currently placed in a separate subfamily,

Antrocarcininae Ng and Chia, 1994) are very
close to each other and also superficially similar
to Pilomedaeus in the indistinct 4M, the absenceof endostomial ridges, the crested meri of ambulatory legs and the discernible sutures between
abdominal somites 3–5, but differ in the flattened
and glabrous carapace (vs. convex and tomentose
in Pilomedaeus), the auriculiform anteroexternalangle of the third maxilliped (vs. not expanded),as well as the normal posterolateral margin (vs.
with distinctive and deep grooves on the posterolateral margins and regions) (cf. Ng and Chia,
1994; Komatsu, unpublished data on G.
lophopus).
Comparisons of the genera discussed are summarized in Table 1. These genera, although currently classified in two subfamilies, actually forma single clade in an unpublished molecular phylogenetic analysis (P. K. L. Ng and J. C. Y. Lai,personal communication). Until this study is
published when the classification of these two
subfamilies is reorganized, we retain Pilomedaeus in Euxanthinae for the time being.
Pilomedaeus okutanii sp. nov.
[New Japanese name: Kebuka-nokoha-ohgigani](Figs. 1–4)

Material examined. Holotype: male (cb14.6cl 11.3 mm), NSMT-Cr 21446, Kurose
Bank, Izu Islands, 33°22.0N, 139°39.1E,
150–190 m, RV Soyo Maru, st. D86, coll. T. Okutani, 19 June 1973.
Paratypes: 1 ovig. female (cb 12.9cl 9.2mm), NSMT-Cr 21447, same data as holotype; 2ovig. females (cb 12.2cl 9.2 mm, cb 13.5cl10.7 mm), NSMT-Cr 21448, Hyotanse Bank, Izu
Islands, 34°18.7N, 139°01.0E to 34°22.5N,
139°05.8E, RV Soyo Maru, st. D1–5, coll. T.
Okutani, 9 July 1968; 1 female (cb 10.9cl 8.6mm), NSMT-Cr 21449, Hyotanse Bank, Izu Islands, 34°19.4N, 139°01.2E to 34°22.0N,
139°05.6E, 115–200 m, dredge, RV Soyo Maru,st. D26–30, coll., T. Okutani, 10 July 1969; 2
males (cb 7.6cl 5.9 mm, cb 8.7cl 6.8 mm), 3
females (cb 9.5cl 6.3–cb 11.9cl 8.9 mm),with sutureswith partial sutures laterallywith sutureswith sutures

crestedserrated or spinate
crestedspinatesmooth or erodedsmoothwith irregular short ridges

rugose or eroded

auriculiform
not expanded
not expanded

auriculiform

flattened and glabrous
indistinct
with channel
without ridges
flattened and glabrous
distinctwithout channel
with oblique ridges
flattened and glabrous
indistinct
without channel
without ridges
convex and tomentose
indistinct
without channel
without ridges

Carapace
Dorsal surface
4M
Posterolataral surface
Endostome
Third maxilliped
Anteroexternal angle of merus
Cheliped
CarpusAmbulatory legs
Merus
Male abdomen
Somites 3–5

Ladomedaeus
Crosnierius
Pilomedaeus

Table 1.

Comparison of Pilomedaeus gen. nov. with its allied genera.

Glyptocarcinus
Antrocarcinus
Cyrtocarcinus

New Genus and Species of Euxanthine Crab

121

NSMT-Cr 21450, Kurose Bank, Izu Islands,
33°20.6N, 139°36.7E to 33°22.7N, 139°41.0E,
130–190 m, RV Soyo Maru, st. 4, coll. T. Okutani, 23 July 1974; 2 young males (cb 6.7cl
5.3 mm, cb 7.2cl 5.6 mm), 1 female (cb 13.2
cl 9.9 mm), 1 young female (cb 5.9cl 4.4 mm),
NSMT-Cr S 574, Kurose Bank, Izu Islands,
33°27.3N, 139°42.6E to 33°27.7N, 139°42.4E,
200–211 m, dredge, RV Shin’yo Maru, coll. H.
Komatsu, 21 October 2003.
Description. Carapace (Figs. 1A, 2A, 4) ovoid,
about 1.3 times broader than long (1.29 in holotype), convex anteriorly, slightly convex transversely. Dorsal surface evenly covered with tomentum, glabrous on front, with acute granules
around lateral border of carapace; sparse, inconspicuous, rounded granules on proto-, metagastric and cardiac regions. Regions distinct in large
males, indistinct in females and juveniles; shallow grooves separating meso- and protogastricregions, H-shaped groove separating gastric andcardiac regions; short, transverse, epigastric ridge
weak; protogastric region indistinctly divided byshallow, longitudinal groove. Front about 0.3
times carapace width, projecting beyond innersupra-orbital tooth, weakly deflexed, divided bymedian V-shaped notch or short fissure; marginsubtruncate; laterally with deep V-shaped notch;
lateral angle well separated from orbit. Upper orbital border granulated, evenly concave; medianand lateral fissures vestigial, but with small Vshaped notch. Lower orbital border evenly concave, with obtusely triangular inner lobe; vestigial lateral fissure, but with small V-shaped
notch. Anterolateral margin convex, clearly separated from posterolateral margin by sharp angle,
armed with 4 prominent, evenly separated,sparsely granulated, triangular teeth behind blunt
exorbital angle; 1st to 3rd teeth gradually increasing in size, 4th larger than 1st, subequal to
2nd tooth. Posterolateral border almost straight inoutline, convergent towards posterior carapace
margin.
Ocular peduncle (Fig. 2D) short, dorsal extension onto cornea tongue-shaped, with small tubercle at anterior base of dorsal extension. Basal

122

Masatsune Takeda and Hironori Komatsu

Fig. 1. Pilomedaeus okutanii gen. et sp. nov., holotype male (cb 14.6cl 11.3 mm), NSMT-Cr 21446. A, carapace, dorsal view; B, same, ventral view; C, right chela, outer view; D, left chela, outer view.

New Genus and Species of Euxanthine Crab123

Fig. 2. Pilomedaeus okutanii gen. et sp. nov., paratype female (cb 13.2cl 9.9 mm), NSMT-Cr S 574. A, carapace, dorsal view; B, same, ventral view; C, same, anterolateral view; D, frontal region, ventral view.

segment of antennule swollen, crested on anterior
and lateral margins; flagellum folding slightly
obliquely. Basal antennal segment robust,smooth, touching front, occupying entire space
between antennular fossa and internal suborbital
angle; flagellum length almost equal to orbital
width.
Third maxilliped (Fig. 3A) wide merus 0.7
times as long as width, about 0.5 length of ischium; surface granulate. Ischium rectangular, about
1.5 times longer than wide; surface smooth; withstout setae along mesial margin.
Chelipeds (Figs. 1C, D, 2A–C) robust, moderately large, covered with short tomentum except
on carinae of carpus and palm and on fingers;
palm height about 0.5 length of palm including
fixed finger. Merus short, broad, carinate on dorsal margin, armed with distal triangular lobe separated from dorsal carina. Carpus produced into
triangular tooth at inner angle; upper surface
armed with some short irregular carinae, carinae
sometimes furnished with rounded granules.

Palm sparsely granulated on outer surface; dorsal
surface with 2 parallel longitudinal carinae; innersurface granulated as in outer surface. Dactylus
weakly curved, with molariform tooth on base of
cutting edge in major chela. Fixed finger withsome large, low triangular or rounded teeth. Fingers brown, getting pale in distal 0.3, without extention onto palm.
Ambulatory legs (Fig. 2A–C) medium inlength, stout, covered with short tomentum except on carinae of merus and carpus; first pairlongest, slightly longer than second pair. Merus
with dorsal margin carinate in distal 0.7, produced into triangular tooth at distal end, with 2–4
spinules in proximal 0.3. Dorsal margin of carpus entirely carinate, with 2 triangular teeth on
proximal 0.3 and distal end; posterior surface
with row of median granules. Propodus subcylindrical. Dactylus straight, subconical, terminatingin acute chitinous recurved tip.
Male thoracic sternum (Fig. 1B) relatively narrow, smooth, tomentose as on carapace; telson

124

Masatsune Takeda and Hironori Komatsu

Fig. 3. Pilomedaeus okutanii gen. et sp. nov. A, paratype female (cb 13.2cl 9.9 mm), NSMT-Cr S 574; B–D,
holotype male (cb 14.6cl 11.3 mm), NSMT-Cr 21446. A, right third maxilliped, ventral view; B, abdomen,
ventral view; C, right G1, abdominal view; D, right G2, abdominal view. Scales: 1 mm.

Fig. 4.

Pilomedaeus okutanii gen. et sp. nov., paratype female (cb 13.2cl 9.9 mm), NSMT-Cr S 574.

New Genus and Species of Euxanthine Crabreaching slightly beyond suture between sternites
4/5; sternite 4 with median longitudinal suture;
median longitudinal suture extending from sternite 7 to 8. Suture between sternites 1/2 completely absent; suture between sternites 2/3 complete;suture between sternites 3/4 vestigial, present onlateral 0.2, marked as shallow furrow on mesial
part; sutures between sternites 4/5 and 5/6 medially interrupted; sutures between sternites 6/7
and 7/8 complete.
Female thoracic sternum (Fig. 2B) relatively
narrow, not granulate, tomentose as on carapace;telson reaching to half length of sternite 4; sutures as in male.
Male abdomen (Figs. 1B, 3B) relatively narrow, not granulate, tomentose as on carapace;somites 3–5 fused but sutures separating somites
discernible. Somites 1 and 2 subequal in width,transversely rectangular. Somites 3–5 trapezoidal, lateral margins concave. Somite 6 rectangular, about 1.5 times wider than long. Telson triangular with rounded tip, about 0.8 times as longas sixth somite, about 1.5 times wider than long.
G1 (Fig. 3C) compressed, weakly curved laterally, tapering, with several spinules along distal
part of mesial and lateral borders; apex rounded,with aperture. G2 (Fig. 3D) relatively short,about 0.3 times as long as G1; distal segment
sickle-shaped, with line of very short setae on
proximal part of mesial surface.
Etymology. The specific name is dedicated toDr. Takashi Okutani who provided us the specimens.
Remarks. The present new species superficially resembles pilumnoid crabs in the tomentose
and convex carapace and the granulated outersurface of chelipedal palm, but can easily berecognised as a xanthoid in the fused somites 3–5of the male abdomen as well as structures of theG1 and G2.
Acknowledgements
We wish to express our sincere thanks to Dr.
Takashi Okutani, a leading taxonomist of Mollusca, of the Japan Agency for Marine-Earth Sci-

125

ence and Technology, Yokosuka, for providing uswith the great number of crab specimens collected by RV Soyo Maru and also to Dr. Hiroshi
Saito of NSMT for providing us with the sampling data. Our cordial thanks are also due to Dr.
Kotaro Tsuchiya and the crews of TRV Shin’yoMaru of the Tokyo University of Marine Scienceand Technology for their cooperation during fieldsurveys. This manuscript was greatly improved
by Drs. Peter K. L. Ng, Jose Christopher E. Mendoza and Tomoyuki Komai.
References
Alcock, A., 1898. Materials for a carcinological fauna of
India. No. 3. The Brachyura Cyclometopa. Part I. The
family Xanthidae. Journal of the Asiatic Society ofBengal, 67: 67–233.
Komatsu, H. and M. Takeda, 2004. Two new species of
the genus Goneplax (Decapoda, Brachyura, Goneplacidae) from East Asia. Crustaceana, 76: 1243–1256.
MacLeay, W. S., 1838. On the brachyurous decapod Crustacea brought from the Cape by Dr. Smith. In: A. Smith(ed), Illustrations of the Annulosa of South Africa;
being a portion of the objects of natural history chiefly
collected during an expedition into the interior of South
Africa, under the direction of Dr. Andrew Smith, in theyears 1834, 1835, and 1836; fitted out by “The Cape ofGood Hope Association for Exploring Central Africa”,
pp. 53–71. Smith, Elder, and Co., London.
Manuel-Santos, M. R. and P. K. L. Ng, 2007. On thegenus Ladomedaeus Števčić, 2005, from the Philippines and Japan, and the status of the Ladomedaeidae
Števčić, 2005 (Decapoda: Brachyura: Xanthoidea).
Raffles Bulletin of Zoology, Supplement (16): 169–175.
Ng, P. K. L. and H.-L. Chen, 2005. On two species of euxanthine crabs from the South China Sea, including adescription of a new species of Crosnierius (Crustacea:
Decapoda: Brachyura: Xanthidae). Proceedings of the
Biological Society of Washington, 118: 319–325.
Ng, P. K. L. and D. G. B. Chia, 1994. The genus Glyptocarcinus Takeda, 1973, with descriptions of a new subfamily, two new genera and two new species from New
Caledonia (Crustacea: Decapoda: Brachyura: Xanthidae). Raffles Bulletin of Zoology, 42: 701–731.
Ng, P. K. L., D. Guinot and P. J. F. Davie, 2008. Systema
Brachyurorum: Part I. An annotated checklist of extant
brachyuran crabs of the world. Raffles Bulletin of Zoology, Supplement (17): 1–286.
Ohe, M. and M. Takeda, 1986. A new deepsea shrimp of
the genus Paracrangon from central Japan. Bulletin of
the National Science Museum, Tokyo, Series A, 12:

126

Masatsune Takeda and Hironori Komatsu

75–81.
Okutani, T., 1972. Molluscan fauna on the submarine
banks Zenisu, Hyotanse, and Takase, near the Izu-Shichito Islands. Bulletin of the Tokai Regional Fisheries
Research Laboratory (72): 63–142, pls. 1–2.
Okutani, T., 1975. Glimpse of benthic molluscan fauna
occupying the submarine bank, Kurose, near Hachijo
Island, Japan. Venus, 33: 185–205.
Sakai, T., 1965. On two new genera and five new speciesof xanthoid crabs from the collection of His Majesty
the Emperor of Japan made in Sagami Bay. Crustaceana, 8: 97–106.
Serène, R. and C. Vadon, 1981. Crustacés Décapodes:
Brachyoures Liste préliminaire, description de formes
nouvelles et remarques taxonomiques. In: Résultats desCampagnes MUSORSTOM. I—Philippines (18–28
Mars 1976), pp. 117–140. Éditions de l’Office de laRecherche Scientifique et Technique d’Outre-Mer,
Paris.
Stevcic, Z., 2005. The reclassification of brachyuran crabs(Crustacea: Decapoda: Brachyura). Natura Croatica,
14 (Supplement 1): 1–159.
Takeda, M., 1973. A new genus and a new species of theParthenopidae from the sea off the Ogasawara lslands(Crustacea, Brachyura). Bulletin of the National Science Museum, Tokyo, Series A, 16: 31–36.
Takeda, M. and B. Galil, 1980. A new species of Calocarcinus (Crustacea, Brachyura) from the submarine
bank off the Izu Islands, central Japan. Bulletin of the
National Science Museum, Tokyo, Series A, 6: 191–194.
Takeda, M. and B. S. Galil, 2005. A new crab species of
the genus Mursia from Japanese waters (Crustacea, Decapoda, Brachyura, Calappidae). In: K. Hasegawa, G.
Shinohara and M. Takeda (eds), Deep-sea Fauna and

Pollutants in Nansei Islands. National Sciece Museum
Monographs, (29): 289–295.
Takeda, M., H. Namikawa, T. Kuramochi, H. Ono, M.
Higuchi and S. Matsumoto (eds.), 2006a. Study on Environmental Changes in the Sagami Sea and Adjacent
Coastal Area with Time Serial Comparison of Fauna
and Flora I. Marine Organism (Brown Algae and Animals (Sponges-Annelids)). Memoirs of the National
Science Museum, Tokyo, (40): 1–408.
Takeda, M., H. Namikawa, T. Kuramochi, H. Ono, M.
Higuchi and S. Matsumoto (eds.), 2006b. Study on Environmental Changes in the Sagami Sea and Adjacent
Coastal Area with Time Serial Comparison of Fauna
and Flora II. Marine Organism (Animals (ArthropodsChordates)). Memoirs of the National Science Museum,
Tokyo, (41): 1–575.
Takeda, M., H. Namikawa, T. Kuramochi, H. Ono, M.
Higuchi and S. Matsumoto (eds.), 2006c. Study on Environmental Changes in the Sagami Sea and Adjacent
Coastal Area with Time Serial Comparison of Fauna
and Flora III. Living Organism and Soil of Coastal
Areas. Memoirs of the National Science Museum,
Tokyo, (42): 1–310.
Takeda, M. and P. K. L. Ng, 1997. The Indo-PacificPilumnidae XI. An unusual new species of Pilumnus(Crustacea, Decapoda, Brachyura) from Japan. Bulletinof the National Science Museum, Tokyo, Series A, 23:
185–190.
Takeda, M. and H. Watabe, 2004. Deepwater carcinoplacine crabs of the genus Mathildella (Crustacea, Decapoda, Brachyura), with description of a new species
from the Kyushu-Palau submarine ridge, southwesternJapan. Bulletin of the National Science Museum, Tokyo,
Series A, 30: 181–189.