Chitons (Mollusca: Polyplacophora) associated with hydrothermal vents and methane seeps around Japan, with descriptions of three new species*

2008·8.1 MB·English

by  Hiroshi Saito

Amer. Make. Bull. 25: 113-124 (2008)
Chitons (Mollusca: Polyplacophora) associated with hydrothermal vents and
methane seeps around Japan, with descriptions of three new species'^
Hiroshi Saito\ Katsunori Fujikura^, and Shinji Tsuchida^
' Department ofZoology, National Museum ofNature and Science, Tokyo, 3-23-1 Hyakunin-cho, Shinjuku-ku, Tokyo 169-0073 Japan,
[email protected]
^ Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka-shi, Kanagawa 237-0061 Japan,
[email protected]
^ Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka-shi, Kanagawa 237-0061 Japan,
[email protected]
Abstract: Three new species ofchitons are described from hydrothermal vent sites and methane seep sites around Japan: Deshayesiella
sirenkoin. sp. fromthehydrothermalvent siteson theseamountsinthe northern MarianaIslandsarea,Placiphorella okutaniin. sp. from
Hachijo Depression in the Izu-Ogasawara (Bonin) Islands area where no active vent/seep area has been discovered, but the possibility of
hydrothermal activityhas been suggested, and Placiphorella isaotakii n. sp. from methane seep sites on the Kuroshima Knoll offYaeyama
Islands.Deshayesiellasirenkoin.sp.aswellastwopreviouslyknownhydrothermalventspecies,Leptochiton tenuidontusSaitoandOkutani,
1990 and Thermochiton undocostatusSaito and Okutani, 1990, are vent/seep associated species, whereas the two Placiphorella maybe guest
species. Additional distributional records are given for the two known species.
Key words: Deshayesiella, Placiphorella, deep-sea, chemosynthetic environment, taxonomy. Pacific Ocean
The number ofthe molluscan taxa described from che- Okutani, 1990. They were collected by a human-occupied
mosynthetic environments has rapidly increased in the last submersible, Shinkai 2000, belonging to Japan Agency for
two decades (Sasaki et al. 2005). Most of these taxa are, Marine-Earth Science and Technology (JAMSTEC). Since
however, gastropods and bivalves (Desbruyeres et al. 2006: then, some additional chiton specimens have been collected
520-523). Since Saito and Okutani (1990) described two chi- from Japanese waters by the Deep Sea Research System,
ton species from the hydrothermal vent site of Okinawa includinghuman-occupied submersiblesor ROVsbelonging
Trough, East China Sea, some chiton species have been re- to JAMSTEC. Here, we describe three new species and fur-
ported from chemosynthetic environments. Sc]uires and ther describe the morphology and distribution of the two
Goedert (1995) reported Leptochiton alveolus (Loven, 1846) previously known species.
(sensu Ferreira 1979 and Kaas and Van Belle 1985) from
Eocene and Oligocene cold methane seep limestones, Olym-
pic Peninsula,Washington. Olu etal. (1997) reported “Poly- MATERIALS AND METHODS
placophora” from the methane seep of Barbados Prism,
1,000-2,000 m, and Sellanes et al. (2004) reported Leptochi- Specimens were collected bythe Deep-sea Research Sys-
ton sp., Stenosemus sp., and Placiphorella sp. from methane tem of JAMSTEC: human-occupied submersibles Shinkai
seepage in the bathyal zone offChile. Schwabe and Sellanes 2000 (abbreviated as 2K) and Shinkai 6500 (6K) and a re-
(2004) have described a newspecies, Lepidozona balenophila, motely operated vehicle Hyper-Dolphin (HPD). Sampling
from another type ofchemosynthetic environment, decom- sites are shown in Fig. 1. Preparation for SEM observation
posing whale carcasses. However, no vent/seep associated followed Saito (2006). All specimens were deposited in the
chiton species, other than the two known species, has been molluscan collection of the Department ot Zoology, Na-
described anywhere else in the world. Those two known tional Museum of Nature and Science (formerly National
species that were described from the hydrothermal vent in Science Museum, Tokyo) (NSMT).
the East China Sea are Leptochiton tenuidontus Saito and The systematic arrangement used in this paper follows
Okutani, 1990 and Therinochiton undocostatus Saito and Sirenko (2006).
Prom thesymposium “Advances in Chiton Research” presented at the joint meeting ofthe American Malacological Societyand Western
Society ofMalacologists, held 29 July to 3 August 2006 in Seattle, Washington.
113
114 AMERICAN MALACOLOGICAL BULLETIN 25 • 1/2 • 2008
SYSTEMATICS 120 E 130° 140° 150°
Order Lepidopleurida Thiele, 1909
Suborder Lepidopleurina Thiele, 1909
Eamily Leptochitonidae Dali, 1889
Genus Leptochiton Gray, 1847
Type species
Chiton cinereus Montagu, 1803 [= Leptochiton asellus
(Gmelin, 1791)], by subsequent designation (Gray, 1847).
Leptochiton tenuidontus Saito and Okutani, 1990
(Eig. 2A-B)
Leptochiton tenuidontus Saito and Okutani 1990: 166-
171, figs. 2-12, pi. 1, figs. 1-4; Kaas and Van Belle
1994: 22-23, fig. 7; Kaas and Van Belle 1998: 185;
Saito 2000: 7, pi. 3, fig. 11; Cosel 2006: 81. Figure 1. Sampling sites. Solid circles indicate hydrothermal vent.
Open circles indicate methane seep. -kNo active vent/seep area has
Leptochiton sp. Saito and Eujikura 2000: 74-75.
been discovered but the possibility of hydrothermal activity has
been suggested.
Type material examined
Holotype: NSMT-Mo 69193, body length ca. 16 mm.
Type locality: hydrothermal vent site on the Iheya Ridge, Gills merobranchial, adanal, without interspace, 6-8 on
central Okinawa Trough, East Ghina Sea, 27°32.70'N, each side.
126°58.20'E, 1395 m, 2K, Dive #426, 21 luly 1989.
Distribution and type ofhabitat
Additional material examined Iheya Ridge and off Kikaijima Island, Nansei Islands,
NSMT-Mo 73838 (ex. JAMSTEC sample No.: RK4-A-1, 1395-1442 m; hydrothermal vent and methane seep.
009550-009553), 4 specimens, body length ca. 18-20 mm,
Remarks
methane seep site otfKikaijima Island in the Amami Islands
This species was described based on a single specimen
area, 28°26.39'N, 130°19.01'E, 1430 m, 2K, Dive #1020, 24
June 1998; NSMT-Mo 73839 (ex. JAMSTEC sampJe No.: with heavily eroded valves missing a large piart of the teg-
mental sculpiture. The remaining small portion ofsculptur-
RK4-A-1, 009548-009549), 2 specimens, body length 22 and
23 mm, methaneseep siteoffKikaijima Island in theAmami ing and other features, especially the characteristic radula
with elongate “toothpick”-like inner small (third) lateral,
Islands area, 28°26.42'N, 130°18.98'E, 1442 m, 2K, Dive
#1021, 25 June 1998; NSMT-Mo 73940 (ex. JAMSTEC allows the additional specimens to be identified as this
species.
sample No.: RK4-A-1, 009544-009547), 3 specimens, body
length ca. 20-22 mm, methane seep site offKikaijima Island, Theholotypewas collected from undersurface ofa rock,
in theAmami Islands area, 28°26.45'N, 130°19.1'E, 2K, Dive whereas all additional specimens were attached on the shells
ofBathymodiolus platifrons.
#1022, 1440 m, 26 June 1998. All nine specimenswerefound
on the shells of Bathyniodiohis platifrons Elashimoto and Family Protochitonidae Ashby, 1925
Okutani, 1994. Genus Deshayesiella Garpenter in Dali, 1879
Additional description Type species
Tegmentum sculptured with round granules densely ar- Deshayesiella (Leptochiton) curvatus (Carpenter MS)
ranged in quincunx order on head valve, lateral areas of Dali, 1879 (nom. nud., = Lepidopleurus (Deshayesiella) cur-
median valves, and postmucronal area of tail valve, with vatus Carpenter in Pilsbry, 1892), bysubsequent designation
elongate granules arranged in quincunx order or, occasion- (Pilsbry, 1892).
ally, in irregular longitudinal rows in central area ofmedian Deshayesiella sirenkoi sp. nov.
valves and antemucronal area of tail valve (Fig. 2A). Each (Figs. 3, 4, 5A-D)
granule with one macraesthete pore and one to four mi-
craesthete pores on anterior slope; size ofmacraesthete pore Type material examined
ca. 5-8 pm, that of micraesthete pore slightly smaller than JTolotypie: NSMT-Mo 73841 (ex. JAMSTECsampleNo.:
macraesthete pore (Fig. 2B). RK8-B-1, 006983), body length 36.4 mm. Type locality: hy-
CHITONS FROM HYDROTHERMAL VENTS AND METHANE SEEPS 115
Figure 2. Leptochitoii teniddontus Saito and Okutani, 1991 (NSMT-Mo 73940) valve VI. A, sculpture on central area, scale bar = 200 pm;
B, close-up ofgranules on central area, scale bar = 100 pm.
drothermal vent site on the Kasuga II Seamount in the ules which are often fused radially, forming larger elongate
northern Mariana Islands area, 21°36.1'N, 143°38.5'E, 400 granules, marked with concentric growth lines; pleural areas
m, 2K, Dive#986, 23 November 1997; paratypes: NSMT-Mo ofmedian and tail valves sculptured with strong, longitudi-
73842, I specimen, body length ca. 45 mm, hydrothermal nal, slightly curving riblets; jugal area densely sculptured
vent site on the Nikko Seamount in the northern Mariana with finer granules. Aesthete pores (Fig. 3A) located on an-
Islands area, 23°04.7'N, 142°19.9'E, 460 m, 6K, Dive #144, terior portion ofeach granule. Each group ofpores consist-
19September 1992; NSMT-Mo 73843 (ex. JAMSTEC sample ing of one macraesthete pore, 10-20 pm in diameter, and
No.: EZIO, 061632-061635), 4 specimens, body length ca. one or two micraesthete pores, 5-8 pm in diameter at both
27-31 mm, hydrothermal vent site on the Daikoku Sea- sides of macraesthete pore. Articulamentum of head valve
mount in the northern Mariana Islands area, 21°19.53'N, thickened, weakly projecting around the anterior margin of
144°1 1.51'E, 428 m, on rock, HPD, Dive #498, 1 November transverse muscle scars. Median valves and tail valve with
2005. widelyV-shaped callus. Eaves wide, nearlysmooth, scattered
with minute aesthete pores. Tegmentum broadly folded un-
Diagnosis der on posterior margin. Sutural laminae (Fig. 5B) strongly
Valvesthick, low, slightlycarinated. Median valveswide, projected forward, triangular, widely separated from each
angulated at antero-lateral corners, weakly protruded at an- other.
terior margin of jugal area. Tail valve with slightly raised Girdle fleshy, thick, brownish. Perinotum (Figs. 3B, 5D)
mucro located anterior to the center, and concave posterior densely covered with elongate, obtusely pointed, flattened,
slope. Pleural areas sculptured with longitudinal, weakly distally ribbed spicules (Fig. 4A), 130 pm x 25 pm, inter-
curving riblets. Girdle with long needles. mingled with long, straight, smooth needles (Fig. 4B), up to
680 pm X 55 pm. Girdle margin fringed with long needles
Description similar to those on perinotum (Fig. 3C). Spicules on hypo-
mm
Body (Fig. 5A) oval, 36.4 in length. Valves (Fig. 5B) notum (Figs. 3D, 4C-E) flat with one to three strong riblets,
thick, low, slightlycarinated. Girdle fleshy, deeplyencroach- 90 pm X 30 pm.
ing at sutures. Gills merobranchial, adanal, without interspace, 16 on
Headvalve semicircular, rounded at postero-lateral cor- left, 18 on right.
mm
ners. Median valves wide, widest at valves IV-VI, slightly Radula (Fig. 3E-F) long, 15.5 in length with 56
carinated, beaked, weakly projected at anterior margin of transverse rows of mineralized teeth. Central tooth oblong
jugal area. Tail valve more than semicircular, wider than with narrow cusp at top, weakly expanded laterally, keeled
head valve; mucro slightly raised, located anterior to the near base. Centro-lateral (first lateral) teeth with well devel-
center; posterior slope concave. Tegmentum granulo- oped plate surroundingbase ofmajor lateral (second lateral)
costate. Head valve, lateral areas ofmedian valves, and pos- teeth, obtuselypointed atantero-dorsal corner. Major lateral
terior area oftail valve sculpturedwith denselypacked gran- teeth with bicuspid head, of which the larger outer cusp is
116 AMERICAN MALACOLOGICAL BULLETIN 25 • 1/2 • 2008
Figure 3. Deshayesielln sircnkoi sp. nov., holotype (NSMT-Mo 73841). A, valve II, granules on pleural area near anterior margin ofjugal
area, scale bar = 100 pm; B, spicules ofperinotum near girdle margin, scale bar = 50 pm; C, marginal spicules, scale bar = 100 pm; D,
spicules ofhyponotum, scale bar = 50 pm; E, radula, central part, postero-dorsal view, scale bar = 100 pm; F, radula, central part, oblicjue
antero-dorsal view, scale bar = 100 pm.
pointed and the smaller, inner cusp is rounded. Major un- Distribution and type of habitat
cinus (fifth lateral) teeth rounded at top with blade ofmod- Known from the seamounts on Kasuga II, Nikko, and
erate width. Bolster (radular vesicle and cartilage) length Daikoku in the northern Mariana Islands area, 400-460 m;
5.2 mm. hydrothermal vent.
CHITONS FROM HYDROTHERMAL VENTS AND METHANE SEEPS 117
Etymology
This species is named in honor of
Dr. Boris Sirenko, who has recently
given a new diagnosis for the genus
Deshayesiella.
Remarks
The features ofthe present species
match the characteristics ofDeshayesi-
ella given by Sirenko (1997). These
features include: valves solid, rather Figure 4. Deshayesiella sireiikoi sp. nov., holotype (NSMT-Mo 73841). A, spicule ofperino-
flat, evenlyrounded; median valves di- tum; B, needle on perinotum; C-E, spicules ofhyponotum; scale bar = 50 pm.
vided into jugal, two pleural, and two
lateral areas (unlike Leptochiton); teg-
mentum of head valve, lateral area of median valves and Type species
postmucronal area of tail valve sculptured with irregular Thermochiton undocostatus Saito and Okutani, 1990, by
granules, strongly marked with concentric lines of growth; original designation.
girdle ratherwide, dorsallycoveredwith small spicules (100- Thermochiton undocostatus Saito and Okutani, 1990
150 pm) and randomly dispersed large spines (320-550 pm);
radula with bicuspid major lateral teeth. Although there are Therttwchiton undocostatus Saito and Okutani 1990:
some slight differences, such as the slightly carinated valves 171-174, figs. 13-23, pi. 2, figs. 1-4; Kaas and Van
and somewhat longer large perinotal spines in the present Belle 1994: 36-38, fig. 13; Kaas and Van Belle 1998:
species, we think they are insignificant for generic assign- 192; Saito 2000: 11, pi. 6, fig. 12; Cosel 2006: 80.
ment. Sirenko (1997) recognized three known species in Type material examined
Deshayesiella: D. curvata (Carpenter in Pilsbry, 1892), Ol- Holotype: NSMT-Mo 69194, body length ca. 13 mm.
droydia bidentata Is. Taki, 1938, and Hanleya sinica Xu, Type locality: hydrothermal vent site on the Iheya Ridge,
1990, as well as two undescribed species (sp. 1 and 2). The central Okinawa Trough, East China Sea, 27°32.70'N,
assignment ofH. sinica may, however, need reconsideration 126°58.20'E, 1395 m, 2K, Dive #426, 21 luly 1989.
because it has rather vaguely regionalized tegmentum with
finer sculpture, and thus is more like members ofLeptochi- Additional material examined
ton in this respect. The present species is easily distinguish- NSMT-Mo 73844 (ex. JAMSTEC sample No.: RK4-A-6,
able from all known congeners and one of Sirenko’s unde- 038998-039004), 6 specimens, body length ca. 3-8 mm. Hy-
scribed species, sp. 1, in having wider median valves, each drothermal vent site on the Hatoma Knoll, 24°51.65'N,
sidewith more angularantero-lateral corner. The features of 123°50.29E, 1497 m, on small chimney rock, 2K Dive #1277,
another undescribed species, sp. 2, have not yet been given 29 May 2001; NSMT-Mo 73845, 2 specimens, body length
in detail; however, the present new species is probably dis- ca. 7 mm, methane seep site on the Kuroshima Knoll off
tinct from Sirenko’s sp. 2 because the latter is distributed in Yaeyama Islands area, 24°08.00'N, 124°11.50'E, 686-688 m,
a different geographic area: the East Pacific off southern on the shells ofBathynwdiolus hirtus Okutani, Fujikura, and
California and in the Gulf of California, Mexico. Sasaki, 2004orBathynwdiolussecuriformisOkutani, Fujikura
Deshayesiella sirenkoi is locally common around the hy- and Sasaki, 2004, and Calyptogena kawamurai (Kuroda,
drothermalventsiteon the Daikoku Seamount (see Pig. 5C). 1943), HPD, Dive #554, 21 May 2002.
Deshayesiella sirenkoi, as well as two known vent species, Additional description
Leptochiton teniiidontus Saito and Okutani, 1990 and Ther- Gills nearly holobranchial (anterior-most gill located
mochiton undocostatus Saito and Okutani, 1990 coulci be under the third valve), adanal, with interspace, 19 gills on
restricted to hydrothermal vent and/or methane seep habi- each side (NSMT-Mo 73845).
tats because each ofthese species was found only from those
environments of more than two sites. Distribution and type of habitat
Offsouthern Nansei Islands, 686-1497 m; hydrothermal
vent and methane seep.
Order Chitonida Thiele, 1909
Suborder Chitonina Thiele, 1909 Remarks
Family Ischnochitonidae Dali, 1889 Characteristic features ofthe present specimens, such as
Genus Thermochiton Saito and Okutani, 1990 undulating sculpture on the valves, granulo-costate dorsal
118 AMERICAN MALACOLOGICAL BULLETIN 25 • 1/2 • 2008
Figure 5. Newspecies ofchitons. A-D, Desliayesiella sirenkoi; E-H, Placiphorella okutanii; I-K, Placiphorella isaotakii. A, E, I, whole animal,
dorsal view, holotypes; B, E, 1,head, median (B:valve II; F, J; valveIII), andtailvalves, dorsalview,holotypes, scalebar= 5 mm; C, habitat,
DaikokuSeamount,arrowheadsindicateposition ofchitons;D, perinotum,showinglongneedles,holotype,scalebar= 1 mm;G,tailvalve,
paratype, scale bar = 1 mm; H, K, anterior margin ofgirdle, paratype and holotype, respectively, scale bar = 1 mm.
CHITONS FROM HYDROTHERMAL VENTS AND METHANE SEEPS 119
scales, and head of the major lateral tooth of radula with difference between macraesthete pore and micraesthete pore
basal pointed projection agree well with those of the hardly discernible. Articulamentum well developed, white,
holotype. heavily calloused anteriorly in head valve, transversely in
median valve, and posteriorly in tail valve; posterior margin
Suborder Acanthochitonina Bergenhayn, 1930
of articulamentum widely covered with folded tegmentum
Family Mopaliidae Dali, 1889
Genus Placiphorella Dali, 1879 in head and median valves, narrowly covered in tail valve.
Sutural laminae well developed, narrowly separated from
Type species each other. Insertion teeth short, thick, rugose on anterior
Placiphorella velata (Carpenter MS) Dali, 1879, byorigi- surface,with 12 slits in headvalve (Fig. 6C), one on each side
nal designation. in median valves, none on tail valve. Slit rays represented by
series of minute pores, clearly visible in apical half ofhead
Placiphorella okutanii sp. nov. valve, median valves, inconspicuous in tail valve. Eaves nar-
(Figs. 5E-H, 6, 7) row, with many minute pores.
Placiphorella stimpsonr. Wu and Okutani 1985: 126-128, figs. Girdle (Fig. 5E) widely expanded anteriorly, becoming
9-18 (not ofGould 1859). narrower toward posterior end. Perinotum (Fig. 6E) covered
with minute spicules (Fig. 7A-B), mammilated at tip, ca. 150
Type material examined pm X 30 pm, and densely implanted with low spiny tufts
Holotyp^e: NSMT-Mo 73777 (ex. JAMSTEG sample No.: consist of5-10 sharply pointed, weakly curved spicules (Fig.
RK4-B-5, 006454), body length 32 mm. Type locality: 7C-D), ca. 400 pm x 50 pm in width, surrounded bybroken
Hachijo Depression in the Izu-Ogasawara (Bonin) Islands short spicules. Bristle, worn off in holotype, with sharply
area. 32°48.8'N, 139°27.0'E to 32°51.3'N, 139°31.6'E, 926- pointed spicules similar to spiny tufts. Hyponotum clothed
817 m, dredge attached to JAMSTEC Deep Tow Camera, with obtusely pointed, smooth spicules (Figs. 6F, 7G), 140-
cruise No.: DK88-3-IZU, 27 August 1988; paratype: NSMT- 165 pm X 30 pm. Anterior hyponotum with numerous
Mo 60008, body length ca. 30 mm, off Miyake Island, Izu- warts, which are provided with 20-30 pointed spicules (Fig.
Ogasawara Islands area, 34°03.0'N, 140°02.2'E, 1210-1235 7H-I), 150-170 pm X 25 pm. Pallial fold well developed with
m, R/V Soyo-Maru St. B2, beam trawl, 5 July 1967. 9 precephalic tentacles, which are occasionally bifurcated.
Spicules on pallial fold similar to obtusely pointed spicules
Diagnosis on hyponotum, but smalleron precephalic tentacles, 110 pm
Valves chalky white, fragile, sculptured with densely X 15 pm (Fig. 7J), somewhat narrower on posterior end, 160
packed, low, rather large, granules. Tail valve with narrow pm X 25 pm (Fig. 7K).
postmucronal areas separated by shallow sinus behind mu- Gills holobranchial, abanal, with interspace, 15 on left
cro. Sutural laminae wide, narrowly separated from each side, 16 on right.
mm
other. Perinotum densely implanted with low spiny tufts. Radula (Fig. 6G-H) small, 6.5 in length, with 40
Bristle implanted around girdle margin. transverse rows of mineralized teeth. Central tooth oblong,
with narrow cutting edge, slightly expanded laterally and
Description bilobed at base. Centro-lateral (first lateral) teeth low, thick-
mm
Body (Fig. 5E) broadlyoval, 32 in length, light buff ened at antero-dorsal corner. Major lateral (second lateral)
in color. teeth with proportionally small tridentate head. Major un-
Valves (Fig. 5F) wide, depressed, subcarinated, chalky cjnus (fifth lateral teeth with rather long blade ofmoderate
)
white, fragile. Head valve crescent in outline, anterior slope width. Bolster (radular vesicle and cartilage) length 3.3 mm.
concave. Median valves very wide, short, oblong in outline, Paratype: Tail valve (Fig. 5G) with narrow posterior
weakly projected forward at jugal portion; lateral areas areas separated by shallow sinus at posterior end.
raised, grooved medially. Tail valve (Fig. 6A) small, inversed Bristle densely implanted along girdle margin (Fig. 5H).
trapezoidal in outline, with narrow postmucronal area sepa- Thick bristles (Fig. 7E), ca. 300 pm in width, implanted on
rated by shallow sinus at posterior end; mucro subterminal, girdle margin and apparently thinner bristles (Fig. 7F), at-
mm
slightly raised. Tegmentum (Fig. 5F) sculptured with densely taining ca. 1 x 100 pm, restricted on dorsal surface
packed, low, somewhat elongate granules on head valve, lat- close to girdle margin, and occasionally on other area on
eral areas of median valves, and postmucronal area of tail perinotum.
valve. Remaining tegmental areas with slightly lower, larger
granules. Aesthete pores minute, 3-6 pm in diameter, dis- Distribution and type of habitat
tributed both on granules and the tegmental plain (Fig. 6B), Only known from Hachijo Depression and off Miyake
which become denser toward the lateral areas (Fig. 6D). The Island in Izu-Ogasawara Islands area, 817-1235 m; un-
120 AMERICAN MALACOLOGICAL BULLETIN 25 • 1/2 • 2008
Figure 6. Placiphorclln okutaiiiisp. nov., liolotype (NSMT-Mo 73777). A, tail valve, scale bar = 1 mm; B, tegmentum ofvalve 111, anterior
margin ofcentral area, scalebar = 200 pm; C, insertion teeth ofheadvalve, scale bar = 2 mm; D, tegmentum ofvalve III, lateral area, scale
bar = 200 pm; E, anterior perinotum, scalebar = 200 pm; F,spiculesofhyponotum,scalebar = 100 pm; G, radula, central part (righthalf),
postero-dorsal view, scale bar = 50 pm; H, radula, lateral part, postero-dorsal view, scale bar = 50 pm.
CHITONS FROM HYDROTHERMAL VENTS AND METHANE SEEPS 121
cause P. pacifica Berry, 1919 (Lecto-
type, SBMNH 34394 designated by
Scott et al. 1990) has almost smooth
surface on the tegmentum, and no
other known species of Phiciphorella
has such an obviously granular teg-
mentum. From Japanese waters, an-
other deep-sea species, Placiphorella
albitestae was described from the Sa-
gami Sea, northern Izu-Ogasawara Is-
lands area. Placiphorella albitestae has
much finer granules on finer, but
sharply raised, growth lines on the
tegmentum, much finer and scarce
spinous tufts on the perinotum, and
shallower bathymetrical range of dis-
m
tribution, from 80 to 200 (Saito
2000
).
Placiphorella isaotakii n. sp.
Figure7.Placiphorellaokutaniisp. nov., sclerites,A-D, G-K, holotype (NSMT-Mo 73777); E, (Figs. 5I-K, 8, 9)
F, paratype (NSMT-Mo 60008). A, B, spicules of perinotum; C, D, spicules of tuft on
perinotum; E, thick bristle (spicules are lost in large part); F, thin bristle; G, spicules of Type material examined
bohafyrppoa=lnloi5at0lu0mfp,olmdH,,nfe1o,arsrpEipcoausnltedesrFio.ofrtuefntdo.nUhpyppeornostcaulme;bJa,rsp=icu1l0e0ofpmpr,ecfeoprhaAl-iDc,teGn-taKc;le;loKw,erspisccuallee JAMHSoTloEtCypes;amNpSlMeT-NoM.o: 7R3K747-8A-(3ex,.
016481), body length ca. 34 mm. Type
locality: methane seep sites on the Ku-
known, possibility of hydrothermal activity is suggested in roshima Knoll off Yaeyama Islands area, 24°07.00'N,
Hachijo Depression. 124°11.00'E, 691-692 m, 2K, Dive #1100, 22 May 1999.
Etymology Diagnosis
This species is named in honor ofDr. Takashi Okulani, Valves solid, sculpturedwith fine elongate granules. Tail
who has been actively working for deep-sea vent/seep mol- valve wide triangular, with terminal mucro. Sutural laminae
luscs, and collected this species for the first time. wide, narrowly separated each other. Insertion teeth low,
hardly separated with slits in head valve. Perinotum densely
Remarks implanted with low spinous tufts. Bristle implanted along
Kaas and Van Belle (1994) synonymized all known girdle margin.
deep-sea Placiphorella species with Placiphorella atlantica
(Verrill and Smith, 1882) and this decision was followed by Description
mm
Clark (1994). However, at least Placiphorella pacifica Berry, Body (Pig. 51) broadly oval, ca. 34 in length, light
1919 and Placiphorella albitestae Is. Taki, 1954 are distinc- buff in color.
tive, and can be separated by the valve shape and sculpture, Valves (Fig. 5J) wide, depressed, subcarinated, solid.
girdle element shape and sclerite arrangement, and other Head valve crescent in outline; anterior slope concave. Me-
features. Among those deep-sea Placiphorella, the present dian valves very wide, short, oblong in outline, weakly pro-
new species most closely resembles Placiphorella “pacifica” jected forward at jugal portion; lateral areas raised, grooved
reported by Smith and Hanna (1952) from Pioneer Sea- medially. Tail valve (Fig. 8A) small, wide triangular in out-
mount, East Pacific, 500-650 m (CASIZ 064802) by having line, with terminal mucro. Tegmentum (Fig. 5J) sculptured
granular tegmentum. However, the granules of the former with densely packed, elongate granules on head valve and
are irregular in shape and arrangement, especially on the lateral areas of median valves. Remaining tegmental areas
lateral areas, and the spiny tufts of the perinotum are very with weak, elongated granules, which are occasionally
prominent and dense. Placiphorella “pacifica” reported by merged into longitudinal threads. Aesthete pores minute,
Smith and Hanna (1952) can be an undescribed species be- 3-5 pm in diameter, arranged roughly in concentric patterns
122 AMERICAN MALACOLOGICAL BULLETIN 25 • 1/2 • 2008
Figure 8. Placiphorella isaotakii sp. nov., holotype (NSMT-Mo 73778). A, tail valve, scale bar = 1 mm; B, tegmentum ofvalve III, anterior
margin ofcentral area, scalebar = 200 pm; C, insertion teeth ofhead valve, scale bar = 1 mm; D, tegmentum ofvalve III, lateral area, scale
bar = 200 pm; E, perinotum, scale bar = 500 pm; F, spicules ofhyponotum, scale bar = 100 pm; G, bristles near anterior margin, scale
bar = 500 pm; H, radula, postero-dorsal view, scale bar = 100 pm.
in pleural areas (Fig. 8B), which are denser and less regularly riorly in head valve, transversely in median valve, and
arranged on lateral areas (Fig. 8D). Differencebetween mac- posteriorly in tail valve; posterior margin ofarticulamentum
raesthete pore and micraesthete pore hardly discernible. Ar- widely covered with folded tegmentum in all valves. Sutural
ticulamentum well developed, white, heavily calloused ante- laminaewell developed, narrowlyseparated from each other.