Dilemma japonicum new species (Bivalvia: Anomalodesmata: Poromyidae): a new record of the genus from the Northwest Pacific

2008·2.7 MB·English

THE NAUTILUS 122(3):166-170, 2008 Pao-e 166
Dilemma japonicum new species (Bivalvia: Anomaloclesmata:
Poromyiclae): A new record of the genus from the
Northwest Pacific
Takenori Sasaki Jose H. Leal
The Universit}' Museum, The Baile\-Matthews Shell Museum
The University ofTok\o P. O. Box 1580
7-3-1 Hongo, BunkAO-ku, Sanibel, FL 33957 USA
Tokyo 113-0033 [email protected]
JAPAN
[email protected]'>o.ae.jp
ABSTRACT from offKeyWest Florida, 229 m, (2) D. spectralis Leal,
TdehsecrfiobuerdthfrspoemcideissaorftitchuelabtievdalvvaelvgeesncuosllDeictleedmomfafcLeenatlr,al20J0a8p.ani.s t20a0t8u,mf(rCroomzioefrf,V1a9n6u6a}tuf,ro9m50o-ff96T1hrme,eaKnidng(s3)IsDl.andisn,exNpeecw-
Thediscoveryofthesespecimens representsasignificant range Zealand, 805 m. Theirgeographicdistributionisdisjunct,
extension for the genus to the Northwest Pacific. The new but shell moipholog)' is surprisinglv similar between the
species is distinguished from the other three known species by species of Florida and Southwest Pacific.
its surface sculpture, shape ot escutcheon, and hinge. The pro- Duiing a research cniise offcentral Japan, in 2000, die
dissoconch, indicative oflecithotrophicdevelopment, and shell senioraudiorcollectedsmall unpaired\";il\es oiCorcidiim-
microstructure, widr outer homogeneous and inner nacreous likc bivalves, but its locality (240-273 m) was obviously
layers, are illustrated for the first time for a species of the too deep for Coreiihim. In addition, the inner shell layer
genus.
is iridescent, indicating presence of a nacreous layer.
Additional Keywords: Septibranchia. Poromxoidea. Japan, This conchological feature alone precluded inclusion of
nacreous lax'er. pi'odissocondi this new species in the Cardiidae. However, lack ofsoft
parts prevented a better resolved familial or supra-
familial placement ofthe new taxon. The publication by
INTRODUCTION Leal (2008) alkm'ed allocation of the unknown bi\alve in
that newly established genus ofthe Porom\idae. In this
Dilemma is a recently estabhshed unusual genus of paper, we describe diis most interesting newspecies and
Anomalodesmata (Leal, 2008). The first described spe repoit a new record of the genus from the Northwest
cies of the genus, D. inexpcctatum (Crozier, 1966), was Pacific.
dredged from bathyal depths offnorthern New Zealand
and originally assigned to Corciihim. family Cardiidae
(Crozier, 1966) due to general similarities in shell shape. MATERIALS AND METHODS
However, its original familial allocation was questionable
from habitat and depth alone, because Corcidum species
inhabit shallowlagoons of tropicalwaters andutilize pho- Four unpaired \al\es were sorted from sediments
tosvnithesis-derived energy by symbiotic dinoflagellates dredgedfrom southeast of Kamogawa, ChibaPrefecture,
(Farmer et al., 2001). Subsequent discoveiy ofobviously Japan, with a biological dredge (RA'T..\nsei-MARU, cmise
related species with preserved soft parts allowed Leal KT-00-05, statio'n 1, 240-273 m, 34°59.963' N,
(2008), using details of the macroanatomy, to demon- 140°27.159' E- 35°00.020' N, 140°28.427' E, May 17,
strate (1) that Corcuhim inexpectatum and the other, 2000). All specimens were dead and disarticulated. The
then newly found species are related and desen'ed shellswere coatedwith platinum andvanadium andpho-
grouping under a new genus, (2) that the new genus tographed with a scanning electron microscope (Hitachi
should be included in the Poromyidae, and (3) that spe- S-2250N), folloudng a standard method (see Geiger et
cies of Dilemma are carnivores. In fact, camivon' is a al, 2007). The holotype and two paratvpes are deposited
feeding habit that is common in the Anomalodesmata, a in the Department ofHistorical Geolog\' and Paleontol-
group ofbivalves mostly present in the deep sea. ogy, The University Museum, The Universits' of Tok"\'o
Three species were originally (Leal, 2008) assigned to (UMUT), and one paratvpe in The Bailev-Matthews
the genus: (1) Dilemma fnimarhernonim Leal, 2008, Shell Museum, Sanibel, Florida (BMSM).
T. Sasaki and l.H. Leal 2008 Pac;e 167
Figures 1—t. Dilcinniii jajionirum new species. 1. Lateral view. 2. Posterior view. 3-4. Interior view. 1-3. Left valve. 4. Right
val\e. 1. Holotxpe, UMUT RM 29689. 2. Paratope L UMUT RM29690, 3. Paratyise 2. UMUT RM2969L 4. Parat)pe 3, BMSM
1T9S3. Abbrexiations: ct = cardinal-like tooth; e = escntcheon; k = keel; It = lateral tooth; p = projection in hinge; s = socket.
SYSTEMATICS than posterior. Surface of anterior region smootli, flat-
tened in macroscopicview, but microscopically punctate
Superfamilv Poroimoidea Dall, LS86 (Figure5). Surfaceofposteriorregion sculpturedbysev-
Family Poromyidae Dall, 1886 eralweakradial ribs (Figure3), andmoreprominentlyso
Genus Dilemma Leal, 2008 by dense commarginal ribs, also pimctate in enlarged
T>pe Species: Dilemmii fnimnrkcrnonim Leal, 2008, vliikeew,onbuatntpeurnicotraraergrioanng(eFdignuearteh'6).inUrmadbiaol dainrgeucltaitoendubny-
b\ original designation.
keel and rib along posterior margin (Figure 7); umbonal
Diagnosis: Sliell compressed anteroposteriorlv and cavity deep; umbo involute (Figure 3). Prodissoconch
expanded lateralK'; lateral outline of articulated valves simple, flattened, sliield-like, ca. 200 |xm in lengtli, with-
cardioid; umbones projecting dorsally and located ante- out prominent sculpture or division into prodissoconch I
riorly; sharp oblique carina dividing anterior and poste- and II (Figure 8). Hinge provided with single cardinal-
rior regions; maximum growth a.\is having ca. 30° against hke tooth and socketon eachvalve (Figures 3-4, 7, ct, s);
anteroposterior a.\is; hinge a\is short with cardinal-like cardinal-like tooth located anteriorto socketon leftvalve
tootla and socket in each valve; lateral tooth reduced, (Figure 3), theii- position reversed on right valve (Figure
present only in right valve; ligament external, double- 4). Lateral toodi small, present only on right valve (Fig-
lavered; lithodesma absent; shell interior lined with ure 4, It). Single shaip projection (Figures 3-4, 7, p)
sheet-like nacreous laver. See Leal (2008) for anatomical prominent on posterior side ofhinge in similar position
characters. and size in right and left\'alves. Ligament and outlines of
muscle scars not obsei-ved in disarticulated valves. Shell
Dilemmajaponicum new species margin consisting of two layers, outer homogeneous
(Figures 1-12) structure and inner nacreous structure (Figure 9-10,
OL, ID. In outer la)'er elongate granules arranged ver-
Diagnosis: Posterior region ol shell sculptured by ticalK' but lacking clear boundary. In inner layer suboval
rough, lamellate, thin, commarginal ribs; surface irregu- tablets fusing in growth region (Figure 11), numerous
larl\- punctate on anterior region but puncta radially ar- sheet-like layers forming nacre (Figure 12).
ranged on posterior region; escutcheon not distinctlyde-
marcated; prominent projection present in posterior re- Type Material: Holotyj^e, 4.7 mm (height: SH) x 2.0
gion ofhinge. nim (length: SL), UMUT RM29689 (Figure 1); Paratype
Description: Slu^ll tliin, fragile, compressed antero- 1, 5.6 mm (SH) x 2.4 mm (SL), UMUT RM29690 (Fig-
mm
posteriorK'; shell height largerthan shell length. Anterior ure 2); ParaHpe 2, 6.0 (SH) x 3.0 nuu (width),
and posterior regions clearly demarcated b\ shaqi keel UMUT RM29691 (Figure3); Paratvpe 3, 9.3 mm (SH) x
(Figure 1. kj: anterior region narrower and less inflated 4.0 mm (SL), BMSm' 17983 (Figure 4).
Pase 168 THE NAUTILUS, Vol, 122, No. 3
Figures 5-8. Dilciiuiiii japoiiiiinn newspecies. 5-6. Detail ofsurface sciilptiiie, 5. Aiiteiior slope, (>. Posterior slope. 7. Detail of
hinae. 8. Prodissoconch. 5-6. llolot>pe, UMUT RM29fi.S9. 7-8. Parat\pe 2, UMIJT RM29691. Abbreviations; ct = cardinal-like
toodi; p = projection in hinge; pd = prodissoconch; s = socket.
Type Locality: Southea.st off Kamogawa, Chiba Pre- cardinal-liketoothandsocket ineachvalve, andnacreous
fecture, Japan', 240-273 m, 34°59.963'-35°()0.020' N, shell interior. Obviouslythere is no othergroup having a
140°27.159'-140°2S.427' E. combination of these characters in bivalves.
From the three previouslvkmov\ii species ol the genus.
Distribution: Known onK liiini tlic t\pe localitw Dilemma inexpcciatum (Crozier, 19fi6) from northern
Etymology: The .species epithet recognizes the coun- NewZealand is most similar to the newspecies in having
try ot the type locality. The epithet japoniciim agrees in rough commarginal sculpture in the posterior region.
genderwiththe nameDilemma, alate-Latinneuternoun However, the two species ditter in two characters: the
derivedfromthe Greek. Ofthepreviouslynamedspecies rows ofpores are absent andthe escutcheon is distinctin
in the genus, inexpectatiim is a neuter gender epithet, D, inexpectatiim. As summarized in Table 1, the four
fntmarkernonun has a genitive ending that is not to be species in the genus can be distinguished among them-
influenced by the gender ofthe genus, and spectmli.s is selves by four shell chai^acters: sculpture in the posterior
an epithet originallv used hv Leal (200S) as a noun in region; the presence or absence ot small pits; the dis-
tinctness ofthe escutcheon; andthe presence orabsence
apposition.
of the posteiior projection of the hinge. The paired pos-
terior projections are present only in D.Japonieiim; they
DISCUSSION are not part oi the hinge teeth system, because they do
not aiiiculate as a tooth and a siK'ket between the right
The inclusion ol the new species in the genus Dihinnui and Icll \al\cs. Uidcntunatelv, the (jutlines of muscle
is well supported by the presence ofthe diagnostic shell scars were unclear in the specimens ol the new species.
characters oi the genus: anteroposteriorly compressed The original description of Dileitima mentioned
shell, cardioid outline in articulated valves, shai"p carina "Shell apparentlynacreous internally . . ."' (Leal, 200S: 3),
dividing the shell into die anterior and posterior regions. but did not oiler any detailed descriptions or discussions
T. Sasaki and ].H. Leal, 200S Pasre 169
Figures9-12. Dilemmajaponicumnewspecies. Sliellmicrostructure.9-10. (Jbsenedatsmallbrokenpartot shell margin,vertical
section of homogenous structure in outer layer (OL) lined by nacreous structure in inner layer (IL). 11-12. Obhque (11) and
horizontal (12) \ie\vs ofnacreous laver at inner shell surface. Parat)-pe 2, UMUT RM29691.
on tlie shell microstnjcture ot the three originally in- somewhat similar to a simple prismatic structure in that
cluded species. The description of the inner nacreous elongategranules are arrangedveitically. The outerlayer
shell la\'er in the present stud\' confirms its presence in is identified as ahomogeneous stmcture, because, imlike
Dilemma. t}'pical prismatic stmcture, ciystals lack shaqj bound-
A shell consisting of outer homogeneous and inner aries. It should be observed, however, that environmen-
nacreous layers (Figure 9-12) supports inclusion of tal changes can alter otherwise organized shell micro-
the new species in Anomalodesmata (Tavlor et al, 1973; stiTJCtures to appear irregular, smoothed, and homoge-
Prezant, 1998). The niicrostiiicture of the outer la\er is neous (R. Prezant, pers. comm.).
Table 1. Shell characters and distribution of the four Dilemma speci
D.Japoiiieiim
D. fniiaarkernonim D ^peelralis D iiiexpeetaliiin new species
Sculpture in posterior Dense, coarse growth Smooth Rough commarginal Rugose, lamellate
region lines ribs and grooves commarginal ribs
Small pits on exterior Absent Present Absent Present
surface
Escutcheon Not clearK" separated .\ot clearK" separated ClearK defined Not clearK separated
Projection posterior Absent .Absent Absent Present
to beak
Distribution OffKev West Florida, Oft \'anuatu. OffThree Kings Offcentral Japan,
USA'. 229 m Southwestern Pacific, Islands, New Zealand, Northwest Pacific,
9.5t)-961 0] S0.5 m 24(V27:3 m (dead)
Pase 170 THE NAUTILUS, Vol, 122, No. 3
TheprodissoconchoiDilemmajaponicumnewspecies versity ofTokyo), other colleagues on board and crew of
WW
indicates non-planktotrophic, lecithotrophic develop- Tansei-Maru (Japan Agency for Marine-Earth Sci-
ment. It measures ca. 200 |JLm inlength (Figure 8), asize ence and Technology, formerly Ocean Research Insti-
that fits into the predicted size range (135-230 |jL,m) of tute, University ofTokyo) during the cnaise KT-00-05.
prodissoconchs of bivalves with lecithotrophic develop-
ment (Ockelmann, 1965; Jablonski and Lutz, 1983). The
LITERATURE CITED
developmental modes of the other species ofthe genus
are unknown. Crozier, M. A. 1966. NewspeciesandrecordsofMoUuscafrom
Although two of the other live-collected species of offThree Kings Islands, NewZealand. Transactionsofthe
Dilemma were found attached to hard substrata, the mi- Royal Society ofNew Zealand, Zoology 8: 39^9.
crohabitat of the new species is unknown. The bottom Fanner, M.A,W. K. Fitt, andR. K. Trench. 2001. Moqshology
sediment from the type locality contained numerous of the symbiosis between Corculum carclissa (MoUusca:
deadshells andotherbiogenic fragments such asthose of Bivalvia) and Symbiodiniuni corculonim (Dinophyceae).
bryozoans andsponges. Because mostbivalvesweredead Biological Bulletin 200; 336-343.
and disarticulated, large part of samples in the dredge Geiger, D. L., B. A. Marshall, W. F. Ponder, T. Sasaki, and
haul was inferred to be transported and accumulated. A. Waren. 2007. Techniques for collecting, handling, and
Specifying the actual microhabitat of the species is an preparing small molluscan specimens, Molluscan Re-
interesting target for future sampling. search 27(1); 1-49
The description of Dilemma japoniciim extends the Jablonski, D. and R. A. Lutz. 1983. Lai-val ecology of marine
benthic invertebrates; paleobiological implications. Bio-
geographic range ofthe genus to the Northwest Pacific,
logical Reviews 58: 21-89.
iSnouatdhdwiteisotnertno pPraecviifiocus(orfefcoVradnsufartoumaonidfFNloerwidaZeaanladndt)h.e Leal,seJs.siHle.bi2v0a0l8v.esA(MroeUmuasrckaa:blAenonmaelwodgeesnmuastao:fPcoarronimvyoirdoaues),
The genus has a broad geographic range, extending with descriptions oftwo newspecies. Zootaxa 1764: 1-18.
across the Panama land bridge and Eastern Pacific bar- Ockelmann, W. K. 196.5. Developmental types in marine bi-
rier. Future sampling of deep-sea hard substrates may valves and their distribution along the Atlantic coast of
yield additional records ofDilemma species from other Europe. In: Cox, L. R. andPeake, F. (eds.) Proceedings
J.
locations in the Pacific and in other oceans. of the First European Malacological Congress, London,
1962, Conchological Society ofCreat Britain and Ireland,
and the Malacological Society of London, London,
ACKNOWLEDGMENTS
pp. 25-35.
The authors are indebted to the reviewers, Riidiger Prezant, R. S. 1998, Subclass Anomalodesmata, In; Beasley,
P, L„ Ross, C, B, and Wills, E, (eds,) MoUusca: The
Bieler, Robert S. Prezant, and Richard C. Willan, for Southern SynthesJ,is, Fauna of Australia, Vol, 5, Part A,
theircomments andsuggestions. RichardWillan and Rii- SCIRO PubUshing, Melbourne, pp, 397-249,
diger Bieler helped with etymologies and confirmed the Taylor, D„ W. D. Kennedy, and A. Hall. 1973. The shell
gender of the name Dilemma. The samples of the new struJ,cture and mineralog)' of the Bivalvia. II. Lucinacea-
species became available with the kind assistance of Dr. Clavagellacea, Conclusions. BuUetin of die British Mu-
Suguru Ohta (formerly Ocean Research Institute, Uni- seum"(Natural History). Zoology 22: 2.5.5-294.