Austinograea Williamsi, New Genus, New Species, a Hydrothermal Vent Crab (Decapoda: Bythograeidae) from the Mariana Back-Arc

Austinograea williamsi, New Genus, New Species, a Hydrothermal Vent Crab (Decapoda: Bythograeidae) from the Mariana Back-Arc Basin, Western Pacific Author(s): Robert R. Hessler and Joel W. Martin Source: Journal of Crustacean Biology, Vol. 9, No. 4 (Nov., 1989), pp. 645-661 Published by: The Crustacean Society Stable URL: http://www.jstor.org/stable/1548594 Accessed: 31/10/2008 12:54

Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use.

Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=crustsoc.

Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission.

JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with the scholarly community to preserve their work and the materials they rely upon, and to build a common research platform that promotes the discovery and use of these resources. For more information about JSTOR, please contact [email protected].

The Crustacean Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Crustacean Biology.

http://www.jstor.org JOURNAL OF CRUSTACEAN BIOLOGY, 9(4): 645-661, 1989

AUSTINOGRAEA WILLIAMSI, NEW GENUS, NEW SPECIES, A HYDROTHERMAL VENT CRAB (DECAPODA: BYTHOGRAEIDAE) FROM THE MARIANA BACK-ARC BASIN, WESTERN PACIFIC RobertR. Hessler and Joel W. Martin

ABSTRACT Austinograeawilliamsi is describedfrom hydrothermalvents in the MarianaBack-Arc Basin, a spreadingcenter in the western North Pacific, at depths of 3,595-3,660 m. This is the first bythograeidcrab describedfrom the westernPacific, extending the rangeof the Bythograeidae approximately12,500 km from the nearestprevious collection sites along the EastPacific Rise. The species differsremarkably from all other brachyurancrabs in lacking eyes or moveable eyestalks;the possible remnantof the eyestalkis fused to the surroundingorbital region of the carapaceand bears no cornea or pigment. The species is furtherdistingushed by the coxa of the third maxilliped,which is nearlycovered by the juxtapositionof the marginof the carapace with the sternumand chelipedalcoxa, a characterproposed herein to be of genericimportance. Other charactersserving to separateA. williamsi from species in the genera Bythograeaand Cyanagraeaare the presence of setose fields on the subhepatic surface and on the ventral dactylarsurface of the chelae in both sexes, densely setose ventral marginson the merus and basi-ischiumof all walkinglegs, chelipeds with fingersthat meet along their entire occluding marginsin both sexes (i.e., do not gape), and a relativelystraight male firstpleopod that bears small spinulesand is longerthan the second pleopod. The third maxillipedsand male pleopods of Cyanagraea,previously unknown, are describedfrom a specimentaken at 13?Non the East PacificRise; the pleopodsare more similarto those of Austinograeathan to those of Bythograea. A key to the generaof the Bythograeidaeis provided.

Crabs of the family Bythograeidae are the assumption that some of the smaller known only from deep watersat marinehy- juveniles describedby Williams(1980) were drothermalvents. The family was originally of this previously unrecognizedspecies. Fi- erected by Williams (1980) to accommo- nally, Williams (1988) described Bytho- date Bythograeathermydron from the Ga- graea mesatlantica from vents along the lapagos vent fields. Subsequent to that Mid-AtlanticRidge. Guinot (in press)erect- species description there have been scat- ed the genus Segonzacia on the basis of this tered reportsof other bythograeidsfrom the species. In the present paper, we describe a Galapagosand from other vent areas of the new genus and species of the Bythograeidae Pacific and Atlantic Oceans. Bythograea from hydrothermalvents on the spreading micropsSaint Laurent, 1984, was described centerof the MarianaBack-Arc Basin in the on the basis of one adult female (the 23.5- western North Pacific Ocean. This find ex- mm carapacelength holotype) and several tends the known rangeof the family Bytho- subadultsor juveniles. Saint Laurent(1984) graeidae approximately 12,500 km to the also erected a second genus and a third west from the nearest previous collection species, Cyanagraeapraedator, to accom- sites along the East Pacific Rise. modate two large adult females taken from the vicinity of black smokers at the 13?N MATERIALSAND METHODS vent fields on the East Pacific Rise. Subse- The material forming the basis of this report was quently, more specimens of C. praedator collected duringa series of dives with the submarine have been collected; three of them, includ- Alvinin April-May 1987. The collectionsites consisted two were material of 3 active vent fieldsalong the spreadingcenter of the ing males, among col- MarianaBack-Arc Basin at about 18?N,just west of lected by physiologists of the Scripps In- the MarianaIsland Arc. These vent fields were spaced stitution of Oceanographyon the French along 3.5 km of the ridgecrest at depthsbetween 3,595 HydronautExpedition to the same locality. and 3,660 m (Hessleret al., 1988). More Saint Laurent erected Most of the crabswere caught in baitedtraps actually recently, (1988) intendedfor obtainingscavenging amphipods (Ingram a fourth species, Bythograeaintermedia, on and Hessler, 1983, fig. 2a). Consequently,the opening the basis of a single megalopa larva and on at the innerend of the entrancefunnel was smallerthan 645 646 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 9, NO. 4, 1989 would accommodatethe largercrabs. The traps were All other paratypesare in the Benthic Invertebrate broughtback to the surfacewithin an insulated box. Collection, Scripps Institution of Oceanography,La Most of the crabswere fixed in 4% bufferedFormalin Jolla, California.Several specimens that were not in- and storedin 80%ethanol. A few wereleft in Formalin. spected in detail, but are almost surely this species The male Cyanagraeapraedator used in our com- because they came from the same samples, were do- parisonwas taken on the 1987 FrenchHydronaut Ex- nated to the Emperorof Japan'sImperial Museum as pedition to 13?Non the East Pacific Rise. well as to the workingcollections of Dr. S. Ohta(Ocean All drawingswere made with the aid of a Wild M5 ResearchDivision, Universityof Tokyo), C. Van Do- dissectingstereoscope with cameralucida. ver (Woods Hole OceanographicInstitution), and G. Somero (ScrippsInstitution of Oceanography). Genus Austinograea, new genus Diagnosis. -No moveable articulated eye- Description stalks. Third maxilliped with only most me- In the following description, we treat only dial portion of coxa visible in ventral view; those characters that serve to distinguish remainder covered by confluence of ventral this species from other crabs in the family border of carapace with sternum and che- Bythograeidae. Corresponding character lipedal coxa. First male pleopod longer than states of B. thermydron (bt) and C. prae- second. dator (cp), the only other Pacific species for which an are Type Species. -Austinograea williamsi, new adequate description exists, enclosed in brackets ] the de- species. [ following scription of the character state for A. wil- Etymology. -From Austin (after Austin B. liamsi. Comparisons with other bytho- Williams, who first described the family) graeids, some of which are incompletely and graea (Greek for sea crab). described because of the relative lack of ma- are deferred to the Discussion. Be- new terial, Austinograea williamsi, species cause we were concerned about al- 1-4 possible Figs. lometric effects, we used specimens of Material.-137 adult or subadultcrabs (69 66, 68 29) equivalent size for comparison with B. ther- collected from 3 vent fields along the crest of the mydron. This was not possible with C. prae- spreadingcenter in the MarianaBack-Arc Basin, west- dator, all specimens of which were consid- ernNorth Pacific,as follows:dive 1825, 10 April 1987, Ilium vent field, 3,595 m, 18?12.805'N,144?42.425'E, erably larger. 1 2; dive 182?(exact dive unknown),date and locality Size. -Male from 10.2 unknown, 1 2; dive 1835, 26 April 1987, Snail Pits carapace length (CL) portion of Burke vent field, 3,660 m, 18?10.948'N, mm (dive 1843, our specimen no. 73) to 144?43.204'E,10 66, 3 99; dive 1836, 27 April 1987, 25.3 mm (dive 1836, our specimen no. 2), Burkevent field(both SnailPits and AnemoneHeaven mean = 18.1 mm; female CL from 8.6 mm portions), 3,660 m, 18010.917-.948'N, 144?43.210- our no. to 25.8 40 12 dive 29 Ilium (dive 1845, specimen 82) .204'E, 66, 99; 1838, April 1987, mm dive our no. vent field, 3,595 m, 18?12.805'N,144?42.425'E, 2 99; (allotype, 1836, specimen dive 1843, 4 May 1987, Alice Springsvent field, 3,640 1) mean = 17.3. No sexual size differences m, 18?12.599'N, 144?42.431'E,4 66, 3 99; and dive evident (Fig. 3). 1845, 6 May 1987, Alice Springsvent field, 3,640 m, Measurements of the holotype male and 15 46 99. 18?12.599'N,144?42.431'E, 66, allotype female (mm) are as follows (* right TypeMaterial. -Holotype 6, carapacelength (CL) 24.5 chela used for measurements): mm, dive 1836, NationalMuseum of NaturalHistory, Smithsonian Institution, USNM 243241; allotype 2, male female CL 25.8 mm, dive 1836, USNM 243242. Paratypes distributedas follows: National Museum of Natural Carapace length (CL) 24.5 25.8 History, SmithsonianInstitution, Washington,D.C., Carapace width 37.8 39.7 USNM 243243, 3 66, 3 92, dive 1836; NaturalHistory of Los Depth cephalo- Museum of Angeles County, Los Angeles, Cali- thorax 14.2 fornia, 4 66, 1 9, dive 1836, and 1 6, 4 99, dive 1845; 13.5 Academy of Natural Sciences, Philadelphia,Pennsyl- Fronto-orbital width 8.6 8.6 vania, 1 6, 1 2, dive 1845;Museum National d'Histoire Propodus* lower Naturelle,Paris, France,MNHN B 20910, 2 66, 2 99, margin 34.2 34.6 dive 1845; Rijksmuseum van Natuurlijke Historie, 22.3 19.9 Leiden,The Netherlands,D 37730, 1 6, 1 2, dive 1836; Dactylus* length ZoologiskMuseum, University of Copenhagen,Den- Palm* height 14.2 13.0 mark, 1 6, 1 2, dive 1845;Zoological Museum, Moscow University, USSR, 1 6, 1 9, dive 1836; and National Carapace (Figs. la, 2a, 3).-Length 0.63 Science Museum, Tokyo, Japan 1 6, 1 9, dive 1845. width [shorter, 0.53 (Williams, 1980, fig. 8) HESSLER AND MARTIN: NEW HYDROTHERMAL VENT CRAB 647

Fig. 1. Austinograea williamsi. Male holotype. a, dorsal view; b, ventral view. Scale bar = 2.0 cm.

(bt)]. Slightly granulate to completely patch [males with at most small setal patch smooth; transverse row of granules marking at lateral edge of subhepatic area; female crest of frontal border at best weakly de- lacking patch (bt); patch lacking in both sexes veloped [with well-developed line of gran- (cp)]; this patch sharply separated from ules marking frontal border (bt)]. Subhe- granulate suborbital margin by distinct su- patic area in both sexes with densely setose ture dorsally and stopping abruptly along 648 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 9, NO. 4, 1989

Fig. 2. Austinograeawilliamsi. Female allotype. a, dorsal view; b, ventral view. Scale bar = 2.0 cm. oblique posteroventral line (pleural suture) (cp)]; not distinctly delimited laterally by on subhepatic surface (Figs. 4a, 5a, 6a). lines of tubercles [distinctly delimited by converging dorsal and ventral tubercle rows Orbital Region (Figs. 4a, b, 5a). -Eyestalks (bt)]. "Floor" of orbital region minutely absent in juvenile and adult; potential eye- granulate, nearly horizontal [distinctly slop- stalk vestige well-demarcated elongate oval ing anteriorly (bt)]. Ventral surface of fron- region in posterior orbital wall lateral to sec- to-orbital border (location of dorsal portion ond antenna (Figs. 4b, 5a) [mobile eyestalks of antennular septum in other brachyurans) (bt, cp)]. Orbital region extending as groove with small, but distinct, median lobe or keel lateral to region containing vestigial eye- [only minute remnant ofantennular septum stalks and antennae [no groove lateral to eye (bt, cp)]. HESSLER AND MARTIN: NEW HYDROTHERMAL VENT CRAB 649

Epistome (Figs. 4a, 5a).-Dorsal margin 3U lacking paired blunt protrusions (Fig. 5b) E 25- A males * into mouth field [obvious blunt lobes ex- E o females into mouth field (bt)]. Plate tending ventrally o) 20 - bordering lateral edge of mouth field (Fig. 0c a) 5a) broad, dorsally rounded [narrower, dor- 0 15- sally acute (bt)]. o o 10- P o Maxilliped 1.-Distal endopodal segment setose 5 1 1 1 1 1 1 1 bearing small, blunt, protrusion 5 10 15 20 25 30 35 40 4!5 see Williams, ("portunid lobe"; 1980); por- Carapace width (mm) tunid lobe blunt, rolled inward and distinctly proximal to distomesial corer (difficult Fig. 3. Carapace length-width relationships of all of williamsi. For males and to see because of dense plumose setation of specimens Austinograea females: y = 0.45 + 0.63x, r = 0.995. proximal lacinia and basal maxilliped segments) [portunid lobe flattened, not rolled, located on or near distomesial lacinia bor- imal to it lobe much Mer- der (bt); distal lobe acutely produced (cp)]. [distal larger (bt)]. us: lateral margin more angular than on bt, Maxilliped 2.--Dactylus distally rounded with main curvature proximal to midpoint and bearing row of uniform stout spines on [main curvature midway (bt, cp)]; medial distomesial border; dactylus and propodus lobe (Fig. 7a,ml) only half length of medial with dense setose fields arising from areas margin [occupying most of margin's length of thin cuticle on dorsal (functionally me- (cp)]; margin of medial lobe facing medially sial) border forming even medial brush (parallel to proximal portion of lateral mar- [dactylus distally lanceolate, lacking row of gin) [facing proximomedially (not parallel short spines (bt); setae of dactylus and pro- to proximolateral margin) (bt, cp)]; distal podus not of uniform length, not brushlike end (Fig. 7a,de) more acutely produced than (cp)]. on bt or cp; inner medial keel with dense tuft of setae [sparsely setose (bt)]. Maxilliped 3 (Figs. 6a, 7a, 8b).-In inner view of distal end of exopodal peduncle (Fig. Chelipeds (Figs. 9-12).-Relatively long, 1.06 width 7a,ex) medial cristate lobe (Fig. 7a,cl) dis- right propodus being carapace 0.89 width tally located [cristate lobe projecting subter- (male), carapace (female) [av- for 6 females = for 3 males = minally (bt, cp)]. Dactylus broadest near erage 0.67, 0.79 for 1 male and 1 female midpoint [broadest near base (bt, cp)], not (bt); average = 0.85 and left in curved [curved toward mouth field (cp)]; (cp)]; right equal size, or dorsal margin not densely setose [densely slightly dimorphic strongly dimorphic; crusher on setose (cp)]; insertion on propodus (Fig. 7a, nearly always right; dimorphism on smaller males arrow) not visible in outer view [propodal prevalent mostly [rela- almost all with obvious base of insertion distinctly visible (bt, cp)]. tively smaller, crusher and cutter Setae of medial margin of propodus and (bt)]. dactylus proportionately shorter than on bt, Cutter (Figs. 9b, 1Oa, b, 1 b, d-h).-Dac- longer than on cp. Coxa: proximal portion tylus: distal end thick; occluding margin ex- of lateral projections not visible ventrally cept for proximal portion smooth, lacking (Fig. 8b), hidden by confluence of sternum ventral teeth [more pointed (bt, cp), with and chelipedal coxa with ventral border of teeth on occluding margin (cp)]; inner oc- carapace [elongate, laterally tapering pro- cluding surface with groove bearing patch jection visible (bt, cp)]; lateral coxal projec- of dense, plumose setae [no dense setation tions visible only when dissected (Fig. 7), (bt, cp)]; occluding margins of dactylus and with single prominent lobe (Fig. 7all) on propodus not gaping, meeting along entire middle region [two lobes (bt); three lobes length [distinct gape in largest individuals (cp)]; embayment in lateral end of coxal pro- (bt, cp)]. Propodus: ventral margin rela- jection for attachment of gill broader than tively straight [concave due to inflation of in bt or cp; lobe distal to embayment (Fig. palm (bt, cp)], height greatest at base of dac- 7a12)barely more prominent than that prox- tylus [height greatest more proximally (bt)]; 650 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 9, NO. 4, 1989

It_ * * _LS la_z

. 'a & % laA _A .4

-z ~- 'I

Irs "

4 --)I- ,L -- I r. G

-t' A --I -

* L4.~rtir It_r; ' Fig. 4. Scanning electron micrograph of frontal and orbital regions of female Austinograea williamsi, CL = 20.3 mm. a, frontal region, distal portions of left first and second antennae removed; b, higher magnification of orbital region outlined in white in a. Note oval plate, possibly a remnant of the eyestalk (e), fused to the surrounding cuticle and lacking visual apparatus. Scale bar = 5 mm for a, 1 mm for b. HESSLER AND MARTIN: NEW HYDROTHERMAL VENT CRAB 651

C4r ~ - - c.," c"'c- ~~r~C c cCc -...00

C'- C Co '' C'' CoQc C) , ?,o c DC,. C c C

Fig. 5. Right frontal region of two genera of the Bythograeidae,distal portions of first and second antennae removed (torn ends are cross-hatched).a, Austinograeawilliamsi, male, CL = 18.7 mm; b, Bythograeather- mydron, male, CL = 16.4 mm. Scale bars = 5 mm. Abbreviations:e, possible eyestalk remnant;lp, plate borderinglateral edge of mouth field;p, protrusioninto mouth field.

innersurface with midcentralpatch of dense, Crusher(in Comparisonto Cutter)(Figs. 9c, plumosesetae [lesspronounced in males (bt), 1 la, c).- Dactylusblunter, with coarse den- absent (cp)]; finger very broad [less broad tition on occluding margin, more curved, on bt, slender on cpl; inner surfaceof finger yieldinggape betweenit and occludingmar- markedlydished or spooned [not markedly gin ofpropodal finger;dense, plumose setae dished (bt, cp)];outer surface on largermales limited to basal third. Propodus relatively with small, often brown-stained,pitted area higher, with height subequal to dactylar just proximal to, and in line with, ventral length. border of dactylus [pitted area lacking (bt, cp)]. Merus:seen in ventral view, produced WalkingLegs (Figs. Ib, 2b).-Ventral mar- cristate ridge on anteromedial margin of gins of all walking legs with tract of dense, constantprominence throughout length [in- plumose setae on ventral margin of basi- creasing in prominence distally (bt); not ischium and merus; number of setae de- present(cp)]. Ischium and merus with patch creasingposteriorly [setal tracts lacking (bt, of dense, plumose setae on ventral surface cp)]. Dorsal and ventralmargins of dactylus in females [lackingin both sexes (bt, cp)]. and propodus, and to minor extent dorsal Fig. 6. Mouth field of three genera of the Bythograeidae. a, Austinograea williamsi, male, CL = 19.5 mm; b, Bythograea thermydron, male paratype (Allan Hancock Foundation 799), CL = 33.2 mm; c, Cyanagraea praedator, male, CL = 45.8 mm. Note amount of coxa of third maxilliped visible in a compared to b and c. HESSLER AND MARTIN: NEW HYDROTHERMAL VENT CRAB 653 de.

I I

Fig. 7. Right third maxilliped of three genera of the Bythograeidae; distal portion of epipod omitted on b and c. a, Austinograea williamsi, male, CL = 18.7 mm; b, Bythograea thermydron, male, CL = 16.4 mm; c, Cyanagraea praedator, male, CL = 45.8 mm. Scale bars = 5 mm. Abbreviations: cl, cristate lobe; de, distal end of merus; ex, exopod; 1i, lobe on middle region of coxa; 12, lobe distal to embayment; ml, medial lobe of merus. Arrow indicates insertion on propodus. 654 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 9, NO. 4, 1989

coxa of

coxa of maxilliped 3

I I

Fig. 8. Sternalview of base of left third maxillipedof three generaof the Bythograeidae,showing the degree to which the coxa of that appendage(stippled) forms part of the sternalsurface. a, Bythograeathermydron, male, CL = 18.4 mm; b, Austinograeawilliamsi, male, CL = 18.7 mm; c, Cyanagraeapraedator, male, CL = 45.8 mm. Scale bars = 5 mm. margin of carpus, with fields of short, spine- pleonite 6 (bt with carapace width 44.7 like setae [lacking on propodus and carpus mm)]. (bt); entirely lacking (cp)]. Difference from Male Pleopods (Figs. 14a-e).--First pleo- bt not as obvious on walking leg 5. pod nearly twice length of second (male Abdomen (Figs. 1b, 2b, 13a).-Pleonite 6 paratype no. 68 with first and second pleo- on male (Fig. 13a) tapering markedly dis- pods 9.2 and 5.1 mm, respectively), narrow, tally [sides nearly parallel (bt, cp)]. Female more or less straight, grooved posteriorly telson proportionately narrower (allotype and anteriorly, with mesial double row of 0.71 width pleonite 6) [wider, 0.81 width short stout sclerotized spines [distinctly HESSLER AND MARTIN: NEW HYDROTHERMAL VENT CRAB 655

Fig. 10. Austinograea williamsi, female chelae. a and Fig. 9. Austinograea williamsi, male chelae. a and b, b, allotype, right and left, respectively; c, inner surface holotype, right and left, respectively; c, right crusher of different female (CL = 19.5 mm), showing dishing from smaller male, CL = 19.5 mm. of inner surface of immovable finger and setose patch on palmar surface. shorter than second, stout, dextrally twisted Female chelae are more slender and more (bt); nearly equal, slightly stouter (cp); scler- ventrally curved than male chelae, and they otized spines not as obvious (bt, cp)]. Sec- bear sharper and more delicate teeth on both ond pleopod with bend about two-thirds dactylus and propodus. In addition, the length at level of small oval area fringed strongly concave propodal inner margin, with short setae (Fig. 14e, arrow); distally merging eventually with a ventral propodal flattened and bladelike [with oval seta-lined keel, is much more pronounced in females. area at about midlength (bt, cp); distally Male chelae, on the other hand, in addition curving (cp)]. to being slightly more inflated (i.e., the pro- Etymology. -We are pleased to name this podal height is relatively greater; see Fig. are species after our friend and colleague, Aus- 12), also longer relative to carapace size tin B. Williams, in recognition of his ex- and are occasionally dimorphic, with dis- cellent contributions to the systematics of tinct crusher and cutter claws. This condi- hydrothermal vent decapods. tion exists mostly in some intermediate- sized males (those points above the line in Fig. 12), but there are also some larger males Sexual Dimorphism with one chela that we would term a "crush- There are no apparent differences in the er." Female chelae are never strongly di- overall size and shape of the carapace in morphic. Females have patches of dense males and females, and carapace shape does plumose setae on the ventromesial margin not change with ontogeny (Fig. 3). However, of the chelipedal basi-ischium, whereas sexual dimorphism exists in the chelipeds. males do not, or bear at most a small setose 656 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 9, NO. 4, 1989

E 20 E A - males A/ 15 o females ^ A

lo A

a o I I I I I I - 5 10 15 20 25 30 35 40 Propodus length, right chela (mm)

Fig. 12. Length-height relationship of propodus of chela, right side only, in all specimens of Austinograea williamsi with right chela intact. For males, y = 1.27 + 0.42x, r = 0.922; for females, y = 1.18 + 0.34x, r = 0.975. Note scatter around line for males, reflecting presence of crusher and cutter morphologies in males.

contrast to B. thermydron from the Gala- pagos vents, many females of which were brooding eggs (Van Dover et al., 1985). The overall ratio of males to females was 1:1 (69 males, 68 females), but the sex ratio g h of crabs from different localities was skewed. For example, dive 1845 to Alice Springs Fig. 11. Chelipedal fingers of Austinograea williamsi, yielded 46 females and only 15 males, while outer surface, illustrating both the smallest individuals dive 1836 to the Burke field resulted in 40 and those at the end of the a and large range. b, right males and 12 females. crusher and left cutter, respectively, male, CL = 12.4 mm; c and d, right crusher and left cutter, respectively, male, CL = 19.3 mm; e and f, right and left cutters, respectively, female, CL = 12.2 mm; g and h, right and KEY TO GENERA OF THE left cutters, respectively, female, CL = 20.0 mm. Scale FAMILYBYTHOGRAEIDAE bars = 5 mm.

1. Coxa of maxilliped 3 partially obscured in ventral view, lateral extension hidden by juxtapo- patch at the base of the merus. Females also sition of ventral margin of carapace in the pter- are more setose on the anterior part of the ygostomial region with anterolateral sternal border and coxa. absent or sternum (compare Figs. lb, 2b). chelipedal Eyestalk merely fused portion of orbital wall. Dactylus with recessed ventral border Natural History ofcheliped bearing dense setae Austinograea (monotypic; A general description of the Mariana hy- Austinograea williamsi, new species) drothermal vents and their communities was given by Hessler et al. (1988). Austinograea - Coxa of maxilliped 3 not obscured in ventral williamsi was found in abundance on the view, lateral extension visible, not hidden by ventral of patches of snails, Alvinoconcha hessleri pterygostomial margin carapace, sternum, and coxa. Mobile Okutani and that chelipedal eyestalks Ohta, 1988, frequently present at all stages of development. Dactylus filled vent openings. As with other bytho- of cheliped without recessed ventral border graeids, A. williamsi occurred more sparsely bearing setae 2 within the surrounding vent field. Obser- 2. Frontal border in dorsal view nearly straight vations of behavior suggest that it is a scav- (Fig. 15a). Widest point of carapace just pos- and a carnivore. terior to frontal border (across anterior one- enger probably fourth to one-third of located No females in our collection were carapace). Eyes carry- in small, clearly defined pockets ing eggs. Nor did we see any egg-bearing Cyanagraea (monotypic; Cyanagraea individuals during the dives. This is in strong praedator Saint Laurent, 1984) HESSLER AND MARTIN: NEW HYDROTHERMAL VENT CRAB 657

- Frontal border in dorsal view rounded (Figs. la, 2a). Widestpoint of carapacenear midline, slightlydisplaced anteriorly. Eyes borderedlat- erallyby largedepressed regions demarcated by dorsal and ventralrow of granules 3 3. Oval patch of cuticle with special texture located ventrolateralto orbit ...... Segonzacia (monotypic;Segonzacia mesatlantica (Williams, 1988))

- No such patch present Bythograea (Threedescribed species of Bythograea:B. thermy- dronWilliams, 1980, B. micropsSaint Laurent,1984, and B. intermediaSaint Laurent, 1988. The status of B. intermediais uncertain due to inadequate information. Guinot (1988) mentioned an unidentifiable specimen in a vial of juveniles of B. thermydronand B. microps.)

DISCUSSION Characters of Generic Importance There can be no doubt that the species described herein is a member of the family 13. Male of three of the The overall resemblance to Fig. pleon genera Bytho- Bythograeidae. graeidae,ventral view, setation not shown. a, Austin- previously described species of Bythograea ograeawilliamsi, holotype; b, Bythograeathermydron, and, to a lesser extent, Cyanagraea is ob- CL = 18.4 mm; c, Cyanagraea praedator. CL = 45.8 = vious. However, there is one character ex- mm. Scale bars 10 mm; a and b are to the same scale. Abbreviation:p16, pleonite 6. hibited by A. williamsi that falls outside the original familial diagnosis offered by Wil- liams (1980). There is no moveable eyestalk terolateral extension of the coxa of this ap- in specimens of Austinograea of any size pendage is obscured from view by the con- (our smallest specimen was a female, CL = fluence of the ventral border of the 8.6 mm, dive 1845); indeed, it is not even pterygostomial region of the carapace with certain that a fused eyestalk remnant is pres- the anterolateral margin of the sternum and ent. The significance of this difference is still the chelipedal coxa. This is not the case in unclear; several authors have pointed out other genera, in which the maxillipedal pos- that there is a tendency in bythograeids for terolateral coxal process is easily seen in reduction of the eyestalk. Larger specimens ventral view (Figs. 6, 8). The male pleopods of Bythograea thermydron have proportion- differ from those of B. thermydron, in that ately smaller eyes, and B. microps was they are relatively straight and not twisted named for a further reduction in the eye- in Austinograea, and in that the second stalks (Saint Laurent, 1984). The eyestalk pleopod is distinctly shorter than the first. in Cyanagraea, although rather large, moves The male pleopods have not been previ- very little, and may indicate incipient fusion ously described for Cyanagraea. The over- with the carapace. To accommodate A. wil- all condition in Cyanagraea is similar to liamsi, we need only modify Williams' fa- that in Austinograea, but in Cyanagraea the milial diagnosis of the Bythograeidae (Wil- second pleopod more nearly approaches the liams, 1980: 444) from "Stalked eyes length of the first (Fig. 14h, i). On Segon- movable, cornea unpigmented" to "Eyes zacia, the broad, setose tip of pleopod 1 and present or absent, cornea unpigmented; stalk the coiled tip of pleopod 2 are unique among distinct or absent." known bythograeids, as is the oval cuticular Several morphological characters prevent patch ventrolateral to the orbit. Although it us from placing the current species within is easy to envision intermediate states for any other genus. Most salient, apart from all of our generic characters, we feel that the absence of eyes noted above, are the until morphological diversity within the By- shape and ventral external exposure of the thograeidae is more fully appreciated, the third maxilliped. In Austinograea, the pos- most conservative approach is the erection 658 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 9, NO. 4, 1989

:Z b C ;i:??? :

\i:??

1 HESSLER AND MARTIN: NEW HYDROTHERMAL VENT CRAB 659 of a fourthgenus to accommodate the Mar- walking legs, maxillipeds, abdomen, and iana species. male pleopods. There are equally as many differencesbe- Species Comparison tween our species and C. praedator.Indeed, Comparisonsof charactersofA. williamsi we feel that Austinograeais closer to By- with B. thermydronand C. praedator are thograea than to Cyanagraea, most ob- presented in the species description, with viously in the shape of the carapace and charactersof the last two species set off in shapeand size of the orbitalregion (Fig. 15). brackets.Bythograea microps and B. inter- However, in some other characters,such as media, both of which have eyes and mobile the shape of the male pleopods, Austino- eyestalks, are still too poorly known to jus- graeaand Cyanagraeaare more similarthan tify detailed comparison. either is to Bythograea(Fig. 14). Allometric changes can be demonstrated Our observationsof previously described for some characterswithin the Bythograei- species differ from published accounts on dae. In males of A. williamsi, the fingersof two points. We found the antennular sep- the crushingcheliped do not gape in smaller tum of C. praedator, previously described individuals, but do to some extent in larger as complete (Saint Laurent, 1984), to be in- ones. In B. thermydron,the male pleopod complete at best. The inner propodal sur- 1 is twisted and S-shaped in large individ- face of the cheliped on males of B. ther- uals, but is less curved in young individuals mydronwas describedas glabrous(Williams, (Fig. 14f) and more closely approximates 1980). We found that some males bear a the condition seen in Austinograea and setal patch in this position, althoughnot so Cyanagraea. large a patch as that of A. williamsi. In comparingA. williamsi to B. thermy- Bythograea mesatlantica was described dron, we avoided allometric considerations from a single, small female (CL 13.8 mm) by limiting comparison to individuals of from 23022'N on the Mid-Atlantic Ridge similar size. This was impossible with C. (Williams, 1988). Guinot (in press) erected praedator,because only threeextremely large the genus Segonzacia for this species on the individuals were available. Potential allo- basis of several additional specimens from metric effects might include the degree of the same area. We have not personally in- setation, the gape of the cheliped, and the spected specimens of this species and there- presence of teeth on the occluding margins fore do not make a detailed comparison of the chelipedalfingers. This is of concern, herein. Some charactersof A. williamsi re- because in all of these features, our speci- semble those of S. mesatlanticamore than mens of C. praedatordiffered from the other those of the EastPacific Rise species B. ther- species. mydron.The orbital region is not as clearly Our largest specimen has a CL of only demarcated in either species as it is in B. 25.8 mm comparedto 33.1 mm for the male thermydron.The last three segments of the holotype of B. thermydron.Although the walking legs are similarly setose. However, trapsat Marianawere size limiting, personal in addition to differingin the diagnosticge- observationby the first author suggeststhat neric characters, A. williamsi also differs A. williamsi does not reach as large a size. from S. mesatlantica in the shape of the The overall shape of A. williamsi and B. chelipeds, the location of setal patches, the thermydronis similar. There are numerous shape of the endopod of the third maxil- and obvious differences,including the shape liped, and the more taperingmale pleonite of the orbital region, carapace, chelipeds, 6. In addition, the male first pleopod of S.

Fig. 14. Male firstand secondpleopods (gonopods) in threegenera of the Bythograeidae.Austinograea williamsi, CL = 19.5 mm. a and b, first and second pleopods, respectively;c and d, highermagnification of distal fourth of first pleopod in posterior and anterior views, respectively;e, distal half of second pleopod. Bythograea thermydron,small male, CL = 18.4 mm. f and g, firstand second pleopods,respectively. Cyanagraea praedator, male, CL = 45.8 mm. h and i, first and second pleopods, respectively.Scale bars = 5 mm for a and b, f and g, h and i; scale bar = 1 mm for c-e (latterscale bar located between Figs. d, e). Arrow in e indicates oval, seta- lined field. 660 JOURNALOF CRUSTACEAN BIOLOGY, VOL. 9, NO.4, 1989

^Hl^-i' ~~~~~~~~~~~~~~~~~'W

^BBBl'ttir^i ?bt-~~~~~

F^^^^Mfc^^ ii

Fig. 15. Male of Cyanagraea praedator Saint Laurent, 1984. a, dorsal view; b, ventral view; c, frontal view. Scale bars = 2 cm. HESSLERAND MARTIN:NEW HYDROTHERMALVENT CRAB 661 mesatlantica is distally truncate and setose, Ingram, C. L., and R. R. Hessler. 1983. Distribution and the second pleopod terminates in a tight and behavior of scavenging amphipods from the cen- tral North Pacific. Sea Research 39: 683-706. spiral (Guinot, personal communication). -Deep Okutani, T., and S. Ohta. 1988. A new gastropod mollusk associated with ACKNOWLEDGEMENTS hydrothermal vents in the Mariana Back Arc Basin, western Pacific.-Venus, This work profited from the cooperation of many Kyoto 47: 1-9. people. M. Boudrias, S. France, and S. Ohta helped Saint Laurent, M. de. 1984. Crustaces d6capodes d'un collect the crabs. H. Craig let us have the crabs from site hydrothermal actif de la dorsale du Pacifique his previous cruise leg to the same area. Our specimens oriental (13?N), en provenance de la campagne fran- of Cyanagraeapraedator were collected on the 1987 9aise Biocyatherm.-Comptes Rendus de l'Acade- French-American Hydronaut Expedition to the East mie des Sciences, s6rie III, 299: 355-360. Pacific Rise at 13?N and were turned over to us by one 1988. Les m6galopes et jeunes stades crabe of the participants, H. Felbeck. Similarly, J. Childress de trois especes du genre Bythograea Williams, 1980 and N. Sanders provided us with Bythograea ther- (Crustacea Decapoda Brachyura).--Oceanologica mydron. D. Guinot and M. de Saint Laurent provided Acta (Special Number, 1988): 99-107. additional information on C. praedator,B. microps, Van Dover, C. L., J. R. Factor, A. B. Williams, and and S. mesatlantica. D. Meier took the photographs. C. J. Berg, Jr. 1985. Reproductive strategies of hy- T. Stebbins assisted with the computer programs that drothermal vent decapod crustaceans. -Biological generated the graphs. A. Williams, M. de Saint Laurent, Society of Washington Bulletin 6: 223-227. and D. Guinot critically read the manuscript. This work Williams, A. B. 1980. A new crab family from the received support from NSF grants OCE83-11258, vicinity of submarine thermal vents on the Gala- BSR86-15018, and DPP86-17149. pagos Rift (Crustacea: Decapoda: Brachyura). - Pro- ceedings of the Biological Society of Washington 93: LITERATURECITED 443-472. 1988. New marine decapod crustaceans from Guinot, D. 1988. Les crabes des sources hydrother- waters influenced by hydrothermal discharge, brine, males de la dorsale du Pacifique oriental (Campagne and hydrocarbon seepage. -Fishery Bulletin, United Biocyarise, 1984).-Oceanologica Acta, Volume States 86: 263-287. special no. 8: 109-118. . (In press.) Description de Segonzacia gen. RECEIVED:11 May 1989. nov. et remarquessur Segonzacia mesatlantica (Wil- ACCEPTED: 11 July 1989. liams): campagne HYDROSNAKE 1988 sur la dorsale m6dio-Atlantique (Crustacea, Decapoda Addresses: (RRH) Scripps Institution of Oceanog- Brachyura).-Bulletin du Museum National d'His- raphy, La Jolla, California 92093; (JWM) Natural His- toire Naturelle, Zoologie, Sect. A. tory Museum of Los Angeles County, 900 Exposition Hessler, R. R., P. Lonsdale, and J. Hawkins. 1988. Boulevard, Los Angeles, California 90007. Patterns on the ocean floor.-New Scientist, 24 March: 47-51.