A new species of Mirocaris ( Crustacea : Decapoda : Caridea : Alvinocarididae ) associated with hydrothermal vents on the Central Indian Ridge , Indian Ocean *

1 Natural History Museum and Institute, Chiba, 955-2 Aoba-cho, Chuo-ku, Chiba 260-8682, Japan. E-mail: komai@chiba-muse.or.jp 2 Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 90007, U.S.A. 3 Japan Agency for Marine-Earth Science Technology (JAMSTEC), 2-15 Natsushima-cho Yokosuka, 237-0061, Japan. 4 Faculty of Fisheries, Nagasaki University, 1-14 Bunkyo, Nagasaki 852-8521, Japan.

Mirocaris was originally established for two taxa from the Mid-Atlantic Ridge by Vereshchaka (1997): M. keldyshi Vereshchaka, 1997 (designated as the type species of the genus), and Chorocaris fortunata Martin and Christiansen, 1995.In their recent review of the genus Mirocaris, Komai and Segonzac (2003) reduced M. keldyshi to a synonym of M. fortunata and redefined the genus.Thus, prior to this study there was only one known species of Mirocaris: the Atlantic M. fortunata (Martin and Christiansen, 1995).
Mirocaris is characterized by the dorsoventrally flattened, non-dentate rostrum and the possession of epipods on the third maxilliped through to the fourth pereopod, with a corresponding setobranch on the first to fifth pereopods.The new species is assigned to Mirocaris based on these features.Although it closely resembles M. fortunata, we confer full specific status for the Indian Ocean population because of the presence of a constant morphological difference in the setation of the chela of the first pereopod.

MATERIAL AND METHODS
Holotype and other paratypic specimens in the collection of JAMSTEC were collected during the YK01-15 cruise from 2nd to 24th February 2002.Ten dives of the submersible Shinkai 6500 were conducted at the Kairei Field (25°19.2'S,70°02.4'E),at depths of 2420 to 2450 m, 22 km north of the Rodriguez Triple Junction.The collection site was an active vent field that included black smoker complexes with dense patches of vent-associated animals, including Rimicaris kairei, Bathymodiolus mussel, Alviniconcha snail, Austinograea crab, and Marianactis anemone.Eight individuals of the present new species were captured by the suction sampler loaded on the Shinkai 6500 at the active vent sites.Of these, six specimens were fixed and preserved in 99.5% ethanol, and the other two were fixed in 2.5% glutaraldehyde solution and preserved in70 % ethanol in an on-board laboratory.An additional 15 specimens, sent by Tim Shank to JWM and KZ at the Natural History Museum of Los Angeles County, were collected by the ROV Jason (lowering number 302) in April 2001 from the Edmond Vent Field (23°52.7'S,69°35.8'E)at a depth of 3,300 m; several specimens of Rimicaris were also collected during that lowering.
The holotype is deposited in the collections of the National Science Museum, Tokyo (NSMT).The remaining specimens, including paratypes, are housed in JAMSTEC (the Shinkai 6500 specimens) and in the Crustacea collections of the Natural History Museum of Los Angeles County (LACM) (ROV Jason specimens).The specimen size is indicated by the postorbital carapace length (CL).
Two specimens from the Edmond Vent Field, used for SEM observation, were destroyed, and therefore they were not designated as paratypes.Etymology: The specific name, indica, reflects the general geographic environs of this new species, only the second of the Alvinocarididae from the Indian Ocean to be described.
Rostrum (Fig. 2A, B) triangular, terminating subacutely or acutely in dorsal view, flattened dorsoventrally, slightly overreaching antennal tooth, directed forward; both dorsal and ventral surfaces not dentate, dorsal surface convex with blunt median ridge.Carapace (Figs. 1, 2C) slightly compressed laterally, with sparse short setae along midline (including rostrum), occurring in greater density in anterior part; cuticle surface with short vertical rows of tiny pits visible only under magnification (about x 30).Orbital margin evenly concave; antennal tooth slightly directed mesially; pterygostomian angle angular, not exceeding antennal tooth, usually with very small tooth.Anterolateral margin between antennal tooth and pterygostomian angle slightly concave, posterior submarginal groove shallow.
Thoracic sternite with pair of slender submedian spines on seventh somite; median spur on eighth somite terminating in acute spine.
Abdomen (Fig. 1) rounded dorsally in all somites; pleura of anterior four somites all broadly rounded; fifth pleuron with small posteroventral tooth.Sixth somite about 1.8 times longer than fifth somite, 1.4 times longer than proximal height; posterolateral process short, terminating in small acute tooth; posteroventral corner produced, terminating in blunt or subacute tooth.First abdominal sternite with pair of small submedian spines, similar spines better developed and more strongly curved mesially on second and third sternites, less developed spines on fourth sternite; fifth sternite with distinct median keel terminating posteriorly in acute spine; sixth sternite flattened, thin, transparent, with small preanal spine.
Eye-stalks (Fig. 2A, B) rather large but degenerated, broadly fused mesially without trace of median separation; no faceted structure apparent on NEW SPECIES OF MIROCARIS 111 corneal region; anterior surface of eye unarmed.Antennular peduncles (Fig. 2A, B) stout, slightly flattened dorsoventrally.First segment with distal width nearly half its length; dorsal surface convex in distal part, but remaining proximal part concave below, continuous with deep groove separating basal segment and stylocerite; distal margin slightly oblique in dorsal view; distolateral tooth strong, reaching nearly to midlength of second segment, overlapped by stylocerite, distomesial tooth much smaller than distolateral tooth; ventromesial ridge with 1 spinule arising at about midlength of segment; stylocerite strong, tapering to slender point reaching or overreaching midlength of second peduncular segment.Second segment with scattered short setae on dorsal surface; distomesial tooth larger than corresponding tooth on first segment.Third segment almost as wide as it is long.Flagella (Fig. 1) rather stout, unequal, inserted side by side on oblique terminal margin of third segment; lateral flagellum shorter than mesial, proximal aesthetascbearing portion occupying about 0.8 of total length of flagellum; each article with tuft of sensory setae; mesial flagellum with annuli much denser than those on lateral flagellum.
Antenna (Fig. 2A, B) with basicerite stout, bearing blunt dorsolateral distal projection and acute ventrolateral distal tooth exceeding former projection.Carpocerite (fifth segment of antennal peduncle) stout, cylindrical, exceeding midlength of scaphocerite.Antennal scale (Fig. 2D) broadly oval with greatest width across level of midlength, 1.8 times longer than wide; lateral margin slightly convex, terminating in short, stout tooth separated by narrow incision and considerably exceeded by strongly produced, rounded blade; dorsal surface with distinct median ridge somewhat diverging against lateral margin.Flagellum stouter than antennular flagella, slightly longer than body, annuli dense.
Mandible (Fig. 3A) with incisor process broad, tapering distally, bearing 8 unequal acute or subacute teeth on mesial margin (distalmost tooth distinctly separated from remaining teeth); molar process slender, unarmed, extending as far as incisor process; basal article of palp with broad notch on mesial surface, distal article stout, shorter than basal article, bearing numerous plumose setae of differing lengths.Maxillule (Fig. 3B) with coxal endite slightly tapering distomesially, with dense setae on mesial margin; basial endite broad, mesial margin with 2 rows of small spines (spines more numerous and denser in dorsal row than in ventral row); ventral surface of basial endite with submarginal row of short setae; palp somewhat curved, slightly bilobed distally, bearing 2 setae; outer seta short, simple, arising subterminally from ventral surface slightly proximal to base of somewhat produced outer lobule; inner lobule small, bearing a long plumose seta distally.
Second maxilliped (Fig. 3F) somewhat pediform, composed of 6 segments.Coxa slightly expanded mesially, with numerous setae on mesial face.Basis and ischium completely fused, this fused segment longest, with row of dorsally curved setae on mesial margin forming basket-like structure.Merus about half length of basis-ischium fused segment, with long plumose setae on lateral face.Carpus short, with long plumose setae on outer surface.Propodus with row of setae on margin; articulation between propodus and dactylus strongly oblique.Dactylus longer than propodus, tapering to rounded apex, bearing dense cluster of short setae on mesial to distal margins.Exopod absent; epipod (Fig. 3F, inset) subtriangular, with stout rudiment of podobranch reaching midlength of basis-ischium fused segment.
Third maxilliped (Fig. 4A) composed of 4 clearly separated segments, slightly overreaching anterior margin of scaphocerite.Coxa (Fig. 3G) stout; epipod composed of laterally produced process and strap-like part with terminal hook corresponding to setobranch on first pereopod.Antepenultimate segment (basisischium-merus fused segment, but fusion between basis and ischium incomplete with suture on ventral surface) somewhat flattened dorsoventrally, strongly sinuously curved in dorsal view, setose, with slender spine at distolateral ventral corner; cluster of short to long setae at proximomesial portion (Fig. 3G).
Penultimate segment (= carpus) weakly curved ventrally, with dense setae on mesial surface.Ultimate segment slightly curved, gradually tapering distally, mesial section with obliquely transverse tracts of short, stiff setae, long setae scattered along lateral edge, and 2 or 3 terminal spines.
First pereopod (Fig. 4B) stout, slightly overreaching (when extended) distal margin of scaphocerite at most, with chela and carpus oriented toward midline.Ischium and merus with scattered plumose setae on lateral and ventral surfaces.Merus somewhat compressed laterally and slightly tapering distally, ventral surface slightly concave for reception of flexed carpus.Carpus shorter than merus, somewhat inflated, irregularly funnel-shaped, dorsal surface bent at right angle near tapered proximal end articulating with merus; distolateral margin slightly produced medially; distomesial margin more strongly produced, forming broadly triangular lobe; mesial face (Figs.4C, 5A, C) concave, with large patch of setae composing grooming apparatus and 2-5 small spines on proximal border of setal patch.Palm (Fig. 3H, I) short, strongly inflated, with patch of short setae on mesial surface ventrally (Fig. 5C).Fingers curved and closing without gap; internal surfaces deeply concave; external surface of both fingers convex; cutting edges uniformly offset, each armed with row of uniform, minute, erect, closely set teeth (Fig. 5B, D); cutting edge of fixed finger bordered with narrow, thin chitinous plate including tip; internal surface with submarginal row of widely spaced short setae along cutting edge; external surface of fixed finger with some submarginal rows of longer setae (Fig. 5B).Dactylus subequal to palm in length, uniformly narrowed distally, considerably flattened in distal half; internal surface also with submarginal row of short, sparse setae along cutting edge; external surface lacking conspicuous rows of longer setae along cutting edge.
Second pereopod (Fig. 4D) slightly thinner than other pereopods, reaching distal margin of scaphocerite at most.Articulation between ischium and merus oblique.Ischium with 1 movable spine strongly pressed on lateral surface and row of setae on dorsal and ventral margins.Merus about 4.4 times longer than maximum height, with row of setae on dorsal and ventral margins.Carpus with sparse setae on dorsal and lateral surfaces.Chela (Fig. 3J) about 1.7 times length of carpus, about 4.0 times longer than wide; fingers slightly longer than palm, each terminating in small corneous claw (Fig. 3D), crossing at tip; external surfaces slightly depressed toward cutting edges, with scattered very short setae and longer setae on distal part of fingers; cutting edges each with row of minute corneous spinules.
Pleurobranchs on fourth to eighth thoracic somites asymmetrically Y-branched, noticeably increasing in length posteriorly, apices directed forward.Arthrobranchs on third to seventh thoracic somites moderately developed, nearly symmetrically U-branched; last arthrobranch on seventh somite distinctly smaller than those preceding.Epipods on first to fourth pereopods strap-like, similar shape to that on third maxilliped (broken on first pereopod of holotype and therefore not shown in Fig. 2E).Setobranchs on first to fifth pereopods corresponding to epipods on third maxilliped to fourth pereopod respectively (Fig. 2E).
Endopod of female first pleopod uniformly tapering with margins fringed sparsely with plumose setae; second and third pleopods lacking appendix interna while fourth pleopods with slender, simple appendix interna, fifth pleopods with well-developed appendix interna bearing cluster of cincinnuri.
Uropod (Fig. 2F) with both rami elongate oval, exceeding posterior margin of telson.Endopod shorter and narrower than exopod.Exopod with straight lateral margin terminating in tiny acuminate tooth; long movable spine arising just mesial to distolateral tooth; suture distinct, sinuous.
Color: In life, body and appendages are generally white, but dorsal surface of carapace and anterior two abdominal somites reddish; eyes yellowish.
Preserved specimens uniformly beige or tan to light grey.
Variation: The telson of the holotype is abnormally deformed (Fig. 1F).The normal condition is represented by the paratypes (Fig. 1G).
Comparison: Morphologically, Mirocaris indica closely resembles M. fortunata, the sole known representative of the genus and a species known only from vent fields of the Mid-Atlantic Ridge.It is generally known that congeneric species of alvinocaridid shrimp share many morphological traits, despite the great distance separating the species (e.g.see Williams, 1988;Kikuchi and Ohta, 1995;Webber, 2004;Komai and Segonzac, in press) other than in M. fortunata, the presence of the setal rows is known only in Nautilocaris saintlaurentae Komai and Segonzac, 2004 from the south-western Pacific vent fields (Komai and Segonzac, 2004).In addition, the new species may attain a larger size at maturity than M. fortunata; the largest known specimen of M. fortunata is CL 10.5 mm (Vereshchaka, 1997;as M. keldyshi), whereas the largest specimen of M. indica is CL 14.3 mm.Hashimoto et al. (2001) and Van Dover et al. (2001) reported two shrimp species from the vent fields on the Central Indian Ridge, Rimicaris aff.exoculata and Chorocaris n. sp.Watabe and Hashimoto (2002) later described the Rimicaris species as new, R. kairei, in spite of the low genetic divergence suggested by Van Dover et al. (2001).In the process of sorting our specimens, we did not encounter species assignable to Chorocaris, but we did find species that could be assigned to Mirocaris (based on characters given by Komai and Segonzac, 2003).Both the species of Rimicaris described by Watabe and Hashimoto (2001) and the new Mirocaris described herein were collected on the same lowering of the ROV Jason (lowering 302).Considering the superficial resemblance between Chorocaris and Mirocaris, there is little doubt that the Chorocaris n. sp. reported by Hashimoto et al. (2001) and Van Dover et al. (2001) actually represents Mirocaris indica.Van Dover et al. (2001) noted affinities between Indian Ocean and western Pacific taxa, such as Bathymodiolus mussels, polynoid scale worms, Austinograea crabs and hairy gastropods of the genus Alviniconcha.However, it is remarkable that the shrimp fauna of the Central Indian Ridge is rather similar to that of the Mid-Atlantic Ridge.Neither Mirocaris nor Rimicaris has been found in the Pacific Ocean.Thus, it has been suggested that the vent fauna of the Indian Ridge is composed of multiple taxa originating from both the Atlantic and Pacific Oceans (Hashimoto et al., 2001).The alternative explanation -that the fauna originated in the Indian Ocean and subsequently spread to the Atlantic and Pacific -would demand an explanation for how the shrimp fauna moved westward, whereas the crab fauna moved to the east (i.e.into the Indo-West Pacific).

DISCUSSION
Our report extends the known range of the genus Mirocaris to the Indian Ocean, as the other representative of the genus, M. fortunata, is thus far restricted to the Mid-Atlantic Ridge between 38°N and 14°N (Komai and Segonzac, 2003).It is remarkable that the distribution pattern of species in Mirocaris and Rimicaris is nearly identical despite the different evolutionary history suggested by Shank et al. (1999).Shank et al. (1999) estimated that Mirocaris fortunata shared a last common ancestor with other alvinocaridid species some 6.7 to 11.7 MYA, while R. exoculata and C. chacei shared a last common ancestor 0.42 to 0.5 MYA.The close morphological similarity between the two known species of Rimicaris also supports the theory that Rimicaris is a relatively young genus.Preliminary molecular data provided by Van Dover et al. (2001) may indicate that R. kairei is conspecific with R. exoculata, in spite of some morphological differences (Watabe and Hashimoto, 2002).According to the hypothesis of Shank et al. (1999), Mirocaris is a relatively old genus, although the speciation between M. fortunata and M. indica may be a more recent event.
Morphology of the mouthparts and thoracic appendages suggests that Mirocaris indica is not a "bacteria farmer" but instead feeds mainly on substrate fauna.The setal rows that serve to distinguish the two species probably enable M. fortunata to pick up particles from substrates for efficient feeding.Indeed, we have encountered many specimens of M. fortunata bearing substrate-derived particles attached to the setal rows on the first chelae (personal observation).Therefore, given the vital function of the setal rows, the morphological difference between M. indica and M. fortunata is possibly highly significant.
In addition to the specimens of Mirocaris indica and Rimicaris kairei collected by the ROV Jason (lowering 302, April, 2001), other shrimp from the Central Indian Ridge were collected on ROV Jason lowerings 297 (two specimens) and 296 (four specimens).These specimens, currently with JWM at the Natural History Museum of Los Angeles County, may represent still more undescribed alvinocaridid shrimp species from the Indian Ocean.
sending specimens from the ROV Jason lowering 302 to the Natural History Museum of Los Angeles County to be examined.ST and JH thank the manager, Mr. Yoshiji Imai, the operation team of the Shinkai 6500, the captain Mr. Sadao Ishida, and the crews of the R/V Yokosuka for their helpful support in collecting specimens.TK very much appreciates Michél Segonzac of Ifremer, France, for making specimens of M. fortunata available for study, and for discussing various aspects of biology of vent endemic shrimps.This study was supported in part by U.S. National Science Foundation grants DEB 9978193 (from the PEET initiative of the Systematic Biology program) and DBI 0138674 (a Biological Research Collections grant supporting the Marine Biodiversity Processing Center at the Natural History Museum of Los Angeles County).
FIG. 5. -Mirocaris indica sp.nov.SEM figures.A, chela and carpus of left first pereopod, lateral view; B, external surface of chela of left first pereopod; C, higher magnification of serrate setae on ventromesial face of carpus (carpal cleaning brush) and similar setae of propodus (white arrow in A); D, higher magnification of teeth on cutting edge of dactylus (approximate area of white arrow in B).Non-type (CL 11.0 mm; LACM, not catalogued).