Chorocaris Martin and Hessler, 1990

Komai, Tomoyuki & Tsuchida, Shinji, 2015, New records of Alvinocarididae (Crustacea: Decapoda: Caridea) from the southwestern Pacific hydrothermal vents, with descriptions of one new genus and three new species, Journal of Natural History (J. Nat. Hist.) 49, pp. 1-36 : 3-5

Genus Chorocaris Martin and Hessler, 1990

Rimicaris: Williams and Rona 1986: 447 (in part).

Chorocaris Martin and Hessler, 1990: 2 ; Komai and Segonzac 2008: 32.

Opaepele: Komai et al. 2007: 239 (in part).

Type species

Chorocaris vandoverae Martin and Hessler, 1990 .

Composition

In addition to the type species, the following five species are here assigned to Chorocaris : Chorocaris chacei ( Williams and Rona 1986) , C. parva sp. nov., Chorocaris paulexa ( Martin and Shank 2005) , Chorocaris susannae ( Komai et al. 2007) comb. nov. and C. variabilis sp. nov.

Emended diagnosis

Rostrum reaching or falling short of midlength of first segment of antennular peduncle, dorsoventrally flattened, triangular to rounded in dorsal view; dorsal surface usually non-carinate, always unarmed; ventral surface nearly flat or slightly convex, unarmed. Carapace without postrostral carina; shallow longitudinal depression on either side of midline in spawning females; antennal tooth distinct, varying from acute to blunt; pterygostomial angle produced anteriorly or non-produced; anterior part of branchial region not strongly inflated; pair of dorsal organs evident in postorbital region in living specimens. Pleon with third pleuron unarmed; fourth pleuron unarmed or with one or more denticles posteroventrally; fifth pleuron with posteroventral tooth and occasionally with additional teeth or denticles on posterolateral margin. Telson with dorsolateral spines arranged in sinuous row with second and/or third spines located distinctly mesial to other spines; posterior margin convex, with row of plumose setae flanked by two pairs of lateral spines. Eyes broadly fused, with faint median notch; anterior surface of each eye flat anteriorly, unarmed. Antennular stylocerite clearly separated from first peduncular segment. Antennae usually not forming operculate structure (see Martin and Hessler 1990); antennal scale with sharp to blunt distolateral tooth, clearly separated from lamella or closely approximated to it; transverse suture present, extending mesially from base of distolateral tooth. First maxilliped with rudimentary bud of flagellum on exopod. First pereopod with welldeveloped grooming apparatus on carpus. Second pereopod without spine on ischium. Third to fifth pereopods increasing in length posteriorly; dactyli each with two or more rows of accessory spinules on flexor surfaces; meri of third and fourth pereopods unarmed; ischia of third and fourth pereopods unarmed (except for males of C. variabilis sp. nov.). No strap-like epipods on third maxilliped or first to fourth pereopods. Appendix interna on fourth pleopod without terminal cluster of coupling hooks. Uropodal exopod with two posterolateral spines mesial to posterolateral tooth; protopod sharply to bluntly pointed posterolaterally.

Distribution

Hydrothermal vents in the western and eastern Pacific and on the Mid-Atlantic Ridge; 1305–3650 m.

Remarks

The genus Chorocaris was established by Martin and Hessler (1990) to accommodate the type species C. vandoverae from hydrothermal vents in the Mariana Back Arc Basin, and Rimicaris chacei Williams and Rona, 1986 from hydrothermal vents along the Mid-Atlantic Ridge. Chorocaris was differentiated from Rimicaris by the more-developed rostrum, the non-inflated branchial regions of the carapace, the cylindrical fused eyestalks and the non-operculate antennae ( Martin and Hessler 1990). A new species Chorocaris fortunata Martin and Christiansen, 1995 was subsequently described from the Lucky Strike vent field on the Mid-Atlantic Ridge, but this species was later transferred to Mirocaris by Vereshchaka (1997). Based on analyses of the COI gene (600 bp), Shank et al. (1999) proposed that Chorocaris is a paraphyletic assemblage with C. chacei being more closely related to Rimicaris exoculata than to C. vandoverae . Martin and Shank (2005) described a new species, C. paulexa , from the Rapa Nui Homer Vent Site on the southern East Pacific Rise. Komai et al. (2007) then described a new species, Opaepele susannae Komai et al. 2007 , from the southern Mid-Atlantic Ridge, and suggested that their new species appears to be intermediate between Chorocaris and Opaepele . Komai and Segonzac (2008) reviewed Chorocaris and Rimicaris , and took a conservative approach by recognizing three species in Chorocaris , namely, C. chacei , C. paulexa and C. vandoverae ; these authors maintained the genus despite the possibility of it being paraphyletic. Recently, a new species, Rimicaris hybisae Nye, Copley and Plouviez, 2012 , was described from the Mid-Cayman Spreading Centre, in the Caribbean; this species was provisionally referred to Rimicaris , but exhibits some similarities to C. chacei (see Nye et al. 2012).

The discovery of the two new species, herein referred to Chorocaris , led us to reassess the characters that differentiate the genera Chorocaris and Opaepele . Comparison of C. vandoverae and Opaepele loihi , the type species of the respective genera, has confirmed the following morphological differences: (1) the rostrum is devoid of a dorsal carina or dorsal teeth or denticles in C. vandoverae , whereas in O. loihi , the rostrum has a distinct dorsal carina, extending to the postrostral part of the carapace, and occasionally bears minute denticles; (2) the uropodal exopod always bears two movable spines at the posterolateral angle in C. vandoverae , whereas there is usually one, rarely two, movable spines just mesial to the fixed posterolateral tooth in O. loihi . The two previously known species of Chorocaris ( C. chacei and C. paulexa ), O. susannae and the two new species of Chorocaris described herein are morphologically akin to C. vandoverae in these regards.

The lack of a dorsal carina on the rostrum and the possession of two posterolateral spines on the uropodal endopod are presumed to be apomorphic within the Alvinocarididae (cf. Komai and Segonzac 2003, 2004, 2008). Consequently, Opaepele loihi is set apart from a group, which includes Opaepele susannae , Chorocaris species including the two new species described herein, and Rimicaris species. Despite the suggested paraphyly of Chorocaris , we recognize Chorocaris as a valid genus for the time being, including C. vandoverae , C. chacei , C. paulexa , C. parva sp. nov., C. variabilis sp. nov. and C. susannae comb. nov. until thorough phylogenetic analysis is performed.

In addition, Opaepele vavilovi has a number of presumably plesiomorphic characters, such as the dorsolateral spines on the telson being arranged in a linear row, the presence of a spiniform tubercle on the rounded anterior surface of each eye, the possession of meral and/or ischial spines on the second to fourth pereopods and the presence of a single row of accessory spinules on the dactyli of the third to fifth pereopods. In these regards, O. vavilovi is similar to species of Alvinocaris , and could be excluded from a group of genera containing Alvinocaridinides , Chorocaris , Opaepele , Rimicaris and Shinkaicaris . Future study has the potential to warrant the establishment of a separate genus for O. vavilovi .