Alvinocaridinides Komai & Chan, 2010, gen. nov.

Alvinocaridinides gen. nov.

Type species. Alvinocaridinides formosa sp. nov., by present designation and monotypy; gender feminine.

Diagnosis. Rostrum short, not overreaching distal margin of antennular peduncle, somewhat depressed dorsoventrally, narrow triangular in dorsal view, dorsal midline elevated in blunt ridge, armed with series of teeth not extending to carapace; tip acuminate; ventral surface nearly flat, usually unarmed. Carapace not extremely inflated laterally, with moderately large antennal tooth and large pterygostomial tooth; postrostral ridge absent; suborbital lobe absent; pair of large yellowish white spots present inside cephalothorax posterior to orbits, visible through transparent cuticle, possibly representing posterior extension of eyes. Fourth and fifth abdominal pleura each with posteroventral tooth, latter with 1 or more denticles on posterolateral margin. Telson with dorsolateral spines arranged in sinuous row (second and/or third spines arising distinctly mesial to other spines); posterior margin with row of numerous plumose setae. Eyes broadly fused mesially, each unarmed on anterior surface. First segment of antennular peduncle with small distomesial tooth. Antennal scale oval, not locked with antennular peduncle; distolateral tooth acute. Exopod of first maxilliped with small triangular projection on mesial margin. No prominent setae on surfaces of scaphognathite and exopod of first maxilliped. No strap-like epipods on third maxilliped to fourth pereopods. Ischium of second pereopod unarmed or armed with 1 ventrolateral spine. Dactyli of third to fifth pereopods each with several spines arranged in 2 or 3 rows on flexor surface; meri unarmed or armed with 1 lateral spine in third and fourth pereopods, unarmed in fifth. Appendices internae on second to fourth pleopods small, slender, tapering distally, devoid of cincinnuli, that of fifth pleopod better developed, with cincinnuli. Protopod of uropod with acute posterolateral angle; exopod with 2 movable spines at posterolateral angle.

Remarks. There are some intermediate forms between Alvinocaris and the Chorocaris + Rimicaris group (e.g. Komai and Segonzac 2005, 2008; Komai et al. 2007). This new genus is superficially similar to Alvinocaris in the well-developed, dorsally dentate rostrum, but several presumably apomorphic characters suggest that it is rather close to Shinkaicaris, Opaepele, Chorocaris and Rimicaris. These apomorphic characters include: ventral surface of rostrum flattened, unarmed or armed only with one minute subterminal tooth at most; postrostral ridge low, not extending to midlength of carapace; dorsolateral spines on telson arranged in sinuous row, with second and/or third spines arising distinctly mesial to other spines; dactyli of third to fifth pereopods armed with several spines arranged in 2 or more rows on flexor surfaces; spination of meri of third to fifth pereopods showing tendency of reduction; and uropodal exopod bearing 2 movable spines at posterolateral angle (see Komai & Segonzac 2003, 2005, 2008; Komai et al. 2007). These apomorphies are not present in Alvinocaris, which has: ventral surface of rostrum carinate, with one or more teeth; postrostral ridge high, extending beyond midlength of carapace; dorsolateral spines on telson arranged in straight row; dactyli of third to fifth pereopods each with single row of accessory spinules on flexor margin; meri of third to fifth pereopods each with 1 or 2 lateral spines (though spination tends to be reduced in A. williamsi and A. chelys sp. nov.). Furthermore, the eyes are unarmed on the anterior facies in these genera, as well as in the new genus, but they possess spiniform denticles on the anterodorsal surface in Alvinocaris.

It is interesting that there is a pair of large light yellowish shiny spots visible inside the carapace just posterior to the orbits in Alvinocaridinides formosa gen. et sp. nov. These shiny spots may represent an extension of the eyes, which is homologous to the “dorsal eye” found in species of Rimicaris (see Van Dover et al. 1989; O’Neill et al. 1995; Chamberlain 2000; Desbruyères et al. 2006). Similar spots are present at least in species of Chorocaris, Opaepele, Mirocaris Vereshchaka, 1997 and Nautilocaris Komai & Segonzac, 2004 (Desbruyères et al. 2006; Tsuchida et al. 2008), whereas such spots are absent in Alvinocaris (Komai and Segonzac 2005). The presence of this spot-like organ also suggests a close relationship between the new genus and the derived genera.

Shinkaicaris differs from Alvinocaridinides gen. nov. in having a better developed, laterally compressed rostrum, which overreaches the distal margin of the first segment of the antennular peduncle, a distinct suborbital lobe and a sharply buttressed antennal tooth. Furthermore, in Shinkaicaris the dorsal rostral series and rostral ridge extend slightly onto the carapace beyond the level of the posterior margin of the orbit, rather than being limited to the rostrum proper in the new genus.

Opaepele is distinguished from Alvinocaridinides by the lack of conspicuous dorsal teeth and middorsal ridge of the rostrum, although these structures are often traceable in young individuals. The second to fifth pereopods completely lack armature on the ischia and meri in Opaepele.

Chorocaris and Rimicaris are easily separated from the new genus by: the rounded or broadly triangular, unarmed rostrum (greatly reduced in adults of Rimicaris); blunt antennal tooth on the carapace (also greatly reduced in adults of Rimicaris); less acuminate pterygostomial angle of the carapace (broadly rounded in Chorocaris chacei, C. vandoverae and Rimicaris spp.); non-dentate fourth and fifth abdominal pleura; and a complete absence of spines on the meri and ischia of the second to fifth pereopods in adults (see Komai & Segonzac 2008).

The new genus differs from Mirocaris and Nautilocaris, in lacking strap-like epipods on the third maxilliped to fourth pereopod and the dactyli of the third to fifth pereopod bearing accessory spinules arranged in more than one row.

Etymology. From the generic name Alvinocaris and the suffix -ides (Greek, son of), in reference to the superficial resemblance of the two genera.