Alijinocaris, Williams and Chace, 1982

Komai, Tomoyuki & Segonzac, Michel, 2005, A revision of the genus AlIJinocaris Williams and Chace (Crustacea: Decapoda: Caridea: Alvinocarididae), with descriptions of a new genus and a new species of AlIJinocaris, Journal of Natural History 39 (15), pp. 1111-1175 : 1114-1123

Genus AlIJinocaris Williams and Chace, 1982

Alυinocaris Williams and Chace 1982, p 137; Williams 1988, p 263; Kikuchi and Ohta 1995, p 771.

Type species. Alυinocaris lusca Williams and Chace, 1982 .

Emended diagnosis

Rostrum more than 0.20 of carapace length, always with series of dorsal teeth extending to anterior part of carapace. Carapace with conspicuous, rather high postrostral ridge extending beyond midlength of carapace; branchial region not extremely inflated laterally; antennal and pterygostomian teeth well developed, sharp; inferior orbital angle not delineated; lateral surface with shallow post-antennal groove extending from base of antennal tooth and slightly to strongly diverging from anterior to posterior with horizontal plane of dorsal margin of carapace continuous with rudimentary hepatic groove. Fourth and fifth abdominal pleura usually dentate posterolaterally. Telson with dorsolateral spines arranged in straight rows; posterior margin rounded, with row of flexible, plumose setae and/or rigid spines. Eyes narrowly fused mesially with indication of median separation, lacking corneal facets, but usually with diffused pigmentation inside; anterior surface dorsally with small acute or subacute tubercle. Antennal scale oval, not locked with antennule, with sharp distolateral tooth. Third maxilliped to fourth pereopods without strap-like epipods. Ischium of second pereopod with one spine ventrolaterally. Dactyli of third to fifth pereopods compressed laterally, each with single row of accessory spinules on ventral margin; meri of third and fourth pereopods with one to four spines on ventrolateral surface; ischia of third and fourth pereopods each with one or two spines ventrolaterally. Second to fourth pleopods each with slender, simple appendix interna in both sexes; fifth pleopod with well-developed appendix interna bearing terminal cluster of cincinnuli.

General description of adult

Integument of body thin, but not membranous, surface shining.

Rostrum compressed laterally, sharply pointed, length and curvature variable intra- or interspecifically; dorsal margin always armed with series of fixed teeth diminishing in size anteriorly and usually extending to anterior part of carapace; lateral carina conspicuous, broadened proximally and confluent with orbital margin; ventral margin usually armed with small fixed teeth. Carapace somewhat compressed laterally; postrostral ridge rather high, extending beyond midlength of carapace; antennal and pterygostomian teeth sharp; inferior orbital angle not delineated; lateral surface with shallow post-antennal groove passing obliquely ventrally and extending to hepatic region. Strong median sternal spine between coxae of fifth pereopods.

Abdomen smooth dorsally; pleural margin of anterior two somites broadly rounded, that of third somite rounded or obscurely serrate or dentate, those of fourth and fifth somites usually each with at least posterolateral tooth. Sixth somite with sharp posterolateral process and posteroventral tooth. Telson elongate subrectangular, with straight row of six to nine dorsolateral spines on either side; posterior margin usually rounded, with row of more than 10 plumose setae or spines.

Eyes on basally movable stalks narrowly fused mesially (cf. Figure 4B ); division of corneal region and stalk unclear; corneal region, shrunken, unfaceted, with irregular, scattered pigment-like masses within stalk; dorsal surface of corneal region slightly folded, anterodorsal surface flattened or slightly concave, margins of that surface and dorsal fold converging in small anterior spiniform tubercle.

Antennular peduncle moderately stout to stout. First segment with conspicuous fossa on dorsal surface subproximally, and with conspicuous distolateral spine; stylocerite slender, reaching or overreaching distal margin of second peduncular segment, slightly depressed dorsoventrally, sharp, separated from first segment by narrow, deep incision and succeeding deep groove; dorsal surface of stylocerite with distinct rounded tubercle subproximally and transverse row of setae somewhat distal to subproximal tubercle. Second segment with distomesial spine. Third segment short. Dorsolateral flagellum longer than carapace, thickened aesthetasc-bearing portion in basal 0.40–0.50; ventromesial flagellum somewhat longer than dorsolateral flagellum.

Antenna with stout basicerite bearing ventrolateral distal spine and ventral submarginal spine. Antennal scale broad, with sharp distolateral tooth. Carpocerite stout. Antennal flagellum much longer than body, with many, close-set annulations.

Mandible ( Figure 2B ) with distinct separation of incisor process and molar process; incisor process broad, armed with row of teeth on mesial margin, dorsal (inner) surface concave; molar process simple, its narrowly rounded tip minutely setose; palp two-articulated, basal article longer than distal article, somewhat curved mesially; distal article broadly spatulate, with numerous short to long setae marginally. Maxillule ( Figure 2C ) with coxal endite asymmetrically oval–triangular, with numerous short setae on mesial to anterior margin; basial endite with narrowed base but broadened distally, armed with numerous short spines arranged in two rows and with mesial spines partially obscured by submarginal row of setae; palp scarcely bifurcated, with long spiniform seta on obsolescent proximomesial lobe, and two or three much shorter adjacent submarginal setae on distal lobe. Maxilla ( Figure 2D ) with subtriangular coxal endite represented by single lobe; basial endite bilobed, proximal lobe subrectangular, distal lobe subtriangular, both with dense marginal and submarginal setae; palp narrow, somewhat twisted; scaphognathite with anterior lobe broad, rectangulo-ovate, fringed with long setae on anterior and mesial margins, shorter setae along entire lateral margin, posterior lobe narrow and somewhat elongate, fringed on blunt tip and adjacent mesial margin by strikingly long setae preceded proximally by shorter setae similar to those on lateral margin. First maxilliped ( Figure 2E ) with small, mesially setose coxal endite; basial endite irregularly fusiform, strongly convex on ventral (outer) surface and deeply concave on dorsal (inner) surface, with numerous submarginal, dorsally curved setae on ventral surface; endopod ( Figure 2F ) concealed by exopod, short, rather abruptly narrowed at about distal 0.30; exopod greatly expanded, leaf-like, fringed marginally by long plumose setae, flagellum completely reduced; epipod large, obscurely bilobed. Second maxilliped ( Figure 2G ) with endopod six-segmented, somewhat pediform but flattened; coxa and basis–ischium fused segment with row of dorsally curved, fine long setae on mesial margin; merus much shorter than basis–ischium fused segment; carpus very short; propodus narrow, lacking row of spines on mesial margin, but with setae; dactylus tapering terminally, articulating with distal part of propodus, with dense setae on mesial face and blunt tip; exopod absent; epipod subovate or subrectangular, with slender, non-lamellate podobranch directed forward. Third maxilliped ( Figure 3A, B ) slender, composed of four segments, reaching slightly beyond distal end of antennal scale; distal two segments slightly arched in lateral view; ultimate segment trigonal in cross-section, tapered distally, bearing two or three small terminal spines, row of short setae on dorsal and ventral margins, and transverse tracts of dense setae along mesial face; carpus (penultimate segment) shorter than ultimate segment, also with transverse tracts of setae on mesial face; antepenultimate segment (fused merus–ischium–basis) subequal to distal two segments combined, somewhat sinuously curved in dorsal view, distal half weakly compressed laterally and proximal half somewhat flattened dorsoventrally, with slender, curved spine at distolateral ventral corner, margins with short to long setae; tufts of long setae at proximomesial portion of antepenultimate segment; coxa with small rounded process bearing terminal setae and feebly bifurcated epipod; exopod absent.

First pereopod ( Figure 3C ) reaching about as far as third maxilliped, moderately to fairly robust, exhibiting polymorphism apparently correlated to growth. Fingers curved downward and inward; length of dactylus quite variable; outer surface of both fingers convex, inner concavity with opposed edges uniformly offset, closing without gape, each armed with fine row of almost uniform erect corneous teeth so closely set as to be contiguous, tip of each finger slightly spooned; row of tufts of short sensory setae on inner surface submarginally along cutting edges. Palm extremely short to moderately long (showing tendency to become proportionally longer and stouter with increase of size), weakly to somewhat inflated. Carpus cupped distally to receive palm; dorsodistal margin with blunt to subacute projection mesially; ventral surface flared into strong lateral ridge terminating in small to large tooth and smaller mesial ridge ending in smaller blunt tooth, surface between ridges with dense grooming setae and one to three small movable spines ( Figure 3D ); mesial face with shallow depression. Merus and ischium strongly obliquely articulated in lateral view; merus sometimes inflated ventrally, occasionally with small subdistal tooth on ventrolateral margin; ischium always unarmed.

Second pereopod ( Figure 3E ) shorter and more slender than first pereopod, not reaching distal margin of antennal scale. Fingers 0.90–1.10 times as long as palm, each terminating in small corneous unguis crossing each other when closed, cutting edges without gape, each pectinate with single row of minute teeth directed obliquely distally and increasing slightly in size ( Figure 3F ). Carpus slightly longer than chela. Merus and ischium obliquely articulated in lateral view. Ischium with one ventrolateral spine.

Third to fifth pereopods ( Figure 3G, I, J ) moderately long for family, generally similar in length and structure, third reaching beyond distal margin of antennal scale by 0.40–0.70 length of propodus. Propodus–carpus combined shorter than merus–ischium combined in third, subequal in fourth, and longer in fifth. Dactyli ( Figure 3H, K ) short (0.08–0.15 of propodus length), armed with four to six corneous spines on flexor margin grading from small proximally to longest and strongest distally. Propodi of third and fourth pereopods with slender spinules arranged in two rows on ventral surface; propodus of fifth pereopod with numerous spiniform setulose setae arranged in three or four rows on distal half of ventral surface. Carpi distinctly shorter than propodus, with dorsodistal process. Meri each with one to four movable spines ventrolaterally in third and fourth, zero to two spines in fifth. Ischia usually each with two (rarely one) ventrolateral spines in third and fourth pereopods, zero to two (usually one) spine in fifth.

First to fourth pereopods each with small pre-coxal spine visible in posterolateral view.

Branchial formula summarized in Table I. Pleurobranchs on fourth to eighth thoracic somites becoming progressively larger posteriorly. Arthrobranchs on third to seventh thoracic somites more nearly uniform in size (that on seventh somite slightly smaller than others). Epipods and exopods absent on pereopods.

Pleopods well developed. First pleopod ( Figure 2H ) with endopod 0.50–0.60 length of exopod, sexually dimorphic; in males ( Figure 2I ), distal part of endopod feebly bilobed, bearing four to six long spiniform setae; in females ( Figure 2H ), distal part bluntly pointed, with fringe of plumose setae similar to those fringing remaining margins. Second to fifth pleopods with endopods developed as in Figure 1H , slightly shorter than exopods; appendices internae on second to fourth pleopods not greatly reduced in size, but slender, that on second pleopod (cf. Figure 2J ) simple, without terminal cluster of cincinnuli, but those on third and fourth pleopods with few cincinnuli; appendix interna on fifth pleopod more stout than others, with terminal cluster of cincinnuli. Appendix masculina ( Figure 2J ) moderately robust for family, slightly shorter than appendix interna, with several (up to 12) long, terminal and subterminal bristles.

Uropod with rami subequal in length, exopod with small movable spine mesial to smaller distolateral tooth and sinuous diaeresis.

Composition (named species only)

Alυinocaris lusca Williams and Chace, 1982 (type species of the genus); A. markensis Williams, 1988 ; A. muricola Williams, 1988 ; A. stactophila Williams, 1988 ; A. longirostris Kikuchi and Ohta, 1985 ; A. breυitelsonis Kikuchi and Hashimoto, 2000; A. aeilliamsi Shank and Martin, 2003 ; A. dissimilis sp. nov.

Remarks

The genus Alυinocaris is distinguished from other alvinocaridid genera by the presence of a relatively well2developed postrostral ridge extending beyond the midlength of the carapace, the possession of a small spiniform tubercle on the anterior surface of the eye, the narrowly fused eyes and the presence of meral spines on the third and fourth pereopods. In the other alvinocaridid genera ( Chorocaris , Mirocaris , Opaepele , Rimicaris , and Shinkaicaris gen. nov.), the postrostral ridge is absent, or if present, does not reach the midlength of the carapace, the eyes are broadly fused mesially without trace of a median separation, and the meri of the third and fourth pereopods are devoid of ventrolateral spines. Furthermore, the styliform, dorsally dentate rostrum separates Alυinocaris and Shinkaicaris from Chorocaris , Mirocaris , Opaepele , and Rimicaris . In the latter four genera, the rostrum is flattened dorsoventrally, and its dorsal surface is slightly carinate and minutely dentate ( Opaepele ) or rounded ( Chorocaris , Mirocaris , and Rimicaris ). The dorsolateral spines on the telson arranged in a straight row, and the possession of a single row of accessory spines on the ventral margins of the dactyli of the third to fifth pereopods, distinguish Alυinocaris from Chorocaris , Opaepele , Rimicaris , and Shinkaicaris gen. nov. In the latter genera, the dorsolateral spines on the telson are arranged in a sinuous row, and the accessory spines on the dactyli of the third to fourth pereopods are arranged in three or four rows on the ventral surfaces. However, most of these features characterizing Alυinocaris appear plesiomorphic, as they are shared with the genus Bresilia Calman, 1896 , the sister group of the Alvinocarididae ( Christoffersen 1986; Komai and Segonzac 2003). Nevertheless, the monophyly of Alυinocaris may be suggested by the possession of a small tubercle on the anterodorsal surface of the eye, as this feature is not known in other alvinocaridid or bresilioid taxa.

As the above comparison suggests, Alυinocaris appears the most basal assemblage within the Alvinocarididae . Further, the structure of the eye-stalks of Alυinocaris is intermediate between the well-developed, clearly separated eyes shown by most carideans and the broadly fused condition exhibited by other alvinocaridid genera. However, Shank et al. (1999) hypothesized that Mirocaris and the other alvinocaridid genera (as Bresiliidae ) are sister groups. A phylogeny of the alvinocaridid shrimps based on morphological data will be discussed in a separate paper.

In Alυinocaris, size-related morphological variation is seen in the length of the rostrum, dorsal angle of the carapace, position of the posteriormost tooth of the dorsal rostral series, width of the telson, width of the antennal scale, shape of the chela of the first pereopod, and stoutness of the third to fifth pereopod. With increasing size, the length of the rostrum reduces, the dorsal angle of the carapace becomes sharper, the dorsal series of teeth extends more posteriorly, the width of the telson and antennal scale become great. Further, the shape of the chela of the first pereopod is variable with increasing size, as the palm is lengthened and thickened, and the dactylus becomes proportionally shorter. The third to fifth pereopods become stouter with increase of body size. The terminal conditions of these characters provide diagnostic features for species discrimination (see below). Note, however, that most of these features are not differentiated in juvenile or subadult specimens, and therefore, morphology-based identification of juvenile or subadult specimens is sometimes extremely difficult.

The armature of the third to fifth abdominal pleura is also highly variable in most species. No apparent correlation with sex or size is recognized in this variation. Certain degree of abnormality is seen in the shape and armature of the rostrum (e.g. A. muricola ). This abnormality of the rostrum may be caused by injury and regeneration.

Species of the genus are generally very similar to one another. Williams (1988) suggested some minor differences that may provide specific significance for discrimination of Alυinocaris species. These include features such as the number of incisor teeth on the mandible, number of spines on the maxillule, shape of the maxilla, number of meral spines on the fifth pereopod, distribution of spines on the ischia of the third to fifth pereopods, and the shape of the endopod of the male first pleopod and appendix masculina. However, we have found most of these features to be unreliable because of interspecific overlap, particularly the shape of the maxilla and the shape of the pleopodal structure that were found to be largely affected by change in size. The number of pereopod meral spines is variable and overlaps among species. Further, in his original descriptions of A. markensis , A. muricola , and A. stactophila, Williams (1988) described the ischium of the second pereopod as unarmed in these three species. However, our re-examination of the type material of these species has shown that there is one movable spine on the ventrolateral face of the ischium of the second pereopod in all three species. Although the armature of the ischium of the second pereopod has been used as one of the diagnostic characters in distinguishing species of Alυinocaris ( Kikuchi and Ohta 1995; Kikuchi and Hashimoto 2000), the presence of this spine is stable within Alυinocaris.

During this study, the following characters were found to be useful in discriminating species of Alυinocaris: the length of the rostrum, the number of dorsal and ventral teeth on the rostrum, the position of the posteriormost tooth of the rostral series, the degree of inflation of the branchiostegal region of the carapace, the degree of the projection of the pterygostomian tooth, armature of the fourth abdominal pleuron, shape of the sixth abdominal somite (represented by the ratio ‘‘length/proximal height’’), shape of the telson, armature of the posterior margin of the telson, stoutness of the antennular peduncle (represented by the ratio ‘‘length/ width’’ of the penultimate segment), shape of the antennal scale (represented by the ratio ‘‘length/width’’), direction of the distolateral tooth of the antennal scale, shape of the chela of the first pereopod; and stoutness of the merus of the third pereopod. However, many of these features should be used with caution, as they exhibit large size-related variations, as mentioned above. Particularly, large variation in the length and armature of the rostrum is observed in A. muricola sp. nov., which diminishes the usefulness of the rostral character for species discrimination.

The following key should be used with caution, as it is designed solely for identifying adult specimens. For identification of juvenile and subadult specimens, the locality records will be helpful, as most species are geographically or bathymetrically separated.

Figures

Figure 1. Diagrammatic Alυinocaris, showing measurements used in text. (A) Carapace and rostrum, lateral; CL, carapace length; DA, dorsal angle; RL, rostral length. (B) Carapace, dorsal: CW, carapace width. (C) Anterior part of carapace and cephalic appendages, dorsal; L, length; W, width. (D) Sixth abdominal somite, lateral; L, length; H, height. (E) Telson, dorsal; AW, anterior width; L, length; PW, posterior width. (F) Antennal scale, dorsal; abbreviations as in (C). (G) Chela of first pereopod, lateral; PL, palm length; PH, palm height.

Figure 2. Alυinocaris muricola Williams, 1988. Left appendages. (A) Antennal scale, dorsal; (B) mandible, dorsal; (C) maxillule, ventral; (D) maxilla, ventral; (E) first maxilliped, ventral; (F) endopod of first maxilliped, dorsal; (G) second maxilliped, ventral; (H) first pleopod, ventrolateral; (I) endopod of first pleopod; (J) appendices interna and masculina of second pleopod, mesial. (A–H) Female from Régab site, west equatorial African margin (CL 16.8 mm; MNHN-Na 14277); (I, J) male from same locality (CL 15.4 mm; Ifremer).

Figure 3. Alυinocaris muricola Williams, 1988. Left thoracic appendages. (A) Third maxilliped, lateral; (B) coxa and antepenultimate segment of third maxilliped, dorsal; (C) first pereopod, lateral; (D) carpus of first pereopod, mesial; (E) second pereopod, lateral; (F) chela of second pereopod, dorsal; (G) third pereopod, lateral; (H) dactylus of third pereopod, lateral; (I) fourth pereopod, lateral; (J) fifth pereopod, lateral; (K) dactylus of fifth pereopod, lateral. Female from Régab site, west equatorial African margin (CL 16.8 mm; MNHN-Na 14277).

Figure 4. Alυinocaris lusca Williams and Chace, 1982. (A) Carapace and cephalic appendages, lateral (rostrum broken off); (B) anterior part of carapace and cephalic appendages, dorsal; (C) same, lateral; (D) carapace, dorsal (rostrum broken off); (E) third to sixth abdominal somites, lateral. (A, B, D, E) Paratype female from Rose Garden area, Galapagos Rift (CL 13.5 mm; USNM 184537); (C) paratype female (CL 8.6 mm; same lot).