Alvinocaris methanophila, Komai & Shank & Van Dover, 2005

Komai, T., Shank, T. M. & Van Dover, C. L., 2005, A new species of Alvinocaris (Crustacea: Decapoda: Caridea: Alvinocarididae) and a new record of A. muricola from methane seeps on the Blake Ridge Diapir, Northwestern Atlantic, Zootaxa 1019 (1), pp. 27-42 : 29-38

Alvinocaris methanophila n. sp. ( Figs 1–4 )

Alvinocaris sp. — Van Dover et al., 2003: figs 6E, F, 8D

Alvinocaris cf. muricola . — Van Dover et al., 2003: 292, table 1.

Material examined. ODP site 996, Blake Ridge Diapir, 32°29.623'N, 76°11.467'W, 2155 m, DSV Alvin, slurp gun. HOLOTYPE: LACM CR 2001 ­ 030.1 , ovigerous female (CL 12.8 mm), dive 3710, Pot 5, 26 September 2001 . PARATYPES: CBM­ZC 8283, 1 female (CL 12.6 mm), dive 3709, Pot 1, 25 September 2001 ; CBM­ZC 8284 , 1 female (CL 6.4 mm), dive 3709, Pot 3; LACM 2001 ­ 030.2 , 1 female (CL 7.0 mm), 1 male (CL 6.8 mm) , same data as holotype; LACM 2001 ­ 032.2 , 3 females (CL 8.3–8.7 mm), 4 males (CL 5.5– 7.3 mm), dive 3711, Pot 1, 27 September 2001 ; LACM 2001 ­ 032.1 , 1 ovigerous female (CL 8.6 mm), 5 females (CL 6.7–9.5 mm), dive 3711, Pot 5, 27 September 2001 ; Van Dover’s reference collection, 3 females (CL 10.2–12.5 mm), 2 males (CL 6.4, 6.4 mm), dive 3711, Pot 9, 27 September 2001 ; MNHN­Na 15767, 1 female (CL 6.2 mm), 1 male (CL 7.0 mm), dive 3712, Pot 2; USNM 1074209 , 1 ovigerous female (CL 9.0 mm), 5 females (CL 7.2–11.3 mm), 1 male (CL 6.9 mm), dive 3712, Pot 6, 28 September 2001 ;

LACM CR 2001 ­ 034.1 , 1 male (CL 7.0 mm), Blake Ridge Diapir, 32°28.135’N, 76°11.486’W, 2167 m, dive 3910, slurp gun, 27 July 2003 .

Non­type: LACM, 1 juvenile (CL 4.7 mm), dive 3711, Pot 5, 27 September 2001 .

Description of adults. Body ( Fig. 1A, E ) moderately robust for genus; integument thin and soft, but not membranous; surface smooth.

Rostrum ( Figs. 1A, C, D , 4A ) directed downward or forward, straight, usually reaching distal margin of first segment of antennular peduncle to midlength of second segment in females (0.28–0.50 times as long as carapace), occasionally reaching or slightly overreaching distal margin of second segment of antennular peduncle in males (0.24–0.60 times as long as carapace); dorsal margin armed with 11–19 teeth, including 5–12 teeth on rostrum proper and 4–7 (usually 5–7) moderately large teeth on carapace posterior to level of orbital margin, posterior­most tooth arising from 0.21–0.34 of carapace length; ventral margin armed with 3–11 (most frequently 5–7) small, occasionally rudimentary teeth on anterior 0.30–0.70; lateral carina sharp. Carapace ( Figs. 1A, B, C , 4A ) 0.54–0.71 times as wide as long; postrostral median ridge moderately high, extending to 0.75–0.80 of carapace length, sloping anteriorly to rostrum, dorsal angle about 155° when sharpest; pterygostomial tooth strongly produced anteriorly particularly in large specimens (CL> 8.0 mm), far beyond tip of antennal tooth; post­antennal groove relatively deep, almost parallel to horizontal plane of carapace; branchial region somewhat inflated, thus lateral face notably convex.

Eighth thoracic sternite with long median spur directed anteroventrally.

Abdomen ( Fig. 1E ) rounded dorsally; pleura of anterior three somites broadly rounded; fourth abdominal pleuron with 1–4 teeth (most frequently 2 or 3) posterolaterally; fifth pleuron similarly armed with 1 strong posteroventral tooth and 2–4 additional smaller teeth. Sixth abdominal somite 1.50–1.70 times longer than height. Telson not reaching posterior margin of uropodal endopod, length 2.90–3.20 times anterior width and 4.90–5.20 times posterior width; armed with 6–8 dorsolateral spines on either side; posterior margin convex, armed with 2 pairs of spines at lateral angles and 12–14 plumose setae all longer than mesial pair of lateral spines.

Eye with cornea imperfectly developed, unfaceted though diffusely pigmented; corneal part ovate in general outline, but fused mesially, each with small upturned tooth or tubercle on anterodorsal surface.

Antennular peduncle ( Fig. 1A, D ) reaching or slightly overreaching distal margin of antennal scale. First segment with conspicuous proximal tubercle on dorsal surface laterally and strong distolateral tooth; dorsal surface with distinct longitudinal groove; stylocerite sharp, overreaching distal margin of first segment, but not reaching mid­length of second segment. Second segment stout, 1.35–1.89 times as long as wide in females (1.63 on average, n = 24), 1.52–1.95 times as long in males (1.75 on average, n = 9), with small distomesial tooth. Lateral flagellum subequal to carapace in length; mesial flagellum somewhat longer than lateral flagellum.

Antenna ( Fig. 1A, D ) with stout basicerite bearing strong ventrolateral and weak ventral teeth. Carpocerite stout, not reaching midlength of antennal scale. Antennal scale ( Fig. 2A ) 0.48–0.50 times as long as carapace, 1.70–1.85 times as long as wide; lateral margin straight or slightly convex, slightly diverging distally against dorsal median ridge; distolateral tooth moderately broad, directed forward, falling short of broadly rounded distal margin of blade.

Mouthparts typical of genus ( Fig. 2B–F ). Second maxilliped ( Fig. 2F ) moderately slender; epipod with simple podobranch. Third maxilliped ( Fig. 2G ) composed of 4 segments, overreaching antennal scale by 0.30–0.50 length of ultimate segment; ultimate segment distinctly longer than penultimate segment (= carpus), trigonal in cross section, truncate terminal margin with 2 or 3 spines ( Fig. 2H ), mesial face with several sets of obliquely transverse tracts of stiff setae; carpus also with grooming apparatus composed of transverse rows of stiff setae on mesial face; antepenultimate segment composed of completely fused merus, ischium and basis, flattened dorsoventrally, sinuously curved in dorsal view, bearing long marginal setae and 1 slender spine at ventrolateral distal angle; dorsal surface of antepenultimate segment with prominent tuft of short to long setae proximomesially ( Fig. 2I ); coxa ( Fig. 2I ) stout, with small projection dorsal to base of epipod; epipod directed laterally, slightly bi­lobed.

First pereopod ( Fig. 3A, B ) reaching about as far as third maxilliped, robust. Fingers curved downward and inward; outer surface of both fingers convex, inner concavity with opposed edges uniformly offset, closing without gape, each armed with fine row of closely set, tip of each finger slightly spooned. Palm stout, greatest height 0.65 length of chela at most. Carpus ( Fig. 3C ) 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 1–3 small movable spines; 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 ( Fig. 3D, E ) shorter and more slender than first pereopod, not reaching distal margin of antennal scale. Fingers subequal to palm in length, each terminating in small corneous unguis crossing each other when closed, cutting edges without gape, each pectinate with single row of minute corneous teeth. Carpus slightly longer than chela. Merus and ischium obliquely articulated in lateral view. Ischium with 1 ventrolateral spine.

Third to fifth pereopods ( Fig. 3F, H, I ) moderately stout, generally similar in length and structure, usually setose, third reaching beyond distal margin of antennal scale by 0.40–0.50 length of propodus. Propodus­carpus combined shorter than merus­ischium combined in third, subequal in fourth, and longer in fifth. Dactyli ( Fig. 3G, J ) short (0.08– 0.15 of propodi length), armed with 3–5 corneous spines on flexor margin. 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 ( Fig. 3G, J ). Carpi 0.50–0.65 of propodi length. Meri each with 1–3 movable spines ventrolaterally in third and fourth, unarmed or with 1 spine in fifth. Ischia with 1 or 2 ventrolateral spines in third and fourth pereopods, unarmed or with 1 spine in fifth.

Male first pleopod ( Fig. 4B ) with endopod about 0.60 length of exopod, distal part faintly bilobed; distomesial lobe with 4 long spiniform setae directed mesially or distomesially on mesial margin; distolateral lobe obsolete; endopod of female bluntly pointed distally, with fringe of plumose setae similar to those fringing remaining margins. Appendices internae on second to fourth pleopods not greatly reduced in size, but slender, that on second pleopod ( Fig. 4C ) simple, without terminal cluster of cincinnuli, but those on third and fourth pleopods with few cincinnuli; appendix interna on fifth pleopod better developed than others, with terminal cluster of cincinnuli. Appendix masculina ( Fig. 4C ) moderately robust, slightly shorter than appendix interna, with about 10 long, terminal or subterminal bristles.

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

Coloration in life. Integument transparent except tips of dactyli of first pereopod, which are opaque white; internal tissues neutral to grayish; eyes pale orange, reflective.

Variations. Like other species of Alvinocaris , A. methanophila n. sp. exhibits considerable variations in the length and armament of the rostrum, armament of the fourth and fifth abdominal pleuron, and the shape of the chela of the first pereopod. The ratio of the rostral length to the carapace length ranges from 0.27 to 0.50 in females and from 0.24 to 0.60 in males. The second segment of the antennular peduncle can be more elongate in males than in females, although the proportional values partially overlap ( Table 1). The shape of the chela of the first pereopod seems to be size­related. In juveniles, the palm is very short and the fingers are more than 4 times longer than the palm ( Fig. 4E ). In all adult specimens, the chela is stout with the palm subequal to or slightly shorter than the dactylus ( Fig. 3B ).

A. methanophila n. sp. A. muricola A. markensis

Branchial region of notably convex notably convex slightly convex

carapace

Second segment of females: 1.35–1.89(1.63; n = 24) 1.91–2.26 (2.08; n=16) 1.79–2.00 (1.88; n=7) antennular peduncle males: 1.52–1.95 (1.75; n=9) 1.92–2.48 (2.18; n=4) no data

length/width

Rostrum length/ females: 0.28–0.50 0.30–0.72 0.45–0.70*

carapace length males: 0.24–0.60 0.40–0.80 0.74

Size of adults ovigerous females CL 8.6–12.8 mm CL 14.5–23.8 mm * CL 13.0– 22.3 mm * males: CL 5.5–7.3 mm CL 6.0– 15.7 mm * CL 6.9–12.9 mm *

Habitat cold seeps cold seeps hydrothermal vents

* after Komai & Segonzac (2005).

Juveniles are further different from adults in the following respects: the branchial region of the carapace is less convex ( Fig. 4D ); the posterior margin of the telson is usually armed with one median tooth; the palm of the first pereopod is very short, while the fingers are elongate ( Fig. 4E ); and the pereopods are less setose and less stout.

Distribution and Habitat. So far known only from methane seeps on the Blake Ridge Diapir, northwest Atlantic, off Carolina, 2155 m. Habitat and associated fauna were described in detail by Van Dover et al. (2003).

Etymology. The name is the combination of methane and the Greek " philos, " to love, for association of the new species with methane seeps.

Remarks. Species of Alvinocaris exhibit only minor or subtle differences from one another ( Williams, 1988; Kikuchi and Ohta, 1995; Komai and Segonzac, 2005). Species determination for juveniles and young individuals is usually very difficult, as the specific features are not fully differentiated in these stages ( Komai and Segonzac, 2005). Therefore, the following comparison is restricted to adults. The new species is very similar to the two Atlantic species, A. markensis and A. muricola , both characterized by (1) the rostrum usually not reaching the distal margin of the antennal scale and bearing normally more than two ventral teeth, and (2) the posterior margin of the telson fringed with a row of long plumose setae, instead of a row of spines. Differences among the three species are summarized in Table 1. The notably convex anterior part of the branchial region of the carapace, one of the characteristics of the new species, is also seen in A. muricola , which occurs also in cold seeps. Alvinocaris methanophila n. sp. can be distinguished from A. muricola and A. markensis by the comparatively broader second segment of the antennular peduncle. The difference in the mean values between A. methanophila n. sp. and the other two species is significant (for the pair of A. methanophila n. sp. and A. muricola , females: t = 11.58732, degree of freedom 37, P <0.01; males: t = 3.26478, degree of freedom 4, P <0.05; for the pair of A. methanophila n. sp. and A. markensis , females: t = 5.65519, degree of freedom 18, P <0.01). Although ranges of the relative length of the rostrum overlap between A. methanophila n. sp. and A. muricola , the female rostrum tends to be shorter in A. methanophila n. sp. than in A. muricola in the same size range (CL 6.2–12.6 mm; see Fig. 5 ). The rostrum of A. markensis is also usually longer than that of A. methanophila n. sp. ( Table 1). It should be noted that, in females, the rostrum barely exceeds half of the carapace length in A. methanophila n. sp. The new species is characterized by its relatively small size. Female Alvinocaris methanophila n. sp. does not attain a carapace length of much more than 13 mm in CL, a size at which female A. muricola and female A. markensis have barely reached maturity. The available largest male of A. methanophila n. sp. is 7.3 mm in CL, while the largest male of A. muricola and A. markensis is 15.7 mm and 12.9 mm in CL, respectively ( Komai & Segonzac, 2005). Furthermore, A. methanophila n. sp. and A. muricola are known from cold seeps, whereas A. markensis occurs exclusively at active hydrothermal vent sites so far.

Alvinocaris stactophila , known only from hydrocarbon seeps on the Louisiana Slope in the Gulf of Mexico, can be readily distinguished from A. methanophila n. sp. by the presence of a row of slender spines between the principal two pairs of spines at the posterolateral angles ( Komai & Segonzac, 2005), instead of a row of plumose setae.

Alvinocaris williamsi , known only from hydrothermal vents at Menez Gwen on the Mid­Atlantic ridge, is characteristic in the relatively robust body and in having broad antennular peduncle and antennal scale ( Komai & Segonzac, 2005). Furthermore, the ventral margin of the rostrum is unarmed or armed only with one tooth in A. williamsi , rather than bearing three to 11 teeth in A. methanophila n. sp.

One female contained in samples collected during Alvin dive 3711 (LACM CR 2001­ 032.5) has not been identified satisfactorily to species level. In spite of its small size (CL 7.1 mm), the specimen is ovigerous (the ovigerous females of A. methanophila n. sp. are 8.6–12.3 mm in CL). Furthermore, it differs from other female specimens here referred to A. methanophila n. sp. in the slightly smaller teeth in the dorsal and ventral series of the rostrum ( Fig. 6A ), less produced pterygostomial tooth on the carapace, less convex branchial region ( Fig. 6A ), the posterior margin of telson armed with small median tooth ( Fig. 6C ), very short palm and elongate fingers of the first pereopod ( Fig. 6D ), and less robust third to fifth pereopods. We do not know whether these differences are of a specific nature, because only a single specimen is available for study. Many of these differences are sizerelated or subject to intraspecific variations in other Alvinocaris species ( Kikuchi & Ohta, 1995; Kikuchi & Hashimoto, 2000; Webber, 2004; Komai and Segonzac, 2005). Therefore, final decision is deferred in the hope that more specimens will be collected in future surveys.

Figures

FIGURE 1. Alvinocaris methanophila n. sp. Holotype ovigerous female (CL 12.8 mm; LACM 2001­030.1) from the Blake Ridge Diapir, northwestern Atlantic. A, carapace and cephalic appendages, lateral view; B, carapace, dorsal view; C, anterior part of carapace and eye, lateral view; D, anterior part of carapace and cephalic appendages, dorsal view (setae partially omitted); E, abdomen, lateral view (setae and eggs omitted).

FIGURE 2. Alvinocaris methanophila n. sp. Holotype ovigerous female (CL 12.8 mm; LACM 2001­030.1) from the Blake Ridge Diapir, northwestern Atlantic. Left appendages. A, antennal scale, dorsal view (marginal setae omitted); B, mandible, dorsal view; C, maxillule, ventral view; D, maxilla, ventral view; E, first maxilliped, ventral view; F, second maxilliped, ventral view; G, third maxilliped, lateral view; H, same, distal part of ultimate segment, lateral view (setae omitted); I, same, basal part of antepenultimate segment and coxa, dorsal view; J, telson and right uropod, dorsal view (marginal setae on uropod omitted); K, posterior part of telson, dorsal view.

FIGURE 3. Alvinocaris methanophila n. sp. Holotype ovigerous female (CL 12.8 mm; LACM 2001­030.1) from the Blake Ridge Diapir, northwestern Atlantic. Left pereopods. A, first pereopod, lateral view; B, same chela, inner view; C, same, carpus, mesial view; D, second pereopod, lateral view; E, same, chela, outer view; F, third pereopod, lateral view; G, same, dactylus and distal part of propodus, lateral view; H, fourth pereopod, lateral view; I, fifth pereopod, lateral view; J, same, dactylus and distal part of propodus, lateral view.

FIGURE 4. Alvinocaris methanophila n. sp. A–C, paratype male (CL 6.8 mm; LACM 2001­ 030.2); D, E, non­type, juvenile (CL 4.7 mm; LACM) from the Blake Ridge Diapir, northwestern Atlantic. A, D, carapace and cephalic appendages, in lateral view; B, endopod of first pleopod, ventral view; C, appendices interna and masculina of second pleopod, mesial view; E, chela of first pereopod, outer view.

FIGURE 5. Scatter plot of the relative length of the rostrum (indicated by rostral length/carapace length against carapace length) between female A. methanophila n. sp. and female A. muricola Williams, 1988.

FIGURE 6. Alvinocaris sp. Ovigerous female (CL 7.1 mm; LACM CR 2001­032.5) from the Blake Ridge Diapir, northwestern Atlantic. A, carapace and cephalic appendages, lateral view; B, anterior part of carapace and cephalic appendages, dorsal view; C, posterior part of telson, dorsal view; D, first pereopod, lateral view.