Alvinocaris niwa Webber, 2004, n. sp.
Creators: name: Webber, W. Richard
Publisher: name: Zenodo
Year: 2004
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Alvinocarididae
Alvinocaris
Alvinocaris niwa

description: Alvinocaris niwa n. sp. (Figs 1–4)
Alvinocaris sp. A. — Webber & Bruce, 2002: 6, fig.
Material examined. —5 Ψ (2 ovigerous) R.V. Tangaroa stn TAN0107/135, Brothers Caldera, 34° 52.89–52.87´S 179° 3.76–3.21´E, 1,346– 1,196 m, benthic sled [NIWA P1388, NIWA 3255 (3 specimens A, D, E); MNZ CR. 9977 (specimen B)]; 2 ɗ, 2 Ψ (1 ovig.) R.V. Tangaroa stn TAN0107/141, Brothers Caldera, 34° 52.96–52.69´S 179° 4.02– 4.93´E, 1,538– 1,197 m, benthic sled [NIWA 3256 (3 specimens 141, 141A, 141C); MNZ CR. 9967 (specimen C)]; 11 ɗ, 10 Ψ (1 ovigerous), R.V. Tangaroa stn TAN0107/228, Rumble V, 36° 8.63–8.57´S 178° 11.77–11.50´E, 877– 655 m, benthic sled [NIWA H837, NIWA 3257 (18 specimens A–D, F–Q, S, T); MNZ CR. 9969 (specimen E), MNZ CR. 9970 (specimen R)]; 1 ɗ, 4 Ψ, R.V. Tangaroa stn TAN0107/230, Rumble V, 36° 8.48– 8.79´S 178° 11.70–11.53´E, 755– 360 m, benthic sled [NIWA 3258 (4 specimens A, B, D, E); MNZ CR. 9971 (specimen C)]; 1 ɗ, 2 Ψ, R.V. Tangaroa stn TAN0107/233, Rumble V, 36° 8.35–8.70´S 178° 11.74–11.58´E, 520– 367 m, benthic sled [NIWA 3259 (3 specimens A–C)]; 4 ɗ, 1 Ψ, R.V. Tangaroa stn TAN0107/235, Rumble V, 36° 8.36–8.08´S 178° 11.76–11.96´E, 672– 367 m, benthic sled [NIWA 3260 (4 specimens B–E); MNZ CR. 9972 (specimen A)]; 24 ɗ, 22 Ψ, R.V. Tangaroa stn TAN0107/325, Rumble V, 36° 8.27–7.96´S 178° 11.74–11.70´E, 485– 415 m, benthic sled [NIWA 3261 (42 specimens A, C, E–AC, AE– AS); MNZ CR. MNZ CR. 9973 (specimen D), MNZ CR. 9974 (specimen B), MNZ CR. 9975 (specimen AD), MNZ CR. 9976 (specimen AT)].
Description: Measurements, male holotype: CL 15.38 mm; RL 3.75 mm (together giving total carapace length of 19.13 mm); CD 11.88 mm; TL approximately 57 mm. Female paratype: CL 15.00 mm; RL 3.63 mm; CD 11.50 mm; TL approximately 57 mm.
Carapace (Fig. 1 a, c, 2a) integument thin, semitransparent laterally in preserved specimens, inconspicuously punctate, small setae scattered over surface; dorsally with seven sharp, forward pointing dorsal teeth (five rostral + two carapace), six in female (rostrum tip broken off), distal tooth almost forming a bifid rostral tip in holotype, a few setae on dorsal margins of teeth; ventral rostral teeth absent (one occasionally present see variation below). Rostral and carapace dorsal teeth supported by thin, low, carinate crest continuing, posteriorly to teeth, as low ridge to about mid point of carapace, contributing to moderately curved carapace dorsal profile; ventral margin of rostrum also carinate, carina shallow in holotype, curving more deeply in female paratype (Fig 1 c); lateral carina on rostrum detectable near rostrum tip, running posteriorly, confluent with orbital margin. Acute antennal spine reaching anterior margin of eye; antennal spine at anterior of short, inconspicuous ridge curving obliquely downwards, fading out about 0.25 length of carapace, shallow depression ventral to antennal ridge; strong pterygostomian spine projecting forwards similar distance to antennal spine in holotype, stronger and outreaching antennal spine in female paratype; low submarginal ridge along ventral and posterior carapace margins, no other ridges or grooves apparent on carapace.
Precoxae (not illustrated) with spine present adjacent to each pereopod, becoming more slender and sharp from pereopod 1–pereopod 5; in holotype an anteroventrally directed, blade­like sternal spine with unevenly serrated tip present between coxae of pereopod 5 (not illustrated), absent from females; cephalothoracic sternites otherwise unarmed.
Abdomen (Fig. 1 a–c) moderately arched dorsally, tapering posteriorly, deeper in female paratype, no carinae on somites, integument with small setae scattered over surface, semitransparent pleurally. Pleuron of somite 1 rounded, unarmed; of somite 2 rounded anteriorly and posteriorly, proportionately much larger and deeper in ovigerous female paratype, unarmed; of somite 3 (AP3) left side rounded, with very small posterolateral spine in holotype, unarmed on right side, unarmed both sides in female paratype; of somite 4 (AP4) left side with a stronger posterolateral spine than AP3, without ventral or posterior spines (similar in female paratype); of somite 5 (AP5) left side without ventral spines, drawn out into a strong posterolateral spine, with two posterior spines somewhat remote from posterolateral spine in holotype, single such posterior spine in female paratype. AP5 of right side in holotype with bifid posterolateral spine. Holotype pleural spine formulae of AP3/ AP4/ AP5 right side is: 0 ventral, 0 posterolateral, 0 posterior/ 0, 1, 0/ 0, 1 (bifid), 3. Formula for female paratype AP3–AP5 right side; 0, 0, 0/ 0, 1, 1/ 0, 1, 0. Dorsal length of abdominal somite 6 about 1.4 times that of somite 5; broad­based mid­lateral spine overlaps base of telson; posterolateral corner of somite spinous.
Sternites of abdominal somites 1–3 in holotype (Fig. 1 b) with submedian pair of narrow, somewhat flexible spines, longer on somites 2 and 3 than on somite 1; somite 4 with single, strong, posteriorly directed spine; somite 5 with large, strong, blade­like, posteriorly directed spine; somite 6 with strong, low­profile spine posteriorly. Sternite spines on somites 1–3 in paratype female reduced to blunt teeth.
Telson (Fig. 2 c) broad, almost straight­sided, tapering moderately (anterior width about 1.4 times posterior width), shorter than uropods; armed dorsolaterally on each side with row of six movable spines of similar size (seven in female paratype), rows slightly sinuous anteriorly; posterolateral corner of telson armed with short lateral spine, second twice its length sublaterally adjacent to lateral spine, posterior margin slightly convex bilaterally, slight inwards curve mesially (paratype female with pronounced, unevenly shaped median indentation); with 22 long, slender plumose setae distributed evenly along margin between sublateral spines (13 setae distributed unevenly in female paratype, several broken off), no gap or spinule apparent between median pair of setae (a wide gap between median pair of setae in female paratype).
Eyes (Fig. 1 a, c) with stalks fused and immobile, anteriorly with deep vertical mesial groove, cornea unfaceted, globular but shrunken in preserved holotype and female paratype, with inconspicuous tubercle on anterodorsal surface.
Antennule (Fig. 1 a, c, 2a) with peduncle fractionally outreaching antennal scale, basal article about 0.75 length of middle article, slightly longer than distal article; stylocerite long, almost reaching distal margin of middle article, strong, sharp, with gentle lateral curve towards tip, isolated from remainder of basal article by deep groove, an L­shaped row of small setae fanning out against dorsal surface immediately distal to proximal dorsal protuberance bearing a few small setae around tip; relatively featureless dorsal surface of proximal article with setae as illustrated including a short longitudinal row, distally with fence of close­set, small setae running across dorsal surface between small, sharp distomesial spine and base of much larger distolateral spine; distolateral spine reaching 0.75 length of middle peduncular article, separated from but parallel with it, strong, blade­like, dished mesially, with sharp tip, smaller subterminal spinule ventral to it (female paratype lacking subterminal spinule but spinule usually present in other specimens—see Variation).
Middle article with distomesial spine, scattered setae dorsally, dense row of plumose setae along ventromesial face curving dorsally at distal margin adjacent to distomesial spine; distal segment unarmed, with setae as illustrated, lateral flagellum slightly stouter and longer than mesial (tips of both in holotype and of lateral flagellum in female paratype, missing), lateral flagellum with long sensory setae inserted between annuli along proximal 0.5 of length.
Antenna (Fig. 1 a, c, 2a, b) with scale about 1.7 as long as wide, distolateral tooth strong, falling somewhat short of blade, blade distally curved and projecting furthest mesially. Peduncle reaching almost 0.75 length of scale, basal segment with two strong spines ventrally, one distal, one median. Flagellum about 3.5 times length of carapace (distal 0.25 missing from holotype).
Mandible (Fig. 2 d, e) with two­segmented palp, distal segment with long setae on lateral margin and posterior face as illustrated, proximal segment with three long, plumose setae on distal lateral margin. Incisor process with an upper portion bearing four blunt teeth but lacking dorsal, subterminal tooth, lower portion projecting further than upper with four stronger teeth (female paratype with similar division into upper and lower portions, upper with one large and two small teeth, lower with four large and one small tooth); molar process distinct from incisor process, separated from it by shallow notch, rounded tip lacking minute setae.
First maxilla (Fig. 2 f) proximal endite oval, curving anteriorly (dished anteriorly) bearing numerous distal marginal setae, densest around narrow anteromesial tip; distal endite with narrow base, distal margin curving anteriorly, armed with two regular rows of about 23 teeth with additional submarginal teeth around proximal and distal ends of spine rows, numerous setae on proximal margin and posterior surface proximal to teeth, longer plumose setae placed irregularly around distal tip and lateral margin of endite; palp with round tip and mesial, subterminal protrusion bearing single long, sparsely plumose seta, two smaller setae on raised subterminal area adjacent to protrusion.
Second maxilla (Fig. 2 g) endites dished anteriorly, densely setose distally on margins and submarginally, proximal endite of two lobes, distal lobe with small, setose protrusion on anterior margin, distal endite spatulate in posterior view, with uniform row of small setae along lateral margin; palp sinuous, strap­like with short row of small, plumose setae on lateral margin, tip unarmed; scaphognathite almost rectangular anteriorly, fringed with long plumose setae anteriorly, shorter plumose setae on mesial and almost entire lateral margins, narrowing to acuminate posterior lobe armed around tip and mesial margin with series of very long, strong setae, a row of small setae proximally on mesial margin.
First maxilliped (Fig. 3 a) endite strongly dished anteriorly, fringed along lateral margin by dense setae, along anterior margin by numerous plumose setae; exopod ovate, fringed by long plumose setae, submarginal ridge laterally with palp flattened against it, group of very densely plumose setae (cluttered with fine detritus) on ridge distal to tip of palp, group of similar setae on opposite face of exopod; palp with row of fine setae on mesial and lateral edges, tipped by one acuminate spine or distal segment; epipod bilobed, unarmed.
Second maxilliped (Fig. 3 b, c) pediform but rather flattened; coxa fringed mesially by long, dense, plumose setae, fused basis­ischium with regular mesial row of long, strong, sparsely plumose setae, opposite margin and that of merus, carpus, propodus bordered by long, plumose setae, dactylus bordered by long, plumose or serrate setae, forming particularly dense, nest­like pad around mesial part of dactyl; epipod leaf­like, unarmed; podobranch rudimentary with tip shallowly bifurcate, mesial branch with a small terminal seta, lateral margin with elbow bearing about four tiny tooth­like projections (not illustrated).
Third maxilliped (Fig. 3 d, e) long, four­segmented, outreaching antennal scale by about 0.5 length of fused propodus­dactylus terminal segment; terminal segment triangular in cross­section, tapered distally, tip with three spines, an irregular row of about four additional spines subterminally on mesial face, five groups of distally directed serrate setae arranged in close­set, transverse tracts along mesial face, groups overlapping to form a longitudinal pad; posterolateral face of terminal segment somewhat dished, angle between posterolateral and anterolateral faces armed with row of seven or eight slender spines or spine­like setae (female paratype with distal two or three setae spine­like, remainder of row consisting of strong setae); carpus with pads of dense setae on distal 0.66, similar and adjacent to those of terminal segment; merus­ischium with row of small but similarly strong setae, otherwise armed as illustrated; coxa with small epipod.
First pereopod (P1) (Fig. 1 a, c, 3f–h) slightly outreaching third maxilliped, robust; fingers curved downwards and outwards, together concave laterally, fixed finger 2.0 times width of dactyl basally, opposing edges of fingers each armed with row of uniform teeth close­set against one another, teeth of fixed finger somewhat longer than on dactyl, row on dactyl angled towards convex side to interdigitate with teeth on fixed finger (Fig h), finger tips slightly spooned by teeth fanned around edge, latter teeth fused to form corneous edge around lateral border of dactyl tip, line of sensory setae on each concave surface running parallel to teeth, setae on fixed finger fewer, more widely spaced than on dactyl; second, shorter, uneven row along dished lateral surface of fixed finger; dactyl fractionally outreaching fixed finger, fingers somewhat shorter than inflated palm [in female paratype (Fig. 1 c) chela hardly inflated, palm barely 0.33 length of fingers]; carpus short, cone shaped, cupped distally to accommodate palm, with smaller blunt protuberance on distomesial margin, lateral ridge produced distally into strong process with pinched tip (process slender, acuminate in female paratype); dense, clearly delineated pad of strong, serrate setae posteriorly between pinched and smaller process with single larger, movable spine adjacent to proximal corner of pad of serrate setae; merus, ischium somewhat flattened, they and distinct basis and coxa armed as illustrated.
Second pereopod (P2) (Fig. 4 a, b) shorter and more slender than first, reaching end of antennal scale; fingers subequal to palm, similar in size and shape, opposing edges without gape, each pectinate with single row of short teeth directed obliquely distally, row beginning about 0.2 from base of each finger, increasing slightly in size distally, terminating in larger spine at each finger tip, terminal spines scissor when chela closed (terminal spine of fixed finger missing from holotype); carpus, merus armed as illustrated, ischium with single spine at about 0.75 length.
Third to fifth pereopods (P3–P5) (Fig. 4 c–g) similar in length although propodus becoming progressively longer, merus progressively shorter from P3–P5 (propodi 6.0, 6.4, 7.6 mm; meri 8.1, 7.5, 6.5 mm), P3 outreaching antennal scale by 0.66 propodus; dactyls short, armed with single terminal corneous spine and two rows of three or four + three or four subterminal corneous spines on flexor surface (Fig. 4 f), smallest proximally, longest distally (subterminal spines damaged in holotype); propodus with irregular, composite row of spines ventrolaterally becoming denser distally, densest against lateral face of dactylus (Fig 4 f, g), row shortest and sparsest on P3, longest and densest on P5; carpus of each pereopod with distodorsal extension over proximal extensor surface of propodus; ischium and merus of P3 stronger than in P4 or P5, meri of pereopods without spines, ischium of pereopods armed with two posteroventral spines.
Pleopods (Fig. 1 a, c, 4h) well developed, endopods about 0.5 length of exopod on first pleopod, subequal with exopod in pleopods 2–5; appendix interna of pleopods 2–4 slender with about three or four, three or four, nine cincinnuli mesially on tips; of pleopod 5 twice as wide, parallel sided, with 30–50 cincinnuli distributed in fingernail­shaped pad at tip on mesial surface; appendix masculina (fig 4h) with about eight slender spines around tip and subterminally.
Uropods (Fig 1 a, c, 2c) with exopod and endopod subequal, slightly outreaching telson, exopod with movable spine mesial to distolateral tooth half its length; diaresis sinuous.
Phyllobranchs (not illustrated) with pleurobranchs progressively larger posteriorly associated with P1–5; smaller more similarly sized arthrobranchs associated with third maxilliped and P1–4.
Var ia t io n: In the following a range of spine counts is given along with observations of other variable characters, in the sequence followed in the forgoing description. Ranges of measurements, and of tooth and spine numbers, are followed by the sample number (in brackets). The sample number (indicating the number of specimens on which spines or teeth of a single character were measured; or the total number of bilateral characters on which spines or teeth were counted) is given in each case, because all the characters assessed are damaged in one of more specimens. A correlation analysis comparing animal size with some other variables, appears at the end of this section.
Carapace: The total number of dorsal teeth ranges 5–17 (n = 83) including 4–12 rostral teeth; 0–6 carapace teeth; 30 of these specimens lack carapace teeth with all their dorsal teeth being confined to the rostrum. Rostral ventral teeth are usually absent (n = 82) but seven have a single, tiny tooth distally. The rostral ventral carina arches ventrally considerably more in some specimens (e.g. female paratype) than in the holotype.
The carapace dorsal profile is noticeably variable; in at least one specimen it is horizontal while in some large specimens it is gently arched and often, a shallow saddle is present in profile at around 0.6–0.7 of the carapace length, posterior to the orbit.
The antennal spine in some specimens extends slightly beyond the eyes. The pterygostomian spine projects considerably further forward in most specimens (e.g. female paratype, Fig. 1 c) than in the holotype.
Abdominal pleura: On abdominal somites 3–5, the numbers of pleura without spines or with one or more spines are as follows (sample number in brackets):
AP3 usually lacks spines but in a minority of specimens (15 %) a spine is present in a posterolateral position. Four pleura have two spines but in three of the four the opposite pleuron bears a single spine; AP4 usually bears a posterolateral spine larger than that of AP3 although a small number bear more than one spine and both the left and right pleura on two shrimps lack spines altogether; AP5 invariably bears a strong posterolateral spine. Only one AP5 pleuron of 172 examined has spines (two) on the lateral margin anterior to the posterolateral spine but 164 have a variable number of spines on the posterior margin of the pleuron, above the strong posterolateral one (for illustrations of variation in pleural armature (in Alvinocaris longirostris) see Kikuchi & Ohta 1995, Fig. 5, page 777).
Pereopod spines: On the ischia and meri of P2–5, the numbers of ventrolateral spines are as follows:
Among the ischia and meri of P2–P5 only the meri of P2 and P5 are invariably without spines. The P2 ischium is usually without spines and that of P3–P5 usually has two spines. The meri of P2–P5 usually lack spines. However, all ischia and the meri of P3 and P4 have a variable number of spines with percentage variations of: ischium P2 11%; P3 11%; P4 13%; P5 56%; merus P3 7%; P4 7%. The greatest variation occurs on the ischium of P5 while the greatest range in spine number is on the merus of P3.
Telson; the number of dorsolateral spines ranges 4–11 (n = 162) on both left and right sides; 26 (n = 79) individuals have a different number on each side, the difference ranging up to three. The number of posterolateral corner spines is usually two but on 14 posterolateral corners (n = 164) there are zero, one, three or four spines with a single specimen having four posterolateral spines on both sides.
The telson posterior margin varies considerably from distinctly bilaterally convex (e.g. holotype) with a ‘v’–‘u’ shaped median indentation of varying size, to a single, convex curve with no indentation; the indentation is sometimes unevenly shaped and not always centred on the telson midline. A tiny spinule is present in the apex of the indentation in the majority of specimens (not present in the holotype) and is also sometimes present at or near the centre in specimens with a single convex margin.
The number and distribution of telson posterior plumose setae is also very variable. The total number of setae ranges 14–29 (n = 68). In ten specimens (n = 78) no central gap is discernable dividing setae into left and right series (e.g. holotype); the remaining specimens have a gap at or near the centre line with setae ranging 5–13 (left side), 4–16 (right side). Only 28 (n = 78) specimens have the same number on left and right sides. A few posterior setae are sometimes submarginal in origin, mostly beneath the median area of the telson tip.
Antennular peduncle; the subterminal spine on the distolateral spine of the proximal segment is variable in acuity. It is represented by no more than a corner in some specimens and in a few specimens is absent e.g. female paratype.
Third maxilliped: the row of seven or eight slender spines along the posterolateral/ anterolateral angle of the fused propodus­dactylus terminal segment in the holotype, does not usually run the full length of the segment. In most other large specimens (e.g. female paratype Fig. 1 c) spines are replaced proximally by longer, strong setae and in most smaller specimens spines are confined to the distal part of the segment or are replaced by strong setae for its full length.
First pereopod; the size of the first pereopod is dimorphic, with the cheliped palm inflated and approximately the same length as the fingers in males, and hardly inflated and about half as long as the fingers in females.
Correlation analysis: These analyses indicate that the rostrum becomes comparatively longer in relation to CL in larger specimens (r = 0.67, n = 81, p<0.001). This holds true for both sexes (females: r = 0.69, n = 41, p<0.001; males: r = 0.67, n = 40, p<0.001). However, no such increase in the number of carapace dorsal spines occurs with increased size (CL) (r = 0.02, n = 81, p>0.5). Nor does the number of spines on the right P3 merus show a significant increase with size (r = 0.05, n = 82, p>0.5).
A clear difference between the sexes occurs in the number of spines on abdominal pleuron 5 which increases with greater CL in males (r = 0.57, n = 41, p<0.001) but not females (r = 0.03, n = 45, 0.288, p>0.5).
There is no significant increase in the number of telson dorsolateral spines with greater CL (r = 0.11, n = 81, p>0.2) but there is a clear indication that the number of setae on the telson posterior margin does increase with CL (r = 0.54, n = 78, p<0.001).
Etymology: The name niwa is after the acronym for the National Institute of Water and Atmospheric Research (NIWA) who collected and holds the material.
Remarks: Alvinocaris niwa n. sp. was collected at eight stations, two on the Brothers Caldera at 1,538– 1,197 m and 1,346– 1,196 m and six at considerably shallower depths on Rumble V Seamount about 480 km southwest of the Brothers (see material examined for depth ranges). The deepest Rumble V station at which A. niwa was taken (stn 228) ranged from 877– 655 m and the shallowest (stn 325) from 485– 415 m. At a maximum of 485 m on station 325, A. niwa is the shallowest species recorded in the Alvinocarididae. A. muricola and A. stactophila from the Gulf of Mexico occur at 534 and 530 m respectively (Williams 1988), the shallowest depth previously reported for alvinocaridids, and the deepest, A. markensis at 3,437 m, was recorded in the same paper. Alvinocaris niwa was also collected at depths ranging over at least 712 m (485–1196 m), a greater depth range than other alvinocaridids. However, fewer specimens of most species have been collected, and at fewer stations, and they may well occur at a wider range of depths than has been recorded.
Alvinocaris niwa shares a number of characters with A. leurokolos Kikuchi & Hashimoto, 2000 but the combination of characters in A. niwa set it apart from all other Alvinocaris species described.
The rostrum in A. niwa is somewhat shorter than in other species of Alvinocaris but is most similar in length to those of A. leurokolos and of A. williamsi Shank & Martin, 2003.
In addition the row of forward pointing dorsal carapace spines in A. niwa has a smoother transition with the profile of the carapace and does not meet it at an angle as it does in these other two species, particularly A. williamsi.
Alvinocaris niwa differs from other species of Alvinocaris in having conspicuous pairs of spines on the first to third abdominal sternites. Komai & Segonzac (2003) describe similarly paired sternal spines in Mirocaris fortunata (Martin & Christiansen, 1995) but as smaller on the first somite than in A. niwa and also occurring on the fourth somite where a single, more robust spine occurs in A. niwa. M. fortunata females also have these paired spines reduced in mature individuals (Komai & Segonzac 2003) as they are in A. niwa.
The stylocerite of the antennular peduncle is longer in A. niwa than in most other species of Alvinocaris but again, those of A. leurokolos and A. williamsi are similar in length. The particularly strong distolateral spine on the proximal peduncular segment of the antennule in A. niwa appears unique among alvinocaridids in having a subterminal spine. This is, however, a difficult character to see, and, although it is almost always present, it varies in its acuity.
A. niwa bears two spines ventrally on the proximal antennal segment, a character it shares with A. leurokolos and A. brevitelsonis (Kikuchi & Hashimoto 2000) but not with A. williamsi which has a single spine as do other Alvinocaris species.
In the key to Alvinocaris species of Kikuchi & Hashimoto (2000), A. niwa keys out closest to A. brevitelsonis which has the merus of P4 with two spines, but 2 spines are only present in A. niwa on two of 159 P4 meri sampled. Nine meri have a single spine while the remaining 148 lack spines altogether (see table of pereopod spines above).
A. niwa is the only alvinocaridid described with a row of spines along the posterolateral/ anterolateral angle of the third maxilliped distal segment. While the length of this row varies, with differing proportions of the spines replaced by strong setae proximally in smaller specimens, at least some spines were consistently found to be present in larger specimens.
The shape and armature of the mandibular incisor process in A. niwa differs from that of other alvinocaridids described, in having the distal half of the row of teeth set back from the proximal half and in lacking a single subterminal spine on its distal border.
The small area of densely plumose setae on both faces of the flattened exopods of the second maxillae and first maxillipeds have not been indicated in other Alvinocaris species but are present in both A. niwa and A. longirostris found north of New Zealand. In Rimicaris exoculata these exopods are more expanded than in other alvinocaridids and are clothed on all surfaces with conspicuous plumose setae (Williams & Rona 1986). These setae may be quite similar to those in the New Zealand Alvinocaris species and probably occur in other alvinocaridids as well.
Two subterminal rows of spines on the flexor surface of the dactyls of P3–P5 similar to those of A. niwa have not been described in other species of Alvinocaris. Species in this genus have a single row (e.g. Fig. 6 e, this paper), although not all authors have included descriptions of their distribution (e.g. A. brevitelsonis and A. leurokolos Kikuchi & Hashimoto, 2000). Chorocaris vandoverae Martin & Hessler, 1990 has multiple spines on its flexor surfaces, not arranged into rows, and R. exoculata is described as having compound rows of these spines (Williams & Rona 1986).
Shank & Martin (2003) were struck by the strong morphological similarities among extant Alvinocaris species living in relatively shallow water vent and seep environments (<800 m) in the Atlantic and Gulf of Mexico. A. williamsi was collected at 850 m in the North Atlantic (these authors may have intended giving 850 m as the maximum depth of relatively shallow Alvinocaris species) and the shallower depths of A. muricola and A. stactophila are indicated above. The two Pacific species A. leurokolos and A. brevitelsonis are likewise shallow water species, both from 705 m (Kikuchi & Hashimoto 2000). A. niwa, the shallowest Alvinocaris yet found, finds its closest affinities among these species, adding weight to Shank & Martin’s (2003) observation.
description: Published as part of Webber, W. Richard, 2004, Alvinocarididae) and new records of alvinocaridids from hydrothermal vents north of New Zealand, pp. 1-26 in Zootaxa 444 on pages 5-18, DOI: 10.5281/zenodo.168897