Alpheus samudra, Grave & Krishnan & Kumar & Christodoulou, 2020

Grave, Sammy De, Krishnan, Apsara S., Kumar, Anil & Christodoulou, Magdalini, 2020, A new species of Alpheus (Crustacea, Decapoda, Alpheidae) from the Arabian Sea, Kerala, India, Zootaxa 4750 (2), pp. 277-285 : 278-284

Alpheus samudra nov. sp.

( Figs. 1–5 )

Type material. Holotype: female (cl 13.7 mm), bycatch from vessels operating on Quilon Bank (between 8– 11°N and 74– 76°E), Sakthikulangara Fishing Harbour, Kollam, Kerala, leg. A. Kumar, 07.ii.2016 ( SIFAN) . Paratypes: male (cl 11.7 mm), same collection data ( SIFAN); female (cl 10.2 mm), same location, leg. A. Krishnan, 08.iii 2016 ( DABFUK) .

Non-type material. Male (cl 13.0 mm), from bycatch, Neendakara fishing port, Kollam, Kerala, leg. A.B. Kumar, 25.ii.2018, from bycatch ( DABFUK); 3 males (cl 10.2, 11.5, 12.2 mm), 1 female (cl 11.6 mm), from bycatch, Sakthikulangara fishing port, Kollam, Kerala, leg. A.B. Kumar, 2015–2016 ( DABFUK).

Description. Medium- to large-sized species of Alpheus (cl reaching 13.8 mm). Carapace tuberculate ( Fig. 1 A–B). Rostrum well developed ( Fig. 1 A–B), relatively long, about 1.5 times as long as broad at base, sharp distally, reaching to distal margin of first article of antenullar peduncle; rostral carina weak, not extending posterior to orbital hoods; lateral margin non-setose. Orbital hoods swollen, anterior margin lacking teeth. Pterygostomial angle rounded ( Fig. 1A ), not protruding; cardiac notch well developed.

Telson ( Fig. 1D ) elongate, gradually tapering lateral margins, about twice as long as proximal width (at widest point); dorsal surface with two pairs of spiniform setae, both inserted at some distance from lateral margin, proximal pair at about 0.4 of telson length, distal-most pair at about 0.7 of telson length; posterior margin broadly convex ( Fig. 1E ), furnished with long plumose setae and short, spiniform setae; each posterolateral angle with single stout spiniform seta in holotype (pair of spiniform setae present in male paratype, inner pair longest).

Antennular peduncle ( Fig. 1 A–B) with second article elongate, about 3 times as long as wide; stylocerite with sharp point almost reaching to distal margin of first article; ventromesial carina ( Fig. 1C ) with sharply delineated triangular tooth.

Antenna ( Fig. 1 A–B) with stout basicerite, bearing blunt distoventral tooth; scaphocerite relatively stout, with marginally concave lateral margin and well-developed blade, with strong distolateral tooth, overreaching blade; carpocerite reaching to about distal margin of second article of antennular peduncle.

Mouthparts typical for genus in external observation. Third maxilliped ( Fig. 3A ) slender, pediform; antepenultimate article granulated, penultimate article relatively short, about 1.5 times as long as wide, ultimate article richly setose, unarmed distally.

Major cheliped ( Fig. 2 A–B) similar in males and females, not significantly larger in males; ischium short, rugose on mesial margin, finely granulated on lateral margin; merus elongate, about 4.5 times as long as wide, distomedial margin unarmed, mesial and lateral margin granulated, distomedial margin rounded; carpus cup-shaped, with broadly rounded distal lobes, faintly granulated; chela elongate, extremely laterally compressed, about 4.0 times as long as wide, about 3.0 times as high as thick; palm about as long as fingers, densely granulated, but patchy in places, without any grooves, crest or notches, highest immediately proximal to dactylar articulation, tapering proximally, area of maximum thickness above middle with lower half of palm blade-like, inferior edge carinate; dactylus about same length as pollex, sharply carinate on inferior margin, curved slightly outwards, plunger reduced ( Fig. 4 ), defined only by proximal lobe; pollex distally curved, continuing with inferior carinate edge of palm, fossa shallow, poorly-developed.

Minor cheliped ( Fig. 2C ) not sexually dimorphic, nearly of equal length to major cheliped; ischium short, granulated; merus elongate, about 5 times as long as wide, granulated, distomedial margin unarmed; carpus cupshaped; chela elongate, slender, with palm sub-cylindrical in cross-section, almost half as long as fingers, surface smooth, without any grooves, crests or notches; fingers subequal in length, tapering to tips, distally slightly arched, with simple, blade-like cutting edges; long setae covering chela and palm.

Second pereiopod slender ( Fig. 3 B–C), ischium somewhat shorter than merus, both with rugose surfaces; carpus with five divisions, with approximate ratios (proximal to distal) of 1:1.4:0.4:0.5:0.5.

Third pereiopod ( Fig. 3 D–F) relatively robust; ischium somewhat granulated, merus about 6.5 times as long as wide, unarmed; carpus about half length of merus, unarmed; propodus about 1.2 length of carpus, unarmed; dactylus spatulate, about 0.6 length of propodus, lateral surface with cluster of setae; mesial surface with 5–6 short rows of setae. Fourth pereiopod (not illustrated) similar to third. Fifth pereiopod ( Fig. 3 G–H) much more slender than third and fourth; ischium unarmed; merus about 6.5 times as long as wide; carpus 0.9 length of merus; propodus subequal in length to carpus, ventromesial margin unarmed, cleaning brush well-developed; dactylus subspatulate, simple, about 0.5 length of propodus.

Male second pleopod ( Fig. 2 D–E) with appendix masculina slightly longer than appendix interna, adorned with setae on mesial and lateral margins, as well as on apex.

Uropod ( Fig. 1F ) with lateral lobe of protopod unarmed (mesial lobe distally damaged in holotype), mesial lobe in male paratype terminating in subacute tooth; exopod broad, somewhat truncate distally, diaresis weakly sinuous, with small triangular tooth adjacent and mesial to short spiniform seta; endopod without special features.

Colour pattern ( Fig. 5 ). On a recently deceased specimen, traces of transverse red banding remain on the pleon, whilst the chelipeds have an olive-green colour, with the fingertips of the major cheliped being peach coloured. Fresh specimens have been noted to also display red banding on the distal part of each pereiopod article, with the uropods and telson being translucent-red.

Etymology. The new species name is from the Sanskrit “samudra”, meaning the gathering together of waters, i.e. the ocean; used as a noun in apposition.

Type locality. Quilon Bank , Kerala, India

Distribution. Currently only known from the type locality.

Ecology. The fishing vessels from which the specimens were obtained operated on a fine sandy to muddy bottom in depths of 275– 375 m.

Taxonomic remarks. The present new species can comfortably be referred to the A. brevirostris species group (sensu Bruce, 1994), on account of the major chela being strongly compressed and sub-rectangular in cross-section. This species group contains about 50 species (see Komai & Ohtomi, 2018), with several deep-water representatives. However, the extreme lateral compression of the chela of the major cheliped, at once separates the new species from all species in the group, except A. leptocheles Banner & Banner, 1975 , a shallow water (<20m) species only known from the mouth of the Sepik River in north-western Papua New Guinea.

Alpheus leptocheles was described on the basis of a male holotype and two female paratypes, with the description noting that the observed sexual dimorphism in the major cheliped would be unusual within the genus ( Banner & Banner, 1975). Although the authors believed that they were dealing with a single species, they also noted that the male was in a different vial than the two females “…this may indicate a separate habitat or even a separate trawl haul”. As the major chela of the female paratypes of A. leptocheles are reminiscent of the usual type observed within the A. brevirostris group, it seems highly likely that two species were mixed up in the type description of A. leptocheles and further comparison is restricted to the holotype only.

The major chela of the holotype of A. leptocheles is extremely laterally compressed, as is the case in the new species. Banner & Banner (1975) state that the palm is “…7 times as high as thick; …” although their illustration ( Fig. 1D ) shows this to be incorrect. At our request, S. Ahyong examined the holotype in the collection of the Australian Museum and confirmed that the drawing is correct with a measured ratio of 3.37 of height vs thickness. This is somewhat similar to the ratio observed in the new species, and can probably not be used as a distinguishing feature between both species. Although Banner & Banner (1975) do not comment on the nature of the plunger-fossa, it was also confirmed that in the holotype this is similar to the new species, in having a poorly developed plunger and a shallow fossa. Both species can however be distinguished by the relative proportion and shape of the fingers of the major cheliped. In A. leptocheles , the finger to palm ratio is 0.36 and the dactylus is relatively robust, whilst in A. samudra nov. sp., the fingers are longer (about 0.50–0.55) and the dactylus is more elongate. Asides from the differences in the major cheliped, A. samudra nov. sp. also differs from A. leptocheles by the rostrum reaching the distal margin of the first article of the antennular peduncle (vs. falling short in A. leptocheles ), the length of the fingers of the minor cheliped being shorter in relation to the palm (2.2 times in A. leptocheles vs. 1.7–1.8 in A. samudra nov. sp.), the general shape of the telson (more elongate and more distally concave in A. samudra nov. sp.), the strongly convex distal margin of the telson (vs. weakly convex in A. leptocheles ) and the distolateral tooth of the scaphocerite over-reaching the blade (vs. tip being level with blade in A. leptocheles ). A further distinct difference between these two species, is that the distal margin of the telson is furnished with a large series of short, spiniform setae in A. samudra nov. sp., whilst these are absent in the holotype of A. leptocheles (confirmed by S. Ahyong).

A potential ecological difference also exists between both species. Alpheus leptocheles was obtained from depths of 1–20 m from the mouth of the Sepik River ( Papua New Guinea), whilst the new species was obtained from depths of 275–375 m, albeit on a similar substrate of fine sand and mud.

Figures

FIGURE 1. Alpheus samudra nov. sp. Holotype, female (cl 13.7 mm): A, frontal region and cephalic appendages, lateral view; B, same, dorsal view;; C, antennule, ventromesial carina; D, telson; E, same, distal margin (partially denuded); F, uropod. Scale bars: 1 mm (A–D, F), 0.5 mm (E).

FIGURE 2. Alpheus samudra nov. sp. Holotype, female (cl 13.7 mm): A, major cheliped, mesial view; B, same, lateral view; C, minor cheliped, mesial view. Paratype, male (cl 11.7 mm): D, second pleopod; E, same, appendix masculina and appendix interna. Scale bars: 1 mm (A–D), 0.5 mm (E).

FIGURE 3. Alpheus samudra nov. sp. Holotype, female (cl 13.7 mm): A, third maxilliped; B, second pereiopod; C, same, chela; D, third pereiopod, lateral view; E, same, dactylus, lateral view F, same, mesial view; G, fifth pereiopod, lateral view; H, same, dactylus. Scale bars: 1 mm (A–B, D–H); 0.5 mm (C).

FIGURE 4. Alpheus samudra nov. sp. Paratype, male (cl 11.7 mm), plunger and socket of major cheliped: A, lateral view; B, mesial view.

FIGURE 5. Alpheus samudra nov. sp. Colour pattern, male, non-type, recently deceased (cl 13.0 mm), Neendakara fishing port, Kollam, Kerala (photo A.B. Kumar).