Acanthanas pusillus Anker, Poddoubtchenko & Jeng, 2006

Anker, Arthur, 2010, New findings of rare or little-known alpheid shrimp genera (Crustacea, Decapoda) in Moorea, French Polynesia, Zootaxa 2403, pp. 23-41 : 24-27

Acanthanas pusillus Anker, Poddoubtchenko & Jeng, 2006

Figures 1 , 2

Acanthanas pusillus Anker, Poddoubtchenko & Jeng 2006: 342 , figs. 1–5.

Material examined. French Polynesia , Society Islands. 1 male (CL 2.30 mm), FLMNH UF Arthropoda 16055, Moorea, between Temae and Afareaitu, 17.5145˚ S, 149.7616˚ W, outer reef slope, deep from within reef rubble, depth: 20 m, coll. C. Meyer, C. McKeon, J. Moore, S. Fay, G. Paulay, processed by A. Anker, 23.X.2008 [fcn BMOO-4077]; 1 ovigerous female (CL 2.20 mm), FLMNH UF Arthropoda 15613, Moorea, east of Opunohu pass, at shark feeding buoy, 17.4817˚ S, 149.8558˚ W, outer reef slope, gentle sand and reef slope, from within coral rubble, outer reef slope, depth: 17–18 m, coll. C. McKeon, J. Moore, G. Paulay, processed by A. Anker, 16.X.2008, fcn BMOO-2811; 1 female (CL 1.85 mm), FLMNH UF Arthropoda 23088, Moorea, off Temae, 17.4784˚ S, 149.7639˚ W, outer reef slope, deep from within rubble, rubble brushing, depth: 18–23 m, coll. G. Paulay, C. McKeon, S. Bush, S. Haddock, processed by A. Anker, 29.X.2009 [fcn BMOO-6707]; 1 male (CL 2.15 mm), FLMNH UF Arthropoda 23090, Moorea, Temae Bay, 149.7517˚ S, 17.4915˚ W, steep reef drop-off, under rubble, rubble brushing and cracking, depth: 7–20 m, coll. C. Meyer, S. McPherson, A. Collins, M. Parker, C. McKeon, processed by A. Anker, 22.X.2009; [fcn BMOO-6487]; 2 males (CL 1.30 mm, 1.32 mm), OUMNH-ZC.2010-01-001, same collection data as for previous specimen.

Description. Anker et al. (2006) provided a detailed description of A. pusillus based on two complete females (one of which was dissected). The finding of several male specimens in Moorea, including an almost complete adult male, enables to complete the original description with the description of two important male features, viz. the cheliped and the pleopod. It also makes necessary a slight emendation of the generic diagnosis of Acanthanas (see below).

Description of the male cheliped of Acanthanas pusillus (cf. Anker et al. 2006, p. 343)

Male chelipeds symmetrical or subsymmetrical, polymorphic depending on age/size. Chelipeds of immature males slightly longer and more robust ( Fig. 1 B) than those of females ( Fig. 1 D). Chelipeds of adult males (CL> 2 mm) significantly enlarged relative to body size ( Fig. 2 A), elongate, robust, with large chelae; basis with slender distal spine on dorsal margin; ischium with row of five slender spines on dorsal margin ( Fig. 2 B); merus robust, widening distally, both dorsal and ventral margins ending in sharp tooth ( Fig. 2 B); carpus cup-shaped, embracing proximal portion of palm; palm smooth, somewhat compressed laterally, ovoid in cross-section, distinctly longer than wide, with numerous fine setae along ventral surface; fingers longer than half length of palm, somewhat twisted, dactylus strongly curved; cutting edges of fingers unarmed (on dactylus) or with inconspicuously irregular teeth (on pollex); lateral side of pollex shallowly depressed and bearing strong subdistal tooth ( Fig. 2 C, D).

Description of the male pleopod of Acanthanas pusillus (cf. Anker et al. 2006, p. 344)

Male pleopod with endopod smaller than exopod, bearing appendix interna and appendix masculine, latter reaching distal margin of endopod ( Fig. 2 E).

Revised diagnosis of Acanthanas (cf. Anker et al. 2006, p. 342)

Body bulky stout. Frontal margin with long, distally acute rostrum and acute orbital teeth. Pterygostomial angle bluntly angular to rounded, protruding. Eyes mostly concealed in dorsal and lateral view, visible in frontal view; anteromesial margin of cornea and base of eyestalk with spiny projections. Antennular peduncle short, stout; first segment with large tooth on ventromesial carina; lateral antennular flagellum feebly biramous. Antenna with moderately robust basicerite; carpocerite short, not overreaching scaphocerite, latter with well-developed blade. Mouthparts typical for family; mandible with two-segmented palp and expanded incisor process. Third maxilliped pediform, lateral plate ear-shaped, acute distally; tip of ultimate segment with spines. First pereiopods (= chelipeds) equal, symmetrical or subsymmetrical, carried extended, sexually dimorphic, feebly enlarged in females and immature males, greatly enlarged, robust in adult males; ischium unarmed in females, armed with slender spines on dorsal margin in adult males; merus ending in acute or subacute distal tooth dorsally and in adult males also ventrally; carpus somewhat elongate, subcylindrical in females, short, cup-shaped, distally widening in adult males, mesial side without setal rows; chelae simple, smooth, feebly enlarged in females, conspicuously enlarged in adult males; linea impressa and adhesive disks absent; fingers without snapping mechanism, unarmed in females, pollex armed with small teeth in adult males. Second pereiopod with five-segmented carpus. Third pereiopod with ischium bearing spine on ventrolateral surface; merus and carpus unarmed; propodus with spinules on ventral margin; dactylus biunguiculate. Fifth pereiopod with cleaning brush consisting of several setal rows on propodus. Sixth abdominal somite with articulated plate at posteroventral angle. Second pleopod with appendices masculina and interna in both males and females (including ovigerous females). Uropod with lateral lobe of protopod [formerly called “sympodite”] bearing two teeth; diaeresis of exopod sinuous. Telson with two pairs of dorsal spines and two pairs of posterolateral spines; posterior margin feebly rounded; anal tubercles absent. Gill/ exopod formula: 5 pleurobranchs (above P1–5); 0 arthrobranch; 0 podobranch; 2 lobe-shaped epipods (Mxp1- 2); 3 strap-like epipods = mastigobranchs (Mxp3 and P1–2), 3 setobranchs (P1–3), 3 fully developed exopods (Mxp1–3), 1 rudimentary exopod (P1).

Colour pattern. As the type specimens, some individuals from Moorea had an intricate pattern of red and white marbling, white and red patches and small red dots ( Fig. 1 D, E); other specimens had a more distinct pattern consisting of a white carapace speckled with tiny red dots, a mostly red abdomen with some whitish or transparent dots ( Fig. 1 A, B; 2A); chelipeds, walking legs and frontal appendages mostly semitransparent whitish; chelipeds speckled with a few red dots; see also description and photographs in Anker et al. (2006, fig. 5).

Type locality. Panglao, the Philippines.

Ecology. Dwelling deep in crevices of dead corals, at depths of 7–23 m (Moorea) or 20–24 m (Panglao).

Distribution. Previously known only from the two type specimens from Panglao, the Philippines (Anker et al. 2006); now recorded for the first time from Moorea, French Polynesia , representing a considerable range extension of both the genus and the species from the “Coral Triangle” into the central-western Pacific Ocean.

Remarks. This remarkable dwarf alpheid differs from all other alpheids by having two sharp spine-like projections on each eye (dorsal projection of left eye is visible in Fig. 1 C). This feature, along with the presence of small, unspecialised chelipeds in the two type specimens (both females) were used by Anker et al. (2006) to separate Acanthanas from the undoubtedly closest genus, Athanas Leach, 1814 . The finding of the first adult male specimen of A. pusillus in Moorea (FLMNH 23090) is particularly interesting for the right cheliped (the left is missing) is similar to the chelipeds defining the Athanas nitescens Leach 1814 group, which may further link Acanthanas and Athanas . The latter genus is a fairly heterogeneous group in need of a revision ( Anker & Jeng 2007), especially because based on this new evidence, Acanthanas appears to be embedded within Athanas sensu lato.

The very small size of A. pusillus (CL 1.3–2.3 mm, TL approximately 5.0–8.0 mm) and highly cryptic life style makes it impossible to spot underwater; the only way to collect it is to break and brush off large pieces of dead corals collected on outer reef slopes or in deeper rubble beds. It appears to be fairly common in rubble beds off Moorea, with multiple specimens collected over a period of several months.

Figures

FIGURE 1. Acanthanas pusillus Anker, Poddoubtchenko & Jeng, 2006, two specimens from Moorea, A – C, male (CL 2.30 mm), FLMNH UF Arthropoda 16055; D – E, ovigerous female (CL 2.20 mm), FLMNH UF Arthropoda 15613; A, dorsal view; B, lateral view; C, frontal region, dorsolateral view; D, lateral view; E frontal region and chelipeds, lateral view.

FIGURE 2. Acanthanas pusillus Anker, Poddoubtchenko & Jeng, 2006, two specimens from Moorea, A – D, larger male (CL 2.15 mm), FLMNH UF Arthropoda 23090; E, smaller male (CL 1.32 mm), OUMNH-ZC. 2010 - 01 - 001; A, lateral view; B, right cheliped, lateral view; C, same, chela; D, same, chela and carpus, mesial view; E, pleopod. Scale bars = 0.5 mm.