Prionalpheus cf. brachytomeus Banner & Banner, 1971

Anker, Arthur, 2010, New findings of rare or little-known alpheid shrimp genera (Crustacea, Decapoda) in Moorea, French Polynesia, Zootaxa 2403, pp. 23-41 : 32-33

Prionalpheus cf. brachytomeus Banner & Banner, 1971

Figures 5 D–F, 6

Prionalpheus sp. – Banner & Banner 1960b: 296, fig. 2 Prionalpheus brachytomeus Banner & Banner 1971: 267

Material examined. French Polynesia , Society Islands. 1 specimen, possibly male (CL 2.55 mm), FLMNH UF Arthropoda 16447, Moorea, Gump station – CRIOBE station road, 200 m from Sheraton Hotel, lagoon, near-shore, silt-covered fringing reef, under rocks and rubble and within crevices in dead corals, depth: 0.5–2 m, coll. A. Anker, 16.XI.2008 [fcn BMOO-5492].

Description. See Banner & Banner (1960b) as Prionalpheus sp.; species formally named, with a detailed diagnosis, in Banner & Banner (1971). Chelipeds and frontal region are illustrated in Fig. 6 C, D. See also Fig. 5 D-F for additional figures and remarks below.

Colour pattern. Hyaline-white, semitransparent, with diffuse reddish pigments at leg base and sternal region; brown hepatopancreas visible through carapace ( Fig. 6 A, B).

Type locality. Viti Levu, Fiji ( Banner & Banner 1971) [Arue, Tahiti in Banner & Banner 1960b, 1962; see under remarks below].

Ecology. The single Moorea specimen was collected in the lagoon, under large coral head, on silty sand, on a near-shore sand flat surrounded by living and dead corals at 1–2 m; the Tahiti specimen came from dead coral heads at 1.5–3 m and the Fijian specimen from outer edge of fringing reef.

Distribution. Previously known only from Tahiti and Fiji ( Banner & Banner 1960b, 1971); herewith reported for the first time from Moorea.

Remarks. Banner & Banner’s (1960b) original material of P. brachytomeus was represented by a nonovigerous female from Arue, Tahiti, and an ovigerous female from Viti Levu, Fiji; both were incomplete, missing many legs, and most importantly, the chelipeds. In addition, both specimens were subsequently destroyed in the fire of the Hawaii Marine Laboratory in 1961 (see Banner & Banner 1962, 1971). Even so the Fijian specimen was designated as “ type ” when the species was formally described as P. brachytomeus ( Banner & Banner 1971) , followed by the remark “ type specimen lost, see: Banner & Banner, 1962 ”. However, it was the Tahitian specimen that was listed in Banner & Banner (1962) ’s report on the losses of type specimens in the fire of the Hawaii Marine Laboratory, and Tahiti was listed as “ type locality” of Prionalpheus sp. (see Banner & Banner 1962, p. 239, Table 2). At the author’s request, Lu Eldredge (BPBM) confirmed the absence of specimens registered as either P. brachytomeus or Prionalpheus sp. from Fiji or Tahiti in the collections of the BPBM and no specimens of Prionalpheus from these areas exist in the collections of the USNM (pers. obs.). Therefore, it is safe to assume that both the Tahitian and the Fijian specimens of P. brachytomeus were destroyed in fire in 1961. Therefore, the Moorea specimen may be presently the only known specimen of this species (but see below), which is the main reason why it was not dissected. The previously unknown chelipeds were also left in situ ( Fig. 6 D); they are equal in size, symmetrical in shape, with moderately enlarged, setose chelae, and appear to be typical to Prionalpheus .

Prionapheus is a nearly pantropical genus, with one species in the western Atlantic ( Martínez-Iglesias & Carvacho 1991), one in the eastern Pacific ( Alvarez et al. 1996) [both known from incomplete specimens], and five species in the Indo-West Pacific ( Banner & Banner 1971; Bruce 1990). According to Banner & Banner (1971), P. brachytomeus is one of two Indo-West Pacific species characterised by the four-segmented carpus in the second pereiopod ( Fig. 3 D), the other being P. s u l u Banner & Banner, 1971; all other species have either five or three segments ( P. triarticulatus ) ( Banner & Banner 1971; Bruce 1990). In the species table provided by Banner & Banner (1971), P. brachytomeus differs from both P. s u l u and P. triarticulatus by the rostrum being relatively broad at the base (narrow in the other species); the ventral unguis of the third pereiopod dactylus being larger (broader) than the terminal unguis; the somewhat shorter teeth on the incisor process of the highly modified mandible; and specifically from the sympatric P triarticulatus by the foursegmented carpus of the second pereiopod (three-segmented in P. triarticulatus ).

The Moorea specimen was tentatively identified as P. cf. brachytomeus because the carpi of both second pereiopods appear to be three-segmented, as in P. triarticulatus (cf. Fig. 5 B, E), although there is a very faint indication of an articulation zone between the first and the second segment in P. cf. brachytomeus . The dactyli of the third pereiopods of P. cf. brachytomeus and P. triarticulatus do not show a significant difference (compare Fig. 5 C, F); however, the ventral unguis appears to be relatively larger compared to the terminal unguis in P. cf. brachytomeus . The rostral base appears to be relatively broader in P. cf. brachytomeus ( Fig. 5 D) compared to that of P. triarticulatus ( Fig. 5 A). Another feature not mentioned by Banner & Banner (1960b, 1971) that may perhaps be used to distinguish P. brachytomeus from both P. s u l u and P. triarticulatus is the unusually close position of corneas ( Figs. 5 D, 6C, see also Banner & Banner 1960b, fig. 2b); in the other two species, the corneas appear to be at some distance from each other ( Fig. 5 A). This was the author’s main criterion to assign the Moorea specimen to P. cf. brachytomeus and not to P. triarticulatus . The ongoing largescale DNA sequencing of all the Moorea alpheids will certainly verify the correctness of the present identification.

Figures

FIGURE 3. Rugathanas borradailei (Coutière, 1903): two specimens from Moorea: A – C, ovigerous female (CL 2.05 mm), FLMNH UF Arthropoda 16326; D – E, male (CL 2.20 mm), FLMNH UF Arthropoda 16327; A, dorsal view; B, lateral view; C, frontal region, dorsal view; D, dorsolateral view; E carapace with appendages, lateral view.

FIGURE 5. Prionalpheus triarticulatus Banner & Banner, 1960, ovigerous female (CL 4.05 mm) from Moorea, OUMNH. ZC. 2010 - 01 - 002 (A – C) and Prionalpheus cf. brachytomeus Banner & Banner, 1971, possibly male (CL 2.55 mm) from Moorea, FLMNH UF Arthropoda 16447 (D – F): A, D, rostral region, dorsal view; B, E, second pereiopod, carpus and chela; C, F, third pereiopod, distal propodus and dactylus. Scale bars = 0.5 mm.

FIGURE 6. Prionalpheus cf. brachytomeus Banner & Banner, 1971, possibly male (CL 2.55 mm) from Moorea, FLMNH UF Arthropoda 16447; A, dorsal view; B, lateral view; C, frontal region, dorsal view; D, chelipeds in situ, ventral view.