Alpheus naranjo sp. nov.

(Figs. 1, 2, 3A)

Type material. Holotype: male (cl 9.0 mm), FLMNH UF 44471, Panama, Caribbean coast , Bocas del Toro Archipelago, off Isla Popa, Popa Reef, 9.233° -82.112°, coll. M. Leray, F. Michonneau & R. Lasley, 24 May 2016 (fcn BBDT-2166). Paratypes: 1 ovigerous female (cl 9.7 mm), OUMNH.ZC. 2018.01.0 3, same collection data as for holotype (fcn BBDT-2166); 1 ovigerous female (cl 10.0 mm, missing major cheliped), FLMNH UF 44448, same collection data as for holotype (fcn BBDT-2112); 1 male (8.4 mm—dissected and illustrated), 1 female (cl 8.1 mm), 1 ovigerous female (8.7 mm, missing minor cheliped), same collection data as for holotype (fcn BBDT- 2744); 1 male (8.6 mm), MNHN-IU-2017-1547, same collection data for holotype (fcn BBDT-2744).

Description. Medium-sized species of Alpheus (cl reaching 10.0 mm). Carapace smooth, not setose. Rostrum well developed, long, relatively slender, more than twice as long as broad at base, sharp distally, reaching or almost reaching distal margin of first article of antennular peduncle, flattening posteriorly and with base abruptly delimited from adjacent rostro-orbital area; rostral carina not distinct; lateral margin furnished with stiff setae (Fig. 1A, B). Orbital hoods moderately swollen; anterior margin of each orbital hood armed with anteriorly directed, sharp tooth, latter reaching or almost reaching mid-length of rostrum; margin between tooth and rostrum somewhat flattened (Fig. 1A, B). Pterygostomial angle rounded, not protruding (Fig. 1B); cardiac notch well developed. Each epistomial sclerite with strong blunt process.

Telson moderately broad, subrectangular, tapering distally, about twice as long as proximal width, with slightly convex lateral margins; dorsal surface with two pairs of very stout spiniform setae both inserted at some distance from lateral margins, first pair at about telson mid-length, second pair at about 0.7 of telson length; posterior margin broadly rounded, without spiniform setae; each posterolateral angle with pair of stout spiniform setae, lateral about one-third of length of mesial (Fig. 1C).

Antennular peduncle with second article short, about 1.2 times as long as wide; stylocerite with sharp point reaching to or beyond mid-length of first article; ventromesial carina with large, broadly triangular tooth, latter with small but well-marked anterior point; lateral antennular flagellum with secondary ramus fused to main ramus over most of its length, except for very short free terminal section, with numerous groups of aesthetascs starting from about fourth article (Fig. 1A, B, D). Antenna with stout basicerite bearing large sharp distoventral tooth; scaphocerite relatively stout, with straight lateral margin and well-developed blade, latter not overreaching strong distolateral tooth; carpocerite reaching slightly beyond scaphocerite and exceeding end of antennular peduncle (Fig. 1A, B).

Mouthparts typical for genus in external observation. Third maxilliped slender, pediform; coxa with distally subacute lateral plate furnished with some long setae; antepenultimate article slightly flattened ventrolaterally; penultimate article about 3.5 times as long as wide; ultimate article with numerous rows of short serrulate setae and longer simple setae, unarmed distally; arthrobranch well developed (Fig. 1E).

Major cheliped similar in males and females, not significantly larger in males; ischium very short, stout, with row of short spiniform setae (typically four) along mesial margin; merus relatively stout, about 2.2 times as long as wide, distodorsal margin ending in very strong, somewhat curved, sharp tooth; mesial margin rugose, with row of short stout spiniform setae (typically at least five, proximal ones more robust); distomesial tooth very strong, sharp, with broad dorsal lobe; carpus very short, cup-shaped, with broadly rounded distal lobes; chela elongate, with palm subcylindrical in cross-section and about twice as long as fingers; palm surface mostly smooth, except for some low setiferous tubercles on ventral surface, extending to pollex, without any grooves, crests or notches; dactylus slightly longer than pollex, distally rounded, not twisted, with large stout plunger; adhesive disks well developed (Fig. 2A–E).

Minor cheliped not sexually dimorphic, slightly stouter in some males, much more slender than major cheliped, but of nearly equal length; ischium very short, stout, with row of short spiniform setae (typically four or five) along mesial margin; merus moderately stout, at least 2.5 times as long as wide, distodorsal margin ending in very strong, somewhat curved, sharp tooth; mesial margin rugose and with row of short stout spiniform setae (usually at least five, on proximal half); distomesial tooth very strong, sharp, with broad dorsal lobe; carpus short, cup-shaped, with strong distal lobe mesially; chela elongate, slender, with palm subcylindrical in cross-section, slightly but noticeably shorter than fingers (about 0.8–0.9 of finger length); palm surface smooth, without any grooves, crests or notches; fingers subequal in length, crossing distally; dactylus very slightly flattened dorsally, more conical distally, with simple blade-like cutting edge; pollex thicker than dactylus, with deep median groove flanked by two blade-like cutting edges; adhesive disks very small (Fig. 2F–H).

Second pereiopod slender; ischium and merus subequal in length; carpus with five joints, their ratio approximately equal to 2.5/1/0.6/0.5/1; chela as long as two distal-most carpal joints combined (Fig. 1F). Third pereiopod slender; ischium with short spiniform seta on ventrolateral surface; merus more than six times as long as wide, not inflated, unarmed; carpus about half length of merus, somewhat more slender, unarmed; propodus much longer than carpus but shorter than merus, with six or seven irregularly inserted, stout spiniform setae on ventral margin, not including one pair of additional spiniform setae near dactylar base; dactylus about 0.25 length of propodus, fairly slender, curved, conical, simple, with some setae subdistally (Fig. 1G, H). Fourth pereiopod generally similar to third, slightly more slender. Fifth pereiopod much more slender than third and fourth; ischium unarmed; merus not inflated, about six times as long as wide; carpus 0.9 length of merus; propodus with five or so spiniform setae along ventromesial margin, and one pair of (or sometimes three) spiniform setae near dactylus, in additional to several rows of cleaning setae on ventrolateral surface, on distal-most portion; dactylus conical, simple, slightly more curved than that of third pereiopod, with some setae subdistally (Fig. 1I, J).

Male second pleopod with appendix masculina subequal in length to appendix interna, with stiff setae on apex and along mesial margin (Fig. 1K). Uropod with both mesial and lateral lobes of protopod ending in sharp point; exopod broad, slightly truncate distally, with strong distolateral tooth; diaeresis strongly sinuous, with strong triangular tooth adjacent and mesial to very thick, black-coloured spiniform seta; endopod narrower than exopod, with short row of small spiniform setae on distal margin (Fig. 1L).

Colouration. Body uniformly bright reddish-orange, with some paler or colourless areas on carapace flanks; antennules and antennae reddish-orange, with deeper red flagella; chelipeds bright orange; walking legs reddish with colourless areas near joints and more distally; tail fan orange-red (Fig. 3A, B).

Etymology. The new species' name refers to its overall bright orange colour ( naranjo, Spanish for orange); used as a noun in apposition.

Distribution. Presently known only from the type locality in the Bocas del Toro Archipelago, on the Caribbean coast of Panama.

Ecology. All specimens of A. naranjo sp. nov. were collected by carefully breaking apart dead colonies of thin-leaf lettuce coral, Agaricia tenuifolia Dana (Agariciidae), on a very shallow coral reef (depth: about 2 m).

Remarks. Alpheus naranjo sp. nov. is morphologically closest to three species, namely Alpheus blachei Crosnier & Forest, 1965 from the tropical eastern Atlantic (Crosnier & Forest 1965, 1966); A. felgenhaueri Kim & Abele, 1988 and A. confusus Carvacho, 1989, both from the tropical eastern Pacific (Kim & Abele 1988; Carvacho 1989). These four species are characterised by the presence of sharp orbital teeth on or very close to the anterior margin of the orbital hoods; the well-developed acute rostrum, not or only slightly flattened posteriorly; the elongate subcylindrical major chela; the enlarged and darkly coloured spiniform seta of the uropod; and the uniform red-orange, red-brown, or orange-yellow body colour (Fig. 3), although the colouration of A. felgenhaueri presently remains unknown. Based on these characteristics, these four species together may form a small clade within Alpheus, which will be referred to as the Alpheus blachei species complex.

Alpheus naranjo sp. nov. can be separated from the remaining three species of the A. blachei complex mainly by the absence of rows of balaeniceps setae on the minor chela, which are present in both sexes of A. blachei and A. confusus, and at least in males of A. felgenhaueri (possibly also in females). The rows of balaeniceps setae are typically present on both fingers of the minor chela and are particularly well developed in A. blachei (confirmed by examination of the comparative material from São Tomé, MZUSP 33940, see also Fig. 3C) and A. felgenhaueri (confirmed by re-examination of the holotype, USNM 143347, see also Kim & Abele 1988: fig. 16g, h). Carvacho (1989) also mentioned (although not illustrated) the balaeniceps condition of the minor chela in the type series of A. confusus . Corroborating Carvacho’s observation, a relatively small ovigerous female from Coiba here tentatively identified as A. confusus (OUMNH.ZC.2014-02-028) also bears a well-developed row of balaeniceps setae on the minor chela (visible in Fig. 3C). Another distinguishing feature of A. naranjo sp. nov., which separates it at once from A. felgenhaueri, A. blachei and A. confusus is the elongation of the minor cheliped fingers, which are noticeably longer than the palm vs. distinctly shorter than the palm in the other three species (cf. Figs. 1F, G, 3A–D; see also Kim & Abele 1988: fig. 16g, h; Crosnier & Forest 1965: fig. 3e). Thus, the minor chela with its relatively long, non-expanded, non-balaeniceps fingers, characteristic of the new species, represents a unique feature within the A. blachei complex.

Additional morphological features that can be used to separate A. naranjo sp. nov. from the other species of the A. blachei complex are found on the frontal margin of the carapace, antennules, antenna, major cheliped, second and third pereiopods, and uropod. For instance, the new species differs from A. felgenhaueri by the large and anteriorly directed orbital teeth, which are much smaller and mesially pointed in A. felgenhaueri; the distolateral tooth of the scaphocerite reaching far beyond the blade vs. hardly overreaching it in A. felgenhaueri (cf. Fig. 1A and Kim & Abele 1988: fig. 16a); and the distinctly more slender second and third pereiopods, which are comparatively much more robust in A. felgenhaueri, this difference being particularly obvious in the proportions of the merus, propodus and dactylus of the third pereiopod (cf. Fig. 1F–H and Kim & Abele 1988: fig. 16i –k). The new species also differs from A. blachei by the noticeably more slender third pereiopod, exemplified by the proportions of the merus (about six times as long as wide in A. naranjo sp. nov. vs. at most five times in A. blachei, cf. Fig. 1G and Crosnier & Forest 1966: fig. 3g); and the much larger spiniform seta on the uropod (cf. Fig. 1L and Crosnier & Forest 1966: fig. 3h). Alpheus naranjo sp. nov. may be separated specifically from A. confusus by the shape of the frontal margin, e.g., by the larger, more marginal and more anteriorly directed orbital teeth and longer rostrum, which typically is reaching the distal margin of the first article of the antennular peduncle vs. reaching only half-length of this article in A. confusus; and the distinctly longer stylocerite, which is easily reaching or sometimes reaching far beyond half-length of the second article of the antennular peduncle vs. not exceeding beyond proximal third of this article in A. confusus (cf. Fig. 1A and Carvacho 1989: fig. 1a). The presence of widely and irregularly spaced, low tubercles along the ventral surface of the major cheliped palm and pollex in A. naranjo sp. nov. (Fig. 2A, B), is another feature not seen in the other three species (cf. Crosnier & Forest 1966: fig. 3c, d; Kim & Abele 1988: fig. 16e, f; Carvacho 1989: fig. 1e, f). Finally, the body colour of A. naranjo sp. nov. is a rich orange-red (Fig. 3A, B), differing from the more deep red-brown with tiny yellowish spots of A. blachei (Fig. 3C) and the more yellow tones of A. confusus (Fig. 3D), both illustrated here for the first time.

The current paucity of the material of the A. blachei complex, especially from the eastern Pacific, currently precludes a more rigorous molecular analysis of this group. Based on morphological grounds, A. naranjo sp. nov. appears to be closest to either A. blachei (in which case it could be its trans-Atlantic sister) or A. confusus (in which case it could be its trans-isthmian sister), and slightly more distant to A. felgenhaueri . The fairly inadequate description of A. confusus by Carvacho (1989) does not make it easy to compare some of the characters of this taxon to those of other species in the A. blachei complex. For instance, Carvacho (1989) provided a very superficial drawing of the frontal region and failed to illustrate or to describe any of the walking legs of A. confusus . He also did not compare A. confusus with the sympatric A. felgenhaueri, possibly because it was described only one year earlier (Kim & Abele 1988). Based on the present evidence, A. felgenhaueri and A. confusus represent two distinct species that can be separated from each other by the shape of the scaphocerite (with a very small distolateral tooth hardly exceeding beyond the blade in A. felgenhaueri vs. with a fairly strong distolateral tooth strongly overreaching the blade in A. confusus) and orbital hoods (distinctly swollen and rounded anteriorly, with minute teeth, in A. felgenhaueri vs. slightly flared and sinuous anteriorly, with larger teeth, in A. confusus) (cf. Kim & Abele 1988: fig. 16a; Carvacho 1989: fig. 1a), and by the proportions of the third pereiopod dactylus (short and stout in A. felgenhaueri vs. more slender in A confusus (A. Anker, pers. obs., OUMNH.ZC.2014-02-028).

The only two western Atlantic snapping shrimps that have some morphological similarities with A. naranjo sp. nov., such as the armed orbital hoods, the general shape of the major and minor cheliped, and the dark-coloured spiniform seta of the uropod, are A. formosus Gibbes, 1850 and A. paraformosus Anker, Hurt & Knowlton, 2008 . Both are readily distinguishable from the new species by their dorsally abruptly flattened post-rostral areas, their orbital teeth distinctly removed from the anterior orbital margin, their much shorter stylocerites, as well as by their very different and characteristic colour patterns, dominated by one median longitudinal yellow or cream band, two lateral brown bands and bluish walking legs (Anker et al. 2008).

In the Indo-West Pacific, A. splendidus Coutière, 1897, A. facetus De Man, 1908 and A. coetivensis Coutière, 1908 [= A. edmondsoni (Banner, 1953)] share some general characteristics with A. naranjo sp. nov., but can be easily distinguished from the new species by a combination of morphological features, and most easily, by their strikingly different colour patterns (Coutière 1908; De Man 1908, 1911; Banner 1953; Bhuti et al. 1975; Banner & Banner 1982; for colour photographs see Anker 2010; Anker & De Grave 2016).