Alpheopsis balaeniceps, Anker, Arthur, 2015

Anker, Arthur, 2015, Alpheopsis balaeniceps sp. nov., an unusual alpheid shrimp (Decapoda: Caridea) from French Polynesia, Zootaxa 3972 (1), pp. 85-92 : 86-90

Alpheopsis balaeniceps sp. nov.

( Figs. 1–3 )

Type material. Holotype: 1 male (cl 3.3 mm), FLMNH UF 23302, French Polynesia, Society Islands, Moorea Island, eastern coast of Cook's Bay, 17°29'16"S 149°49'7 W, depth: ~ 1 m, under corals, on fine sand and rubble, coll. A. Anker, 26.x.2009 [BMOO-07149].

Description. Small-sized species of Alpheopsis (cl of holotype <3.5 mm). Carapace with well-developed rostrum, much longer than wide, acute distally, tip almost reaching second half of second article of antennular peduncle; orbital hoods broadly rounded anteriorly, without orbital teeth; pterygostomial angle with minute, anteriorly protruding tooth ( Figs. 1 A–C).

First four abdominal somites with pleura rounded posteroventrally; fifth pleuron with acutely projecting posteroventral angle; sixth somite with subacute posterior lobe and well-delimited, triangular, articulated plate ( Fig. 1 D). Telson narrow, distally tapering, about 2.5 as long as anterior width; dorsal surface with two pairs of slender spiniform setae inserted at some distance from lateral margin, at about 0.45 and 0.75 telson length, respectively; posterior margin less than half as long as anterior margin, rounded; each posterolateral angle with one pair of stout spiniform setae, mesial much longer than lateral ( Fig. 1 E).

Eyes completely concealed by orbital hoods, not visible in dorsal view, anterior-most portion exposed in lateral view; eyestalk with well-pigmented cornea and small tubercle on anterior margin ( Fig. 1 A, B). Epistomial sclerites with sharp processes ( Fig. 1 F).

Antennular peduncles stout; stylocerite slender, with acute tip, latter almost reaching distal margin of second article; ventromesial carina with stout, anteriorly directed, acute tooth; second article stout, slightly wider than long; lateral flagellum biramous; fused portion short, with three joints; accessory ramus long, with three groups of long aesthetascs ( Fig. 1 A, B, G). Antenna with basicerite ending in sharp tooth distoventrally; scaphocerite ovate, with anteriorly rounded blade and sharp distolateral tooth, latter not reaching beyond anterior margin of blade; anterior margin of scaphocerite not exceeding distal margin of antennular peduncles; carpocerite reaching almost to end of scaphocerite ( Fig. 1 A, B).

Mouthparts not dissected, appearing typical for family in external view. Third maxilliped slender, pediform; coxa with distally subacute lateral plate above mastigobranch; penultimate article at most three times as long as wide proximally, widening distally; ultimate article tapering to slender corneous tip, with stiff subdistal seta dorsally; arthrobranch well-developed ( Fig. 1 H).

Chelipeds unequal in size, subsymmetrical in shape, carried extended when not in use ( Figs. 2 , 3 ). Major cheliped moderately robust; ischium with distodorsal spiniform seta; merus slightly more than four times as long as wide, depressed distoventrally, broadening distally, distomesial angle blunt; carpus short, cup-shaped, with dorsal constriction; chela moderately slender, with fingers about 0.9 times as long as palm; palm about 2.5 times as long as high, subcylindrical in cross-section, smooth; fingers balaeniceps-like, not gaping, with distally crossing tips; pollex with row of plumose setae on lateral and mesial surfaces, posterior half of cutting edge with seven irregularly spaced, subtriangular teeth, becoming gradually larger distally; dactylus slightly flattened dorsoventrally, slightly expanded latero-mesially, with row of plumose setae on lateral and mesial surfaces, subdistal portion of cutting edge with few small irregular teeth ( Fig. 2 A–C). Minor cheliped noticeably smaller than major cheliped, generally similar to major cheliped in proportions and armature on fingers, somewhat more slender; chela with fingers subequal to palm in length, balaeniceps-like, not gaping; dactylus and pollex with rows of plumose setae on lateral and mesial surfaces, rows being somewhat shorter compared to those of major cheliped fingers; cutting edge of dactylus with one subtriangular tooth at about 0.7 of dactylar length and one minute subdistal tooth; cutting edge of pollex with at least six small widely spaced subtriangular teeth, becoming gradually larger distally, and one very small subdistal tooth ( Fig. 2 D, E).

Second pereiopod slender; ischium slightly longer than merus; carpus five-articulated; ratio of carpal articles (from proximal to distal) approximately equal to: 3: 1: 1.3: 1.2: 1.5; chela longer than distal carpal article, with fingers subequal to palm in length, simple ( Fig. 1 I). Third pereiopod slender; ischium with two stout spiniform setae on ventrolateral surface; merus about six times as long as wide, unarmed; carpus about 0.6 length of merus, with small spiniform seta on distoventral margin; propodus with three slender spiniform setae along ventral margin and two distal spiniform setae, one much longer than other, adjacent to dactylus; dactylus about 0.4 length of propodus, slender, simple, subconical, gradually curving distally, with some setae subdistally ( Fig. 1 J, K). Fourth pereiopod generally similar to third pereiopod in proportions and armature on ischium and propodus. Fifth pereiopod more slender than third and fourth pereiopods; ischium with one spiniform seta on ventrolateral surface; merus at least eight times as long as wide, unarmed; carpus about 0.7 length of merus, with stiff seta on distoventral margin; propodus long and slender, with at least four small spiniform setae along ventral margin (some not visible in lateral view), one long distal spiniform setae adjacent to dactylus, and several rows of serrulate setae on distolateral surface; dactylus about 0.35 length of propodus, similar to that of third pereiopod ( Fig. 1 L).

Male second pleopod with appendix masculina exceeding appendix interna; apex with several long stiff setae ( Fig. 1 M). Uropod with lateral lobe of protopod ending in sharp tooth; endopod and exopod broadly ovate; distolateral tooth strong, sharp; adjacent spiniform seta long, slender; diaresis with subtriangular tooth adjacent to spiniform seta and sinuous mesial portion ( Fig. 1 N).

Gill-exopod formula: five pleurobranchs above first to fifth pereiopods; five mastigobranchs on coxae of third maxilliped and first to fourth pereiopods; five setobranchs on coxae of first to fifth pereiopods; one arthrobranch at base of third maxilliped; no podobranchs; three exopods on first to third maxilliped.

Colour pattern. Carapace semitransparent with red chromatophores forming two diffuse transverse bands in posterior half and with smaller field of red chromatophores in frontal area; abdomen semitransparent with six transverse bands of red chromatophores, one across each somite; antennular and antennal peduncles, uropods and telson almost entirely covered with red chromatophores; chelipeds mostly hyaline-whitish except for some reddish areas with red chromatophores; walking legs and pleopods mostly colourless ( Fig. 3 ).

Etymology. The new species name refers to the characteristic balaeniceps-like setae on the cheliped fingers (term “ balaeniceps ” first proposed by Coutière (1899) to describe the minor cheliped in some species of Alpheus ); used as a noun in apposition.

Type locality. Moorea, French Polynesia.

Distribution. Presently known only from the type locality in French Polynesia.

Remarks. Alpheopsis balaeniceps sp. nov. is the only known species in the genus characterised by the presence of balaeniceps-type rows of plumose setae on the cheliped fingers. Two species of Alpheopsis , namely A. idiocarpus Coutière, 1908 and A. biunguiculata ( Banner, 1953) , were described based on fragmentary specimens lacking both chelipeds ( Coutière 1908, 1921; Banner 1953). However, A. idiocarpus can be separated from A. balaeniceps sp. nov. by the second pereiopod carpus subdivided into three articles ( Coutière 1921: fig. 2a) and not five, as in the new species and most other species in the genus, whereas A. biunguiculata differs from A. balaeniceps sp. nov. by the shorter descendant rostrum and much stouter biunguiculate dactyli of the third to fifth pereiopods ( Banner 1953: fig. 5a, b, e, f).

The shape of the frontal margin of the carapace, including a relatively long pointed rostrum and the absence of orbital teeth, as well as the general shape of the chelipeds suggest that Alpheopsis balaeniceps sp. nov. belongs to the A. aequalis Coutière, 1897 species complex. This species complex contains, besides the taxonomically problematic A. aequalis (Indo-West Pacific) with two questionable synonyms ( A. aequalis truncata Coutière, 1903 and A. consobrinus De Man, 1910 ), at least four other species: A. yaldwyni Banner & Banner, 1973 , A. tetrarthri Banner, 1956 (both Indo-West Pacific), A. labis Chace, 1972 (western Atlantic), and A. allanhancocki Wicksten, 1992 (eastern Pacific). As already mentioned, none of these species has balaeniceps-type setae on the chelipeds. Examination of Papua New Guinean material identified as A. aequalis [ sensu lato] and A. yaldwyni confirmed that the cheliped fingers in these specimens only possess simple setae on the lateral and mesial surfaces, in addition to rows of spaced simple setae along the cutting edges, not homologous to the plumose balaeniceps-type setae found in A. balaeniceps sp. nov. In addition, A. balaeniceps sp. nov. differs specifically from A. aequalis by the absence of stout spiniform setae on the cheliped merus (present in A. aequalis ); the presence of a spiniform seta on the ischium of the fifth pereiopod (absent in A. aequalis ); the presence of two spiniform setae on the ischium of the third pereiopod (unarmed in A. aequalis ); and the red-banded colour pattern (colour described as orange-yellow by Coutière (1899) for A. aequalis ) (cf. Coutière 1899; Banner 1953; Banner & Banner 1973; Miya 1983). It must be noted that A. aequalis [ sensu lato] appears to contain several species and needs a thorough revision.

Alpheopsis balaeniceps sp. nov. can be easily separated from A. yaldwyni by the more slender chelipeds, with a non-flattened, subcylindrical chelae (noticeably stouter and with a dorso-ventrally flattened chelae in A. yaldwyni ) and the pterygostomial angle bearing a minute tooth visible only at the highest magnification (compared to a conspicuously projecting tooth in A. yaldwyni ). The new species also differs from A. tetrarthri , for instance, by the second pereiopod having a five-articulated carpus (four-articulated in A. tetrarthri ); the much longer, horizontal rostrum (short and descendant in A. tetrarthri ); the much longer stylocerite, reaching almost to the distal margin of the second article (not reaching mid-length of the second article in A. tetrarthri ) (cf. Banner 1956). Finally, A. balaeniceps sp. nov. can be separated from A. labis and A. allanhancocki by the much narrower telson (conspicuously wider in A. labis and A. allanhancocki ) and from A. allanhancocki also by the presence of a minute tooth on the pterygostomial angle (illustrated and described as blunt in A. allanhancocki ), in addition to several features on the chelipeds (cf. Chace 1972; Wicksten 1992).

To the author’s best knowledge, Alpheopsis balaeniceps sp. nov. represents the first species with balaenicepstype setae on the cheliped fingers outside the genus Alpheus . In Alpheus , the balaeniceps setae are present, with various degrees of development, on the minor cheliped of numerous species belonging to several species groups. The balaeniceps condition is typically present only in males or, more rarely, in both sexes, and may help to distinguish closely related species, suggesting that this character is highly flexible, adding to the general plasticity of the alpheid chelipeds. As explained by Banner and Banner (1982), in the most extreme form, the dactylus of the minor chela is conspicously broadened and somewhat excavate, whereas in the least developed form, the dactylus shows lesser degrees of lateral expansion and only bears a low crest befringed with setae. This latter condition is found in A. balaeniceps sp. nov., however, and in contrast to Alpheus , on the dactylus of both chelipeds. Whether the female chelipeds of the new species also have balaeniceps setae is presently unknown. Curiously, the exact function of the balaeniceps minor chela in alpheid shrimps remains a mystery.

Figures

FIGURE 1. Alpheopsis balaeniceps sp. nov., holotype, male (cl 3.3 mm) from Moorea, French Polynesia (FLMNH UF 23302); A—frontal region, dorsal; B—same, lateral; C—detail of pterygostomial angle, lateral; D—posterior abdominal somites, lateral; E—telson, dorsal; F—epistomial sclerites; G—tooth on ventromesial carina of first article of antennular peduncle, lateral; H—third maxilliped, lateral; I—second pereiopod, lateral; J—third pereiopod, lateral; K—same, detail of distal propodus and dactylus, mesial; L—fifth pereiopod, lateral; M—pleopod, lateral; N—uropod, dorsal. Scale bars as indicated except for C, F, G (drawn without scale).

FIGURE 2. Alpheopsis balaeniceps sp. nov., holotype, male (cl 3.3 mm) from Moorea, French Polynesia (FLMNH UF 23302); A—right (major) cheliped, lateral; B—same, chela with fingers closed, lateral; C—same, chela fingers open, lateral; D—left (minor) cheliped, lateral; E—same, chela, mesial. Scale bars as indicated.

FIGURE 3. Alpheopsis balaeniceps sp. nov., holotype, male (cl 3.3 mm) from Moorea, French Polynesia (FLMNH UF 23302), in life; A—dorsal view; B—lateral view. Photographs by the author.