Pachelpheus pachyacanthus, Anker, 2020

Anker, Arthur, 2020, A remarkable burrow-dwelling alpheid shrimp, new genus and new species, from the tropical eastern Pacific (Malacostraca: Decapoda: Caridea), Zootaxa 4731 (1), pp. 75-88 : 78-85

publication ID	https://doi.org/ 10.11646/zootaxa.4731.1.5
publication LSID	lsid:zoobank.org:pub:6C467606-BFA4-468D-ABBB-ECE3B2AE59C6
persistent identifier	https://treatment.plazi.org/id/03ED87ED-FFC7-9D76-8FB8-FCDBFD1EF874
treatment provided by	Plazi
scientific name	Pachelpheus pachyacanthus
status	sp. nov.

Treatment

Pachelpheus pachyacanthus View in CoL sp. nov.

Figs. 1–7 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7

Type material. Holotype: male (cl 3.5 mm, tl 12.3 mm), FLMNH UF 51917 , Panama, Pacific coast, Coiba Archipelago, Isla Rancheria ( Isla de Coibita), southern side near STRI boat “parking” area and large rocky outcrop, 7°38’13.1”N 81°42’17.7”W, depth: 1–1.5 m, subtidal sand flat with some muddier patches, coral rubble and small colonies of living corals, suction (yabby) pump, in burrow (host not collected), leg. A. Anker & P.P.G. Pachelle, 24 Feb. 2019 [fcn PAN-282] GoogleMaps . Paratype: female (cl 3.6 mm, tl 12.6 mm), FLMNH UF 51918 , same collection data as for the holotype [fcn PAN-280] GoogleMaps .

Description. Small-sized (cl 3.5–3.6 mm) alpheid shrimp with moderately slender, somewhat compressed body ( Fig. 1 View FIGURE 1 ). Carapace smooth, not setose, unarmed; frontal margin with broadly rounded rostral projection, without well-defined rostrum, orbital teeth absent; pterygostomial angle rounded, slightly produced anteriorly ( Fig. 2A, B View FIGURE 2 ). Cardiac notch well developed ( Fig. 1 View FIGURE 1 ).

Pleon with first to fifth pleura broadly rounded posteroventrally; second pleuron greatly expanded in females ( Figs. 1 View FIGURE 1 , 7 View FIGURE 7 ). Sixth pleonite with slight dorsal constriction near proximal margin; posteroventral angle with small triangular articulated plate; posterolateral margin not produced posteriorly, blunt; preanal plate rounded ( Figs. 1 View FIGURE 1 , 2C View FIGURE 2 ).

Telson fairly broad, subrectangular, strongly tapering distally, about 1.9 times as long as maximal (proximal) width; dorsal surface with two pairs of small cuspidate setae inserted at small distance from lateral margin, first pair slightly posterior to telson half-length, second pair at about 0.7 of telson length; posterior margin about 0.4 of maximal telson width, broadly rounded, with two pairs of spiniform setae, mesial noticeably thicker and at least three times longer than lateral; three or four long plumose setae arising between mesial spiniform setae ( Fig. 2D, E View FIGURE 2 ).

Eyes fully concealed in dorsal view, partly exposed in lateral view; cornea relatively small, well pigmented; anteromesial margin of eyestalk unarmed ( Fig. 2A, B View FIGURE 2 ). Epistomial sclerites unarmed.

Antennular peduncle stout; stylocerite somewhat appressed to first article, tip blunt or at most subacute, not reaching distal margin of first article; ventromesial carina with anteriorly directed tooth; second article squareshaped in dorsal view, somewhat elongate on mesial side; lateral antennular flagellum much thicker than mesial antennular flagellum, with fused portion consisting of four subdivisions, secondary ramus (= accessory flagellum) well discernable, at least six groups of aesthetascs present on last subdivisions of fused portion and on secondary ramus ( Fig. 2A, B, F View FIGURE 2 ).

Antenna with basicerite very stout, its distoventral margin with large subacute tooth; scaphocerite relatively small, subrectangular, not reaching end of antennular peduncle, with straight lateral margin and rather narrow blade, latter not overreaching small distolateral tooth; carpocerite stout, cylindrical, reaching well beyond scaphocerite and end of antennular peduncle; flagellum long, longer than half of body, relatively thick, although thinner than lateral antennular flagellum ( Fig. 2A, B, G View FIGURE 2 ).

Mouthparts typical for family. Mandible with robust molar process; incisor process terminating in five cutting teeth, second-dorsal largest; palp with faint subdivision between narrow first (proximal) and expanded second (distal) article ( Fig. 3A View FIGURE 3 ). Maxillule with endopod (palp) bilobed, dorsal lobe furnished with slender seta, ventral lobe somewhat square-shaped, without seta; dorsal endite somewhat square-shaped, ventral endite with thick setae ( Fig. 3B View FIGURE 3 ). Maxilla with narrow scaphognathite; endopod simple, without setae; dorsal endite with moderately deep cleft; ventral endite short, with some elongate setae ( Fig. 3C View FIGURE 3 ). First maxilliped with dorsal and ventral endites fused; exopod with moderately expanded caridean lobe; endopod not clearly subdivided, furnished with row of setae mesially, with two long thick setae distally; epipod bilobed ( Fig. 3D, E View FIGURE 3 ). Second maxilliped with typically shaped endopod, propodal and dactylar articles distinct, forming apical “head”; exopod thickened in its proximal half; epipod rather small ( Fig. 3F View FIGURE 3 ). Third maxilliped slender, pediform; coxa with lateral plate subrectangular, almost ear-shaped; antepenultimate article slender, not twisted or markedly flattened; penultimate article about four times as long as wide; ultimate article slightly tapering distally, with numerous rows of short serrulate setae, tip with two robust spiniform setae; arthrobranch present, rather small ( Fig. 3G, H View FIGURE 3 ).

Chelipeds moderately enlarged, equal in size, symmetrical in shape, carried extended with dactylus in ventrolateral position ( Fig. 1 View FIGURE 1 ); basis and ischium short, robust, unarmed; merus short, stout, flattened ventrally, unarmed distally and on margins, distodorsal margin forming blunt angle; carpus wide, short, cup-shaped, mesial surface without setal rows; palm slightly enlarged, somewhat swollen, smooth, subcylindrical in cross-section; fingers short, about 0.6 length of palm, slightly gaping when closed, tips crossing; cutting edge of pollex with large tooth occupying about 0.4–0.6 of pollex length, fitting into broad distal hiatus on opposing dactylar cutting edge; cutting edge of dactylus with large tooth occupying about 0.2–0.3 of dacylar length, fitting into broad proximal hiatus on opposing cutting edge of pollex ( Fig. 4 View FIGURE 4 A–D).

Second pereiopod not particularly slender or elongate; ischium long, unarmed; merus much shorter than ischium; carpus with five subarticles, first longest but not longer than sum of others, ratio of subarticles approximately equal to 3.5/1.0/1.0/1.1/1.8; chela longer than distal-most carpal subarticle, simple, cutting edges of fingers furnished with rows of microscopic setae ( Fig. 5A, B View FIGURE 5 ).

Third pereiopod robust; ischium armed with one very stout, thick cuspidate seta on ventrolateral surface; merus about four times as long as maximal width, armed with two stout, thick cuspidate setae on ventrolateral surface; carpus slenderer than merus, about 0.6 times as long as merus, distoventral margin armed with one slender spiniform seta; propodus noticeably longer than carpus, ventral margin with two slender spiniform setae on proximal half and pair of spiniform setae near dactylar base; dactylus moderately slender, somewhat elongate, conical, simple, slightly curving distally, about 0.7 length of propodus ( Fig. 5C, D View FIGURE 5 ). Fourth pereiopod generally similar to third, except for armature of merus, latter armed with three stout, thick cuspidate setae on ventrolateral surface ( Fig. 5E View FIGURE 5 ). Fifth pereiopod somewhat slenderer than fourth pereiopod; ischium armed with one very stout, thick, flattened, distally truncate (possibly abraded) cuspidate seta on ventrolateral surface; merus about five times as long as maximal width, armed with four stout, thick, flattened, distally truncate (possibly abraded) cuspidate setae on ventrolateral surface; carpus slenderer than merus, about half-length of merus, distoventral margin armed with one spiniform seta; propodus much longer than carpus, ventral margin unarmed, one stout spiniform seta present on mesial side near dactylar base, distal half of propodus with rows of progressively longer serrulate setae on ventrolateral surface, forming grooming brush; dactylus moderately slender, somewhat elongate, conical, simple, more conspicuously curving distally, about half-length of propodus ( Fig. 5 View FIGURE 5 F–I).

First male pleopod with small endopod furnished with some setae ( Fig. 2H View FIGURE 2 ). Second male pleopod with appendix masculina much longer than appendix interna, furnished with numerous stiff setae apically and subapically, as illustrated ( Fig. 2I, J View FIGURE 2 ). Second female pleopod without appendix masculina, with appendix interna only.

Uropod with lateral lobe of protopod very broad, rounded; exopod narrowly ovoid, slightly truncate distally; distolateral tooth absent; diaeresis (transverse suture) straight, lateral section conspicuously thickened, elevated, armed with two very stout, thick spiniform setae; distal and distomesial margin with row of slender spiniform setae above marginal plumose setae; endopod more narrowly ovoid, distal and distomesial margin with row of slender spiniform setae above marginal plumose setae, less numerous than on exopod ( Fig. 2K, L View FIGURE 2 ).

Gill formula as given for genus (see above).

Colouration. Body semi-translucent, more or less evenly covered with large reddish spots (= groups of red chromatophores); yellow ovary visible through partial translucence of carapace in the female ( Figs. 6 View FIGURE 6 , 7 View FIGURE 7 ).

Etymology. The new species’ name is a combination of two Latinised Greek words pachy (from the ancient Greek pakhús = thick) and acanthus (from the ancient Greek akanthos —spine) and refers to the presence of unusually thick cuspidate setae on the third, fourth and fifth pereiopods, as well as two very thick spiniform setae on the lateral portion of the uropodal diaeresis; used as an adjective.

Ecology. Both specimens were collected from burrows of unknown hosts, on a shallow subtidal sand flat, about 30 m from the sandy shore and 50 m from a large rocky outcrop, at a depth of about 1–1.5 m at incoming tight. The sand flat is characterised by relatively fine, more or less compacted sand, with an abundance of fragmented coral rubble, and some muddier patches, larger pieces of rubble, and small living colonies of the coral Pocillopora damicornis (L.).

Remarks. The presence of cuspidate setae on the meri of the third to fifth pereiopods is not uncommon in alpheid shrimps, being also known in several so-called “lower” alpheid genera, such as Jengalpheops (monotypic), Potamalpheops (most if not all species) and Athanopsis Coutiere, 1897 (some species), but also in “higher” or more derived genera, such as Parabetaeus Coutière, 1897 (all species), Betaeus (most if not all species) and Betaeopsis (both species) (e.g., Powell 1979; Nomura & Anker 2001; Anker & Jeng 2002; Anker & Dworschak 2007; Anker 2012, 2015). However, in Pachelpheus pachyacanthus sp. nov., the cuspidate setae of walking legs are conspicuously thickened and attain truly unusual dimensions; in fact they are readily visible in the general view of the shrimps, especially on the merus of the fifth pereiopod ( Figs. 1 View FIGURE 1 , 6 View FIGURE 6 , 7 View FIGURE 7 ). The most robust of these cuspidate setae seem to be somewhat flattened and their sockets are relatively shallow, i.e. not accommodating them completely; in addition, those of the fifth pereiopod seem to be somewhat abraded ( Fig. 5G, H View FIGURE 5 ). Their function remains enigmatic, but it is possible that they are somehow involved in the shrimp’s clinging either to the host or to the burrow walls, and perhaps also in digging. It also remains unknown whether these setae are actually homologous to the more conventional cuspidate setae, which are conical, not flattened, never abraded, and lodged in deeper pits.

Pachelpheus pachyacanthus sp. nov. is presumably an infaunal symbiont (“commensal”), dwelling in burrows of yet unknown hosts. Other infaunal animals collected at the type locality of P. pachyacanthus sp. nov. include the snapping shrimps Alpheus cf. naos Anker, Hurt & Knowlton, 2007 and A. hephaestus Bracken-Grissom & Felder, 2014 , mud and ghost shrimps ( Upogebiidae , Axiidae ), pea crabs ( Pinnotheridae ), stomatopods ( Squillidae ), thalassematid echiurans, sipunculans, and a single galeommatid bivalve, indicating a highly diversified burrowing fauna (material under study). Interestingly, the single species of the genus Jengalpheops , which may be a close relative of Pachelpheus gen. nov., is an obligate symbiont of burrows of large ghost shrimps, Glypturus armatus (A. Milne- Edwards, 1870) ( Anker & Dworschak 2007).