Heteroptychus galapagos, Baba & Wicksten, 2019

Baba, Keiji & Wicksten, Mary K., 2019, Chirostyloidean squat lobsters (Crustacea: Decapoda: Anomura) from the Galapagos Islands, Zootaxa 4564 (2), pp. 391-421 : 397-400

Heteroptychus galapagos n. sp.

( Figs. 5 , 6 )

Type material. CDF NA064-031-01-01-A, holotype, ovigerous female (CL 6.1 mm), ROV Dive H1436, East Darwin Seamount, 1°40.5'N, 91°41.2'W, 1012 m, 28 June 2015.

Description. Carapace: 0.8 × as long as broad, greatest breadth 1.9 × distance between anterolateral spines. Dorsal surface polished, somewhat convex from anterior to posterior, without depression between gastric and cardiac regions, ridged along posterior half of lateral margin, greatest breadth measured at posterior third. Lateral margins convexly divergent posteriorly; anterolateral spine well developed, directed anterodorsally and straight forward; smooth except for 2 small, distinct protuberances, one at anterior end of anterior branchial margin, another at anterior end of posterior branchial margin. Rostrum slightly overreaching apex of eye, 0.7 × as long as broad, distinctly concave on lateral margin; distally blunt and setose, with interior angle of 22°, length one-quarter postorbital carapace length, breadth less than half (0.4) that of carapace measured at posterior margin; dorsal surface flattish but feebly concave basally, horizontally directed straight forward. Lateral limit of orbit rounded, without spine. Pterygostomian flap with posterior height 0.4 × anterior height, anteriorly produced to sharp spine; anterior surface well inflated (convex from dorsal to ventral), with 2 low ridges in midline.

Thoracic sternum: Excavated sternum with triangular anterior end followed by rounded longitudinal ridge in midline. Sternal plastron 0.6 × as long as broad, lateral extremities divergent posteriorly, sternite 6 as broad as sternite 7. Posterior margin strongly concave. Anterior margin of sternite 3 deeply excavated in semicircular shape, with small, shallow median notch, lacking submedian spines.

Abdomen: Smooth, polished. Somite 1 tergite gently convex from anterior to posterior. Somite 2 tergite 2.0 × broader than long; pleural lateral margins moderately concave, strongly divergent posteriorly, with blunt terminus. Pleura of somites 3–4 ending in rounded margin. Telson 0.4 × as long as broad; posterior plate slightly concave on posterior margin, 1.1 × longer than, and 0.8 × as broad as anterior plate.

Eyes: Ovate, 1.7 × as long as broad. Cornea 0.8 × as long as remaining eyestalk.

Antennule and antenna: Ultimate article of antennular peduncle 4.1 × longer than high. Antennal peduncle overreaching apex of rostrum by half length of article 5. Antennal scale fused with article 2, 1.2 × broader than article 5, terminating in distal quarter of article 4. Distal 2 articles unarmed; article 5 1.4 × longer than article 4, breadth 0.5 × height of ultimate antennular article. Flagellum of 18 segments.

Mxp: Mxps 1 with bases widely separated. Mxp 3 basis with distal denticle on mesial ridge; ischium crista dentata with 5 widely spaced denticles; merus 2.6 × longer than ischium, relatively thick mesio-laterally, unarmed, with rounded ridge along flexor margin; extensor and flexor margins subparallel; carpus unarmed.

P1: Left P1 missing, right P1 with distal part missing, presumably taken for tissue sample. Ischium with basally broad, distally blunt distodorsal process and proximal dorsal lobe-like process. Merus with short distodorsal and smaller distoventral spine mesially, length 1.7 × that of carapace. Carpus 0.8 × as high as broad, 1.4 × longer than merus, with row of 8 small spines along mesial margin, and 4 spines on distal margin at juncture with palm.

P2–4: Long setae on propodi, thickly setose along prehensile margins of propodus and dactylus. P2 and P3 subequal. P4 merus 0.4 × as long as P3 merus; length-breadth ratio, 7.5 on P2 and P3, 3.6 on P4; P2 merus 1.1 × carapace length, 1.4 × length of propodus. P3 merus 1.4 × length of P3 propodus. P4 merus 0.7 × length of P4 propodus. P2 and P3 carpi 3.1 × longer than P4 carpus, 1.2 × longer than P2 and P3 propodi; P4 carpus 0.4 × length of P4 propodus. Propodi 1.6 (P2 and P3), 1.4 × (P4) longer than dactyli, flexor margins concavely curving in lateral view, unarmed. Dactyli 0.5 × (P2 and P3), 1.7 × (P4) length of carpi; flexor margin with row of 15 or 16 sharp spines proximally diminishing and nearly perpendicular to margin, ultimate and penultimate spines subequal.

Eggs: Number of eggs carried, 36; size, 1.32 × 1.40 mm to 1.45 × 1.14 mm.

Coloration: Information not available.

Etymology. Named for the type locality of the species; used as a noun in apposition.

Remarks. Only two specimens referable to this genus are identified as new species, H. galapagos and H. nautilus (see below). These species share a non-articulated antennal scale, which links them to H. lemaitrei Baba, 2018 and H. claudeae Baba, 2018 , both from the western Pacific.

Heteroptychus galapagos n. sp. differs from H. lemaitrei Baba, 2018 in having two instead of only one protuberance on the carapace lateral margin, in having the rostrum horizontally directed straight forward rather than dorsally directed, in having the anterolateral spine of the carapace directed straight forward and dorsally instead of anteromesially and horizontally, in having the lateral orbital angle rounded instead of bearing a distinct spine, and in having the pterygostomian flap surface with two distinct ridges instead of being smooth without ridges.

Heteroptychus claudeae has no protuberance on the carapace lateral margin, the anterolateral spine of the carapace closer to the lateral limit of the orbit, the rostrum more or less depressed distally rather than spiniform, the pterygostomian flap much lower in the posterior half (posterior height/anterior height, 0.1 versus 0.4).

Morphological differences between H. galapagos and H. nautilus are outlined under the remarks of H. nautilus (see below).

Distribution. East Darwin Seamount, Galapagos Islands at 1012 m depth.

Figures

FIGURE 5. Heteroptychus galapagos n. sp., holotype, ovigerous female (CL 6.1 mm), CDF NA064-031-01-01-A. A, carapace and anterior part of abdomen, dorsal. B, carapace, lateral. C, anterolateral part of carapace showing pterygostomian flap. D, sternal plastron, with excavated sternum and basal parts of Mxp1. E, telson. F, left antenna, ventral. G, right Mxp3, ventral. H, same, lateral. Scales = 1 mm.

FIGURE 6. Heteroptychus galapagos n. sp., holotype, ovigerous female (CL 6.1 mm), CDF NA064-031-01-01-A. A, right P1, dorsomesial. B, same, proximal part, lateral. C, same, distal articles omitted, ventral. D, same, proximal articles omitted, mesial. E, left P2, lateral. F, same, distal part, setae omitted. G, left P3, lateral. H, same, distal part. I, left P4, lateral. J, same, distal part. Scales = 1 mm.