Heteroptychus nautilus Baba & Wicksten 2019, n. sp.
Creators: name: Baba, Keiji
name: Wicksten, Mary K.
Publisher: name: Zenodo
Year: 2019
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Chirostylidae
Heteroptychus
Heteroptychus nautilus

description: Heteroptychus nautilus n. sp.
(Figs. 7, 8, 17B)
Type material. CDF NA064-009-01-01-A, holotype, ovigerous female (CL 5.2 mm), ROV Dive H1435, East Wolf Seamount, 1°12.928'N, 91°5.5395'W, 1049.4 m, with bamboo coral Isididae (Alcyonacea), 27 June 2015.
Description. Carapace: 0.9 × as long as broad, greatest breadth 2.1 × distance between anterolateral spines. Dorsal surface polished, moderately convex from anterior to posterior, without depression between gastric and cardiac regions, ridged along posterior half of lateral margin, greatest breadth measured at posterior fourth. Lateral margins convexly divergent posteriorly; anterolateral spine well developed, horizontal, directed anteromesially; smooth without any process and protuberance. Rostrum 0.8 × as long as broad, directed somewhat antero-ventrally, with lateral margins distinctly concave; distally blunt, bearing setae, with interior angle of 20°, length 0.3 × postorbital carapace length, breadth less than half that of carapace measured at posterior margin; dorsal surface flattish but feebly concave basally. Lateral limit of orbit feebly angular, without distinct spine. Pterygostomian flap very low on posterior half (posterior height 0.35 × anterior height), anteriorly produced to sharp spine; anterior surface well inflated (convex from dorsal to ventral), with 2 low, blunt ridges on midline.
Thoracic sternum: Excavated sternum with triangular anterior end followed by tiny spine on rounded longitudinal ridge in midline. Sternal plastron 0.6 × as long as broad, lateral extremities divergent posteriorly, sternite 6 nearly as broad as sternite 7. Anterior margin of sternite 3 deeply excavated in semicircular shape, with small median notch; submedian spines absent.
Abdomen: Smooth, polished. Somite 1 tergite gently convex from anterior to posterior. Somite 2 tergite 2.1 × broader than long; pleural lateral margins moderately concave, strongly divergent posteriorly, ending in rounded margin. Pleura of somites 3–4 ending in rounded margin. Telson 0.4 × as long as broad; posterior plate slightly concave on posterior margin, 1.2 × longer than, and 0.8 × as broad as anterior plate.
Eyes: 1.4 × as long as broad. Cornea as long as remaining eyestalk.
Antennule and antenna: Ultimate article of antennular peduncle 3.9 × longer than high. Antennal peduncle slightly overreaching apex of rostrum. Antennal scale fused with article 2, 1.3 × broader than article 5, slightly falling short of midlength 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 16 or 17 segments.
Mxp: Mxps 1 with bases moderately separated. Mxp 3 basis with obsolescent distal denticle on mesial ridge; ischium crista dentata with obsolescent denticles; merus 2.2 × longer than ischium, relatively thick mesio-laterally, unarmed, with rounded ridge along flexor margin; extensor and flexor margins subparallel; carpus unarmed.
P1: 7.4 × carapace length, barely setose except for fingers. Ischium with basally broad, distally blunt distodorsal process and smaller proximal dorsal process overhanging basis. Merus with short distodorsal spine mesially, smooth elsewhere, length 1.6 × that of carapace. Carpus 0.8 × as high as broad, 1.5 × longer than merus, with row of 5 or 6 obsolescent spines along mesial margin and distinct distodorsal spine mesially. Palm 6.1 × longer than broad, 0.6 × as high as broad, 0.9 × as long as carpus; mesial margin roundly ridged, not cristiform, with 5 or 6 obsolescent spines. Fingers slender, gaping in proximal half, opposable margins straightly fitting to each other in distal half when closed, distally spooned; movable finger 0.4 × length of palm, with prominent, blunt process at proximal third, distal to position of opposite lower process on fixed finger.
P2–4: Long setae on carpi, thickly setose along prehensile margins of propodi and dactyli. P2 merus slightly longer than P3 merus, P4 merus 0.5 × as long as P3 merus; length-breadth ratio, 6.5 on P2, 6.3 on P3, 3.2 on P4; P2 merus as long as carapace, 1.4 × length of propodus. P2 carpus slightly longer than P3 carpus, 1.1 × longer than P2 propodus; P4 carpus one-third length of P3 carpus, 0.4 × length of P4 propodus. Propodi 1.4 (P2), 1.5 (P3), 1.4 × (P4) longer than dactyli, flexor margins concavely curving in lateral view, unarmed. Dactyli 0.6 × (P2 and P3), 1.8 × (P4) length of carpi; flexor margin with row of 16 or 17 sharp spines proximally diminishing and nearly perpendicular to margin, ultimate and penultimate spines subequal or ultimate slightly larger.
Eggs: Number of eggs carried, 27; size, 1.21 × 1.40 mm to 1.26 × 1.48 mm.
Coloration: Overall seashell pink, anterior part of carapace reddish.
Host: Collected with bamboo coral Isididae (Alcyonacea).
Etymology. The specific name is a noun in apposition, taken from the E/V Nautilus by which the species was collected.
Remarks. The specimen is apparently in the process of molting, as is observed on some of the flexor spines on the P2–4 dactyli, which are not demarcated at their bases. In all known species of Heteroptychus as well as all the other genera of the Chirostylidae, all the spines are basally demarcated from the flexor margin of the dactylus.
Heteroptychus nautilus n. sp. resembles H. claudeae Baba, 2018 known widely from the western Pacific in having the antennal scale fused with article 2 and in having a smooth carapace lateral margin. In H. nautilus, the carapace is longer relative to breadth (0.9 versus 0.7 ×), the posterior part of the pterygostomian flap is higher (posterior height/anterior height, 0.35 versus 0.1); the anterolateral spine of the carapace is more remote from the lateral limit of the orbit or lateral orbital spine when viewed dorsally (this distance is fully as large as instead of distinctly smaller than the basal breadth of the anterolateral spine). Heteroptychus nautilus n. sp. is distinguished from H. galapagos n. sp. by the absence of tubercles on the carapace lateral margin and the anterolateral spine directed anteromesially and horizontally, instead of straight forward and dorsally.
Distribution. East Wolf Seamount, Galapagos Islands at 1049.4 m depth.
description: Published as part of Baba, Keiji & Wicksten, Mary K., 2019, Chirostyloidean squat lobsters (Crustacea: Decapoda: Anomura) from the Galapagos Islands, pp. 391-421 in Zootaxa 4564 (2) on pages 400-403, DOI: 10.11646/zootaxa.4564.2.5, http://zenodo.org/record/2589236