Uroptychus minutus Benedict, 1902

Baba, Keiji & Wicksten, Mary, 2015, Uroptychus minutus Benedict, 1902 and a closely related new species (Crustacea: Anomura: Chirostylidae) from the western Atlantic Ocean, Zootaxa 3957 (2), pp. 215-225 : 216-220

Uroptychus minutus Benedict, 1902

( Figs. 1–3 )

Uroptychus minutus Benedict, 1902: 296 , fig. 39.— Chace, 1942 10 (key).

Type material. Syntypes: 1 male (2.2 mm), 4 ov. female (1 ov. 1.9 mm; mouthparts missing on right side), “Albatross” Sta. St 2120, off Trinidad, 73 fms [134 m] ( USNM 7833)

Description. Small species. Carapace: 0.8 × as long as broad; greatest breadth 1.4–1.6 × distance between anterolateral spines. Dorsal surface unarmed and barely setose, moderately convex from side to side, slightly or somewhat so from anterior to posterior, without depression. Lateral margins divergent posteriorly to point one-fifth from posterior end, with 6 spines: first anterolateral, relatively small, subequal to or slightly smaller than lateral orbital spine, directed anteriorly, reaching or slightly falling short of tip of lateral orbital spine; second to sixth also small (those on branchial margin slightly larger), nearly equidistant from one another, second somewhat posterior to midpoint between, and, somewhat dorsal to level of, first and third; last spine situated at proximal fifth of lateral margin. Rostrum straight horizontal or directed slightly ventrally, broad triangular (1.0–1.2 × broader than long; in ovigerous female 1.8 mm, 0.9 ×), with interior angle of 22–30°, ending in sharp tip; dorsal surface concave; lateral margin smooth, slightly concave; length 0.6–0.7 × that of remaining carapace, breadth 0.7 × length of posterior carapace margin. Lateral orbital spine slightly larger than, situated slightly anterior to level of, anterolateral spine of carapace, directed anteriorly. Pterygostomian flap anteriorly angular, produced to small spine, smooth on surface, posterior portion about as high as anterior portion when measured between linea anomurica and ridge along ventral margin.

Sternum: Sternites 1–3 triangular between bases of Mxp1, surface with sharp ridge in midline. Sternal plastron 0.7–0.8 × as long as broad, lateral extremities divergent posteriorly between sternites 4–7. Sternite 3 slightly depressed from level of sternite 4 in ventral view; anterior margin medially subtriangular, flanked by deep concavity, lacking pair of median spines and median sinus, laterally sharp angular. Sternite 4 having anterolateral margin slightly convex and anteriorly angular; posterolateral margin half as long as anterolateral margin. Anterolateral margins of sternite 5 convex, 1.5 × longer than posterolateral margin of sternite 4.

Abdomen: Almost glabrous. Somite 1 weakly convex from anterior to posterior. Somite 2 tergite 3.5 × (male), 2.2–2.6 × (females) broader than long; pleural lateral margins strongly concave, moderately divergent laterally, posterolaterally angular. Pleura of somites 3–5 with rounded lateral margin. Telson half as long as broad; posterior plate 0.8 × length of anterior plate, posterior margin very feebly concave, not emarginate.

Eye: 1.3–1.8 × longer than broad, slightly narrowed distally, slightly overreaching rostral tip; lateral margins slightly diverging proximally. Cornea 0.4–0.5 × length of remaining eyestalk, not dilated.

Antennule and antenna: Ultimate article of antennular peduncle 3.0–3.1 × longer than high. Antennal peduncles not reaching rostral tip. Article 2 with distinct lateral spine. Antennal scale 1.4–1.7 × broader than article 4; length variable, from ending in point distal third of article 5 to tip of that article. Article 4 with very small or obsolescent distomesial spine. Article 5 with distinct distomesial spine, 1.4–1.6 × longer than article 4, breadth 0.5 × height of ultimate article of antennule. Flagellum of 4 segments, much shorter than articles 4 and 5 combined, terminating at most at rostral tip.

Mxp: Mxp1 with bases broadly separated. Mxp3 basis without distinct denticle on mesial ridge. Ischium flexor margin distally produced to small, distinct spine; crista dentata with about 20 denticles. Merus relatively narrow, about 1.4 × length of ischium, with distolateral spine; flexor margin unarmed. Carpus with distolateral spine, and 2 very small spines visible or invisible on proximal part of extensor margin.

P1: With sparse long setae. Ischium with sharp dorsal spine, ventromesial margin with distinct subterminal spine. Merus 1.1 × length of carapace (male), with several spines in addition to 4 distal marginal spines (1 dorsal at distal third, 1 mesiodorsal around midlength, 2 mesial equidistant between proximal and distal ends, 1 or 2 mesioventral on distal half), and 5 spines on distal margin including ventromesial spine. Carpus slightly narrowed proximally, 0.90–0.96 × length of merus; with 1 dorsal spine at midlength (proximally followed by 2 additional smaller spines in male), 2 or 3 spines on mesial margin, and 6 terminal spines (3 dorsal, 1 mesial, 1 lateral, 1 mesioventral). Palm slightly broader at midlength, equally broad at distal and median parts or gently narrowed proximally, 2.6 × (male), 2.3–2.7 × (females) longer than broad, 1.5 × (male), 1.3–1.5 × (females) length of carpus, moderately depressed (height 0.7 × breadth), mesial margin roundly ridged; proximally with denticle-like small dorsal spine, smooth elsewhere. Fingers incurved distally, crossing when closed, not gaping in both sexes; movable finger 0.5–0.6 × as long as palm, opposable margin with low, blunt process at proximal third; opposable margin of fixed finger with no distinct process.

P2–4: Sparsely with long setae, relatively thick mesio-laterally (0.8 × thicker than high). Ischia with 2 very small dorsal spines. Meri successively shorter posteriorly; P2 merus 0.9–1.0 × length of P2 propodus; P3 merus 0.7–0.8 × length of P3 propodus; P4 merus 0.7–0.8 × length of P4 propodus; length-breadth ratio, 3.0–3.2 on P2, 2.7–2.8 on P3, 2.4–2.5 on P4; dorsal margins with 3 equidistant spines on P2, 3 spines on P3, unarmed on P4 (where present, distalmost spine situated at distal end; ventral margin with terminal spine only). Carpi with 1 or 2 small distal extensor spines; carpus-propodus length ratio, 0.4 on P2–4, carpus-dactylus length ratio, 0.7–0.9 on P2, 0.7–0.8 and P3, 0.7 on P4. Propodi 1.5–2.1 × longer than dactyli on P2, 1.7–1.8 × on P3, 1.6 × on P4; flexor margin nearly straight, with pair of slender terminal spines preceded by 4, 3, 2 or 3 spines on P2, P3, P4, respectively. Dactyli proportionately broad; flexor margin feebly concave in lateral view, with 9 (rarely 10) sharp, very slightly inclined spines, ultimate spine shorter and more slender than penultimate, antepenultimate separated from penultimate by its basal breadth.

Eggs. Number of eggs carried, 2–8; size, 0.54 × 0.60 mm –0.50 × 0.52 mm (7 eggs), 0.59 × 0.78 mm –0.72 × 0.61 mm (2 eggs), 0.56 × 0.50 mm – 0.56 × 0.54 mm (8 eggs).

Remarks. The P2–4 (walking legs), all detached from the body, are presumed to be as such in the above description. The only character to determine which is which pereopod is the length-breadth ratio of the merus, and possibly the flexor marginal spines, which may be four on P2 and fewer on P3 and P4. The seven pereopods not illustrated here cannot be determined by their position, so the actual morphological variations cannot be shown in the description.

It is worth noting that sternite 3 in U. minutus has an unusual anterior margin that is medially triangularly produced. To the best of our knowledge, this shape is possessed by U. marissae n. sp. (see below) and U. bellus Faxon, 1893 (confirmed by examination of the type material). In the other known species including Indo-Pacific species the anterior margin is excavated, with or without a pair of median spines and/or a notch.

The excavated sternum (sternites 1–3) is sharply ridged in the midline like those of Uroptychodes Baba, 2004 from the Indo-West Pacific.

Figures

FIGURE 1. Uroptychus minutus Benedict, 1902, syntypes (USNM 7833). A, E, male 2.2 mm; B, C, D, G, H, J, K, ovigerous female 2.0 mm; F, I, ovigerous female 1.9 mm. A, carapace and anterior part of abdomen, dorsal. B, same, dorsal. C, same, lateral. D, sternal plastron, with sternites 1 – 3 and basal parts of Mxp 1. E, right pleura of abdominal somites 2 – 4, dorsolateral. F, left pleura of abdominal somites 2 – 4, dorsolateral. G, telson. H, left antenna, ventral. I, right antenna, ventral. J, right Mxp 3, ventral. K, same, lateral. Scale bars: A – I, 1 mm; J, K, 0.5 mm.

FIGURE 2. Uroptychus minutus Benedict, 1902, syntypes (USNM 7833), A – D, presumably male P 1. E, F, presumably female P 1. A, dorsal. B, mesial. C, ventral. D, lateral. E, F, dorsal. Scale bar: 1 mm.

FIGURE 3. Uroptychus minutus Benedict, 1902, syntypes (USNM 7833). Detached pereopods (walking legs). A, B, presumably P 2. C, D, presumably P 3. E, F, presumably P 4. Scale bar: 1 mm.