Onychatrium, Bruce, Niel L. & Cumming, R. L., 2015

Bruce, Niel L. & Cumming, R. L., 2015, A new genus of Stenetriidae Hansen, 1905 (Asellota: Isopoda: Crustacea) from the Great Barrier Reef, Australia and the southwestern Pacific, Zootaxa 3941 (4), pp. 485-508 : 486-488

Onychatrium gen. nov.

Type species: Onychatrium forceps sp. nov., here designated.

Species included. Onychatrium forceps sp. nov., type species and Onychatrium torosus sp. nov., both from the Great Barrier Reef; Onychatrium entale ( Nordenstam, 1946) comb. nov., Tapateuen (=Tabiteue Island), Gilbert Islands, Kiribati; and Onychatrium thomasi ( Bolstad & Kensley, 1999) comb. nov. Madang, Papua New Guinea. Onychatrium echiurum ( Nobili, 1906) comb. nov., Tumaotu Islands, Eastern French Polynesia, belongs to the genus, but as there is no type material for Nobili’s specimens ( Nobili 1906; see Bruce & Buxton 2013), and the description and figures are minimal; the species is here regarded as species inquirenda.

Diagnosis. Cephalon lateral margins smooth, lateral spines acute, antennal spines sub-equal in length to lateral, acute; antennal spines about as long as pseudorostrum. Pseudorostrum trapezoid, wider than long. Male pereopod 1 merus quadrate with inferodistal process; carpus inferodistal process triangular, projecting at approximately 45° to axis of merus–propodus; propodus elongate (length at least 1.6–2.5 times longer than proximal width), propodal palm transverse or oblique, narrow, 0.4–0.6 as wide as propodus width, with toothed lobe bearing 3–5 teeth, with large terminal RS; palm with large inferior process, curved distally or straight, tapering to rounded apex with terminal RS; dactylus elongate, 4.7–7.3 as long as proximal width, 1.1–1.4 length of propodus; superior margin distal one-third densely setose, inferior margin with 15–18 regularly-spaced robust setulate setae. Male pleopod 2 appendix masculina widest at mid-length, distally narrowed, sub-acuminate, without setae; lateral margin with 0 or 1 proximally directed spines.

Description of male. Eyes large (more than 12 ommatidia), reniform. Body dorsal surface smooth or sparsely setose, widest at pereonite 1; pereonite 1 length 0.8–1.6 times pereonite 2 length; pereonites 2–4 lateral margins concave to straight, anteriorly acute. Pleotelson length subequal to width; lateral margins smooth, sub-parallel, posterolateral spines prominent, margin posterior to spines rounded with weak apical point; dorsal surface smooth, not setose.

Antenna length equal or longer than total body length, article 1 lateral spine absent.

Pereopod 1 basis superior margin with regularly spaced setae along length, superior submarginal ridge with row of setae. Merus inferior submargin with a dense patch of long setae, superodistal margin not produced, densely setose, with long setae. Carpus inferodistal margin produced. Propodus superior side robust, superior margin with submarginal row of long setae; inferior margin with submarginal row of long setae, both laterally and mesially.

Pleopod 1 protopod rectangular, lateral margin setae present; rami lateral margins evenly convex. Pleopod 2 protopod 3 times longer than wide, proximal lobe extending mesially c 1.6–1.7 medial width; distal apex sharply pointed; endopod length 0.4–0.5 protopod length. appendix masculina lateral margin groove present.

Female. Generally similar to male but for primary sexual characters. Pereopod 1 basis submarginal ridge with regularly-spaced short setae, 1 long seta. Ischium superodistal margin produced, apex blunt with 1 large, long seta. Merus superodistal margin produced, process sub-triangular, with dense long setae mesially, inferodistal margin not produced. Carpus inferodistal margin produced. Propodus widest distally; superior side robust, with central oblique area of steep narrowing, inferior side a wide, thin blade; inferior margin densely setose, with a row of setulate setae and a row of simple setae. Propodal palm straight, slightly oblique; width c equal to inferior margin length, 0.6 propodus maximum width; with 5–8 teeth increasing in size towards inferior end; inferior end of palm with single robust flagellated seta and single robust serrate seta adjacent to first tooth.

Pleopod 2 lateral margins parallel or becoming closer together distally, with 3 setae; apex with 1–2 setae, inferior surface sparsely setose.

Remarks. Oynchatrium gen. nov. is characterised by the putative apomorphies of the male pereopod 1 propodus having a prominent, elongate and distally acute process arising from the proximal inferior margin together with an elongate dactylus, the tip of which extends well beyond the propodal process; the palm is narrow, with a weakly to strongly toothed lobe; three of the four species have a prominent, triangular and distally acute inferior carpal process that projects strongly away from the axis of the propodus at an angle of approximately 45°; the merus of all species has a slender inferodistal process that is variable in size between species; pleopod 2 appendix masculina is widest at mid-length, with a narrowly rounded sub-acuminate apex that lacks a setose fringe.

The female pereopod 1 of Onychatrium has a single serrate robust seta on the palm, positioned adjacent to the large robust seta at the inferodistal angle of the palm ( Fig 4 F). All species of Hansenium Serov & Wilson, 1995 and Machatrium Bruce & Buxton, 2013 (where illustrated) have a series of submarginal serrate setae on the female palm, with the exception of Machatrium thungku Bruce & Buxton, 2013 , which has none.

Onychatrium gen. nov. belongs to a group of stenetriid genera that is characterised by having a trapezoid pseudorostrum. The principle differentiating and diagnostic characters that separate Onychatrium from Hansenium Serov & Wilson, 1995 , Machatrium Bruce & Buxton, 2013 and Stenetrium Haswell 1881 are presented in Table 1 . Onychatrium , Hansenium and Machatrium typically have a prominent carpal process on pereopod 1. In Onychatrium the process is triangular in profile and projects at an approximate angle of 45° to the meral–propodal axis, whereas in Machatrium the carpal process extends at a weak angle (approximately 10° to axis) along the propodus; in Hansenium the process is serrate and is not triangular.

Liocoryphe Serov & Wilson, 1995 has a prominent spoon-like carpal process on the male pereopod 1. This genus is clearly distinguished from Onychatrium gen. nov. as well as Hansenium and Machatrium , in having small eyes (4–6 vs 12 or more ommatidea), the head lacking antennal and lateral spines, the dorsum lacking chromatophores and the pseudorostrum is anteriorly rounded, not trapezoid.

Onychatrium torosus sp. nov. has a weak carpal process, a relatively long meral process and also has an elongate merus. In the context of the stenetriid genera these are significant differences from the other congeneric species but the diagnostic propodal process, elongate dactylus and appendix masculina morphology permits the species to be held in this combination.

Species of Onychatrium gen. nov. are largely and most readily separated by differences in the morphology of the male pereopod 1. There are also subtle difference in the proportions of the cephalic spines, and the shape of the space between the antennal and lateral spine. The shape and size of the various processes and lobes on the male pereopod 1 develop with maturity, and this is illustrated in Figure 10 for the three species for which there are multiple specimens.

Distribution. The genus is known from the southwestern Pacific, and has been recorded only from coral-reef habitats: Great Barrier Reef, Queensland; Madang, Papua New Guinea ( Bolstad & Kensley 1999); Tuamotu Islands, French Polynesia ( Nobili 1906, 1907) and Kiribati ( Nordenstam 1946).

Etymology. Derived from the Greek ‘onychos’ (onyx) meaning talon or claw, combined with ‘– trium’ indicating family affinity; gender neuter.

Character Onychatrium gen. nov. Hansenium Machatrium Stenetrium 1. Antenna (A2), article 1 without lateral spine without lateral without lateral spine with lateral spine

spine

Figures

FIGURE 4. Onychatrium forceps sp. nov. A, B, male holotype (MTQ W 34194), A, pereopod 1, B, enlargement of pereopod 1 dactylus; C, D, male paratype (MTQ W 33826), C, pereopod 2, D, pereopod 7; E – H, female paratype (MTQ W 34196), E, pereopod 1, F, enlargement of pereopod 1 dactylus, G, pereopod 2 carpus and dactylus, H, pereopod 7 merus, carpus and dactylus.

FIGURE 10. Onychatrium male pereopod 1 variation—comparison between species, developmental stages and locations. Onychatrium entale (Nordenstam, 1946) comb. nov. holotype (SMNH 798). Onychatrium forceps sp. nov., A, paratype (MTQ W 31594), B, paratype (MTQ W 32851), C, holotype (MTQ W 34194), D, paratype (MTQ W 34196), E, paratype (MTQ W 34198). Onychatrium torosus sp. nov., A, holotype (MTQ W 34204), B, paratype (MTQ W 14076), C, paratype (MTQ W 34206), D, paratype (MTQ W 32760). Onychatrium thomasi (Bolstad and Kensley, 1999) comb. nov., A – C, topotypes (MTQ W 19749), D, topotype (MTQ W 19762).