Harryplax severus, Mendoza, Jose C. E. & Ng, Peter K. L., 2017

Mendoza, Jose C. E. & Ng, Peter K. L., 2017, Harryplaxseverus, a new genus and species of an unusual coral rubble-inhabiting crab from Guam (Crustacea, Brachyura, Christmaplacidae), ZooKeys 647, pp. 23-35 : 27-33

Harryplax severus sp. n. Figs 1 A–D, 2, 3

Type material.

Holotype ♀ (7.9 × 5.6 mm) (ZRC 2016.0253), Guam, Piti Reef margin, 1-1.5 m depth, in rubble; coll. H.T. Conley, September 1998; paratype ♂ (5.4 × 3.9 mm) (ZRC 2016.0254), Guam, Glass Breakwater near mouth of Apra Harbour, 4.6-7.6 m depth, among rocks; coll. H.T. Conley, 18 July 2001.

Comparative material.

Christmaplax mirabilis Naruse & Ng, 2014, holotype ♂ (11.0 × 7.9 mm) (QM W29223), stn. CI-D04, Christmas Island, Thundercliff Cave, coll. T. Naruse & Y. Fujita, 15 February 2012; paratype ♀ (11.3 × 8.3 mm) (ZRC 2014.0814), stn. CI-D07, same locality as holotype, coll. Y. Fujita & T. Naruse, 16 February 2012.

Description.

Carapace (Figs 1A, B, 2A) transversely subovate, 1.38-1.41 times as wide as long, dorsal surface slightly convex, mostly smooth but becoming more granulate at periphery, regions poorly defined; H-shaped gastric grooves barely discernible. Front well produced anteriorly, ventrally deflexed; frontal margin bilobed, lobes separated by wide V-shaped concavity, anterior margins slightly concave, mesial angles more produced than lateral. Supraorbital margin granulate, forming slightly obtuse angle with base of front, continuing uninterrupted into anterolateral margin (without notches); infraorbital margin much shorter, junction with supraorbital margin sunken; orbit small, laterally unarmed, infraorbital angle produced as large ridge-like tooth, mesial surface slightly concave for accommodating distal end of second antennular article when folded, tooth produced anteriorly beyond orbit when viewed dorsally. Exorbital angle not clearly marked in dorsal view. Anterior half of anterolateral margin, arcuate, cristate, lined with round granules; posterior half with two strong teeth with sharp, incurved apices, first tooth distinctly larger, more curved than second tooth which marks junction between antero- and posterolateral margins. Posterolateral margins almost straight, convergent posteriorly; surfaces covered with tiny granules. Suborbital, subhepatic, and pterygostomial regions covered with many fine and some larger granules; pterygostomial region with a granulate ridge anterior to Milne Edwards’ aperture. Epistome short, with medial transverse depression, posterior margin with a small median projection. Endostomial ridge strongly developed. Lateral margins of buccal cavern subparallel, slightly convergent anteriorly, concave.

Antennules (Figs 1B, 2A) well developed; basal antennular segment large, high, upper half forming cavity for second article; second and third articles long, can be partially retracted into antennular fossae, second article longer than third, which has distal end wider than proximal end; mesial surface of internal orbital tooth accommodating joint of second and third antennular articles when folded. Antennal basal article subrectangular, longer than wide, not completely filling orbital hiatus, barely mobile, subsequent two articles elongate, cylindrical; flagellum long, reaching first anterolateral tooth when folded laterally along anterolateral margin.

Eye (Figs 1A, B, 2A) reduced, immovable; visible from dorsal view of carapace, with small granules on dorsal surface; cornea discernible on rounded, bulbous distal end of eyestalk, visible from dorsal and anterior views.

Third maxillipeds (Figs 1B, C, 2B) forming narrow triangular median hiatus when closed; ischium long, median length about twice of that of merus, with shallow longitudinal sulcus; merus quadrate, anterior margin slightly concave, distolateral angle rounded, distal and mesial margins granulate; exopod slender, lateral margin convex, mesial margin with subdistal triangular tooth, flagellum long.

Male thoracic sternum (Fig. 3A, B) transversely narrow. Sternites 1 and 2 fused, triangular in outline, apex acute, lateral margins straight; sternite 2 separated from sternite 3 by distinct transverse suture; sternites 3 and 4 almost completely fused except for incomplete sutures laterally which continue mesially as shallow but distinct V-shaped groove; sternite 4 long, narrow, ratio of width (measured at lateral extremes of episternites) over length (measured as distance between tip of closed telson to center of suture 3/4) = 3.5; male pleonal locking mechanism (press-button) present as round tubercle on sternite 5 midway between sutures 4/5 and 5/6; sutures 4/5 and 5/6 interrupted medially; suture 6/7 fused medially, median marked by trapezoidal area that is less calcified than surrounding area of sterno-pleonal cavity, appearing like a ‘hole’; suture 7/8 fused medially and connecting with median line; sternite 7 widely exposed when pleon closed, wider than long; small portion of sternite 8 exposed between lateral edges of pleomeres 2 and 3 when pleon is pressed closed against thoracic sternum. Median line present on exposed region of sternite 4, absent within sterno-pleonal cavity except at level of sternites 7 and 8. Penis (Fig. 3C) protruding from gonopore on P5 coxa, anterior to coxo-sternal condyle. Female thoracic structure (Figs 1C, D) similar to that of male; vulva (Fig. 3H) on sternite 6, round, relatively large, anterior border contacting sture 5/6, operculum similarly round, no sternal projections.

Chelipeds (Figs 1A, 2 C–E) distinctly asymmetrical, right chelipeds more robust and specialized in examined material; no obvious sexual dimorphisms. Anterior margins of basis-ischium and merus of both chelipeds lined with sharp conical spines of varying length, upper margin of merus distinctly convex, cristate, lined with pointed granules, lower outer margin gently convex, cristate, granulate, terminating distally with granulate tuberosity. Carpus finely granulate, with strong, sharp, broadly triangular, lamellar tooth on inner margin (larger in major cheliped). Major chela wide, inflated; external and internal surfaces mostly smooth; upper surface weakly granulate, inner margin of upper surface carinate, with transverse concavity immediately beneath it on inner surface; lower margin lined with sharp granules. Fingers stout, fixed finger shorter than movable finger, almost straight except for upcurved tip, cutting margin with large, proximal molariform tooth with flattened occlusal surface, followed distally by 2 smaller cutting teeth, lower margin armed with large conical granules; movable finger gently curved downwards, with large, but eroded, flat tooth proximally, followed distally by 1 cutting tooth. Minor chela relatively narrower, more granulate, fingers less robust, without molariform teeth, lower margin with conical spines.

Ambulatory legs (Figs 1A, 2F, G) long, slender, sparsely setose; P4 longest, combined merus-to-dactylus length 1.57-1.60 times carapace width, merus approximately five times as long as wide; P2 and P5 shortest. Meri flattened, margins lined with sharp granules, which are much smaller on P5; other articles unarmed, except for propodus of P2 which is lined with small sharp granules; carpus subcylindrical, curved proximally; propodus straight, flattened; dactylus subcylindrical, tapering distally, terminating in sharp curved claw, flexor margins with row of closely packed short stiff setae.

Male pleon (Fig. 3D) with all somites and telson free; somites 1 and 2 short; somite 1 partially concealed under posterior margin of carapace, with transverse ridge; somite 3 widest, subsequent somites trapezoidal, progressively narrowing, producing a combined lateral margin that is concave; telson subtriangular, lateral margins convex, apex rounded. Female pleon longitudinally oval with all somites and telson free; relatively narrow, somite 3 widest, telson wider than long; in female holotype, pleopods developed, setose throughout entire length.

G1 (Fig. 3E) slender, slightly sinuous, unarmed, mesial and lateral margins each with row of stiff simple setae towards distal end; aperture terminally placed. G2 (Fig. 3F) about one-third length of G1, distal segment petaloid in shape.

Etymology.

The specific epithet, severus (L., harsh, rough, rigorous), alludes to the rigorous and laborious process by which this crab was collected. It is also an allusion to a notorious and misunderstood character in the Harry Potter novels, Professor Severus Snape, for his ability to keep one of the most important secrets in the story, just like the present new species which has eluded discovery until now, nearly 20 years after it was first collected. The name is used here as a noun in apposition.

Remarks.

Harryplax severus sp. n., shares some key morphological features with Christmaplax mirabilis which are presumably adaptations to a stygobitic lifestyle (i.e. reduced eyes, well-developed antennules and antennae, and long, slender ambulatory legs), which in turn suggests that the environmental conditions under which the former thrives are probably similar to the conditions in underwater caves. Harryplax severus is clearly a chalicophilous species, as it was collected deep in coral rubble or under subtidal rocks. It is also possible that it is a cavity dweller, or coelobite (viz. Choi and Ginsburg 1983; Choi 1984; Takada et al. 2007, 2008), living within the interstices of coral rubble and rocks. It is clearly a part of the poorly known coral reef cryptofauna and its reclusive nature accounts for its rarity and absence in conventional reef surveys. The new species is only known from the type locality, Guam, thus far.