CHASMOCARCINIDAE Serène, 1964

Davie, Peter J. F., 2013, A new species of Megaesthesius Rathbun, 1909 (Crustacea: Brachyura: Chasmocarcinidae) from deep water off Western Australia, Zootaxa 3647 (3), pp. 470-478 : 471-473

CHASMOCARCINIDAE Serène, 1964

MEGAESTHESIINAE Štev č i ć, 2005

Megaesthesius Rathbun, 1909

Megaesthesius Rathbun, 1909: 112 ; 1910: 344.—Tesch, 1918: 202 (key).—Sakai, 1939: 577; 1976: 552.—Serène, 1964: 175; 1968: 92 (list).—Takeda & Miyake, 1969: 460; 1972: 86.—Takeda, 1973: 54.

Type species: Megaesthesius sagedae Rathbun, 1909 , by monotypy; gender masculine.

Diagnosis. Carapace quadrangular, rounded anteriorly; lateral margins almost parallel; anterolateral margins forming regular curve with anterior margin. Front narrow, bilobed. Orbits long, transverse, occupying whole anterior border. Eyestalks very long, embedded in orbits; swollen proximally, tapering distally; cornea elongated, distally bent posteriorly, slightly protruding laterally, pigment reduced to speck. Antennules thick, cannot be folded because swollen basal segment fills fossa; palp with long thick plumose setae. Buccal cavern anteriorly narrowed, outer borders arcuate. Third maxillipeds with merus subtriangular; palp short, articulated apically. Chelipeds subequal, shorter than ambulatory legs. Ambulatory legs slender, with spinules on borders. Male abdomen with somites 3–5 fused.

Included species: Megaesthesius sagedae Rathbun, 1909 ; M. yokoyai Sakai, 1939 ; M. westralia sp. nov.

Megaesthesius westralia sp. nov. ( Figs 1 , 2 , 3 A–E)

Megaesthesius sagedae —Poore et al., 2008: 46.

Material Examined. HOLOTYPE: MV- J54597 , male (3.8× 3.4 mm), 22°04.00' S, 113°48.40' E, off Ningaloo, Western Australia, depth 101–106 m, Southern Surveyor voyage SS10-2005, Sherman sled, 10.12.2005.

Description of holotype. Carapace subquadrate, 1.1 times wider than long, strongly convex antero-posteriorly, flat from side to side; regions indistinct except for moderately well-defined urogastic-cardiac regions separated by broad shallow grooves; surface moderately rugose; conspicuously but sparsely granular anteriorly, laterally near margin, posterolaterally. Frontal margin strongly pronounced; bilobed; frontal region swollen either side of median sulcus. Supraorbital border moderately deflexed but margin clearly visible in dorsal view; weakly sinuous medially, broadly rounded laterally; sloping slightly below first anterolateral tooth; mostly smooth but granular near anterolateral margin. Lateral carapace margins almost parallel; bearing 4 teeth separated by clear notches. First lateral tooth commencing above lateral outer orbital rim; forming broad flat lobe with weakly granular rim; second tooth of similar form, shorter, separated from first by shallow notch; third tooth the largest, almost as long as first 2 teeth combined, separated from second tooth by deeper u-shaped sulcus, rim minutely granular only; fourth tooth in form of large granular tubercle, relatively more widely separated from third tooth, single large granule between teeth. Posterolateral border behind last tooth marked by sparse strong moderately sharp granules. Posterolateral walls of carapace diverging; visible dorsally.

Infraorbital border ( Fig. 2 B) smooth; straight over most of length, laterally curved; lacking obviously developed tooth at inner end. Eyestalk long; frontal edge armed with row of low, well-spaced, blunt to subacute tubercles; embedded in orbital hiatus, immovable; distal quarter of length projecting free of orbit, curved to lie closely abutting lateral carapace wall ( Fig. 2 B), just visible dorsally as lateral protrusion below antero-lateral margin; cornea poorly defined, rounded, with only hint of pigment. Antennule greatly enlarged, projecting, lacking fossa, flagellum fringed with long fine setae; wholly visible dorsally except for basal segment. Antennal basal segment similar but slightly stouter than second; flagellum long, but not quite reaching level of antero-lateral tooth of carapace, with 2 long fine setae apically. Pterygostomial region coarsely granular distolaterally otherwise microscopically granular; strongly swollen, separated from subhepatic region by broad sulcus; strong clear suture line. Buccal cavity with raised lateral rim; margins broadly curved, narrowing anteriorly. Third maxillipeds ( Fig.

2C) wide, only narrow median gape; ischium longer than merus, subquadrate, armed with 4 or 5 spinules on outer lateral margin; merus subtriangular, uneven row of minute spinules near outer lateral margin; palp short, reaching only about half length of inner margin of merus.

Chelipeds missing. Walking legs ( Fig. 2 D, E) long, slender; sparsely fringed with plumose setae; last leg shorter than preceding. P 4 merus c. 4.9 times longer than wide; anterior posterior borders with sparse row of small spinules. Carpus unarmed. Propodus/dactylus each with some large stout, movable spines along posterior border. P5 merus c. 4.4 times longer than wide; stout, movable spines absent on propodus, smaller on dactylus.

Thoracic sternum broad; sternite 3 separated from fused sternites 1/2 by clear suture line; sternite 3 fused to sternite 4, with only remnants of suture present near articulation of maxilliped 3; sternites 3–4 transversely sunken medially. Surface of sternites 4–8 coarsely granular. Penis with outer (ventral) surface calcified, forming a movable, plate-like structure between anterior supplementary plate and sternite 8. Male abdomen with third to fifth somites fused, though sutures partially visible; telson broadly rounded, c. 1.6 times wider than long, slightly shorter than sixth somite; sixth somite twice as wide as long. G1 ( Fig. 3 A–C) stout, tapering to about middle, margins subparallel until distal end; apex markedly tapering towards mid-line; apical opening broad, elongated, triangular; row of short stout moveable spinules along long edge above opening. G2 ( Fig. 3 D) sinuous, subequal in length to G1; markedly constricted medially to form long curved flagellum.

Etymology. Named for Western Australia.

Distribution. Only known from the type locality off Exmouth, Western Australia; from a depth of 101– 106 m.

Remarks. Megaesthesius westralia sp. nov. can be easily separated from the type of the genus, M. sagedae Rathbun, 1909 , by the lack of spination on the lateral and frontal margins of the carapace, the presence of truncate lobes on the lateral margins, the shape of the male abdomen, particularly a narrower sixth somite (compare Fig. 3 E and 3N), a more slender G1 (compare Fig. 3 A–C and 3J–L), and also a more slender and much longer G2 with a long curved flagellum.

Megaesthesius westralia sp. nov. is more closely related to M. yokoyai Sakai, 1939 , in that both share a similar carapace form, with bluntly truncate, unarmed lobes/teeth on the lateral margins, although the shape and length of these appear to differ between the two species. In M. westralia sp. nov. the third lobe is particularly long, extending further posteriorly, and is clearly demarcated from the last broad tubercular tooth. Also the last tooth in M. yokoyai seems to be lower, less pronounced, and more spinous than in the new species. M. westralia sp. nov. also has longer, more slender walking legs, and in particular the propodus and carpus of P3 are noticeably longer (compare Fig. 2 D and 5B). The male abdomens also differ between the two species, with M. yokoyai having a much longer telson, and a relatively wider, narrower somite 6 (compare Fig. 3 E and 3I). The G1 structure of the two species is also different (compare Fig. 3 A–C & 3F–H), with M. westralia being constricted more proximally, and the tip being more acutely and obliquely pointed.

The shape of the male G2 is particularly noteworthy. In Megaesthesius westralia sp. nov. it is of similar length to the G1, and medially constricted with a long curved flagellum; however in the only other figure or description of the G2 (for the specimen of M. sagedae from off Madagascar by Crosnier1975), it is apparently shorter (about twothirds the length of the G1), stouter, and while tapering to a slender tip, the flagellum is markedly broadened at its base, and not obviously constricted at this point. Unfortunately, Serène’s (1964) refiguring of M. sagedae from the Gulf of Thailand was of a female specimen, so there is currently no information on the gonopod morphology of this species from either Singapore, its type locality, or elsewhere in Southeast Asia. It is highly unusual to have such a marked difference in G2 morphology within a genus, and this bears closer examination as part of a larger revisionary study. Unfortunately, although Takeda & Miyake (1969) gave excellent figures of the G1 of M. yokoyai they failed to either describe or figure the G2. Because of the close morphological similarities of M. westralia with M. yokoyai it seems likely that these two species will prove to share similar G2 morphology.

Figures

FIGURE 1. Megaesthesius westralia sp. nov., holotype male. A, carapace in dorsal view; B, fronto-ventral view showing third maxilliped and buccal frame; C, latero-ventral view; D, frontal view.

FIGURE 2. Megaesthesius westralia sp. nov., holotype male. A, anterodorsal view of frontal and orbital regions; B, ventral view of infraorbital margin showing eyestalk; C, third maxilliped; D, fourth pereiopod; E, fifth pereiopod.

FIGURE 3. Male gonopods and abdomens for the three known species of Megaesthesius. A – E, M. westralia sp. nov.: A, left G 1 in sternal view; B, left G 1 in abdominal view; C, left G 1, detail of tip; D, left G 2; E, male abdomen with lateral view of sternites 7 – 8 and penis. F – I, M. yokoyai Sakai, 1939: F, left G 1 in abdominal view; G, H. distal part of G 1 in abdominal and sternal view; I, male abdomen (somites 1 – 2 not shown) (After Takeda & Miyake, 1972). J – N, M. sagedae Rathbun, 1909: J, left G 1, abdominal face; K, detail of tip; L, left G 1 sternal face; M, left G 2 abdominal face; N, male abdomen (after Crosnier 1975).