Ovimaja, Ng & Forges, 2015

Ng, Peter K. L. & Forges, Bertrand Richer De, 2015, Revision of the spider crab genus Maja Lamarck, 1801 (Crustacea: Brachyura: Majoidea: Majidae), with descriptions of seven new genera and 17 new species from the Atlantic and Indo-West Pacific, Raffles Bulletin of Zoology 63, pp. 110-225 : 181-184

Ovimaja n. gen.

Diagnosis. Carapace pyriform, much longer than broad; dorsal surface evenly inflated, covered by low granules or tubercles; gastric and branchial regions delimited by grooves ( Fig. 34C, F, G). Intestinal region without median spine ( Fig. 34C, F, G). Pseudorostral spines relatively short, diverging ( Figs. 34C, D, F, G, 38B, C). Supraorbital eave with anterior part longitudinally narrow, rectangular, not prominently expanded; antorbital spine sharp ( Fig. 38B, C). Intercalated spine distinct, separated from supraorbital eave and postorbital spine by distinct gaps; postorbital spine strong, lobiform; hepatic region with 1 distinct spine, much shorter than postorbital spine; 2 or small spines below ( Fig. 38B, C). Lateral carapace margin with about 3 short spines and some sharp tubercles, branchial region with 1 low spine, obscure in large specimens ( Fig. 34C, F, G). Posterior carapace margin with 2 very short median spines ( Fig. 34C, F, G). Eyes long, slender, with long ovoid cornea bent at 30° from horrizontal ( Fig. 38B, C). Antennal flagellum short, slender. Basal antennal article longer than broad, rectangular; surface with several rounded granules, with 3 low, blunt spines distally, outer lateral margin entire or with a few small rounded granules; inner lateral margin with 3 large rounded projections with overlaps antennular fossa; proximal outer angle rounded; antero-external crested rim of antennular fossa overlaps halfway into distal part of basal antennal article, forming a hook-like structure ( Fig. 40O). Epistome longer than wide, anterior margin with 2 large rounded tubercles; posterior margin composed of 4 rectangular plates separated by deep fissures ( Figs. 40O, 42H). Suborbital margin separated from basal antennal article and margin of postorbital tooth by deep fissures ( Fig. 40O). Outer surface of third maxilliped covered by setae in adults; ischium subrectangular, slightly longer than broad; postero-external angle of merus relatively broad, “inserted” into shallower concavity on outer margin of ischium; anterointernal part of ischium rounded, auriculiform ( Fig. 46O, P). Male chelipeds relatively short in adult males, surfaces of merus and carpus smooth; carpus long; propodus of palm short, smooth, fingers longer than palm; fingers long, slender, gently curved, with basal gape when closed ( Figs. 34C, F, G, 54O, P). Ambulatory legs relatively short, slender; merus without dorsal subdistal spine; carpus very short, cordiform with deep median groove; dactylus elongate, slightly curved, covered with long setae except for corneous tip ( Figs. 34C, F, G, 56L). Thoracic sternum wide; surfaces of somites 5–8 almost smooth; sternites 3 and 4 slightly depressed; margin between sternites 2 and 3 demarcated by very deep notch, forming waist-like structure; anterior margin of sternoabdominal cavity not forming complete rim ( Figs. 51I, 52L). Male abdomen subrectangular, with 6 free somites and telson; somites 6 wider than all other somites and telson ( Fig. 51I). Male press-button abdominal locking mechanism laterally positioned on sterno-abdominal cavity ( Fig. 52L). Female abdomen dome-shaped, covering most of thoracic sternum. G1 very long, slender, distal part elongated, curved outwards, with 2 distal folds and sharp tip, distal part with scattered very short setae ( Fig. 35F–L).

Type species. Paramithrax (Leptomithrax) compressipes Miers, 1879 , by present designation.

Etymology. The genus name is derived from an arbitrary combination of the Latin “ ovum ” for egg and Maja , alluding to the egg-like appearance of the carapace of the type species. Gender feminine.

Remarks. Ovimaja n. gen. is very different from Maja s. str. and the various genera recognised here. Aside from the distinctively elongated carapace ( Fig. 34C, F, G), the structure of the basal antennal article is unique, with the inner margin lined with three prominent projections ( Figs. 40O, 42H); the anterior margin of the epistome has two large projections ( Figs. 40O, 42H); the male anterior thoracic sternum is waist-like, with the region between sternites 2 and 3 deeply constricted ( Figs. 51I, 52L); the carpus of the ambulatory leg is very short, enlarged and flattened ( Fig. 56L); and the G1 is very long and curved with the distal part very slender ( Fig. 35I–L). Ovimaja has a very different male abdominal locking mechanism from the other genera. In these genera, including Maja s. str., the press-button is submedian in position ( Fig. 52A–K). In Ovimaja , the pressbutton is positioned laterally on thoracic sternite 5, near the margin of the sterno-abdominal cavity ( Fig. 52L).

One fossil species described from South Australia, Maja robinsoni Jenkins, 1985 , is almost certainly a species of Ovimaja . Described on the basis of nine carapaces (the holotype 31.0 × 24.0 mm) from the mid to late Early Miocene (ca. 16–23 million years ago). The carapace of M. robinsoni (cf. Jenkins, 1985: pl. 2 figs. 2a, 3) is similar to O. compressipes , but the granules on the posterior half of the carapace appear to be more closely packed (less packed in O. compressipes , Fig. 34C, F, G) and the lobulation on the epistome and margins of the antenna is less distinct ( Jenkins, 1985: pl. 2 fig. 2d, e) (cf. Fig. 40O).