Dromia Weber, 1795

Published, First, 2009, New records of crabs (Decapoda: Brachyura) from the New Zealand region, including a new species of Rochinia A. Milne-Edwards, 1875 (Majidae), and a revision of the genus Dromia Weber, 1795 (Dromiidae), Zootaxa 2111, pp. 1-66 : 14-15

Dromia Weber, 1795 sensu stricto

Definition Carapace wider than long (1.2 – 1.4), surface sculptured. Rostrum tridentate, median tooth less than or equal in length to lateral teeth. Female sternal grooves normally end apart in pits between bases of P2. Cheliped with epipod. Sternite 4 T-shaped (inverted), episternites extend laterally between coxae of chelipeds, P2 to form cross-piece of “T”, stem of “T” formed by sternite narrowing anteriorly, tip truncate, ratio of distance across episternites to sternite length 1.0–0.9. Walking legs (P2–P4) not knobbed or ridged. Length of propodi, dactyli of first two pairs of legs (P2, P3) usually equal, inner margins of dactyli armed with 5–7 small spines. P4 dactylus opposed by single propodal spine, no spine on the outer propodal margin. Fourth leg (P5) shorter than second (P3), dactylus usually opposed by two propodal spines, with another spine on the outer propodal margin. Margin of telson rounded. Uropod plates well developed, visible externally, used in male abdominal locking mechanism by fitting in front of serrated flanges on the bases of first legs (P2). Joint between last two abdominal somites freely movable.

Remarks. One of the enduring difficulties in deciding the limits of Dromia has been the importance attached to the female sternal groove character. These grooves, which mark the suture between sternite 7 and 8, are of more than passing interest because they encompass the apertures to the spermathecae and are intimately involved in copulation, sperm transfer and fertilization. They may be short so that the apertures are posterior to the female gonopores or long so that the apertures are anterior to the gonopores in the coxae of the second walking legs (P3). In addition the grooves may terminate wide apart or very close together. In the type species, Dromia personata , the sternal grooves are somewhat intermediate, ending apart in front of the gonopores, between the bases of the first pair of walking legs (P2). This condition is found in all the species currently included in the genus, except for D. bollorei , in which the sternal grooves end close together in pits on an elevated protuberance between bases of the first walking legs (P2). However, sternite 4 in this species is similar to that of the other Dromia species , having wing-like lateral projections between the coxae of the cheliped and first leg (P2). The only problem species is D. monodi , which has all the characters of Dromia , but whose body shape is remarkably similar to Sternodromia spinirostris . Forest (1974) commented that immature specimens of D. monodi and S. spinirostris are difficult to separate so that Guinot & Tavares (2003) transferred Dromia monodi Forest & Guinot, 1966 to Sternodromia Forest, 1974 .

McLay (1993) synonymised Sternodromia with Dromia because of the similarity of the modifications to the two camouflage-carrying limbs (last two pairs of legs, P4 and P5) as well as in body shape, and chose to overlook the sternal characters. This was ill-advised because we should regard the sternum as a more conservative feature than the arrangement of propodal spines on the carrying legs. The truncated sternite 4, the slit-like apertures of the spermathecae and sternal grooves terminating close together at the base of a large conical swelling between bases of first legs are sufficiently important as to warrant a separate genus. Also the abdominal locking mechanism in S. spinirostris differs from that typical of Dromia : the uropods are smaller and the coxae, with which they engage, are only shallowly indented. The same argument applies to D. monodi and therefore this species should not be placed in Sternodromia . Given the importance of the sternal characters it seems wiser to keep D. monodi in Dromia and regard conformation of the carapace as being convergent. If we use sternal differences to justify Sternodromia as a genus, then we must use the same argument to exclude D. monodi .

The following key for identifying the genera resulting from the revision of Dromia sensu lato uses the shape of the sternite 4 between the bases of the cheliped. This sternite is T-shaped with the episternites forming the top of the “T,” articulating with the cheliped coxae on each side, and the long axis of the “T” extending forward between the coxae of chelipeds.