Sesarmidae, Dana, 1851

GUINOT, DANIÈLE, TAVARES, MARCOS & CASTRO, PETER, 2013, Significance of the sexual openings and supplementary structures on the phylogeny of brachyuran crabs (Crustacea, Decapoda, Brachyura), with new nomina for higher-ranked podotreme taxa, Zootaxa 3665 (1), pp. 1-414 : 141-142

Family Sesarmidae

The male gonopore is sternal. It is located posterior to suture 7/8, adjacent to the variously expanded episternite 7, and anterior to the P5 coxa. The penis is rather long, well developed, generally basally calcified, sometimes foliaceous. It shows as a longitudinal, oblique or horizontal tube variously oriented ( Guinot 1979a: fig. 52D; N.K. Ng, Davie, Schubart & Ng 2007: fig. 4B; see also Karasawa & Kato 2001: fig. 2.20). The penis is described as proximally calcified in species of Selatium Serène & Soh, 1970 (Schubart et al. 2009: 4).

An unusual arrangement has been observed in Metasesarma aubryi ( Fig. 35 ; see Additional thoracotreme protection in a depression at the base of the G1). The long penis, which emerges from a large sternal gonopore, is directed horizontally in a rest position, remaining exposed above the G1 when the abdomen is lifted. It shows a sclerotised proximal half, its distal half lying in a small depression excavated at the base of the G1 endopodite and close to the foramen receiving the G2. The whole penis is bound posteriorly by an internal folding of abdominal somite 2, a folding that bears both G2. The penis, instead of being located on the surface of sternite 8 below the G1, is positioned above the G1 and coadapted with two other elements, the G2 and the abdominal somite 2 margin. Such disposition is similar in the “tree climbing crab” Aratus pisonii (see Cuesta et al. 2006 for larval data; Diaz & Bevilacqua 1986 for larval development; Conde & Diaz 1989, and Conde et al. 2000 for life history). Aratus seems to contain more than one species (N.K. Ng & Guinot, manuscript). This arrangement, which seems to vary in details among the sesarmid genera, appears not to have been previously described, and will need to be checked in all sesarmid genera, especially in fresh material.

Hartnoll (1965: 121, 145) recorded in Aratus pisonii (from Jamaica) the absence of an abdominal-locking mechanism without a trace of a button or socket. It was suggested that the wide male abdomen of Aratus , that somewhat resembles that of a mature female, this “feminisation”, could perhaps account for the loss of the locking system. Indeed, the absence of a typical abdominal-locking system is found in thoracotremes with a well-developed abdomen, such as Heloeciidae and Mictyridae . Sesarmine and ocypodine species often have vestigial, inefficient press-buttons (rarely functional in adult sesarmids, often disappearing with age), but the disappearance of an effective locking system does not seem to change the abdomen overall appearance ( Guinot & Bouchard 1998). A comparative study of all these crabs is needed, including species of Uca sensu lato, where the abdomen is only loosely applied, and Minuca Bott, 1954, with a pair of carinae on sternite 4 (Guinot 1979: 153; Guinot & Bouchard 1998: 668, fig. 24C).

Figures

FIGURE 35. Metasesarma aubryi (A. Milne-Edwards, 1869) (Sesarmidae), male 18.1 × 21.5 mm, Red Sea (MNHN-B13230): penis in rest position (abdomen lifted), exposed above the G1. cx4, P4 coxa; e6, episternite 6; g, male gonopore; G1, first gonopod; G2, second gonopod; i.a.2, internal folding of abdominal somite 2; i.s., incomplete sulcus of thoracic sternite 7; p, penis; 7, 8, thoracic sternites 7, 8; 6/7, 7/8, thoracic sternal sutures 6/7, 7/8. Scale bar: 3 mm.