Symethinae Goeke, 1981

Guinot, Danièle & Quenette, Gwenaël, 2005, The spermatheca in podotreme crabs (Crustacea, Decapoda, Brachyura, Podotremata) and its phylogenetic implications, Zoosystema 27 (2), pp. 267-342 : 312

Superfamily RANINOIDEA De Haan, 1839

The superfamily Raninoidea De Haan, 1839 , which contains the single family Raninidae or frog crabs, and assigned to the Podotremata by Guinot (1978, 1979a), was subdivided into six Recent subfamilies ( Ranininae De Haan, 1839 , Raninoidinae Lörenthey & Beurlen, 1929 , Notopinae Serène & Umali, 1972, Symethinae Goeke, 1981 , Cyrtorhininae Guinot, 1993, Lyreidinae Guinot, 1993 ), principally based on morphology of the thoracic sternum ( Guinot 1993b: figs 1-6). Dawson & Yaldwyn (2000) adopted these views. Tucker (1998) also followed Guinot (1993b) but elevated the Symethinae to familial rank, and recognized the exclusively fossil subfamily Palaeocorystinae Lörenthey & Beurlen, 1929 , which ranged from the Lower Albian to the Cenomanian and is supposed to represent the rootstock of the Raninidae .

The superfamily consists currently of less than 50 extant species, distributed in 12 genera. Basal relationship of raninoids to the heterotremethoracotreme assemblage, indicated by some morphological features, spermatological analysis of Ranina ranina (Linnaeus, 1758) ( Jamieson 1989) and molecular sequences ( Spears et al. 1992), was partly refuted by further spermatozoal studies (Jamieson 1994; Jamieson et al. 1994). Nevertheless, subsection Raninoida, with the two superfamilies Raninoidea and Cyclodorippoidea , was recently attributed to the Eubrachyura ( Martin & Davis 2001). This change in the classification of the Raninoidea was made mostly on the basis of arguable molecular evidence while rejecting morphology and functional morphology, in particular that related to the female genital condition. It is nevertheless possible that the Archaeobrachyura may eventually prove to be an “interim” scheme, between the basal Podotremata (Dromiacea and Homolidea) and the Eubrachyura. Cladistic analysis and the study of molecular sequences are in progress to critically examine the phylogenetic relationships of the Archaeobrachyura (Guinot unpubl. data). The distinctive morphology of the raninoids seems to reflect a high degree of specialization for burrowing, which has characterized the group throughout its long geologic history since its apparition in the Early Albian.

In raninid crabs the abdomens are only weakly sexually dimorphic, with all the somites that are not folded down in males. The sterno-abdominal depression is small, only posterior, and a single subfamily, the Lyreidinae , shows an efficient holding of the abdomen ( Guinot & Bouchard 1998; Bouchard 2000). The posterior part of the sternum shows an acute dorsal flexion and, in ventral aspect, only the anterior sternites form a flat shield. Posterior sternites are narrower, sometimes linear; sternite 8 is variously reduced. The spermathecae, which are generally small and medially located, lie in a depression and are more or less recessed to the bottom or sides of the pit-like depression ( Fig. 24 ), so that they are not immediately obvious or even not visible at all. In order to see their paired apertures, it is necessary to bend the crab and examine the bottom of the depression and its more or less steep opposite sides.

The axial skeleton of the raninoids is very peculiar ( Bourne 1922), the extreme dorsal flexion of last sternites resulting in a marked modification of the axial skeleton in that region. Posterior sternites are separated medially and to a variable extent by a high longitudinal wall (“median apodeme” of Gordon 1963 and Hartnoll 1979), marked externally by the median line that is present at least on sternites 7 and 8, and often also on sternite 6 and a part of sternite 5. Each spermatheca, with its aperture occluded by a membranous area, is only composed by a chamber that is enclosed on each side of this wall. As a result, the spermathecae are variously placed together, that led Gordon (1966) to suppose a single, unpaired spermatheca. The paired spermathecal structure was remarkably determined by Hartnoll (1979), the gonopods described by Minagawa (1993), and the reproductive biology studied by Minagawa et al. (1993, 1994).

Figures

FIG. 24. — Posterior part of thoracic sternum in four Raninoidea; A, Lyreidus tridentatus De Haan, 1841 (Lyreidinae); B, Ranina ranina (Linnaeus, 1758) (Ranininae); C, Cosmonotus grayii White, 1847 (Notopodinae); D, Symethis corallica Davie, 1989 (Symethinae). Abbreviations: ccx3-ccx5, articular condyles of P3-P5; g, female gonopore; l.m., median line; pr5, projection from sternite 5 for abdominal maintaining; s, spermatheca; 6-8, thoracic sternites 6-8. Scale bars: A, C, D, 2.5 mm; B, 1 cm.