Egg capsules of the little skate, Psammobatis extenta (Garman, 1913) (Chondrichthyes, Rajidae)

Abstract

Corresponding author: rochaf_bio@yahoo.com.br The elasmobranch Rajidae family, known usually as skates, is the most numerous group among cartilaginous fishes, having almost 245 species with very conservative morphology (EBERT and COMPAGNO, 2007). Elasmobranch egg capsules are widely recognised as important in species identification and provide relevant information concerning their reproductive biology (ODDONE et al. , 2004). The genus Psammobatis GUNTHER, 1870 comprises eight species, four of them recorded in Brazil (PARAGO, 2001): P. extenta (GARMAN, 1913), P. rutrum JORDAN, 1890, P. bergi MARINI, 1932, and P. lentiginosa (BIGELOW and SCHROEDER, 1953). Psammobatis extenta , the little skate, is endemic to the continental shelf of the western South Atlantic, ranging from Cabo Frio, Rio de Janeiro, Brazil (22°56’S) to Patagonia, Argentina (~ 45°S) (PARAGO, 2001). Recent studies of P. extenta have focused mainly on its reproduction, morphology and feeding habits (BRACCINI and PEREZ, 2005; BRACCINI and CHIARAMONTE, 2002a; 2002b) and detailed information concerning its egg capsules is lacking. Only the egg capsules of P. scobina (PHILIPPI, 1857), from the Southeastern Pacific, have been accurately described (CONCHA et al. , 2009); short descriptions of the egg capsules of P. rudis , P. normani and P. bergi have been presented with their reproductive biology (MABRAGANA and COUSSEAU, 2004 and SAN MARTIN et al. , 2005). The present study describes the egg capsules of Psammobatis extenta , a small common rajid species of the western South Atlantic. Sixty-one egg capsules were removed directly from the 35 female Psammobatis extenta oviduct, thus avoiding species misidentification. The females were collected by bottom trawlers on the Sao Paulo continental shelf, at 30 to 50 m depths (between 24°12’S; 46°04’ W and 24°08’S; 46°50’W), during 2002, except in autumn. The general morphology, color, texture, presence and number of eggs, presence and position of attachment fibrils, presence and shape of velum and keel and finally, presence, position and shape of ventilation fissures were all recorded. The measurements taken were: capsule length (without horns), maximum width, anterior and posterior horn lengths, capsule height, and thickness and width of lateral keel. Both the terminology and morphometrics follow TEMPLEMAN (1982), ODDONE et al. (2004) and TREOLAR et al. (2006). Only fully developed capsules were used in the calculations made and the terms ‘anterior’/‘posterior’ and ‘dorsal’/‘ventral’ refer to the position of the egg capsules in the female oviduct (TEMPLEMAN, 1982). Capsules were fixed in formalin and preserved in 70% ethanol. The total length of the females was measured from the tip of the snout to the tip of the tail. Differences between anterior and posterior horn lengths and between right and left egg capsules were verified with Student´s t-test and the relationship between the females´ total length and the body length of the capsules was investigated by linear regression (SOKAL and ROHLF, 1995). The capsule measurements are presented in Table 1. Only one egg capsule was found in each female’s oviduct and just one egg was found per capsule. Almost 75% of the gravid females had capsules in both oviducts and when only one capsule was present, it was in the left oviduct (except for one female, which had developing horns on the right shell gland). The fully developed egg capsules are rectangular, with a horny process on each corner and a brownish copper in color (Fig. 1). The capsule walls are symmetrically convex, with the highest point situated centrally. Both capsule walls present a