Short Communications: The Egg Case of the Oviparous Elasmobranch, Raja Erinacea, Does Osmoregulate

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Abstract
Like many other elasmobranchs (Wourms, 1977), the little skate, Raja erinacea, lays individual eggs covered with a specialized structure termed the egg case or ‘Mermaid’s Purse’. Collagen is the major component of the egg case, which is secreted by the shell gland in the anterior region of the oviduct (Wourms, 1977). Smith (1936) and Price & Daiber (1967) suggested that the egg case provided an osmotically isolated environment until the embryo was capable of urea retention and osmoregulation. However, Reed (1968a) showed that the encapsulated embryos of the skate, Raja binoculata, possess the enzymes of the Krebs omithine-urea cycle and are able to retain near-adult levels of both urea and trimethylamine oxide, even at very early stages (Reed, 1968b). The retention of urea seems to be secondary to a urea-impermeable embryonic membrane since Needham & Needham (1930) found that the egg cases of Syllium (= Scyliorhinus) canicula are permeable to urea, and this has been substantiated by recent work on the egg cases of the same species (Hornsey, 1978; Foulley & Melinger, 1980). Despite the ability to maintain high urea levels, it appears that the full complement of elasmobranch osmoregulatory mechanisms is not present in at least the early stages of encapsulated elasmobranch development since Libby (1959) found that R. eglanteria cannot survive in sea water if it is removed from the egg case before day 20 of the 64-day developmental period. In this species, at day 20 a mucous plug in the egg case is dissolved and sea water enters. This opening of the egg case to sea water during the later stages of development is also found in other oviparous elasmobranch species (Wourms, 1977).