Rajidae

M. A. Treloar, L. J. B. Laurenson & J. D. Stevens, 2006, Descriptions of rajid egg cases from southeastern Australian waters., Zootaxa 1231, pp. 53-68 : 53-54

[[ Family Rajidae ]]

Skates (Family: Rajidae ) are oviparous; a reproductive mode that enables females to encapsulate oocytes in morphologically structured shells that suit their environment. Generally, one oocyte from each ovary passes through the oviduct to the oviducal gland. The fertilised ovum then enters the egg case, which is generally one third to half formed. The remainder of the egg case then forms around the ovum. Two egg cases are produced (one in each oviduct) and deposited consecutively onto the substrate. The embryo develops inside the egg case and is nourished from the nutrients of the yolk until most of the yolk is absorbed. The neonate is then strong enough to leave the egg case (Hamlett and Koob 1999). In Australia, there are at least 43 skate species (10 genera) with most exhibiting a high level of endemicity (Last and Yearsley 2002). Several nomenclature changes have occurred in the Australian skate fauna recently with some species remaining undescribed. This may result from rajids forming one of the largest groups of elasmobranchs and having a high species diversity along with morphological conservatism (McEachran and Dunn 1998).

To date, there are several published identification keys or descriptive morphological studies on skate egg cases (Clark 1922; Breder and Nichols 1937; Breder and Atz 1938; Ishiyama 1958; Hitz 1964; Templeman 1982; Stehmann and Merrett 2001; Ebert 2005), although few exist for Australian species (Whitley 1938; Whitley 1944). Despite Whitley’s studies, there is still confusion with identifications of most Australian skate egg cases.

Egg cases have specific characteristics that aid in species identification. Differences between species have been widely noted, although generic characters may also exist. Identifying egg cases can lead to more knowledge about the adaptive differentiation of a species and may shed light on the taxonomic status of many species (Ishiyama 1958).