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 : 67

[[ Family Rajidae ]]

Discussion

This key is suitable for identifying both wet and dry skate egg cases from southeastern Australia. The key was developed using fresh (extracted from the uteri) egg cases, where correct species identification was assured and egg cases were undamaged. However drying identified fresh specimens showed that the key would still work on dried egg cases. When dry, the keratin-like substance of the egg case becomes brittle and is prone to damage. At room temperature, dried egg cases shrink by about 30% of their original size. This key was also used on previously unknown specimens collected from the beach. It was found if the egg cases were relatively undamaged, identification was possible. Heavily damaged and predated egg cases were unidentifiable.

The egg case of the different species had a variety of characteristics which allowed them to be distinguished. Although egg case body size sometimes overlapped between species, other features could be used to distinguish them, including horn and apron lengths, shape, and presence/absence of a lateral keel and attachment fibres (irrespective of whether the egg case was fresh or not).

Further identifications of other Australian skate egg cases are necessary to elucidate any phylogenic relationships between a skate’s genus and its egg cases. It is interesting that Dipturus cf gudgeri egg cases have horns that are almost fully enclosed in the apron. This feature may be an attribute of a subgenus in the genus Dipturus (Ishiyama 1958). An egg case with enclosed horns was also found in a Japanese species that belonged to the genus Dipturus . Ishiyama (1958) and H. Ishihara (Taiyo Engineering, Tokyo, Japan, personal communication) reported some phylogenetic differences in egg cases between genera in Japanese species. This suggests that egg case morphology may help in determining phylogenetic relationships in rajid species, although further work on egg case assemblages is required both in Australia and worldwide.