Oviparous elasmobranch development inside the egg case in 7 key stages

Embryological stages of oviparous elasmobranch during development can be difficult to identify, requiring magnification and/or fixation of an anaesthetized embryo. These restrictions are poorly suited for monitoring the development of living elasmobranchs inside their egg cases. There are two major aims of this study. The first was to observe elasmobranch embryonic development non-invasively and produce a non-invasive developmental key for identifying the life stages for an elasmobranch inside the egg case. To this end, 7 key developmental stages were identified for the greater spotted catshark, Scyliorhinus stellaris, and are provided here with diagrams from multiple perspectives to demonstrate the key features of each stage. The physiological and ecological relevance of each stage are discussed in terms of structure and function for embryonic survival in the harsh intertidal zone. Also discussed is the importance of the egg case membrane and the protective embryonic jelly. The second aim of the study was to understand the applicability of the 7 developmental stages from S. stellaris to other oviparous elasmobranchs. Thus, changes in embryonic body size and egg yolk volume at each stage were measured and compared with those of the closely related, lesser spotted catshark, Scyliorhinus canicula. We find nearly identical growth patterns and yolk consumption patterns in both species across the 7 developmental stages. Thus, although the 7 developmental stages have been constructed in reference to the greater spotted catshark, we suggest that it can be applied to other oviparous elasmobranch species with only minor modification.