Abstract
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.
Highlights
Elasmobranchs are a group of cartilaginous fishes from the subclass Elasmobranchii, which consists of sharks, rays, skates, guitarfish and sawfish [1,2,3,4]
To demonstrate the applicability of our 7 key stages based on S. stellaris to other oviparous elasmobranchs, and to be able to correlate our non-invasive scale with earlier embryological studies [13], we have examined how each stage identified for S. stellaris compares with that of S. canicula in terms of growth and yolk consumption
The S. stellaris egg cases were oval to rectangular in shape with colors ranging from yellowish green to brownish green when wet, to darker shades of the same colors when dry
Summary
Elasmobranchs are a group of cartilaginous fishes from the subclass Elasmobranchii, which consists of sharks, rays, skates, guitarfish and sawfish [1,2,3,4]. They can be differentiated from teleost fish (true fish with bony endoskeleton) by several characteristics; their cartilaginous endoskeleton, the possession of five to seven gill openings, rigid dorsal fins or spines, lack of swim bladder, and a body that is covered in denticles or placoid scales [2]. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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