Normal embryonic development in the clearnose skate, Raja eglanteria, with experimental observations on artificial insemination

Abstract

Clearnose skates, Raja eglanteria, are a common species of skate found seasonally along the Atlantic coast of North America from Cape Cod to mid-Florida and in the Gulf of Mexico from mid-Florida to eastern Texas. Clearnose skates inhabit the west central coast of Florida during winter months, when Gulf temperatures are conducive to mating and egg-laying (approximately 16°C–22°C). Specimens collected during this time will breed in captivity and, if maintained at 20°C, mated females will store sperm and continue to lay fertile eggs for up to six months. With fertile eggs available from day of laying until hatching, the first complete description of batoid elasmobranch embryonic development with a timetable has been established and is presented here. If eggs are maintained at 20°C, appearance and progression of embryonic events proceed over a reproducible timeline. The timeline has been divided into discrete time periods into which developmental events have been grouped. These include cleavage and expansion of the blastodisc (day 1–day 4), embryonic axis and neural tube formation (day 4–day 7), pharyngeal pouches and gill filament development (day 10–day 28), expansion of fins and transition from external filaments to internal gills (week 4–week 7), and epidermal pigmentation and yolk absorption (week 8–week 12). Fully formed offspring hatch after an incubation period of approximately 12 weeks (85 ± 6 days). To examine possible mechanisms of transport and storage of sperm in the reproductive tract of females following copulation, experiments resulting in the first artificial insemination for any elasmobranch fish are described. These experiments provide evidence for functional roles of the alkaline gland, whose secretions stimulate sperm motility and may enhance migration of sperm to the oviducal glands, and the clasper gland, whose secretions may provide nutrition during storage of viable sperm in the oviducal gland.