Abstract
Elasmobranchs display various reproductive modes, which have been key to their evolutionary success. In recent decades there has been a rise in the number of reported cases of foetal abnormalities including fertilised, double-embryos held within one egg capsule, hereafter referred to as twins. Previously, the occurrences of twin egg cases have been reported in two batoid and one shark species. We report the first cases of twins in three species of oviparous elasmobranchs: the undulate ray (Raja undulata), the nursehound (Scyliorhinus stellaris), and the small-spotted catshark (Scyliorhinus canicula). We investigated the genetic relationships between the twins in S. stellaris, and S. canicula using microsatellite markers. Whilst the S. stellaris twins displayed the same genotypes, we found that the S. canicula twin individuals arose through heteropaternal superfecundation. This is the first reported incidence of such a paternity in elasmobranchs. The relationship between environmental change and reproductive strategy in elasmobranchs is unclear and further research is needed to determine its effect on the prevalence and mechanisms of formation of elasmobranch twins.
Highlights
Elasmobranchs comprise almost 1,200 species [1,2] of sharks and batoids [3] that display complex reproductive modes, characterised by low numbers of offspring that are born, or hatched, as active, fully-formed individuals [4]
We report the first incidence of an egg case containing two embryos in the oviparous Raja elasmobranch, the undulate ray (R. undulata)
The S. canicula and S. stellaris eggs all had two yolk sacs, indicating that two oocytes were released into the same oviducal gland for shelling in a single egg case
Summary
Elasmobranchs comprise almost 1,200 species [1,2] of sharks and batoids (guitarfishes, sawfishes, skates and rays) [3] that display complex reproductive modes, characterised by low numbers of offspring that are born, or hatched, as active, fully-formed individuals [4]. All extant elasmobranchs employ internal fertilisation, with unique organ systems that increase the efficiency and likelihood of fertilisation, whilst minimising sperm wastage and the predation of unfertilised eggs [4,5] Their diversity of reproductive traits is suggested to be a major selective advantage that has contributed to the group’s success [6].
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