The complete mitogenome of the ray Pastinachus atrus was recovered from a partial genome scan using the HiSeq sequencing system. The P. atrus mitogenome has 18,162 base pairs (61% A + T content) made up of 13 protein-coding genes, 2 ribosomal subunit genes, 22 transfer RNAs, and a 2516 bp non-coding AT-rich region. This mitogenome sequence is the first for a ray from Australian waters, the first for the Genus Pastinachus, and the 6th for the family Dasyatidae.
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.