The secret of the mermaid's purse: phylogenetic affinities within the Rajidae and the evolution of a novel reproductive strategy in skates.

The systematics of the skates in the family Rajidae have been contentious for over 250years, with most studies inferring relationships among geographically clustered species, and non-overlapping taxa and data sets. Rajid skates are oviparous, and lay egg capsules with a single embryo. However, two species exhibit a derived form of egg laying, with multiple embryos per egg capsule. We provide a molecular assessment of the phylogenetic relationships of skates in the family Rajidae based on three mitochondrial genes. The resulting topology supports monophyly the family. However the genusRajais polyphyletic, and several species assemblages need to be revised. We propose a new assemblage, the Rostrajini, which organizesrajid species into three well-supported tribal lineages for the first time. Further, these data provide an independent assessment of monophyly for the two species exhibiting multiple embryos per egg capsule, supporting their status as the unique genusBeringraja. In addition, we find that among the different size classes of egg capsules, ranging from 1 to 8 embryos per capsule in this genus, there is variation in frequency and survivorship. InBeringraja binoculata, the strategy of having two embryos per egg capsule occurs most frequently and with the highest fitness.