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

- The Rajini tribe is revised to exclude Rostroraja alba and AAA.
- A new tribe Rostrorajini is proposed resulting in three monophyletic rajid lineages.
- mtDNA joins sister taxa Beringraja binoculata and B. pulchra.
- The shared trait of multiple embryos per egg capsule arose once in skates.
- B. binoculata have 1-8 embryos per egg capsule in nature, with 2 occurring most frequently in captivity.

The systematics of the skates in the family Rajidae have been contentious for over 250 years, 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 genus Raja is polyphyletic, and several species assemblages need to be revised. We propose a new assemblage, the Rostrajini, which organizes rajid 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 genus Beringraja. 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. In Beringraja binoculata, the strategy of having two embryos per egg capsule occurs most frequently and with the highest fitness.