The adaptability of facultative parthenogenesis and 'multiple embryos per eggcase' as alternative reproductive strategies in Chondrichthyes.

Chondrichthyans (sharks, rays and chimaera) are a fascinating case study through which to examine reproductive biology. While most vertebrate lineages have converged upon either placental viviparity or oviparity, chondrichthyans exhibit unparalleled diversity in reproductive mode and other aspects of reproductive biology. Despite this, our understanding of reproductive variation in this clade remains incomplete. Namely, several 'unusual' reproductive traits have been repeatedly observed in some chondrichthyan taxa, but we have little to no understanding of their adaptive value. Here, I focus on two traits (facultative parthenogenesis and the deposition of multiple embryos with separate yolks within a single eggcase (MEPE)), which theoretically result in exponential increases in fecundity. I discuss the theoretical fitness consequences of these traits, the range of species and eco-evolutionary contexts in which they have been documented and major open questions regarding their prevalence and evolutionary consequences. It appears likely that both facultative parthenogenesis and MEPE are adaptive in at least some chondrichthyan taxa and ecological contexts. However, additional data are needed to determine the true effect of these traits on lifetime fecundity, the frequency with which they occur, potential population-level effects and cues or triggers that might favour switches from 'typical' sexual reproduction to parthenogenesis or MEPE.