It's a shark-eat-shark world, but does that make for bigger pups? A comparison between oophagous and non-oophagous viviparous sharks

Abstract

Oophagous reproduction (i.e., consumption of unfertilized ova in utero) in sharks has been hypothesized to result in fewer but larger pups relative to those produced by viviparous sharks with different modes of maternal nutrient transfer. We compared pup and litter sizes reported in the literature for 106 shark species with lecithotrophic viviparity, oophagy, and placental viviparity as methods of maternal nutrient transfer during pregnancy. Using a suite of permutational tests, we accounted for the effect of maternal size to test whether oophagous strategies do indeed result in larger pups and smaller litters relative to sharks with lecithotrophic and placental viviparous reproduction. Our results demonstrated that litter size was significantly reduced in species with oophagous reproduction relative to sharks with lecithotrophic and placentally viviparous reproduction. Further, the influence of oophagous reproduction on pup length was more variable, and generally pup length of oophagous species was only larger than sharks with lecithotrophic viviparous reproduction. However, when maternal investment was expressed as litter mass (minimum pup mass by litter size), the effect of oophagy was neutralized. We found further evidence that pup length at birth was directly modulated by litter size and habitat, suggesting pup length at birth may also be adapted to conditions at nursing grounds. Our study supports the hypothesis that both placentally viviparous and lecithotrophic viviparous species maximize their reproductive fitness by allocating nutrients to larger litters of pups, whereas oophagous species maximize reproductive fitness through smaller litters of pups that may be well adapted to their nursing grounds.