The biology of the evolution of viviparity in asterinid sea stars

Abstract

Viviparous asterinids exhibit great diversity in reproductive and offspring provisioning strategies, which raises fascinating life-history questions. This thesis investigates the biology of parent-offspring size variation, offspring release, nutrient provisioning and morphological adaptions in three viviparous asterinid sea stars, Cryptasterina hystera, Parvulastra vivipara and P. parvivipara. These species have contrasting lecithotrophic and matrotrophic provisioning of developing offspring in the gonads. In C. hystera (lecithotrophic), the juveniles (655 µm diameter) develop from large eggs (440 µm diameter). In P. vivipara and P. parvivipara, juveniles vary greatly in diameter (500–5000 µm) and develop from small eggs (84–150 µm diameter) through sibling cannibalism (matrotrophy). In these species, larger parents had greater reproductive output and produced more, but not larger, offspring. The species with matrotrophic offspring provisioning had a higher reproductive output than the lecithotrophic species. Parvulastra parvivipara released juveniles in 1–5 cohorts and exhibits continuous reproduction. Cryptasterina hystera retained a few large offspring in the gonad after 30 days of synchronous release. The degree of parental investment measured as matrotrophy index (the ratio of juvenile to egg dry mass) ranged from 597–55082 (P. parvivipara) and 1.7–6.2 (C. hystera), indicating a continuum in offspring provisioning. Potential specializations for viviparity and provisioning of nutrients for offspring were investigated using confocal microscopy and histology. The early larvae were closely associated with the inner gonad wall, supported by thin processes from somatic cells. The arrangement of P. parvivipara progeny in the gonads was observed three-dimensionally using micro-computed tomography. The juveniles were orally opposite to each other, presumably as a defensive strategy to protect themselves from being eaten. Confocal microscopy revealed 2–6 developmental stages in each gonad. The size variation of offspring intensifies when siblings start cannibalism post-metamorphosis.