Temporal pattern of offspring release and degree of parental investment in two viviparous asterinid sea stars with an overview of matrotrophy and offspring size variation in echinoderms that care for their offspring

ABSTRACT The temporal pattern of juvenile release by two species of viviparous asterinid sea stars that incubate their young in the gonads was documented. Parvulastra parvivipara released juveniles (400–3000 µm diameter) in 1–5 cohorts. Parents produced large juveniles (>1000-µm) irrespective of adult size. Released juveniles were larger than the retained juveniles. Most Cryptasterina hystera offspring were released in one large clutch of similarly sized juveniles (732-µm mean diameter). After this initial release, the presence of large juveniles (944-µm mean diameter) in the gonads of C. hystera indicates that they are supported by matrotrophy, potentially through sibling cannibalism. The degree of parental investment additional to the egg in both species was estimated by using a matrotrophy index (MI, the ratio of juvenile and egg dry mass). As the eggs of P. parvivipara and C. hystera could not be isolated, the eggs of their congeners (P. exigua and C. pentagona, respectively) were used as a proxy to estimate the MI, the first application of this index to a marine invertebrate. The MI ranged from 597 to 55082 in P. parvivipara and 1.7–6.2 in C. hystera for juveniles across the different size classes. Matrotrophy and size variation of offspring may be characteristics of echinoderms that incubate their young.