Viviparity and intragonadal cannibalism in the diminutive sea stars Patiriella vivipara and P. parvivipara (family Asterinidae)

Abstract

The Australian asterinid sea stars Patiriella vivipara and P. parvivipara have the most derived lifehistory pattern seen in the Asteroidea. They are simultaneous hermaphrodites, have intragonadal fertilisation, and incubate their young in the gonads to an advanced juvenile stage. As characteristic of brooding echinoderms, P. vivipara and P. parvivipara are diminutive, with P. parvivipara being the smallest known sea star. These species have the most restricted distribution known in the Asteroidea. Reproduction of two populations P. vivipara in Tasmania, Midway Point and Tesselated Pavement, was examined in specimens collected in 1991, 1992 and 1995. Reproduction of P. parvivipara in South Australia was examined in specimens collected from one population in 1991 and 1994. The gonads are ovotestes. Both species typically contain 6 to 8 predominantly female gonads and 1 predominantly male gonad. In the predominantly female gonads, only a few eggs and a small amount of sperm are produced at a time. Detection of sperm in these gonads requires histological examination. The amount of sperm in the predominantly female gonads appears sufficient to fertilise all the ova produced. The low allocation to male function and the simultaneous presence of mature eggs and sperm suggests that both species may be self-fertile. The amount of sperm in the predominantly male gonads however, would not be expected in exclusively self-fertilisers, suggesting that some outcrossing occurs. The significantly higher reproductive output of P. vivipara at Midway Point than at the Tesselated Pavement indicates that the Midway Point population is particularly important for the conservation of this species. P. vivipara and P. parvivipara have small, 140 to 150 μm-diam eggs. Embryogenesis is asynchronous, with progeny at different stages of development in the gonads. The end-point of brooding is variable in P. vivipara, with juveniles emerging from the parent at sizes ranging between 1.5 and 5.0 mm diam. Juvenile P. parvivipara are born at a diameter of up to 2.5 mm. At birth the juveniles of both species are up to 25–30% of the parent's diameter. Birth involves distension of the gonopore, with a marked separation of the ossicles and softening of the connective tissue around the pore. The juveniles are considerably larger than the ova and depend on extraembryonic nutrition to support their growth. Once the mouth opens, the juveniles prey on their intragonadal siblings. Cannibalism accounts for the substantial post-metamorphic growth. Viviparity lies at the extreme end of the broadcasting-brooding continuum of life histories in Patiriella, and the life-history traits of P. vivipara and P. par-vivipara are compared with those of other Patiriella spp. to assess the changes associated with evolution of viviparity.