Asexual reproduction and somatic growth of the fissiparous sea star Allostichaster polyplax in New Zealand

Allostichaster polyplax is a fissiparous (capable of reproducing asexually by fission) sea star found in intertidal and shallow subtidal habitats in New Zealand and Australia. A localized population of A. polyplax occurred at a density of 2.7?individuals?m^?2 in an intertidal boulder field at Otago Harbour, South Island, New Zealand. The percentage of sea stars undergoing fission, recorded from January 2010 to March 2011, peaked in January (midsummer) in both years. The frequency of fission decreased for larger sea stars (mean arm length, R?>?25?mm), suggesting an ontogenic shift from asexual to sexual reproduction. Effects of fission (i.e. arm regeneration) and food availability (small gastropods) on growth, storage and sexual reproduction were examined in a factorial laboratory experiment. There was no effect of arm regeneration or food on total arm growth. Storage capacity, measured as pyloric caeca index (PCI), was greater in full-armed than regenerating sea stars, and greater in fed than starved individuals. Fed individuals had a similar PCI as field-collected ones. Gonads were observed in two individuals (males) in the full-armed and fed treatment. Fission did not occur during the experiment. Feeding rate on snails was greater for full-armed than regenerating sea stars. Water temperature had a weak positive effect on the feeding rate of regenerating sea stars. Our findings suggest that fission and subsequent growth of ramets can sustain demographically stable and geographically isolated populations of A. polyplax. The role of sexual reproduction in population dynamics and connectivity of this species (and fissiparous sea stars in general) remains equivocal and requires further research on genetic differentiation within and among populations.