Spatial variability in sexual and asexual reproduction of the fissiparous seastar Coscinasterias muricata: the role of food and fluctuating temperature

Populations of the fissiparous seastar Coscinasterias muricata were surveyed around New Zealand to determine the frequency of asexual reproduction. Seastars from an intertidal popula- tion from the Maori Bay (North Island) and 2 shallow subtidal-intertidal sites in Otago Harbour on the east coast of the South Island had split, were asymmetrical, had multiple madreporites and skewed sex ratios when gonads were present. In contrast, none of the populations from 13 subtidal populations from the fjords on the west coast or a subtidal population from Stewart Island had characters indicat- ing that they had recently split. Examination of genetic diversity using allozymes agreed with mor- phological interpretations; the populations from the Maori Bay and the east coast (Otago Harbour) had significantly lower genotypic diversity than expected, while the fjord populations had genotypic diversity that conformed to expectations under sexual outcrossing. We investigated the effects of food supply and fluctuating temperature on growth, energy storage, gonad development and fission of C. muricata from a fissiparous population in Otago Harbour. Growth, gonad development and the py- loric caeca index increased with increased food supply. No effects were found due to the fluctuating temperature regime. Fission occurred in all treatments except when the seastars were starving, as in- dicated by decrease in size. Gonad development and energy storage under intermediate food supply were similar to measurements of individuals in the field, and suggest food limitation for the popula- tion studied. Since fissiparity decreases with increasing size in C. muricata and growth rates and go- nad development increase with supply of food, we propose that the occurrence of apparently fewer fissiparous individuals in habitats where food is abundant is correlated to food supply. Rapid growth would allow individuals to more quickly reach the size threshold at which fission begins to be sup- pressed. Conversely, where food supply or feeding time is limiting growth, the pattern would be a relative increase in abundance of small fissiparous individuals. However, populations from the fjords and Stewart Island do not seem to undergo fission at all. A possible explanation could be that the asexual mode of reproduction of this species is not triggered in the sheltered subtidal fjord environ- ment. Another possibility could be that natural selection acts against asexual reproduction by fission in the fjord environment, or that the ability to split was lost due to founder effects and random genetic drift when the fjords were successively colonised following the rise in sea level after the last ice-age.