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

Abstract

Populations of the fissiparous seastar Coscinasterias muricata were surveyed around New Zealand to determine the frequency of asexual reproduction. Seastars from an intertidal population 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 indicating that they had recently split. Examination of genetic diversity using allozymes agreed with morphological 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 pyloric 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 indicated 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 population studied. Since fissiparity decreases with increasing size in C. muricata and growth rates and gonad 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 suppressed. 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 environment. 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.

Potential for clonal animals in longevity and ageing studies

“…Another interesting feature of clonal and colonial animals that may be of relevance to their longevity is that they are commonly smaller than their sexually reproducing counterparts. For instance, individuals from clonal populations of the sea star Coscinasterias muricata are smaller and grow slower than those from the obligate sexually reproducing populations of the same species (Sköld et al 2002(Sköld et al , 2003. Here, mean length of longest arm of clonal individuals from Otago harbour and from North island Maori bay were 25 and 60 mm respectively.…”

“…The molecular mechanisms behind this elevated longevity seem to be upregulation of Sirtuins (Wood et al 2004), insulin/IGF-1-like receptors (Gami and Wolkow 2006) and reduced oxidative stress (Sohal and Weindruch 1996). Phenotypes of especially clonal sea stars resemble such caloric restriction, with smaller-sized adults and reduced feeding (Emson and Wilkie 1980;Barker et al 1991;Sköld et al 2002Sköld et al , 2003. In sexually reproducing animals, there is on the other hand a positive correlation between body size, metabolic rate and postnatal lifespan when different species are compared (Finch 1990) and at first this relationship may contradict the observed effects of caloric restriction.…”

Genetic subdivision of a sea star with high dispersal capability in relation to physical barriers in a fjordic seascape

“…Sexual recruitment is irregular, and in addition to sexual reproduction, C. muricata propagate asexually through fission (Crump & Barker 1985, Johnson & Threlfall 1987. However, not all populations show signs of fission, and no evidence of asexual reproduction has been observed in the fjords of the South Island of New Zealand (Sköld et al 2002).…”

Asexual propagation in the marine bryozoan Cupuladria exfragminis

“…Colonies may use the lowered temperatures as a cue to indicate times of higher food levels that occur during upwelling times. Higher food levels have been observed to stimulate asexual propagation in a Seastar (Sköld et al, 2002) where it was believed to assist in the regeneration of new individuals. Similarly, higher food levels could help cupuladriid fragments regenerate, particularly in providing energies to defend against deleterious processes (see next section).…”