Plasticity in the brooding cidaroid Ctenocidaris nutrix

Phenotypic plasticity refers to the ability of species to display morphological, physiological, or behavioral modifications in response to changes in environmental conditions, without any change in the individual genotype. Phenotypic plasticity may be active and physiologically regulated, involving trait changes through stress-response pathways. It can also be passive and solely induced by growth-limiting factors such as nutrients and food resources. Active plasticity is often adaptive as it can enhance or stabilize fitness, or limit fitness loss. Phenotypic plasticity is a key feature when assessing species response to climate change.
The evolution of phenotypic plasticity is usually expected to be more beneficial to broadcasting species (adaptation to a heterogeneous environment) than to brooding species. In the present work, we report a significant plasticity in a direct developer, the brooding sub-Antarctic species Ctenocidaris (Eurocidaris) nutrix (Thomson, 1876) (Echinoidea, Cidaridae) formerly believed to be composed of two distinct (sub)species. The species was initially described from the remote Kerguelen archipelago where it is common on the shoreline and over the surrounding shelf. Two distinct morphologies were described: the longispina and nutrix forms, notably differing in the size of their spines. A recent study revealed that the two forms shared a single COI haplotype in the Kerguelen archipelago, regardless of depth or location. This could indicate that the observed morphological difference is associated to phenotypic plasticity..
Therefore, phenotypic plasticity was investigated through morphological, anatomical, and trophic (using stable isotopes and gut content studies) analyses in populations around the Kerguelen archipelago. In addition, specimens from two close sites (about 20 km apart) contrasting in terms of swell exposure were further characterized by their respective tegument and gonad biomass, as well as biomechanics of the skeleton. Gut content and isotope analyses suggested slight significant trophic differences between sites, the species mostly feeding on sponges. Significant anatomical differences concerning relative spine length and apex diameter were recorded and appeared linked to depth or swell exposure. Echinoids from exposed and shallow sites showed shorter spines, and wider apex relative to the test diameter. On the contrary, specimens from sheltered or deeper sites had longer spines, and narrower apex. Comparing the two close sites revealed that specimens from the exposed shallow site had heavier gonads compared to those from the sheltered shallow site. These results suggest that phenotypic plasticity occurred in a brooding cidaroid echinoid. As formerly demonstrated in broadcasting euechinoid species, morphological plasticity appeared linked to environmental parameters, mainly depth and swell exposure.