Internal brooding affects the spatial structure of intertidal sea anemones in the Arctic-boreal region

Phylogenetic Relatedness within the Internally Brooding Sea Anemones from the Arctic-Boreal Region

Phylogenetic analyses based on mitochondrial 16S rDNA, nuclear 28S rDNA, and morphological and ecological traits of Aulactinia, Urticina and Cribrinopsis sea anemones inhabiting the Arctic-boreal region indicate discordances between trees derived from molecular sequences and those based on morphological traits. Nuclear genes were more informative than mitochondrial and morphological datasets. Our findings indicate that 16S rDNA has limited applicability for phylogenetic analyses at lower taxonomic levels and can only be used for distinction of families. Although 28S rDNA allowed for the classification of distinct genera, it could not confirm that species of Urticina and Cribrinopsis, which appeared to be closely related, were correctly separated into two different genera. The nuclear tree revealed inconsistencies between specimens belonging to European Urticina crassicornis and Pacific U. crassicornis; the latter seems to be a different species. In contrast to Pacific U. crassicornis, the specimens collected from different localities in the Barents Sea are on the same tree branch. The same was observed for specimens of Aulactinia stella. Both species brood their young internally. The dispersal of sea anemones with brooding juveniles seems to be less limited than expected and might be sufficient to settle habitats more than a thousand kilometers away.

Remarkable Shifts in Offspring Provisioning during Gestation in a Live-Bearing Cnidarian

Animals display diverse means of producing and provisioning offspring, from eggs to embryos and juveniles. While external development predominates, many forms of embryonic incubation have evolved, including placentation in mammals and a number of understudied variants in basal metazoans that could help understand evolutionary diversification. Here we studied the brooding sea anemone Aulactinia stella, using behavioural, morphological and biochemical indicators of offspring phenotype to characterize gestation and elucidate parental and sibling relationships. The pronounced variance in juvenile weight within broods was not strongly related to any of the typical external predictors (adult weight, clutch size, sampling date, environmental conditions). Lipid concentration was significantly higher in the tissues of the small juveniles than in those of large juveniles or adult, and fatty acid profiles tended to set small juveniles apart. Finally, intra-brood feeding on external resources was documented in larger juveniles. These results are consistent with ontogenetic shifts in nutrition, from vitellogenic provisioning to post-zygotic nourishment to a prenatal form of nursing upon acquisition of feeding organs, highlighting matrotrophic and conflict-driven mechanisms acting on offspring phenotype during gestation.