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

Primarily for intertidal species, internal brooding has been reported as a frequently used reproductive strategy of sea anemones. While brooding reduces dispersal, this strategy seems to be beneficial in a harsh environment, such as the intertidal zone. We focused on the species composition, habitat preference and spatial structure of the internally brooding sea anemones, Aulactinia stella and Urticina crassicornis, which both form aggregations in the intertidal zone of the Barents Sea. We analyzed the color patterns of polyps within aggregations and the distance between polyps at different ages. Juveniles settled near the individual parent that released them. The distance between the U. crassicornis parent individual and its juveniles increased with increasing size of the juveniles. This tendency leads to a spatial structure based on the aggregations comprising one or two reproducing individuals that are surrounded by small, identically colored anemones. Our findings revealed that monochromatic aggregations dominated, which suggests that U. crassicornis similar to A. stella may reproduce asexually. A. stella appeared to be more resistant to wave action and more able to colonize the sandy ground compared to U. crassicornis, which prefers large boulders providing shelter, avoids sites exposed to strong wave action and often occurs in dense aggregations in the habitat patches favorable for survival and reproduction. It seems that internal brooding allows for the successful colonization of harsh and heterogeneous habitats, such as the intertidal zone, by counteracting the washout by waves and tidal action.