Hidden diversity in sympatric sponges: adjusting life-history dynamics to share substrate

Marine biodiversity is difficult to assess accurately in part because of the existence of sibling species, which are difficult to discern. This is particularly tricky when sibling species live in sympatry. We investigated biological and ecological traits in 2 sympatric sibling sponge species inhabiting the shallow north-western Mediterranean: Scopalina lophyropoda Schmidt, 1862 and S. blanensis Blanquer & Uriz, 2008. Growth, fissions, fusions, and survival were monitored twice monthly for 2 yr. S. lophyropoda slightly increased in area over the 2 yr period, whereas S. blanensis did not show effective growth, since gains in autumn to winter were offset by losses in spring to summer. Survival was significantly different in both species. By the end of the study (24 mo), 74% of the individuals of S. lophyropoda and 34% of S. blanensis survived. All individuals of S. lophyropoda and all but 5 of S. blanensis underwent fissions or fusions at least once during the study. The frequencies of multiple fissions and fusions were higher in S. blanensis than in S. lophyropoda. These 2 sympatric sibling species share common traits such as a high dynamism (higher than any other previously studied encrusting sponge species) and intra-species variability in growth. However, they showed contrasting ecological strategies (conservative in S. lophyropoda vs. opportunistic in S. blanensis), which favours species coexistence. This example shows for the first time how seasonality promotes the coexistence of sibling sponge species in the Mediterranean, and may represent an important step towards understanding species coexistence mechanisms.