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 lophyropodaSchmidt, 1862 andS. blanensisBlanquer & Uriz, 2008. Growth, fissions, fusions, and survival were monitored twice monthly for 2 yr.S. lophyropodaslightly increased in area over the 2 yr period, whereasS. blanensisdid 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 ofS. lophyropodaand 34% ofS. blanensissurvived. All individuals ofS. lophyropodaand all but 5 ofS. blanensisunderwent fissions or fusions at least once during the study. The frequencies of multiple fissions and fusions were higher inS. blanensisthan inS. 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 inS. lophyropodavs. opportunistic inS. 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.