Our study focused on the Mediterranean species Hemimycale columella and Crella elegans, which have overlapping ecological distributions but contrasting population densities and resilience. We formulated the hypothesis that differential reproductive traits were the main cause underlying these ecological differences in the study area. The issues addressed were whether recruits compete for the substrate, either because their respective reproductive cycles overlap or/and larval performance differs between species, and whether a contrasting investment in reproduction contributed to explain their diverse densities. Both species were simultaneous hermaphrodites and incubated their larvae. The reproductive period was notably shorter in C. elegans than in H. columella, while investment in reproductive tissue was higher in individuals of H. columella than in C. elegans. In contrast, C. elegans larvae harbored larger amounts of lipids and yolk inclusions than H. columella larvae. Moreover, the former contained amazing collagen masses densely packed among their inner cells. H. columella is likely to produce a higher number of poorer equipped larvae while C. elegans seems to produce a lower number of more resistant, better fitted with energy reserves, and thus potentially more successful larvae. These reproductive patterns agree with a higher and stable density of C. elegans in the study site compared with a more variable abundance of H. columella.

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Despite their abundance in benthic ecosystems, life cycles and reproductive features of most sponge species remain unknown. We have studied the main reproductive features of two demosponges, Dysidea avara and Phorbas tenacior, belonging to phylogenetically distant groups: Orders Dictyoceratida and Poecilosclerida, respectively. Both sponges are abundant and share habitat in the Mediterranean rocky sublittoral. They brood parenchymella larvae with different morphology and behaviour. Sampling was conducted monthly over a two-year period in a locality where both species coexist. The two species reproduced in spring-summer, and presented species-specific reproductive features despite being subject to the same environmental conditions. D. avara has a shorter reproductive period than P. tenacior, ending before the peak of temperature in summer, while the reproductive period of P. tenacior lasts until beginning of autumn. Brooding larvae were present in June-July in D. avara, and in August-October in P. tenacior. Larval size, reproductive effort and number of larvae produced (measured the month with the maximum production) were significantly higher in D. avara than in P. tenacior. A higher reproductive effort and larval traits point to a more opportunistic life strategy in D. avara than in P. tenacior. A lack of overlap in the timing of larval release, as well as different reproductive traits, may reduce competition and facilitate the coexistence of these two sympatric and abundant sponges.

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