Intertidal vermetid reef as a shelter for invasive bivalves in a tropical bay

Vermetid reef building species play an important role as ecosystem engineers modifying the physical environment, creating microhabitats and affecting local hydro-sedimentary patterns. We explore the association of native vermetid reefs (Petaloconchus varians) with the invasive bivalves Isognomon bicolor and Leiosolenus aristatus. We also examined the different utilization of the reef habitat, since L. aristatus is a boring species, comparing their population structure on sheltered and exposed rocky shores. Ten sites (five sheltered, five exposed) were sampled at Ilha Grande Bay (Rio de Janeiro, Brazil). I. bicolor and L. aristatus were the most frequent with mean densities of 20,000 (±3400) and 2100 (±600) ind.m2, respectively. There were no significant differences (t = 1.41; p = 0.17) between sheltered and exposed sites in relation to the densities of I. bicolor. However, there were significant differences (t = 3.14; p = 0.03) in relation to the densities of L. aristatus, with higher values at exposed sites. Mean size of both species was significantly (p < 0.01) higher in sheltered areas where the vermetid weight was significantly higher (t = 2.36; p = 0.02). Although native vermetid reefs act as a shelter for both invasive bivalves on sheltered and exposed rocky shores, they prevent the growth of I. bicolor which reaches smaller sizes in relation to populations outside of the reefs. Our results indicate that the two invasive bivalves might be differently affected by hydrodynamic processes.