Comparison between death and living molluscs assemblages in a Mediterranean infralittoral off-shore reef

Abstract

The agreement between the death (DA) and the living (LA) molluscs assemblages is analyzed in an infralittoral off-shore reef in the Central Tyrrhenian Sea (“Secche di Tor Paterno” Marine Protected Area) with extensive coralligenous and Posidonia oceanica biocoenoses. These biocoenoses host a highly diverse assemblage of small sized species. The work has a high species resolution and takes into consideration the richest but also more taxonomically difficult size class (1–6 mm). DAs contain allochthonous species which mainly live in deeper soft substrates but also contain species which were under-represented in the LA samples because they are endolithic or cemented to hard substrates, although such species are numerically rare. Multivariate analysis shows there are significant differences between living and death assemblages within habitats, but also that DAs are able to discriminate the two types of habitat. Fidelity of species richness and evenness is better in the coralligenous biocoenosis rather than in the P. oceanica meadow. This may be due to the reduced transport of shells out of the rhizome layer due to its reduced hydrodynamism due to the effect of the canopy. The good fidelity in the coralligenous biocoenosis suggests that the situation is close to complete ergodic mixing that draws individuals that coexisted or potentially coexisted in the same habitat into the DA. The lower fidelity in the P. oceanica meadow suggests incomplete ergodic mixing which can occur when within-habitat spatial mixing does not result in homogenization of dead skeletal material because transportation is non-uniform. Marked differences in species abundance between the LA and the DA are observed in both habitats and the result is consistent with previous works in similar habitats. Among the factors influencing fidelity, we hypothesize that trophic guild and life span of species may influence species abundance because carnivores and scavengers are more abundant in the living assemblage while microalgae herbivores are more abundant in the death assemblage. This difference in the fidelity of guild abundance may be connected to the different life span of species because many herbivores have short life span while carnivores and scavengers have long life span, although this issue needs to be further tested. On the contrary, temporal changes in community composition and individual preservation rate are not considered to influence fidelity significantly. The study suggests low bottom transport of dead shells in these habitats. This work opens good prospects for biodiversity assessments on the basis of DAs because it allows reduced diving and field time and better faunistic description due to time-averaging. However, dead specimens tend to lose important diagnostic characters and may require more skilled personnel for sorting and identification.