Donax trunculus (Bivalvia: Donacidae) as a potential biological indicator of grain-size variations in beach sediment

Donax trunculus (Linnaeus, 1758) is one of the most common bivalve molluscs inhabiting the sandy shores of the Mediterranean Sea. This species, which is preferentially distributed on sediments at depths between 0 and about 2 m, is considered to be a substrate-sensitive organism because of its sensitivity to sediment grain size variations during its life cycle, in particular during its early growth stages. This study aims at investigating the relationship between D. trunculus’ density distributions and sediment grain size, in order to evaluate its use as a biological indicator of grain-size variations in beach sediment. For this purpose, a specific environmental study was performed by ISPRA (Italian National Institute for Environmental Protection and Research) along six beaches located in the Central Tyrrhenian Sea between Ladispoli and Anzio (Rome, Italy). On each beach, samples of D. trunculus and superficial sediments were collected in the infra-littoral zone at 3 different depths (0, 0.5, and 1 m) between June 2002 and March 2004. The results pointed out that the distribution of D. trunculus populations is strongly influenced by the sediment's grain size variations. A significant positive correlation was found between species density and the 2.00-phi and 2.50-phi grain size classes; but, in particular, the predominance of the 2.00-phi grain size class in the beach's sediment seems to positively influence the species density and distribution.This study suggests that sediment grain size is the chief factor controlling the distribution of D. trunculus populations; suitable grain sizes allow the settlement and the subsequent growth of well-structured populations. Generally, species density declined in areas subjected to grain-size variations in beach sediments, suggesting a predictable biological response to both natural and human-caused stressors. The present study also highlighted that where nourishment was carried out using suitable sediments, the species reappears on the beach after a few months, as it finds an optimal substrate for settlement. In this case, finding D. trunculus specimens a few months after replenishment indicates that the nourishment operations have been successful from an environmental point of view. As reported by Carignan and Villard (2002), a species that is strongly associated with particular habitat features could be a useful indicator; in particular, a species – like D. trunculus – that is negatively associated with human disturbances could be considered a “positive” indicator of ecological integrity.Long-term monitoring of D. trunculus abundance or density on sandy beaches (adequately carried out during the species’ recruitment period, e.g. between July and September), could help pinpoint the presence of natural or human-made phenomena that are leading to grain-size variations in the sediment. In conclusion, we can assume that D. trunculus can be used as a practical biological indicator to detect grain size variations on sandy beaches in coastal monitoring programmes; it could be a useful tool for researchers, managers and administrators to assess erosion phenomena while minimizing times and costs, also in view of an integrated coastal zone management.