HERBIVORY IN THE SOFT CORAL Sinularia flexibilis.

Ike Olivotto*, Chiara Carla Piccinetti, Roberta Ricci, Chiara Pennesi, Giuseppe Radaelli, Cecilia Totti, Alessandra Norici, Mario Giordano.
*Dipartimento di Scienze della Vita e dell'Ambiente,
Università Politecnica delle Marche,
via Brecce Bianche, 60131 Ancona,
Italy.
E-mail: i.olivotto@univpm.it

Many corals are able of both autotrophy, thanks to their photosynthetic symbionts, and heterotrophy. The contribution of heterotrophy to coral nutrition can be substantial in several species and in conditions such as bleaching and low irradiance. Coral heterotrophy can be supported by various substrates, including particulate organic matter, zooplankton, bacteria and phytoplankton, including diatoms. Heterotrophy is necessary to maintain an appropriate elemental stoichiometry of the animal, provided that the algae typically export to the animal mostly organic C skeletons poor in other nutrients. The ability of different coral species to feed on plankton has been investigated using different approaches and methodologies, including microscopy, removal rates in feeding tanks, labeling of the prey with radioactive/fluorescent markers and use of specific sequences to identify ingested preys. Most of these studies have been conducted on zooplankton; the data on phytoplankton predation by corals are scant, although the importance of phytoplankton in food webs and in coral feeding  is of paramount importance.

Diatoms are an important component of phytoplankton in all ecosystems and in coral reefs and are responsible for a large proportion of marine primary production.

The information available on the predation of phytoplankton by corals is inconsistent and sometimes contradictory. Part of these inconsistences is due to methodological difficulties.

The aim of this study was to obtain solid proofs of the uptake of diatoms by soft corals. The utilization of a variety of methodologies allowed us to compare the effectiveness of several approaches and demonstrate that some differences in previous reports were due to the use of not fully adequate methods. We selected two species, the centric diatom Thalassiosira pseudonana and the soft coral Sinularia flexibilis, for the investigation of whose biology a large array of tools exists. Morphological analyses clearly show that the diatom is ingested by the coral; molecular markers did not prove to be sufficiently selective/specific to afford clear results. We therefore suggest the use of multiple methodologies to investigate heterotrophy in corals, since no single method can ensure unequivocal results.