Arca noae

Abstract: Molecular characteristics, especially 18S rDNA sequences, may be of great value for the study of bivalve evolution and its numerous morphological convergencies once the reliability of these data can be evaluated. The analysis of 11 published complete molluscan sequences and two new ones,Arca noae andAtrina pectinata, reveals considerable differences in relative substitution rates. The gastropod and eulamellibranch species have the fastest and Atrina species have the slowest rates. Two methods are used to assess the information contents of the dataset in addition to bootstrap analysis, spectral analysis, and the “pattern of resolved nodes” technique. Tree reconstructions by parsimony, neighbor-joining, and maximum-likelihood differ in regard to the position of the eulamellibranch family Mactridae and ofCrassostrea. Although there is a signal for the monophyly of Bivalvia, Mactridae cluster with Gastropoda in most runs, rendering Bivalvia diphyletic. The position ofCrassostrea was extremely variable, probably due to the high substitution rate of this species.Atrina roots deeper thanArca in all trees, although a corresponding signal in spectral analysis is absent. Phylogenetic signals among the three pectinid species are low but sufficient to resolve the branching pattern. The tree inferred from the 18S rDNA and from morphological data has Bivalvia monophyletic with a basal polytomy of Mactridae,Crassostrea, and the remaining Pteriomorphia, whereArca branches off before Atrina and the Pectinidae.Argopecten is sister group to the other two pectinids; 18S sequence data will have great impact on our understanding of bivalve phylogeny, but only when more sequences of similar substitution rates are available.

Abstract: Mediterranean populations of the bivalve Arca noae were found to be specifically fouled by the demosponge Crambe crambe. The surface of the shells forms spines which favour settlement of the sponge larvae. The reproductive strategy of this symbiotic sponge contrasts with other Mediterranean sponges, which produce numerous small larvae, while Crambe crambe produces relatively small numbers of quite large larvae containing secondary metabolites. The selective shell fouling of Arca noae suggests that this is behavioural farming of a chemically defended sponge by a bivalve which does not sequester or synthesize de novo metabolites. Some of the observed benefits of this symbiosis are the enhanced survival of the bivalve, and the sponge being able to live in a site free of competitors. This mutual symbiosis is optional for the sponge but essential for the bivalve, since the fouled bivalves can use the secondary metabolites produced by the sponge to deter predators. We tested the effects of sponge epibiosis on predator preferences. Feeding trials were carried out on the starfish Marthasterias glacialis and the snail Hexaplex trunculus in Hornillo Bay, SE Spain. When A. noae fouled by the sponge Crambe crambe were offered to starfish and snails, they survived significantly more than unfouled bivalves. Field studies in the Mar Menor lagoon indicate that the aposymbiotic populations of A. noae have disappeared from this hypersaline lagoon due to the biological invasion by the non-native snail Hexaplex trunculus. This symbiotic relationship was also effective against octopus. Aposymbiotic bivalves were rapidly eaten when presented to octopuses, while fouled bivalves were consistently rejected.

Abstract: This study provides information about differences in composition of ingested zooplankton amongst bivalve species coexisting in the same area in a period from May 2009 to December 2010. The study was conducted at the Mali Ston Bay (42°51′ N, 17°40′ E)—the most important bivalve aquaculture area in the eastern Adriatic Sea. Stomach content analysis was performed on cultured species—Ostrea edulis and Mytilus galloprovincialis, and commercially important bivalve species from their natural environment—Modiolus barbatus and Arca noae. Results confirmed carnivory in bivalves, both from natural and cultured populations, but cultured species had higher numbers of zooplankters than those living on the seabed. The most abundant taxa were bivalve larvae, followed by tintinnids, copepods, unidentified eggs and gastropod larvae. Recorded numbers of bivalve larvae in M. galloprovincialis stomach were the highest so far reported and show that mussels impact the availability of natural spat.