Selection or random picking? Foraminiferal tests in Sabellaria alveolata (Linnaeus, 1767) bioconstructions

This work investigates whether the honeycomb worm Sabellaria alveolata (Polychaeta) actively selects foraminiferal tests to build its arenaceous bioconstructions and if the tests are chosen based on a defined criterion. To this purpose, both foraminiferal content and structure of communities were compared across samples of bioconstructions and samples of neighbouring sediment collected from two sites of southern Sicily (Central Mediterranean). Results document a higher concentration of foraminiferal tests within the tubes than in sediment, with a clear preference for biconvex and spherical morphologies. We hypothesize that the high proportion of biconvex and spherical tests in the bioconstruction is probably due to the active selection operated by the polychaete combined with the different buoyancy of foraminiferal tests kept in suspension by the wave motion. Among the grains agglutinated in the tubes, we also observed sporadic specimens of Amphistegina lobifera Larsen, a non-indigenous foraminifer native to the Red Sea.