Single-species bacteria in sediments induce larval settlement of the infaunal polychaetes Polydora cornuta and Streblospio benedicti

Larval settlement of the infaunal spionid polychaetes Polydora cornuta and Streblospio benedicti is mediated by sediment-associated microorganisms. To investigate if larval preference for certain sediment is guided by individual sediment-associated bacteria, 13 bacterial isolates (5 phyla) obtained from the natural habitat of adult polychaetes (Wadden Sea, Germany) in 2008 were screened in still-water, no-choice settlement assays. Two isolates (α-Proteobacterium-Strain DF11 and Flavobacterium-Strain 54) significantly triggered larval settlement in comparison with sterile sediment. In still-water, multiple-choice settlement assays comprising natural and sterile sediment and sediment re-inoculated with isolates DF11 and 54, significant preferences for natural sediment and sediment containing bacterial isolates at 10⁸ cells g^?1 were observed. Larval settlement was influenced by bacterial abundance in sediment but the correlation was not strictly positive; thus, maximum larval settlement in response to single bacterial species may occur at certain optimum densities. Non-viable or suspended bacteria and water-soluble bacterial products did not induce larval settlement, suggesting that sediment-associated bacterial settlement cues for P. cornuta and S. benedicti were either produced in situ and/or consisted of heat-labile bacterial products.