Implication of the biofilm ageing for the settlement of the vermetid gastropod Dendropoma cristatum (Biondi 1857)

Dendropoma cristatumreefs are key-intertidal habitats of the central Mediterranean. Knowledge onD. cristatumbiology is presently scant, particularly on its early life stage. The development of embryos occurs inside the maternal shell, and the crawling larvae settle on hard substrates shortly after hatching. Epilithic biofilm is known to have implications in the settlement of many marine invertebrates; however whether biofilm plays a role in driving the vermetid settlement is unknown. In this study the microbial assemblage ageing is tested as apotentialtriggerforD. cristatumsettlement. A field experiment was set-up to compare the larval settlement rate on removable limestone cubes (5x5x2 cm), covered by a 28-, 32-and 40-days old vs. a 4-days old biofilm. Limestone showed physical properties suitable forD. cristatumsettlement. Cubes were fixed in equal number along the seaward edge of two reefs (NW Sicily) and the vermetid recruits attached within a 20-day period were counted. Overall, 709 recruits were recorded on the cubes, 76.4% of which settling on 28-40 days and the rest on 4-d biofilmed cubes. The number of settlers on 28- and 32-d cubes was 6-7 fold higher than those of 4-d, and doubled (i.e. 14-fold) in 40-d. SEM observations revealed the higher biological complexity of the older biofilm, rich in diatoms and cyanobacteria embedded in a mucal web. Our findings suggest a key-role of biofilm ageing in the settlement preference ofD. cristatumlarvae. Molecular analyses are underway to identify microorganisms and metabolites potentially facilitatingD. cristatumsettlement.