Induction of settlement in crab megalopae by ambient underwater reef sound

The larvae of a number of crab species have been found to orientate and swim toward ambient underwater sound emanating from coastal settlement habitats. This current study examined whether ambient underwater sound also has the potential to trigger settlement responses in crab larvae. The effect of exposure to reef sound on the settlement behavior and time to metamorphosis (TTM) was examined in the megalopae of 5 common crab species, 3 from temperate waters and 2 from tropical waters. The megalopae of all 5 crab species showed marked changes in swimming behavior and a significant decrease in TTM when exposed to replayed ambient underwater reef sound compared with a silent (control) treatment. Megalopae exposed to sound decreased swimming activity earlier and displayed crawling behavior that was a precursor to both settlement and metamorphosis. Sound exposure decreased the median TTM by 33 h in Hemigrapsus sexdentatus and by 75 h in Grapsidae sp. 2. The consistent results among all species examined indicate that ambient underwater sound is likely to be an important settlement cue for the megalopae of many crab species. The wider ecological significance of acoustic settlement cues in crab larvae, in relation to other settlement cues and processes, now needs to be determined.