Ammonia Induces Settlement Behavior in Oyster Larvae.

Oyster larvae exposed to solutions of NH4Cl exhibit stereotypical settlement behavior similar to that which normally precedes cementation and metamorphosis. Un-ionized ammonia is the active chemical species. At pH = 8.0, the threshold concentration of NH4Cl (pH = 8.0) for newly competent larvae is 2.5 mM; maximum activity is at 7.9 mM, corresponding to calculated NH3 concentrations of 100 {mu}M and 310 {mu}M, respectively. Induction of settlement behavior is rapid, with >90% of larvae exposed to 310 {mu}M NH3 responding within less than 5 min. After 15 to 30 min, larvae become habituated to NH3 and resume swimming so that the percent exhibiting settlement behavior after 30 min is <10%. Other weak bases, such as methylamine and trimethylamine, induce similar behavior suggesting that NH3 acts by increasing intracellular pH. Evidence that NH3 and L-3,4-dihyrodxyphenylalanine (L-DOPA) induce settlement behavior through different mechanisms is presented. Ammonia may be a natural environmental cue that promotes oyster settlement behavior and, ultimately, recruitment.