Induction of Larval Settlement by KCl in Three Species of Bugula (Bryozoa)

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Invertebrate Biology, Vol: 114, Issue: 4, Page: 345-351

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Wendt, Dean E.; Woollacott, Robert M.
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Elevation of the K+ concentration in seawater (added as KCl) induces larval settlement and metamorphosis in Bugula simplex, B. stolonifera, and B. turrita. All three of these bryozoan species have similar bell-shaped dose-response curves: 5 mM excess K+ is sufficient to increase settlement and metamorphosis significantly over seawater controls in all the species and optimal responses to excess K concentrations occur ations at 10-25 mM, 5-10 mM, and 10-15 mM for B. simplex, B. stolonifera, and B. turrita, respectively. Percent settlement in all three species is decreased at levels greater than 25 mniM excess KCl. Similar bell-shaped curves have been observed for KCl-induced settlement of larvae from other, but not all, invertebrate groups. Larvae of B. turrita do not settle at a significantly higher level than seawater controls when exposed to excess K+ concentrations >25 mM even in the presence of a second favorable artificial inducer, filmed chicken eggshell membrane, indicating that K+ at high concentrations interferes with the normal settlement process. Furthermore, pulse exposures for 5 min, 15 min, and 30 min to 10 mM excess K+ are ineffective at inducing settlement over levels in seawater controls. When larvae of B. turrita were induced to settle by 10 mM excess K+, completion of metamorphosis was not significantly different from that of controls lacking excess KCl (86% and 91%, respectively). However, if larvae induced by 10 mM excess K+ are not removed from the 10 mM excess K+ after settlement, the success ratio of metamorphosis is significantly lower than in controls (72% and 91%, respectively). Our results confirm previous studies demonstrating that settlement of bryozoan larvae can be induced by excess K+. Bryozoan larvae examined to date become responsive around 5-10 mM excess K+. These data suggest that shared under- lying features exist in the mechanism triggering settlement and metamorphosis in bryozoans and that excess K+ is a useful experimental tool for inducing settlement of bryozoan larvae.