Einflu� von Benetzungsspannung und lonen auf die Substratbesiedlung und das Einsetzen der Metamorphose bei Bryozoenlarven (Bowerbankia gracilis)

Abstract

Settlement and metamorphosis in larvae of Bowerbankia gracilis depend on the wettability of the substratum. The wettability of a solid can be characterized by it's specific contact angle, ϑ. Larvae settle on solids with angles >17°, but not on surfaces with high wettability properties. In an attempt to explain this phenomenon, we consider the larvae as a second liquid and their attachment as a second wetting. In such case, water and larvae would compete in wetting the substratum. Adhesion can be accounted for by assuming the wetting tension of solid to water to be lower than that of the larval surface (|σsolid/water| < |σsolid/larvae|). Adhesion would thus be favoured energetically. The rate of settlement and that of metamorphosis are demonstrated as depending on the degree of contact angle, ϑ, in an all-or-none fashion. Settlement of larvae on high-wetting solids is prevented by high capillary pressure accruing at the point of contact. Metamorphosis is initiated by contractions occurring after adhesion. Application of CsCl or KCl induces such contractions and metamorphosis in free-swimming larvae. MgCl2 prevents the onset of metamorphosis but not adhesion. Two mechanisms are considered as possibly accounting for induction of metamorphosis: (1) mechanical stimulation by shearing the cilia at the point of contact, and (2) a chemical stimulation of the cell membrane after application of K+ or Cs+.