The resistance to dislodgement ofSargassum muticumgermlings under defined hydrodynamic conditions

Abstract

The attachment capabilities of a brown seaweed,Sargassum muticum(Yendo) Fensholt were investigated in the laboratory on both smooth and rough substrata and in surging water motion reaching flow velocities of 57 cm s and 90 cm s.Newly released zygotes have no means of adhesion until they develop rhizoids, which can adhere on contact with the substratum. Attachment tenacity, as measured by the germling's ability to withstand dislodgement at a given velocity of water flow, is slight at first, but increases with time. This is not simply a function of time spent in residence on the substratum, for germlings cultured suspension stick very quickly and tenaciously when allowed to settle. Tenacity of adhesion seems to be a reflexion of the amount of mucopolysaccharide adhesive present. This increases gradually during the first few days of the germling's life, regardless of whether it is attached or not.Once secure attachment is achieved it is not weakened even by the death of the germling. A film of bacteria and micro algae on the surface of the substratum is not a requirement for germling adhesion, but its presence enhances the tenacity of adhesion.Smooth substrata are the least favourable for germling survival in flowing water. On rough substrata the ‘peaks’ of the microtopography and surfaces facing into the current are much less favourable than the troughs between the ‘peaks’, or surfaces in the lee of ‘peaks’. Germling survival increases with increasing substratum roughness up to an optimum rugosity at which the mean depth of the depressions on the surface is 420 μm, and the vast majority are deeper than the height of the germlings.