A preliminary study of buoyancy behaviour in Avicennia marina propagules

Propagules of A. marina (Forssk.) Vierh. were shown to be either ‘sinkers’, i.e. sink on shedding their pericarp, or ‘floaters’ i.e. remain buoyant after shedding their pericarp. While propagules from different estuaries showed big variations in buoyancy, generally mangroves in the north produced a high proportion of ‘floaters’ while those in the south produced mainly ‘sinkers’. The buoyancy of propagules appeared to be determined by the relative densities of the different parts of the cotyledons. Anatomical sections revealed that there was little difference between the different parts of ‘sinkers’, whereas in ‘floaters’ parts which were buoyant had a high proportion of intercellular air spaces and those which sank had closely- packed cells high in reserves. Some of the ‘sinkers’ were found to become buoyant after a period of submersion, although propagules from all estuaries did not behave in this way. Apparently those propagules which had a density close to that of sea water tended to become buoyant later and it was suggested that utilization of reserves reduced the original density to a level which enabled these propagules to float. The implications of variations in buoyancy of A. marina propagules for the further distribution of this mangrove along the southeast Cape coast are discussed.