CONSEQUENCES OF MORPHOLOGY AND TISSUE STRENGTH TO BLADE SURVIVORSHIP OF 2 CLOSELY-RELATED RHODOPHYTA SPECIES

In Barkley Sound, British Columbia, Canada, the wide-bladed red alga M azzaella splendens occurs intertidally at low to intermediate wave exp osure sites but not at adjacent high wave exposure intertidal sites th at are occupied by the narrow-bladed sister species Mazzaella Linearis . This study used morphological and biomechanical characteristics of b oth species to determine whether or not drag and acceleration forces c ould prevent blades of M. splendens from surviving at high exposure si tes and hence account, in part, for their distributions. For each spec ies, these hydrodynamic forces were calculated for gametophyte and spo rophyte phases and, when possible, short and long thalli. The most fre quent break location when thalli were pulled by a spring-scale was the junction between the stipe and holdfast. The following predictions we re made by a model that compared hydrodynamic forces which a blade wou ld experience to the measured force required to break the stipe/holdfa st junction: (1) long blades of M. splendens should not occur at high wave exposure sites, (2) within the range of wave exposure occupied by M. splendens, sporophytes should be more abundant than gametophytes w hen and where wave exposure is greatest and (3) long blades of M. line aris are predicted to occur at high wave exposure sites. All 3 of thes e predictions agree with other studies of natural populations. Two pre dictions do not agree with field observations: (1) long M. linearis ga metophytes are predicted to have greater survivorship at higher water velocities than long sporophytes but, in natural populations, sporophy tes are actually more abundant when wave action is greater and (2) sho rt blades of M. splendens are predicted to survive very high water vel ocities but, in reality, are absent from high wave exposure sites in B arkley Sound. The latter contradiction suggests that recruitment of M. splendens at high wave exposure sites is prevented at a life history stage prior to the development of short blades.