DIEL VERTICAL MOVEMENT BY MESOGRAZERS ON SEAWEEDS

Diel vertical movement is well documented for many zooplankton. The ec ology of small benthic herbivores which use seaweeds as food and habit at, known as 'mesograzers', is similar in some regards to zooplankton, and we hypothesised that mesograzers might also exhibit diel patterns of movement on host algae. We studied 3 non-swimming species of mesog razer, the sea hare Aplysia pasvula, the sea urchin Holopneustes purpu rascens, and the prosobranch mollusc Phasianotrochus eximius. All exhi bited diel movement on host algae. This behaviour occurred on differen t host algae, despite variation in algal morphology and other characte rs. Possible factors causing diel movement by mesograzers include pred ation, nutritional gain, avoidance of photodamage, micro-environmental variation near host algae, and reproductive strategies. These are dis cussed with regard to mesograzers and related to theories for diel ver tical movement by zooplankton. The relative ease of experimental manip ulation of benthic seaweeds and mesograzers makes them suitable as mod el systems to test theories for diel vertical movement.