Diel vertical movement by mesograzers on seaweeds

Diel vertical movement is web documented for many zooplankton. The ecology of small benthic herbivores which use seaweeds as food and habitat, 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 mesograzer, the sea hare Aplysia parvula, the sea urchin Holopneustes purpurascens, and the prosobranch mollusc Phasianotrochus eximius. All exhibited diel movement on host algae. This behaviour occurred on different host algae, despite variation in algal morphology and other characters. Possible factors causing diel movement by mesograzers include predation, nutritional gain, avoidance of photo-damage, micro-environmental variation near host algae, and reproductive strategies. These are discussed with regard to mesograzers and related to theories for diel vertical movement by zooplankton. The relative ease of experimental manipulation of benthic seaweeds and mesograzers makes them suitable as model systems to test theories for diel vertical movement.