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The grazing activity of consumers causes shifts between alternative states in a variety of terrestrial and marine ecosystems. One of the best examples of a consumer-driven shift occurs on temperate marine reefs, where grazing by high densities of sea urchins results in a shift from a foliose algal-to a crustose algal-dominated state. In this study, we focussed on 2 largely untested but important issues during the transition from a foliose algal-to a crustose algal-dominated state: (1) whether sea urchins impact foliose algal community composition by differentially removing species and (2) whether any impacts of grazing vary with 2 different densities of aggregating sea urchins. We tracked the movement of a high-density front of the sea urchin Heliocidaris erythrogramma and then performed experimental manipulations of H. erythrogramma at 2 unusually high but naturally occurring densities. Non-metric multidimensional scaling (nMDS) followed by analysis of similarities (ANOSIM) showed differences in the foliose algal community composition, and therefore differential removal of species, between permanent plots before and during grazing (surveys), and between grazed and ungrazed plots (experiment). Of the 6 abundant foliose algae, 2 had relatively low survivorship (Amphiroa anceps and Zonaria diesingiana), while 2 had relatively high survivorship (Delisea pulchra and Corallina officinalis) when grazed by high densities of sea urchins. Grazing by different densities of H. erythrogramma resulted in differences in the foliose algal community composition and for the chemically-defended D. pulchra there appeared to be a threshold sea urchin density required before its removal. Our results show that an intermediate community state composed of grazer-resistant foliose algae and crustose algae can occur, which may have important consequences for community composition.

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ABSTRACT-Diel vertical movement is well 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 exhlbit 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 photodamage, micro-environmental vanation 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 dlel vertical movement.

Although sea hares are well known for acquiring algal secondary metabolites from their diet, the effects of diet on fitness and the susceptibility of sea hares to predators are poorly understood. We examined the effects of diet on the performance of the sea hare Aplysia parvula, and measured predation by reef fishes on sea hares raised on different seaweeds. Diet-switching, body size, the presence of conspecifics, and the presence of epiphytes on dietary algae were also investigated experimentally. A. parvula were fed the chemically rich red algae Delisea pulchra or Laurencia obtusa in laboratory experiments, and consumption, growth, egg production, conversion efficiency, and survivorship were measured. Sea hares fed L. obtusa consumed more algae, grew faster, laid more eggs, and had higher survivorship compared to sea hares fed D. pulchra. Most other factors investigated -body size, the presence of conspecifics or epiphytes -were in general unimportant relative to the effects of diet. The disparity in fitness of A. parvula fed D. pulchra versus L. obtusa is primarily due to different levels of consumption of each seaweed, as conversion efficiencies were similar. Predation on sea hares raised on different diets and containing different types and levels of acquired secondary metabolites was measured in field experiments in which sea hares were exposed to mixed assemblages of reef fishes. A high proportion of both chemically rich (fed red algae) and chemically poor sea hares (fed the green alga Ulva sp.) were eaten by fishes (with predation rates of 25 to 55% over 1 to 2 h), although this sea hare was relatively unpalatable compared to squid tissue. Juvenile A. parvula were eaten at a significantly greater rate than adults. Sea hare ink did not deter some fishes, which consumed both de-inked and untreated sea hares. Predation by fishes was similar in adjacent habitats, but varied between sites, and was not restricted to A. parvula as reef fishes also ate juvenile A. dactylomela of equivalent size. The benefit A. parvula gain from acquiring algal secondary metabolites is unclear, as these compounds appear ineffective as defences against some fishes. A. parvula may sequester algal secondary metabolites as part of a broader defensive strategy that includes crypsis and escape movements.