Population structure of turbinid gastropods on wave-exposed subtidal reefs: effects of density, body size and algae on grazing behaviour

Abstract

Knowledge of the population structure and feeding capabilities of herbivores is critical to evaluate their influence on energy flow and community structure in their habitats. We tested for patterns in abundance (24 reefs) and size (12 reefs) of turbinid gastropods across 4 locations spanning >6° latitude (~1000 km) in Western Australia, and we tested the effect of density (1 to 5 individuals) and size (38.8 to 747.1 g wet wt) of Turbo torquatus on consumption of macroalgae with different thal- lus structure (Functional Groups 3 to 5). Turbinid gastropods were found at all locations (up to 2.4 ind. m -2 ); 82.4% of all individuals were T. torquatus. One location (Marmion, Perth) had considerably higher abundances than all other locations. Populations of T. torquatus at the 2 southern locations had a broad range of sizes (9 to 119 mm total shell length (TSL)), although 1 location was dominated by small, and the other, by large, individuals. In contrast, both northern locations were strongly dom- inated by 1 size class (40 to 60 mm TSL). T. torquatus consumed Ulva lactuca, Hypnea valentiae, Hennedya crispa and juvenile Ecklonia radiata at rates of 150 to 450 mg blotted fresh weight d -1 , depending on density of gastropods and species of algae. There was a positive relationship between the rate of consumption and size of T. torquatus when fed U. lactuca and H. valentiae. The present study has produced 3 main insights: (1) densities of turbinids on offshore, wave-exposed, subtidal reefs are similar to those in other coastal habitats; (2) patterns of abundances and sizes are consistent with broad-scale processes, such as ocean climate, fishing pressure and eutrophication; and (3) T. torquatus can consume a variety of macroalgae at rates that suggest it has the potential to exert top- down control of macroalgae, although low densities of gastropods preclude strong effects.