Factors that drive sea urchin feeding behavior

JUAN P. LIVORE

Adelaide

Simposio; International Temperate Reefs Symposium; 2009

The University of Adelaide

Sea urchins have been identified as major contributors to community structure in rocky reef habitats all over the world, including temperate waters. These herbivores can form dense fronts that switch their feeding behaviour from trapping drift algae to actively grazing the benthos and depleting any standing algae in their path. The removal of predator pressure has most commonly been suggested as the main factor driving these increases in sea urchin density and therefore the creation of barren areas. In South Australia sea urchins lack the presence of major predators, yet up to date there has been no record of sea urchin fronts or barren areas. In this manipulative field experiment we test the hypothesis that more than one factor could be driving this change in sea urchin behaviour. Sea urchin density and food availability were manipulated and the effect these factors had on a developing algal community was measured. Our results showed that algal biomass was significantly higher at lower sea urchin density. Trapped sediment, which has been shown to be positively related to turf algae biomass, was higher when food availability was lower. Although there were no significant differences in gonad index, which is an indicator of general urchin condition, there was a trend towards lower index at high density and low food availability. This study shows that bottom-up factors such as spatial subsidies as well as sea urchin density can influence this herbivore?s feeding behaviour.