Population biology and grazing processes of the sea urchin Centrostephanus tenuispinus (Clark, 1914) inhabiting coral and macroalgal dominated reefs

Sea urchins play a key role as herbivores and bio-eroders, impacting the organization and structure in many marine benthic habitats. Hall Bank reef (32°2.002´S and 115°42.957´E) off Western Australia is unique having high coral cover, which is unusual for high latitude reefs. Although the high density of Centrostephanus tenuispinus is believed to be the reason for the absence of macroalgae, lack of knowledge on the biology and ecology of this species hampers our understanding of the functioning these reefs. The main objective of this study was to evaluate the role of C. tenuispinus in structuring and functioning of Hall Bank reef with respect to macroalgae-dominated Minden Reef. Benthic surveys and monthly sample collections were carried out at two sites, to assess temporal variation of substrate cover, urchin density, reproductive patterns, feeding habits and bio-erosion. Higher density (2.94 ± 0.14 m-2) of C. tenuispinus with small tests at Hall Bank reef indicated low food availability, while lower densities (0.14 ± 0.01 m-2) of larger urchins at Minden Reef indicated high productivity in the habitat. A clear synchronised annual reproductive cycle was recorded. The gametogenic cycle was initiated by decreasing seawater temperature and day length in March, leading to spawning in winter. Minden Reef urchins had a higher Gonadosomatic Index, coinciding with high food availability. Less seasonal variability in substrate composition influenced the reduced variability in diet at Hall Bank reef while pronounced seasonal variation in the diet of Minden Reef urchins coincided with seasonal changes of substrate cover. Bio-erosion rates were positively correlated with sea water temperature. The annual bio-erosion rate in Hall Bank reef was 1017.69 g CaCO3 m-2 y-1. Changes in the population of Centrostephanus tenuispinus, being the dominant grazer and bio-eroder in Hall Bank reef, can greatly influence the structure and species composition of the reef, and could lead to ecological phase shifts in this reef system at extremes. The comprehensive knowledge acquired through this study on Centrostephanus tenuispinus provides baseline data for the region on this particular species which could be used in monitoring and managing reefs with current trends in climate change