Reproductive seasonality of Sea Urchin <i>Centrostephanus tenuispinus</i> on high latitude coral and macroalgal reefs in the south-eastern Indian Ocean

Abstract Sea urchins can have considerable ecological impacts on benthic habitats through grazing and bioerosion and many are exploited as fisheries resources. Of the abundant temperate sea urchins, Centrostephanus tenuispinus is among the least studied. We determined the reproductive seasonality of C. tenuispinus off western Australia at Hall Bank where a high density (2.94 ± 0.14 individuals m −2 ) of small to medium size (66.23 ± 0.24 mm mean test diameter) urchins has been found to suppress macroalgae recruitment and facilitate hard coral dominance of the benthos and at Minden Reef where, typical of most reefs in the region, a low density (0.14 ± 0.01 individuals m −2 ), larger sized (100.69 ± 0.45 mm) population occupies a habitat dominated by dense macroalgae. Centrostephanus tenuispinus exhibited a clear synchronized annual reproductive cycle. Gametogenesis began in autumn coincident with lowering sea water temperature and decreasing day length and spawning occurred in late winter and spring. The larger urchins from Minden Reef had significantly larger gonads and a higher % GSI (percentage gonadosomatic index) value than Hall Bank Reef. % GSI increased significantly at both sites between winter and summer, but the magnitude of the increase was much greater at Minden Reef (76%) compared to Hall Bank (10%). The results indicate that both populations have the same reproductive cycle but raise questions about the relative contribution the two populations make to the reproductive output of the species in southwestern Australia.