Investigating the diets and condition of Centrostephanus rodgersii (long-spined urchin) in barrens and macroalgae habitats in south-eastern Australia

Abstract

Sea urchins are important herbivores that can graze macroalgae, creating ‘barren’ areas. These barrens are believed to offer little food for urchins due to the absence of attached macroalgae, leading to malnourishment as indicated by a low urchin gonad index. To investigate the diet and resultant gonad index of the long-spined sea urchin Centrostephanus rodgersii in New South Wales, Australia, we dissected 100 C. rodgersii individuals collected in macroalgae and barrens habitats along >470 km of SE NSW coastline. Diverse items were present in the digestive tracts of urchins from all habitats. These items included brown (42% barrens, 46% macroalgae), green (28% barrens, 42% macroalgae) and red algae (15% barrens, 12% macroalgae) and corallines (29% barrens, 37% macroalgae), molluscs (28% barrens, 29% macroalgae) and crustaceans (26% barrens, 22% macroalgae). There was no difference in urchin gut fullness between habitats (85% barrens, 90% macroalgae). Importantly, the gonad index only differed in macroalgae compared to barrens habitats at one location, with no differences detected at the other 4 locations. These results suggest that C. rodgersii has a diverse diet that is similar in both habitats, which could explain the similarities in gut fullness and gonad index. Our results suggest that C. rodgersii eat a broad diet including invertebrates and drift algae, and hence may not be malnourished in barrens. The finding of comparable gonad index between barrens and macroalgal areas further supports this conclusion. These findings challenge the prevailing perspective, indicating that sea urchins have sufficient food to survive and reproduce in different habitats.