Isotopic niche of Centrostephanus rodgersii from barrens overlap with those from macroalgal forests in SE Australia

Sea urchin ‘barrens’ are thought to offer fewer food resources than macroalgal habitats. As such, sea urchins from barrens are expected to have a smaller isotopic niche compared to those in dense macroalgal forests. Sea urchin isotopic niches in macroalgal forests are also expected to show little overlap with those from barrens given the differing availability of resources. To investigate, we analysed δ 13 C, δ 15 N and δ 34 S values from muscle tissues of 150 Centrostephanus rodgersii individuals collected along >470 km of SE Australian coastline. Using a conservative Bayesian isotopic niche analysis, we found moderate niche overlap from barrens relative to macroalgal forests (45%) but lower overlap from macroalgal forests relative to barrens (18%). This suggests that resource use in barrens can show similarities to that in macroalgal forests while sea urchins in macroalgal forests use different resources. However, the trend reversed at 1 out of 5 locations while overlap was approximately equal at 2. Similarly, isotopic niches in barrens were approximately half the size (4.98 ± 0.72‰ 3 ) of macroalgal forests (10.48 ± 1.51‰ 3 ), but this trend again reversed at 2 locations while niches were approximately equal at 2. The smaller isotopic niches of sea urchins from barrens generally had higher δ 15 N ranges than those from macroalgal forests, indicating resource use from higher trophic levels. These results suggest that rather than being general, C. rodgersii resource use in barrens and macroalgal forests is specific to location, and despite their name, some ‘barrens’ can provide similar food resources to macroalgal forests.