Diel movement patterns and abundance of the sea urchin Tripneustes gratilla with comparisons to Echinometra mathaei and their associated ecology at Ningaloo Reef, north-western Australia

Sea urchin herbivory can profoundly modify the benthic habitat and community assemblage of coral reefs making their abundance and movement patterns key features to understanding the role they play in regulating community structure. This study examines the diel movement patterns of Tripneustes gratilla, with comparisons to Echinometra mathaei, and their co-occurrence with other invertebrates, at sites with a high population density on the Ningaloo Reef, Western Australia. Movement rates were measured using long-run underwater stereo-video, while density was measured using belt transects at multiple sites. The size and gut content index of Tripneustes gratilla were also investigated. We observed abundances of T. gratilla (4.29 and 6.02 individuals/m²) exceeding those from areas where outbreak densities have caused significant changes in macroalgal cover. The majority of T. gratilla were in the 60–70 mm size range, and their gut content was significantly higher when collected in the morning indicating that feeding activity may be higher during the night. We also observed greater overall movement patterns for T. gratilla than E. mathaei regardless of site or time of day, with both species exhibiting their highest movement rates at night or in the evening at sites 1 and 2, respectively. Considering the observed movement capacity of T. gratilla in this study, their relatively high abundances, and their significant grazing impact found in previous studies, this species may exert localized pressure on communities in similar habitats. Therefore, we recommend regular monitoring of both macroalgal and urchin populations on Ningaloo Reef.