Density of Tripneustes gratilla in different seagrass meadows in Changuu Island (Unguja Island, Zanzibar Archipelago, Tanzania)

To assess the sea urchin distribution across the seagrass species, we measured the sea urchin density in three monospecific meadows (T. ciliatum, C. serrulata and H. uninervis) and two mixed meadows (dominated by T. hemprichii and by S. isoetifolium) in the study site. We selected three patches per meadow type. Each patch was 25 square meters, delimited with a transect tape. We counted the number of sea urchins at high tide, and divided this number by the surface of the patch to calculate sea urchin density per square meter of meadow (individuals per square meter).