Spatial and temporal distribution of feeding of Aspidochirotida (Holothuroidea) on Heron Island, Great Barrier Reef

Abstract

Aspidochirotida are distributed through the reef flat of Heron Island, contributing 34-44 g wet biomass per square metre. On the reef flat, feeding by Holothuria edulis and Stichopus chloronotus is concentrated on sediments 2-5 cm from coral outcroppings, while Holothuria atra and Holothuria leucospilota feed at 5-16 cm from coral outcroppings. Reef flat sediments actively fed upon by Aspidochirotida do not differ significantly (p>0.05) in total organis, chlorophyll a, or phaeophytin from sediments not fed upon. Spatial variation in the apparent food quality of sediment is low and probably does not contribute to the distribution of Aspidochirotida on the reef flat. In the lagoon, Aspidochirotida are highly aggregated around the bases of coral patch reefs, where their biomass is 214 g wet weight per square metre.Biomass of Aspidochirotida approaches zero beyond 15 m from coral patch reefs. Sediments at the bases of coral patch reefs in the lagoon do not differ in total organic, chlorophyll a, or phaeophytin from sediments at 20 m distant. However, sediments at the bases of coral patch reefs contain significantly more protein and are significantly skewed toward the coarser grain sizes (phi -1.5 to 1.0). This suggests either that Aspidochirotida in the lagoon aggregate in areas of higher quality sediment, or that intense feeding by Aspidochirotida alters the composition of sediment near coral patch reefs. Most Aspidochirotida feed continuously. Holothuria atra, H. edulis, H. leucospilota and Stichopus variegatus do not exhibit any diel rhythm in feeding or the passage of sediment through the gut, whereas S. chloronotus demonstrates a clear diel rhythm, extending oral tentacles more frequently and passing more sediment late in the day. The aggregated spatial distribution of Aspidochirotida, and the spatial separation of some species, is not driven by resource availability and niche partitioning, but rather some other factor, such as the availability of shelter.