Movement and dietary preference of the sea urchin Tripneustes gratilla (Echinodermata) on a patch reef in Hawai‘i

Background/Question/Methods

On coral reefs overgrown with invasive algae, the physical removal of most of the algal biomass followed by enhancement of native herbivores may be a viable restoration technique for controlling algal cover, saving existing corals, and allowing for the return of ecosystem diversity. The sea urchin Tripneustes gratilla shows promise as a native biological control agent for invasive algae in Hawai‘i. Efforts are currently underway to artificially enhance the abundance of T. gratilla on degraded patch reefs after most of the algal biomass has been physically removed. The purpose of this experiment was to provide resource managers with baseline movement and dietary preference information in relation to food availability. Fifty-one urchins were externally tagged with uniquely numbered Peterson discs and followed for approximately three months to investigate their feeding preferences and movement patterns. 

Results/Conclusions

The urchin population dispersed slowly from a release point. Although average movement of individual urchins was approximately 1 m per day, overall dispersal was slow. After 12 weeks at liberty, 53.3% of urchins were less than 5 m from the release point and 76.7% were less than 10 m from the release point. Urchin movement may be directional. A number of urchins in this study descended the reef slope, suggesting that some T. gratilla may emigrate from patch reefs to which they have been transplanted. Finally, T. gratilla were observed eating crustose coralline algae, even though macroalgae were abundant in the area. This behavior was not quantified, but if substantial, may affect reef infrastructure.