The growth and survival of the sea urchin Tripneustes gratilla, a possible biocontrol agent for invasive macroalgae

Abstract

Tripneustes gratilla is a common echinoid in the tropical Indo-Pacific. It is found in wide variety of sub-tidal coral reef habitats, where it consumes turf algae, macroalgae, and seagrass (Shokita et al. 1991, Vaitlingon et al 2003, Stimson et al. 2007). Stimson et al. (2007) demonstrated that it is a generalist herbivore in Hawaiʻi, able to consume coralline algae, turf algae, endolithic algae, and macroalgae. This echinoid is a mobile benthic grazer, and Shokita et al. (1991) observed migrations in the range of hundreds of meters involving juveniles moving to deeper water. Tripneustes gratilla is among the fastest growing echinoid species recorded (Ebert 1985, Bacolod and Dy 1986, Juinio-Menez et al. 2008, Regalado et al. 2010). Invasive macroalgae have overrun coral reefs in some areas of Hawaiʻi (Smith et al. 2002, Smith et al. 2004), and T. gratilla has been proposed as a possible biocontrol agent to control the invasive macroalgae. Subsequent studies have added support to this idea (Conklin and Smith 2005, Cunha 2006, Stimson et al. 2007), because T. gratilla has demonstrated the ability to consume the invasive macroalgae in substantial quantities. Valentine and Edgar (2010) recently reported an 'outbreak' of T. gratilla on Lord Howe Island in Australia, where densities exceeded 4 individuals per square meter. They concluded that T. gratilla possesses an 'ecosystem engineer' function, because 'outbreak' sites were characterized by significant decreases in foliose macroalgae, with an increase in crustose coralline algal cover. There was also no apparent impact on sessile invertebrates, mobile invertebrates, or fish populations (Valentine and Edgar 2010), which would make T. gratilla an ideal biocontrol agent for invasive macroalgae. However, if T. gratilla is to be utilized as a biocontrol agent, more information is required about its growth and survival in areas and environments where the invasive algae are plentiful, and where biocontrol is required