Does the Initial Diet of Hatchery-Reared Tripneustes gratilla (Linnaeus) Impact their Effectiveness as a Biocontrol for Invasive Seaweeds

Abstract

Steps to identifying an appropriate native biocontrol species for invasive non-native seaweeds may initially only include testing palatability of an invasive species in initial screening because, as a native, the biocontrol species would be generally acknowledged to graze native algae. Additionally, invasive species are typically the dominant species in large areal infestations with few remaining natives. However, in mixed stands of native reef plants and invasive seaweed, dietary preferences of an outplanted grazer become critically important. Hatchery- reared biocontrol species are often fed an optimized diet with the goal of rearing the species quickly and with reasonable yields for outplanting. As a result, additional critical questions arise as to whether or not a biocontrol species prefers a hatchery-diet over target invasive species, at least in initial stages. Testing for any preference is an important means to examine the effectiveness of an outplanted grazer prior to its introduction field sites of mixed native algae – invasive seaweeds. The use of mesocosms for control testing can be an effective tool to examine the extent to which native grazers can be used for biological control in mixed stands of native and invasive species. This project examined dietary preferences of the hatchery-reared, native urchin Tripneustes gratilla a biocontrol species that has been outplanted in Kāne‘ohe Bay and Waikīkī to graze algal biomass of six invasive species on isolated patch reefs. In this study, feeding preferences of hatchery reared and wild T.gratilla were examined to establish feeding preferences on native versus introduced seaweeds found in the extensive Waikīkī reef platform. Urchins were placed in tanks with continuous flow and offered a pair-wise combination of a native and an invasive algal species over a 24 hour period at the Ānuenue Fisheries Research Center (AFRC). In total, 15 urchins per replicate were used and three replicate runs were conducted between Oct. 25th and Nov. 15th, 2014. In most cases, urchins grazed the invasive species A. spicifera and G. salicornia more readily than native species with the exception of native Microdictyon setchellianum and Gracilaria coronopifolia. A second experiment examined preferences of AFRC urchins when given all seven choices of both native and invasive species simultaneously. AFRC urchins demonstrated a high preference for native G. parvispora followed by invasive A. spicifera and G. salicornia. Future work could include caging experiments in areas of mixed plant assemblages to test the outcomes of our previous studies. Such tests are important to validate the use of urchins as biocontrol agents for regions with mixed native and invasive algal assemblages and underscore the importance of surveying for the native flora in a region prior to outplanting this urchin.