40 DECEMBER 2012 • WORLD AQUACULTURE • WWW.WAS.ORG Limpets are shallow-water gastropods in the genus Cellana. Various species are shown in Figure 1. The opihi, as it is known in Hawaii, is a cultural icon. They have been harvested as a highvalue snack called pupu (snail) by Hawaiians for centuries (Fig. 2). With increases in population, intensive harvesting led to scarcity in many areas of the Hawaiian Islands. The major annual commercial catch decreased from 68 t in the 1900s to about 4.5 t in 1978 (Iacchei 2011). Scarcity has driven prices up to about US$150/4-L container, with shell. Thus, this high value has led to increased interest in aquaculture of the species. The enabling studies reported here will allow assessment of the commercial potential of opihi aquaculture. Previous work focused on the life history of the yellow-foot and black-foot opihi (Corpuz 1981). We focused on the giant opihi Cellana talcosa because it grows the fastest (Corpuz 1983). The giant opihi lives in deep water, not on wave-splashed rocks, raising the possibility that it may be grown in tanks rather than on large hemispheres sprayed with seawater, or in ebb and flow reef tanks used by earlier researchers, both of which are too costly for commercial aquaculture. Collecting and Transferring Animals Giant and yellow-foot opihi were collected in remote areas of the Hawaiian Islands. Figure 3 represents typical rough-water opihi habitat where they must be collected. Opihi have evolved to cling tightly to rocks in high-energy water. Opihi are called fish of death because they are dangerous to collect. Every year people collecting opihi are swept off the rocks and drown. When beginning these studies, challenges arose immediately. The first collection of animals yielded 58 percent mortality within four days, mostly caused by injury that occurred when prying opihi from rocks. Opihi were collected from rocks at low tide in about 1 m of water by scraping with a rounded putty knife. We learned to detach them quickly from rocks before they became alarmed and would cling more tightly. Opihi were transported to the laboratory after being placed in a cooler and covered with a wet towel. Collection mortality has been reduced by more careful techniques and, by the fourth collection, there was 83 percent survival. An additional high mortality of 53 percent occurred while moving animals from one holding tank to another in the first trial. Animals attached tightly to the walls of the tank and were difficult to remove. Our first approach was to test several tank liners: 1) a rough, 0.5-mm thick, polyethylene plastic sheet sold as a drop-cloth for painting2, 2) a clear plastic sheet3, and 3) nylon netting used to screen rotifers4. Later, soft plastic colanders were used (Fig. 4). Tanks overlaid with a plastic liner or containing colanders eliminated mortality because animals were easily removed from walls by gently deforming the plastic or soft colander, causing the opihi to fall off, thereby facilitating collection. Benthic Diatoms are the Preferred Natural Food Feeding preferences of opihi in the wild was determined by analysis of stomach contents. Stomach contents contained benthic diatoms (about 30 percent), bacterial clumps, and other unidentifiable particles. Twenty diatom species were identified, but the most frequently occurring were the benthic diatoms Bacillaria sp., Fragilaria sp., Melosira sp., Navicula sp. and Rabdonema sp. (Gilmartin and Revelante 1974, Saboski 1976, Lee 2008). Aquaria covered with plastic sheets were supplied with seawater (approximately 15 L/min) that had been filtered with a sand filter and exposed to direct sunlight. Biofilm grew spontaneously. Among substrates, biofilm covered most of the surface of the rough plastic with a layer about 1 mm thick within 7 days, whereas 10-14 days were required for coverage of smooth plastic or nylon. After 2 weeks the biofilm on the rough plastic was approximately 3-4 mm thick. The most frequently occurring species in the biofilm were diatoms Nitzschia sp., Rhizosolenia sp., Melosira sp., Cosinodiscus sp. and Navicula sp. There were also some bacterial clumps and the initial growth of some macroalgae. Similarities between stomach contents and biofilm composition suggest that opihi prefer to feed on benthic diatoms. When placed in tanks with the biofilm mat, animals were observed to feed. Feeding was estimated by the area the animals cleared every day. The mean feeding rate among the animals tested was 0.47 percent dry matter of opihi body weight per day. Biofilm was used for feeding opihi between feeding trials and played a key role in developing artificial feeds. Enabling Studies for Aquaculture of the Hawaiian Opihi, the Limpet Cellana Nhan Thai Hua and Harry Ako1 Limpets are shallow-water gastropods in the genus Cellana. The opihi, as it is known in Hawaii, is a cultural icon. They have been harvested as a high-value snack called pupu (snail) by Hawaiians for centuries. With increases in population, intensive harvesting led to scarcity in many areas of the Hawaiian Islands. Scarcity has driven prices up... Thus, this high value has led to increased interest in aquaculture of the species. The enabling studies reported here will allow assessment of the commercial potential of opihi aquaculture.
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