by Anthony Mau, University of Hawaii Graduate Researcher
There has never been a more exciting time for the aquaculture of ‘opihi ‘alinalina (Cellana sandwicensis), which is currently underway at the Oceanic Institute. There is major demand for ‘opihi by the local seafood retailers in Hawaii, especially on Oahu—where scarcity has become a real issue. Irrespective of this project’s commercial significance, there is also a serious implication for Native Hawaiians to use ‘opihi aquaculture for stock enhancement.
Under a previous CTSA-funded project, Dr. Harry Ako and Dr. Nahn Hua were successful in developing the first grow-out and maturation diet. This major accomplishment set-up the current project with a good understanding of dietary requirements. Another key component of previous work was figuring out a good spawning method that could be replicated, and past work showed using salmon gonadotropin releasing hormone analog (sGnRHa) was effective in controlling maturation and spawning.
During the current project’s first year, our research team has focused on growing the highest quality broodstock. We first collected animals in Puna, Hawaii with >90% survival, and stocked them into dark circular tubs fitted with an overhead irrigation spray nozzle. The tank design was critical in keeping animals alive through the year, which was engineered to provide high surface area above the water level, while keep the animals wet with oxygen rich seawater.
After collecting wild animals, muscle and gonad tissues were analyzed for biochemical composition to aid in diet formulation and understanding of maturation. After 5 months of acclimating on a diet sourced from fishmeal, soybean meal, and krill meal containing 35% protein, the animals were stocked into a maturation trial. This trial investigated if there was an effect of an ARA-supplemented on the maturation of ‘opihi. We found there to be no significant difference (P = >0.05) between the gonadal somatic index (GSI) of the control and ARA-supplemented group. We did, however, observe a sudden increase in feed intake for both groups in October as air temperature dramatically decreased. We understand that the spawning season of wild ‘opihi starts as early as September, henceforth we believe this shift in behavior marked the onset of gametogenesis. Further supporting this was a steady increase in mean GSI for both groups in subsequent months. The highest average gonadosomatic index (GSI) was 15.58% in February, which followed the coolest month of the year. Therefore, we suspect maturation in influenced by environmental factors (i.e. water and air temperature, photoperiod, etc.). In addition, there appeared to be a near significant difference in GSI between male and females (P = 0.07), which suggests the GSI range of mature animals will not be the same.
Upon successful maturation of ‘opihi, spawning trials were conducted in effort to close the lifecycle. After injecting animals with a priming dose of sGnRHa at 250 ng/g BW, synchronous spawns were induced with a resolving dose of 500 ng/g BW at 100% survival. Males typically spawned 4-6 hours post-injection, and females spawned soon after. Spawning behaviors changed in the presence of sperm, and animals began stacking on each other, which appeared to be a form of natural induction between male and female. After collection and fertilization of gametes, development was monitored to the veliger larvae stage (average width and length of 161.28 ± 15.49 µm and 216.35±16.29 µm). Spawn rates were 10.8-12.5% following this method. Across five different trials, fertilization rates and hatching rates were highly varied. Higher fertilization and hatching rates most likely reflect higher egg quality, and our best larval rearing results came from eggs with 90% fertilization and 85% hatching. This March, we successfully reared and stocked 25,000 late veliger larvae into various settlement tanks. We are also underway with an experiment to look at the effect of specific inducers of settlement and metamorphosis. A total of 360 larvae were pipetted into 12 petri dishes filled with 30 ml of filtered sea water and antibiotics 10 µg/ml. The four treatments include: mixed diatoms (MD), MD + GABA (1 mM), MD + crustose coralline algae, and MD + adult mucus. After one week, we observed good settlement (up to 66.7%) and metamorphosis (up to 13.3%) to post-larval ‘opihi. The juvenile animals will be documented for survival and growth rates over the next few months.
The continued support from CTSA has been well received. The direction of this project is looking bright, and we are confident there will be some new and exciting discoveries regarding ‘opihi aquaculture in the future. The next phase will be off-season spawning and transfer of technology to local farmers. Stay tuned!
Source: CTSA
