Isolation and evaluation of local microalgal isolates as feed for larval rearing of Crassostrea tulipa towards optimisation of mangrove oyster seed production in Ghana

This study explored the potential of three local microalgal isolates as feed for oyster larvae in laboratory-rearing experiments, towards the optimisation of seed production of Crassostrea tulipa to support its large-scale farming along the West African coast. Three species of local microalgae—Rhodomonas sp., Nannochloropsis sp. and Pseudanabaena sp.—were isolated from waters off the coast of Ghana, West Africa, following a serial dilution technique. The growth performance of the isolates was assessed in the laboratory through daily estimation of cell density until the stationary phase was observed. Characterisation of the microalgal isolates was carried out by estimation of their biovolume, carbon content and energy content. Biovolumes of the microalgae were calculated from the estimated equivalent spherical diameters using proposed geometric shapes and formulae. Carbon weight and carbon energy content were subsequently calculated using derived conversions. The three microalgal isolates showed potential for large-scale cultivation in the laboratory with marked differences in daily increases in cell densities. Nannochloropsis sp. and Rhodomonas sp. recorded the highest and the lowest peak densities of 2.4 × 10⁵ and 1.5 × 10⁵ cell mL⁻¹, respectively, from an initial inoculating cell density of 1.05 × 10⁵ cell mL⁻¹. The estimated mean biovolumes of Rhodomonas sp., Nannochloropsis sp. and Pseudanabaena sp. were 238.9, 8.182 and 42.42 µm³, respectively, and the corresponding derived carbon energy contents were 1.7 × 10⁻⁶, 7.13 × 10⁻⁸ and 1.05 × 10⁻⁷ J, respectively. Results from a laboratory rearing experiment indicated that the individual microalgal isolates supported the growth and survival of oyster larvae at different scales, but a mixed diet of the three promoted superior growth and survival of C. tulipa larvae. The three local microalgal isolates- Rhodomonas sp., Nannochloropsis sp., and Pseudanabaena sp.- were well adapted to laboratory culture conditions, and the observed differences in growth and survival of the oyster larvae fed on these algal diets could be due to the differences in diet properties and biochemical compositions of the different species. A combination of the three algal diets, however, provided complementaery nutrients for the optimal growth and survival of C. tulipa larvae. The outcome of this study shows that local microalgal isolates have the potential to support hatchery rearing of C. tulipa, which is essential for the development of commercial mangrove oyster aquaculture in West Africa.