Agar-based pellets as feed for sea urchins (Paracentrotus lividus): rheological behaviour, digestive enzymes and gonad growth

Abstract

The algal polysaccharide agar has long been used as a food binder due to its structure, rheological behaviour, stability and interactions – properties that help to generate firm, round, disk-shaped pellets that may be used in recirculating sea urchin-rearing systems. Three algae-based diets (Ulva lactuca, Gracilaria gracilis, Cystoseira sp.) containing 3% and 6% agar were tested on the sea urchin Paracentrotus lividus in order to examine the effect of varying percentages of agar on pellet stability in water and sea urchin gonad growth. The kinetics of water absorption and solute leaching of pellets were measured by immersing quadruplicate samples of the pellets in water for 1, 2, 3, 4, 5 and 6 days. Our results show that the pellets had good water stability, were readily consumed by sea urchins and the presence of agar did not hamper sea urchin gonad growth. Animals fed Ulva-containing pellets reached a more advanced gametogenic stage with respect to animals fed Cystoseira- and Gracilaria-containing pellets. Moreover, the presence of agarase activity in the digestive system indicated that agar may be an energy source. Pellets are relatively low cost and easy to prepare and store. They may represent a useful resource for rearing sea urchins under intensive conditions.