Development of the radula and digestive system of juvenile blacklip abalone (Haliotis rubra): Potential factors responsible for variable weaning success on artificial diets

We investigated the structural and physiological changes in the radula and digestive system in juvenile blacklip abalone Haliotis rubra, between 80 and 158 days post settlement (PS) to determine if variable growth on artificial diets (and prevalence of runts) are due to an inability to efficiently ingest and digest the diet. Between 80 and 102 days PS, L5 teeth appeared on the radula and there were fewer lateral serrations, consistent with the adult form of the animal, suggesting that this development is in preparation for feeding on macroalgae. Digestive gland complexity (tubule number and density) increased between 80 and 102 days PS and is consistent with greater enzyme production and increased digestive efficiency. Of the enzymes studied, laminarinase and lipase exhibited the highest activities in animals fed a diatom diet, both significantly increasing with age of the abalone. High laminarinase activities reflect higher utilisation of the algal polysaccharide chrysolaminarin in the diatom diet. Ingestion of artificial diet had no adverse effects on the morphological development of the digestive system, but trypsin activity in abalone fed the artificial diet was significantly higher than diatom-fed abalone of similar age, indicative of higher levels of protein in the artificial diet. Similarly, lipase activity was significantly lower in abalone fed the artificial diet and may reflect an inability to digest the fish oil component, which is not found in their natural diet. Future development of artificial diets, especially for juvenile abalone, should focus on the levels and type of lipid provided. Runt abalone (i.e. under-developed compared to their siblings) had radulae similar to much younger 80 days PS abalone and digestive tissue degradation at 137 days PS is evidence that runts have limited ability to ingest food and are nutritionally compromised.