Differential survivorship, reproduction, growth and nutrient allocation in the regular echinoid Lytechinus variegatus (Lamarck) fed natural diets

Size frequency distributions, growth and reproductive output of the regular echinoid Lytechinus variegatus (Lamarck) differ among sub-populations occurring in microhabitats dominated by Thalassia testudinum Koenig, Syringodium filiforme (Kutz) and sand/Enteromorpha compressa Greville. These factors may be related to documented differences in microhabitat-specific variations in temporal and spatial patterns of food availability. The effects of five different diets on growth, reproductive condition and nutrient allocation were investigated in the laboratory by feeding sea urchins ad libidum diets of prepared foods containing elements of one of five different natural foods, or a diet consisting of an equivalent mixture of all foods, over a seven month period. Feeding rates varied with food type. Individuals fed monospecific diets had significantly lower feeding rates than those fed either mixed or epibiont diets, both of which simulated generalist diets. Sea urchins fed mixed or epibiont foods had significantly higher survival rates and a higher percentage of mature gametes. Growth, as measured by the percentage of increase in test diameter, was highest in individuals fed the seagrasses Thalassia testudinum and Syringodium filiforme. However, test indices (wet weight test/total wet weight) were similar for individuals on all diets, indicating that while individuals feeding on other diets showed smaller increases in test diameters, test thickness increased in these individuals. Gonadal indices showed considerable variability between individuals fed the different diets. Sea urchins fed a diet of the green algae Enteromorpha compressa and those fed mixed diets had the highest gonadal indices. Individuals fed diets of Syringodium filiforme and those fed decayed seagrass diets had gonadal indices which were significantly lower than all other treatments. Sea urchins fed Syringodium filiforme had significantly higher gut indices than individuals fed mixed, Thalassia testudinum or algal diets. Sea urchins simultaneously presented with paired combinations of the six diets exhibited no initial preference for any food resource. However, sea urchins exhibited a preference for epibionts over Thalassia testudinum, Enteromorpha compressa and decayed seagrass when preference was determined by amounts of each of two foods consumed in a 24 h period. No other preferences were observed. Total organic concentration was highest for decayed seagrass and lowest for epibionts and Enteromorpha compressa. The diet comprised of Thalassia testudinum contained the highest concentration of protein and lipid. Moreover, sea urchins absorbed higher amounts of energy from these organic constituents than from other diets. The highest absorption efficiencies for total organic material were recorded for individuals fed epibionts and decayed seagrass. The lowest absorption rates for total organic material were recorded for sea urchins fed the seagrasses Thalassia testudinum and Syringodium filiforme. Absorption efficiencies for protein, as well as total energy absorbed per day, were negative for individuals fed Syringodium filiforme. This study indicates that feeding rates, survivorship, growth, reproductive activity and nutrient allocation may vary in response to qualitative characteristics of natural diets. This provides a basis to evaluate the contribution of diet to the regulation of individual and population level traits among Lytechinus variegatus in discrete microhabitats.