Effects of diet and temperature on ingestion, absorption, assimilation, gonad yield, and gonad quality of the purple sea urchin ( Strongylocentrotus purpuratus)

Sea-urchin ingestion rate, absorption, gonad yield, and gonad quality can be influenced by a variety of factors, including food quantity, food quality, and various environmental conditions. We examined the combined effects of two factors (diet and temperature) on ingestion rate, absorption, assimilation efficiency, and gonad enhancement of the purple sea urchin, Strongylocentrotus purpuratus, over a 12-week period. Four diets [three kelp species ( Macrocystis integrifolia, Nereocystis luetkeana, and Saccharina latissima) and one prepared feed] and three temperatures (8.0 ± 0.4, 12.3 ± 0.3, and 16.2 ± 0.4 °C: mean ± SD) were tested in a completely crossed factorial design. Ingestion rate, gravimetric absorption, and assimilation efficiency of urchins fed the various diets were calculated at weeks 6 and 12. In addition, a number of gonad attributes were quantified at the beginning and end of the experiment including wet weight, percent water content, gonad index, texture, firmness, colour, and flavour. Ingestion rate, gravimetric absorption, and assimilation efficiency were generally significantly affected by both temperature and diet. Wet-weight ingestion rates were significantly higher with the three kelp treatments than with the prepared diet at both weeks 6 and 12. This particular dietary difference was not apparent, however, when ingestion rates were expressed on a dry-weight basis. The highest ingestion rates were recorded for animals held at 12 °C at week 6 and at 16 °C at week 12. Absorption rates and assimilation efficiencies were significantly higher at 16 °C than at 8 °C at week 6 and significantly higher at 16 °C than at the other two temperatures at week 12. Final gonad weight and gonad index were significantly greater in urchins fed the prepared diet than in those fed any of the three kelp species, with little difference among the kelp treatments. Final gonad weight was significantly affected by temperature, but this effect was dependent on diet; temperature had little impact on gonad weight (and index) in the three kelp treatments, but 12 and 16 °C produced significantly heavier gonads than 8 °C for urchins fed the prepared diet. Gonads of urchins fed kelps had more (significantly more in the case of M. integrifolia and N. luetkeana) water than those fed the prepared diet. Gonad colour, texture, and firmness were generally not significantly affected by temperature, diet, or the interaction of these two factors. Gonad flavour was generally (som