Effect of variation in the protein value of the red macroalga Palmaria palmata on the feeding, growth and gonad composition of the sea urchins Psammechinus miliaris and Paracentrotus lividus (Echinodermata)

The sea urchins Psammechinus miliaris and Paracentrotus lividus were fed three macroalgal diets with varying protein content over a 90-day period. These diets were the red alga Palmaria palmata, which had been grown in seawater enriched with two levels of ammonia-N and contained a protein content of either 41% (High-N) or 32% (Low-N) and the brown alga Laminaria saccharina (23% protein). A significantly different response to the macroalgal diets by the two sea urchin species was found in terms of food consumption rate, food conversion efficiency, gonadal growth and gonad nutritional composition. P. miliaris was able to efficiently utilise the High-N P. palmata, whereas P. lividus exhibited a higher food conversion efficiency (FCE) and specific growth rate (SGR) when fed the Low-N compared with the High-N P. palmata. P. miliaris had a significantly higher food consumption rate and higher gonad index (GI) compared with P. lividus, irrespective of diet type. The lowest FCE, SGR and GI were shown by both species fed L. saccharina. No interspecific difference in gonadal nitrogen content was observed, although a positive relationship was shown between dietary and gonadal nitrogen content for both species. All three macroalgal diets produced bright orange or yellow orange gonad colour in P. lividus. High-N and Low-N P. palmata improved gonad colouration in P. miliaris compared with L. saccharina. The results suggest that P. miliaris can efficiently assimilate high protein plant-based diet, whereas P. lividus is less capable of benefiting from high dietary protein levels and this should be taken into consideration when culturing this species either in land-based or sea-based polyculture systems.