Growth rates and RNA: DNA ratios in Paracentrotus lividus (Echinodermata: Echinoidea) fed on benthic macrophytes

Abstract

We measured growth rates and RNA: DNA ratios of young sea urchins Paracentrotus lividus cultured for 6 months in the laboratory where they were fed on 12 macrophytes ( Rissoella verruculosa, Cystoseira mediterranea, Corallina elongata, Stypocaulon scoparium, Padina pavonica, Ulva rigida, Cystoseira compressa, Colpomenia sinuosa, Posidonia oceanica, Codium vermilara, Asparagopsis armata, and Dilophus spiralis). Average increases in test diamter ranged from 0.3 mm · month dash1 for urchins fed on Dilophus spiralis to 1.9 mm · month 1 for urchins fed on Rissoella verruculosa. Average increases in wet mass ranged from 0.10 g wet mass · month 1 for urchins fed on Dilophus spiralis and Asparagopsis armata to 1.20 g wet mass · month 1 for urchins fed on Rissoella verruculosa. In terms of proteins, average growth rates ranged from 0.23 mg protein · month −1 for urchins fed on Codium vermilara to 3.08 mg protein · month −1 for urchins fed on Cystoseira mediterranea. In all three cases, growth rates were significantly ( p = 0.0001) affected by the nature of the offered macrophytes. There was a high correlation between increase in test diameter and (1) increase in wet mass and (2) increase in proteins ( r = 0.98 and 0.83, respectively). RNA: DNA ratios of Paracentrotus lividus (0.47 < r < 2.02) were within the range of what has previously been reported for larval fishes, bivalves, cephalopods, and crustaceans. Average RNA: DNA ratios were significantly affected by the food regime. However, results of a posteriori LSD tests were different for growth rates and RNA: DNA ratios. There were significantly ( p = 0.0001, 0.0001, and 0.0004, respectively) positive correlations between growth rate (in terms of diameter, wet mass, and proteins) and RNA : DNA ratio. However these relationships accounted for only 23.8, 23.6, and 19.4%, respectively, of the variance of RNA : DNA ratios. These results suggest that the use of RNA : DNA ratios would be inefficient in predicting growth rates of field collected Paracentrotus lividus.