RNA concentration and the RNA to protein ratio in cephalopod tissues: sources of variation and relationship with growth rate

RNA and protein concentrations, and the RNA to protein ratio, were measured in four species of cephalopods, to evaluate sources of variation and the p otential for using RNA concentration and the RNA to protein ratio as growth indices. In field samples of Loliga forbesi and Eledone cirrhosa, RNA conc entrations and the RNA to protein ratio were higher in immature animals tha n in mature animals. In Loligo forbesi, values were also higher in males th an in females and higher in smaller individuals than in large individuals. Both these trends are consistent with expected differences in growth rate, i.e. RNA is higher in faster growing animals. Mature female Eledone cirrhos a, a species in which the female is larger and presumably grows faster, had higher RNA concentrations than mature males. However, no such difference b etween the sexes was seen for immature E. cirrhosa or Todarodes sagittatus. Methods for transport and maintenance of Loligo forbesi in captivity in the Azores are described. Many of the captive squid showed poor growth and sur vival but results from these animals nevertheless confirmed that RNA concen trations were higher in males than in females and higher in animals with sm aller gonads than in animals with large gonads. Higher protein concentratio ns were found in males than in females, and protein concentration was also positively correlated with feeding rate and digestive gland indices. Octopu s vulgaris held in captivity grew rapidly and consistently and RNA concentr ations were lower in bigger animals than in smaller animals. Neither experi ment provided direct support for the hypothesis that RNA concentration or t he RNA to protein ratio is directly related to growth rate. Systematic vari ation in protein concentration, e.g. in relation to recent feeding, leads u s to suggest that protein concentration (mg/g fresh body weight) is likely to provide a more reliable index than the RNA to protein ratio. (C) 1999 El sevier Science BN. All rights reserved.