Evaluation of biochemical indices for assessing growth and condition of the deepwater squid Moroteuthis ingens

Abstract

This study investigated whether biochemical indices (RNA, DNA and protein concentrations and RNA:protein and RNA:DNA ratios) of the mantle muscle and digestive gland were related to growth and condition in wild-caught squid Moroteuthis ingens. The influence of size, sex and reproductive maturity on these relationships was also examined. Levels of RNA, DNA and RNA:protein ratios in the digestive gland were much higher than those in muscle, perhaps due to the high metabolic activity of the gland. The digestive gland of females, however, showed no correlation for any of the indices. There was a general trend of smaller immature males and females having higher levels of biochemical indices compared to larger mature individuals, which was most probably a trade-off between somatic and reproductive growth. RNA concentration and RNA:protein ratio demonstrated the greatest number of correlations with measures of growth and condition, suggesting that they are the most appropriate biochemical indices for M. ingens. There was weak support for using RNA:protein and RNA:DNA ratios to assess the condition of M. ingens. Despite this, there was little support for the biochemical indices being directly related to growth rates. Faster growing females had lower levels of RNA, DNA, protein and RNA:protein ratios than slower growing individuals, negating the rationale that these indices are a measure of instantaneous growth. These results, however, may represent a lag between whole body growth and RNA levels. Alternatively, biochemical indices may simply not be suitable for field studies of cephalopods. © Inter-Research 2005.