Biochemical measurements from the mantle tissue showed that recent growth in
Euprymna tasmanica was highly variable, and although the variability was partly explained by water temperature, there was little seasonal effect on instantaneous growth. Body size was more influential in explaining the variation of growth of E. tasmanica, with concentrations of RNA decreasing with increasing body weight. Since the majority of RNA is found in ribosome’s, which are building blocks for proteins (Pierce, et al., 1999), small individuals with greater concentrations of RNA will have higher rates of protein synthesis that fuel relatively high instantaneous growth rates. This negative relationship between body size and growth rate is a common occurrence in cephalopods e.g. Loligo forbesi (Pierce, et al., 1999); Eledone cirrhosa
(Houlihan, et al., 1998); Sepioteuthis australis (Ho, et al., 2004) Euprymna tasmanica
(Moltschaniwskyj and Carter, 2010); Moroteuthis ingens (Semmens and Jackson, 2005) and fish (Houlihan, et al., 1986) and is thought to be associated to the relative proportion of body the occupied by the intestine (Weatherley, 1987). For example, as the body grows not only can the fractural rates of protein synthesis of individual tissue decline, but the relative proportions of the tissue in the body will change with a relative increase in slow synthesising white muscle (Houlihan, 1991). This change is of particular importance in cephalopods that have mantles contributing up to 70% of the total body weight (Moltschaniwskyj and Martinez, 1998).
Although less influential than body size, temperature had a significant effect on the instantaneous growth rate; individuals that experienced warmer water had greater
concentrations of RNA and faster instantaneous growth rates. This positive relationship between growth and water temperature is not only common in cephalopods (Moltschaniwskyj and Martinez, 1998; Pierce, et al., 1999; Domingues, et al., 2006), but many aquatic species (see Houlihan, 1991 for a review). In this study individuals caught in summer were generally smaller (Chapter 2) making it difficult to determine if the increase in RNA concentration was a response to body size or from experiencing warmer water temperatures. However, when the effects of total body weight were accounted for, the season of capture was still a significant factor influencing growth rate, with the concentration of RNA from individuals caught in 2008 being significantly greater in spring and summer compared to winter. These results are comparable to Loligo forbesi caught in Scotland where the RNA concentration in the mantle tissue of mature males was greater in summer than in winter (Pierce, et al., 1999). During 2009 no significant difference in RNA concentration was found among each seasons of this study and given the average seasonal temperatures were similar between 2008 and 2009 (Chapter 2), it is likely that water temperature is not the sole environmental factor effecting RNA concentration.
Other environmental parameters such as food availability, food quality (Pierce, et al., 1999; Vidal, et al., 2006) and even photoperiod (Vidal, et al., 2006) may all impact on the RNA concentration in cephalopods. While all animals from this study were collected during the dark, the processes that control food availability in the wild are influenced by several biotic and abiotic parameters (Vidal, et al., 2006) and are therefore complex and difficult to measure. While recent feeding activity has a direct effect on fish metabolism (Carter, et al., 1993; McCarthy, et al., 1994), there is little evidence from manipulative experiments that biochemical
parameters are correlated with recent feeding history in cephalopods (Castro, et al., 1992; Houlihan, et al., 1998; Moltschaniwskyj and Jackson, 2000).
In addition to the environmental parameters, weight, sex, reproductive and somatic condition can also influence the biochemical levels in cephalopods (Ho, et al., 2004; Rosa, et al., 2005). While these parameters made little or no contribution in explaining the variation in protein concentration, 46% of the variation in RNA:protein was explained by sex and weight. Although a large percentage of variation is still unaccounted for, this was the strongest relationship recorded in this study. In a laboratory study, body weight explained 84% of the variation in RNA:protein in reared E. tasmanica (Moltschaniwskyj and Carter, 2010). However some caution is needed when comparing results of cephalopods held in captivity as growth processes can be modified in captivity (Pecl and Moltschaniwskyj, 1999). A possible explanation for the difference between the two studies was the size of individuals used, with Moltschaniwskyj and Carter (2010) using smaller individuals from 0.02-7.2g, while individuals in this study ranged from 1.4-18.05g. Despite these differences, both studies confirm that the capacity for growth decreases with individual size, as evidenced by the decrease in RNA:protein ratio with total body mass.
A similar pattern between gender group was also found in both studies, with larger females having smaller RNA:protein values for their size compared to males. This suggests that as individuals grow, the capacity for growth in female E. tasmanica decreases at a faster rate than that of males, which is similar to wild Loligo forbesi, where the RNA content of males was higher than females (Pierce, et al., 1999). The size at which E. tasmanica starts to allocate energy to reproduction was also sex specific, with males starting to allocate energy to
reproduction at approximately half the body size as females. The sexual difference in body size at the onset of maturation is thought to be a factor of the different reproductive demands between males and females, with females needing to invest in larger body sizes due to the increased physical and physiological constraints of egg production compared to sperm production (Rodhouse and Hatfield, 1992; Ceriola and Jackson, 2010).