On the timing of moulting processes in reproductively active Northern krill Meganyctiphanes norvegica

Abstract

The interactions between moult phasing, growth and environmental cues in Northern krill (Meganyctiphanes norvegica) were examined through analysing populations at seasonal, weekly, and daily timescales. The analyses were carried out on resident populations of krill found in three different neritic locations that experience similar environmental signals (the Clyde Sea, Scotland; the Kattegat, Denmark; Gullmarsfjord, Sweden). Seasonal analyses were carried out on the Clyde Sea population and showed that moulting frequency increased significantly moving from winter to summer. The proportion of moulting females in summer samples was often more than double the proportion of moulting males, suggesting that females had a comparatively shorter intermoult period (IMP). Weekly samples taken from the Kattegat showed a similar pattern. However, although the difference between the proportion of female and male moulters was significant in one week, it was not another, mainly because of the variability in the proportion of female moulters. Such variability in females was equally evident in the daily samples taken at Gullmarsfjord. It suggests that females have a shorter IMP (12.5 days) than males (18.4 days) and are more likely to moult in synchrony. Nevertheless, the daily samples revealed that males are also capable of moult synchronisation, although less frequently than females. Shortened IMPs in females were not a result of the abbreviation of specific moult stages. Accordingly, reproductive activity did not alter the course of the normal moult cycle. There was no significant difference between the total body lengths of males and females indicating that females achieve the same levels of growth despite moulting more frequently and having to provision the energy-rich ovaries. This is in contrast to most other crustaceans where the energy costs of reproduction reduce female growth. The fact that females were less abundant than males, probably by suffering a greater level of mortality, suggests that different behavioural strategies, particularly vertical migration regimes, were adopted by each sex to maximise growth and reproduction.

Growth and Moulting in Northern Krill (Meganyctiphanes norvegica Sars)

“…In the course of our studies on Northern Krill, sex dependent differences in moulting became conspicuous at three locations and at different time scales (Buchholz et al, 2006). Not only did sexual maturation result in a synchronisation effect on females at the population level (Tarling and Cuzin-Roudy, 2003) and modulate female vertical migration patterns (Tarling et al, 1999b), but it also increased their moult frequency compared to males, at least temporarily.…”

“…In establishing any scheme on the physiological basis of growth, the adjustment of the moult cycle according to seasonal environmental signals must be considered. IMP has been found to be invariably around 13 d in adult krill of ca 30 mm total length in a number of separate studies carried out at 10 C (Buchholz et al, 2006;Cuzin-Roudy and Buchholz, 1999 and unpublished data). This value also falls close to a regression line of a combination of laboratory experiments and field assessments of IMP ( Fig.…”

“…Cuzin-Roudy and Buchholz (1999) adapted the scheme for Meganyctiphanes norvegica. Table 6.1 is an overview of the main phases based on Buchholz (1991) and applies to both M. norvegica and E. superba, including an update to the estimated duration of each of the moult stages in M. norvegica (Buchholz et al, 2006). The stage nomenclature was devised to match the Drach system as closely as possible, this applies also for the detailed version in Table 6.2.…”

“…However, IMP may be varied by temporal variation of most major moult stages at the same time in response to a suite of other cues, particularly reproductive processes (Buchholz et al, 2006). Laboratory maintenance is the only way to differentiate the effect of the two parameters IMP and INC on growth rate: an adjustment of growth to environmental cues can be brought about by variation of either or both of these parameters.…”

“…Alternatively, a more sophisticated staging system can be applied, where individual moult stages are identified in laboratory incubated specimens (Buchholz, 1991;CuzinRoudy and Buchholz, 1999). Combining both approaches provides insights into moulting patterns over short and long timescales and between different populations, as demonstrated in Buchholz et al (2006). The approach has been successfully adapted for mysids (Gorokhova, 2002).…”