Moult cycle-related changes in feeding rates of larval krill Meganyctiphanes norvegica and Thysanoessa spp

Knowledge of crustacean moulting is derived mainly from benthic decapods, which often show profound changes in physiology and behaviour through the moult cycle. In contrast, euphausiids are suggested to be little impaired by moulting, enabling a swarming pelagic life. The aim of this study was to quantify moult cycle-related changes in the feeding activity of 2 euphausiids, Meganyctiphanes norvegica and Thysanoessa spp. Late furcilia larvae and early postlarvae were kept individually over 6 to 7 wk and fed with either a high or low concentration of Artemia salina nauplii or particulate fish food. The intermoult period, similar to9 d for M norvegica and similar to8 d for Thysanoessa spp., increased with body weight, but did not differ with food source. Moulting was partially synchronised, with up to 50 % of the individuals moulting within 48 h of each other. Daily feeding rates on A, salina decreased on the day before moulting, but increased during the next few days with highest values on Days 1 to 3 after moulting. The deviation from the mean feeding rate over the whole moult was more pronounced at the higher food concentration, reaching up to 40 %. Likewise, the defecation volume was reduced on the moulting day and the following day to similar to50 % of the mean, but increased to 180 % of the mean on Day 3 after moulting. Thus, the moult cycle induces significant changes in feeding rates of larval euphausiids with a similar succession of events and intensity as observed in decapods. Feeding rates, extrapolated from spot measurements on a few individuals, are unlikely to represent average values over the whole moult cycle, especially when populations moult synchronously. We propose a protocol to increase the precision of field estimates on feeding rates.

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