Seasonal variation in starvation resistance of early larval North Sea shrimp Crangon crangon (Decapoda: Crangonidae)

Abstract

The North Sea shrimp Crangon crangon (Linnaeus, 1758) has seasonal dimorphism in egg size, with larger winter eggs and smaller summer eggs. In the laboratory, we compared the tol- erance of nutritional stress in Zoea I larvae hatching from the different types of eggs (referred to as 'winter larvae', WL, and 'summer larvae', SL, respectively). Starvation tolerance was quantified as median point-of-no-return (PNR50) and point-of-reserve-saturation (PRS50). PNR50 is defined as the time when 50% of starved larvae have lost the capability to recover (after subsequent feeding); PRS50 is the time when 50% of fed larvae attain the capability to develop through the rest of the moulting cycle using internally stored energy reserves. These critical points in the moulting cycle were esti- mated by fitting sigmoidal dose-response curves of cumulative mortality to the time of initial starva- tion or feeding, respectively. Significant seasonal variation was observed in the initial biomass at hatching (16.2 vs 14.7 µg dry mass in WL and SL, respectively) as well as in the development dura- tion of continuously fed larvae (fed controls, FC; average Zoea I stage durations: 4.4 vs 5.0 d). Like- wise, WL showed a consistently shorter development duration after 1 to 4 d initial starvation and sub- sequent feeding (PNR treatments). In treatments with 3 to 5 d initial starvation, mortality was also significantly lower in WL than in SL. Both larval groups showed an increasingly delayed moult to the Zoea II stage with increasing time of initial starvation, but this effect was significantly weaker in WL than in SL. As a consequence, the mean PNR50 value was higher in WL than in SL (4.8 vs 3.5). When zoeae were continuously starved from hatching onwards (starved controls, SC), WL were able to sur- vive significantly longer than SL (8.8 vs 6.4 d). In experiments with differential periods of initial feed- ing and subsequent starvation (PRS experiments), 50% of the WL exceeded their PRS after only 1 d of food availability, while SL required at least 2 d of feeding to become independent of further food supply. PSR50 values of WL and SL differed significantly (1.0 vs 1.6 d). Our results indicate a shorter development and stronger starvation resistance in WL compared to SL. Seasonal variation in egg size and initial biomass and physiological condition of early larvae allow for an extended period of repro- duction, including larval hatching under conditions of low or unpredictable planktonic food avail- ability in winter and early spring.