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

摘要

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 tolerance 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 (PNR_(50)) and point-of-reserve-saturation (PRS_(50)). PNR_(50) is defined as the time when 50 % of starved larvae have lost the capability to recover (after subsequent feeding); PRS_(50) 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 estimated by fitting sigmoidal dose-response curves of cumulative mortality to the time of initial starvation 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 duration of continuously fed larvae (fed controls, FC; average Zoea Ⅰ stage durations: 4.4 vs 5.0 d). Likewise, WL showed a consistently shorter development duration after 1 to 4 d initial starvation and subsequent 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 PNR_(50) 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 survive significantly longer than SL (8.8 vs 6.4 d). In experiments with differential periods of initial feeding 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. PSR_(50) 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 reproduction, including larval hatching under conditions of low or unpredictable planktonic food availability in winter and early spring.