Krill Reproduction and Moulting
The peak time for krill to reproduce occurs during the three month period from January to March. Antarctic krill males deposit a sperm sack near the genital opening of the female, and the eggs will fertilize as they pass through the genital opening. The fertilized eggs will then sink to the nutrient rich bottom where the embryos develop and hatch into a nauplius larva. The next larval stage is the metanauplius and in this stage is when the krill will begin to ascend towards the surface. It will continue to develop as it ascends and it takes the krill roughly three weeks to reach the ocean surface. The krill resembles a smaller version of the mature adult when it has reached the surface and this stage is called the furcila stage. Females are known to lay anywhere from 6,000 to 10,000 eggs at a time (Hempel and Hempel, 1986, Field observations on the developmental ascent). As the krill matures through its different life stages, it moults, or shed, the old exoskeleton in order to make room for the new body mass and appendages that are developing. While they are developing, krill will shed about every 13 to 20 days and Antarctic krill will reach maturity in about 3 years. Antarctic krill also have another unique ability called shrinkage. If there is not enough nutrients to support the krill's larger body mass, the krill will shed their exoskeleton and become a smaller size for that food deprived period so to lower the amount of stress upon its metabolic functions in a less nutrient-rich environment. Antarctic krill will also moult when they are scared. When one krill starts to moult within a swarm, the other krill will moult as well and flee the area, leaving their old exoskeletons behind and giving the appearance to a predator that there is still a large population of krill in the area (Hamner et. al., 1983, Behavior of Antarctic krill).
It has been determined through experiments described in Hirano et. al. that photoperiod (day length) is an important proximate cue for Antarctic krill to signal that it is time to reproduce. Climate change hasn't done anything to affect daylength yet so Antarctic krill should still be able to reproduce on time. The ultimate cause of this is most likely to maximize interaction with their primary food source, phytoplankton, for this is also the time when phytoplankton bloom is at its peak and there is still plenty of ice algae for the newly hatched krill to easily find and consume. As far as moulting goes, a possible proximate cue for that could be temperature because the water would become warmer as the krill-to-be ascends from the bottom and develops on its way up. With ocean temperatures increasing, krill could potentially develop faster than they should at deeper levels of the ocean and not have access to the food sources they need in order to survive at that developmental stage, leading to more krill dying at an earlier age and not reaching reproductive maturity (Poulsen et. al., 2011, Altered developmental timing).
Is the peak of krill based on ocean temperatures or photoperiod?
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