Egg-release behaviour in Antarctic krill
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The process of egg release is a complex and crucial step in the life cycle of euphausiids, especially with regards mortality and recruitment success. We examined this process in Antarctic krill (Euphausia superba) in terms of the functioning of the female genital apparatus and associated swimming behaviour. A tethering technique combined with video analysis was used to make observations of three females during the release process. We found eggs were steadily extruded over a period of up to 10 h, during which time the krill released between 1,600 and 4,000 eggs and the ovary reduced by a half in length and a third in height. Eggs were mainly released individually or, less commonly, in batches of between 2 and 4 eggs. Release rates were between 5 and 30 eggs min−1. The steady release of eggs fits well with histological evidence that mature oocytes must pass individually through tight genital ducts with narrow apertures, before coming in to close contact with the sperm plug for fertilisation, and then expulsion from the thelycum as fertilised eggs. During spawning, the female alternated between slow and rapid rates of pleopod beating with egg release occurring at the moment of beat acceleration. At the point of release, the descent of the egg was accelerated through downward beats of the 7th thoracic leg. The cyclic pattern in pleopod beat-rate during spawning may alter swimming performance and contribute to the widely reported sex- and maturity-based biases within krill swarms.
KeywordsEuphausia superba Southern Ocean Scotia Sea Fecundity Spawning Behaviour
Our thanks to Captain Jerry Burgan and the crew of RRS James Clark Ross, and to Chief scientist, Peter Ward, and the JR116 science team for their support of our work at sea. Christopher Gilbert assisted with technical aspects of video analysis, Juliet Corley produced schematic drawings of krill egg-release behaviour. Robin Ross provided samples of spawning females for histological analysis. We are grateful to Robin Ross and Stephen Nicol for their constructive criticisms of an earlier draft of this manuscript. This work was carried out with the support of the Antarctic Funding Initiative Collaborative Gearing Scheme (CGS6/16). The contribution of GT was carried out as part of the FLEXICON project of the DISCOVERY 2010 programme at BAS.
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