International eel symposium, 2003 - Quebec August 11, 2003

Speculations on the Adaptive Strategies and Survival-To-Recruitment of Anguillid Leptocephali in Relation to Long-Term Trends in Ocean-Climate Change

Knights, B.* School of Biosciences, University of Westminster, 115 New Cavendish Street, London W1M 8JS, England, knightb@westminster.ac.uk

Presenter email address: knightb AT westminster.ac.uk

Abstract Text: Evidence on adaptive strategies of Elopomorphan leptocephali is reviewed, with special to N. Hemisphere anguillids. It is proposed that SubTropical Gyre fronts provide cues for high-fecundity reproduction and enhance nutrient circulation and food supplies. Predators are scarce in such oligotrophic waters, but little is known about the nutrition of leptocephali. They can, however, grow rapidly in size for relatively low energy costs by laying down an acellular gelatinous matrix. This reduces the predator size-spectrum, but also provides support and a large energy reserve. Marine snow of Type A (appendicularian houses) and Type B (mixtures of aggregated POM) could form important nutrient sources. The unusual charge, transport mechanisms and high surface area of the epithelium could enhance the formation of aggregates and nutrient absorption. C :N ratios suggest a reliance on pico- and bacterioplankton, important components of nutrient circulation in oligotrophic oceans and major gyre currents and eddies. Energetic calculations imply larger leptocephali can survive relatively long periods without feeding. Decadal cycles of warming-cooling probably cause variations in spring-overturn and nutrient circulation (and changes in major current systems), causing variations in starvation and advection mortality associated with long-term trends in eventual continental recruitment. Studies to test the hypotheses proposed are discussed.