Wednesday, June 24, 2009

Pond Plankton

Mention the word ‘plankton’ and the drifting life in the surface layer of the oceans springs to mind – stuff that blue whales eat - but there’s plenty of plankton in the surface layer of lakes and ponds too. This picture (top) shows the larval stage – known as a nauplius – of a freshwater crustacean called a copepod (bottom photo), possibly Cyclops (that single red eye is a strong clue!). All those long bristles probably improve the nauplius larva's buoyancy, slowing the rate that it sinks and giving it extra leverage when it beats its limbs to stay in the surface layers. Copepod nauplius larvae go through a series of up to six moults before they become a recognisable Cyclops and this in turn goes through a further five moults before it reaches its final adult stage. The whole process takes the best part of a month, during which the nauplius and developing Cyclops are favourite food items on the menu of every passing predator; in this respect a nearly-mature tadpole is probably the blue whale of the pond. Life is hazardous when you’re small and edible!


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