Wednesday, 10 October 2012
The meek inheriting the Earth
Water fleas are not the biggest or fiercest of pond-life. They are food for most predators: you can even buy them in bags of bloated, farmed fleas for feeding to tropical aquarium fish. The wild water fleas of Druridge Bay tell a more successful story. The water flea, top left, is a species called Daphnia obtusa, a 3-4mm long crustacean and common in shallow, muddy pools along the Bay. You can spot them as jerky specks swimming indecisively hither and thither, although “swimming” is too elegant a description as they row themselves with their antennae (the long arms in the photo). Their legs are enclosed within the carapace which gives them their rounded shape, each limb equipped with a fine-meshed net of bristles for raking algae out of the water for food. Each Daphnia may not be large, but the photograph to the right shows a swarm of thousands, each speck a Daphnia, in a dune pool near Cresswell. In the good times they reproduce by cloning. Most of the population are female and they develop embryos asexually. In the left hand photo you can see several baby Daphnia, the greenish blobs to the right of the adult’s sinuous gut. This cloning allows rapid population growth. As water levels drop and ponds begin to dry out the Daphnia produce tougher, drought resistant eggs which linger in the mud long after the adult hordes have been wiped out. When the rains come and ponds refill these eggs hatch and the Daphnia numbers boom, benefitting from the absence of fish. It is a successful strategy; Daphnia obtusa is found throughout Europe and North America as well as just north of Cresswell.