Cladoceran fashions

Nice look at a morphological response to predation pressure in Daphnia in Swedish lakes. Water fleas studied in the lakes in the Orekilsalven River basin show two very different types of responses. In lakes in the upper reaches of the system, female Daphnia have relatively large bodies topped by a fairly small "cap" (an anterior extension of the carapace). Those found further down the river system show reduced body size, but a larger cap.

Hans Lord from the Department of Zoology at University of Gothenburg thinks that the different morphologies are an indication of the ostracods response to a widening suite of predators. The larger cap makes the cladocerns harder for invertebrate predators to manipulate. Smaller body size, on the other hand, makes the animals less visible to sight-feeding fish. The cap, being transparent, gives protection against invertebrate predators without making them more visible to fish.

Male Daphnia don't show the same morphology - no cap. This is likely due to different selective pressures. Males need to be able to find females and mate quickly. A big cap gets in the way.

In females, the large caps are a summertime phenomenon. This could be the result of different types of predatory pressures with changing seasons, or it may refect the greater viscosity of cold water. Can't swim through thick water wearing a big hat.