Predatory Promesostoma species (Plathelminthes, Rhabdocoela) in the WaddenSea

Laboratory experiments were performed on the food ecology of four congeneri c species of free-living plathelminths, Promesostoma caligulatum, P. marmor atum, P. rostratum, and P. meixneri, all inhabiting an intertidal sandflat near the island of Sylt (North Sea). Their prey spectrum is within the micr ocrustaceans: P. caligulatum preferred ostracods, while the other three spe cies favoured copepods, with species-specific differences for copepod speci es and size classes. Daily consumption of prey species varied with the size of both the predator and the prey. On average, P. marmoratum consumed 0.76 Harpacticus flexus per day while this rate decreased to 0.06 in P. meixner i, the smallest predator. When these Promesostoma species were fed with Tac hidius discipes, a smaller prey species, their predation rates were about 2 5% higher. While the larger predators preferred the larger harpacticoids as prey, the small P. meixneri preferred small cyclopoids over larger harpact icoids. In terms of biomass, P. marmoratum's mean consumption of T. discipe s per day was about half the predator's own weight. This average varied wit h prey density and temperature. A comparison of these consumption rates wit h the field densities of the predators and their prey shows that the plathe lminth predators may consume as much as 10% per day of their copepod prey p opulations, thus strongly influencing these prey populations on these sandf lats. The predation pressure of P. caligulatum on ostracods was about 1% pe r day of the prey population. Since ostracods usually have fewer generation s per year, the total effect on the population dynamics may be similar to t hat on copepods. Therefore, nocturnal swimming of copepods in the water col umn may be interpreted as an attempt to escape plathelminth predators. (C) 1999 Elsevier Science B.V. All rights reserved.