Predation within the marine environment has been well-studied and shown to be of major importance in shaping patterns of biodiversity. Typically larger predators, such as fishes, are examined because of the ease of manipulation and strong detectable results, whereas effects of smaller micro-predators are much more difficult to evaluate. Here, we examined the distribution and prey consumption of the polyclad flatworm, Euplana gracilis, in the Chesapeake Bay. Euplana gracilis is a common, micro-predator, but no data exist on its ecological function. Flatworms were found to actively prey upon a single species, the tube-building amphipod Apocorophium lacustre, in lab trials when tested against several other commonly encountered species. To examine natural population densities of flatworms, large-scale field sampling was conducted via benthic grabs and E. gracilis abundances were found to be significantly correlated with A. lacustre particularly in areas close to the shoreline. Some predator-prey interactions were examined including timed observations of consumption, predator size, and tube protection. Flatworm body size was found to correlate strongly with the number of prey consumed over time. Tubes constructed by amphipods were examined as a means of refuge when in the presence of E. gracilis, but provided very little protection as flatworms could easily penetrate tubes in search of prey. Our results are the first to show predation of an estuarine/marine polyclad flatworm on amphipods as well as provide some insight into the dynamics of this previously unknown predator-prey relationship.
