PRATT, M.C.; GRASON, E.W.; Bowdoin College, Brunswick, ME; Bowdoin College, Brunswick, ME: Do invasive species provide the opportunity to watch evolution in action in predator-prey interactions.

Introduced species can become dominant if they are released from predation, outcompete prior residents, or occupy an open niche. The bryozoan Membranipora membranacea was introduced into the Gulf of Maine in 1987 and within two years became the dominant epiphyte. Encrustation by Membranipora may increase the likelihood of breakage of kelps, possibly leading to large changes in the ecosystem. Membranipora may have become dominant so quickly because it had little competition in a relatively unoccupied niche; however, lack of predation has also probably played a major role. Where Membranipora is native, there tends to be a specialist nudibranch predator that keeps the population in check. In European populations, Polycera quadrilineata prefers Membranipora while Onchidoris muricata is known to prefer another bryozoan, Electra pilosa. Electra, Membranipora, and Onchidoris are all now found in the Gulf of Maine while Polycera is not. We tested whether Onchidoris would (1) eat Membranipora at all, (2) eat Membranipora or Electra faster, and (3) show a preference for eating Membranipora or Electra when given a choice. Onchidoris did eat Membranipora, and it generally ate Membranipora faster than it ate Electra. However, when given a choice, Onchidoris preferred Electra. Onchidoris typically reproduces in the spring and grows over the winter, but has recently been found reproducing in the winter in New Hampshire. While Membranipora is the dominant epiphyte in the late summer and fall, it does not survive the winter as well as Electra. Thus, it will be interesting to see if Onchidoris maintains its current life-history pattern and continues to concentrate on eating Electra, or if the life cycle shifts so that it can take advantage of the large Membranipora food source in the summer and fall.