Changes in native species after biological invasion: Effects of introduced green crabs on native soft-shell clams and an estuarine community

Biological invasions have ecological and economic consequences worldwide, and a key to effectively managing the impacts of invasions is understanding how invaders affect native species. By examining how native behavior and morphology changes in response to invaders, changes in native traits and abundance after invasion can be better understood, and future methods for native protection can be developed. In Maine, predation by introduced green crabs, Carcinus maenas, has contributed to declines in the abundance of valuable native soft-shell clams, Mya arenaria. This dissertation describes an historical analysis, laboratory experiments, and field experiments that were employed to determine the mechanisms responsible for changes in clam behavior, morphology, and survival rates following the invasion of crabs. Analysis of clam shells from museum collections indicated that siphon length and shell thickness had increased in clams after green crab arrival. To identify potential mechanisms responsible for this pattern of change in clam morphology after crab invasion, experiments examined changes in clam behavior and morphology after interactions with the introduced crab. The experiments demonstrated that clams burrowed deeper in the sediment and siphon length increased in response to crabs, and those clams that burrowed deeper were less vulnerable to predation by crabs. In addition, clams were capable of detecting and responding to chemical cues released from crab predation by burrowing deeper in the sediment. The clams that responded to crabs by burrowing were found to grow at slower rates, potentially due to growth allocation towards siphon tissue at the cost of body growth or less efficient feeding at greater depths. When clams were exposed to chemical cues under hatchery conditions, shell growth rate increased, suggesting clam responses to crabs may differ with ontogeny or environment. Green crabs were found to reduce diversity and abundance of organisms in the soft-sediment community. Hence, the tents found to be effective for protecting clams may also increase the abundance of other infaunal invertebrates. From this work, there is a better understanding of the mechanisms responsible for changes in native clams since the green crab invasion, and potential methods for effectively protecting the valuable clam species have been identified.