Date of Award

Spring 2007

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

James E Byers


The burgeoning field of phenotypic plasticity and inducible defenses has documented a wide variety of predator-induced defenses. I this dissertation I have explored induced defenses in the marine mussel Mytilus edulis as they are affected by (a) shared evolutionary history with invasive crab predators, (b) specificity of responses to multiple predators (singly and combined) with different foraging strategies, and (c) spatial and temporal variation in the expression of predator specific induced defenses in situ.

Mytilus from southern New England expressed induced shell thickening when exposed to waterborne cues from the crab Hemigrapsus , but "naive" northern mussel populations do not respond. Yet, both populations thicken their shells in response to a long-established crab, Carcinus. These results are consistent with the rapid evolution of an induced response to the recent invader Hemigrapsus.

Mytilus developed significantly heavier shells only in the presence of waterborne cues from Carcinus, thicker shells in response to Carcinus, the seastar Asterias, and the whelk Nucella, and heavier adductor muscles in response to cues from Nucella and Asterias. These induced defenses subsequently protected mussels from Carcinus, but only Asterias exposed mussel were defended from Asterias. However, mussels exposed to the combined cues from Asterias and Carcinus expressed neither inducible defense nor deterred foraging by the sea star or crab. Furthermore, Mytilus did not thicken shells in response to cues from the native crab Cancer irroratus or the combined cues from Carcinus and Cancer, yet mussels did increase adductor muscle in response to combined cues from Asterias and Cancer. Thus, multiple predator assemblages can disrupt predator specific induced defenses (resulting in risk enhancement for mussels), but these effects cannot be reliably predicted from the predator's functional grouping.

Finally, in field experiments, I found that mussels expressed predator specific responses to Carcinus in mid-intertidal cages (but not Asterias) and mussels in low intertidal cages increased adductor muscle only in response to Asterias, and only during a year with high tissue growth. Together these results suggest that inducible defenses can be influenced by shared evolutionary history with predators and the functional diversity of predator assemblages.