Chemical and morphological responses of an intertidal seaweed to waterborne cues from an herbivorous snail

The intertidal rockweed Fucus distichus responds chemically and morphologically to mechanical damage from herbivores. Recent evidence suggests that a related species, Ascophyllum nodosum, responds chemically to waterborne cues that indicate the presence of herbivores. To determine if waterborne cues from herbivores induce morphological or chemical responses in F. distichus we grew juvenile F. distichus in outdoor mesocosms for 70 days from August to October in the presence of waterborne cues from: 1) the snail Littorina sitkana feeding on adult F. distichus (experimental treatment), or 2) adult F. distichus (control). We compared these treatments in terms of morphology, growth, tissue carbon and nitrogen, phlorotannin levels, and palatability to L. sitkana. There was no significant difference between treatments in morphology, growth, tissue carbon or phlorotannin levels, suggesting that juvenile F. distichus either cannot detect or do not respond to waterborne cues from L. sitkana. However, tissue from the experimental treatment was significantly preferred by L. sitkana in a choice feeding trial and consumption was positively related to tissue nitrogen levels. This suggests that waterborne cues from L. sitkana do not induce a defense response in juvenile F. distichus but L. sitkana can detect small differences in F. distichus tissue nitrogen levels. Results suggest the possibility of a feedback loop in which utilization of snail metabolic wastes by F. distichus influences subsequent feeding preferences.