Effects of Low Tide Conditions on Susceptibility to Herbivory for Saccharina sessilis

Abstract

The intertidal zone experiences different abiotic conditions during high and low tide. Low tide stressors include, but are not limited to, temperature fluctuations and wind desiccation. Saccharina sessilis provides a canopy for intertidal animals to escape to during these warm, stressful low tides. Some of these animals include herbivores of S. sessilis : K. tunicata, I. wosnesenskii, and Lacuna species snails. This macroalgae will accumulate damage over a tide series. The focus of this study looks at whether being damaged will alter S. sessilis susceptibility to herbivory. First we assessed S. sessilis risk of herbivory in the field at Pile Point, WA by counting the number of S. sessilis, K. tunicata, I. wosnesenskii, and Lacuna in a given area in the S. sessilis zone. Next, we carried out stress/no stress trials where we simulated a low tide and either desiccated samples of S. sessilis or left them out of water. Both treatments lasted either 60 or 90 minutes. Area, wet weight, and percent brown were measured before and after each simulation Data showed the stressed samples lost more mass, area, and accumulated more visible damage than the not stressed samples. We then put these paired samples though herbivory trials in a container without herbivores or with a set of herbivores (K. tunicata, or I. wosnesenskii). Once 50% of the S. sessilis tissue in the herbivore container was consumed, the trial was terminated for the pair of containers. The area and wet weight of the remaining tissue was measured before and after the herbivore trials. Analysis showed that K. tunicata preferred damaged S. sessilis tissue over undamaged tissue and that I. wosnesenskii had no preference for damaged or undamaged S. sessilis tissue. Overall, we conclude that repeated wind exposure leads to shrinking algae, and ultimately their algal canopy. We also conclude that S.sessilis’s susceptibility to herbivory is sensitive to the differences of the feeding preferences of its herbivores.