Canopy-forming plants and algae, in addition to their role as food, can positively affect higher trophic levels by providing protection from predation or stressful abiotic conditions. In the low rocky intertidal zone of the Pacific Northwest, the chiton Katharina tunicata is closely associated with the canopy-forming alga Hedophyllum sessile. In a 27-month field experiment, I used experimental devices to mimic characteristics of the Hedophyllum canopy and monitored the abundance and physiology of Katharina to determine the mechanism behind this association. The data show that this association is due to selection of shaded microhabitats by Katharina, and that neither predation nor Hedophyllum per se influenced the distribution of the chiton. Exclusion of visual predators had no effect on chiton abundance, and tests of the effects of shade independent of Hedophyllum presence indicated that the shade produced by the alga was critical to the chiton. Seasonal fluctuations in levels of 70 kDa 0.16 X 10(-19) g) heat shock proteins (Hsp70), indicate that Katharina experience sublethal physiological stress during the spring and summer. The corresponding use of shaded areas during spring and summer low tides suggests that the chiton's distribution is limited during these seasons by the availability of refugia from sublethal abiotic stresses. Subtle variations in environmental conditions are also important to this association; the strength of the positive interaction varied annually, becoming more positive in warm summers (1998 and 2000) and neutral in a cooler summer (1999). On a finer scale, physiological measurements indicate that the response of chitons to summertime temperatures depends on both absolute temperature change and the animal's recent thermal history. In particular, the seasonal onset of daytime low tides in the spring is associated with increased Hsp70 levels, indicating that this is a significant physiological event for Katharina. The importance of facultative positive interactions that allow consumers to escape from sublethal abiotic stresses will likely be variable in time but can have large impacts on communities both by affecting the small-scale distribution of consumers over the short term and potentially by affecting the persistence of organisms over the long term.
