L Journal of Experimental Marine Biology and Ecology 249 (2000) 249–262 www.elsevier.nl / locate / jembe The intertidal distribution of two algal symbionts hosted by Anthopleura xanthogrammica (Brandt 1835) a,b, * Amanda Bates a Simon Fraser University, Burnaby, B.C., V5A 1S6, Canada b Bamfield Marine Station, Bamfield B.C., V0R 1B0, Canada Received 4 June 1998; received in revised form 16 March 2000; accepted 28 March 2000 Abstract This paper quantifies the spatial distribution of zooxanthellae (ZX) and zoochlorellae (ZC), two algal symbionts common to the temperate anemone, Anthopleura xanthogrammica, in relation to shore height. Anemones in tidepools and crevices had varying algal proportions: . 0.90 ZC (green anemones), 0.10 to 0.90 ZX (mixed), and . 0.90 ZX (brown). Brown anemones are primarily found in the high intertidal and the upper region of tidepools. Mixed anemones are most common at intermediate shore heights and green anemones are exclusive to the low shore and at increasing depth in tidepools. Microhabitat was also important to algal proportion, as anemones in crevices had greater proportions of ZC than anemones in tidepools at the same shore height. In a reciprocal transplant experiment, A. xanthogrammica were moved between high and low shallow tidepools. All anemones moved from a low to a high tidepool exhibited a shift from ZC to ZX populations, while the anemones transplanted from high to low tidepools maintained ZX dominance. This is the first documentation that field algal populations can shift from ZC to ZX in Anthopleura. The field survey and transplant study results support the hypothesis that the relative abundance of ZX and ZC in A. xanthogrammica is influenced by the environmental gradient associated with shore height and microhabitat.  2000 Elsevier Science B.V. All rights reserved. Keywords: Algal symbiosis; Algal distribution; Anthopleura xanthogrammica; Environmental gradient; Zoochlorellae; Zooxanthellae 1. Introduction The coexistence of multiple algal types within a given host is rare (Muscatine, 1971), and such systems are therefore intriguing. Anthopleura elegantissima and A. xantho- grammica, anemones found on the Pacific northwest coast, commonly host two different *Correspondence address: Bamfield Marine Station, Bamfield, B.C., V0R 1B0, Canada. 0022-0981 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0022-0981(00)00203-3