Temporal variation in the symbiosis and growth of the temperate scleractinian coral Astrangia poculata

Abstract

Seasonal variation in the algal symbiosis and growth of Astrangia poculata, a facultatively symbiotic temperate scleractinian, was explored in Rhode Island, USA. Coral pigmentation and growth were measured simultaneously and repeatedly in both zooxanthellate (corals with symbionts) and azooxanthellate (symbiont free) colonies at 2 sites (~10 km apart) over a 15 mo period using non-destructive digital image analysis methods. A chlorophyll density proxy based on coral pigmentation was derived from multivariate analysis of color data from coral images, and polyps were enumerated to measure colony growth. Among zooxanthellate corals, predicted chlorophyll density exhibited significant seasonal fluctuations that were positively related to temperature, with maxima occurring during late summer and early autumn. Pigmentation dynamics in azooxanthellate corals were more variable, although many of these corals displayed temporal fluctuations in pigmentation. Growth also exhibited seasonal fluctuations positively related to temperature, and ceased during the coldest 3 to 4 mo of the year. Corals lost live polyps during the winter as a result of tissue thinning and dormancy, which rendered the colony unable to fend off settling organisms. Although zooxanthellate colonies were able to grow faster than azooxanthellate colonies, coral pigmentation explained only 23% of the variation in growth rate, emphasizing the importance of heterotrophy as the primary source of nutrition for A. poculata at this northern margin of its range.