Cloning and Aggression among Sea Anemones (Coelenterata: Actiniaria) of the Rocky Shore

¹ Biology Department, Bates College, Lewiston, Maine 04240

Among the common rocky shore anemones of Pacific North America, New Zealand, and Tropical Australia, clonal growth is significantly correlated with other features of the biology including aggressiveness, habitat, and body size. Individual size is more variable among aclonal species and among species living on the lower shore; and aclonal individuals are larger on average than clonal individuals. Aclonal species are usually non-aggressive inhabitants of the lower shore, while clonal species are usually aggressive residents of the upper shore. To explain the link between cloning and aggression, a geometric model is developed that compares the scaling of interference budgets for isometric aclonal growth, and for two-dimensional growth of a compact, encrusting clone. The ratio of exposed perimeter to feeding surface area (P/S) declines more steeply through clonal spread than through aclonal growth; and therefore, the costs of interference relative to energy intake rates should be lower for a short, squat clone than for a single, bulbous individual of the same volume. Association with mutually tolerant individuals can also reduce the per capita costs of interference (The Three Musketeers effect—a special case of the geometric model). Cloning may be more common among anemones living higher on the shore because clonal individuals are generally smaller than their aclonal counterparts, and because predators that prefer small anemones tend to be more abundant downshore.