Effect of the bat star Asterina miniata (Brandt) on recruitment of the giant kelp Macrocystis pyrifera C. Agardh

Abstract

The effect of the common bat star, Asterina miniata (Brandt), on recruitment of the giant kelp, Macrocystis pyrifera C. Agardh, was addressed through laboratory grazing experiments, a field experiment, and modeling of feeding behavior. In the laboratory, Asterina miniata significantly decreased the density of sporophytes that developed from 1-wk-old gametophytes as well as the percent cover of 2-, 6-, and 7-wk-old sporophytes. All grazed blades remaining at the end of these experiments subsequently died. Small scale variability in spore settlement and sporophyte development were also evident in the laboratory. Bat star density significantly affected short-term kelp recruitment during a large-scale field experiment. Small scale variability in kelp recruitment and bat star density was also evident during this experiment. Simple modeling suggested that high densities of Asterina miniata could graze nearly 100% of the bottom over the 90-day experiment. However, visible recruitment was seen in less than 30 days, and over this time, 36% of the substratum was predicted to have remained ungrazed. Macrocystis pyrifera of 1 to 3 cm in length may thus obtain a refuge in size from bat star grazing through rapid growth. These results indicate this generalist grazer can affect giant kelp recruitment but that even under high grazing pressure numerous plants survive. Bat star grazing probably does not contribute to large-scale differences in adult plant density but may contribute to small-scale patterns of dispersion.