Predator ontogeny affects expression of inducible defense morphology in rotifers.

Many prey organisms show induced morphological responses to predators including changes in protective spine length, such as in rotifers, although previous studies have mainly focused on how prey become larger than the predator gape-size optimum. Here we show that a large-sized predator makes prey rotifers escape below the gape-size optimum of the predator by reducing spine length. In experiments and field studies we show that during part of their ontogeny fish larvae feed intensively on the common rotifer Keratella cochlearis, and that larval fish predation reduces rotifer spine length both through induction of shorter spines and selective predation on long-spined individuals. We also describe a global scale pattern in spine length of K. cochlearis, showing an increasing variance in spine length with latitude. This pattern may be explained by differences in fish reproduction from once per year at high latitudes to several times per year at lower latitudes. That spine length is adaptively adjusted to the ontogeny of a dominant predator taxa provides a novel view on our understanding of factors affecting temporal and spatial variations in prey defense morphology.

Food limitation and seasonal population declines of rotifers.

Keratella cochlearis exhibited consistent seasonal abundance patterns during a four-year study in Little Rock Lake, Wisconsin, U.S.A. In each year, spring population peaks were followed by strong summer reductions. Concomitant with population declines, there were reductions in rotifer egg ratios. Keratella taurocephala abundance patterns were similar to those of K. cochlearis during 1984 and 1985, but not in 1986 and 1987, when spring peaks and summer declines were not apparent. However, summer declines in the egg ratio of K. taurocephala were observed during each year. The reduction in rotifer populations simultaneously with decreased egg ratios suggested that population declines were caused by food limitation. Food-addition experiments conducted in situ in small enclosures indicated that food was limiting for K. cochlearis when its populations were declining, but not during other periods of the year. Keratella taurocephala did not show a consistent response to food addition.