Meeting Abstract
The fairy shrimp genus Branchinecta has two, giant, predatory species (B. gigas and B. raptor) that prey primarily on the normal sized congener B. mackini, and to a lesser extent on B. readingi. These prey species occur commonly in temporary pools across western North America, while the two predatory species are limited to far fewer locations. I observed that the prey species are far larger and more physically robust in basins where the predator is present than in other sites. Though field collections and lab cultures, I found that the prey species growth rates (length and volume) were different depending upon whether a predator species was present or not, and was different in the presence of each predator species. Prey species growth rates not only increased greatly in the presence of the predatory species, but matched the growth rate of the predator species present. This increased growth rate only occurred when the predator species was present, and could be induced when water from cultures or wild populations of the predatory species was added to predator free prey cultures, even from populations that had never encountered the predator species in the wild. Predator species actively consumed smaller prey species individuals of subsequent cohorts, but ignored the larger, first cohort individuals, unless no other prey was present. This suggests that the prey species have found a “size refugium”, where they may be too large for predator consumption, and that this change in growth rate is triggered by some sort of kairomone produced by the predator. The fact that populations which have never encountered the predator species also demonstrate this change in growth, suggests that these species encountered each other early in their evolution.
