Predation efficiency in visual and tactile zooplanktivores

Gelatinous zooplanktivores (medusae, siphonophores, and ctenophores) and visual zooplanktivores (fish) interact through competition, predation, and commensalism. In the search for key factors governing the outcome of competition, we examined the instantaneous predation efficiency and its light dependency. The visual predator Gobiusculus flavescens and the tactile predator Bolinopsis infundibulum were used as experimental models for the two predation modes. The predation rate of G. flavescens was adequately described by Holling’s curvilinear disc equation, and that of B. infundibulum was proportional to prey density. However, because of superfluous feeding, the feeding rate of B. infundibulum differed from the predation rate and approached the asymptotic limitation at high prey levels. The predation rate was reduced for G. flavescens at irradiances ,5–10 mmol photons m22 s21, whereas light had no significant impact on the feeding pattern of B. infundibulum. Provided sufficient light, the predation rate of G. flavescens was several orders of magnitude higher than that of B. infundibulum. These results are consistent with the results of other studies, which suggests that the maximum clearance rate (Cmax) of visual and tactile predators is described by the power functions Cmax 5 3.42 3 1027 L2.94 and Cmax 5 6.02 3 1028 L1.77, respectively, where L is the length of the organisms in centimeters. We conclude that visual predation is most efficient at high visibility and low prey densities. As visibility decreases and prey density increases, the competitive efficiency of tactile predators increases. Medusae, siphonophores, and ctenophores are voracious predators of zooplankton in marine ecosystems. Episodic and more persistent aggregations of gelatinous predators have been frequently reported (Graham et al. 2001). Competing for similar resources, gelatinous planktivores may reduce fish production and constrain harvest (Purcell and Arai 2001). Some ecosystems have converted from supporting viable commercial fisheries to hosting exceptional numbers of gelatinous planktivores and few fishes (Mills 2001). Numerous studies have investigated the causal factors for these remarkable blooms—for example, environmental degradation (Arai 2001), climate changes (Brodeur et al. 1999), overfishing (Daskalov 2002), and the introduction of alien species (Kideys 2002). Most gelatinous zooplanktivores are characterized by high feeding, growth, and reproductive rates, as well as restricted predation (Alldredge 1984). From this perspective, it is not surprising that these animals are frequently dominant zooplanktivores. Indeed, one might wonder how planktivorous fish are able to compete with gelatinous planktivores. We addressed the question with a comparative study of the instantaneous predation efficiency for visual and tactile predators. Specifically, the functional responses (i.e., predation rate vs. prey density) of the visual zooplanktivore Gobiusculus flavescens (a fish) and the tactile zooplanktivore Bolinopsis infundibulum (a ctenophore) were examined. A literature review of predation efficiency in visual and tactile zooplanktivores indicated that these two animals can be considered as representative models of the two predation modes. Eiane et al. (1999) hypothesized that tactile predators, which are adapted to feed continuously, should increase their com1 Corresponding author ([email protected]).