Several animals are renowned for their cognitive skills, such as tool use, metacognition or social learning. However, it remains puzzling why some species excel whereas others - sometimes even closely related ones - do not. Archerfish show a remarkable assembly of skills in the context of their unique hunting behavior in which they down aerial prey with shots of water. Hoping to find ecological factors behind these skills, we have over the past years regularly traveled to archerfish mangrove habitats in Thailand (Figure 1A). One of our most consistent findings was the presence of other surface-feeding fish, particularly the similar-sized halfbeak Zenarchopterus buffonis, wherever we spotted groups of archerfish (Figure 1A; Supplemental movie S1). We describe here that Zenarchopterus is superbly equipped with water-wave detectors, rapidly detects the impact of prey even in the dark, is active at all times, is usually more numerous than archerfish and supplements its capabilities with visual skills. Without sophisticated additions to their hunting technique archerfish would thus lose most of their downed prey to halfbeaks. We suggest that the evolution of several skills of archerfish may have thus been influenced not only by intraspecific competition [5] but also by competition with other surface-feeding fish species.

SUMMARYArcherfish are renowned for shooting down aerial prey with water jets, but nothing is known about how they spot prey items in their richly structured mangrove habitats. We trained archerfish to stably assign the categories 'target' and 'background' to objects solely on the basis of non-motion cues. Unlike many other hunters, archerfish are able to discriminate a target from its background in the complete absence of either self-motion or relative motion parallax cues and without using stored information about the structure of the background. This allowed us to perform matched tests to compare the ways fish and humans scan stationary visual scenes. In humans, visual search is seen as a doorway to cortical mechanisms of how attention is allocated. Fish lack a cortex and we therefore wondered whether archerfish would differ from humans in how they scan a stationary visual scene. Our matched tests failed to disclose any differences in the dependence of response time distributions, a most sensitive indicator of the search mechanism, on number and complexity of background objects. Median and range of response times depended linearly on the number of background objects and the corresponding effective processing time per item increased similarlyapproximately fourfold -in both humans and fish when the task was harder. Archerfish, like humans, also systematically scanned the scenery, starting with the closest object. Taken together, benchmark visual search tasks failed to disclose any difference between archerfish -who lack a cortex -and humans.