Hunting in archerfish - an ecological perspective on a remarkable combination of skills

A critical view on five rules of how behavioural experiments could be run

In everyday life it can be tempting to follow some convenient rules, but it is important to sometimes pause and to consider whether these rules are actually useful: Do they help or obscure our understanding of the full beauty of animal behavior?

Archerfish foraging success varies with immediate competition level but not group size

Group living can lead to kleptoparasitism, the theft of resources by competitors. Under such conditions, foragers may alter their behavior to minimize competition. However, it is unclear how such behavioral changes impact foraging performance. Archerfish (Toxotes spp.) are a good model for investigating the behavioral responses to kleptoparasitism, as their hunting method (shooting waterjets at insects perched above the water) leaves them vulnerable to theft. They must hit the target prey with sufficient force to dislodge it; thus, the prey may land some distance away from the shooter. Kleptoparasitism rates increase with group size in archerfish, and individuals alter their behavior around conspecifics. We investigated whether group size affected shooting success, using 7-spot archerfish T. chatareus. We considered a fish's shot to be successful if it knocked a fly, placed on a transparent platform above the tank, into the water. The probability of shooting success was modeled as a function of group size, aiming duration, nearest neighbor distance and position, and trial number. We found no effect of group size, aiming duration, or nearest neighbor distance or position on shooting success. Shooting success increased as trials progressed, likely due to the fish becoming more familiar with the task. We also found no change in the kleptoparasitism rate between group sizes. Instead, the likelihood of the shooter consuming the prey depended on the types of competition present at the time of shooting. We suggest that archerfish shooting behavior can be influenced by the presence of conspecifics in ways not previously considered.

The archerfish predictive C-start

A very quick decision enables hunting archerfish to secure downed prey even when they are heavily outnumbered by competing other surface-feeding fish. Based exclusively on information that is taken briefly after the onset of prey motion, the fish select a rapid C-start that turns them right towards the later point of catch. Moreover, the C-start, and not later fin strokes, already lends the fish the speed needed to arrive at just the right time. The archerfish predictive C-starts are kinematically not distinguishable from escape C-starts made by the same individual and are among the fastest C-starts known in teleost fish. The start decisions allow the fish-for ballistically falling prey-to respond accurately to any combination of the initial variables of prey movement and for any position and orientation of the responding fish. The start decisions do not show a speed-accuracy tradeoff and their accuracy is buffered against substantial changes of environmental parameters. Here, I introduce key aspects of this high-speed decision that combines speed, complexity, and precision in an unusual way.

Prosocial behavior in competitive fish: the case of the archerfish

Humans are social creatures, demonstrate prosocial behaviors, and are sensitive to the actions and consequent payoff of others. This social sensitivity has also been found in many other species, though not in all. Research has suggested that prosocial tendencies are more pronounced in naturally cooperative species whose social structure requires a high level of interdependence and allomaternal care. The present study challenges this assumption by demonstrating, in a laboratory setting, that archerfish, competitive by nature, preferred targets rewarding both themselves and their tankmates, but only when the payoff was equal. With no tankmate on the other side of the partition, they exhibited no obvious preference. Finding evidence for prosocial behavior and negative responses to unequal distribution of reward to the advantage of the other fish suggests that in a competitive social environment, being prosocial may be the most adaptive strategy for personal survival, even if it benefits others as well.

Short-range hunters: exploring the function and constraints of water shooting in dwarf gouramis

Ballistic predation is a rare foraging adaptation: in fishes, most attention has focused on a single genus, the archerfish, known to manipulate water to shoot down prey above the water surface. However, several gourami species also exhibit apparently similar 'shooting' behaviour, spitting water up to 5 cm above the surface. In a series of experiments, we explored the shooting behaviour and aspects of its significance as a foraging ability in the dwarf gourami (Trichogaster lalius). We investigated sex differences in shooting abilities to determine whether gourami shooting is related to the sex-specific bubble nest manufacture where males mix air and water at the surface to form bubbles. We found that, actually, both sexes were equally able to shoot and could learn to shoot a novel target. In a second experiment, we presented untrained gouramis with opportunities to shoot at live prey and found they successfully shot down both fruit flies and crickets. Finally, we explored the effect of target height on shooting performance to establish potential constraints of shooting as a foraging ability. The frequency of attempted shots and success of hitting targets decreased with height, whereas latency to shoot increased. We also observed that repeatable individual differences account for variation in these measures of shooting performance. Together, our results provide evidence that gourami shooting has a foraging function analogous to that of archerfish. Gourami shooting may serve as an example of convergent evolution and provide opportunities for comparative studies into the, as yet unexplored, ecology and evolution of shooting in fishes.

Archerfish coordinate fin maneuvers with their shots

Archerfish down a variety of aerial prey from a range of distances using water jets that they adjust to the size and distance of their prey. We describe here that characteristic rapid fin maneuvers, most notably of the pectoral and pelvic fins, are precisely coordinated with the release of the jet. We discovered these maneuvers in two fish, the jets of which had been characterized in detail, that had been trained to shoot from fixed positions at targets at different heights and that remained stable during their shots. Based on the findings in these individuals, we examined shooting-associated fin movement in 28 further archerfish of two species that could shoot from freely chosen positions at targets at different heights. Slightly before the onset of the water jet, at a time when the shooter remains stable, the pectoral fins of all shooters switched from asynchronous low-amplitude beating to a synchronized rapid forward flap. The onset and duration of the forward and subsequent backward flap were robust across all individuals and shooting angles but depended on target height. The pelvic fins were slowly adducted at the start of the jet and stopped moving after its release. All other fins also showed a characteristic sequence of activation, some starting ∼0.5 s before the shot. Our findings suggest that shooting-related fin maneuvers are needed to stabilize the shooter, and that these maneuvers are an important component in the precise and powerful far-distance shooting in archerfish.

Archerfish vision: Visual challenges faced by a predator with a unique hunting technique

Archerfish are well-known for their ballistic hunting behaviour, in which they shoot down aerial prey with a well-aimed jet of water. This unique hunting strategy poses several challenges for visual systems. Archerfish face significant distortion to the appearance of targets due to refraction at the air/water interface, they search for prey against a complex background of foliage, they change prey targeting behaviour as conditions change, and they must make high speed decisions to avoid competition. By studying how archerfish have overcome these challenges, we have been able to understand more about fundamental problems faced by visual systems and the mechanisms used to solve them. In some cases, such as when searching for targets, the visual capabilities of archerfish are functionally similar to those of humans, despite significant differences in neuroanatomy. In other cases, the particular challenge faced by archerfish magnifies fundamental problems generally faced by visual systems, such as recognizing objects given strong viewpoint dependent changes to appearance. The efficiency of archerfish retrieving fallen prey to avoid kleptoparasitism, demonstrates that their visual processing excels in both speed and accuracy. In this review, we attempt to provide an overview of the many facets of visually driven behaviour of archerfish, and how they have been studied. In addition to their hunting technique, archerfish are ideal for visual processing experiments as they can be quickly trained to perform a range of non-ecologically relevant tasks. Their behavioural flexibility moreover, introduces the opportunity to study how experience-dependence and choice affects visual processing.