Visual discrimination in fishes

Fishes possess various sensory systems such as hearing and a hydrodynamic sense, chemical senses, a temperature sense and passive or active electrosensitivity. Among the vertebrates they are the inventors of eyes. In a series of experiments we investigate the specific adaptations of the fish eye to diverse environments. We already measured the visual acuity of the European eel (Anguilla anguilla) and the Indian mudskipper (Periophthalmus novemradiatus) under different conditions. These two fish species are characterized by completely different lifestyles, however both are able to survive in water as well as in air. The mudskipper almost possesses a terrestrial lifestyle.

The video shows a two-alternative-forced choice experiment run with mudskippers in order to determine their visual acuity. The animal is entering the experimental compartment from the left side and chooses the stripe pattern for which it is rewarded. By stepwise increasing the difficulty of the task the threshold of the mudskipper can be assessed.

Hydrodynamic analysis of fast starts

C starts and S starts, also called fast starts, are fast escape responses of fishes which are elicited by an acoustical or visual stimulus. In this project we analyze if the water movements that are caused by these escape responses can be used by predators to detect the escaping fish. Many predators such as seals or fishes possess senses which enable them to detect minute water movements. However it is not known until now if they can analyze and categorize spatial patterns of water movements. Therefore we measure water movements that occur in their natural habitat with the method Particle Image Velocimetry (PIV).

The video shows two fast starts and the related water movements. The first sequence illustrates a C start, the second a S start. The water movements are illustrated as vector graphs. In both sequences three zones of fast water movements, so called jets, can be seen which adopt more or less the form of vortex rings: jet 1 is generated by the tail fin, jet 2 by the body and jet 3 by the body in a late phase. Theoretically a predator could extract valuable information out of these jets.