The vertical component of a fish's spatial map

Fish orient by using a memorized map of their surroundings and, as they live in aquatic environments, this map must be three dimensional. Studies have shown that fish are able to encode horizontal information; however, we have little information about the cues that are learnt and remembered in the vertical axis. We used the blind Mexican cave fish, Astyanax fasciatus, to test whether fish can orient in the vertical axis and whether they use hydrostatic pressure to do so. After learning the cues associated with a specific height in the aquarium (evident by a reduction in swimming speed), the fish showed significant dishabituation (their swimming speed increased) when moved vertically upwards. When we controlled for the increase in light intensity from the bottom to the top of the tank the effect persisted, indicating that the most likely cue used by the fish to orient vertically was hydrostatic pressure. A post hoc experiment revealed that the change in swimming speed in response to altered light levels probably represents a change in the level of activity rather than an orientational response. This may define a photokinetic mechanism which operates in conjunction with a negatively phototactic response (revealed in previous experiments) by which the fish orient away from higher light levels and swim faster as light levels increase. This would result in the fish spending more time in lower light levels and would prevent the fish exiting the cave.