Laterality in a non-visual sensory modality — the lateral line of fish

Although a range of animal taxa preferentially use one eye for particular tasks [1–3] little is known about laterality in other sensory modalities, particularly in non-terrestrial vertebrates [4]. Several species, including fish, have been shown to preferentially use one eye over the other for specific tasks. For example, some fish use their left eye — and hence the right hemisphere of their brain — when observing social stimuli [5], but they use their right eye and left hemisphere when encountering novel objects [6]. Fish also have a mechanosensory organ at their disposal, the lateral line organ (LLO), which allows for detection of water movements. Here, we show that a similar lateral bias to novel objects occurs in blind Mexican cave fish (Astyanax fasciatus), which exhibit a clear preference for using the right sided LLO. This preference wanes once the fish are familiar with the landmark. Similar to vision, the lateralised use of the right LLO can be related to use of the left hemisphere of the brain in fish, because the neurones associated with the lateral line course bilaterally from the hindbrain nucleus to the midbrain with contralateral predominance [7]. Thus, our results show behavioural laterality in a nonvisual sense. As eye formation in blind Mexican cave fish arrests early during development, these fish have no access to visual cues. However, they are able to navigate effectively using their LLO by detecting perturbations in the flow field, which is generated around the fish as it moves through the water [8]. We, therefore, wondered whether behavioural lateralization exists in the LLO. In the first experiment (see supplemental data for details), individual cave fish were introduced into a square arena that contained a novel plastic landmark at the centre of one of the walls. We recorded the number of times that each fish passed the landmark using its left or right flank during the first 30 min of exposure to the landmark. Swimming speed was also recorded during this period using an automated tracking device [9]. The fish were left in the experimental arena over night to allow them to become familiar with the tank. The following day, LLO side preference and swimming speed in each fish were recorded as before. Familiarity with the environment can be measured in Mexican cave fish by monitoring swimming speed; in a familiar environment these fish swim more slowly [10]. On day 1, we