Stubbing your toe on a table leg is not an enjoyable experience. Although we can all be clumsy at times, for most of us this is not a frequent occurrence as we're usually pretty good at navigating our way through complex environments. However, we're not the only ones living in cluttered environments; fish also face obstacles in their habitats, be it a coral reef or a reedy lake bottom. So, how do they manoeuvre their way around these hurdles? It's a question that interested Brooke Flammang, a post-doc in George Lauder's group at Harvard University, USA: ‘Up until now, the majority of studies of fish swimming have looked at them just swimming in a completely open environment in a tank without anything around them, to just really

understand the fish itself. But we wanted to put the fish into the context of its environment, to understand how it was moving in a more natural setting.’ So, with the help of several students, Flammang began her investigation (p. 3084).

To begin, Flammang created an obstacle course by placing several posts in a flow tank – essentially a treadmill for fish – taking care that there was a route that the fish could take without ever having to bump into the posts. She then put some bluegill sunfish to the test and filmed the outcome. Flammang suspected that these fish would use vision and/or their lateral line (a sense organ that enables them to detect water flow) to aid their quest. To test this, she deprived some of the fish of one or both of these senses during their time in the flow tank by keeping them in the dark and/or treating them with cobalt chloride to temporarily disable their lateral lines.

The results were surprising. Even when the fish were able to both see and use their lateral lines, they were actively tapping the posts with their pectoral fins, wrapping their fins around the obstacles, and they never took the direct route through the poles. As fish use their pectoral fins to swim at slow speeds, Flammang was surprised: ‘If you have animals swimming using their pectoral fins and you put them into an environment where they're going to be potentially contacting things around them with their fins, then you would think that that contact would hinder their locomotion.’ But perhaps they were just using the posts to push themselves forward? Flammang doesn't think so: ‘If they had been pushing off they would have had to have had a flat fin ray pressed against the obstacle; they wouldn't have been able to generate that pushing force and maintain that nice flexible bend around the tube.’ In addition, when the fish were coaxed to swim faster and use their body and tail fins to swim against the flow, the pectoral fins were still touching the obstacles.

So what information are they getting from these touches? Flammang doesn't know, but suspects ‘it is important to the fish to have an idea of where it is peripheral to obstacles, and potentially the size and shape of the obstacles.’ In support of a role in feeling their way around obstacles, when the fish were deprived of both vision and the lateral line, fin taps became more frequent. In short, they depended on the only sense left to them – touching. It seems that, for a fish, touching, feeling and seeing are all useful when navigating your way through a complicated habitat.

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Pectoral fins aid in navigation of a complex environment by bluegill sunfish under sensory deprivation conditions
J. Exp. Biol.