A sixth sense

Mechanisms underlying vertebrate electroreceptor development

Our sense of hearing, and the information used for balance, results from the movement of tiny hairs on specialised hair cells in our inner ears in response to fluid motion, caused by vibrations (for sound) and head movement (for balance). In fish and aquatic-stage amphibians, mechanosensory (motion-sensing) hair cells just like those in the inner ear are also found on the body surface, collected in tiny sense organs called neuromasts, arranged in characteristic lines over the head and body. Neuromast hair cells detect local water flow around the animal: the information is used for schooling behaviour, finding prey and detecting predators.

Some fish and salamander-type amphibians also possess a second set of modified hair cells that detect changes in weak electric fields in water. Electroreception, a real sixth sense, is used for finding prey (for example, a shark can detect a flounder buried in the sand because of tiny changes in the electric field caused by the flounder’s heart beating), for orientation (e.g. for long distance migration relative to the earth’s magnetic field) and, in some species, for communication. Frogs, all land vertebrates, and the ancestors of teleost fishes (the group including most modern bony fish species) lost these electroreceptors. This means that most modern fish cannot detect electric fields, but, interestingly, electroreceptors were independently 're-invented' at least twice in two different groups of teleosts, including catfish and electric fish. Teleost electroreceptors work in a slightly different way from the ancestral type, raising the possibility that teleost electroreceptors are modified neuromast hair cells.

We plan to shed light on the course of electroreceptor development and evolution by investigating the mechanisms underlying their embryonic development in paddlefish (an ancient, extraordinary river fish with more electroreceptors than any other vertebrate) and catfish (one of the teleost fish that re-invented electroreceptors).

Our overall aim is to provide a greater understanding of the mechanisms underlying sensory cell diversification both in embryonic development (i.e. the formation of mechanosensory versus electrosensory hair cells) and evolution.

Dr Clare Baker
University of Cambridge

Clare was awarded a Research Project Grant in November 2011; providing £194,949 over 36 months.

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Rosette-like electrosensory organs shown here distributed in fields over the head of this paddlefish embryo, as well as the thin lines of mechanosensory organs that run between these fields (and also in a line down the body). Below is a close up of these electrosensory organs, the eye is visible towards the left of the image (images courtesy of Dr Melinda Modrell, University of Cambridge).

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