The European eel is a surprisingly enigmatic creature. Its breeding behaviour in particular has confounded people for over 2,000 years. To some, its proliferation seemed inexplicable. The great Greek thinker Aristotle, for example, believed that eels spontaneously emerged from mud. A few hundred years later, Pliny the Elder had his own imaginative ideas about eel reproduction – he thought they bred by rubbing their bellies against rocks.
The ability for animals to generate, store and release electricity is more common than many people realise. All living animals produce electrical impulses on an infinitesimal scale; they are the medium by which messages are sent along nerves and are discharged whenever muscles contract. But some fish have developed banks of modified tissue, which can produce electricity on a much greater scale.
Lurking in the depths of a select few lakes in England and Scotland is our rarest freshwater fish. Known as the vendace, this truly is a relic from the past. The few isolated populations that remain are the last vestiges of a species that was much more widespread during the last Ice Age. In our warmer modern world, only large, deep glacial lakes can provide the cold, clean, well-oxygenated conditions that these fish need to survive.
At the bottom of a freshwater stream in Britain, a thin, elongated fish undulates through the water. It looks a bit like an eel, but closer examination reveals it is something much different, and much stranger. It only has a single nostril (on the top of its head) and its large suctorial mouth is bristling with sharp, inward-pointing teeth of varying sizes, arranged in neat concentric circles.
Large, lazy and very strange-looking – meet the ocean sunfish. This extraordinary animal is certainly very peculiarly-proportioned. Tall but vertically flattened, with a pale, circular body that seems to end abruptly behind its huge dorsal and anal fins, the sunfish almost looks as though it is simply a massive severed head with a short, frilly tail attached.