Nick Baker’s hidden Britain
Meet the Cornish sucker fish
Draw back a curtain of bladderwrack, and you may reveal a fish that looks like it was designed by Dr Seuss. This cartoonish blob doesn’t look or behave like any other fish I know. Deep purple blotches, a red tail fin, yellow lips and electric turquoise spots usually convince the finder that it is something special, but there’s more to the Cornish sucker (or Cornish clingfish, Lepadogaster purpurea) than meets the eye.
This fish has to be one of my favourite UK weirdos and, as ever, when an animal looks so peculiar, it has a good ecological backstory. Clearly, it’s designed for something other than free swimming – more for creeping about on the bottom or over rocks. Here, it feeds on any small creature it can find, from snails to crustaceans.
The sucker fish has a strange flattened profile that is rather tadpole-shaped, with an almost duck-shaped head. There are no scales, just slimy smooth skin, and it lacks a dorsal fin. In fact, other than the tail, none of the usual ‘finny’ protuberances are obvious in this species.
This fish is found mainly along rocky shores in the south and south-west, sticking to the intertidal zone – a harsh place to make a living. Here, the species is exposed by receding tides twice a day and, when submerged, frequently takes a violent beating from the swirling waves. It must be like living in a washing machine on a particularly high spin cycle. When rockpooling, you’ll find that this fish is both slippery and sticky. You will struggle to get it out of a crevice or off a rock or weed, only to find it sticking to your hands like chewing gum. If you were to place one into an observation tank, it’s likely to obligingly attach itself to the tank’s transparent side. Then you’d get to see the magic in action – the sucker that gives this fish its name.
On the fish’s underside, the paired pectoral and pelvic fins don’t look like fins, but have become fused into a continuous disc. For those of us who’ve been frustrated with the sucker discs used to attach mirrors, sat-navs or bird feeders to perfectly smooth windows or bathroom tiles, this achievement is miraculous.
What the clingfish is able to do in a glass tank, it can do just as well on uneven, pitted rocks.
A sucker is quite a simple device. It creates a seal, then produces a negative pressure by expanding the volume of the sucker cavity (the fish does this with muscular contractions). This only works as long as the friction seal between the disc’s edges and the surface remains intact. Suckers fail when the edge slips, the seal breaks and water from outside gets in.
The sucker works on any surface due to the disc’s microscopic structure. The whole organ is flexible, and its rim is effectively a fin under the muscular control of the fish. Close scrutiny reveals the surface of the sucker edge is made of hexagonal knobs, each covered with microscopic hairs. These find and fill the tiniest imperfections in the surface of the rock, creating the frictional force that holds the disc edge firmly in place.
If you remove one of these fish from the water, in order to take a closer look, make sure you put it back exactly where you found it – especially from spring to summer, during the species’ breeding season.