“There is no such thing as a fish.” – The Oxford Encyclopaedia of Underwater Life (2005)

What is a fish?

No such thing as a fish? This really means that unlike mammals and birds, not all the creatures we call fish today descend from the same common ancestor. Or put another way, if we go back to most recent common ancestor of everything we now call fish (including the incredibly primitive lungfish and hagfish), we find that they also were the ancestor of all four-legged land vertebrates, which obviously aren’t fish at all. So, it’s a term to use with caution. After all, in the 16th century, seals, whales, crocodiles and hippos were called fish and cuttlefish, starfish, crayfish, jellyfish and shellfish still are.


Sprattus sprattus

European Sprat or Sprattus Sprattus

What’s a sardine? Good question. Sardine is a generic term to describe around 20 different small, soft-boned, oily fish. In Britain, they are usually pilchards, sometimes called – optimistically – the true sardine, although their Latin name (Sardina pichardus) reveals the confusion. Sometimes what you get in a sardine can is a herring, sometimes it is a sprat (which glories in the scientific names Sprattus sprattus sprattus).

Little fish

The world’s smallest fish (and smallest vertebrate) is the stout infantfish (Schindleria brevipinguis). Only 8mm long and transparent, it has no teeth or scales and their only pigment is in their eyes. They live in coral lagoons in eastern Australia. They reach maturity about six weeks after hatching. We know this by counting the rings in their ear bones (or otoliths), just as you would count the rings in a tree. This method works for ageing all bony fish.


English: Some smoked European sprats, in Germa...

Some smoked European sprats, in Germany called ‚Kieler Sprotten‘.

Billingsgate fish market originally sold corn, coal, iron, wine, salt, pottery as well as fish. In 1699 an Act of Parliament declared it a “free and open market for all sorts of fish whatsoever” except for eels. Eels could only be sold by Dutch fishermen moored in the Thames. This was their reward for helping to feed the people of London during the Great Fire. The European eel population is in freefall: it has reduced by 99 per cent since the Seventies as a result of pollution, parasites, overfishing, dam building and changes in the Atlantic currents. Nowadays, “everybody knows” that eels are born in the Sargasso Sea. But this has never been proved. No one has ever seen an eel spawn or seen one die there. Nor has a single eel ever been bred in captivity. When you catch an eel, its reproductive system shuts down completely. Sigmund Freud, seeking an answer to this phenomenon, worked in Trieste cutting up hundreds of eels to see how their sexual organs worked. When he had finished, he published a thesis concluding that all of his research was a waste of time. He turned to psychology instead.

Gender bending

Nemo in the film Finding Nemo wasn’t a clownfish but a close relative, the False clownfish. They look similar, except the True clownfish has more intense colours and thick black edging to its stripes. Both species live in a symbiotic relationship with anemones, having evolved a thick mucous coat to protect them from being stung. All True clownfish are born male but some turn into females. The rather odd family grouping inside an anemone is a female (who was previously a male), a dominant male and up to four males who cannot reproduce. If the female dies, the dominant male becomes a female. If a dominant male dies then a non-breeding male becomes able to breed.

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