Beneath London’s waterways lurk creatures both strange and ancient—from the plague-carrying Signal Crayfish to the burrowing Chinese Mitten Crab. Did you know the Thames once teemed with native crayfish, kept as live snacks by the Romans? Now, invasive species dominate, ignored by Londoners but prized as delicacies elsewhere.
On 17 May, dive into the hidden world of London’s crustaceans: ghostly translucent Leptodora drifting in reservoirs, parasitic fish lice with vampiric suckers, and even land-loving sandhoppers hiding in gardens. Discover why these creatures matter—and which ones are quietly reshaping our ecosystems.
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Crustacea
Crustaceans are arthropods like insects and spiders but instead of either six or eight legs respectively they have many more. Even the humble water louse (Asellus) has nineteen pairs of limbs. In London, crustaceans vary enormously from the tiny planktonic water fleas and copepods in our lakes up to our largest invertebrate, the freshwater crayfish. It seems likely that even in Roman London they were caught in the Thames and its tributaries but also kept alive at home in earthenware pots as a ready source of live food. Our native crayfish the White-clawed crayfish Austropotamobius pallipes can live for six or even seven years. Thankfully it is now protected. We have four other invaders that are not. The Turkish crayfish Astacus leptodactylus which can be found in the ponds on Clapham Common, the Virile crayfish Orconectes virilis in the River Lee, the Louisiana red swamp crayfish Procambarus clarkii in Regents Park canal and the Signal crayfish Procambarus leniusculus in the Rover Colne. The latter species was responsible for introducing the “crayfish plague”, a fungal infection which led to sixty per cent of our native species dying over a thirty year period.
We also have in London quite a different alien invader, the Chinese Mitten crab Eriochier sinensis which, since it arrived in 1936, has been busily burrowing into the muddy banks of the Thames all the way up to Staines, even including rivers such as the Crane, Hogsmill and Ash. Crayfish are not too fussy as to what they eat, anything dead being fair game. Consequently a piece of meat on some string dangled into the water is usually all that is needed to catch them. The Chinese mitten crab and most of our alien crayfish are much prized edible species elsewhere in the world yet here Londoners tend to ignore them all.
Two other very common aquatic crustaceans are water lice (Asellus) and water shrimps (Gammarus). Water lice are amongst the most common of all freshwater animals with us all year, found wherever there is mud and caught in nearly every pond net. This month you may see them coupled with the female, who is likely to be carrying a whitish bag of eggs beneath her. Similarly water shrimps are found most places there is fresh running water. Again, this month they are seen attached to each other. She will be in berry and he is in attendance on top to some extent helping her to move around. We are all familiar with woodlice, but there is another land crustacean in London, the terrestrial sandhopper Arcitalitrus dorrieni. This is closely related to the sandhoppers you see jumping around among dried seaweed on the strandline of a seashore. It only arrived in London in 1998 and turns up occasionally here and there, preferring to live under flower pots with woodlice in my own garden.
Quite different are the parasitic crustaceans such as the fish louse (Argulus). They are quite often found attached to fish and look like small flattened jelly-like discs with two ghoulish eyes. These “eyes” are in fact suckers and between them is a spine which is used to penetrate the skin and suck blood. They seem to think nothing of leaving one fish and seeking out another, their spines deterring fish from eating them. The most unusual of all London’s crustaceans has to be Leptodora kindtii. It is about two centimetres long, has an elongated phantom-like shape, strong jaws and a single dazzling Polyphemus-like eye. It is often almost colourless and glass-like floating just below the surface in reservoirs where it feeds on its fellow planktonic crustaceans. Often it is only its shadow that ever gets noticed.
