Nick Baker’s Hidden Britain
The popular naturalist, author and TV presenter reveals a secret world of overlooked wildlife
Stuck on you
COMMON FISH LOUSE
The parasitic freshwater crustacean that likes to draw close to its fish host – before sucking its blood
A THIN DISC, TRANSPARENT AND smooth, beady black eyes gazing up at you from your pond net. Place it in water and it starts to animate, performing somersaults propelled by numerous limbs on its underside. What is this peculiar creature that looks like a well-sucked Jelly Tot come to life?
Out of context, the common fish louse (Argulus foliaceus) looks alien. It doesn’t fit the description of any of the usual freshwater creatures, and few freshwater field guides include the species.
The reality, though, is that they are a quite common, highly specialised, parasitic crustacean. Stealing the blood of their host is their business – unsurprisingly, therefore, they spend most of their lives attached to a fish. Their beauty is in their adaptation, their form aligned to their furtive function.
Part of their life-cycle involves leaving their host as adults to mate and reproduce. This is when they turn up in pond nets and we can get a proper look at them.
Find a clear-sided or white container for optimum viewing. An adult is 7-8mm long and 5mm wide. The first thing you’ll notice is a pair of dark eye spots. It is probably these that give them their scientific name Argulus, which is derived from the manyeyed Argus in Greek myth – get a fish louse under a microscope and you’ll see that its eyes are compound.
The rest of the body is patterned with spots and blotches. Some are pigment that breaks up its outline, others are its internal organs: a central stomach, ovaries or testes can be seen, as well as the central digestive tract. When the side chambers to its stomach (called diverticula) are filled with fish blood, they form a symmetrical pattern of a dark and sinister beauty.
“For most of their life, fish lice are almost invisible”
While the topside is all about creating as little resistance to water flow as possible, the underside holds all the nefarious mechanisms which allow the fish louse to live its vampiric life. Just below the eyes are two rings – these sucker organs are actually modified limbs which, combined with a flexible body, plus bristles and spines provided by modified antenna and leg bases, enable the louse to grip and stick to its host.
The usual crustacean mouthparts have been modified into a tube-like proboscis through which the louse penetrates the circulatory system of the fish. In addition to this, there is what is referred to by some as the ‘sting’. Between the eyes and the mouth, this needle is associated with glands that are thought to deliver an anaesthetic amongst other things – so the unfortunate host can’t feel the louse.
It can disengage when it needs to, slipping over the surface of its host’s skin before propelling itself along with four or five paired swimming legs on its abdomen. This strange movement often involves summersaults and flips.
For most of their life, fish lice are almost invisible, as all good parasites should be – to their host in particular. Their smooth hydrodynamic profile offers little resistance to water flow or objects and the fact that they are translucent allows the pattern of the host’s skin to show through, offering the perfect camouflage. It’s not in their interest to draw attention to themselves, so when plugged into their host they appear as nothing more than slimy bumps, pimples with a perverse purpose.
Heavy infestations in nature are rare, but when they do occur they can be devastating. A 28cm-long tench was once found with 4,250 Argulus foliaceus attached.
LOOK CLOSER
Marine stowaways
Sea lice (Caligus sp.) are a closely related parasitic crustacean that are found on marine species of fish. While they can’t survive for very long in freshwater, they do often enter river systems on the backs of migrating fish such as salmon and sea trout, falling off after a few days. Their presence on a fish tells us therefore that it has only recently left the ocean on its return migration up stream.