British Wildlife of the Week
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. Its gills have no covers, so water taken in through the mouth passes through the pulsing pouches and is then expelled through a line of seven round holes along each side of its flank. It has no scales and, should you cut its body open, you would discover that it has no proper bones either; it is cartilaginous like sharks and rays, which gives it a robust, rubbery form. This primitive-looking creature is a river lamprey.
The river lamprey is a member of a small group of animals called the jawless fish – a descendent of the primeval fish-like progenitors that eventually evolved into true fish hundreds of millions of years ago. The jawless fish were once the pinnacle of evolution, the most advanced and revolutionary creatures in the sea. They used their jawless mouthparts – a muscular sucker lined with rasping tongue and teeth – to scrape away at their food or swallow huge chunks of meat whole. However, it wasn’t long before true fish – the ancestors of the cartilaginous fish and the bony fish that we know today – appeared in the oceans. They grew proper jaws and were able to outcompete their jawless rivals. Although the jawless fish didn’t die out completely, they were nonetheless swept to the sidelines. Today, only a few species remain – the strange, slime-producing hagfish, and the lampreys.
Britain is home to three species of lamprey: the aforementioned river lamprey, the brook lamprey, and the sea lamprey. Lamprey larvae, otherwise known as ammocoetes, are tiny, blind, transparent, and possess mouthparts. They are so unlike their parents, in fact, that early scientists described them as distinct animals in their own right. These worm-like larvae spend several years buried in the soft sediment at the bottom of rivers and streams, filtering detritus and tiny organisms by channelling water through their mouths and securing prey on a mucus thread. They take three to seven years to mature, after which their mouthparts transform into suckers and they develop eyes.
River and sea lampreys then migrate downstream into the sea and become ectoparasites, latching on to marine fish and mammals and grating through their flesh using their multitude of teeth to feed on their blood. As they tear through the skin and muscle of the victim, they secrete an anticoagulant that keeps the blood flowing freely. It’s easy to understand why these primitive creatures have earned the nickname ‘vampire fish’. Once attached to a host, a lamprey is almost impossible to detach, although it will fall off by itself when satiated. However, this leaves its victim with a gaping wound that is slow to heal and prone to infection. Host fish often die as a result of this form of parasitism.
After one year of hematophagous feeding, river and sea lampreys return to freshwater, embarking on pilgrimages upriver to spawn. The journey is slow and difficult, as they must fight currents and obstacles. They can overcome waterfalls by using their oral sucker to cling on to the wet rocks and then hoisting themselves up through the rushing water. However, this exhausting reproductive gesture is the last they make, and after spawning almost all lamprey die.
Although parasitic lampreys are the most well-known, the majority of species don’t engage in this grisly lifestyle. Non-parasitic species do not feed when fully grown; they simply live off the reserves acquired as larvae and consequently have short adult lives. The smallest and most common of the UK’s trio, the brook lamprey, is non-parasitic and spends its entire life in freshwater. Like other lampreys, it develops eyes and a suction disc during its metamorphosis from its filter-feeding larval stage. At the same time, its intestinal tract degenerates and loses its function, which means that this species, strangely, only develops its teeth precisely when it is no longer able to eat. Unsurprisingly, the brook lamprey’s teeth are blunter than its parasitic relatives, since it doesn’t need to use them to latch onto other animals. Its sucker-like mouth is used instead to cling to rocks and grip stones in order to build its underwater nest.
Although the lamprey is not strictly a fish, it is certainly eaten as a fish. The flesh of the lamprey is reputed to be very nutritious and tasty, albeit rather fatty, and during the Middle Ages, they were widely eaten by the upper classes throughout Europe. In 1135, King Henry I of England is said to have died after eating ‘a surfeit of lampreys’ against his physician’s advice (although many historians today believe that he died from food poisoning). Hundreds of years later, lampreys still retain a royal connection – in 1953, Queen Elizabeth II’s coronation pie was made using lampreys, and she was presented with another lamprey dish to celebrate her Golden Jubilee in 2012.
The lamprey is a blast from the past – a true relic. These creatures have a fossil record dating back some 450 million years, virtually unchanged, pre-dating all mammals, birds, reptiles and amphibians. Yet somehow they still survive in a world dominated by higher, more advanced animals.
In the next British Wildlife of the Week, we’ll be looking at the classic fairytale toadstool – the fly agaric.
(Also, be sure to check out The Nature Nook’s other Halloween-themed articles this week, including a look at how burying beetles take care of their young, why vampire bats don’t deserve such a bad reputation, and how to look after your very own ghost mantis.)