Riverwatch

Riverwatch: Crusty creatures

Much of river life becomes torpid during the winter. Some river life, such as fishes and particularly younger life stages, seek out gentler flows and more tangled habitat in which they can expend less energy fighting stronger currents and subsist on reduced availability of food. Other creatures, such as many insects, enter resting phases in their life cycles, or else like pond skaters go into hibernation or nearhibernating stasis as adults. Bats above the water’s surface go into full hibernation in sheltered, dry places, whilst birds such as swallows, swifts and martins and many warbler species migrate to warmer climes.

However, some creatures remain present and active all year round. Water snails are one such group as too, though more cryptic in the sediment yet very common and widespread, are various species of freshwater mussels from tiny Pea Mussels (Pisidium species) to the large Swan Mussel (Anodonta cygnea).

More conspicuous, at least if you turn over stones or disturb leaf litter, are a range of crustaceans. Amongst the under-stone dwelling crustaceans most commonly encountered in our rivers are crayfish and shrimps, whilst Water Slaters are more common in poorer water quality or hiding in decaying vegetation in river edges.

The native White-clawed Crayfish (Austropotamobius pallipes) is, as described previously, now largely extirpated in many parts of Britain by the spread of more aggressive, plaguecarrying alien crayfish species, particularly the American Signal Crayfish (Pacifastacus leniusculus).

The most common of the freshwater shrimp species found in our rivers is Gammarus pulex, the ‘pulex’ name denoting its flea-like ability to jump when stranded on damp land. This and several other species also occur in ponds, particularly those with inflowing streams and springs.

A recent species that has invaded British rivers from its native Ponto-Caspian region of eastern Europe is the ‘Killer Shrimp’ (Dikerogammarus villosus), the shrimp progressively advancing across western Europe and appearing in British waters in 2010 and spreading subsequently now quite widely. Killer Shrimps can colonise many types of habitat, tolerating a range of temperatures as well as oxygen concentrations and even mild salinity, living a predatory lifestyle feeding on a variety of invertebrates and breeding all year round. More gentle in habit, and also native, are the Water Slaters. Slaters look similar to woodlice, to which they are a close aquatic relative. For those who wish to look more closely, the Common Water Slater (Asellus aquaticus) can be distinguished by the two white spots on its head, whereas the less common One-spotted Water Slater (Asellus meridianus) has, unsurprisingly given its name, only one spot. The Common Water Slater is by far the more common of the two, found in a wide diversity of ponds and river margins. As Water Slaters can exist in water with a low oxygen content and subsist by eating rotting vegetation, performing the important services of recycling stored nutrients and energy into the food chain, they tend to fare well in richer and more polluted waters that are also less favourable to their potential predators.

Even these apparently boring creatures are truly fascinating, Water Slaters moulting periodically as do all crustaceans. Shedding the exoskeleton (external skeleton) enables them to grow and, in so doing, enables them to completely regenerate lost limbs and recover from other major injuries. Fascinatingly, Water Slaters are also marsupials, the female slater laying eggs during warmer months into her brood pouch, or marsupium, comprising overlapping, flat blades extending from the anterior legs enabling the mother to hold and protect her developing brood under her body until the larvae are sufficiently developed to emerge as miniature versions of the adults.

As the crusty exoskeleton, giving the crustaceans their name, contains a significant amount of calcium carbonate (which we most commonly experience as chalk or limestone), the crustaceans fare best in hard water and are consequently scarcer in acid waters and so generally absent from more mountainous uplands or acid heathland.

Mark Everard