The level of the salty, sediment-laden water in the Upper Solway is dropping with the ebbing tide, turning increasingly brackish as the waters of the rivers Esk and Eden dominate. The edges of the saltmarshes stand proud above glistening mudflats that are sweeps of ochre, sienna-brown and grey; the smooth glossiness of the upper slopes shades down into roughness, speckled with stones and weed.
That smoothness, as seen from the comparative safety of the saltmarsh, is an illusion, for if I step – squelch, slide – out onto the glutinous surface and look down, the surface is pockmarked with tiny holes and the wriggling trails of wandering creatures. Other animals have been here before me: there are bird footprints – the precise three-toed prints of waders, the webbed prints of gulls and aggressively sharp footsteps of oystercatchers; rough-edged holes where beaks have probed; and a line of paw-prints of an otter which has trotted from low-water mark to leap up onto the short, silty grass of the saltmarsh.
Some of these are signs that animals have been busy on the surface while the tide is out, but the minute holes hint at other animals that are here for much longer periods, burrowed down below. For to live on this apparently featureless and changeable shore means there is nowhere to hide – except within the mud.
To find the mud-dwellers I can search for where the density of holes is greatest or, since waders are specialists in finding their prey I can be guided by their footprints and probe-marks – and use a spade.
The divot is heavy and sticky; at the top are sections through U-shaped burrows opening to the surface. Each is the home of a tiny pale brown mud-shrimp: mud-shrimp – a uninspiring name for a small animal that is so beautifully adapted to where it lives – the crustacean, Corophium volutator. It is not as iconic as a polar-bear, nor as obviously charismatic as a curlew – yet, as the ‘keystone species’ of the mud-flat, it has its own special charisma.
The elegance of limbs
Unlike shrimps in rock-pools or cooked and ‘potted’ in butter, Corophium belongs to a group of related animals, the amphipods, that are dorso-ventrally flattened – from top to bottom – rather than laterally, from side-to-side. Also unlike shrimps and lobsters, it doesn’t have a carapace – its head and the middle segments of its body aren’t covered by a single curved piece of armour but all the segments are visible from above. I place some of the mudshrimps in water in a pie-dish. With a hand-lens it’s easy to see how different parts of its body are perfectly adapted for different functions. There are flexible joints between the segments so that – lacking a carapace – the animal can straighten or flex its body; each segment has a pair of jointed limbs, and groups of adjacent limbs are modified in size and shape to carry out different functions.
At the head end the appendages are adapted to ‘read’ the surroundings, and find and deal with food. The two pairs of long, flexible antennae that are so characteristic of Corophium have a sensory function, but the second pair are also used as rakes to pull food towards the mouth. Behind the antennae and on the underside of the body, are small ‘feeding limbs’ with different sizes and shapes –the maxillae, maxillipeds and gnathopods (‘jaw feet’) – all specialised for separate roles in the capture, handling and ingestion of food. Inside the mouth (unseen by my lens), a pair of hard mandibles act as macerating-machines.
Some of these feeding limbs have a second function, too – they have small thin-walled flaps of gills at their base to take up dissolved oxygen from the water.
Under the middle section of the body are the legs, the pereiopods (‘transporting feet’), by which Corophium crawls on the surface of the mud, and further back, tucked beneath the abdomen, are small, feather-like limbs, the pleopods (‘swim feet’). The pleopods beat like paddles when the Corophium swims, or they can beat more slowly to send a current of oxygenated water over the gills. I am happy to be re-visiting old friends – it was watching how these pleopods beat perfectly in unison that so captured my imagination so long ago.
At the back end of the body, a broad flat telson and a couple of leaf-like uropods form the tail fan, a small version of a lobster tail: if the abdomen is suddenly flexed forward, the fan acts as a paddle that drives a current of water forward – and the animal backwards: a useful method of escape from a predator.
This then, is what a live Corophium looks like under a lens.
In retrospect I realise that Corophium set me on the road to becoming a research scientist. When I was an undergraduate in London, the field-trip for our marine biology course took us to Swansea and the Gower for ten days, where we were offered a range of short projects to help us learn about research: the constructing of hypotheses, the practicalities of ‘materials and methods’, the gathering and interpreting of data. This brought, too, the realisation that the lives and activities of intertidal animals worked to a different schedule than our own, their lives governed by the shifting clock of the diurnal tides: depending on your project, evenings might be spent in the lab, not in the pub. I chose to investigate how Corophium responded to the falling oxygen levels in its burrow – did it become quiescent and sit out the low-tide period, or did it pump the water in its burrow more rapidly over its gills?
So how do you watch an animal that lives in a burrow? You fill a narrow glass chamber with muddy sand from a Corophium colony, and hope the animal will construct its burrow next to the glass… You fill the chamber with seawater of different oxygen concentrations, you hope you can see the delicate pleopods and count their rate of beating … A short project, naively simple through ignorance, yet a mixture of logic, planning – and hope. As the lecturer in charge drove me back to the lab one evening, he asked, ‘Do you really enjoy doing this?’ Yes, I really did!
(What were the results and conclusions of this research? I can’t remember. All I remember is the delicate, synchronised beating of the pleopods, and the animal’s elegant, questing antennae.)
I didn’t go on to research the physiology of marine animals but, years later, when I moved to Glasgow University, one of the other lecturers in the department was Peter Meadows, whose early research, in the 1960s, had been on the burrowing and feeding behaviour of Corophium (1).
Mudflats might look fairly uniform to us, but Corophium swim about and use their antennae to test the nuances of the size of sediment. Peter Meadows had written that the animal is ‘apparently quite deliberate’ in choosing where it burrows: some patches of mud are perfect, others are rejected like Baby Bear’s porridge. The sediment must be not too fine or the burrow might collapse, and not too coarse. Most important, the particles must be coated with organic matter – a slimy biofilm of bacteria, microalgae, and other organic material: a larder as well as a building block. Having found the right conditions, the shrimp burrows into the mud using its antennae and pereiopods, and stabilises the compacted particles of the walls with a sticky secretion (2).
Dimensions and planes
Our own lives are carried out on the surface of our world; few humans penetrate above or below that single plane. But an animal that burrows in the intertidal zone, has an extra freedom – to pass through that plane in either direction, from solid to liquid, liquid to air, depending on the tide; to live, crawl, swim, mate, feed, in three dimensions.
There is, too, the extra dimension of time. For an intertidal animal, time is synonymous with tide. There must be periods of waiting, when the animal keeps a low profile, hiding from predators, ‘house-keeping’, digesting, defaecating – and anticipating the return of the water that brings fresh oxygen and food (and sometimes, sex and procreation).
This coming and going of the sea, and the behaviour and activity of these marine creatures – and of all the animals that depend upon them – are influenced by the moon and sun, on a daily and seasonal schedule.
Now, in late April, I walk out onto the shore below Campfield saltmarsh; the water of the Firth has shrunk to a grey ribbon between me and Scotland, and rainclouds are sweeping in from the West. The surface of the mud is squiggled with trails, and I squat down to watch mud-shrimps as they part-crawl, part-swim in the shallow film of water that remains on the surface of the mud: the moon has sent the signal that it is the time for mating, so most are probably males, searching out the burrows of females.
Food-chains and feeding
Fortunately for them there are few waders around. The upper reaches of the Solway Firth have multiple conservation designations (3), reflecting the enormous importance of the mudflats in providing food for local and visiting waders such as dunlin, knot and redshank. Corophium and the mud-snail Hydrobia are special favourites, and the birds scurry hither and thither, following the falling tide, rapidly probing the mud.
But to think of the Corophium colonies merely as unwitting suppliers of thousands of tasty snacks for visiting birds is too simple: we have to turn the idea on its head – the invertebrate animals and single-celled organisms, the algae and bacteria are the core of the life of the mudflat, and without them the ‘mudflats’ would be a sterile shifting slurry of mud and sand, washed this way and that at the whim of storms and tides.
The shrimps themselves feed in several ways: by using their mandibles to scrape and gnaw the biofilm that coats the sediment particles; by raking around the burrow-mouth with their large second antennae; and by using bristles on their gnathopods to filter and sort organic particles like diatoms from the water circulating through their burrows – the pleopods may pump 25-100mls of water per hour through the burrow (1). Look again at the contrasting colours of the mud on the spade. The deeper mud is black and anaerobic, containing little oxygen, but around the mud-shrimps’ burrows it is pale and yellowish-brown, oxygenated by the animals’ activity.
A foreign shore?
Corophium are almost unknown, rarely seen – for why would anybody bother to venture out onto a sticky muddy shore?
Indeed, to most people mudflats and saltmarshes are probably as foreign as another country and, as David Attenborough has said, “No-one will protect what they don’t care about; and no-one will care about what they have never experienced”.
For a while, now, various people have been arguing that we ‘need new words’, simpler concepts to talk and write about the places that we share with other organisms (4, 5). Richard Mabey, writing about barn owls, found he was constructing “extravagant phrases. … I was rather pleased with my poetic metaphors….” Then, delightfully, he saw that he should think of the owls as his neighbours. “For much of my working life I have been trying to find way of talking about other organisms that neither reduces them to mechanical objects nor turns them into sentimentalised versions of ourselves. Neighbours are fellow creatures, but independent souls. You share their territory (their parish) and often their fortunes, but you can care about them in full knowledge they may not even recognise you.”(6)
We can think of Corophium as the engineers of the mudflats, through their feeding on biofilms, their burrowing and ‘bioturbation’ of the sediments, but they are so much more than ‘mechanical objects’ or food for wading birds. We may not share their territory but we can visit them, physically or in our minds, and perhaps this will help us to care about these important creatures – and their neighbourhood.
- Peter Meadows & Alison Reid (1966) Journal of Zoology, 150, pp 387-99.
- Rachel Hale: The 3-D structure of Corophium burrows; https://www.youtube.com/watch?v=z75cJSLLgz
- George Monbiot: https://www.theguardian.com/commentisfree/2017/aug/09/forget-the-environment-new-words-lifes-wonders-language
- eg Mark Cocker https://www.newstatesman.com/culture/2015/06/death-naturalist-why-new-nature-writing-so-tame; Peter Reason https://peterreason.net/2015/09/18/on-nature-writing-the-cockermacfarlane-exchange/
- Richard Mabey: https://www.littletoller.co.uk/the-clearing/owl-winter-richard-mabey/