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Life’s a Beach: Wildlife Beneath the Sand

One of the benefits of being an island nation like Ireland is that you’re never too far from a beach. But how many of us look at what’s going on literally beneath our feet when we hit the Irish coast? Let’s take a closer look at the wildlife of an Irish beach.

Beaches like Inchydonney in West Cork harbour a host of wildlifeGiven our national preoccupation with the Irish weather it’s hardly surprising that when a bit of sun has the temerity linger we make the most of it. Warm, sunny periods in late spring, summer and into the autumn see us heading for the beaches in droves – and one of the great things about living on an island is that for most of us the beach isn’t too far away.

But while we’re busy sunning ourselves on the sand the beaches’ permanent residents are going about their daily lives, often quite literally beneath our feet, and for them the beach is anything but a relaxing environment.

When most of us think about coastal wildlife we conjure up images of whales and dolphins, of seals and seabirds, of rocky promontories whose highly adapted denizens are easy to spot on the exposed rock or trapped in the myriad rock-pools left behind by the receding tide.

While the communities on rocky shores are of course fascinating, accessible and above all visible, they are only part of the story. A host of intriguing species specialise in living on, or more accurately under our sandy shores – but before taking a closer look at some of them we need to understand a little bit about the make-up of our beaches and some of the environmental conditions that prevail there.

Substrate composition and wave action

Take a close look at a handful of sand the next time you’re on a beach and you’ll notice that it’s made up of many different types and sizes of particle. Scientists often classify these particles according to the Wentworth scale which lists five different grades of sand ranging in particle size from 0.125mm to 2mm in diameter; in practice the sand on our beaches usually contains a variety of particle sizes that extend above or below this specifically defined range. The main constituent of the sand around the coastline of the British Isles is silica fragments with some silt, clay and substances like shell fragments, diatoms and calcareous algae mixed in.

Grade Name Particle Size (mm)
Boulder >256
Cobble 64 256
Pebble 4 64
Granule 2 4
Very Coarse Sand 1 2
Coarse Sand 0.5 1
Medium Sand 0.25 0.5
Fine Sand 0.125 0.25
Very Fine Sand 0.0625 0.125
Silt 0.0039 0.0625
Clay <0.0039

The Wentworth Scale of particle sizes showing the 5 different grades of sand

On sandy shores wave action influences many factors, including the stability of the substrate, the particle size that can settle, the shore’s gradient, drainage properties, oxygen availability and organic content. These factors all affect the type and the density of life that a particular stretch of beach can support.

Generally speaking the more exposed the shore, the greater the effects of wave action, the coarser and less stable the substrate, the steeper the gradient from high to low water and the less organic matter the substrate will contain – but this oversimplifies things. Substrate consistency also varies significantly both up and down the beach and along its length according to a large number of interacting physical conditions. These relate to things like the wavelength and height of different wave types as they converge along the shore, the angle at which they strike, the influence of adjacent promontories, land-masses, the sea-bed itself and many other factors.

As well as all of this, of course, the sandy shore is subjected to the same twice-daily fluctuation in environmental conditions that its rocky counterpart endures with the ebb and flow of every tide. Inter-tidal species also have to cope with three sets of potential predators: terrestrial predators like waders and other birds come in to feed as the tide moves out, fish and other marine predators move in with the rising tide, and inter-tidal predators are ever-present.

Suffice it to say that your average sandy beach is a harsh, complex and constantly fluctuating environment that presents a unique set of challenges to the species that live there.

Getting under the sand makes sense

Because of the constant shifting of the surface layer by waves and wind a sandy shore offers little in the way of a firm anchor for plants or animals to cling to. The sand’s surface is also exposed to extreme fluctuations in temperature, salinity and to the drying effects of the sun and the wind when the tide recedes. While some organisms; notably the small amphipod crustaceans known as sand hoppers, a number of terrestrial insects and a few terrestrial plants; have adapted to eke out a living on the sand’s surface around the high-water mark, out on the beach itself the only real option for survival after the tide goes out is to burrow beneath the sand. Here conditions, while still difficult, are at least a bit more stable.

A phenomenon known as capillary action draws water up into the interstitial spaces between the grains of sand, keeping the actual water level under the surface of a beach significantly higher than the low-water mark. The finer the substrate the smaller the spaces between the grains and the more pronounced the effect of capillary action. In general more sheltered beaches, with their correspondingly smaller particle sizes, have better water retention properties than exposed shores and will contain more organic matter, providing suitable conditions for denser, more diverse populations than exposed shores.

What about plants?

Sand hoppers often occur in huge numbers on Ireland's sandy shoresSandy beaches are inhospitable places for plants. Rocks and large stones embedded in the sand low on the shore sometimes offer a stable enough anchor for seaweeds. They are frequently colonised by species like the sea bootlace (Chorda filum), which itself provides an anchor for epiphytes like Litosiphon pusillus. On sheltered beaches the substrate may be stable enough to support beds of eel-grass (Zostera marina), which will sometimes be exposed during extreme low water, while high on the beach, above the strand-line, terrestrial plants like the dune-forming marram grass (Ammophilia arenaria), the sand sedge (Carex arenaria) and the sea mayweed (Tripleurospermum maritimum) are found. But for the most part the shifting substrate keeps larger plants at bay, and the only marine plants found on the beach itself are microscopic algae which either occupy the interstitial spaces between sand grains or are attached to the individual grains themselves. Along with plankton washed in by the incoming tide and the organic debris deposited in the substrate, these form the first link in the sandy shore food chain.

Minute animals live between the grains of sand

The familiar cast of a lugworm on an Irish beachMany of the animals in the sand are tiny, flattened or threadlike forms that exist in the water between the sand grains. Scientifically dubbed the microfauna and meiofauna, these animals include things like protozoans, harpacticoid copepods (tiny crustaceans), nematodes and gastrotrichs, and contain some of the smallest known representatives of most invertebrate groups. They can exist in huge numbers, going largely unnoticed because of their small size, but playing a vital role in the ecology of the beach community by making the energy captured by interstitial algae, on which they feed, available to larger animals in the ecosystem which prey on them in turn. These larger animals, the more visible macrofauna, are the worms, molluscs (bivalves and snails), crustaceans (shrimps, crabs and relatives) and echinoderms (starfish and relatives), which represent the more familiar residents of our sandy shores.

Our sandy shores support a diverse macrofauna

The lugworm (Arenicola marina)High on the shore, around and just below the strand line, the sand hoppers like Talitrus saltator and Orchestia gammarella are common. These amphipod crustaceans jump when disturbed, propelling themselves remarkable distances with energetic flicks of their curved tails. Living so high on the shore they are immersed in seawater relatively infrequently and have evolved to extract oxygen from the air. They live in burrows in the sand and emerge at night to scavenge among the rotting weed of the strand-line. Here you also find terrestrial beetles and flies which feed on the rotting organic debris thrown up by the tide.

As you move onto the mid-shore, the area between the average high tide mark and the average low tide mark, the open expanse of sand seems barren and lifeless at first, but if you look more closely you will see signs of a thriving animal community under your feet. Tracks, holes or depressions in the sand, casts, water spurts and small protruding tubes of sand all betray the presence of buried animals waiting for the turn of the tide.

The predatory ragworm Nereis diversicolorIt is hardly surprising that worms figure prominently in an environment where burrowing beneath the sediment is the norm. On more sheltered shores the ragworm Nereis diversicolor is abundant from the mid-shore down. These fast moving, free-living polychaetes (bristleworms), are active hunters and use their strong, pincer like jaws to catch and grip other worms, small crustaceans and just about anything else they can seize. They are occasionally seen on the surface, but usually burrow in search of their prey.

Another common polychaete with a completely different lifestyle is found on the lower half of sheltered beaches. The lugworm (Arenicola marina), which is responsible for the familiar spaghetti-like casts often seen at low tide, is a sediment feeder that lives in a permanent “U” shaped burrow in the sand. The head end of the U is full of sand. When submerged, the worm pumps water through the burrow from the tail end forward across its gills to replenish its oxygen supply. The water is then filtered through the sand at the head end of the burrow, trapping food particles in the water. This sand is ingested by the worm which digests any edible matter and voids the remaining sand as the cast at the tail end of the burrow.

The sand mason (Lanice conchilega) builds a protective tube of sand particlesSeveral species of worm found on our shores build protective tubes in which to live. A narrow tube of sand protruding from the beach on the lower shore and sporting a fringe of sandy branches is likely to be the home of the sand mason (Lanice conchilega). The fringe of branches helps to protect the delicate tentacles that wave in the water to collect food particles when the worm is submerged. Although only about 4cm of the tube will protrude above the surface it can be up to 30cm in length.

Molluscs are common on our sandy shores, and bivalves in particular are well represented. Bivalves have two flattened shells (or valves) held together by a hinge-like ligament and include the mussels, cockles, oysters and scallops that we often eat as seafood. Burrowing bivalves use their powerful muscular foot to loosen the sand, anchor themselves and pull the shell down into the substrate. Bivalves are predominantly filter feeders and those buried under the sand must have access to the seawater above to breathe and to feed. To do this they extend two tube-like siphons – one to draw water in (the inhalant siphon), and one to let water back out (the exhalant siphon) – to the surface of the sand when covered by the tide.

The razor-shell is a bivalve mollusc often found burried low on the sandy shoreThe common cockle (Cerastoderma edule) is a burrowing bivalve that lives buried 2 to 3cm below the surface of the sand. It is often collected and, after being allowed to filter in clean seawater for a few hours, can be eaten. Another unmistakeable bivalve is the curved razor shell (Ensis ensis) which is forced to venture close to the surface of the sand to feed because of its short, fused siphon. The razor shell compensates for this with an ability to burrow very rapidly into the sand at the first sign of trouble thanks to its elongated form and a very long, powerful foot.

Crustaceans of the sandy shore include the amphipods like the sand-hoppers already mentioned and sand-shrimps (Gammarus spp.) often found under stones low on the shore. Opposum shrimps, or mysids, like the ghost shrimp (Schistomysis spiritus), are sometimes present at the edge of the sea or in pools left behind by the receding tide, as is the edible shrimp (Crangon crangon), which lies buried in the sand by day with only the tips of its antennae protruding.

The shore crab (Carcinus maenas) is often associated with the rocky shore, but is equally at home on the sandThe shore crab (Carcinus maenas) is the most widespread crab on any shore, including sand, where it burrows beneath the surface when the tide is out. It is a scavenger that scours the beach for tasty morsels when covered by the tide, though it will also take live prey if the opportunity presents itself. Lacking the protective carapace of its brethren the common hermit crab (Pagurus bernhardus) takes up residence in an empty mollusc shell for protection, moving into progressively larger accommodation as it grows. Sometimes found on the sand of the lower shore or in sandy pools it is frequently joined in its shell by the small ragworm Nereis fucata, and the anemone Calliactis parasitica and hydroid Hydractinea echinata will sometimes hitch a free ride on the outside of the shell.

A slight imprint in the sand close to low water may betray the presence of the burrowing starfish (Astropecten irregularis), which lives buried just below the surface. It feeds on worms, bivalves and crabs, swallowing them whole and ejecting the indigestible parts some time later. The sand brittlestar is easily recognised with its distinct central disc and five long arms. It leaves a mark like a birds foot on the surface of the sand where it lies buried just below the surface low on the shore. Sea urchins are also represented here: heart urchins are heart shaped with soft, backward-pointing spines in place of the erect, rigid spines of their rocky-shore counterparts. They live buried up to 10cm deep in the sand of the lower shore.

Sea anemones are perhaps among the last creatures you would expect to find buried in the sand but there are some burrowing species, two of which, Peachia hastata and Halcampa chrysanthellum, occasionally occur at low water on very sheltered shores.

Beaches are important to visiting species as well

Our beaches attract many subtidal species that come in with the tide to take advantage of the sheltered conditions and rich food supply. They are vitally important nursery grounds for young flatfish, including familiar species like plaice (Pleuronectes platessa), flounder (Platichthys flesus) and sole (Solea solea). Small flatfish can often be found buried in the sand at the waters edge or in sandy pools on the beach.

Wading birds also take advantage of the wealth of invertebrate life on the sandy shore, and fly in to feed on the rich pickings with the ebb of the tide. Along with estuarine mudflats our beaches form a network of vital feeding grounds for many migratory species.

Rocky shore interlopers

Mussels (Mitilus edulis) soon colonise any hard, permanent substrate on the sandy shoreWherever you get expanses of rock embedded in the sand or fringing the beach animals and plants of the rocky shore will quickly colonise them. Species like the edible mussel (Mytilus edulis) and the acorn barnacle (Semibalanus balanoides) are often seen blanketing rocks on the periphery of sandy shores, and familiar rocky-shore seaweed like wracks (Fucus spp.) and kelps (Laminaria spp) are quick to take hold where conditions are suitable. They, of course, attract settlement by other species, and it’s not uncommon to find thriving rocky shore communities alongside, and sometimes right in the middle of, a sandy beach.

Overall the seashore is a wonderfully diverse and intriguing environment encompassing many different habitats and supporting a complex community of species. It certainly offers plenty of scope for the inquisitive nature-lover who grows tired of basking in that oh-so-fickle of commodities, the Irish sunshine.

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