The secret world of crabs
To some people, crabs are ugly, aggressive scavengers that nip the toes of unsuspecting bathers in cartoons. To biologist John Walsby they are beautiful, complex creatures whose lives are full of intriguing secrets.
Long before the fishes ruled the oceans, about 350 million years ago, there was a time when ferocious lobster-like creatures called eurypterids, some more than two metres long, were the undisputed masters, roaming the sea floor like heavily armoured assault vehicles. These were the ancestral crustacea, and most of their descendants (shrimps, hill, crayfish and the like), still inhabit the seas.
However, one branch of the crustacean family tree, the crabs, moved away from the ancestral home and headed for the bright lights and action of that most risky of habitats: the land between the tides. It was an evolutionary step which involved major modifications to the standard submarine version of crustacean design that had stood the test of time for millions of years.
A few of these new models went even further, managing in most respects (apart from breeding) to become fully terrestrial. But they are exceptions; the majority hover around the fringes of the sea, some higher, some lower on the shore.
Although superficially very different, crabs retain many of the features of their prawn and crayfish-like ancestors, and are therefore grouped alongside them in the Sub-order Decapoda (meaning “ten legs”). All decapods have five pairs of legs, including walking and pincer legs, and these are joined to the cephalothorax (the fused head-trunk). Other features shared by decapods are stalked compound eyes, the same number of body segments, similar arrangement of appendages around the mouth and similar larval development.
How is it, then, that crabs, which look so different from shrimps, should be classified in the same group? The radical change in crab shape comes from the adoption of a completely different lifestyle. The prawns, shrimps, crayfish and lobsters are all long, cylindrical animals with relatively slender legs, that can move rapidly off the bottom through open water. Most have pincers that reach out in front of the head and very long, whip-like feelers that can test the surroundings well in front of the animals. The armoured carapace enclosing the gills, heart, reproductive organs and food processing parts of the body is tubular, and behind this the long, muscular abdomen (that portion which we separate as the crayfish tail), with its broad, terminal tailfan paddle, serves as a powerful propulsion unit.
The smaller shrimps and prawns often cruise slowly through open water, propelled by swimming appendages under the abdomen, while the larger rock lobsters and crayfish amble across the sea bed on their walking legs. But it is the ability of all shrimp and lobster-like crustacea to thrust themselves backwards or forwards with explosive flicks of the tail that allows them to escape from a threat or lunge at prey. They are like streamlined torpedoes with primed weaponry and sensors at the front, and engines and propellers aft.
To make an effective assault on the beaches, a different strategy was required. Long, tubular bodies do not fit into sharp, angled spaces between rocks, and tail-kick propulsion does not work when the tide is out. The torpedo shape was therefore abandoned, the body flattened out and tapered to the back, and the gait spread wide. The once important muscular tail remains only as a vestige, tucked away neatly under the body, its use restricted to that of an egg carrier by the females during the breeding season.
The legs are strengthened, short feelers substituted for long, cumbersome ones and the eyes set farther apart to suit the new direction of movement. Rather than moving forward, the wedge-shaped crabs scurry sideways and backwards, edging along crevices and backing under boulders with their menacing, heavily armoured pincers acting as shields and weapons to both ward off intruders and despatch prey.
The crabs have become the crustacean commandoes of cracks and crevices. They are masters of surprise attack and covert action, and by venturing into the intertidal zone, where they can survive and remain active for short periods out of water, they are able to escape the predatory attentions of such animals as octopus and fish. However, there is a cost to living on land.
Unlike crayfish and lobsters, the strongly armoured crabs, with their heavy chalky skeletons, must be able to support the whole weight of their bodies when they leave the water. For many, this implies a loss of a speedy escape mechanism, though others have coped by lightening their shells and becoming fleet-footed. But they must also cope with the physical assault of breaking waves, as well as stones and sand tossed around at the water’s edge, and when the tide is out they must then guard against drying out.
Some crab families, such as the xanthids, are built like tanks: slow moving but well protected. Others, especially the grapsids, while still well armed with aggressive pincers, have longer legs and lighter bodies and rely on speed and sensitivity to avoid capture or damage. A third group of weak-legged, slow-moving crabs relies on stealth and cryptic colouration, or other means of camouflage, to avoid capture.
Almost anyone who has scrambled around a rocky shore in search of a fishing spot will have caught a glimpse of one of our most alert, athletic and aggressive crabs: the purple rock crab Leptograpsus variegatus. Immature specimens are coloured green, with splashes of purple, but the adults, which venture well above the high tide mark of exposed rocky shores, are strikingly marked in bold purple over a creamy white background.
The old males have much larger pincers than do the females and are especially impressive as they confront us with malevolent stares, bubbling mouths and bold displays of aggression as we peer down into their crevice retreats.
For many a child, this crab in its cleft represents a challenge: a defiant creature to be extricated with a stick. But it is always a fruitless exercise, for, of all crabs, this species is the most difficult to dislodge. Its resistance has probably developed to save it from the stresses of water shooting along gullies as crashing waves explode on the rocks, but it takes no less kindly to being deliberately disturbed’ by humans.
The crab’s eight walking legs are spread wide to grasp the rock not only on the floor of a crevice but also the ceiling, for the last pair can turn upwards, above the others. Moreover, each leg is tipped with a sharp, hard claw and fitted out with strong spines as effective as any mountaineer’s crampons.
In the daytime these crabs frequently bask in the sun, but the best view that we normally get is of them dashing for cover, frequently even throwing themselves off large rocks into pools or on to the boulder jumble below, seemingly without injury. Only the patient observer with half an hour to spare and the self-control to remain motionless will be able to study these cautious crabs at close quarters as they venture out into the open again to sun themselves. Frequently, they spar with each other, possibly establishing a pecking order for the temporary occupation of the prime sites.
At night, under torchlight, they seem less timid, and it is then that they can be seen feeding. They regularly eat the green seaweed that grows in high tidal pools, and frequently follow the tide down to avail themselves of the richer pickings lower on the shore. There they take mussels, barnacles, sizeable grazing snails, limpets and chitons, which they wrench from the rocks and crush with their nutcracker pincers. After moulting, with its consequent loss of calcium in the old shell, they also eat considerable amounts of coralline turf and other chalk-secreting algae to make up the deficit.
At other times they show themselves to be adventurers. They are the most terrestrial of New Zealand crabs, the adults sometimes scaling steep cliffs to feed on unsuspecting land animals. On the Poor Knights Islands they have been observed at night, climbing up into the bush and there feeding on the rare flax snail Placostylus.
Perhaps the smartest grapsid is the marble crab, Hemigrapsus edwardsi, found sheltering under largish boulders towards the upper levels of the beach on semi-protected shores. With its shiny body, indigo-purple marbled colouration above and pure white below, it is a most handsome animal.
Its close relative, Hemigrapsus crenulatus, sometimes called the hairy-handed crab, is found lower on the shore and extends into beaches where the silt load is greater. There it hides beneath stones and debris or moves slowly and almost unseen through the partially settled silt in shallow puddles and sluggish drainage channels. Variably coloured with dull, light greens, tan and fawny grey, this crab has developed and perfected the style of camouflage now used by land armies to disguise its presence.
Intriguingly, the silted shallows of sheltered beaches offer, at the same time, both nourishment and survival difficulties. Hemigrapsus crenulatus uses brushes of hairs to cope with both. In sunny bays the wet depressions and seepage runnels left by the ebbing tide quickly warm up, and the microscopic plants and animals that live between the sediment particles flourish to such an extent that the development of greenish-brown living scums of diatoms (microscopic plants) is soon very obvious. This is an abundant food source—if you can exploit it.
There would be little profit in eating large amounts of mud for the small quantities of microscopic life contained in it, and it is thought that the hairy-handed crab and the stalk-eyed mud crab, Macrophthalmus hirtipes, use the soft, velvety covering of fine, dense hairs on the palms of their pincers to separate out that food. It seems that when the velvet brushes are stroked through the broth of slushy sediment, some of the micro-organisms are retained, but the hairs are too fine to hold most of the heavier non-living silt particles. The crabs then clean their palms with their mouths, rather like a child sucking the sleeve of its jersey after accidently dipping it in the soup.
While the sediment provides a good food source, it can also be an impediment to respiration by clogging the gills of animals that live on these sheltered shores. Hemigrapsus crenulatus has efficient cleaning bars, rather like windscreen wiper blades, that regularly sweep across its bank of feathery gills, but perhaps even more important is its primary sediment filter. Along the tops and edges of its walking legs are soft brushes of fine hairs that, in water, act like net curtains to strain out the sediment.
Where the substrate is firmer on sheltered shores, another fast-running grapsid, Helice crassa, the mud crab, always abounds in teeming numbers in every estuary and harbour. They all dig burrows, and millions of finger-sized holes are all that most beach strollers ever see. In such an open habitat, with little or no ground cover, these crabs are very vulnerable on the surface and very reliant on their burrows, dashing down into them at the slightest threatening movement.
As the tide falls, uncovering the burrow mouths, each crab immediately busies itself clearing out sand and mud that was washed into its shelter by lapping waters. This is priority work, for access to its safe-house must be easy in times of danger, and on hot, windy days it may also need a damp retreat in which to moisten drying gills.
Helice carries out the burrow diggings in “armfuls” held beneath the body, and drops each moist ball on a dump-site a short distance from the opening. These piles of cleanings are conspicuous when, in the process of enlarging their burrows, the crustacean miners excavate anaerobic mud and leave this black spoil on the yellowish surface sand.
Like the hairy-handed crab, Helice relishes any carrion, such as a gaping cockle that has been rolled in by the tide. Normally, though, it relies on separating micro-organisms from the surface sand particles.
When feeding on the surface substrate, Helice slowly makes short excursions away from the burrow, returning briskly from time to time to check the position of the opening should it need to dash for cover or defend it against a neighbour’s incursion.
As it moves along, each crab leaves behind a trail of small sandy pellets. These are not faecal droppings, but scoops of sand that have been cleaned of micro-organisms before being dropped back to the ground. The pincers repeatedly spoon up small portions from the surface and wipe them off on the top of the mandibles. As these laterally opposed jaws are agitated, the moist sand moves down between them and only the microscopic life between the grains is retained to be eaten.
The grapsids are by far the most versatile of crab families in New Zealand, with representatives found in a diverse array of shoreline habitats, and the members of one genus having returned to a free-swimming ocean life.
Beneath stranded logs and other debris along the extreme high-tide mark of sheltered shores, right through to the uppermost stones of semi-exposed boulder beaches, the shiny-bodied boulder crab, Cyclograpsus lavauxi, can always be found. Like many shore crabs, it is mainly active at night, when it emerges to feed on drift seaweed, fragments of shoreline plants and small invertebrate animals.
In sharp contrast, at the bottom of the shore, and generally seen during the daytime swimming amongst the large brown seaweeds, is the red rock crab Plagusia chabrus. Unmistakable and impressive, Plagusia has the front and sides of its carapace deeply serrated and is boldly coloured in various shades of red above and creamy white below. It has a depressed body, a broadly spread gait and flattened oar-like legs that enable it to glide easily through the water.
It is most active at night, when it moves up into the intertidal, whether the tide is in or out, to feed on grazing snails, barnacles, mussels and seaweeds. It only has small pincers, but can be aggressive if cornered, though its bravery deserts it when confronted by an octopus. Then it takes flight and, when pursued, even in daylight, will shoot out of the water on to the exposed shore, even though this exposes it to other dangers.
Although Plagusia is an able swimmer, it is no match in the water for the jet-propelled cephalopod with its many elastic, entwining suckered arms. But out of the water, the mollusc is lumbering and the fleet-footed crab can outrun it with ease. This behaviour suggests that many millions of years ago when ammonites, belemnites and other cephalopods were dominant marine carnivores, the ability to move out of the sea on to land may have had considerable survival advantages for the decapod ancestors, leading to the evolution of crabs as we know them today.
While Plagusia glides easily through the water amongst seaweeds, excellence in swimming belongs to two other grapsids: Planes cyaneus and P. marinus. These crabs have abandoned life along the shore to return to the sea, where they live as stowaways or pirates, plundering the life on the ocean’s flotsam. Like sailors that joined the navy to see the world, these little crabmariners are found throughout the oceans, often on floating logs that have been at sea long enough to have good growths of goose barnacles. These distant crustacean relatives probably provide both food for the crabs and places for them to shelter from pelagic fish.
As visually striking as the MA tree purple rock crab, but quite different in behavior and mobility, is the black-fingered crab, Ozius truncatus. Both claws of its powerful pincers are black and shiny as if stove enamelled, and they stand out conspicuously against the wine-red colour of the adults.
In contrast to the speedy grapsid crabs, members of the family Xanthidae are slow-moving, heavily armoured, powerful crabs that are usually found beneath fairly stable boulders with some grit and sand below. Ozius, the most common, is found in the middle and lower parts of cleanish boulder shores, sheltering out of sight in cavities beneath the larger rocks. When suddenly uncovered, it often greets the fossicker with a bold display of aggression, confronting the intruder, throwing its pincer legs open wide and brandishing its nippers.
Ozius is often an opportunistic feeder, quick to reach out from the shelter of its under-boulder cavern to take grazing snails that are knocked off the rocks above by waves or by the passing feet of humans. Cupping the shells beneath its body with its walking legs, it has the brute strength in its pincers to crush smaller shells or progressively break open larger ones. When the soft-bodied snail inside is exposed, the crab takes the flesh in its mandibles and hauls the animal from the remains of the shell. Often the inhabitant is not a snail at all, but a hermit crab squatter, and though these try to escape from between the cage of legs as their mobile homes are smashed around them, they suffer the same fate as the snails.
While there is great strength in the black pincers, they are sometimes used with delicacy and precision. Ozius possesses the strength to wrench chitons and limpets off the rocks they cling to, but can only do so if the rather blunt tip of a pincer claw can be hooked under the shell edge of these animals.
With chitons this is particularly difficult, for the animal’s jointed shell plates and flexible, scaly skirt give it a perfect fit, even to rock with the roughest surface topography.
Ozius counters this problem by initially using its large pincer claw to gently tickle the edge of the chiton’s skirt. Whether the chiton’s response is from coyness or irritibility is un‑ certain, but it draws back the skirt at the point that is being interfered with. The withdrawal causes a ruck to form along the hem of the skirt, and that is all that the crab needs to allow it to hook the tip of its pincer claw underneath. Now it can employ its brute strength to raise the skirt and wrench the chiton from the rock.
Not a xanthid, but closely related, one of the commonest under-boulder crabs, Heterozius rotundifrons, is frequently not noticed because of its cryptic colouration and cunning behaviour. Of the several common names it is known by, the “pebble crab” describes its looks, and its response to being uncovered, perfectly.
The crab’s rounded outline and fawn-grey colouration make it look like nothing more than just another pebble amongst the litter of broken shells, grit and small stones that accumulates beneath boulders on semi-sheltered shores. And it completes this illusion by tucking its legs neatly under the body and staying perfectly still when its covering boulder is turned over. So effective is its masquerade that frequently, only after several minutes, when “pebbles” start to move, do fossickers become aware of the crabs that had escaped their notice.
As they walk away and shuffle under larger real pebbles, some are seen to have one pincer much larger than the other, hence another common name, the “big-handed crab”. These are the males, and in mature specimens the pincer is boldly coloured with a splash of yellow. Like a peacock’s tail feathers and other impractical adornments in nature, or a medieval knight’s heraldic shield, it probably serves both to advertise itself to females and to intimidate adversaries or other male suitors.
Sometimes an old male will brandish its large pincer, slowly waving it up and down, and to this endearing action some crab watchers wave back and refer to Heterozius affectionately as the “howdy” crab.
Crabs are masters of the art of camouflage, mimicking their natural environment with the texture and colour patterning of their bodies, and also using sophisticated adornment techniques and cunning behaviour.
The common crabs of muddy harbours all have the same dull fawn-grey colours of the harbour mud, and skulk at the mouths of their burrows or in cloudy water at the slightest sign of danger.
Below the waves along open beaches the clean sand is a pinky-yellow colour when wet, often speckled with darker iron particles, and this is the colour pattern of the broad carapace of the common paddle crab, Ovalipes catharus. To complete the disguise, the paddle crab partially buries itself, leaving just its eyes protruding and its pincer arms spread wide, ready to cut down a hapless fish—or nip an unprotected toe.
Low on the shore at sheltered, silty beaches, several of the slow-moving xanthid crabs are even better camouflage artists. They avoid drawing attention to themselves by accumulating obscuring coats of silt amongst the close stubble of hairs that they wear over their legs and backs.
Such a covering is of no use in clean water below low tide or under deep overhangs at the foot of rocky shores, because here the rock surfaces are extravagantly decorated with a patchwork of multicoloured growths and encrustations. In this environment of bright orange, red and yellow sponges, blancmange pink and mauve compound ascidians, red, white and blue tubeworms and bryozoans of many different hues, a crab must be as loud and garishly painted as the surroundings to avoid being noticed.
Two of the crabs which live in this environment disguise themselves by expertly covering themselves with the very organisms that grow on the surrounding rocks. Petalom era wilsoni, the sponge-carrying crab, cuts and wrenches off an appropriate sized piece of living sponge or compound ascidian to cover its carapace, and places it on its back to disguise its presence. To hold these lumps of living camouflage securely in position, the crab uses its last two pairs of walking legs, which are deployed upwards over the carapace and are equipped with sharp, hook-shaped claws. As the piece of sponge continues to grow, it soon conforms in shape and fit to the crab’s back, perfectly disguising the crab as just another piece of sea floor encrustation.
The decorator crabs (Notomithrax species) grow sponge or algal gardens, according to where they live, stitching these organisms amongst their body hairs and thereby hiding from the probing eyes of predators.
On shores that have boulders scattered over the bedrock, rather than piled several layers deep, those rocks in the middle and lower shore frequently have dozens, sometimes hundreds of flattened grey-green or bluish grey crabs hiding beneath them. As the boulders are lifted, these small-bodied crabs, Petrolisthes elongatus, scuttle away in all directions.
This crab and a couple of close relatives found lower on the shore are known as half-crabs because they retain some features of their lobster-like ancestors that have been lost in all of the true crabs. The abodomen is not so obviously reduced; it is only loosely folded beneath the body, and still has a telson or tail fan. These crabs also retain the long whip-like feelers.
Most crabs are rather solitary and territorial, which is typical of predators, so it is unusual to find one that aggregates in such large numbers. The difference is that while half-crabs (also known as stone crabs and porcelain crabs) might catch, shred and devour the occasional passing worm, they feed mainly like barnacles, filtering micro-organisms from sea water. Sites that offer suitable conditions for filter feeding—with low sediment and good water flow—are therefore occupied by as many half-crabs as can find suitable shelter from roving predators.
When the tide is out, they rest and huddle together deep in their under-boulder crevices, holding water between their packed bodies, for they are much less tolerant to desiccation than true crabs. But when the tide returns they move to positions around the edges of the boulders where they can filter the passing water currents but still be close to safe havens should predators like fish or the black-fingered crab pass by.
Out of water, the porcelain crab’s filter feeding apparatus is scarcely noticeable, but when it is submerged, a pair of very long thoracic appendages (the third maxillipeds) alongside the mouth are unfolded and extended. These filtering arms are each tipped with a fan of long feathery hairs arranged rather like a miniature grass rake. When the arms are stroked through the water these hairs strain out plankton.
If the seawater is flowing, the filter rakes are just held across the current to strain out particles carried along in it, but at slack water the right and left arms beat downwards alternately to screen out food. Fully unfolded, the filtering arms are too long to pass food to the mouth, and at the end of each stroke they must be cleaned by smaller appendages which pass the food to the mandibles.
Although the large, flat pincer arms are invaluable as both protective shields and defensive weapons when the porcelain crab retreats from a predator into its crevice, it appears that they also have an important and exquisite role when the maxillipeds are beating to filter food. The pincer arms are extended, slightly flexed, and lie neatly on the ground on either side of the head, forming a box into which the filtering fans beat down. This arrangement increases feeding efficiency: during each downward stroke, a filter rake is forced through the water rather than just pushing it away to one side.
The broad sweps of smooth sand along open surf beaches and inside free-draining large harbours are not good places for most crabs, for there is little or no shelter from the predatory attentions of fish. But paddle crabs flourish along these stretches because they are both good swimmers and efficient burrowers.
Paddle crabs have their last pair of walking legs modified into broad paddle blades that provide much of the propulsion during swimming, serve as spades when the crabs dig backwards into the sand to hide, and act as stabilisers when they are digging up and opening bivalve shellfish.
At the front, their slender pincer legs are shaped like scimitars and are used to strike down bottom-feeding fish as they fossick for small invertebrates in the sand. The crab is well equipped to kill and butcher its prey. The strong, pointed claws of the second walking legs serve as skewers, while a close inspection of the opposing teeth of the pincer jaws shows that some are very similar to the carnassials of a dog: knife-sharp and designed for shearing through flesh.
As well as lying in wait in the shallows for a small foraging fish or a bather’s big toe to come near, paddle crabs are also plunderers of shellfish stocks. Inside harbours they dig up and crush cockles, pipis and wedge shells, and along open beaches tuatua and toheroa are taken.
When hunting for shellfish, the paddle crab is most active at night, and identifies where the prey are buried from the position of their siphons at the surface or by probing the sand with the pointed claws of its second walking legs—in the same way that gum-diggers once sought buried kauri gum with long spikes.
Having located the shellfish, the crab braces itself with its fourth walking legs stuck firmly into the sand and the paddle blades of the fifth legs splayed on the surface as stabilisers (like those on modern earth excavators). The crab then digs furiously with its second and third legs. Even deep burrowers are not safe, for the paddle crab can dig down to at least 150mm to uncover them.
Especially in mature paddle crabs, the right and left pincers differ in size and shape. One is slightly smaller, with the movable finger more hooked for holding and the small, sharp teeth right along the opposable edges suitable only for cutting.
The larger pincer (and it may be either right or left) has fewer but larger cutting teeth along the outermost half of the pincer jaws, but broad, flattened molars at the back. The latter are strong enough to crush most small shellfish, though some larger bivalves, such as a mature toheroa, can resist crushing. However, even these animals are not immune to being opened by the paddle crab, though it takes considerable ingenuity, dexterity and strength.
All bivalve shells are thinnest around the gape margin, where new shell is being formed. By manipulating a tuatua or toheroa so that the edge is placed between the mandibles, the crab is able to bite out a couple of notches at each end of the shell into which it can thrust the points of its pincers.
Larger prey are often rammed, hinge down, against the sand to force the wedge-shaped pincers further into the shell. Steadily, the gape is opened wider, and by twisting the larger pincer slightly, the scissor blades of the smaller one are freed just enough to snip through the bivalve’s shell-closing adductor muscles. As the elastic hinged shell springs open, luncheon is served.