Peter Davie is a leading marine taxonomist and ecologist, and an expert on crabs. He has published more than 160 scientific papers, named two new crab families and identified more than 125 new crustacean species. He was formerly Senior Curator of Crustacea at Queensland Museum, Australia and has just published Crabs, A Global Natural History
Crustaceans, a diverse taxon of animals that today includes krill, prawns, woodlice, barnacles and of course crabs, first evolved during the explosion of life in the sea at the start of the Palaeozoic Era, 540 million years ago. However, it took more than another 300 million years before crabs themselves finally appeared. This followed the ‘Great Dying’ extinction event at the end of the Permian. Global warming following largescale volcanic activity in today’s Siberian region led to a major change in the ecology of the sea and the loss of 96 per cent of all species.
Ocean chemistry had to recover greatly before marine invertebrates could once again absorb and build skeletons from calcium. Modern stony corals appeared about 230 million years ago, and the ‘true crabs’ of today, first appear in the fossil record about 30 million years later. Being able to build a robust, calcium-fortified exoskeleton was crucial for the evolution and diversification of crabs.
Boxer crabs are so named because of their habit of carrying a tiny sea anemone in each claw – tiny white pom-pom gloves with which they appear to ‘box’ each other during territorial contests. These anemones are also venomous, and small fish have been paralysed after contact with the tentacles. (Photo: Roger Steene)
Research by Israeli scientist, Dr Yisrael Schnytzer, has uncovered some extraordinary behaviour. The crabs keep the tiny anemones as slaves, using them to mop up particles of food and organic matter that they then steal using their specially modified first legs (a behaviour termed ‘kleptoparasitism’). The crabs actively starve the anemones to keep them small and manageable — allowed their freedom, these sea anemones grow to be more than 250 per cent larger. If by chance one anemone is lost, the crab will cut, and transfer to its empty claw, a piece from the remaining anemone, and wait for it to grow back whole again – an example of deliberate cloning!
The first recorded name given to any crab dates back to 3,000 BCE. Sumerians referred to their local freshwater Potamon crab species of southern Mesopotamia as alul, which translates as ‘deceptive digger’. It was also around this time that Sumerians first recognised the constellation of Cancer (the crab) as one of their 12 astrological signs, and the stars that define this constellation mark the Tropic of Cancer, the most northerly point from the equator that the summer sun reaches before the slow return of winter.
Aristotle first described 18 crustaceans in his History of Animals, written in 350 BCE. Today there are more than 7,200 named species of crabs alone, placed in over 100 families. They come in a bewildering range of sizes, shapes, colours, patterns and lifestyles. More are being discovered every year, with modern genetic studies giving us wonderful new insights into their evolutionary patterns and relationships, as well as revealing the presence of many ‘cryptic species’ previously not recognised.
What makes a crab a crab?
The term ‘crab’ is often used loosely, and not everything that carries the name qualifies as a ‘true crab’. For example, horseshoe crabs are not even crustaceans but distant and ancient relatives of spiders. Other groups such as hermit crabs, king crabs, mole crabs and porcelain crabs are much more closely related but, again, are impostors. True crabs belong exclusively to a group called Brachyura, which literally means ‘shortened tail’. Unlike most other crustaceans, a crab’s tail is tucked away neatly under its shell, and its primary function is in reproduction. Females use their broad, flattened flap to protect and nurture their eggs, while the narrow male abdomen is typically locked onto the sternum to protect its copulatory organs.
Brachyurans have an exoskeleton, which supports the body, provides mechanical strength and protection from the environment, and affords an internal surface for muscle attachment. The crab body comprises a series of jointed somites or segments – a bit like earthworms, but much more sophisticated. These somites are fused in various ways to form specialised functional units such as the head, thorax and abdomen (or tail). While these body units are obvious in the larvae, once moulted into the adult form, the conjoined head and thoracic somites are hidden under a large, thick, protective shell (carapace).
Crabs can exhibit a huge variety of carapace shapes. This spindle crab (Ixa inermis) takes width to the extreme, and could be mistaken for a piece of inedible broken coral (Photo: Roger Steene)
Crabs have ten legs (pereiopods) arranged in five pairs. The first pair are highly modified into the claws (chelipeds) for which crabs are synonymous. Claws are used for various purposes: first and foremost, for feeding, but also defence, attack, display and grooming. They also come in a great range of shapes and sizes, enabling different combinations of speed and power to suit different tasks. Crabs that prey on sedentary, hard-bodied prey, such as bivalve molluscs, have the most powerful chelipeds, with the highest mechanical advantage for crushing. Species exploiting more mobile quarry, such as polychaete worms and fish, have lighter but longer claws that can move very quickly and are lined with cutting teeth.
The last four pairs of legs are typically used for walking, though they can be specially modified for swimming, digging, or carrying. Typically, each leg consists of six segments, and while the movement of each element is restricted to a single plane, the direction of each plane is perpendicular to that of its neighbour, enabling a wide range of motion. While crabs are famous for walking sideways, many species can also walk forwards, backwards, or diagonally as needed. Of course, all these legs need to be well coordinated. How effective this arrangement can be, is illustrated by a species of sandy beach ghost crab (Ocypode) that holds the world land-speed record for crabs — it can run sideways at up to 4m (13ft) per second! It achieves this by both leaping into the air, and periodically turning 180 degrees, thus alternating the legs doing the pulling for those pushing.
It is unlikely that this highly venomous flower urchin enjoys being held prisoner, but when porter crabs lock onto them with the claws on their back legs they have no choice but to go along—even while the crab is mating with a female on her back below him (Photo: Roger Steene)
Crabs play critical roles in the healthy ecology of coral reefs, mangrove swamps and shallow coastal waters. Armies of tiny crabs keep our beaches clean, either by scavenging anything dead, or by sifting masses of sand through their mouths at every low tide, pulling out the microscopic detritus of animals and plants. On coral reefs, the vital role of crabs in healthy reef ecology has been long underestimated. Many reef species have specially modified rasping or cutting claws, enabling them to graze on algae and experiments have shown that crabs, like some fish, play a vital role in preventing algae from quickly overgrowing and smothering coral. The myriad of crabs that live in tropical mangrove forests is now known to keep these communities productive and healthy. Their many burrows take fresh seawater and oxygen down deep into the mud to promote healthy tree growth, and their voracious leaf-eating also recycles the fallen leaves back into the mud as nutrients for a host of other animals, as well as fertilising the mangroves themselves. Thus crab activity is of huge importance in keeping the energy the trees pull from the sun safely in the system, and not washing out to sea with each tide!
Crab larvae are a major component of the zooplankton, and thus a vital part of marine food webs. A single female, such as this striped box crab (see below) can release millions of tiny larvae during a single mass hatching (Photo: Roger Steene)
Although crabs are predominantly bottom-dwelling, their larvae play an equally important role in the sunlit surface waters. Most crabs produce free-living microscopic larvae that can spend weeks growing, feeding, and moulting in the near-surface plankton. Some larger species, such as the common commercial Indo-West Pacific giant mud crab (Scylla serrata) and its allies, can produce up to six million eggs per female. It is, therefore, not hard to understand how crab larvae are a crucially important component of the plankton community upon which other marine animals depend. By contrast, the desert-adapted Australian inland crab (Austrothelphusa transversa) can remain dormant (‘estivating’) in underground burrows for several years while waiting for summer rain. The egg development phase progresses underground while the mother sleeps, and around 40 young crabs are ready to hit the ground running when wet weather arrives and the streams start to flow.
A fatal poisoning in the Philippines led to extensive study of the genus Demania (see a Demania splendid pictured) and they were found to contain saxitoxin, tetrodotoxin and palytoxin. Crab toxins are originally derived from either bacteria or phycotoxins produced by dinoflagellates, diatoms and cyanobacteria (Photo Roger Steene)
The toxic reef crab or devil crab, Zosimus aeneus, is said to kill within a few hours of being eaten, and is reported to be used by Pacific islanders as a means of suicide
Crabs navigate their lives with purpose – even if primal needs drive that purpose. It has been well established that brachyurans have sophisticated nervous systems that control almost every aspect of their lives.
Several studies have revealed remarkable abilities to problem-solve mazes, and memorise landmarks. A good example is the European shore crab (Carcinus maenas), an important predator and scavenger in intertidal and shallow subtidal environments. Placed into a complex multi-turn maze (similar to the type used to test mice), the crabs, after only four tries at the labyrinth, each held a week apart, were going straight to the end in under eight minutes, without any wrong turns.
The spanner or red frog crab (Ranina ranina) buries itself in sandy areas, where it acts as an ambush predator. It is the basis of a commercial fishery in eastern Australia (Photo: Roger Steene)
Not only that, after an absence of two weeks, the crabs still had the route memorised and would skip directly to the end of the maze, even when the reward was no longer there to give them a sensory incentive.
Crab fisheries and aquaculture
Multimillion-dollar industries are based on crab fishing and aquaculture, with crabs exported to markets across the globe. Crab food festivals are staged in small coastal towns worldwide to celebrate the sea’s bounty and promote tourism. Around 1.5 million tonnes of true crabs are fished annually, about 20 per cent of all crustaceans that were caught and farmed globally.
A conical gorgonia crab (Xenocarcinus conicus) is perfectly camouflaged on this red gorgoniid fan coral – splashes of cream and yellow on its body and legs even match the colour of the coral polyps (Photo: Roger Steene)
However, not all crab fisheries have been well managed in the past, and some have been forced to close or have been severely curtailed due to overfishing. Sustainable fisheries must be based on knowledge of the biology and lifestyles of the crabs concerned, and catch limits must be strictly controlled. This is especially important for deep-sea species, which are often very long-lived, grow very slowly, take a long time to become sexually mature, and may not reproduce every year.
Farming crabs yields about a million tonnes of crabs annually, with China responsible for around 95 per cent of the world’s production. More than 700,000 tonnes come from just one species, the Chinese mitten crab (Eriocheir sinensis).
The giant Japanese spider crab (Macrocheira kaempferi) is the largest known arthropod – it can reach 3.8m (12ft) in claw span, 40cm (16in) in carapace width, and 19kg (42lb) in weight (Photo: Shutterstock)
The aquaculture of mitten crabs developed rapidly once methods were developed to make larval rearing practical. Crab ‘seed’ (megalopae, the first bottom-dwelling settlement stage) are produced in special hatcheries and then transported to farms spread across the country for grow-out. The numbers are astonishing – in 2003, 522,893kg of live megalopae were produced. These nascent crabs are so tiny that a kilogram equates to about 140,000 individuals, meaning that the annual stocking production exceeds 7.3 x 1010 (many billions of baby crabs!).
Conservation and climate change
Crabs, along with a host of other animals and plants, are finding themselves in a fight for their very existence. Increasing levels of carbon dioxide from burning fossil fuels is steadily, and measurably, changing the fundamental chemistry of our oceans. Too much CO2 leads to the formation of carbonic acid in the water and reduces the availability of carbonate, a critical component of shell-building for many invertebrates. Tiny, delicate planktonic larval stages are especially vulnerable. If larvae do not survive to adulthood, this will not only affect the recruitment of new generations, but because plankton communities are at the base of marine food webs, the whole oceanic ecosystem will be disrupted, and fish stocks will become depleted. A new prescient study has found that some larvae of Dungeness crabs in the US Pacific Northwest are already smaller than usual and are suffering pitted and folded shells and damage to tiny sensory setae that could lead to such issues as slower movements and impaired swimming.
Zebrida adamsii is named for its characteristic brown and white zebra stripes. They live between the protective spines of a variety of echinoids (sea urchins), upon which they also feed. It seems the urchins gain no advantage from their presence, but the crabs rarely do much harm either (Photo: Roger Steene)
The warming of the oceans is an equally serious threat. Corals are bleaching and dying across huge areas. Without adequate and timely recovery, there will be a devastating cascade effect on the myriad invertebrates that depend on this biological system.
Warmer seas will also have the effect of speeding up metabolism and, therefore the need for oxygen, but warmer water also has less ability to hold oxygen, so larger marine animals may begin to move away from equatorial waters due to respiratory stress.
Sponge crabs such as this one (Lewindromia unidentata) disguise their presence by carefully hollowing out pieces of sponge to snugly fit their dome-shaped bodies, but every time they moult and grow larger, the work of making a new home has to start again (Photo: Roger Steene)
Warming also means such quickly rising sea levels that many coastal ecosystems will have no time to adapt. Intertidal mangrove swamps, and their rich diversity of crabs and other specialised fauna and flora, will be submerged, as will beaches that have formed over millennia. Rapid climate change will have many more such consequences, and the evidence of science must not continue to be ignored and underestimated.
Most people know little about the rich diversity of crab shape and habit, or the crucial part they play in sustaining the wellbeing of the planet. Crabs: A Global Natural History draws on the latest research breakthroughs in classification, evolution, physiology, ecology and behaviour to provide new insights into the lives of crabs, to which only a few experts are normally privy, showing them as you have never seen them before.