Extraordinarily Crabby

Carcinus maenas © 2018 Duncan Greenhill

Carcinus maenas © 2018 Duncan Greenhill

I think it was the first creature on the shore I learned to identify; certainly the first I learned the Latin name for: Carcinus maenas, the green crab or shore crab. My family lived in Birmingham, which is about as far from the sea as you can get in the UK, and we went on holidays typical of the early seventies: a week in a caravan on a windy headland site, and the days spent at the beach.

I would head down the shore to explore, bucket in one hand, a nylon net with a bamboo cane handle in the other. Tides didn’t matter unless they were rising; as long as I could reach a rock pool, any rock pool, I was happy. It would be a long time before I would find a crab that was not a shore crab; zonation was still a mystery to me, and both the velvet swimming crab, red-eyed and aggressive, and the edible crab, its claws black-tipped, lived lower down the shore. The shore crab, tough and adaptable, able to tolerate the physical conditions of the shore in a way the other species could not, would prosper higher on the shore where they would not. But also where it should not, its larvae hitching a ride in the ballast water of ships to reach Australia, South Africa, and North America.

I would turn over rocks and try to catch them as they scuttled away for a new refuge. Once on one particular rocky headland, I found a cleft in the rock too deep for the grasping fingers of a child, and a crab just visible under the overhang at the bottom. On this occasion I was without my net but I did have a small ball of string in my pocket, the reasons for which are lost from my memory. I pulled a large mussel free from a clump and threw it against the rocks. I tied the smashed shell to the end of the string and lowered it close to the hiding place of the crab. I waited. The crab began to eat. I lifted the string. Slowly, slowly to the surface until I could grasp the crab and lift my prize from the pool for a more detailed examination. I returned him a few minutes later and left the mussel as compensation for disturbing him.

Carcinus, like many common animals, fools us with its ubiquity. We disregard it. We overlook it simply because it is so common, but its life has characteristics that can draw us in, if only we look closely enough. Its diet is wide; as both predator and scavenger it eats many things. One of the favourite prey items is the mussel, Mytilus edulis, but this choice has consequences for both crab and habitat.

Mytilus edulis shell

Mytilus edulis by H. Zell (CC BY-SA 3.0 from Wikimedia Commons)

Carcinus prefers a certain size of mussel, but how to breach the shell? The claws, or chelae, of shore crabs are not identical. They differ in both size and musculature. It has a larger claw, the ‘crusher’ claw, that is the more forceful of the two, but at the cost of speed. The smaller ‘cutter’ claw doesn’t develop as much force, but can close faster and is more dextrous. The claws have bumps, called teeth, on the inner edge. The claws on the male are larger than those of the female; they are needed for more than just securing a meal.

A crab finds a mussel and picks it up, taking one or two seconds to assess whether this mussel is worth the effort of attempting to gain access. If it is and the mussel is small the crab crushes it with the crusher claw and feeds. A larger mussel needs a different approach. The crab steadies the mussel in its cutter claw, the smaller end of the mussel uppermost, and applies pressure to the narrow end. The shore crab doesn’t squeeze steadily but gives a few pulses of pressure before moving the mussel slightly and trying again. The crab is an engineer – it’s not brute force that will secure the meal, but the propagation of stress fractures through the structure of the shell. If the shell is too robust or large for this approach then the crab changes strategy. This time the claw is forced between the shell halves and pieces are chipped off, gradually dismantling the shell and eating the flesh as it becomes available. In this way Carcinus can chip its way into any size of mussel. A mussel can’t find refuge from predatory crabs simply by growing larger.

The meal is not cost-free. Optimal foraging theory, the idea that animals maximise the energy they gain for the lowest cost they expend, doesn’t quite hold for Carcinus, even though as an experimental animal it was one of the examples used in support of the theory in the 1970s and 80s. Carcinus should pick mussels of an intermediate size. Too small and the mussel isn’t worth the effort of breaking in; too large and the time spent to crack it reduces the gain in energy from feeding on it. They should pick intermediate-sized mussels, but they don’t – they pick mussels slightly smaller. Remember those stress fractures? Well, they not only occur in the mussel shell, but also in the crab’s claws. The teeth on the claw become worn. The claw weakens, and in some cases can be lost completely. By feeding on smaller prey it prioritises the longevity of the claw over the immediate benefit of more food now.

Carcinus maenas underwater

Carcinus maenas – note the five distinctive ‘teeth’ along the side of the carapace and the three bumps between the eyes (CSIRO Image Library CC BY-SA 3.0)

The damage or even the loss of a claw does not have to be fatal. They can be replaced, but only when the crab moults – when it sheds its shell and forms a new one with growing room to spare. The loss of a claw may not be due to the wear and tear from feeding. Carcinus can choose to shed a claw or limb if it needs to, for example, when escaping a predator in a similar way to a lizard shedding its tail. This deliberate shedding is called autotomy.

It can take up to three moults to replace a lost claw, during which time it may have to feed on less armoured prey, particularly if it’s lost its crusher claw. Time may be critical. Carcinus has a limited number of moults and the length of time between moults gets longer as they grow larger and older. For a large or old crab the loss of a claw means that it may never fully replace it simply because it doesn’t have enough moults left. One way to speed up the replacement process is to ‘swap sides’. The old cutter claw develops into a new crusher claw and the new claw becomes a cutter claw.

The loss of a claw doesn’t just restrict the diet; for males it has other consequences. The females can only mate when they moult, and as they become ready to moult they release a pheromone into the water. Males will seek the females out and fight for access to them, and then protect her both before and after moulting. For this they need their claws. They show their dominance by holding them wide in front of them, with pincers parted, and use them when fighting. A large crab with a missing claw may just hold its own against smaller males, but against an intact male of a similar size it will lose.

After mating the female creates a cavity in the sand in which to lay her eggs and attach them to her pleopods, the appendages under her abdomen. A female can lay up to 165,000 eggs and the egg mass is carried beneath the abdomen, which she fans to oxygenate the eggs. At first coloured orange the eggs turn a brown and then a dull grey as they mature. Once hatched, the live in the plankton for two to three years, feeding on other planktonic organisms, moulting through four stages as a spiny zoea before finally moulting into a megalopa, the stage that will eventually leave the plankton and once again live on the bottom.

Crabs can’t increase their size continuously; they moult to grow. Their bodies are encased in a hard shell, and like a child with too-tight shoes, eventually they need to trade up to a larger size. This brings problems because the shell is both armour and skeleton. Without it the crab can neither protect itself nor easily move. Before it sheds its shell the crab will start to break down its existing shell, which will soften. If you’ve ever found a crab on the shore whose shell is soft, called a ‘peeler’ crab by fishermen, then that’s the reason why – it’s close to moulting. It’s vulnerable in this state: put it back where it has good cover and can hide. The cells of the epidermis will pull away and separate from the inside of the shell and start to secrete the layers of a new carapace. The crab takes up water, the pressure splitting the old exoskeleton along the sides and at the rear, and the crab wiggles itself free from its former shell. This shedding of the shell is called ecdysis, and leaves the animal with a new soft, paper-thin exoskeleton. The crab’s tissues take in more water while the new shell hardens, after which the crab expels the water and shrinks, leaving room in which to grow before it again needs to moult.

One of the common names for this crab is the green crab, but it isn’t always green. Some are various shades of a reddish-orange. All newly moulted shore crabs are green but the pigment in the carapace that gives it the green colour can be degraded by light. The longer a crab spends between moults the more likely it is that its colour will change towards the ‘red’ form, and the more likely it is to have other creatures living on its shell. The colour isn’t the only difference. The red forms have heavier and thicker shells and can tackle larger mussels than a green crab of the same size. But there is also a downside. Unlike the green-coloured crabs these can’t tolerate environments with low oxygen or large changes in salinity. That makes a summer rock pool an unfriendly place for a ‘red’ shore crab, and so they tend to be found lower down the shore or below the low water mark.

It’s been close to half a century since I first encountered a shore crab, but its familiarity hasn’t lessened my fondness for it. I still head down the shore bucket in hand but this time not alone. Two years ago my daughter sent me a father’s day card using one of those online sites that make the card from your own image. She chose a photo from our summer holiday. On the cover was my own water-wrinkled hand holding a shore crab.

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