Edible crab

An edible crab's life is a sequence of discrete jumps rather than continuous growth, because an animal in a rigid shell can only get bigger by leaving it.

IT STARTS AS PLANKTON. Larvae hatch and drift for weeks through several stages before settling to the seabed as tiny crabs, which is when the population's distribution is actually decided — settlement depends on where the currents put them, not on where the parents were.

THEN IT MOULTS, REPEATEDLY. Growth happens only at the moult: the crab absorbs water, splits the old shell along a seam, backs out complete with the linings of its gills and gut, and expands before the new shell hardens. Juveniles moult several times a year; large adults may go two years or more between moults, and very large males may effectively stop.

THE PERIOD AFTER A MOULT IS THE VULNERABLE ONE. A newly moulted crab is soft, defenceless and hides — which is why they are found under rock and in holes rather than moving in the open, and why the soft-shelled stage is the basis of the peeler crab bait fishery.

CLAWS REGENERATE ACROSS MOULTS. A crab that sheds a limb regrows it over successive moults, smaller at first and eventually full size. That is why a returned crab missing a claw is not necessarily a wasted animal, and why declawing is nevertheless a real welfare and stock question.

AND LONGEVITY IS SUBSTANTIAL. Edible crabs are long-lived and slow-growing at size, so a large animal represents many years, and grounds fished hard for large crabs do not refill quickly.

Crab reproduction is tied to the moult in a way that decides both the timing and the vulnerability.

MATING HAPPENS WHEN THE FEMALE IS SOFT. A male guards a female approaching her moult, mates immediately after she sheds while her shell is still soft, and then guards her while it hardens. That pairing behaviour is why crabs are sometimes found in twos under the same rock.

THE FEMALE STORES SPERM AND SPAWNS LATER, extruding the eggs onto the underside of the abdomen where they are carried externally, glued to the pleopods. A female in this state is described as berried, and the egg mass can number in the hundreds of thousands to over a million.

SHE THEN CARRIES THEM FOR MONTHS, largely buried and largely not feeding, through the winter in much of the range. That is an extended period of immobility and vulnerability, and it is why a berried hen returned to the water matters more than one crab usually would.

FEMALES MIGRATE TO SPAWN. Females in several populations move substantial distances — tens to hundreds of kilometres in some documented cases — generally offshore and along the coast, so a berried female taken from a ground may represent recruitment somewhere else entirely.

LARVAE RELEASE IN SPRING AND EARLY SUMMER into the plankton, where they drift for weeks before settling.

AND THE PRACTICAL CONSEQUENCE IS SIMPLE. A berried female is carrying a season's recruitment and has invested months in it. Returning her carefully is the single highest-value handling decision in the fishery, and a landing prohibition on berried females is a common measure in several jurisdictions — a rule this record notes exists without stating what any current version of it says.

A crab's size is not a length. It is CARAPACE WIDTH — the distance across the widest point of the shell — and that is not a convention this site has adopted, it is what minimum size limits are written in. Measuring a crab from front to back, or including the claws, produces a number that answers no question anybody is asking and may produce a legal error.

ORDINARY ANIMALS encountered inshore run twelve to fifteen centimetres across. Many are below the minimum landing size that applies in their area and go straight back.

A GOOD ONE is over twenty centimetres across, which is a heavy crab and a good meal.

THE EXCEPTIONAL END reaches carapace widths approaching thirty centimetres and weights over three kilograms.

WHAT THE NUMBER CONCEALS IS THAT CRABS DO NOT GROW SMOOTHLY. Growth happens only at moulting: the animal sheds the entire shell, takes up water to expand rapidly, and then hardens at the new size over days to weeks. Between moults it does not grow at all. The interval lengthens with age until a large adult may moult only every few years or effectively stop, which is why the biggest animals are very old and why a population fished down does not recover on any convenient timescale.

AND SIZE DOES NOT TELL YOU WHETHER IT IS WORTH TAKING. A crab that has recently moulted is legal-sized and nearly empty, because it has grown into a shell it has not yet filled. Practitioners judge this by weight in the hand and by pressing the shell, and a light crab for its size goes back. That is a quality judgement and a conservation one at once: taking an animal that is mostly water wastes it entirely.

THE MINIMUM SIZE ITSELF IS SET AGAINST SIZE AT MATURITY and differs between areas, sexes and management regimes, and it changes. Establish the current figure for where you are before keeping anything, and carry a measure — judging twelve centimetres by eye is not a skill anybody has.

Crabs injure very few people and recreational crabbing kills a great many crabs, and almost none of that is on purpose. Both halves are worth taking seriously.

THE GRIP IS FROM BEHIND. Place thumb and fingers on the top and bottom of the shell at the REAR, between the back legs, and lift. Neither claw reaches there. Approaching from the front, or picking a crab up by a claw, is how people are nipped — and a large edible crab's crusher claw is genuinely capable of breaking a finger, not merely of hurting.

NEVER LIFT ONE BY A CLAW. A held claw is shed. The animal escapes, it has lost a limb it will take a moult or more to regrow, and you are left holding a claw.

A CRAB OUT OF WATER IS NOT DROWNING. Crabs tolerate air well for a period. What they do not tolerate is heat, and this is the fact that matters most: the commonest cause of death in recreational crabbing is a bucket. Animals crowded together in water that is warming in the sun deoxygenate it within an hour, and a bucketful of crabs tipped back at the end of an afternoon is frequently a bucketful of dead crabs.

SO: FEW ANIMALS PER BUCKET. Seawater changed often. Shade. A short time before they go back. Return them by lowering rather than dropping — a crab dropped several metres onto rock or water is damaged.

BERRIED FEMALES GO BACK EVERYWHERE. A female carrying a mass of eggs beneath her abdominal flap represents an entire brood rather than an individual, and returning her is a legal requirement across essentially the whole range as well as the obvious thing to do. Sex is read from the flap folded under the body: narrow and pointed in males, broad and rounded in females.

SOFT CRABS GO BACK TOO, for their sake and yours. An animal that has recently moulted is defenceless, poor eating and vulnerable, and it is the one you would want to still be there next year.

AND IF ONE IS BEING KEPT FOR THE TABLE, the welfare position has moved. Several jurisdictions now recognise decapod crustaceans as sentient in law, and current guidance favours chilling to reduce responsiveness followed by rapid mechanical dispatch over placing a live animal into boiling water. That guidance has changed materially in recent years; establish the current position rather than repeating what was standard a decade ago.

Crab seasonality is genuinely different in kind from fish seasonality, because the dominant cycle is not reproductive or migratory but a matter of the animal periodically discarding and replacing its own skeleton.

WHY A CRAB MUST MOULT. An arthropod's exoskeleton cannot grow. To get bigger, a crab has to construct a soft new shell beneath the old one, split the old one, extract itself entirely — legs, claws, gills and all — swell with water to a larger size, and then harden. It is a dangerous and exhausting process and the animal is defenceless for a period around it.

WHICH PRODUCES A CLEAR SEQUENCE OF CONDITIONS, and every one of them matters to somebody who has caught one. In the run-up, the crab is a peeler: the new shell is formed under the old and the animal hides, stops feeding and is at its most vulnerable — and this is exactly the stage that makes peeler crab the most valued bait in British sea angling, because a fish encountering a defenceless crab treats it as an opportunity it may not get again. Immediately after moulting the crab is soft, and it hides. It then hardens over weeks, filling out gradually, and only after that is it a full, heavy animal with meat in proportion to its shell.

SO A RECENTLY MOULTED CRAB IS POOR EATING and should be returned regardless of its size, because the shell is large and the animal inside it is not. That is a quality judgement and a conservation one simultaneously: the crab that is not worth taking is also the crab that has just made the investment that will make it worth taking later.

THE SEASONAL FRAME AROUND THIS IS ORDINARY. Crabs are most accessible inshore in warmer water and move deeper as it cools, and moulting occurs chiefly in the warmer part of the year — so soft and newly hardened animals are common exactly when the animals are most accessible. Timing is driven by temperature rather than by date and shifts with latitude and with year.

BERRIED FEMALES — carrying eggs beneath the abdomen — are a separate matter entirely and are returned as a matter of course wherever this is fished. What is legally required varies by jurisdiction and is current local information.

Edible crabs occupy two quite different habitats at different stages, and confusing them is the reason people think the species is more shore-dwelling than it is.

THE ANIMAL A SHORE GATHERER MEETS IS A JUVENILE. Small edible crabs are abundant under boulders, in gullies and among rock and weed on the lower shore, and that is genuinely their habitat — a cryptic, shelter-dependent stage using the same broken rocky ground that holds every other intertidal crustacean. It is not, however, where the population lives.

THE ADULT POPULATION IS OFFSHORE, ON MIXED AND SEDIMENT GROUND. As they grow, edible crabs move down and out onto the shallow and mid shelf, occupying rough ground, mixed sediment, sand and gravel with structure, generally in tens of metres of water. That is where the fishery operates and where the biomass is. A large edible crab on the shore is unusual; a large one in a pot in thirty metres is ordinary.

SO THE HABITAT IS BEST DESCRIBED AS A GRADIENT THE ANIMAL WALKS ALONG, and it walks a long way. Mature females in particular undertake substantial directed migrations along coasts, covering distances that are surprising for a walking animal, generally against the prevailing residual current so that larvae released later drift back toward the ground the female came from. Habitat for this species is therefore a stretch of coast rather than a spot.

SHELTER STILL MATTERS BUT DIFFERENTLY. Adults bury in sediment and tuck into ground rather than occupying a defended crevice as a lobster does, which is why the two share ground without competing for the same resource — and why a pot set on clean sand with nothing on it catches crab and not lobster.

MOULTING SETS A SEPARATE HABITAT REQUIREMENT that is easy to overlook. A crab that has just moulted is soft, defenceless and effectively immobile for a period, and it needs somewhere to hide while the shell hardens. Ground with adequate refuge supports moulting; open ground does not, and crabs concentrate in shelter when preparing to moult. That is also why a soft crab in a pot is a poor thing to take: the animal is at its most vulnerable and its worst condition simultaneously.

AND TEMPERATURE MOVES THE WHOLE PICTURE SEASONALLY, with crabs generally shallower in the warmer months and deeper in winter — a movement that shapes when inshore ground fishes at all.

Edible crab reproduction has a long, visible, immobile stage that makes the female extraordinarily vulnerable at a specific point in her life, and it is the single most practically important thing about it.

MATING HAPPENS WHEN THE FEMALE IS SOFT. A female edible crab can only mate immediately after moulting, while her shell is still soft, and the male guards her before and after the moult to ensure it. She stores the sperm and uses it to fertilise eggs later — sometimes considerably later, and sometimes for more than one brood.

THE EGGS ARE THEN CARRIED EXTERNALLY FOR MONTHS. A berried female carries an enormous mass of eggs attached to the underside of her abdomen — a very large number of them — and broods them through the winter. During that period she buries in sediment, feeds little or not at all, and moves hardly at all. That is a substantial fraction of a year spent effectively immobile with the entire brood attached.

WHICH IS WHY A BERRIED CRAB SHOULD GO BACK, WHATEVER THE RULE SAYS. Taking a berried female removes not one animal but a whole brood, at the point of maximum investment and minimum ability to escape. Many jurisdictions prohibit landing berried crabs; where the rule does not exist, the arithmetic still does. What the local rule is remains a matter for the competent authority.

THE LARVAE ARE RELEASED INTO THE PLANKTON IN SPRING and drift for an extended period through several developmental stages before settling onto the shore as tiny crabs. Survival through that phase is low and variable, and it is where recruitment is actually decided.

THE FEMALE MIGRATIONS ARE PART OF THE REPRODUCTIVE STRATEGY. Mature females undertake long directed movements along coasts, generally against the residual current, so that larvae released later drift back toward the ground she came from. That is a reproductive behaviour rather than a feeding one, and it means the population's breeding structure operates over a stretch of coast rather than a piece of ground.

AND MOULTING AND REPRODUCTION COMPETE FOR THE SAME YEARS. A female cannot moult while berried, so growth and reproduction alternate, and a heavily reproducing female grows slowly. That trade-off is why large females are old and why they matter.