The story is that in common with many( but not all) crab species, female brown crabs are only receptive for mating just after they have moulted. A male guards a female that he can detect is about to moult then, after she has shed her old armour and they have mated, continues to guard her while she hardens up her new armour.
Equivalent behaviour can be seen with pairs of swimming crabs, but the males’ long legs enable them to hang onto a female while remaining reasonably mobile. The short stumpy legs of brown crabs make this much more difficult so, as here, the male usually wedges a female into a hole or crevice and the couple stays put. The males’‘ gallantry’ and the females’‘ imprisonment’ benefit both partners of course, preventing their genetic material being lost if a soft and vulnerable female is eaten by a predator.
The female brown crab does not lay her eggs immediately after mating, but waits for a few months and may even store the sperm until the next year. She attaches the eggs she eventually produces, up to three million of them, underneath her abdomen flap. She then broods them for eight months or so before they hatch into planktonic larvae. A couple of months and a few moults later, miniature brown crabs settle on the seabed.
The relatively cumbersome nature of brown crabs that limits courtship options also affects their choice of prey. It is reported as including a variety of smaller crabs, but I have only ever seen them eating molluscs such as whelks, mussels and clams, for which their powerful claws are ideal digging and crushing equipment.
If you’ re thinking“ I’ ve always known these as‘ edible crabs’ you’ re right,
Brown Crab excavating a crater in its search for food
that name is often used. However, the alternative‘ brown’ name is gaining traction and I reckon it’ s better, less based on exploitation and a kinder description of these lumbering but impressive heavyweights. �
Paul’ s photos and video clips capturing UK marine life stories are on Instagram @ paulnaylormarinephoto
Brown Crab and much more lightly-built neighbour, a Velvet Swimming Crab
Dispute between two crabs
A Brown Crab starts to dig in the seabed for food
Rockpool photo of a newly moulted crab and its old armour, showing the increase in size
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