Carcinus Maenas Megalopa
Supervision of Dr. Thomas Schaarschmidt and Jørgen Olsen from SNM.
What you see here is a baby crab. It is of the species of common shore crab, distributed all over the world in waters near the shore. If you ever went crab fishing during summer holiday with a cracked mussel, a clothespin, and a piece of string, this is probably the species of crab you caught.
Females can produce up to 185,000 eggs, and larvae develop offshore in several stages before their final moult to juvenile crabs in the intertidal zone. One of these molting stages is the Megalops modelled here. You can see its carapace is starting to widen, and its tail beginning to fold in under the body. At this stage it is easy to see that crabs and lobster are closely related - lobsters are just crabs with slender carapaces and tails that never folded! Young crabs live among seaweeds and seagrasses until they reach adulthood
The common shorecrab is a widespread invasive species, listed among the 100 "world's worst alien invasive species". It grows to a carapace width of 90 mm (3.5 in), and feeds on a variety of mollusks, worms, and small crustaceans, potentially affecting a number of fisheries. So you are allowed to fish as many out as you want to.