One of the most common crabs at Vancouver Island's shores: the purple shore crab, Hemigrapsus nudus.
The purple shore crab belongs to the Grapsidae, a family of crabs with a rather square carapace, usually not more than a few centimeters wide.
The typical environment of the purple shore crab: mud, sand, little rocks and debris. They prefer the upper tidal zone, are quick and well adapted to living out of the water.
After turning over a rock I saw up to 25 crabs getting away. It is very important to gently return the rock. Otherwise animals and seaweeds living upon or under the rock will die. Grapsid crabs are swift and not aggressive, however you frequently find specimens with amputated legs or claws. Maybe because they are very abundant and are fighting for space. And a lot of crab species are cannibals.
This is obvious a relative of the purple shore crab: the Asian shore crab, Hemigrapsus sanguineus (NL: blaasjeskrab). It is a recent import (since 2003) on the Dutch shore. It originates from Japan's surrounding waters. It is not as abundant as his relative: the penicillate shore crab, Hemigrapsus takanoi (NL: penseelkrab).
A pest or not?
The penicillate shore crab has replaced the common shore crab, Carcinus maenas, on the Dutch upper tidal shores. However, the much bigger common shore crab is still abundant.
The common shore crab (Carcinus maenas).
In Canada and the USA, where the introduced common shore crab is called the European green crab, it is as in the Netherlands: it has difficulty competing with native grapsid crab for space under rocks (G.C. Jensen, 1995).
The common shore crab is considered a pest in South-Africa: 'it poses a threat to many local molluscs. When it invaded the east coast of South-Africa, it caused millions of dollars of damage to the shellfish industry' (G.M. Branch, 2005). A quotation from Australian Marine Life by G.J. Edgar (1997): 'the species, introduced from Europe in the nineteenth century, is an active predator and has probably affected the populations of a number of local animal species.'