Hemigrapsus sexdentatus, known as the common rock crab (sometimes described as "common shore crab"), is a large-eyed marine crab of the family Grapsidae, endemic to the coasts of New Zealand. However, it is not found in the Chatham Islands or the southern islands. It can grow to around 45 millimetres (1.8 in) (female) or 55 millimetres (2.2 in) (male) shell width.
Females are usually in berry (carrying eggs) in the winter months (approx April to September), and they carry up to 26,000 eggs (size 0.35 mm). Males usually have visible bulges of white muscle protruding from the joint of the nippers.
From the eyes to halfway down each edge of the carapace (shell) are three tooth-like notches. The rear most pair of legs are the shortest, the third from rear are the longest. There is a lot of variation in colouration but there are two basic colour types; A paler form with pale to dark red patches on a white to light grey base colour, and a darker form where the patches merge and are a dark purple to blue-black colour with smaller patches of the light coloured base colour showing through. In both forms the colour tends to be darker towards the front, fading and becoming more sparse towards the back. The underside of the crab is white. The legs and claws are white with red to purple pigmentation to the same degree and hue as the carapace.
Rock crabs are known to feed on sea snails by crushing their shells through the use of a chela (a "nipper" or pincer-like appendage). Paul Bourdeau, of the Stony Brook University, calls them, "the meanest organisms that I have ever come across".