![Thumbnail: [Volume 86, 1959 page 202]](/journals/rsnz/images/rsnz_86/thumbnails/rsnz_86_03_0235_0202_ac_02.gif)
Adaptive Colouration
Hingston's concept of “colour conflict” (1933) can be applied to the colouiation and form of the two crabs. According to this theory most animals combine a need for concealment in some situations with a need for conspicuousness in others. One solution to the problem is for the animal to have the generally exposed parts of the body cryptically coloured and conspicuous patterns on parts of the body that are revealed by a special posture.
When Helice crassa is crouched close to the ground and partly buried, all the appendages are folded close to the body and the crab assumes as small and as flat a form as possible. The eye-stalks are shut into their grooves in the front of the carapace, and the pincers are folded under the mouth parts, which are pressed toward the ground. The back of Helice crassa and portions of the appendages still exposed are coloured crytic muddy green, blue green or raw sienna and are difficult to distinguish from the sand on which the animal lies. But when the same crab adopts the “Aufbaumreflex” attitude, the ventral surface of the animal is exposed with every part unfolded and extended and the crab looks as big and obvious as possible. The pincers, which are the most obvious and wicked-looking feature, are light yellow in colour and edged along the top with sharply contrasting darker green. The mouth parts are also light yellow with darker edges. Small patches of bright orange pigment are present at the joints of the chelipeds and walking legs visible from the front. The light colour and jagged form of the opened pincers make them stand out from the general darker shades of the rest of the animal, and the orange patches are very obvious. With the eye-stalks also raised to their full height, there can be no doubt about the conspicuousness of both the animal and its intentions.
Hemiplax hirtipes adopts a similar concealing attitude to that of Helice crassa with all obvious parts tucked out of sight, the body shape being reduced to as small and compact a mass as possible with only the dark green upper body colouring exposed. Often, however, the sand is lighter in colour than the crab and, unless it
![Thumbnail: [Volume 86, 1959 page 203]](/journals/rsnz/images/rsnz_86/thumbnails/rsnz_86_03_0236_0203_ac_02.gif)
can bury itself, the crab can still be seen. Hemiplax hirtipes usually shows warning behaviour first and tries to retreat or bury itself only if this fails. As a consequence it is more brightly coloured in the warning fashion than it is camouflaged for concealment. When disturbed, this crab turns in the direction of the disturbance and stiffens on outstretched legs with mouth parts lifted from the ground and with the pincers flexed and obvious. Moreover, the crab moves as the source of disturbance moves so that the startling pattern of the pincers is always on display. In this species the pincers carry a band of red and a band of white pigment which contrast sharply with the darker greens and browns of the body and the colour of the sand or mud which may be deepened by the crab's shadow. Closer approach brings about a striking and startling change of attitude, the colourful chelipeds being rapidly extended and the large pincers opened. If such intimidation fails the crab finally tries to run away and bury itself.
It is possible that the initial warning reaction of Hemiplax hirtipes and its fairly conspicuous general colouration are real warning signs of unpleasant taste properties in the crab (proaposematic colouration). For, whereas on numerous occasions predators were seen catching and devouring individuals of Helice crassa, on only one occasion was a predator (in this case the crab Hemigrapsus sexdentatus) seen to catch and eat an individual Hemiplax hirtipes.