Introduction

Observations of the biology of the crabs Helice crassa (Dana) (family Grapsidae) and Hemiplax hirtipes (Heller) (family Ocypodidae) were carried out during September, 1954, and from February to August, 1955—i.e., throughout a late summer, autumn, winter, spring Most of the work was done on the mudflats around the Otago Peninsula, but the two crabs are common in similar localities throughout New Zealand.

The two species occupy roughly similar habitats estuarine mud flats where they are confined between the tide marks and live in burrows or under stones. Hemiplax hirtipes is also occasionally found on sheltered rocky shores. The two crabs are similar in appearance.

Helice crassa is a smallish crab, about three-quarters of an inch in width across the carapace, olive-green to tawny-brown in colour with rather square proportions when seen from above.

In Hemiplax hirtipes the carapace is roughly an inch in width and its colour varies from pale blue-green, with a few very small spots, to tawny-brown or reddish-brown with either large masses of purple spots, a pattern of coalesced spots or large purple-blue patches. The legs are yellow-green or blue-green and the chelipeds deeper or lighter red on the dorsal edge, white on the ventral edge. The general proportions are rectangular with the long axis extended laterally.

Both crabs typically feed by extracting the organic matter from surface mud or sand, which is picked up indiscriminately by the chelipeds and sorted by the mouth parts. They also eat large pieces of dead organic matter, the bodies of dead lugworms, pieces of ascidians, etc.

Crabs of both species can be found occupying burrows. Helice crassa individuals were observed constructing burrows in the field and in the laboratory. The technique used by these crabs is similar to that described by Dembowski (1926) for the fiddler crab and Cowles (1908) for Ocypoda arenaria. On no occasion did I see Hemiplax hirtipes constructing a burrow, and I suspect that the crabs of this species occupy burrows built by the other.

In a vertical transect of mud bank, Hemiplax hirtipes burrows are concentrated near the lower tide limit and Helice crassa burrows near the upper tide limit, although there may be considerable overlap between the two distributions. Helice crassa is active, feeding and burrowing, mainly when exposed at low tide; Hemiplax