A comparative study of some aspects of the biology and ecology of Sesarma catenata Ort. and Cyclograpsus punctatus M.Edw., with additional observations on Sesarma meinerti De Man

In the estuary of the Kowie River in the Eastern Cape Province there occur two amphibious grapsoid crabs, Sesarma catenata Ort. and Cyc/ograpsus punctatus M. Edw. They live together in many places, commonly inhabiting the same burrows in the muddy banks of the river. The present exploratory study of these two species has been undertaken to enhance our knowledge of their biology as well as our understanding of the relations between these two very similar animals which live in such close proximity one to another. THE HABITAT The Kowie River empties into the sea at Port Alfred, and is broad, winding and shallow. The tidal limit lies 13 miles from the mouth and over most of this length extensive, gently sloping mud banks are left exposed at low tide. The level of mean high tide is marked by a low salting cliff which is surmounted with vegetation. At Port Alfred itself the river runs between steep, stony, artificial dykes and on either side lie lagoons (Fig. 1). The lagoons on the East Bank (1, 2 and 3) are almost completely cut off from the river although an exchange of water does take place via seepage points in the dyke. The two western lagoons (4 and 5) are fully tidal. Large areas of the intertidal zone in the estuary are colonised by the tall, coarse marsh grass Spartina capensis Nees (S. stricta Roth.). This usually disappears about the level of High Water Neaps (HWN), to be replaced by a zone of typical salt marsh plants, dominated by Arthrocnemum, especially A. pillansii Moss, which has long stems, prostrate and woody at the base and terminating in erect green succulent tips. With the Arthrocnemum there may be min­ gled Spartina and Triglochin bulbosum L., particularly in low wet situations, and towards High Water Springs (HWS) Chenolea diffusa Thunb. A short distance below HWS Limonium scabrum (Thunb.) O.K. appears and spreads upwards, presently to give way to grasses, shrubs and trees. The zonation of plants on the banks of four of the five lagoons is very much the same as that along the river banks. The banks of Lagoon 4 (known locally as Little Beach) are sandy and devoid of vegetation. The two lagoons which lie furthest from the sea (3 and 5) contain masses of drifted algae, in particular the slender, branched brown alga Graci/aria verrucosa (Huds.) Papenf. The distribution of the two crabs in the area round Port Alfred is shown in Fig. 1. Beyond the town, where the river is not restrained by artificial dykes, the two species occur together