Ecological studies on the shores of Moçambique. 1. The fauna of intertidal rocks at Inhaca Island, Delagoa Bay

Abstract

The fauna of the 2, 000-mile shore line of South Africa has been extensively investigated (Stephenson et al., 1937-48, and Day et al., 1950-56), but very little is known of the ecology of the adjacent 1, 000-mile stretch of the Moçambique coast of East Africa. The exposed rocks nearest to Inhaca Island (lat. 26 0'S.), that have been described, are at Isipingo near Durban, Natal, nearly 300 miles south (Eyre and Stephenson, 1938), and the nearest sheltered rocks in Durban Bay (Day and Morgans, 1956). Natal is considered to be a "sub-tropical" shore on which the tropical component is high. Many tropical Indo-Pacific species from the coast of Moçambique have been described by taxonomists in their publications on the South African fauna, cited at the end of this paper. The equatorial waters of the southerly directed Moçambique current warm the east coast of Africa, and in fact, some tropical animals occur as far south as latitude 32 S. On the other hand, a fairly sharp zoogeographical boundary between tropical and warm-temperate faunas has been found at latitude 25 S. on the coast of Queensland (Endean, Kenny and Stephenson, 1956). It is, therefore, of interest to enquire how far the tropical influence has extended to Inhaca on the fringe of the tropics, and whether the intertidal fauna exhibits marked differences from Natal and from northern Moçambique. The Australian workers have shown that, in Queensland, wave action and salinity are of more importance than temperature in determining the geographic ranges of shore animals. To a lesser degree along the coast of Moçambique, wave action is somewhat reduced by submerged coral barriers and sandy shoals, and rivers tend to diminish salinity and deposit silt. These factors vary around the short periphery of Inhaca Island to an extent that is comparable with that over many hundreds of miles of the exposed coast of Moçambique, since one side of the island is exposed to the Indian Ocean and the others are sheltered by Delagoa Bay. The sites selected for the investigation of the rock fauna at Inhaca may therefore be germane to the general problem of the distribution of marine intertidal organisms. The investigation was carried out from 1952 to 1956 at the Estaçao Biologia Maritima, Inhaca Island, through the kindness of the Portuguese Government of Moçambique. An interim report was published three years ago (Kalk, 1954). Collections in northern Moçambique were made in November 1955.