The ecology of Saint Paul's Rocks (Equatorial Atlantic)

St Paul's Rocks are a remote group of islets, barely 400 m across, in the equatorial mid-Atlantic (o°55'N 29°21'W). The results of a primarily zoological survey of the Rocks, carried out in September 1979 by the Cambridge Expedition to Saint Paul's Rocks, are reported. In addition, earlier studies of the Rocks' biology are discussed in the light of recent research. The terrestrial fauna is dominated by three species of sea-bird and a land crab. Also reported from on land are a few species of insects, ticks and spiders, an endemic pseudoscorpion, and a centipede; the majority of these species are associated with the seabirds.
The islets are devoid of tracheophytes, bryophytes and lichens.
The marine ecology of the Rocks is discussed with respect to habitat. Five habitats are distinguished in the supralittoral and littoral zones; the fauna and to a limited extent the flora of each is described. Sublittorally the Rocks consist almost entirely of steep escarpments extending to 60 m depth and beyond. The shallow sublittoral (< 60 m deep) is divided into four habitats: (1) Palythoa zone; (2) Caulerpa zone; (3) sub- Cauferpa zone, and (4) areas of unstable substrates. The Palythoa zone occupies a band extending from the base of the littoral to a depth of c . 5m and is dominated by a mat-forming hexacoral, Palythoa caribaeorum. Below this zone to a depth of c . 33 m the rock faces are thickly invested by a species of green-alga, Caulerpa racemosa. At depth this Caulerpa dominated zone is succeeded by an invertebrate dominated community. Unstable substrates are represented by a few pockets of coarse sand and some patches of rubble. The communities in each of these habitats are described. A primarily taxonomic review of the deep-water fauna is also included. Finally, the influence of ocean currents on the zoogeographical relationships of the Rocks' fauna is briefly examined.