Seventy-one species of shore fishes are recorded from Ascension in a checklist which includes notes on ecology and behaviour. The extraordinary abundance of blackfish, Melichfhys nigrr, and the aggressive nature of moray eels, Lycodontis moringa, towards man are briefly mentioned. The number of species recorded is low for a tropical island and seems to reflect Ascension's geographical isolation and lack of habitat diversity; 15.7% of Ascension shore fishes (excluding one fish not identified to species level) appear to be endemic to the island, with a further 17.1°/o known only from Ascension and St Helena; 30.0% are recorded only from the western and central Atlantic, while 7.1% are recorded from the eastern and central Atlantic; 30.0% are widespread Atlantic species. Two new species are described: Taenioconger camelopardalis and Helcogramma ascmsionis.

Fifty species of fishes are recorded from Saint Paul's Rocks in a checklist which includes notes on ecology and behaviour. The poor species diversity, which is the lowest of any tropical island studied to date, seems to result from the isolation of the Rocks and also their small size and lack of habitat diversity. Large pelagic and semi-pelagic predators, especially Galapagos sharks, are unusually common; 12.1% of identified shore fishes at St Paul's appear to be endemic to the Rocks, while 6.1% are known only from the tropical islands of the mid-Atlantic ridge (St Paul's, Ascension, St Helena); 15.2% are recorded only from Brazil and St Paul's 30.3% are widespread western Atlantic forms that have the central Atlantic as the eastern limit of their range; a further 36.4% are widely distributed throughout the tropical Atlantic. Three new species are described: Anthias salmopunctatus, Stegastes sanctipauli and Enneanectes smithi.

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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.

Nematocysts are the stinging organelles of jellyfish, sea anemones and other cnidarians. Each one consists of a closed capsule filled with fluid. In the resting state, part of the surface of the capsule is inverted, forming a tubular thread, which is everted explosively on excitation. The mechanism of explosion is not yet understood, it may be relevant to exocytosis in general, as the nematocyst is a specialized type of exocytotic secretion originating in the Golgi apparatus. Picken and Skaer observed that the capsular fluid showed a large depression of freezing point, suggesting that the osmotic pressure might be as high as 140 bar. They tentatively ascribed the explosion to a sudden increase in permeability of the capsule wall, allowing a rapid osmotic influx of water. However, there is evidence that the material of the thread itself may be capable of some degree of extension and it has been suggested that osmosis plays no part in discharge. We present here a new theory: that the capsule wall is permeable to water even in the undischarged state; discharge is initiated by an increase in the osmotic pressure of the capsular fluid which is brought about by removal of bound calcium ions.