(1) The tidal cycle and that of solar elevation interact to produce patterns in seabed irradiance during the day which change over the springs-neaps cycle. In locations where high water springs is at midday, there is a single peak in seabed irradiance in the middle of the day at neap tides and two peaks in seabed irradiance (one in the morning, the other in the evening) at spring tides.

(2) These cycles also interact to produce a springs–neaps cycle of daily mean irradiance, which is relevant to the growth of benthic algae. The pattern of this cycle will also vary from place to place. In locations where low water springs is in the middle of the day the pattern is straightforward: maximum daily irradiance always occurs at spring tides. However, where high water springs is in the middle of the day, the cycle can reverse as the days get longer. The timing of the switch in the cycle depends upon the water clarity.

(3) Variations in attenuation over the springs–neaps cycle will also be important. In the Menai Strait, a springs–neaps cycle in K is often (but not always) observed, higher attenuations occurring at spring tides when seabed sediments are stirred up by the stronger tidal currents. This will reduce seabed irradiance at spring tides and tend to favour greater irradiance at neaps. In our model runs, K was kept constant because at present it is difficult to predict these changes. However, in our observations there is evidence of greater seabed irradiance at neap tides associated with greater water clarity. Variations in cloud cover during the day can also affect the regular cycles predicted by the model.

The authors are grateful to Professor Ernest Naylor for encouragement and for information on animal behaviour and light. Members of the Marine Optics Group at Menai Bridge helped with the sampling during the experimental work in July 1994.

Sphaerechinus granularis (Echinodermata: Echinidea) is involved in the erosion of ‘coralligène’ concretions in the Mediterranean. In shallow water (10 m), a high abundance of this species (>20 ind 25 m−2) is associated with small diameter individuals (56·7 ±7·7 mm). In deep clean waters (>40 m), the abundance is lower (<1 ind 25 m−2) and the mean diameter is higher (86·0±9·3 mm). Daily erosion of Corallinaceae by this species is related to the urchin diameter (r=0.87). Local variations in urchin abundance and diameter influence the amount of CaCO3 eroded annually. In shallow waters, the eroded CaCO3 mass reaches 210 g m−2 y−1 vs 16 g m−2 y−1 in coralligène concretions in deep clean waters. Sphaerechinus granularis is an important biological agent which substantially erodes the Mediterranean coralligène concretions.

Thirteen carefully prepared drogued buoy assemblies have been deployed in the eastern subtropical North Atlantic giving ~20 buoy years of Lagrangian data at a depth of 200 m. The buoy results together with hydrography have revealed the structure of the eastward flowing Azores Current (AC). The main jet had a transport of 26 Sv (near 28°W) with compensating counterflows in March 1992. Jet and counterflows were readily seen in the ADCP current structure and evident in the upper layer temperature (salinity) structure on an isopycnal surface. Buoys and hydrography showed that the adjacent westward flowing counterflows resulted in recirculation both north (anticlockwise circulation) and south (clockwise circulation) of the AC - the Subtropical Recirculations. South-south-west flow (3 cm s−1) occurred in the central region of study west of the Canary Islands. The long-term movement measured in the south or northern North Equatorial Current region was 3·5 cm s−1 west-south-west. The mean south displacement per year of the buoys was 2·2° of latitude to the south and most of this displacement occurred in the first half of the year (February-August). The maximum westward displacement rate occurred six months later in October and November. The Subtropical Front/Azores Current region was identified as a zone of increased levels of kinetic energy (>100 cm2s−2) at a latitude near 34°N at the 200 m level, stretching for >2000 km, and as a marked horizontal winter sea surface temperature contrast at a latitude near 36°N. Winter mixing created an outcropping region of water with density in the range 26–4<σ0<26·6 kg m−3 along the Subtropical Front near 34°N with a maximum vertical extent of 200 m.

The evidence for variations in winter phosphate concentration as a component of the ‘Russell Cycle’ has been re-examined. The change from high phosphate concentrations prior to 1930 is the largest apparent variation in concentration through the period 1923–1987. However, these values are the result of a retrospective application in 1938, of a salt correction factor of 1–35. The justification for this factor is examined. It is concluded that there is sufficient uncertainty in the accuracy of the pre-1948 measurements of phosphate concentration to preclude their use in long time-series analysis. A statistical analysis of the post-1948 data shows that a simple first order autoregression model provides a perfectly adequate description of the data; the data can be considered as essentially random, with a small component of carry over from one year to the next. The evidence is weak that variations in winter concentrations of phosphate support the Russell Cycle hypothesis.

Forty-three species of choanoflagellates were seen during a study of the heterotrophic nanoflagellates in Southampton Water, between 1991 and 1994. The majority of the species were loricate choanoflagellates (Acanthoecidae), and this group accounted for –8% of the total heterotrophic nanoplankton during the period of study. Aloricate choanoflagellates (Codosigidae and Salpingoecidae) were less diverse, and accounted for only 3–4% of the heterotrophic nanoplankton. The seasonal occurrence of loricate choanoflagellates was studied using light microscope whole mounts, whilst more detailed studies of lorica structure were made using electron microscope whole mounts. Two new species, described in this paper, had a markedly seasonal occurrence and were only recorded during a period of 6–10 weeks each spring.

The harpacticoid copepod genus Tigriopus is characteristic of shallow upper shore rock pools in both hemispheres. Tigriopus are necessarily physiologically tolerant as the pools feature extreme physicochemical conditions; their lower limit on the shore is apparently set by competition/predation. There are several species, differentiated by relatively minor morphological distinctions; speciation appears to be favoured by restricted gene flow between populations, even over short distances. This paper briefly reviews the literature concerned with the taxonomy, physiology and ecology of the genus, and compares environmental conditions and salinity/thermal tolerance data for three species: T. brevicornis from Scotland, T. fulvus from Madeira, and a previously undescribed population of the Southern Ocean ‘T. angulatus’ group from subantarctic South Georgia. Particularly interesting was a low upper lethal temperature in ‘T. angulatus’ (21·9°C) that probably restricts the species to relatively large pools that buffer environmental thermal extremes. Unlike the other species, T. fulvus cannot withstand freezing conditions. Evidence is presented to show that the South Georgian Tigriopus differs slightly, but significantly, in its morphology from two sibling species that inhabit the subantarctic Kerguelen and Crozet Islands but which cannot interbreed. If the South Georgian copepods are indeed a separate species and do not occur elsewhere (e.g. the Falklands), this indicates rapid speciation within the genus; the coasts of South Georgia were covered by an icecap until 10,000–14,000 years ago and no rock pool habitats were then available. It is suggested that Tigriopus may have reached South Georgia by rafting within Enteromorpha tubes.

Changes in four macrobenthic assemblages of the muddy-fine sand Abra alba community along the north-west French coast were compared using several numerical analyses (distogram, Eigen vector filtering, clustering and multi-dimensional scaling). The faunistic composition of these four assemblages continuously changed during the 1978–1992 period. These modifications are characterized by more or less rapid and abrupt successions of distinct groups of species, but the amplitude and the frequency of these changes are different between sites. Comparison of the results provided by different analyses also reveal that these four macrobenthic assemblages did not fluctuate in parallel, and that local conditions might play a key role on their temporal variations. However, it is shown that a major mesoscale climatic event, the alternation of mild and cold periods, probably also influenced the temporal variations of these four macrobenthic assemblages. Thus it is likely that there is a combination of both local and mesoscale events influencing these communities. In certain cases, changes in local conditions were the major source of variations. Sometimes the impact of the mesoscale conditions was more apparent, especially at the northern sites where the amplitude of the variations of temperature was greater.

Laboratory experiments were carried out upon Elphidium crispum to determine the mechanism by which this species remains epifaunal, by investigating geotaxism, phototaxism, temporal influence and responses through ontogeny. Different size-classes of specimens were tested for phototaxism and geotaxism, over four and five months respectively. Specimens of the smallest size-class tested (250–355 μm) were significantly phototaxic and geotaxic. Significant positive phototaxism was demonstrated in all tests except one, with specimens in the size-class 500–1000 μm giving particularly significant results for the months of June and July 1994. Specimens were significantly geotactic for three of thirteen tests: both negative and positive geotaxism were demonstrated.

Behavioural patterns of crustaceans are known to vary within the 24 hour cycle and in relation to environmental signals. Light and chemical stimuli induce specific behavioural responses. Retinal and extra-retinal photoreceptors use different motor responses to illumination selectively. Light responsiveness is modulated at various levels, from the light admittance to the retina, up to the integration in higher order interneurones and motorneurones. An endogenous circadian rhythmicity contributes to the various elements of the system.

Patterns of the distribution of intertidal fauna in relation to the diversity of habitats were studied in the UNESCO Man and Biosphere (MAB) reserve of the archipelago of Molene and Ouessant (west Brittany, France). A classification of environmental features was developed in order to determine species richness, abundance, biomass and trophic structure of the macrobenthic fauna in this intertidal area which covers 1120 ha. The four descriptors of the environment were: (1) substratum (rock, boulders, sediment, pebbles); (2) height on the shore, based on the macrophytic zones: Pelvetia canaliculata/Fucus spiralis; Fucus vesiculosus / Ascophyllum nodosum; Fucus serratus; and Himanthalia elongata/Zostera marina; (3) wave exposure (sheltered, semi-sheltered, exposed); (4) percentage of algal cover (<50%, >50%). Each type was defined by a four-figure number and matched to a map codification. A two-way analysis of variance (ANOVA) and a posteriori Newman-Keuls test (P=0.05) were applied to the series of species richness and biomass. Among the 308 species sampled, 40 represented 90% of the total biomass. Maximal number of species was reached in boulder fields in the Himanthalia zone, under exposed conditions and high algal cover. The lowest number of species was observed in pebble fields of the Pelvetia zone and in the sedimentary areas. Trophic structures were dominated by suspension feeders and herbivores at high levels on the shore and by carnivores at the lowest levels. High vegetation cover and wave-exposed conditions were shown to increase with increasing faunal diversity. Links between patterns of habitat distribution and faunal diversity and biomass in different spatial scales are discussed.

In September 1993 the Irish Sea floor near the Sellafield nuclear reprocessing plant was surveyed by towed video to assess the distribution, abundance and behaviour of large burrowing invertebrates believed to be important in the transport and mixing of radionuclide-contaminated sediments. Sixteen stations were surveyed, covering a range of sediment types. Burrow openings and mounds were counted and assigned to species where possible. Particular attention was paid to the similarities and differences between the features observed and those produced by the same species in Scottish sea lochs, the source of most relevant ecological data. Several different burrowing communities were identified from bottom topography. Offshore muddy sands supported dense populations of the thalassinidean crustacean Callianassa subterranea, in burrows similar to those described from the North Sea. Coarse sediments closer inshore had C. subterranea at lower density, with the thalassinidean Upogebia deltaura and the crab Goneplax rhomboides also present. Softer muds were dominated topographically by ejecta mounds of the echiuran Maxmuelleria lankesteri and burrows of the Norway lobster Nephrops norvegicus. The thalassinideans C. subterranea, Jaxea nocturna and Calocaris macandreae were also present at low density. Surface bioturbation features in this habitat were very similar to those described from sea lochs. Population densities estimated from surface bioturbation features were always lower than counts of specimens from box-cores. The visual survey therefore gave minimum estimates of population density. Available data on rates of bioturbation by the species concerned are collated to give rough estimates of biogenic sediment transport in the habitats surveyed.

The impact of colonization by Sargassum muticum (Phaeophyta: Fucales) on the resident macroalgal assemblage in tidepools was investigated in northern Spain. Sargassum muticum was allowed to colonize and spread in some tidepools, and was periodically removed in others. Changes in the abundance of macrophytes were recorded at three sites over two years. The colonization by S. muticum and the subsequent changes induced on the native assemblage was extremely variable among sites. Where the invader colonized profusely, it induced changes in the macroalgal composition of tidepools. The leathery group of macrophytes was significantly affected and changes were also observed in the foliose group. It was suggested that S. muticum may take advantage of their perennial and opportunistic traits in the interaction with groups of algae with different survival strategies. However, the spatial patchiness of the colonization may mitigate the impact of the invasion at local and regional scales.

The taxonomy of six species and one subspecies of Natatolana, which have previously been recorded from the north Atlantic Ocean and Mediterranean Sea, is reviewed. Natatolana caeca, N. gallica, N. gracilis and N. neglecta are redescribed from the type material. The location of possible type material of N. borealis is reported and a redescription of this species is provided, based on material from the type locality. Natatolana neglecta var. imicola is recognized as a valid species. Natatolana schtnidti is regarded as a junior synonym of N. imicola based on comparison of type material and this material is used to redescribe the species. Distribution records for these species are summarized from examination of material held in various collections and a key to separate these species is provided.

Xenophyophores are a group of giant deep sea protists characterized by intracellular barium sulphate (BaSO4) crystals. X-ray diffraction, electron diffraction and electron microscopy studies have been performed on barium sulphate crystals from three xenophyophore species (Aschemonella ramuliformis, Reticulammina labyrinthica, Galatheammina lamina) obtained at bathyal and abyssal depths in the north-eastern Atlantic. Two populations of crystals were observed. The first were tablets, ~2μm in length and rhombic or hexagonal in outline. In both cases, the tabular face was of index (100). The second population consisted of much smaller particles (<0·5 μm) of poor crystallinity. A comparison of the larger xenophyophore crystals with synthetically grown crystals indicated that the former probably grew at low supersaturation (S<25) in solutions of low to moderate ionic strength (I<l·0 M). Some preliminary observations of the cellular organisation of A. ramuliformis are reported. The protoplasm is multinucleate and characterized by what seems to be a system of extracellular lacunae formed by imaginations of the cell wall. Similar features have been observed in the deep sea foraminiferan Rhizammina algaeformis. Possible origins of the BaSO4 crystals and the taxonomic relationship between xenophyophores and certain foraminiferans are discussed.

Using video and image analysis techniques we analysed the response of swarms of the mysid Paramesopodopsis rufa to food and predatory attack. After food was added to the tank, mysid aggregations initially (up to 45 s) increased in volume. Subsequently volume decreased significantly until it was smaller than the initial level. After a period of ~12 h during which no food was added, the swarm expanded to near its original volume. These changes are interpreted as resulting from a need by individuals to optimize food capture and protection. The pattern of volume changes in response to food was independent of aquarium size but the magnitude of the changes was reduced in a smaller tank.

Predatory attacks on mysid swarms were simulated using a model fish which ‘swam’ a single pass along the tank driven by an electric motor. Models (latex-covered real preserved fish), represented two different attack styles: ambush (seahorse Hippocampus abdominalis); and lunging (Australian salmon Arripis trutta). ‘Swimming’ speeds resembled those measured for real fish i.e. 0·5–2·0 cm s-1 for seahorse and 15 cm s-1 for salmon. Again changes in swarm volume were recorded. No obvious response by the swarm to the approaching seahorse was apparent; the aggregation simply parted to allow the fish through. No tailflips were seen among the mysids. The swarm resumed the original volume rapidly after the fish had passed. Approach of the salmon was evidently detected from further away, escape reactions were frequent and energetic, and tailflipping was common. After the fish had passed, it took longer for the original swarm volume to be restored. These results suggest that seahorse ambush techniques have evolved to minimize warning of the predator's approach. When the salmon ‘menaced’ the swarm at seahorse speeds, it appeared the mysids still reacted as though it was a ‘normal’ salmon attack so the fright response was not just a function of speed of attack.

Dynamic changes in swarm volume seem to be a useful simple index of motivational state of individuals.

Male fiddler crabs, Uca pugilator (Crustacea: Decapoda), respond to conspecifics by claw waving, and to predators by freezing or escape. In field experiments it was found that this distinction was not made on the basis of angular size and speed, nor was shape important. The remaining possibilities were either the absolute size of the stimulus, determined from angular size and distance, or the position of the stimulus relative to the horizon. To distinguish between these, a crab was placed in a glass dish, and moved black stimuli on a white background, at a distance of 22 cm. Stimuli below the crab's horizon hardly ever evoked escape. However, identical stimuli partially or wholly above the crab's horizon produced escape responses whose frequency varied with the angular size of the stimulus. Halving the distance of the stimulus showed that it was angular and not absolute size that determines escape frequency; and experiments with a tilted horizon showed that it is the position of the stimulus relative to the eye equator that is important, rather than the geographical horizon itself. It has been concluded that crabs categorize stimuli as dangerous or otherwise by their position relative to the crabs’ visual horizon.

The 1989 discovery of a large dorsal eye on the hydrothermal vent shrimp, Rimicaris exoculata, debunked the prevailing opinion that all animals living around deep hydrothermal vents were blind. Recent dives with DSV ‘Alvin’ recovered new bresiliid shrimp [Chorocaris (Lucky Strike) and Chorocaris (Broken Spur)] from two new vent fields on the Mid-Atlantic Ridge. The Lucky Strike species has recently been named Chorocaris fortunata and may be the same as the Broken Spur species - the retinal morphologies reported here are very similar. Like R. exoculata, C. fortunata has a visual apparatus adapted to the very dim light of its environment. Although in both species enlarged, non-imaging eyes appear to have evolved from the stalked compound eyes typical of caridean shrimp, those of C. fortunata are forward-facing whilst that of R. exoculata is located dorsally. The massive array of photosensitive membrane (rhabdom) of C. fortunata lies beneath a smooth cornea within a matt, white, reflecting matrix and occupies 80% of the available volume of the photoreceptors compared to the expected 10–15% of shallow water decapods. All screening pigment is located out of the light path at the bottom of the retina. There is no ultrastructural evidence for cyclic rhabdom shedding or renewal. Thus the cellular organization of C. fortunata is remarkably similar to that of R. exoculata and distinct from its surface-dwelling relatives. This suggests that in these species imaging optics have been sacrificed to achieve the increased visual sensitivity necessary to detect the very dim light emitted from the throats of the black smoker chimneys around which they live

The species composition, abundance and vertical distribution of micronektonic cnidarians has been investigated in the upper 2800 m at Discovery Station 9969, located in the cold regime in the eastern Weddell Sea, Antarctica. In total 22 siphonophore and 20 medusan species were identified. Overall siphonophores were more abundant than medusae, but the latter usually contributed much more to the biovolume. On average, cnidarians contributed more than 50% to the total biovolume of each catch; and they contributed >70% to the combined total of all the samples. Despite the high water content of these animals, these large biovolumes meant that the cnidarians formed ∼25% of the total carbon in each catch. The possible impact of these gelatinous cnidarian populations on the ecosystem is discussed.

Six species of naked amoebae, ranging in size from 45 to 3258 μm3, were isolated from benthic sediments of the Clyde Sea area. The number of fluorescently labelled bacteria (FLB) consumed by these amoebae was determined at 5, 10, 15 and 20°C. Consumption rates varied markedly, ranging from 0·2 to 194·7 bacteria h−1 (equivalent to 0·16–155·8 μim3 bacterial biomass). However, only some of these values were considered to be optimal; several of the uptake values were too low to account for the measured growth rates of amoebae. Using the optimal consumption rate data, the mean specific rates increased from 0·042 bacteria h−1 μm3 at 5°C to 0·131 bacteria h−1 μm−3 at 20°C. This is the first study to detail the ingestion rates of a range of bacterivorous marine gymnamoebae.