Two samples of Neanthes succinea (Frey & Leuckart 1847) from the Mediterranean Sea were investigated. Twenty-one loci were analysed; seven of them were polymorphic in both populations. The level of heterozygosity was 2.2% and 4.4% in populations from the Tyrrhenian and Adriatic coasts respectively. The value of Nei's genetic identity index (1=0.965), together with the mean F(IT) (=0.350), shows that the samples can be considered reproductively isolated populations. F-statistics indicate that ME-1 is the discriminant locus between populations.

This paper reports the results of experiments designed to investigate the relationship between thermal stability of DNA and the physical and chemical conditions experienced by various species of invertebrates, with particular reference to hydrothermal vent organisms. The major conclusion is that thermal stability correlates positively with environmental temperature within the DNA regions encoding ribosomal RNA, but not throughout the genome. Increased thermal stability (based it is supposed on an increase in G-C content) of rDNA relative to total DNA confers protection to coding sequences which would otherwise be susceptible to damage during transcription. An increase in G-C content of rDNA will lead to an increase in G-C levels in rRNA which forms a major structural component of ribosomes, thus conferring increased thermal and chemical stability to these sites of amino acid synthesis within the cell.

The presence of antibacterial activity in the haemocytes of the shore crab, Carcinus maenas (L.) (Crustacea: Decapoda), was investigated using a selection of Gram-positive and Gram-negative bacteria from geographically diverse waters. Preliminary investigations into the relationship between this activity and the prophenoloxidase activating system (proPO) were also carried out. Antibacterial activity against both Gram-positive and Gram-negative organisms were found to reside exclusively in the granular haemocytes and eight of the twelve bacteria tested were susceptible to this effect. Additional studies, using Psychrobacter immobilis (= Moraxella sp.), revealed that the factor (or factors) responsible was 90% effective within 60 min and was also heat stable, independent of divalent cations, and non-lytic in character. Although antibacterial activity resides in the same cell population that carries the proPO system, there appears to be no relationship between antibacterial activity and phenoloxidase itself. Other components of the proPO system, however, may be involved.

Some of the limits to the use of serology to identify prey species in the digestive tracts of cephalopods have been evaluated. Cuttlefish, Sepia officinalis, were given meals of krill slurry (Euphausia superba). Protein extracts of contents from four regions of the digestive tract, stomach, caecum, digestive gland and intestine, were tested for prey antigenicity. Digestion times (loss of antigenicity) ranged from 1 to 8 h depending on sampling site. Stomach and caecum emptied rapidly, but meal antigenicity persisted longer in the digestive gland. The Sepia experiments provide a basis for interpretation of results from natural predation by cephalopods).

A light and electron microscopic study of the enterocytes and gut-associated lymphoid tissue (G ALT) of the flounder Platichthys flesus showed regional differences in the structure of the epithelial cells. Epithelial villi decreased in height and spatial complexity, but increased in overall thickness in the posterior intestine. There were no specialized cells comparable with mammalian M cells. Periodic acid Schiff (PAS) activity increased in the enterocytes and goblet cells in the anterior intestine where alkaline phosphatase activity was most pronounced in enterocytes. Adjacent cells were joined by tight junctions.

Gut-associated lymphoid tissue was composed of isolated leucocytes in the epithelium and lamina propria with no accumulations. Numbers of intra-epithelial leucocytes were not statistically different in the various regions of the intestine. Five leucocyte types were distinguished among the gut-associated lymphoid tissue although lymphocytes and granulocytes were the most common. Granulocytes were positive for the Romanowsky stains with PAS/Alcian blue positive granules. Blast cells similar to large lymphocytes were unevenly distributed through the different regions of the intestine. Plasma cells were more numerous in the epithelium than in the lamina propria. Macrophages were rarely detected.

To resolve the sources of amino nitrogen which is accumulated as intracellular solute during hyperosmotic volume regulation, components of protein metabolism were monitored during compensation for a change from 15 to 30% salinity in the blue mussel, Mytilus edulis L. Net solute gain stemmed primarily from a marked reduction in total output from the metabolic pool of free amino acids, most of this ‘saving’ resulting from slower whole-body protein synthesis, and the remainder from lower nitrogenous excretion. Indeed, total inputs to the metabolic pool of free amino acids actually decreased over the period of net solute gain at 30%. Associated contributions from dietary assimilation, de novo synthesis and the direct uptake of dissolved amino acids were each negligible, indicating that breakdown products from endogenous body protein were the only significant source of amino nitrogen accumulated during hyperosmotic regulation. This accumulation represented more than 3% of soft-tissue protein nitrogen within the whole animal. We therefore impress that the excretion, during hyposaline adjustment, of all nitrogen previously accumulated as solute in response to equivalent hypersaline change, represents a major component cost of cell volume regulation, and which helps to explain stress and even mortality consequent upon what may be small but frequent fluctuations of salinity.

Rapid shell closure in articulate brachiopods, occurring by a twitch contraction of the the ‘quick’ adductor muscles, is a response to disturbance or to physiological requirements of the organism. The relative simplicity of the closing system permits a detailed analysis of the functional architecture of the mechanism and the underlying principles of skeleto-muscular organization, in terms of (1) basic kinematic properties of the system (speeds and times of closure), (2) hydrodynamic reactions resisting closure, and (3) considerations of muscle physiology and mechanics.

Analyses of shell closure in the brachiopods Terebratulina retusa from the Firth of Lorn, Scotland, and Terebratalia transversa from Puget Sound, USA, reveal (1) shell-closing times of the order of 50 to 70 ms, (2) closing velocities of the order of 3·5 radians s-1, from initial gapes of about 0·05 to 0·2 rad, and (3) muscle moment forces and hydrodynamic reactions with magnitudes of the order of 5 × 10-4 N m (5 g cm). Muscle tensions developed in the ‘quick’ adductor muscle are of the order of 105 N m2, and contraction velocities are of the order of one muscle length per second. Hydrodynamic reactions are a fundamental constraint on the closing mechanism, as determined by the concordance of actual closing events with predictions of a hydrodynamic model.

Populations of Carcinus maenas (Crustacea: Decapoda) from an estuary and from the open shore were compared for salinity tolerance and rhythmic locomotor activity. Estuarine crabs were shown to be predominantly green; red and orange individuals were rare within the estuary in contrast to the open shore. Red crabs were less able to tolerate prolonged exposure to low salinity than green crabs. Spontaneous circatidal rhythms of locomotor activity were similar in red and green forms, but red crabs from the open shore reacted earlier, and were more active upon salinity reduction, than green crabs from the estuary which also showed rapid habituation to episodes of salinity change.

Planes minutus (L.) (Crustacea: Brachyura) swims well (forwards only) but has limited endurance; at 28°C crabs swam at the surface for 35–45 min before sinking. The legs of the neuston crab are flattened and rotated into the ‘advanced’ orientation of the legs to the cephalothorax, characteristic of swimming brachyurans. All legs are involved in swimming; pairs 1 and 2 mainly support the crab in the water column, pairs 3 and 4 provide forward propulsion too. In leg-pair 4, the plane of extension and flexion is close to the horizontal and the leg provides lift on the recovery stroke. Plumose hairs are present on the anterior borders of the legs, adding to the propulsive area. Stout spines line the anterior and posterior margins of the legs; they allow the crab to hang upside down from floating substrata, but are also used to hold food items. The chelae of Planes have cutting rather than crushing ‘teeth’. Crabs of the size studied (0·128−0·736 g wet wt) used oxygen (at 30°C) at about twice the rate when swimming (mean 1·014 ml O2 g wet wt−1 h1) as did resting crabs (mean rate 0·495 ml O2 g wet wt−1 h1). No oxygen debt is incurred during swimming. The mean crab density was 1·096 g ml−1 (SD 0·060). Neustonic crabs would not move more than 5 cm away from their floating substrata to catch food. They show a novel ‘revving up’ procedure in which leg pairs 1 to 3 beat while the animal is attached to the substratum by spines on the tips of leg-pair 4. This allows much of the locomotor apparatus to overcome inertia and reach working speed, before an attack is carried out. Crabs ate postlarval and juvenile fish, euphausids, isopods, sea skaters and squid. Gelatinous animals were gener-ally avoided, though salps were captured and the stomach contents eaten. Planes will take prey as big as itself and stores food for later consumption.

A sampling series was undertaken with an opening/closing mid water trawl to assess the responses of midwater crustaceans to illumination and to compare these with previous reports resulting from the use of an open net. An electric light fitted on an RMT8 midwater trawl was switched on for alternate hauls of a series of 31 tows at a depth of 800 ± 25 m at 20°30'N 19°40'W. Turning the light on had no significant effect on the numbers or sizes of the numerically dominant species in the catches.

Physical factors affecting the fate and functional capabilities of the pedal mucus of Patella vulgata L. deposited in the intertidal have been examined. Mucus dehydration, rehydration and degradation were assessed. Mucus dehydration is dependent on ambient wind speed, temperature and relative humidity. Of these, over naturally encountered ranges, wind speed produces the greatest water loss over a 6-h period and it is surmised that mucus deposited at mid-shore will lose >60% of its water when exposed by the tide on days when the air is not still. The more dehydrated pedal mucus is, the longer it subsequently takes when immersed to reach a fully hydrated state.

The occurrence and intensity of infection of caligid species on the five species of wrasse commonly found in British waters are reported from twelve localities in the British Isles and from seven fish-farms in Scotland where wrasse are used to remove sea lice from salmon (Salmo salar L.). A low prevalence of infection was recorded for all wrasse species except ballan. Chalimus of Caligus centrodonti Baird were identified from fins of ballan, rock cook and goldsinny wrasse, this being the first host record for the latter two species. Single chalimus stages of Caligus elongatus Nordmann were identified from a ballan wrasse and a goldsinny wrasse and are similarly first records from these host species. No C. centrodonti were found on farmed salmon and conversely no Lepeophtheirus salmonis (Krøyer) were identified from wrasse kept in contact with infected salmon. This suggests that transfer of caligids between wrasse and salmon and vice versa does not pose problems for fish health on sea-farms.

The distribution, morphology and sensory cell polarity of neuromasts were followed in developing larvae and recently settled juveniles of Dover sole (Solea soled) and plaice (Pleuronectes platessa) using scanning electron microscopy. In plaice the number of neuromasts increased at a similar rate on both sides of the fish throughout development, and most head neuromasts became incorporated into canals. On the trunk presumptive canal neuromasts were outnumbered by accessory superficial neuromasts in early juveniles. In sole the number of neuromasts increased at a similar rate on both sides until the start of metamorphosis. Subsequently a proliferation of superficial neuromasts on the abocular side of the head resulted in 80% of all the neuromasts on this side of the fish being located on the head in early juveniles. Both species had developed asymmetry of the head canals by the early juvenile stage, while canal formation on the trunk commenced later on the abocular side than on the ocular side. Mean numbers of sensory hair cells per neuromast showed little change until metamorphosis in both species, after which there was a two- to three-fold increase in hair cell number. The polarities of the sensory hair cells showed more variability on the head than on the trunk in both species. In general, hair cells were aligned for maximum sensitivity along the axes of the canals which would subsequently enclose them, but in the case of the superficial neuromasts on the abocular side of the head in sole the polarities followed surface features such as the nares, papillae and fin rays.

A survey was conducted from 1986 to 1987 to determine the spatial and seasonal distribution patterns of Ichthyophonus hoferi Plehn & Mulsow, 1911 in marine fishes, especially yellowtail flounder, Limanda ferruginea, from the Nova Scotia shelf, Canada. Ichthyophonus hoferi was found in 56 of 6759 (0·83%) yellowtail flounder, in one of 613 (0·16%) haddock, Melanogrammus aelgefinus, but in none of the other 1485 fishes representing seven species sampled from this area. Within the yellowtail flounder population, the fungus was distributed in patchy manner with infection prevalence ranging from 0·4% at Banquereau Bank to about 13% at Brown's Bank. Infection prevalence was the same in both the male and female flounder. There was no apparent seasonal cycle in infection, although pathogen recruitment into L. ferruginea was from early summer through to early fall months.

A general description of the trophic inter-relationships and the food web within Bridgwater Bay, Somerset, England, is presented. This Bay is a lower estuarine site within the Bristol Channel. Particular attention is given to the use of the area by fish and larger crustaceans which are the dominant animal groups. Using data collected over a ten-year period of monthly sampling, the relative average biomass of all fish and larger crustaceans found within the Bay is calculated for each month of the year. These averages are used to produce food webs which also show species relative abundance for each calendar month.

It is shown that the trophic structure follows a seasonal cycle due to migration. The application of the concept of stability within such a dynamic system is discussed. The role of compartmentalization of the ecosystem into a number of loosely connected groups as an aid to overall stability is discussed. While there is no evidence that contemporaneous compartments exist, it is argued that inter-specific interactions are temporally limited because of the different periods of annual residence of the species. It is suggested that the stability and resilience to interference shown by these estuarine systems is enhanced by temporal partitions.

The evolution of a Phaeocystis bloom off Plymouth in spring 1990 is described. Data were obtained (1) during weekly visits to station El to measure physical parameters, nutrient concentrations, primary production and grazing pressure and (2) by regular tows of an Undulating Oceanographic Recorder around a circuit of five stations including El to detect stratification, monitor the geographical distribution of chlorophyll and determine vertical profiles at each station.

The data show that the Phaeocystis bloom emanated from near-shore, its spread towards the south-east correlating with the wind direction prevailing at the time. Due to bad weather, seasonal stratification had not been established at the time of the bloom so that a major diatom outburst had not occurred and nutrients remained high as the Phaeocystis moved offshore.

Zooplankton populations at El, averaged over 50 m, decreased whilst Phaeocystis was present, resulting in very low grazing pressure. Termination of the bloom was not, therefore, due to consumption by herbivores. It was, instead, caused by Phaeocystis colonies becoming nutrient-depleted and releasing single cells which were transported downwards. As a result of this, chlorophyll concentrations near the bottom remained high for some time after the Phaeocystis had disappeared from the surface water.

A scenario is proposed to explain the observations in terms of our current understanding of the ecophysiology of Phaeocystis.

Distributions of suspended matter, light attenuation and chlorophyll a were measured in the Shannon Estuary over the period 1988–1990. Light attenuation was found to be highly correlated with levels of suspended matter, and the availability of underwater irradiance was found to be the dominant influence on phytoplankton biomass and chlorophyll levels over most of the estuary. Though levels of suspended matter, and hence light attenuation, were highest in the upper estuary, depths of mixing were relatively shallow resulting in less light limitation of phytoplankton growth than elsewhere. Turbidity maxima in the upper estuary were associated with chlorophyll maxima, the magnitude of which appeared to be related to river discharge.

Specimens of a hydroid species previously unrecorded in Britain have been found during the course of a long-term study of the coelenterate fauna of an intertidal mussel bed in Morecambe Bay, Lancashire. Specimens were sent for examination to Professor J. Bouillon who indentified them as Rhizogeton nudum Broch.

The location and nature of the mussel bed, and the survey technique, have been described elsewhere (Jones & Miller, 1966; Jones & Clare, 1977; Jones, 1987). Rhizogeton nudum was observed in April and May 1987–91 and March 1989, on boulders where it was invariably associated with Clava multicomis (Forskal) and, more commonly, on mussel shells attached to these boulders. The few colonies recorded on boulders may have spread by stolon growth from attached shells.

Respiration rates normalized to chlorophyll a of laboratory cultures of Brachiomonas sp., Gyrodinium aureolum, Synechococcus sp. (clone WH 7803), Pycnococcus provasolii (clone W48–23) and Phaeocystis sp. showed a positive relationship with growth photon flux density (Ig). It is argued that this constitutes an energy conserving strategy of adaptation to conditions of low irradiance. The ratio of respiration to light-saturated rate of photosynthesis (R/Pmax) also showed a decrease with decreasing I. Algal cell size, within the investigated range (0·2 to 1380 pg C cell-1), showed little influence on biomass-specific respiration rates.

A new species of Fenestrulina Jullien, 1888, F. asturiasensis sp. nov., is described from the northern coast of the Iberian Peninsula. The main diagnostic features of this species are: four to five oral spines, a completely porous frontal wall and non-stellate frontal pores. These features have allowed the author to differentiate it from F. malusii (Audouin), another species of the genus Fenestrulina Jullien recorded in the same region.