Sessile organisms capable of asexual reproduction may be expected to show much genetic differentiation among local populations: mating between distant individuals is unlikely and genetic drift will contribute to differentiation since habitats can be colonized by one or a few individuals. This study investigates genetic differentiation of populations of the sea anemone Metridium senile (L.) in Great Britain. Individuals of M. senile are sessile and reproduce both sexually, by free-spawning of gametes, and asexually, by regeneration of fragments torn from the pedal disc. Metridium senile is one of the most common and widespread of British sea anemones (Manuel, 1981); reports place it at an enormous number and variety of sites around Britian (unpublished results of surveys by the Underwater Conservation Society of the United Kingdom). The extensive geographic range and variability of the species have confounded attempts to determine the taxonomic status of the ecological and morphological forms, but make it an interesting system for genetic analysis.

In some earlier papers we have discussed the distribution of water and the total amounts of the major inorganic solutes in the cells of Arenicola marina (L.) equilibrated to full-strength and to diluted sea water (Freeman & Shuttleworth 1977b, c). We concluded that adaptation to dilute media by this osmoconforming worm was dependent entirely upon the regulation of cell volume. In dilute media substantial amounts of water do enter the cells but down to about 30% sea water this is only 55% of that which would have entered if the osmotic equilibrium was accomplished by entry of water alone. The influx of water is restrained by simultaneous loss of solute, an increasing amount of solute being lost or rendered osmotically inactive as the dilution increases. For lugworms in 35% sea water, for example, the volume of cell water has doubled compared with worms in 100% sea water, but the increase would have been far greater if the intracellular solutes had not declined by 30%.

INTRODUCTION

Cephalopods may be divided into five types according to their buoyancy. Members of several families such as the Octopodidae, Loliginidae and Ommastrephidae are negatively buoyant and must swim to stay in midwater and are therefore highly muscular animals. Others have mechanisms to make them neutrally buoyant so they can remain suspended in midwater without effort. Nautilus, Spirula and cuttlefishes have low pressure gas-filled chambers and their flesh is muscular and non-buoyant (Denton & Gilpin-Brown, 1973). Squids of one family, the Gonatidae, have a low density oil in their livers to give buoyancy but most of their body is muscular. Some oceanic octopods have very watery tissues in which lighter chloride ions replace sulphate ions (Denton & Shaw, 1961). In 12 of the 26 teuthoid families the buoyancy is provided by low-density ammonia-rich solution in their body and head tissues or in an expanded coelomic cavity (Clarke, Denton & Gilpin-Brown, 1979). These ammoniacal squids are extremely abundant in the oceans of the world and form a large part of the diet of birds, cetaceans, seals and fish (Clarke, 1977). When their biomass is estimated from their utilization by predators it is important to know their properties as food and, in particular, their calorific values. As pointed out by Croxall & Prince in a review of the calorific values of cephalopods (1982), all the known values are of muscular, negatively buoyant species because they are of value as food for humans but no measurements have been made on the ammoniacal or oily species which are probably as important, or even more important, in the economy of the ocean (Clarke, 1983).

While the sprat, Sprattus sprattus (L.), may live for up to a total of six years, at the end of which time the total length can exceed 150 mm, the fishery in British waters is based primarily on the second, third and fourth year classes (Robertson, 1936, 1938; De Silva, 1973). Historically, the sprat was a major contributor to the fishery of the Bristol Channel and Severn Estuary (Matthews, 1933; Lloyd, 1942). Although this fishery has declined during recent decades, the sprat has remained an important component of the teleost fauna of the region. This point is illustrated by the observation that S. sprattus was by far the most abundant of all teleosts in plankton samples taken throughout the Bristol Channel between the spring and autumn of 1974 (Russell, 1980). While length–frequency data were provided by Russell (1980) for these recently spawned sprat and there is comparable information for those sprat which form the basis of the commercial fishery in the Bristol Channel (Lloyd, 1942), no such data are available for those members of the population which are known sometimes to enter the Severn Estuary in considerable numbers (Lloyd, 1941).

The Sr, Mg, and Ca chemistry and mineralogy of the shell, beaks, and inorganic compounds of the renal appendages of Nautilus pompilius Linné 1758 reveal a complex physiochemical system of biomineralization. The chemistry of the shell and septal aragonite is similar, and establish that N. pompilius discriminates against the concentration of Sr and Mg in sea water by 78% and more than 99% respectively. Beaks consist of high-Mg calcite (4.4% MgCO3). Renal appendages contain either aggregates of crystals (uroliths) of Mg-oxalate dihydrate with nuclei of hydroxyapatite or disassociated particles of hydroxyapatite or both. There is no evidence that uroliths or hydroxyapatite particles serve as temporary reservoirs of Ca during calcification of septa.

A high degree of variation in hydrographic conditions is found in the so-called Iroise Sea, within less than 100 km of the west coast of Brittany. Tidal current maximal velocity, especially, ranges there from about 0·5 knot to more than 8 knots (locally, near the island of Ushant), i.e. practically as wide a range as found over the whole of north-west European shelf seas. Pelagic ecosystems accordingly exhibit a high degree of variety, related not only to classical inshore-offshore gradients, but also to the extent of vertical mixing or stratification. Areas where different physical and biological conditions prevail are generally separated by rather clearcut boundaries. The better-known of these is the Ushant thermal front, which runs in summer across the whole entrance to the English Channel, but also extends into the Iroise. In addition, freshwater runoff results in thermohaline stratification, or at least in the existence of thermohaline vertical gradients, in the two major bays of the west coast of Brittany. The relevant area is limited seawards by a thermohaline front, the Iroise inner front (Grail & Le Fèvre, 1967; Le Fèvre & Grall, 1970), beyond which are found the well-mixed waters inshore of the Ushant front. Fig. 1 sums up these hydrographic patterns in the area taken here into consideration.

INTRODUCTION

G. O. Sars found the first specimens of Rhabdopleura in 1866. The specimens were found by dredging at 220 m at Skraaven in the Lofoten Islands. However, the first Rhabdopleura was described by Allman in 1869 (a, b, c) which he called Rhabdopleura normani in honour of A. M. Norman who together with J. G. Jefferys found them while dredging at 165 m off the Shetland Islands in 1868.

Despite concern over declining catches of bass, Dicentrarchus labrax (L.), and disagreement over the cause, overfishing or erratic year classes (Kennedy & Fitzmaurice, 1972; Holden & Williams, 1974; Kelley, 1979), little is known of the early life history of this fish in the wild. Only occasional post-larvae have been recorded previously in young fish collections off Plymouth (Russell, 1935). These were caught between March and June, 2–24 km offshore and commonly below 20 m. The marine spawning grounds in the Plymouth area are unknown. Although Kennedy & Fitzmaurice (1968) found bass eggs at Splaugh Rock and Youghal Harbour on the south coast of Ireland and Barnabé (1978) observed spawning near the breakwater at Sète in the Gulf of Lions, bass eggs have not previously been found near Plymouth. Hartley (1940) recorded the growth and diet of bass in the Tamar Estuary. He noted that adults were very rare in the estuary which was used only as a nursery ground for this species.

The sand smelt (Atherina presbyter Cuvier) is a small, pelagic inshore fish common on some coasts of Britain. The biology of the sand smelt population in Southampton Water, Hants., U.K., has been described in detail (Turnpenny, Bamber & Henderson, 1981), but individuals less than 3 months old were not found during the course of that study.

At Aberaeron, Dyfed, mid-Wales, the two trochid species, Monodonta lineata and Gibbula umbilicalis breed during the summer months. M. lineata showed a discrete spawning period in late July in both 1980 and 1983, whilst G. umbilicalis spawned in August in both years, though with gamete release prolonged into September in 1980. In both species gonad development begins in April/May, and by June the characteristic bimodal oocyte diameter distribution of mature females has been established. Vitellogenesis and the development of a jelly coat around the oocyte begin when the oocytes have a diameter of 70–80 μm, with mature oocytes reaching 140–180 μm in diameter, excluding the jelly coat.

Eggs of Dicentrarchus labrax (L.) have been collected from the mouth of rivers or inshore waters of salinity greater than 30 p.p.t. (Barnabé, 1978; Kennedy & Fitzmaurice, 1972). The main spawning period is between April and June (Kelley, 1979; Kennedy & Fitzmaurice, 1968), though ripe males have been reported as early as February and as late as mid-July (Kennedy & Fitzmaurice, 1972). The fertilised eggs 1·15—1·51 mm in diameter, are pelagic, take between two and five days to hatch and the prolarvae are 3·25—4·05 mm in length when they emerge (Barnabé, 1976; Bertolini, 1933, pp. 310–331; Lumare & Villani, 1973; Jackman, 1954). Postlarval stages have been recorded from the Bristol Channel in mid-June and mid-July (Russell, 1980) as well as off Plymouth between mid-March and mid-July (Russell, 1935, 1976; Russell & Demir, 1971). Although a few juveniles have been encountered, from the middle and lower reaches of estuaries, during their first summer (Kennedy & Fitzmaurice, 1972), it is not until the autumn/winter that relatively large numbers of o + bass have been reported from British estuaries (Claridge & Potter, 1983; Hardisty & Huggins, 1975; Hartley, 1940; Murie, 1903; Van den Broek, 1977; Wheeler, 1979). Juvenile bass have also been recorded from lagoons and coastal waters, during their first summer, indicating that these are important nursery areas (Arias, 1980; Barnabé, 1978; Chervinski & Lourie, 1972; Ferrari & Chieregato, 1981; Gandolfi, Rossi & Tongiorgi, 1981).

Studies concerned with the sub-lethal aspects of the toxicity of copper to the mussel have, in many cases, been compromised through the use of static experimental systems (see Davenport & Redpath (1984) for review). Manley, Gruffydd & Almada-Villela (1984), using a through flow system, reported a significant reduction in shell growth at a concentration of 10 μg I-1 added copper with recovery after metal addition had ceased. Strömgren (1982) investigated the effects of a series of concentrations of the metal (amongst others) on Mytilus, but there are discrepancies between his results and the work of others which have been fully discussed elsewhere by Davenport and Redpath.

In a recent paper we reported on the discovery of an adhesive antennular secretion used by exploring cyprids of Balanus balanoides (L.) and hinted at its possible implication in settlement behaviour (Walker & Yule, 1984). This secretion is thought to be modified integumentary protein. Since the integumentary protein of adult B. balanoides is widely accepted as a powerful stimulus to settlement (Knight-Jones, 1953; Crisp & Meadows, 1962, 1963; Larman, Gabbott & East, 1982), we devised settlement experiments in May 1984 to investigate whether cyprid antennular secretion acts in a similar manner. The outcome of these experiments with B. balanoides cyprids is presented here.

Recruitment of Semibalanus balanoides at Robin Hood Bay, North Yorkshire, between 1969 and 1981 ranged from almost total failure to ca. 30 spat/cm2. A similar annual pattern over 100 km of adjacent coast appears to implicate climatic or hydrographic factors. Adult fecundity did not vary greatly, but as there was substantial variation in the number of cyprids reaching the shore, events during the planktonic phase (particularly wind-driven larval dispersion) appeared to be of prime importance.

Thereafter, settlement density was modified by cyprid behaviour, except when larvae were very abundant, by the availability of space and the temporal pattern of the settlement. Mortality during early shore life was highly variable, photographic studies in 1981 showing between 1% and 92% of settlers dying within ten days. Under highly desiccating conditions 70% of a single day's input to the upper shore died in the first 24h. Such severe mortality could be compensated for by later inputs as settlement extended over 4 weeks or more, primarily as a result of unexpectedly long and asynchronous release.

Cyathura is a genus of Anthurid isopods represented by some fourteen estuarine species around the world (Miller & Burbanck, 1961). They are unusual amongst the isopods in that the majority of individuals in a population show protogynous hermaphroditism, adults breeding as females in their first year, then changing to males over the following winter (Legrand & Juchault, 1963; Burbanck & Burbanck, 1974). The adults are tubicolous, virtually incapable of swimming, and have limited mobility, resulting in disjunct, stable populations, some of which, in the case of C. polita (Stimpson), are known to have maintained the same locations and densities over a period of 20 years (Burbanck & Burbanck, 1975). The embryos develop within a marsupium or brood pouch on the thoracic somites of the female, and are released as interstitial juveniles 1 mm or more in length. With populations averaging 200 to 400 individuals per square metre, these animals are convenient for autecological studies. Indeed, Burbanck & Burbanck (1979) considered that as hardy, euryokous animals, consistent in their tolerance to fluctuating environmental conditions, and convenient for study, Cyathura species are ideal ‘indicator organisms’ in estuaries.

The genus Fucus L. is widely distributed through the cool temperate waters of the north Atlantic and north Pacific oceans. The very large number of sub-generic taxa (> 100) previously recognized was reduced by Powell (1963) to only 6 species. In his revision, Powell recognized F. distichus L. emend. Powell as the most variable of these. He classified the intra-specific variation in this species into 4 sub-species: F. distichus distichus, F. distichus anceps (Harv. et Ward ex. Carruthers) Powell, F. distichus evanescens (C.Ag) Powell and F. distichus edentatus (Pyl.) Powell. Because of reports that intermediate forms existed in Iceland (Jonsson, 1903) and the Faeröes (Börgesen, 1902), Powell (1957 a,b) decided to use sub-specific status rather than specific designation for these 4 taxa. However, within North America the 4 sub-species of F. distichus were often assigned specific rank (e.g. Taylor, 1957).

The distribution patterns of many marine benthic invertebrates are not well known, and when records exist they are usually in the form of presence and absence data with little or no information on the relative density of populations from area to area.

This paper is the third and last in a series employing cohort growth-analysis techniques to describe the secondary production of the major component species in the macrobenthic communities of the Bristol Channel. Previous studies were made in soft-bottom areas of a Venus community (Warwick, George & Davies, 1978) and an Abra community (Warwick & George, 1980). The hard-bottom areas of the Channel support what might classically be denned as a Modiolus community (sensu Thorson 1957; see Warwick & Davies, 1977), but within these communities Modiolus itself is inconspicuous because of its small size. This community is found in its purest form on the rock-floors of the central region of the Channel, but gravel accumulates in areas of slower current seaward of this region and may often have admixtures of finer sediment accommodating a mixed-bottom subgroup of the Modiolus community, containing additional species more typical of soft bottoms. The intention of this study was to examine a site representative of the community in its purest form. Quantitative grab sampling in the rock-floor areas was naturally impossible, but fortunately areas exist in which extensive reefs of the tubeworm Sabellaria spinulosa Leuckart are found, and the structure of these reefs is sufficiently brittle that quantitative grab samples can be collected. Such sites have a more diverse fauna than the open rock areas, the interstices of the reef providing a refuge for a large number of smaller species.

The behaviour of herring subjected to stationary and vibrating obstacles in their swimming path was recorded in daylight and darkness using an infra-red sensitive TV system. The herring avoided stationary obstacles using visual stimuli and usually collided with such obstacles in darkness. They showed strong avoidance to a continuously vibrating sound source in darkness but the speed of turning was relatively slow and there was no evidence for habituation. As they approached the source they usually responded when the sound pressure reached 10–20 Pa, about 70 dB above the threshold. Particle velocity within the lateral line canals was calculated and was 40–60 dB above threshold. Herring seem to be able to adapt their avoidance behaviour to suit the urgency of the situation.

Observations on the density of pelagic marine fish eggs have indicated that they are near neutral buoyancy for most of development and have a tendency to become denser towards hatching (see Russell, 1976). However, most of this information has been based on limited laboratory observations on the relative buoyancy of eggs; only a few experiments have given more precise estimates of the absolute density of eggs based on the salinity of water in which they are neutrally buoyant (e.g. Franz, 1910; Sundnes, Leivestad & Iversen, 1965). Recently, a new technique (Coombs, 1981), based on the use of a stable column of continuously graded sea-water concentration has allowed more precise and routine measurements of the density of fish eggs throughout development.