At Looe, Cornwall (south-west England), engineering problems have resulted in the Sewage Treatment Works operating on saline rather than the typical freshwater sewage input. Saline water enters the system at all stages of the tide, except low water, mainly through the open-jointed sewers embedded in the mud of the Looe River Estuary, and causes a cyclical salinity regime within the works (Jones & Johnson, in press). A consequence of this seawater intrusion is the replacement of the freshwater macroinvertebrate community commonly associated with biological filters by two species of euryhaline amphipods. In particular, Gammarus duebeni Liljeborg is permanently established at various sites in the works including the percolating filters and humus tanks (Jones & Wigham, 1988). The ability of G. duebeni to colonise and establish a breeding population at the works is probably a reflection of its wide tolerance to several environmental factors. This species is extremely euryhaline (Forsman, 1951; Sutcliffe, 1967; Pinkster, 1970) and can withstand marked hypoxic exposure (Bulnheim, 1979; Ritz, 1980). However, in addition to fluctuations in salinity and oxygen saturation, treatment works amphipods are exposed to relatively high levels of heavy metals associated with domestic and industrial inputs (Jones & Johnson, in press). Within the humus (secondary settlement) tanks, zinc concentrations are particularly elevated compared with the adjacent estuary, although there are considerable fluctuations in zinc levels experienced by animals due to the tidally based dilution of the incoming sewage (Jones & Johnson, in press).

The caridean prawn Palaemon longirostris (Milne-Edwards) (Palaemonidae) occurs predominantly in the upper regions of large river estuaries in Europe, and reaches its northern limit of geographical distribution along southern parts of Britain (Smaldon, 1979). All post-larval stages are thought to live under dilute and fluctuating salinity conditions, although there are conflicting reports as to whether ovigerous females need to migrate to the sea at times of egg hatching (Gurney, 1923; Fincham & Furlong, 1984). The species has been known to be extremely euryhaline for some considerable time (Gurney, 1923), yet the salinity tolerance limits of larval (Antonopoulou & Emson, 1989) and post-larval stages (Campbell & Jones, 1989) have been established only recently. Apart from studies on salinity tolerances, and some preliminary measurements of blood and urine osmolality (Parry, 1957), the osmoregulatory adaptations of P.longirostris to its dynamic habitat have not been investigated previously. The present paper describes the osmoregulatory responses of P.longirostris acclimated to a wide range of salinities, and also reports on the ability of individuals to compensate for sudden acute salinity change.

Anemones of the genus Actinia exhibit a well documented, stereotyped aggressive sequence (Bonnin, 1964; Brace & Pavey, 1978; Brace et ah, 1979) involving specialized, nematocyst-bearing structures called acrorhagi, which can be directed against an opponent. Experiments on the Australian species, A. tenebrosa Farquhar, and on the clonal anemone, Anthopleura elegantissima (Brandt) (which similarly bears acrorhagi), indicate that aggression occurs principally between genetically dissimilar (allogeneic) opponents (Ayre, 1982; Francis, 1973a, b; Bigger, 1980). Once acrorhagial contact has been made with an allogeneic individual, massive discharges of nematocysts ensue, which can cause severe necrotic lesions.

Infestation of cage cultured Atlantic salmon by the external parasitic copepods Caligus elongatus (Nordmann) and Lepeophtheirus salmonis (Kröyer) is a serious cause of loss of production in the commercial sea water culture of this species. The copepods feed on the mucus, skin and blood of their hosts (Kabata, 1974; Brandal et al., 1976) causing irritation and lesions. Loss in production due to infestation by lice occurs directly by the mortality of fish from osmotic shock and indirectly from a probable reduction in growth, from secondary infections such as vibriosis (Wootten et al., 1982) or by increasing vulnerability to ultraviolet radiation damage (McArdle & Bullock, 1987).

Studies on the lateral lines of the sprat {Sprattus sprattus (L.); Clupeidae; Clupeiformes) have shown that the patterns of excitation of the neuromasts in the canals on the head changed dramatically with relatively small changes of position of the fish with respect to a source (Denton & Gray, 1983; Gray, 1984). It was concluded that, in the case of the sprat, this sensory system was capable of providing the information needed to maintain position in relation to other fish in a school; cf. the work of Partridge & Pitcher (1980) on saithe (Pollachius virens; Gadidae; Gadiformes). The structure of lateral lines varies greatly between fish of different species and even between different parts of the same fish. It seems likely that these differences have evolved to meet different behavioural needs. An attempt has been made to understand the mechanisms of lateral line excitation in general. This has led to the development of methods to calculate first approximations of likely excitation patterns from a knowledge of the morphology of particular systems (Denton & Gray, 1988).

One of the more recent aims of deep-sea biological investigations has been to assess the rates and processes involved in the recolonisation of deep-sea sediments by the in situ fauna (Grassle, 1977; Desbruyères etal., 1980, 1985; Levin & Smith, 1984). The spur to such initiatives has been the prospect of deep-sea mineral exploitation and the dumping of radioactive and other chemical wastes (Desbruyères etal., 1 985), in addition to the testing of hypotheses about deep-sea community regulation (Levin & Smith, 1984; Smith, 1986). These experiments have shown that perturbated or defaunated sediments incubated for periods of several months are readily recolonised by deep-sea animals, although the process is much slower than in comparable shallow-water situations (e.g. Levin, 1984; Zajac & Whitlach, 1982a, b). Furthermore, the resulting community of colonists may be quantitatively and qualitatively different from the ‘background’ fauna (Grassle, 1977; Levin & Smith, 1984). Similar experiments have examined the effect of large ‘food-parcels’ on the in situ sediments and fauna (Smith, 1986), and a review of the responses of benthic faunas to disturbed sediments has been published by Thistle (1981).

Corophium bonnellii (Milne-Edwards) has been suspected of being parthenogenetic for more than fifty years (see Crawford, 1937) and this suggestion has been supported by recent breeding experiments (Moore, 1981). Many thousands of specimens have been examined by taxonomists and ecologists from a wide geographical area in both hemispheres, yet only Toulmond & Truchot (1964) have claimed to have found a male. No description or figures were presented by Toulmond & Touchot (1964) to support their contention.

It is well established that the lipid content of the herring Clupea harengus from the waters around the British Isles and Ireland undergoes a marked seasonal cycle, being highest during the summer months of active feeding and lowest after a winter fast (Wood, 1958; lies & Wood, 1965; Lovern & Wood, 1937; Molloy & Cullen, 1981; Wallace, 1986). In the case of spring-spawning herring, the minimum lipid content coincides with the spawning period and the depletion of lipid over winter corresponds with the development of gonads (Lovern & Wood, 1937).

The common dogwhelk (Nucella lapillus) is a dioecious species. However, on exposure to low concentrations (<1 ng I-1, Gibbs et al., 1987) of tributyltin (TBT) females develop certain male sexual characteristics, notably a penis and vas deferens. This phenomenon, first reported by Blaber (1970) in dogwhelks from the Plymouth area, and later termed 'imposex' (Smith, 1981), is now recognised as the most sensitive and straightforward way to identify contamination of coastal areas by TBT. The degree of imposex may be readily quantified as the relative penis size index (RPSI), which expresses the relative bulk of the penes in females and males from a given sample. It is calculated as (mean cubed female penis length/the mean cubed male penis length) x 100% (Gibbs et al., 1987). Gibbs et al. (1987) have examined the development of imposex in greater detail, and have divided the process into six stages (known as the vas deferens sequence, or VDS) based upon the degree of development of the vas deferens and penis in the female animal. This classification enables an assessment to be made of the reproductive ability of an animal. Stages 1 to 4 show a progressive development of the penis and vas deferens. At a VDS of 5, vas deferens tissue proliferates over the opening of the reproductive tract, blocking it, and subsequently causing sterility due to the inability of the animal to release egg capsules. Aborted egg capsules then gradually accumulate in the capsule gland (stage 6). The vas deferens sequence index, VDSI, is the mean VDS in a given sample of females.

The marine mussel, Mytilus edulis, is a widely distributed bivalve, especially abundant on wave washed exposed rocky shores (Lewis, 1964). Investigations on the reproductive and settlement cycles of M. edulis in Irish waters have concentrated to date on sheltered shore populations (Wilson & Seed, 1974; Seed & Brown, 1975; Rodhouse et al., 1984; McKenzie, 1986). An exception to this is a brief account of settlement in Bantry Bay (Cross & Southgate, 1983). Elsewhere in Europe, investigations on the biology of exposed-shore mussels is restricted to the extensive studies of Seed (1969) on the north-east coast of Britain.

The demographic parameters of a population (the number of age-classes present; growth rates; mortality as a function of age and recruitment levels) are of considerable interest to marine biologists. If individuals can be aged from growth rings in their hard parts, then the estimation of demographic parameters is relatively straightforward. If this is not possible, the next best alternative is to tag or mark individuals and use data on the recapture of these to give the information required. For many marine invertebrates, neither of these options is practical and we must resort to estimating the demographic parameters by making assumptions about recruitment and the size variation between individuals of the same age and then infer the age structure of the population from its size structure. This was first done by Petersen (1891) who interpreted each mode on a size/ frequency histogram as representing a single age-class. More recently, extensive use has been made of methods which assume that the sizes of individuals of the same age will be normally distributed. The size/frequency histogram can then be decomposed into a number of normal distributions, each of which represents a single age-class. This can be done graphically (Harding, 1949; Cassie, 1954; Bhattacharya, 1967) or with computerbased numerical methods (Macdonald & Pitcher, 1979). The graphical methods seem to be the most popular and are frequently taught to undergraduate students. The same methods can be used to dissect a size/frequency distribution into components other than age-classes (Harding, 1949), but the principles are the same.

The horse-mussel Modiolus modiolus (L.) is a bivalve molluse inhabiting coastal marine environments down to about 200 m, where it may occur in very large communities (Wiborg, 1946; Tebble, 1966; Comely, 1978). It can also be found low in the intertidal zone where it lives in rock pools or in the hold-fasts of Laminaria (Wilson, 1977; Davenport & Kjørsvik, 1982). It is a boreal species. In the Atlantic it ranges from the White Sea to the Bay of Biscay, off Iceland, the Faroes and down the east coast of North America to North Carolina. In the Pacific it occurs from the Bering Sea to Japan and California (Wiborg, 1946). Comparatively little work has been done on Modiolus modiolus, probably because it is of little commercial importance and lives predominantly subtidally in relatively inaccessible environments.

More than 65% of the demersal fish and shellfish caught by British vessels are landed at Scottish ports, most of the catch being taken from waters north of 56°, i.e. in the northern North Sea. The major resource supporting these fisheries is the benthos, but the distributions of both infaunal and epifaunal benthic invertebrates in the North Sea are poorly documented. Moreover, the environmental factors determining the composition and structure of North Sea benthic assemblages are not well understood. There have been several limited surveys of the benthos (Stephen, 1934; Ursin, 1960), and recently Dyer et al. (1982, 1983), Cranmer (1985) and Cranmer et al. (1984) attempted a more quantitative and systematic survey. Various schemes have been proposed to explain observed regional differences in types of benthic assemblage, usually based on the biological characteristics of water masses, and / or a limited and fragmentary knowledge of the sediments of the North Sea {e.g. Glemarec, 1973; Dyer et al, 1983).

Experiments on intact retinae from the eyes of the deep-sea fish Diretmus argenteus show that the region of the retina that receives light from above and possesses very long rods has different photosensitive pigments from the region that receives light from below. In both of these regions the retina has several banks of rods. The optical densities of the photosensitive pigments in the retina at the peaks of absorption are almost the same (about one) in these two regions. However, the region of the retina with very long rods also contains a stable yellow pigment that absorbs heavily in the blue and near ultra-violet (optical density about three at the wavelength 390 nm). If, as seems certain, this stable pigment is largely in the long rods it will filter the light reaching the other layers of rods and act, for this region, as yellow lenses do for whole retinae in other fish. The lens of Diretmus was shown to be transparent in the visible and near ultra-violet.

The disposal of wastes to sea is regulated internationally by the Oslo and Paris Conventions, which were ratified by the United Kingdom in 1975 and 1978 respectively. Under these Conventions the direct disposal of certain substances, for example mercury and cadmium compounds, is prohibited, unless they are present in wastes as trace constituents. Of particular concern in the case of mercury is its possible incorporation in the aquatic food chain and transfer to man via marine foodstuffs. There are many examples of accumulation of mercury by fish in contaminated areas e.g. Liverpool Bay (Norton & Murray, 1983; Franklin, 1987); Haifa Bay (Hornung & Krumgalz, 1984); Minimata Bay (Fujiki, 1980) and in river systems such as the Ems (Essink, 1980,1988).

Macoma carlottensis Whiteaves, 1880 is abundant (up to approximately 350 nr2) in subtidal infaunal communities on the continental shelf of western United States (Lie & Kelly, 1970; Word, 1979) and in British Columbia fjords (Ellis, 1969). Of thirteen different species of Macoma identified from the coastal waters of southern British Columbia, M. carlottensis is the most tolerant to variations in water depth, sediment particle size, current exposure, and salinity (Dunnill & Ellis, 1969). Observations of the species maintained in sediment trays in our laboratory suggest thatM. carlottensis is an infaunal, primarily sub-surface deposit feeder which generally lies at a depth of 1-4 cm with its left valve downwards. Filter feeding was observed in some instances as indicated by extension of the siphons from 0-5-3 cm above the substrate.

Egg-producing Lernaeocera branchialis (L.) are usually found in the gill cavity of cod (Gadus morhua (L.)) and whiting (Merlangius merlangus (L.)) (Kabata, 1979). The early larval development of the copepod includes two free-swimming nauplius stages followed by the first infective stage, the copepodite (Sproston, 1942). On an intermediate host, usually flounder (Platichthys flesus (L.)), the copepodite moults into a chalimus larva which is attached by its frontal filament to the gill tips of its host. After four successive chalimus stages, the parasite reaches maturity and copulation occurs on the flatfish host.

The periodicity of microgrowth patterns observed in thin sections and acetate peel replicas of radial sections of bivalve shells are well documented for both intertidal and subtidal species (Pannella & MacClintock, 1968; Rhoads & Pannella, 1970; Evans, 1972, 1975; Kennish, 1980; Richardson et al., 1979 et seq.; Richardson, 1987a, b, 1988, b). In many species daily or tidal lines are well preserved in the shell, for example, the hard-shell clam Mercenaria mercenaria (Pannella & MacClintock, 1968), the cockle Cerastoderma edule (Richardson et al., 1979) and the clam Tapes philippinarum (Richardson, 1987a, 1988b), while in others they are poorly preserved or lacking. Such an example is the common mussel Mytilus edulis (L.) where fine growth lines seen in acetate peel replicas have previously been reported to be poorly preserved, with long continuous sequences of such lines seldom occurring (Lutz, 1976). The angular relationship of growth lines to the growing margin and to the crystalline elements of the shell structure are presumed to be the factors influencing the resolution of growth increments (MacClintock, 1967; Pannella & MacClintock, 1968). Pannella & MacClintock (1968) suggest that the larger the angle between depositional surfaces and the structural elements of a shell layer the more evident will be the growth increments. Both oysters and mussels have an unfavourable angular relationship between growth increments and the margin of the shell (Pannella & MacClintock, 1968) and this may explain why the growth lines are difficult to observe. The present paper describes improved methods used to examine the internal micro478 growth patterns in the shell of the common mussel Mytilus edulis and reports on experiments to investigate the periodicity of the patterns.

Many field observations have been made on predation by a variety of fish species on the demersally spawned eggs of the Atlantic herring (Clupea harengus L.) (Dunn, 1898; Bowman, 1922; Hempel & Hempel, 1971; Toresen, 1985). Field observations on the predation of herring larvae by fish are comparatively rare although jellyfish medusae have been identified as larval predators (Van der Veer et al, 1983; Moller, 1984). Recently several experiments have been carried out on the predation of medusae and other planktonic organisms (Bailey & Batty, 1983) and of fish on herring larvae (Christensen, 1983; Fuiman & Gamble, 1987). Despite the lack of field observations Hunter (1984) considered that predation, most probably by fish, was the major cause of mortality in fish larvae.