INTRODUCTION

In the central regions of the English Channel surface tidal streams are strong, often exceeding 160 cm s−1 at peak springs, with maximum values of over 250 cm s−1 off the headlands of Portland Bill and Cap de la Hague (Sager & Sammler, 1975). Correspondingly high values for bottom stress (Pingree & Griffiths, 1979) result in a scoured sea-bed covered with a minimum of sediment, the underlying bedrock being exposed in places. Study of current-generated bedforms in relation to the prevailing tidal currents has indicated the presence of a bed-load parting zone across the Channel from the Isle of Wight to Cap de la Hague (Stride, 1963; Johnson et al. 1982).

The morid fishes are important members of the deep-water associations of fish on the continental slope of the Rockall Trough. Three species, Lepidion eques, Halargyreus johnsonii and Mora moro, are distributed between about 500 and 1250 m while Antimora rostrata has its centre of abundance between about 1750 and 2200 m. Aspects of their biology including changes in abundance and length composition with depth, reproduction and fecundity and age composition are described.

INTRODUCTION

In a trawling survey of the bottom-living fish of the Rockall Trough at depths from about 400 to 2900 m, 4 species of the family Moridae were sampled. Three species, Lepidion eques, Halargyreus johnsonii and Mora moro were distributed on the upper and mid slopes while a fourth species Antimora rostrata occurred on the lower slope and continental rise. Several different trawls were used to sample the fish populations of the slope and Gordon & Duncan (in the Press) have discussed the relative selectivity of each of these nets for sampling different species of fish. In many of the bathymetric zones fished morids were important both in terms of biomass and abundance. Some preliminary observations on aspects of the biology of Lepidion eques have been published (Gordon, 1979 a, b) and Mauchline & Gordon (1980, 1984) have described the diets of the morid fish from the Rockall Trough in some detail. In this paper other aspects of their biology will be described.

The European sea-urchin, Echinus esculentus L., occurs from Finnmark in the north to the Canaries in the south. Around Britain, it is found from the Isle of Portland, English Channel, throughout most of the fully saline waters of the western and northern coasts, and in the North Sea as far south as Flamborough Head. Though it occurs between tide-marks in a few places, it is generally common from about 5 m depth below c.d. to about 100 m, though it is recorded from about 200 m depth (A. M. Clark, personal communication). It is a principal browser on the reefs on which it occurs, and feeds preferentially on kelp sporelings and other algae when it occurs in the photic zone, and on encrusting invertebrates and protochordates below the depth at which algae occur (De Ridder & Lawrence, 1982). E. esculentus is said to migrate inshore prior to spawning, and then retreat to deeper water in late spring (Elmhirst, 1922; Stott, 1931; personal observation), but a rigorous study of these apparent movements has yet to be undertaken. Echinoids fare better on a diet principally of algae than on one of animal material (Lawrence, 1975), so it might be assumed that the winter inshore migration could bring an urchin into a nutritionally richer area for the final developmental phase of gametogenesis.

In common with many teleosts, herring show marked ‘fast startle’ responses when subjected to sound stimuli from a vibrating source (Blaxter, Gray & Denton, 1981). These comprise a sharp C-shaped contortion of the body; the fish respond to transients of as little as half a cycle of sound with a latency of about 25 ms and the turn is usually away from the sound source. There is good evidence that the Mauthner cells are implicated in other species (see Eaton & Hackett, 1984) but this has not been established in the herring, although they are known to be present.

The lateral-line system of water-living lower vertebrates is provided with mechanoreceptors enabling the animals to detect water displacements, either caused by moving objects such as prey, predators or neighbours in a school or by deformations of pressure waves from the swimming animal caused by other objects. Cyclostomes, some fish and water–living amphibians have their lateral-line organs situated superficially in the epidermis as free neuromasts, while most fish besides these neuromasts possess a canal system in the dermis. Ordinarily the lateral line canal system consists of a few canals on the sides of the head and a trunk canal. In herring, however, the canal system is confined to the head and opercule. It forms a very richly branched system with numerous pores which connect the canal fluid with the surrounding sea water.

Goldman, McCarthy & Peavey (1979b) argued that growth rates of phyto-plankton in apparently oligotrophic ocean waters may near maximal. Their hypothesis was succinctly restated by Goldman (1980): ‘…the chemical composition of phytoplankton is extremely variable under exacting laboratory conditions of nutrient limitation and approaches the ‘Redfield’ proportions (C:N:P of 106:16:1) when neither nitrogen nor phosphorus is limiting so that near maximal growth rates are attained. In marine surface waters the chemical composition of particular matter often is in the Redfield proportions, thus implying that natural phytoplankton growth rates may be close to maximal.’ We argue on theoretical, experimental and observational grounds that this implication may not necessarily be correct.

The observed diminution in photosynthetic efficiency upon vitamin B12 limitation in Monochrysis indicates that nutrient limitation may profoundly influence the photosynthetic status of a natural population and must therefore be taken into account in productivity studies (Droop et al. 1982). It also suggests that photosynthetic efficiency could be used as a criterion between nutrient and light control of growth in natural populations. However, measurement of photosynthetic efficiency is cumbersome under field conditions and it would be advantageous if some other associated variable more quickly measured, such as, e.g. in vivo chlorophyll a fluorescence, could be used as a reliable indicator.

Crustacean cuticle consists essentially of chitin impregnated and coated with protein which is tanned with quinone (Dennell, 1947a). The outer surface is most heavily tanned, and the cuticle is further strengthened by calcification. The various theories as to the mechanism of calcification in crustacean and other biological material have been reviewed briefly by Digby (1967). Most appear unsatisfactory for various reasons, and evidence was outlined that calcification might arise from the formation of base by processes which are essentially electrochemical in origin. The quinone-tanned protein of the cuticle is electrically semiconducting and supports electrode action in suitable gradients of potential (Digby, 1965), and small potential differences may arise by diffusion or by active processes. Thus the deposition of calcareous salts might arise partly at least by action comparable to that which takes place at a metallic cathode. In support of this, the position of the initial calcareous deposits in Carcinus maenas (L.) was found to change with the gradient of sea-water salinity in the manner expected if some control were exercised by diffusion potentials, acting across a thin semiconducting layer to generate small changes of pH (Digby, 1968).

During a study of the reproductive cycle of a subtidal population of Chaetozone setosa Malmgren at an 80 m station off the Northumberland coast (Christie, 1981), the discovery of morphological and reproductive differences in an intertidal population lead to an extended comparative study of three Northumberland populations. The two additional populations chosen for comparison are situated at Low Newton by the Sea and Holy Island, Northumberland. All three populations are located within a maximum distance of 65 km from each other which makes these populations very close on a geographical scale.

Whereas there is a wealth of literature on the feeding of herbivorous holo-plankton, notably copepods, feeding by herbivorous larvae of benthic animals has been somewhat neglected. This paper considers the functional feeding response of veliger larvae of Ostrea edulis L., Crassostrea gigas (Thunberg) and Mytilus edulis L. fed on several micro-algal species. It also provides some data on the influence of temperature on feeding, and assesses the energy needs of the larvae in relation to their potential ingestion rate and to the availability in the sea of micro-organisms on which they feed.

Apart from isolated records of species of the genus Protankyra and a brief reference to a ‘Leptosynapta sp.’ from 1985 to 2360 m depth from the northern Bay of Biscay (Sibuet, 1977), the holothurian family Synaptidae is unknown in the deep sea. Yet this family of small, deposit-feeding forms are often numerous in soft sediments of shallow water. The present paper describes two previously unknown species of synaptid discovered from recent benthic sampling in the Rockall Trough (N.E. Atlantic). These species belong to the genus Labidoplax, four other species of which have been described from coastal areas in the north eastern Atlantic.

The English Channel was the site of the discovery of the phylum Kinorhyncha in 1841 by the French zoologist Felix Dujardin. Although initially reported from St Malo, Rennes, and Paris by Dujardin (1851), St Vaast la Hougue, France was the site from which the first species, Echinoderes dujardinii Claparede, 1863 was described 12 years later. Since then, E. dujardinii has been reported from many localities in northern Europe as well as from the Mediterranean, the Black Sea, and the Canary Islands (cf. Higgins, 1983). Reports of this species from various North Pacific Ocean localities are either incorrect or higher questionable (Higgins, 1983).

There are five common species of wrasse (Labridae) in British coastal waters, two of which are hermaphrodite, (Labrus bergylta Ascanius and Labrus mixtus L.) and three are gonochorists with discrete sexes (Ctenolabrus rupestris) (L.), Centrolabrus exolelus (L.) and (Crenilabrus melops (L)) (Dipper & Pullin, 1979; Dipper, 1981). L. bergylta (Sjolander, Larson & Engstrom, 1972; Potts, personal observation) and L. mixtus (Wilson, 1958; Potts, 1984) lay their eggs in shallow depressions excavated in coarse paniculate substrates often closely associated with sublittoral reefs. Ctenolabrus rupestris, the smallest of the species is most usually associated with sublittoral crevices, and is the only one to produce pelagic eggs despite showing territorial aggression in the spawning season, a behaviour mostly confined to species with demersal eggs. Little is known about Centrolabrus exoletus, but from the striking sexual dichromism at maturity and its association with rocky reefs it may be supposed that it exhibits some form of parental care for the demersal eggs (Potts, 1984). Crenilabrus melops shows parental care of its eggs which are laid in large and complex algal nests, a feature which is common to all this genus (Fiedler, 1964; Quignard, 1966; Potts, 1984). The corkwing wrasse is one of the commonest coastal species which at maturity show sexual dimorphism and dichromism, a feature which led to taxonomic confusion among earlier workers and which is discussed by Potts (1974).

Pelagic zooplankton and fish release faecal material which rapidly sinks through the water column and contributes various amounts and types of organic and other chemical constituents to bottom sediments. The major importance of this process in linking biological events in the water column with inputs to sediments has been well demonstrated in a number of sea areas through the use of field experiments with pumps or traps (Bishop et al. 1977; Gagosian, Volkman & Nigrelli, 1983; Honjo, 1978; Knauer, Martin & Bruland, 1979; Prahl, Bennett & Carpenter, 1980; Wakeham et al. 1980; Wakeham, Farrington & Volkman, 1983). In general, field studies have shown that the deposition of organic matter of pelagic origin occurs with considerable alterations both quantitative and qualitative (Gagosian, Nigrelli & Volkman, 1983; Prahl et al. 1980; Wakeham, 1982). Such studies, however, have not clearly defined how individual biological processes competing in the water column influence the overall composition of organic matter in vertically transported paniculate material.

Symbiont-bearing solitary and colonial radiolaria occur abundantly in diverse water masses throughout the major oceans of the world, including oligotrophic surface water (e.g. Strelkov & Reshetnyak, 1971; Casey, 1971, 1977; Anderson, 1983; Swanberg, 1979, 1983). Studies of their nutrition (Anderson, 1978, 1983; Swanberg, 1983) suggest that the algal symbionts may serve a substantial role in sustaining host nutrition. Our studies on the role of symbiotic algae in radiolaria have shown that Amphidinium-type, dinoflagellate symbionts (Anderson, 1983, p. 118) associated with the solitary species, Thalassicolla nucleata, and a colonial form Collosphaera sp. contribute photosynthetically-derived carbon to the host. The amount assimilated by T. nucleata is proportional to the primary productivity of the symbionts (Anderson, Swanberg & Bennett, 1983 b). Little is known, however, about the effects of environmental variables such as light intensity and quality on the primary productivity of the symbionts and the kinds of photosynthetic products accumulated within the host cytoplasm.

Five species of rockling: Gaidropsaurus vulgaris (Cloquet), G. mediterraneus (L.), Rhinonemus (Enchelyopus) cimbrius (L.), Ciliata mustela (L.) and C. septentrionalis (Collett), are commonly recorded from British waters. With the exception of G. mediterraneus, which has not been recorded from the North Sea, the other four species occur all round the British coasts. Two additional species have recently been added to the list of British fishes, Antonogadus macrophthalmus (Günther), a southern boreal species and Onogadus argentatus (Reinhardt), a northern boreal species (Wheeler, 1969; Russell, 1976). The identity of adult A. macrophthalmus is, however, still in confusion. It is not clearly known if it is a different species from A. megalokynodon Kolombotovic (a species which occurs in the Mediterranean and adjacent Atlantic waters), since both of them are considered as synonyms of G. biscayensis (Collett) (Bini, 1970; Tortonese, 1970; Svetovidov, 1973). Cohen & Russo (1979) have since relegated Onogadus and Antonogadus to synonyms of Gaidropsaurus, though the exact number of species is uncertain.