INTRODUCTION

Wild-caught squids have seldom survived for very long in laboratory aquaria. Tardent (1962) maintained Loligo vulgaris Lamarck, 1789 for a maximum of 60 days. Choe & Oshima (1963), Choe (1966) and LaRoe (1971) reared squids of the genus Sepioteuthis from eggs to adult size. Between 1975 and 1982 several successful attempts to maintain Loliginidae (e.g. Matsumoto, 1976; Hanlon, Hixon & Hulet, 1978, 1983; Yang et al. 1980, 1983) and Ommastrephidae (Flores et al. 1976; Flores, Igarashi & Mikami, 1977; O'Dor, Durward & Balch, 1977) were made. But to date only ten squid species have been maintained for more than forty days (review: Yang et al. 1980; Boletzky & Hanlon, 1983). Loligo opalescens Berry, 1911 holds the record for longevity in captivity at 233 days from egg to adult (Yang et al., 1983).

INTRODUCTION

The shore crab, Carcinus maenas (L.), rapidly clears bacteria from the circulation to the tissues by virtue of the phagocytic and clumping capacity of circulating haemocytes and fixed phagocytic cells (Smith & Ratcliffe, 1978, 1980a, b; White & Ratcliffe, 1982). If re-infection resulting from the re-entry of sequestered micro-organisms into the circulation is to be prevented then the host must eliminate or incapacitate the foreign particles in some way. The migration of laden host cells to the exterior does not appear to be an important means of disposal of segregated non-self material in the Crustacea (Merril, Mongeon & Fisher, 1979; White & Ratcliffe, 1982), so that the ability to resist infection must reside with the cellular killing capacity, perhaps augmented by humoral antibacterial activity. Indeed, our earlier observations on the disappearance of bacteria from C. maenas gills seem to indicate that their loss resulted from the lytic action of host haemocytes (White & Ratcliffe, 1982), as does the elimination of micro-organisms from the hepatopancreas of the crayfish, Parachaeraps bicarinatus (McKay & Jenkin, 1970a).

INTRODUCTION

A common characteristic of alga-invertebrate symbioses is their specificity; in general, a given symbiotic invertebrate species forms a stable association with just one species or genus of alga. One approach to examine the basis and significance of the specificity is to compare the performance (i.e. survival, growth and reproduction) of invertebrates experimentally infected with a range of algae. To date, such investigations have been made almost exclusively with algae which do not naturally form an association with the host species (e.g. Schoenberg & Trench, 1980; Mews & Smith, 1982; Douglas, 1983), and the relevance of the conclusions to the natural situation is not always clear.

Introduction

The morphology of the Cardiidae is extremely variable and their taxonomy is uncertain because of the difficulty in recognizing the differences that are genetic and the variations that are due to environmental conditions. This is the reason why Cerastoderma glaucum (Bruguière) (Fig. 1) and C. edule have been for a long time considered as ecotypes of one species or two taxons belonging to a super-species C. edule (Lubet, 1976).

INTRODUCTION

Venus striatula (da Costa) (= Venus gallina L.) occurs in high densities in shallow water on a bottom of clean fine sand in the Bay of Douarnenez. The environmental conditions are characterized by an obvious sediment instability due to a high exposure to westerly swells (Guillou & Le Moal, 1978). This species is preferentially distributed in the fine sands (Petersen, 1913; Thorson, 1957), but is relatively tolerant towards certain ecological factors and so is sometimes found in very muddy sediments. It also shows an extended latitudinal range, since it is recorded from the Lofoten Islands to the Canaries, in the Mediterranean and the Black Sea (Tebble, 1966).

INTRODUCTION

It is apparent, in a review on the decapod hepatopancreas (Gibson & Barker, 1979) that there is some consensus of opinion that the epithelium consists of E-, R-, F- and B-cells and M-cells (Al-Mohanna, Nott & Lane, 1985). Also, it is agreed that the gland produces enzymes and absorbs, digests and stores nutrients and excretes waste material. However, the apportionment of these functions to the different cells and the descriptions of the cytological processes involved are variously explained. Thus, the activity of proteases and amylases has been demonstrated in the secretion produced by the gland but the source of these enzymes is attributed to different cells and various modes of secretion are proposed. Also, no secretion granules of the zymogen type have been seen.

There are probably two main reasons for the inconsistent interpretation of the activities of the cells. First, the different stages of the feeding and moult cycles are not taken into account and both these affect the cytology of the gland. Second, some of the functions have been deduced from observations of the fine structure without any experimental treatments to demonstrate more directly the processes involved. In the present work all the animals are taken at the same moult stage and observations are made throughout the feeding cycle. Also, aspects of the function are studied with markers which are administered in the diet and injected into the blood. The activities of hydrolytic enzymes associated with the different epithelial cells have been studied but these will be the subject of a separate publication dealing with the cytochemistry of the digestive processes.

INTRODUCTION

The heart of tunicates is of physiological interest for several reasons: it is a simple peristaltic tube; it lacks valves; and the direction of the contractions changes periodically thus reversing the direction of blood flow. The phylogenetic importance of tunicates, being invertebrate chordates, also make physiological details of interest.

INTRODUCTION

In the deep oceans where the water depths are in excess of 4000 m the benthic environment is widely separated from the productive euphotic zone and there is an exponential decline in pelagic biomass with increasing depth (Vinogradov, 1968; Wishner, 1980; Angel & Baker, 1982). The sea-bed acts as a sediment trap stopping the downward drift of particulates. Here resuspension and biological and geochemical processes in the benthic boundary layer cause enrichment of the waters above the sediments. Wishner (1980) has noted some of the evidence for near-bottom increases in phosphorus, nitrogen, ATP and sediment resuspension in the deep sea. In parallel with this enrichment and probably related to it, the pelagic macroplankton biomass is increased, at least to distances up to 100 m off bottom (Wishner, 1980; Angel & Baker, 1982). This suprabenthic zone seems to be inhabited by a distinct community of nektonic and planktonic species quite different to that inhabiting comparable depths in the abyssopelagic zone. Marshall & Merrett (1977) reviewed what was known of this community, with special reference to the fishes. Grice & Hulsemann (1970) and Grice (1972) have described new suprabenthic copepod species and Angel & Baker (1982) recorded an entirely novel pelagic ostracod fauna of 25 new species in a 5–20 m off bottom haul at 20°N, 21°W in 4005 m of water.

Eight species of the Gobiidae were recorded in weekly samples taken between July 1972 and June 1977 from the intake screens of Oldbury Power Station in the inner Severn Estuary. Buenia jeffreysii, Aphia minuta, Gobius paganellus and Gobius niger occurred infrequently and Crystallogobius linearis was only common in the late spring and summer of 1975 and 1976. Pomatoschistus microps, which peaked in numbers in the winter, increased in abundance each year between 1972 and 1977. The most numerous gobies were those belonging to the Pomatoschistus minutus complex which were separated into P. minutus and P. lozanoi only between June 1974 and July 1975. Seasonal trends in the abundance of these two species were similar, with elevated numbers occurring between July and September 1974 and in January 1975. Gonadosomatic indices and other data indicate that the decline in numbers after the latter month represented an emigration to spawning areas further down the estuary or in the Bristol Channel. Although P. minutus attained a larger body size than P. lozanoi, both these species and P. microps typically had a one year life cycle with only a small number of individuals surviving for a further few months. The fecundity of P. lozanoi, which ranged from 410–2453 (mean 1270), was lower than that reported for P. minutus in this and other studies. Analysis of stomach contents showed that the diet of both species was similar, consisting predominantly of gammarids and mysids. While both species were infected with Ligula intestinalis, the incidence of infection and the parasite index was greater in P. lozanoi than in P. minutus.

INTRODUCTION

The dust cart on the Isles of Scilly did not end its service at the local tip, over the cliffs at Deep Point on the eastern side of St Mary's on 14 October 1983. Instead it floated off and continued in service as a Lagrangian drifter, indicating the residual transport of water around the islands. As a hazard to shipping its position was given out as a navigation warning and it was allegedly sighted on the western side of Scilly. The interesting oceanographic question is whether the dust cart was (or would have had it not sunk) circumnavigating the islands in a clockwise or anticlockwise sense. Accordingly this experiment has been repeated using satellite tracked Argos drifting buoys, drogued with parachutes (~ 10 m diameter), and set at depths of about 15 m. The releases showed that the residual transport was clockwise around the islands. In one instance a drogue circumnavigated the islands in about 12 h (i.e. ~ one tidal period) at an average speed of about 100 cm s−1 (Fig. 1).

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).

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.

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.

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.

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.

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).