INTRODUCTION

Orientation is crucial for almost all organisms, from bacteria to mammals, for selecting habitat or feeding grounds, deciding up from down, or just keeping track of your position, or your neighbour's. The marine environment is an especially rich source of material, particularly for the study of migration, where many animals cover enormous distances, and appear do this with reference to very few sensory cues. The mechanisms underlying these processes, and the benefits they bring to the organism, are only now beginning to be understood.

Vertical migration interacts with oscillatory tidal flows to produce horizontal transport. When the vertical migration period is an exact multiple of the tidal period then net uni-directional horizontal transport can occur. This happens most effectively in ‘selective tidal-stream transport’ for which vertical migration into the flow is specifically synchronized with the dominant (usually M2) tidal constituent. When migration and tide are not synchronized there is no net transport and, instead, a horizontal displacement oscillation of the organism takes place at the beat period between the vertical migration and tidal periods. The most common form of vertical migration is synchronized, not with the tide, but with the day-night cycle. Diel migration in M2-period tidal currents induces no net transport, but can produce excursions of several tens of kilometres over just a few days. Diel vertical migration can, however, interact with the sun-generated part of the semi-diurnal tide to produce net horizontal transport, the direction of which is controlled by the phase of the S2 tidal currents. The spatial distribution of phase implies that regions of horizontal convergence and divergence will result from diel migration. Diel migration in diurnal-period tides causes the preferred direction of transport to reverse at six-monthly intervals.

Proximate aspects of diel vertical migration in the freshwater and marine environment are compared using data from the literature. Examples of migrations in both environments are presented, from which it is concluded that relative changes in light intensity before sunrise and after sunset are primary causes of migrations. Experiments have shown that photoreactive behaviour is enhanced in the presence of predators but inhibited by shortage of food. These factors are called secondary causal factors. A hierarchy of causal factors is proposed. In lakes fish exudates suffice but in marine biotopes like bays, it is possible that fish have to be actually present for enhancement to take effect. To what extent the presented stimulus-response mechanism holds for mesopelagic animals in oceans is discussed on the basis of vertical distributions of euphausiids.

Late stage juveniles and adults of Callinectes sapidus in Chesapeake Bay, USA, and Maja squinado off the Ria de Arousa, Spain, were compared for ontogenetic changes in movement patterns (speed, distance, orientation) and habitat selection (depth, substrate) using ultra-sonic telemetry and published information. After settling in submerged grass beds in the lower Bay, 20-mm juvenile C. sapidus disperse long distances into low salinity sub-estuaries to feed and grow to maturity in two years. Within the Rhode River sub-estuary, juvenile C. sapidus moved with a mean speed of 12 m h1 in nearshore shallows (1·1 m); whereas adults averaged 24 m h·1 in the deeper (2·9 m) channel areas and moved freely in and out of the main estuary. Individuals of both life stages exhibited a pattern of slow meandering (juveniles, 2 m h1, adults 10 m h·1) within a limited area, alternating with faster, directionally-oriented movement (both stages >50 m h·1) between meandering sites. Juvenile and adult males over winter in deeper water nearby, while inseminated females migrate long distances into high salinity areas to incubate the eggs. Callinectes sapidus completes the migration cycle only once per 2·5-y generation. Maja squinado settles on rocks in shallow kelp forests in the coastal zone, where they grow to maturity in 2 y. Juveniles moved slowly (0·5 m h·1) while meandering without directional orientation on shallow (4 m) small patch reefs during summer. After the pubertal moult in summer, adults also meandered slowly (1 m h·1) mostly on rocks at slightly greater depth (7 m). In late summer and autumn, newly mature and older adults moved with directional orientation into deeper (10·40 m) water for the winter, until migrating back to the shallows for the summer; whereas juveniles remained inshore on rocks for the winter. Adult M. squinado live several years after puberty and complete the seasonal migratory cycle several times during their lives.

INTRODUCTION

Individuals of the Antarctic krill Euphausia superba Dana assemble in spring into dense formations (estimated 20,000 m3) which swim for long distances and at considerable speed (estimated 20 cm s−1) to search for patches of phytoplankton (Hamner, 1984). Recruitment to schools and maintenance of a defined position in a travelling formation, a remarkable social behaviour, requires some kind of communication system between individuals.

Vision, as a sophisticated image-processing system, habituating strongly with time, appears inappropriate for the task of continuously monitoring position in the formation. This is not in contradiction of the fact that the prominent eyes of Euphausia orient the shrimp with respect to the axis of light coming from above, and direct the photophores 180° away from light (Land, 1980). The function of the photophores is only partially explained (e.g. predator avoidance by counter-shading, Grinnell et al., 1988), their role in formation swimming being doubtful because they are reportedly active only during dawn and dusk. Of course the eyes help Euphausia to assemble in schools and to evade predators (Strand & Hamner, 1990) as well as fishing nets (Everson & Bone, 1986a,b), but a simple mechanical reflex seems more suitable to control individual position in the formation.

The orientation of tail-flip escape swimming in a range of adult decapod and mysid crustaceans is reviewed. In mechanical terms, tail-flip swimming constitutes unsteady locomotion in which a single ‘appendage’, the abdomen, produces thrust by a combination of a rowing action and a final ‘squeeze’ force when the abdomen presses against the cephalothorax. In small crustaceans, a symmetrical ‘jack-knife’ tail-flip is more typical. Tail-flip flexion is controlled by two giant-fibre pathways, and also by a non-giant-neuronal network. The direction of thrust in the sagittal plane, and hence the elevation, translation and rotation of the tail-flip are dependent upon the point of stimulation and on the giant-fibre pathway activated. The laterality of the stimulus also affects the orientation of swimming, which is directed away from the point of stimulation. In large decapods such as the lobsters Nephrops norvegicus and Jasus lalandii steering is produced by asym-metrical movements of various abdominal appendages, and by rotation of the abdomen about the cephalothorax. In slipper lobsters the flattened antennae provide steering surfaces. In smaller decapods, such as the brown shrimp Crangon crangon, and in mysids, such as Praunus flexuosus, steering is effected by a rapid rotation of the whole body about its longitudinal axis during the initial stages of tail-flip flexion. The effectiveness of tail-flip swimming is considered in the context of predator-prey interactions under natural conditions and in relation to artificial threats from fishing gear.

INTRODUCTION

The problems of observing the behaviour of mid-water animals are considerable. The depths involved are generally too great to allow free diving, and because the population density is low, useful observations from submersibles and remote cameras are rare. Animals brought to the surface are often in too poor a condition to behave at all normally. The hyperiid amphipods are one of the best groups to work with in trying to overcome these difficulties. They are relatively common and frequently observed from submersibles, where they congregate round the lights. They are tough, survive capture and behave well in aquaria at the surface (Land, 1992a). Compared with other groups, a reasonable amount is known about their habits, especially their associations with gelatinous animals on which they prey or have commensal relationships (Harbison et al., 1977; Madin & Harbison, 1977; Diebel, 1988). It is also known that they sometimes form swarms (Lobel & Randall, 1986).

This brief review describes the sense organs involved in orientation by cephalopods with a view to providing a basis for understanding the mechanisms involved. As in many other animals, vision is probably the most important sense involved in orientation. Cephalopods have paired eyes, sometimes providing binocular vision, and generally similar to those of many vertebrates in terms of their gross structure and performance. The main feature apparently lacking in most cephalopods is colour vision, although many have polarized vision. The second most important sense in cephalopods is the sense of balance provided by the statocysts. These paired organs, are functionally similar to the vertebrate vestibular system, and detect both linear and angular accelerations, giving the animal information on its spatial orientation and rotational movements. The statocysts may also be involved in hearing, for there is no doubt that they can detect some vibrations carried in the water and ground. The recently discovered cephalopod lateral line system also detects water-borne vibrations, but this is more probably used for locating other animals in low light conditions. Behavioural experiments have shown that olfaction is also important in cephalopod orientation, but, although there is good morphological data on the structures involved in olfaction, there are little supporting physiological data. Of the remaining senses used in orientation, touch is important for bottom living species, but less so for the free swimming animals. There is no evidence, so far, for electroreception or magnetic sensitivity in cephalopods.

This article is a comparison and a synthesis of laboratory and field studies focused on the influence of larval and early juvenile behaviour on the recruitment mechanisms of sole in the northern part of the Bay of Biscay. Laboratory studies concerned the individual behaviour of larvae and juveniles, in particular, changes in swimming activity controlled by endogenous factors and also reactions to light, current, pressure and feeding conditions. Experiments were performed in different types of tanks fitted with special devices to vary light, pressure and current. Specific methods using visual observations and an infra-red converter as well as actographic techniques with infra-red barriers were utilized. The relative contribution of endogenous and external factors on swimming activity and orientation vary during ontogenesis. Field studies concern a time series of fine-scale vertical distribution of larval stages. Endogenous rhythms and light seem to play an important role in the control of diel vertical migration from early larval stages, whereas semi-diurnal and tidal behaviour could develop mainly after metamorphosis. A simulation is used for the study of the interactions between individual behaviour and tidal currents. Based on laboratory and field observations we propose and discuss a hypothesis of transport and migration. In the Bay of Biscay, such a hypothesis involves diffusion processes enhanced by active behaviour.

During the studies on sole recruitment in the Bay of Biscay, transport process attracted considerable attention and led to important research on migration and dispersal. The last step to complete the study of the spatial component of the life cycle of sole in the northern part of the Bay of Biscay is the offshore migration of young adults towards the spawning grounds. the present study deals with this pre-spawning migration. the main question concerns the kind of linkage between nurseries and spawning grounds. The principal results are based on extensive tagging experiments carried out in autumn 1992 in two distinct nursery grounds, Bay of Vilaine and Pertuis (4620 soles of 2-group and older). One year later (January 1994) catches of tagged individuals reached 18.1% and 11.9% of the initial samples, respectively. From September to February a 50-km net displacement of the centre of the population was observed towards the offshore deeper area. A small trend to move onshore appeared in spring. In parallel with these net movements individuals were dispersed around the centre of gravity of the population. This dispersion was higher in a direction parallel to the coast. Individuals from different nurseries were mixed over the same spawning areas of the Bay of Biscay, and some individuals were also captured in and close to the English Channel. Changes in distribution were in phase with environmental changes and some elements concerning topographic preferences also appeared. Some evidence for orientation was observed but random dispersal seemed more important. No particular link between a given nursery and a spawning ground in the vicinity appeared.

Yellow-phase American eels, Anguilla rostrata, were displaced 10–17 km between tidal fresh water and salinity-stratified water and followed to determine the degree of home site fidelity, swiftness of homing and whether environmental factors affect orientation or homing behaviour. Twenty-one eels were tracked continuously (eight eels displaced up the estuary, eight down the estuary and five controls) in the Penobscot Estuary, Maine, USA for 4–80 h each, while recording position every 20 min. Nine of the 16 displaced eels (56%) returned to their capture site either during the track or shortly afterwards. Three of the remaining seven eels made substantial progress towards their capture site within the time observed. Eels homed with equal frequency whether displaced up or down the estuary in an average of 220 h ±87 (SE). After release, no eel moved farther away from its capture site. The eels were active mostly at night but used only the appropriate tidal currents rather than directed swimming to move about their home range and to home. Control tracks provided a home-range estimate in this habitat of 6.7 ±1.6 km of estuary or 325 ±64 ha. Lunar phase or position of the moon did not appear to influence either homing behaviour or activity patterns. This study shows that orientation and homing to a specific site in eels occurs relatively quickly and with no initial errors in orientation.

Ciona intestinalis has a well developed nerve plexus associated with the dorsal strand, as first described by Marco Fedele. The dorsal strand plexus is immunoreactive with antisera against gonadotropin-releasing hormone. Immunoreactivity is seen in the cell bodies, which lie peripherally, and in processes which run throughout the dorsal blood sinus, enter the branchial sac and penetrate the brain via the visceral nerve. The plexus provides a rich innervation of the gonoducts, and processes have been seen in the gonads. The pericardium is not innervated by processes from the plexus and the rectum is poorly innervated, but the full extent of the plexus in the viscera remains uncertain. While this study confirms many of Fedele's observations, it does not support the view that the dorsal strand plexus is equivalent to the vertebrate visceral nervous system.

The recently described gutless Astomonema southwardorum Austen, Warwick & Ryan 1993, from North Sea methane seeps lives in symbiosis with oval, extracellular bacteria completely filling the lumen of a modified gut. The bacterial strand is tightly lined by a thin layer representing very long intestinal cells of the host. The bacteria are 5.5–6.0 µm in maximum length and 3.5–4.0 µm in width. In the anterior body the alimentary tract is completely reduced. The structure and size of the symbiotic prokaryotes, as well as their extracellular location in the lumen of a non-functional gut, differ substantially from those in A. jenneri, the single species of this genus thoroughly studied electron-microscopically (Ott et al., 1982). These structural discrepancies suggest a careful reassessment of the genus Astomonema.

Coelenterazine chemiluminescence is now established as the most common chemistry responsible for bioluminescence in the sea, being found in seven phyla. However, the organisms which synthesize coelenterazine have yet to be identified. In order to deter-mine whether the luminous midwater shrimp Systellaspis debilis (A. Milne Edwards) (Arthropoda: Decapoda) is capable of luciferin biosynthesis, a developmental series of eggs was assayed for its luciferin, coelenterazine. The advantages of this system are that S. debilis eggs are autonomous and therefore have no external nutrient supply, the embryos can be ranked for developmental stage and the large egg size allows clutch numbers to be determined accurately. Recombinant apo-aequorin, which requires coelenterazine for luminescence, was used to quantify coelenterazine during the developmental sequence. An increase of almost two orders of magnitude was detected in coelenterazine content per egg between the first and final stage of development (mean values of 1 pmol and 71 pmol). This demonstrates de novo biosynthesis of coelenterazine for the first time.

Induction of metallothionein synthesis in Littorina littorea exposed to cadmium (400 µg I-1) is tissue dependent. Concentrations of the metal-binding protein increased by a factor of four in the gills and by a factor of three in the kidney.

Gel filtration chromatography of heat-treated cytosolic extracts reveals that cadmium, accumulated in both the gills and the kidney, is bound principally to the newly formed metallothionein. Cadmium saturation of the protein (at an MT:Cd molar ratio of 1:5) and the approach of steady-state condition for Cd accumulation was indicated in the latter tissue, but there was little evidence for Cd equilibrium in gills.

Metallothionein levels in the kidney of L. littorea can be determined on a routine basis in laboratory experiments and in field samples by differential pulse polarography of wholecytosol preparations: heat-treatment/centrifugation is sufficient to remove most of the interference from high-molecular-weight thiolic proteins. In contrast, the use of gelfiltration chromatography is recommended alongside polarographic analysis of the gill cytosol to enable quantification of, and compensation for, such interferences, and hence to ensure that only metallothionein is determined.

Measurements of metallothionein induction in the kidney of Littorina may therefore prove useful in the determination of sublethal biological response to metal contamination. Background metallothionein concentrations of 3–4 mg g-1 (measured polarographically) serve as a baseline against which samples from contaminated sites can be assessed.

INTRODUCTION

Exposure of marine organisms to metals such as cadmium can induce the synthesis of low molecular weight, cysteine-rich proteins, metallothioneins (MTs), which are capable of sequestering and detoxifying excess intra-cellular metal.

Deep-water suprabenthic species of Mysidacea were studied in the Catalan Sea (western Mediterranean). Twenty-one samples were taken at depths from 385 to 1859 m, using a Macer-Giroq type sledge. Sixteen mysid species have been collected. Boreomysis arctica was the commonest species and Parapseudomma calloplura, Paramblyops rostrata and Erythrops neapolitana were also notable for their high occurrence. The genus Dactylamblyops, Mysidella biscayensis and Pseudomma affine were recorded for the first time in the Mediterranean. The greatest total abundance of mysids was found over the middle slope. On the upper slope P. calloplura and E. neapolitana formed the dominant species, and deeper B. arctica dominated. Species richness and diversity (H’) declined with increasing depth. The decrease in H’ was attributable to the growing dominance of B. arctica over the middle and lower slopes. Swimming coefficients near the bottom showed intraspecific differences. In general, the species collected inhabit the water column between 0.1 and 0.5 m above the bottom. The bathypelagic species Eucopia hanseni had high swimming coefficients. Amongst the suprabenthic species, B. arctica showed the highest swimming coefficients, and rose from the near-bottom during night on the upper and middle slope stations.

A species of balanomorph barnacle with solid plates and basis, Solidobalanus fallax, is recorded for the first time in the English Channel off Plymouth, at 44–56 m depth. It occurs exclusively on the valves of the queen scallop, Aequipecten opercularis. Only one out of 15 scallops carries the barnacle. Morphological details of the species are re-described in comparison with Balanus crenatus and B. spongicola, with which it co-occurs. Solidobalanus fallax ranges from Angola through West Africa and Morocco to Algeria but has not been reported previously from Europe. Some specimens were incubating embryos in late May, indicating a potential for local recruitment. Solidobalanus fallax is unlikely to be a recent immigrant but its abundance may have increased as a result of changing climate.

Most studies on the pore signature of calanoid copepods have been made on adult femalessemicolon very little is known of the ontogenetic development of the pore signatures and their sexual differences. Males, females and copepodids of five species in the metridinid genus Pleuromamma are examined. Interspecific differences in the pore signatures occur as early as copepodid III. Differences in the signatures of the copepodids arise from two sources: (1) species-specific rates of development of the adult signaturesemicolon (2) development of the species-specific components of the adult signature. The cephalosomal signature is complete in copepodid V, and the metasomal and urosomal signatures are complete in copepodid VI, the adult stage. Sexual differences in the pore signature are found in copepodid IV and are primarily evident in the urosome of the adult. The species-specific components of the urosomal signature are greater in the female than in the male.

The armoured planktonic dinoflagellates present at a site adjacent to a floating fish-farm in Loch Eriboll, North Scotland, were studied by surface net-sampling, mainly at two-weekly intervals, over a period of four years 1990–1993. A total of 62 species was recorded. Some dinoflagellates were present at all times of the year but they reached their lowest numbers in May, during the spring diatom bloom, and their highest numbers in mid-summer. There was usually a seasonal pattern in which the most abundant dinoflagellate during the early summer was the heterotrophic species Protoperidinium ovatum, and this was followed in the later summer by the autotroph Ceratium fusus. In 1993 the pattern was rather different, with Gonyaulax species abundant in June and, later in the year, both Protoperidinium cerasus and P. excentricum having periods of abundance. Potential toxin-producing dinoflagellates, Alexandrium tamarense (PSP) and Dinophysis acuminata (DSP), were found in small numbers.

This paper reports the results of a hydrographic survey and the successful deliberate deep droguing of a meddy (Pinball) and the seeding of its core with two ALACE floats. The drogued buoy results give important kinematic properties of the eddy core in real timesemicolon the ALACE have allowed the position of the meddy to be tracked for seven months. Pinball was found against the continental slope near Lisbon canyon. The maximum core salinity was 36·564 psu, at a depth of 1260 m, but the maximum rotation rate with period ~2·5 d was in the upper core near 700 m, where temperatures reached 13·2°C. The azimuthal transport to a radius of 50 km was ~13 Sv. Pinball moved from the continental slope near Lisbon to the central Tagus Abyssal Plain, returned towards the continental slope and then moved westwards crossing the central Tagus Abyssal Plain a second time. At times it had a marked remote sensing infra-red sea-surface signature. It moved ~550 km over 204 d and the near real-time data meant that, in principle, this eddy could have been re-surveyed, redrogued or reseeded with floats during this period.