Showing papers in "Journal of the Marine Biological Association of the United Kingdom in 1989"

The Macrobenthic Infauna of the Offshore Northern North Sea

Abstract: Between 1980 and 1985 ninety-seven stations were sampled by Smith-Mclntyre grab from the offshore northern section of the North Sea Four hundred and nine infaunal species were identified from the 76 selected macrofaunal stations The number of species per station varied from 25 to 80 with a maximum abundance of 9,600 individuals m−2 The biomass ranged from 013 to 1886 g dry weight m−2 At most stations, however, biomass varied between 1 and 4 g dry weight m−2 Diversity and abundance were highest in the 120–140 m zone, characterised by fine sand containing variable amounts of silt The highest biomasses were recorded in two areas; firstly where stronger currents predominate and the sediments are coarser (east of Shetland and west of the Norwegian Trough), and secondly in the fine sandy deposits of the centrally located area In the silty sediments (Fladen Ground and smaller depressions) there was a predominantly subsurface deposit-feeding community, whereas in the coarser area east of the Shetlands carnivores predominated Over the remaining area surface deposit feeders were dominant

A Sibling Species Group of Hydractinia in the North-Eastern United States

Abstract: Many symbiotic organisms are narrowly distributed on one or a few host species. These associations are intriguing, as they invite the development of hypotheses regarding the pattern and process of speciation and serve as laboratories for the testing of methods of phylogenetic reconstruction (Kraus, 1978; Futuyma & Slatkin, 1983; Stone & Hawks worth, 1986). The evolution of host-specificity in the sea may be expected to be severely constrained by the difficulty of achieving reproductive isolation in taxa whose gametes are freely released into the water column and/or whose larvae are potentially widely distributed (Scheltema, 1977). Yet this difficulty may well be overestimated, given the recent demonstrations of limited gamete (Pennington, 1985; Yund, in press) and larval dispersal (Knight-Jones & Moyse, 1961; Ryland, 1981; Olsen, 1985; Jackson & Coates, 1986; Grosberg, 1987). Indeed, if gamete and larval dispersal are as limited as has recently been contended (Jackson, 1986), local isolation of populations may be a routine occurence, offering repeated opportunities for speciation.

On the structure of the inshore fish community of England and Wales

Abstract: Records of fish captures on power station intake screens were used to analyse the structure of the English and Welsh inshore fish community. The study was undertaken as part of a programme to predict fish captures at future power station sites. It was found that 118 of the 122 fish species known to live inshore were recorded from the screens of 12 coastal power stations. The maximum number of species at one site was about 80, achieved at fully marine localities along the English Channel coast. This number declined with increasing latitude and decreasing salinity.Fish subdivide their use of the inshore habitat both spatially and temporally and the proportion of species complement within the major subdivisions of the habitat was remarkably similar at all sites. On average, 28 species were present simultaneously and these would include 4 pelagic, 8 demersal and 16 benthic species.

Seasonal Changes in the Lipid Composition of Herring (Clupea Harengus) in Relation to Gonad Maturation

Abstract: 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 succession of generations and growth of the caligid copepods Caligus elongatus and Lepeophtheirus salmonis parasitising farmed Atlantic salmon smolts (Salmo salar L.)

Abstract: Infestation of cage cultured Atlantic salmon by the external parasitic copepods Caligus elongatus (Nordmann) and Lepeophtheirus salmonis (Kroyer) 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).

Escape Responses of Herring Larvae to Visual Stimuli

Abstract: Predation is now considered the main cause of mortality in larval and juvenile fish (Hunter, 1984) and is therefore the most important factor controlling recruitment to the adult population. Marine fish larvae are prey for many different types of predator including medusae, crustaceans and larger fish. When predatory attacks are sensed both adult and larval fish may respond by making a 'C-start', a very fast, simultaneous contraction of the trunk musculature that deforms the body into a C-shape within 20 ms (Eaton & Hackett, 1984). This startle response is mediated by the Mauthner cells, a pair of prominent neurones in the hind brain or by other reticulo-spinal cells located in the same region. As a result of reciprocal inhibition and decussation of the cell axon, stimulation on one side of the body results in contraction of all the muscle on the contralateral side.

An Analysis of the Microgrowth Bands in the Shell of the Common Mussel Mytilus Edulis

Abstract: 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.

Possible wavelength discrimination by multibank retinae in deep-sea fishes

Abstract: 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 Epifauna of the Northern North Sea (56°–61°N)

Abstract: 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).

Modelling of Phytoplankton Population Dynamics in an Enclosed Water Column

Abstract: To understand the working of the marine pelagic ecosystem one needs to study the logical frame of the complex food web. This study is much easier in an enclosed water column than in the open sea where biological interactions are often concealed by physical events.

The Effects of Tributyltin on Dogwhelks (Nucella Lapillus) From Scottish Coastal Waters

Abstract: 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.

Reproduction and Settlement of Mytilus Edulis on an Exposed Rocky Shore in Galway Bay, West Coast of Ireland

Abstract: 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.

Cadmium and the phosphate granules in Littorina littorea

Abstract: Previous studies with the marine gastropod Littorina littorea have shown that cadmium is concentrated in the digestive gland and that cytosolic metallothionein-like proteins sequester a large proportion of the accumulated burden. In the current investigation, an electron microscope cryo-preparation method and X-ray microanalysis have revealed that some cadmium, in association with sulphur, is localised in the region of the limiting membrane of the mineralised granules which detoxify certain metals as an insoluble phosphate salt. It appears, therefore, that there could be interaction between the sulphur and phosphate systems for binding metals within the basophil cells of the digestive epithelium.

Energy values of marine benthic invertebrates from the western English Channel

Abstract: Energy values of many cold-water marine invertebrates are available (see Wacasey & Atkinson, 1987 and Brey et al, 1988), but energy values of temperate-water marine invertebrates have not been systematically investigated In an attempt to determine the energy content of the macrobenthic communities from the bay of Morlaix (western English Channel), the caloric values in a large number of species living in these communities has been measured This paper is presented as a contribution to the knowledge of the energy content of macrobenthic invertebrates from a temperate sea

Salinity Effects on Brood Maturationo of the Mysid Crustacean Mesopodopsis Slabberi

Abstract: Mesopodopsis slabberi were sampled from the River Tamar estuary (Plymouth, UK) during July-September 1985. Average number of brood pouch young per female was 15.2 (range 10–21). Embryos in three stages of development were variously exposed(in vitro) to ten test dilutions of seawater (3.5–35.0%o salinity). Developmental time from egg to juvenile was ca 16 days and was not influenced by salinity level, but success in completing development was. Stage I embryos successfully metamorphosed to juveniles in 7.0–24.5%o with best survival (>50% survival days) within the range 14.0-24.5%c, compared with 7.0–31.5%o for those tested from stage II. Adults tolerated the full range of salinities but ovigerous females were more euryhaline than males, and highest tolerance was within the range 10.5–24.5%0. These findings are discussed in relation to the ecology of M. slabberi and it is suggested that the narrower range of salinities required for optimal (>50% survival days) brood development may be a factor in seasonal on-shore/off-shore population movements noted in the literature.

The Shell of Cardium Edule, Cardium Glaucum and Ruditapes Philippinarum: Organic Content, Composition and Energy Value, As Determined by Different Methods

Abstract: Assessments of the quantity of organic matter in shells were made both by ignition at 475°C and by chemical extraction in O•1M trichloroacetic acid. The quantity of organic matter obtained by ignition at 475°C was over-estimated by 2 to 4•8 times, depending on the species studied (Ruditapes philippinarum, Cardium edule, Cardium glaucum). A technique of extraction by O•lM trichloroacetic acid, filtration on Whatman CFjC filter and weighing of remaining ashes after ignition (450°C) is proposed, Energy values of the shell organic matter varied from 17•00(±0•60) Jmg 1 to 24•0 (±2'87) Jmg 1 depending on the species and the geographic location. In the energy budget, these methods have made it possible to obtain an estimate of the energy mobilised in the production of shell organic matter. Variability in the energy values was the result of variation in the relative proportion of the different biochemical components. The proportion of proteins (Kjeldahl method) varied from 66•7% to 89•7% according to the species and geographical locations. The lipid content varied from 0.84% to 2•88% and carbohydrates from 0•15'70 to 0•29%.

Feeding Potential in the Lecithotrophic Larvae of Adalaria Proxima and Tritonia Hombergi : An Evolutionary Perspective

Abstract: Vance (1973a, b) argued that among the possible range of developmental strategies available to marine invertebrates, only the extremes of obligate planktotrophy and obligate lecithotrophy are evolutionarily stable. Vance's model, relating reproductive 'efficiency' to egg size (in terms of energetic content), predation rate, and prefeeding (lecithotrophic) vs feeding (planktotrophic) larval periods, has been a source of much discussion and debate since its inception (e.g. Underwood, 1974; Vance, 1974; Christiansen & Fenchel, 1979; Obrebski, 1979; Williams, 1980; Jablonski & Lutz, 1983; Strathmann, 1978, 1985; Todd, 1985). Subsequent publications have continued to dwell mainly on potential selective factors and the extremes of larval developmental type (i.e. obligate planktotrophy or obligate non-pelagic lecithotrophy). For the most part, these investigations have ignored questions concerning how a transition from one larval type to another would be accomplished in morphological and functional terms. Nonetheless, the consensus persists that small eggs and planktotrophy are the primitive (or ancestral) condition, and that lecithotrophy is the more advanced evolutionary derivative (see Strathmann, 1978,1985).

Shell morphology of larval and post-larval mytilids from the north-western Atlantic

Abstract: Comparison of early shell morphological features of six mytilids from the north-western Atlantic revealed differences useful for species identification and classification. Brachidontes exustus, lschadium recurvum, Geukensia demissa, Amygdalum papyrium, Mytilus edulis, and Modiolus modiolus larvae and post-larvae were cultured in the laboratory. Scanning electron micrographs of the shell and hinge during early ontogenetic stages showed that all species had a long provinculum with taxodont dentition. In addition, provinculum length and number of teeth increased during the larval period in the six species. The small, numerous provincular teeth of Mytilus edulis and the bold, comparatively few provincular teeth of Amygdalum papyrium clearly differentiated these two species. Most species had a low umbo, round posterior margin, and more pointed anterior margin, although Amygdalum papyrium was distinguished by a high, prominent umbo. Distinction of Geukensia demissa and lschadium recurvum larval shells was difficult due to similarity in their shapes and hinge dentition. However, discriminant analysis using larval shell length, shell height, provinculum length, and number of teeth aided in classification of these and other sympatric species.

Long-Term Trends In The Occurrence Of Phaeocystis sp In The North-East Atlantic

Abstract: The effects of nutrient enrichment of natural water bodies range from small increases in plant biomass and production, to gross deterioration of water quality. The input of nutrients (e.g. nitrogen and phosphorus) to the sea off NW Europe (especially the North Sea) has increased dramatically over the last three or four decades (Folkard & Jones, 1974; Bennekom et al., 1975; Postma, 1978; Cadee, 1986a) but there is uncertainty about the effects on the ecosystem. One possible effect might be to induce changes in the phytoplankton community. Such an effect has been reported for the North Sea, where increases in flagellate algae have been observed (Gieskes & Kraay, 1977; Postma, 1985; Cadee, 1986b; Batje & Michaelis, 1986). Phaeocystis is one such alga, and its purported involvement in the formation of large quantities of foam, observed on European beaches (Batje & Michaelis, 1986; Weisse et al, 1986), together with evidence that the alga is a source of atmospheric sulphur compounds (Barnard et al, 1984) (with implications for atmospheric acidity), has attracted particular attention and concern

Patterns of Excitation of the Lateral Line of the Ruffe

Abstract: 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).

On the pelagic occurrence and dispersal of the benthic amphipod Corophium volutator

Abstract: Field data on the pelagic occurrence of Corophium volutator were obtained in 1981–82 in the Dollard (Ems estuary). The pelagic population density averaged over a tidal half cycle was in the range 0–15 m-3. The pelagic population represents about 0.06% of the benthic population. On most sampling days pelagic occurrence resulted in a net flood surplus transport.

Shell Shape Variation in Littorina Saxatilis and L. Arcana: A Case of Character Displacement?

Abstract: Variation in shape of the shell of Littorina saxatilis and Littorina arcana has been investigated using shells from four shores where the species co-occur and seven where the former only is present. Samples were taken from shores along the south-west and south coasts of Britain, between Cardigan Bay and Kent. Measurements of eight shell variables and of the operculum area were analysed using canonical variate analysis. The results of the analysis suggest that the shells can be separated into three groups, namely L. arcana, L. saxatilis where it is sympatric with L, arcana, and L. saxatilis where it is allopatric from L. arcana. The separation of the two groups of L. saxatilis is based primarily on their relative globosity, whereas the separation of L. saxatilis from L. arcana is based primarily on the relative size of the operculum and width of shell whorl one (the first above the body whorl). Sexual dimorphism has been considered and shown not to contribute significantly to the between-shore pattern of variation observed. The relationships between important variables were further investigated using ANCOVA, and the relationship of foot area to the shell variables was also analysed at four of the shores. It is concluded that there is evidence of character displacement in L. saxatilis, shown by an increase in globosity in the presence of L. arcana, and that this may partly explain the continuing uncertainty over the taxonomic and ecological identity of members of the rough periwinkle group.

Osmoregulation of the Estuarine Prawn Palaemon Longirostris (Caridea: Palaemonidae)

Abstract: 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.

Seasonal and year-to-year changes in the phytoplankton from the Plymouth area, 1963–1986

Abstract: Samples of phytoplankton from nets towed in the surface waters off Plymouth over a period of 24 years were subjected to a correspondence analysis. There was a high species diversity throughout the year, and a strong seasonal cycle. A total of 104 species was divided into eight groups showing different seasonal and year-to-year occurrence. Over the 24 years the seasonal pattern of species changed in a fairly systematic way and there were two periods (1968–70 and 1983–85) when the changes were most marked. These patterns could be related to changes in the weather patterns, and were similar to those found by other workers in other groups of organisms and in other areas.

Assimilation of Dietary Sterols and Faecal Contribution of Lipids by the Marine Invertebrates Neomysis Integer, Scrobicularia Plana and Nereis Diversicolor

Abstract: Feeding by marine invertebrates affects dietary lipids as they pass through the gut (Volkman et al, 1980s; Tanoue et al, 1982; Prahl et al, 1984a, b,1985; Neal et al, 1986; Harvey et ah, 1987,1989). Not only do animals appear to alter the dietary lipids but they also contribute their own lipids to the egested material. Faecal pellets are thus likely to have a lipid composition which has contributions from the ingested food material, the animal itself and the microbial populations residing in the animal's alimentary system.

In Situ Foraging and Feeding Behaviour of Narcomedusae (Cnidaria: Hydrozoa)

Abstract: Narcomedusae are a small and mostly oceanic group of hydromedusae whose tentacle morphology and comportment sets them off behaviourally and perhaps ecologically from most other medusae. Their tentacles are relatively few in number (2–40), stiff, and noncontractile, with points of insertion located well above the bell margin. Eleven species representing eight narcomedusan genera (Aegina, Aeginura, an undescribed aeginid, Cunina, Pegantha, Solmaris, Solmissus, and Solmundella) were observed and collected in situ in the NW Atlantic, Arctic and Antarctic, using scuba and manned submersibles. In life, the tentacles of narcomedusae are nearly always held upwards over the bell or projected laterally. The major prey were other gelatinous zooplankton, especially salps and doliolids. In the laboratory, these relatively large prey were caught on the tentacles which bend inward and coil at the tips to bring food to the mouth. By extending the tentacles perpendicular to the swimming path, these medusae achieve a relatively large encounter area, thus increasing the probability of contact with prey, for the amount of protein invested in tentacles.

Mytilicola Intestinalis (Copepoda: Cyclopoida): A Ten Year Survey of Infested Mussels in a Cornish Estuary, 1978–1988

Abstract: Presentation des resultats d'une etude de 10 ans sur les relations entre M.i. et les moules de l'estuaire de la Lynher (Cornouailles), afin de determiner si M.i. est un commensal ou un veritable parasite

Observations on the reproduction of Modiolus modiolus in Isle of Man waters

Abstract: 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 & Kjorsvik, 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.

Rapid Hatching Induced By Light Intensity Reduction In The Polyopisthocotylean Monogenean Plectanocotyle Gurnardi From The Gills Of Gurnards (Triglidae), With Observations On The Anatomy and Behaviour of The Oncomiracidium

Abstract: The fully developed eggs of the polyopisthocotylean monogenean Plectanocotyle gurnardi, from the gills of the gurnards Aspitrigla gurnardi and Eutrigla gurnardus, hatch rapidly (within 1–2 s) when the light intensity is reduced by extinguishing the light, casting a shadow, or interposing neutral density filters in the light path. The bodies and fins of the hosts are likely to provide such stimuli since gurnards are slow-moving, diurnally active, bottom-dwellers that use pectoral fin rays to forage in the sediment. After hatching, the oncomiracidia respond to light, gravity and possibly to water currents but spend much of their time attached to the substrate. ‘Resting’ larvae are reactivated by shadows and by mechanical stimuli. The complex behavioural repertoire of the larva is discussed, particularly in relation to host location and the anatomy of the oncomiracidium.