The genus Circeis St-Joseph was based upon C. armoricana St-Joseph (1894), but Caullery & Mesnil (1897) suggested that the latter is a mere variety of Spirorbis spirillum (L.), which Chamberlin (1919) therefore regarded as the type of the genus. These two species are indeed closely related to each other, but distinguished by the sculpturing of the collar setae (Knight-Jones, Knight-Jones & Al-Ogily, 1975). They belong to a genus which is very distinct from Spirorbis, as was pointed out by Knight-Jones, Knight-Jones & Kawahara (1975), who regarded C. armoricana as the type species.

Ergenstrema mugilis Paperna, 1964, the only member of the genus, has been collected in the Mediterranean from the gill rakers of the thin-lipped grey mullet Liza ramada (Risso, 1810) by Paperna (1964) and Lambert & Sanfilippo (1977); this parasite, under the name of ‘Dactylogyridea sp.’, was collected also from the same microhabitat of the same host from the coast of Albania by Ergens (1960). At Plymouth, parasites clearly belonging to the genus Ergenstrema have been collected from both the gill rakers and lamellae of a different host, the thick-lipped grey mullet Chelon labrosus (Risso, 1826).

In investigating the parasites of adult thick-lipped grey mullets Chelon labrosus (Risso, 1826) at Plymouth these fishes were found to be infected exclusively with an ancyrocephaline gill monogenean identified as Ligophorus angustus Euzet & Suriano, 1977. When, at a later date, some juvenile C. labrosus were examined ancyrocephalines were again collected; however, quite unexpectedly, these parasites were not juvenile L. angustus but belonged exclusively to a new species of ancyrocephaline subsequently reported as Ergenstrema labrosi Anderson, 1981.

It seemed then that there might be a situation of considerable biological interest in which young fishes were characterized by one host-specific monogenean and older mullets by another. Although a change in the species composition of the total parasite fauna of mullets with age has been reported previously by Reshetnikova (1955) for golden grey mullet Liza aurata of the Black Sea, a change of monogenean species has never previously been recorded.

From an examination of the literature (e.g. Hickling, 1970) on the life history of the British thick-lipped grey mullet it was inferred that the situation described above might be associated with the migratory habits of the fish and in particular the different environments inhabited by young and old mullets; while there may be many factors (e.g. oxygen, temperature, pH, etc.) which differ between the open sea and tidal pools, inhabited by old and young fishes respectively, the most obvious one is that of salinity. Hence, in addition to establishing the identity and frequency of the ancyrocephaline infections with host age, it was decided to make investigations on the salinity tolerances of L. angustus and E. labrosi.

The sterol biochemistry of the highly advanced molluscan class – the cephalopods – is poorly understood. Few analyses of their component sterols have been published in which GC-MS has been employed (Voogt, 1973; Idler et al. 1978; ApSimon & Burnell, 1980) and these have only involved 5 species.

From the available data the cephalopods appear to have a much simpler sterol make-up than the other less-advanced molluscs. Cholesterol appears to be easily the predominant sterol (ca. 90%) with minor amounts of up to 10 other common marine sterols. Of the species analysed, four (Sepia officinalis, Octopus vulgaris, Eledone aldrovandi and Illex illecebrosus) had a very similar major and minor sterol composition. Only the more primitive Nautilus sp. (Idler et al. 1978) had a noticeably different minor sterol composition.

Voogt (1973) reported cephalopods to be able to synthesise sterols though molluscs generally appear only to be able to carry out this biosynthesis slowly (Goad, 1978). Cephalopods are extremely active carnivores and thus would be expected to have a diverse diet. If their component sterols are of a dietary origin, a considerable variation in their minor sterol composition might be expected on the basis of the range in sterol composition reported for pelagic organisms (e.g. Morris & Culkin, 1977), many of which may be possible dietary components.

Detailed knowledge however of cephalopod diets is limited. Quite apart from the fact that healthy specimens are rarely caught in nets, those that are caught often feed voraciously on the other organisms trapped in the net prior to being brought on board for examination.

The behavioural experiments described in this paper were carried out to complement earlier physiological work on the acoustico-lateralis system of the clupeid fishes. Previous work, summarized by Blaxter, Denton & Gray (1981a), has shown that the sense organs of the clupeid utriculus are highly sensitive to alternating pressures. Since the utriculus signals the polarity and amplitude of each peak in a complex pressure wave form separately, independently of the rate of change of pressure over a range equivalent to a frequency band of 30–800 Hz, it is well suited to give information on brief noises. In the clupeids the lateral line is hydrodynamically connected with the ear and we have suggested that the signals arising from the inner ear and the neuromasts of the lateral line are used in conjunction to provide information about direction and range.

Herring (Clupea harengus L.) show a marked ‘startle’ response when subjected to sound stimuli from a vibrating source (Blaxter, Gray & Denton, 1981c). The response, which occurs at a consistent threshold, is elicited by the first cycle of a tone burst or noise, the threshold to one cycle being the same as for a burst of many cycles. Furthermore, if the onset of the tone burst is ‘ramped’ so that it takes several cycles for the full amplitude to be reached, the threshold is markedly raised. Under some conditions the fish move away from the source, showing that they can determine its direction even though the stimulus is confined to a single cycle. The relative roles of sound pressure and particle velocity in providing information about the amplitude and direction of the noise have not been firmly established. Present evidence suggests that pressure is important in triggering the response and probably in measuring the strength of the stimulus. A vector such as particle velocity provides the directional information used in the response. The response can be very fast with a latency of about 25 msec and a contraction time of about 20 msec (Blaxter, Denton & Gray, 1981 b). In these very fast reactions the Mauthner cells probably constitute the motor pathway (Diamond, 1971).

Callista chione (L.). is a fairly large, handsome bivalve of the family Veneridae. It is found around the coasts of Devon, Cornwall and Wales, though its distribution extends southwards to the Mediterranean. It is usually uncommon but has been found to occur at the eastern end of Whitsand Bay, near Plymouth at a density of about 1 per m2. Several specimens were taken in 1975 while testing a water lift system for the use of aqualung divers. Most of the shells were observed to bear a noticeable growth check in the form of a ridge several millimetres from the shell margin (Fig. 1). This ridge was not found in the older specimens in the laboratory's museum collection. To try to find out the cause of this growth check a tagging experiment was undertaken in 1976, although it was not possible to measure and mark out large numbers; recaptures in the following two years have been sufficient to provide some evidence for the average growth increment of different sizes of shell.

In attributing variations in growth and metabolism within a population to environmental fluctuations, and in interpreting the biological response to environmental fluctuations, it is important to recognize the variations that are intrinsic to the population or species. Consequently, it is common practice in studies on the growth and metabolism of calanoid copepods to separate instars and, in the case of adults, males and females when analysing measurements. Although studies on calanoids, in particular species of Calanus and Euchaeta, have demonstrated the existence of pronounced sexual differences in parameters such as dry weight, biochemical composition and respiration rate (e.g. Comita, Marshall & Orr, 1966; Marshall, 1973; Bamstedt, 1975, 1979; Gatten et al. 1979, 1980), the possibility that similar differences might occur in pre-adult instars has not been examined, this despite the fact that in many species in this group it is possible to distinguish the sex of copepodites IV and V, the two instars preceding the adult.

Symbiotic luminous bacteria have been described in, and cultured from, a number of species offish and cephalopod. Indeed only in these two groups are extracellular luminous bacteria believed to be utilized as a source of light (see Buchner (1965) and Herring (1978) for references). Despite several earlier investigations of such symbioses in cephalopods the bacteria in these animals have not been adequately identified, nor has the extent of their role been clarified. The ultrastructural relationships between bacteria and the tissues of the squid accessory nidamental gland have been investigated in the non-luminous species Loligo pealei (Lesueur) (Bloodgood, 1977) and Sepia officinalis L. (Van den Branden et al. 1979) but no comparative work on luminous species has been undertaken apart from that on Heteroteuthis dispar (Rüppell), whose photophore does not contain typical luminous bacteria (Dilly & Herring, 1978; cf. Leisman, Cohn & Nealson, 1980). The order Sepioidea contains five families, among which are the two families Sepiolidae and Spirulidae. Though the presence of luminous bacteria is known in some sepiolids (as well as in certain loliginids (order Teuthoidea)) some doubt remains about the source of light in the photophore of Spirula spirula Hoyle. The steady luminescence of this species has prompted speculation that bacteria may be involved (Harvey, 1952). In this paper we compare the anatomy and ultrastructure of the photophores of both Sepiola and Spirula in order to clarify some of these problems.

Stromateoids are fishes of the open ocean commonly found in association with gelatinous zooplankton (Mansueti, 1963; Haedrich, 1957). One genus, Tetragonurus, has been found inside the branchial chambers of salps (Emery, 1882; LoBianco, 1909) and cloacal chambers of pyrosomes (LoBianco, 1909; Fitch, 1949, 1951).

Waves and currents render the shallow sea a chaotic environment. Seaweeds broadcast into the water tiny dissemination phases whose powers of locomotion are at best puny in relation to the water forces that act upon them.

Eulalia viridis (L.) is one of a group of polychaete species which are being used in an experimental analysis of the regulation of gametogenesis and reproduction. The back-ground to these studies and some of the preliminary results have been discussed recently by Olive (1980). Despite the abundant data demonstrating the frequent occurrence of well-defined reproductive cycles in marine invertebrates and observations on the correlations between reproductive events and environmental conditions, the mechanisms by which the reproductive cycles of individual species are controlled are very poorly understood. It has generally been accepted since the publication of the important paper of Orton (1920) that the temperature cycle in temperate and boreal regions plays an important role in determining the reproductive cycle, but the nature of the control mechanism is not, in general, understood and is likely to vary markedly between species. Environ-mental conditions such as relative daylength or biological factors such as the quantity and identity of the phytoplankton organisms (Himmelman, 1978, 1979) could also play important roles in regulating the sequence of cellular events which culminates in successful reproduction. These general problems have been discussed by Giese (1959) and more recently by Barnes (1975), Giese & Pearse (1975) and specifically in relation to Polychaeta by Clark (1979) and Olive (1980).

The respiratory function of the naked body of sea slugs (Opisthobranchia) has long been recognized. The gross anatomy of the gills of dorid nudibranchs was first published by Hancock & Embleton (1852) and was later revised and corrected by Eliot in 1910 in his supplement to the Ray Society monograph by Alder & Hancock (1845–55). Herdman (1890) and Rao (1936) give histological details, but do not seriously add to the understanding of the function of dorid gills or their circulation system. While the gill ciliation of bivalves (Atkins, 1938) and prosobranchs (Yonge, 1947) has been thoroughly examined this is not so in dorid nudibranchs and only the ciliation of the notaspid Pleurobranchus membranaceus has been described in any detail (Thompson & Slinn, 1959; Thompson, 1976) and the dorsal mantle of some dorids (Kress, 1981). Much of the general anatomy of opisthobranchs is reviewed by Hoffman (1939).

The nutrient requirements of many algae are well documented, with most algae being found to need only one or two vitamins. In the case of their predators, only a handful of microinvertebrates have had their diets and even fewer their vitamin requirements denned. Thus, any progress in this field together with the mode of nutrient uptake, must lead to a better understanding of the component-transfer in the food chain.

Brachionus plicatilis Müller has been reported to require vitamin B12 (Droop & Scott, 1978). The work described in this paper sought first, to confirm this and then to establish the mode of uptake. The alga Brachiomonas submarina var. pulsifera Droop was used to investigate the first of these questions as the writer had previous experience with it in steady-state energetics work. The alga Dunaliella SMBA Strain 246 was used to examine the mode of uptake by the rotifer since this had also been found to be a good food organism. The possibilities considered were uptake: (a) via the algal cell; (b) directly from solution; (c) a combination of both. Mode of vitamin B12 uptake is of particular interest as it may be representative of the pathway of many of the rotifer's water soluble micronutrients.

Before considering the influence of the vitamin on Brachionus investigations were made into the relationship between the vitamin and the two food-algae. It has already been established that Brachiomonas has a definite requirement (Droop, 1961) and Dunaliella no requirement for vitamin B12 (Turner, 1979). It was of interest to see if they took-up the vitamin in the manner shown by Monochrysts, a B12 requirer, and Phaeodactylum a non-requirer (Droop, 1968).

The ellobiopsid genus Thalassomyces is of uncertain systematic position, having been allied at various times with the fungi, algae and protozoa, and little is known of its biology. Discovery of a flagellate stage suggested to Gait & Whisler (1970) that the genus is a member of the achlorophyllous Dinophyceae (Dinoflagellata). T. fagei (Boschma, 1949) appears to be restricted to euphausiids (of which there are 85 valid species in 11 genera-see Mauchline & Fisher, 1969). According to Vader (1973), T. fagei has been recorded in different areas from various euphausiid species, i.e. in 11 (perhaps 12) species of Euphausia, two of Nycliphanes, three of Thysanoessa, and in Nematoscelis difficilis and M. novegica. Other Thalassomyces species are found in mysid, decapod and amphidpod crustaceans (Vader, 1973).

The Iceland scallop (Chlamys islandica) (O. F. Müller) has its main distribution within the subarctic transitional zone where it is found on gravel or sandy bottoms at depths ranging from 10–100 m and usually in places with strong currents (Ekman, 1953; Wiborg, 1962).

Seasonal changes in the rate of shell formation frequently affect the surface morphology of molluscan shells, with reductions in feeding activity and calcification rates at low temperatures producing winter rings that can be used for age determination. Such annuli are well-known in lamellibranchs and have greatly facilitated studies on their growth, longevity and other age-related processes (as reviewed by Wibur & Owen, 1964; Rhoads & Panella, 1970; Comfort, 1957, 1979). However, they occur less frequently in gastropods and many species that do show such checks are relatively short-lived, or else their growth ceases or becomes erratic on sexual maturity.

Skeletal form is the key to silicoflagellate taxonomy. Variations in skeleton morphology have been frequently described for the genera Dictyocha and Distephanus (Lemmerman, 1908; Deflandre, 1932; Hovasse, 1932; Frenguelli, 1935; Boney, 1973, 1976), and the wide ranging skeletal variations observed with clonal cultures of Dictyocha fibula Ehrenb. underline the potential plasticity of skeleton formation in this species, (Van Valkenberg & Norris, 1970). Sufficient information now appears available to indicate where taxonomic confusions may arise, particularly when single skeletons are observed either in phytoplankton samples or in fossil deposits. Paired skeletons are predivision stages (Gemeinhardt, 1930; Hovasse, 1932; Marshall, 1934; Boney, 1976). The juxtaposition of the two skeletons in this predivision stage allows a more critical assessment of the taxonomic significance of an aberrant form.