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Considering the response of organisms to their environment is difficult; it is made more so if population numbers cannot be closely monitored. In such cases different methods of population assessment are required. This thesis uses lipids as a measure of Atlantic salmon (Salmo salar L.) quality and investigates its usefulness in indicating fish condition. The first study examines the relationship between fish total lipid content and W[sub]R condition factor; this study clearly demonstrates that there is a significant positive relationship between the condition factor of a fish and its total lipid content. In the following study the lipid storage between the different tissues of the Atlantic salmon is considered. This indicates that the red muscle and the adipose tissues hold higher concentrations of lipid than the white muscle. However, the white muscle makes up the majority of lipid tissue mass in the Atlantic salmon so contains the bulk of stored lipid in a fish, at low concentration. The next study investigates the effect of spawning on Atlantic salmon condition. Salmon can be seen preferentially conserving lipid in their musculature and drawing down the lipid stored in their adipose tissues. The following study looked at one key lipid group, triacylglycerides, in salmon. Triacylglycerides are energetically important in fish and this study found that the spawning process depleted triacylglyceride reserves, but that the red muscle conserves triacylglycerides even after spawning. The final study considers the relationship between maternal quality and egg quality, identifying that longer Atlantic salmon produce eggs with more lipid after spawning migration. Egg lipid concentrations were comparably maintained between fish. Monitoring quality in this way is a useful tool to determine population wellbeing and help indicate where populations are compromised.

Investigations into the supply, settlement and recruitment of the barnacle Semibalanus balanoides (L.) to shores in Fife, East Scotland were undertaken over three consecutive years (2004 – 2006). Several designs of a passive larval trap, based on earlier published designs, were employed to quantify the delivery (supply) of cyprids to the substratum. Pump samples from the water column were collected to provide a measure of intertidal cyprid concentration. Cyprid concentration was found to exhibit both spatial and temporal variation, but was also found to be significantly correlated with cyprid supply, as quantified by the traps. In some years, pump and trap samples suggested that cyprid abundance in the intertidal was mediated by wind-driven processes. An artificial substratum was used to quantify cyprid settlement, allowing investigations into the supply-settlement relationship. Supply and settlement were found to be two very distinct biological phases. Supply describes the flux of larvae to the substratum. Settlement provides a measure of the amount of larvae that decide to settle on the substratum, as initial attachment for barnacle larvae is not permanent. Whilst supply saturation is unfeasible, daily saturation of the substratum by settlers was observed at many sites along the Fife coast. Levels of settlement saturation were found to vary both temporally, between years, and spatially, over scales of km. Varying levels of the desperation of larvae to settle, mediated by dwindling energy reserves, was thought to explain some of this variation. The supply data also provided some evidence of the possible movement of cyprids along the Fife coast from Fife Ness to Tentsmuir. However, wind-induced transport may also play a dominant role on the Fife coast. A comparison of supply and recruitment of larvae into adult populations revealed that both pre- and post-settlement events may influence recruitment. The relative importance of these factors was shown to be density-dependent. Negative intraspecific interactions were only seen in denser aggregations of adults. In less crowded aggregations, recruitment reflected initial patterns of larval supply.

This thesis describes the results of a three year study which collected larvae of the acorn barnacles Semibalanus balanoides on a rocky coastline in Fife, East Scotland. The nauplii larvae of S. balanoides are released from their parent in springtime in the United Kingdom and develop in the plankton for approximately one to two months. During this period they are transported some distance from the parent population and eventually return to the intertidal shoreline as a cyprid larval stage. The ‘decision’ to settle is a crucial point in the life history of the organism as most sessile organisms cannot move once this has occurred. The supply of larvae to intertidal shorelines was historically neglected until the 1980s. Patterns of settlement were largely considered irrelevant to distribution patterns of adults relative to postsettlement processes such as predation and competition. Despite the resurgence in “supply-side ecology” in the past 20 years there has been little development towards the measurement of larval supply. Consequently there has been very little description of fine scale or large spatio-temporal studies involving larval supply. This study demonstrates the first study directly addressing larval supply independently from larval settlement at mesoscales (metres to kilometres; days to years). Improvements were made to the passive larval trap described by Todd (2003). A conical opening was combined with the spiral trap design and a number of inlet areas were trialled (0.25cm², 0.5cm², 1cm² and 2cm²). These were tested across typical wave regimes measured with a new autonomous pressure sensing wave transducer and the 1cm² inlet was selected as the most appropriate trap design due to a balance between an increased per unit inlet larval capture and sufficient absolute larval capture so as to identify daily variation in larval supply to a site. Larval settlement has been studied extensively and is often used as a direct measure of rates of larval supply. It is widely assumed therefore that rates of settlement are a direct reflection of rates of supply, as long as settlement substrates and adult conspecific responses remain uniform. This thesis provides a means of accurately characterising the supply-settlement relationship for S. balanoides independent of substrate based responses and demonstrates that this is not the case. The relationship was found to be asymptotic, even at sites where there was low larval supply. It was concluded that density-dependent larva-larva interactions were present during the settlement of larvae and were relevant at daily temporal scales, limiting the rates of larval settlement proportional to larval density. There was no obvious effect on this relationship due to wave action however differences were observed between sites and years. Saturation of preferential environments within the tiles was observed resulting in a settlement preference cascade, with larvae being forced to settle in increasingly less preferable areas of the tiles with increasing larval density. Larva-larva interactions are demonstrated as having a considerable effect on the rates of settlement of S. balanoides.

The main descriptive characters of sabellid polychaetes are presented. Easily misunderstood morphological features are clarified by a series of schematic drawings. The importance of scale is emphasised and some aspects of population ecology and reproduction are addressed. An introduction to Euchone Malmgren (Polychaeta: Sabellidae: Sabellinae) is given and the diagnostic character is discussed. A new taxon, Euchone x, is described from the North Sea and Northern Norwegian Sea. Jasmineira candela Grube from the Adriatic Sea is redescribed and Jasmineira oculata Langerhans from Atlantic waters is reinstated. Some little-known records of Chone in Northern Atlantic waters are presented and one anomalous taxon is described. A historical overview of sabellid systematics is given. Because the binomial system of taxonomic nomenclature still used today was devised more than a century before evolutionary thinking, finds of organisms that do not 'fit' into the traditional systematic hierarchy are not unexpected. An introduction to phylogenetic systematics is given and some current issues are discussed. The concept of phylogenetic taxonomy is introduced. Phylogenetic analyses were carried out of taxa mainly within Euchone with some members of Chone, Jasmineira and Amphicorina. Euchone sensu Malmgren is revealed as being an artificial taxon. Five taxa previously assigned to Euchone are recognised as a discrete sister group to Chone, Euchone and Jasmineira. The data set is further used to demonstrate how phylogenetic taxonomy might be applied to the study group of organisms. Some behavioural observations were made on captive live specimens of Euchone x and the ecological implications are discussed. Some observations are provided on its reproduction and an enigmatic parasitic organism found within the radiolar crown is investigated. Although several important aspects of the taxonomy of the group were resolved, there remain many future challenges in the taxonomy and systematics of soft-bottom sabellin fan worms.

1. Juvenile trout, Salmo trutta L., from three parental groups - sympatric Sea trout and freshwater-Resident trout, and isolated trout from above a waterfall impassable by upstream migrating anadromous trout - were reared under three ration regimes to manipulate growth rates. The development of seawater tolerance was studied by measuring drinking rates after periodic salinity challenges during the first two years of juvenile growth. No trout were observed to undergo the parr-smolt transformation in any of the parental form/ration combinations after two years in freshwater. However, a considerable proportion did mature during this time period. The proportion of maturing trout was directly related to ration level but was also influenced by parental form, with isolated trout demonstrating a greater tendency to mature early. Seawater tolerance increased with age in all groups. However, mean drinking rates upon salinity challenge were generally lower, from Experiment 2 onwards, in Resident trout than in either of the other two groups. 2. Eight immature sea trout (finnock) were radiotracked in the River Eden, Fife, during September, October and November 1994. The individual finnock displayed considerable variation in patterns of movement; two remained in freshwater for at least 27 days whereas others moved downstream out of the river within days or even hours of release. In general, this highlighted the transient nature of the freshwater migrations of some finnock, indicating that they move in and out of rivers over brief periods of time and apparently do not necessarily remain in freshwater continuously throughout the winter. 3. The hypo-osmoregulatory ability of finnock during the winter was assessed in two experiments. The number of finnock was limited in Experiment 1. Therefore, this was designed as a preliminary assessment of the physiological response of finnock to acute freshwater-seawater transfer. Osmoregulatory abilities were assessed by measurement of drinking rates, plasma ion and plasma cortisol concentrations after acute freshwater-seawater challenge and compared with freshwater-adapted and seawater-adapted control groups. Finnock displayed physiological responses typical of euryhaline teleosts upon seawater challenge; a rapid increase in drinking rate, an increase in plasma ion concentrations (but only to levels similar to, or slightly greater than, those of seawater-adapted fish), and increased plasma cortisol concentrations. The second experiment, in which numbers of finnock were greater, made use of the same techniques to assess the longer term acclimation of finnock to both freshwater-seawater and seawater- vi freshwater challenge, to establish whether finnock might suffer from a more subtle reduction in seawater-tolerance which would not have been necessarily apparent in the acute challenge of Experiment 1. Finnock did not appear to be physiologically compromised by seawater challenge during the winter months, and therefore, a breakdown in hypo-osmoregulatory abilities alone cannot be considered a reason for finnock returning to estuaries and rivers during the winter. 4. The physiological effects of low to medium levels of infestation of the ectoparasitic copepod Lepeophtheirus salmonis (Kroyer) on wild sea trout post-smolts were assessed at intervals during the development of the parasite. A mean infestation level of 18 parasites caused significant disruption to the osmoregulatory ability of hosts, as demonstrated by significantly higher plasma osmolality and chloride ion concentrations when compared with naive post-smolts. In addition, since no skin lesions were apparent on the hosts, these physiological effects were considered to be the consequence of larval attachment to the gill filaments, thereby possibly puncturing the epithelia and also damaging vital branchial ion excretory cells. 5. The modern molecular genetic RAPD-PCR technique was used to screen DNA of Lepeophtheirus salmonis collected from wild and fanned salmonid hosts from around the Scottish coasts. This technique indicated markedly different patterns of genetic variation amongst L. salmonis of farmed and wild origin, and between different farms. A number of genetic markers were found to be exclusive to, or at considerably higher frequency amongst, sea lice collected from farmed salmonid hosts. This technique established the possibility of assigning provenance to L. salmonis collected from wild hosts.

The marine bryozoan Electra pilosa typically inhabits ephemeral substrata in the intertidal and shallow subtidal, and is probably the ecologically most successful bryozoan species in British waters. Modular organisms like E. pilosa frequently evolve pronounced phenotypic plasticity to cope with the ecological challenges resulting from passive larval dispersal into unpredictable habitats, and temporal variability of the environment colonized by the immobile adult stage. E. pilosa colonies on wave-exposed shores differ morphologically from those found on sheltered shores in possessing numerous long-spined zooids. The present study demonstrates that spine formation in E. pilosa is environmentally inducible by wave-related abrasion by macroalgae; additionally, the spines also have a fortuitous anti-predator effect in discouraging predation by the nudibranchs Adalaria proxima and Polycera quadrilineata. It is suggested that the inducible spines of E. pilosa constitute an adaptation for the protection of feeding polypides in high-energy environments, and that plasticity for the trait is of adaptive value in this organism which exploits a diverse range of habitats. Although a number of traits in this species clearly are subject to considerable phenotypic plasticity, other attributes apparently are highly deterministic, heritable and genotype-specific. Electra pilosa displays pronounced among-genotype variation in colony growth rate, and the present study shows that this variation is due to proximate factors which affect growth rate and covary with genotype. This study also presents the first evidence of senescence at the zooid level in E. pilosa: Zooids deteriorate systematically over time, as indicated by decreasing polypide life spans and increasing polypide regeneration times, but in contrast to this, whole-organism senescence does not appear to occur in this species.

This study investigated the reproduction, larval growth and metamorphosis of the nudibranchs Onchidoris bilamellata (L.) and Goniodoris nodosa (Montagu) under controlled laboratory conditions. In addition, the rare occurrence of spawning events of O. bilamellata taking place in the field outwith the winter and spring reproductive period was studied. Onchidoris bilamellata and Goniodoris nodosa differ in the size and number of eggs produced per spawning event. In the present study O. bilamellata does not exhibit a clear spawning pattern, whereas G. nodosa lays increasingly smaller eggs and larvae as the season progresses. The diameter of the eggs of O. bilamellata is not correlated with the organic content per egg. The data available for G. nodosa were insufficient to analyse this relationship. The rates of larval growth and development are reported for Onchidoris bilamellata and Goniodoris nodosa. These variables do not differ significantly for O. bilamellata between cultures maintained under various light regimes, ranging from continuous darkness to continuous illumination, and are comparable to those of G. nodosa. The shell growth pattern exhibited by larvae of G. nodosa is sigmoid, similar to that of other opisthobranchs. Spawn masses laid during the winter and spring months were collected fresh from the field and compared to spawn masses collected from the field in July and September, outwith the typical spawning period of Onchidoris bilamellata in the British Isles. The shell size of the July and September hatchlings was significantly smaller than that of the winter and spring hatchlings. Furthermore, the larvae hatched from spawn masses laid in September cultured in the laboratory exhibited high mortality rates and the overwhelming majority did not survive through metamorphosis. The significance of the spawning activity of O. bilamellata past the typical spawning period of this species is discussed. The induction of metamorphosis of Onchidoris bilamellata was investigated using seawater containing elevated concentrations of potassium ion, and the results indicate that the optimal concentration inductive of metamorphosis is 19 mM K+ ASW, Metamorphosis experiments were also performed with the natural prey of O. bilamellata, the acorn barnacle Semibalanus balanoides (L.). The results suggest that pediveligers can be induced to metamorphose at a distance from the inductive substrata. Attempts were made to identify the natural inductive cue of Goniodoris nodosa, but this was not successful.

Within life-history trait variations for two herbivorous intertidal lacunids, Lacuna pallidula, a direct developer and Lacuna vincta a planktotroph, were compared and related to their ecology and to marine invertebrate life-history theory. Aspects of life-history theory covered included; reproductive investment, the relationship between egg size and egg fecundity, the Egg-Juvenile-Period (EJP), the implications of egg size for offspring status and maternal effects. Similar patterns of growth and reproductive investment were observed for adult females of both species, although absolute rates of growth and reproduction were differently affected by macroalgal diet. Further, differences in response to the favourability of macroalgal diet were observed for the two species, notably in the manner in which eggs were packaged. Variations in both egg size and egg numbers in spawn masses were observed for the two species, among populations within both species and within Lacuna pallidula populations. For Lacuna pallidula, these variations were shown to be mediated by both maternal macroalgal diet and population source. Maternal diet directly affected the size of hatching offspring but not the size of eggs produced. Consequently, egg size was not a good indicator of hatching size for either species. The EJP was determined for both species for a range of temperatures. Greater variations in the EJP and juvenile size were observed in Lacuna vincta. This was attributed to the ability of the larvae of this species to delay metamorphosis and to display positive growth during the delay phase. Both temperature and microalgal diet were shown to affect patterns of growth and development in Lacuna vincta larvae. Sources of naturally occurring cues for inducing settlement and metamorphosis in Lacuna vincta larvae were investigated to understand further the distribution patterns of this species on macroalgal types. Extension of the work investigated the suitability of various artificial cues for inducing metamorphosis and the effects of larval age and nutritional status of larvae upon latency of response to established inducing cues. Overall, L. pallidula displayed greater variation in traits and was more sensitive to environmental change than L. vincta. This finding is discussed in light of the two species larval strategy.

Sessile marine invertebrate larvae can recognize suitable settlement substrata by using various environmental cues, including organic/microbial "biofilms". In laboratory assays the effect of biofilm age on the settlement of a range of fouling species was assessed: the species included barnacle, bryozoan, spirorbin and ascidian larvae. The larvae of most species responded differentially to films of varying ages. A general trend of reversal of the effect of filming on the settlement response of the barnacle Balanus amphitrite amphitrite- from inhibitory to facilitatory - was noted with increasing film age. The settlement of the solitary ascidian Ciona intestinalis clearly was facilitated on filmed substrata of all ages. The larvae of the arborescent bryozoan Bugula flabellata generally were inhibited by biofilming (irrespective of film age), and the settlement of the polychaete Spirorbis spirorbis was found to be enhanced on "older" (12d) films only. In a series of manipulative panel experiments in a tidal rapid on the west coast of Scotland the effects of biofilming on subsequent larval settlement in the field were quantified. Inhibitory and facilitatory responses to biofilming were noted and these varied markedly between species and between two local habitats of contrasting flow regimes. The larvae of certain year-round settling species were found to vary in their response to biofilmed substrata depending on the season. Quantitative data were obtained for biofilms on artificial substrata by enumerating bacteria, diatoms, protozoans, fungi and the percent cover of microbial exopolymeric substances. These permitted comparisons of biofilms both in field and laboratory experiments. The results of the present study emphasize the importance of microbial surface film cues in explaining recruitment patterns of macrofouling assemblages during the first stages of substratum colonization.

This study concerns the reproductive and larval ecology of the nudibranch Adalaria proxima. Attainment of competence is demonstrated to be cue specific. Embryos metamorphose in response to choline, yet remain precompetent to elevated potassium and the natural cue until after hatching. It is hypothesized that the choline-mediated metamorphic pathway (or choline-sensitive portion of the natural pathway) becomes functionally complete ontogenetically earlier than do larval chemoreceptors. Larvae metamorphose in response to sea water 'conditioned' (CSW) by the bryozoan Electra pilosa. A range of littoral organisms failed to induce metamorphosis and it is suggested that A. proxima displays a high degree of cue specificy. It is reported that CSW-mediated metamorphosis is dose dependent and effected in a disparate manner to that of potassium. No evidence for a bacterial role in metamorphosis was found. These results are intended to facilitate future isolation of the natural cue. Both starved (lecithotrophic) and fed (facultatively planktotrophic) larvae may successfully delay metamorphosis for 28-31d post-hatching (at 10-C). Larval fitness appeared to be unaffected by nutritional status, which is suggested to reflect the transitional evolutionary nature of facultative planktotrophy. The lowered fitness commensurate with an extended pelagic period is hypothesized to confer a lowered dispersal potential. A. proximais semelparous, producing up to eleven spawn before dying. Significant variation in reproductive traits is demonstrated within and between six U.K. populations of A. proxima Intrapopulation decreases in egg size, fecundity and hatching success with spawn laying sequence are suggested to reflect phylogenetic constraints. Interpopulation differences in egg size and fecundity are considered consistent with the predicted limited larval dispersal potential. Population egg size was correlated to larval size but not latitude or fecundity. Possible causative factors of the observed interpopulation variation in reproductive traits are discussed, and most probable causes hypothesized.

Populations of Arenicola marina from different localities have been reported to show variation in certain characteristics. There are morphological differences such as colour, size and annulation patterns, and there is also variation in the duration and timing of the spawning seasons on different beaches. An extensive study of gametogenesis in individuals from three selected study sites was carried out by regular sampling throughout the year. Each site shows a different spawning pattern, either epidemic such as at the East Sands, St. Andrews, or an extended season, such as at the Eden Estuary, St. Andrews. These two sites are less than three kilometres apart and yet there are significant differences in the size of the gravid adults. The third site, Fairlie Sands on the west coast of Scotland shows a very unusual type of spawning season in that a small percentage of the population are gravid throughout most of the year. Genetic variation was assessed using the technique of starch gel electrophoresis of allozymes. There is a degree of genetic differentiation between the three sites. Allele frequencies at the GOT locus are different between east and west coast sites and a third rare allele is present at Fairlie Sands. It is suggested that gene flow mediated by larval transport may be limited and this would cause inbreeding in some subpopulations. The hormones involved in spawning have been investigated with particular reference to a putative female oocyte maturation hormone. Comparative work between Arenicola marina and A. defodiens has demonstrated the presence of an oocyte maturation factor in the prostomia of both species. While this factor acts directly on A. defodiens oocytes in vitro, the presence of a cofactor or secondary product produced in vivo is required in order to reinitiate meiosis in A. marina oocytes. Preliminary studies on the nature of reproductive endocrine substances present in the prostomia of female A. marina have been carried out.

The cosmopolitan distribution of the anascan bryozoan Electra pilosa manifests itself in the phenotypic plasticity of growth morphology expressed. Longterm field observations of the bryozoan on the essentially ephemeral substrata of the marine macroalgae, Fucus serratiis and Laminaria digitata, at two contrasting sites in Scotland, confirm this. Despite its potential for exponential indeterminate growth, Electra pilosa rarely realises this in the field. Observations show that any pattern of growth is complicated and distorted by the influences of physical (colony abrasion and the dynamics of the substratum) and biotic disturbance (predation by the nudibranch Adalaria proxima). Rather than effect a visible induced defensive response after sustaining injury from mechanical damage or Adalaria proxima, Electra pilosa was demonstrated to maintain a high specific growth rate. It is suggested that this would preclude the settlement of potential competitors and ensure the fitness of this competitively weak, opportunistic species. Adoption of a stellate colony morphology was not concomittant with higher predation susceptibility but rather represented a plastic adaptive response to an ephemeral, dynamic environment. The lack of a metamorphic response to anything other than the presence of live Electra pilosa clearly demonstrated that metamorphosis of Adalaria proxima is by some component of the live, intact bryozoan. This component is neither water- soluble or extractable, and the inductive property is eliminated by homogenisation and freezing. Although hypotheses for this are evaluated, the mechanism of induction and the reasons for this relationship remain unclear. Adaptations to this close nudibranch-bryozoan association are therefore evident in the life history strategies of both species.

Nudibranchia is the major Order of the gastropod Sub-class Opisthobranchia. Nudibranchs demonstrate a considerable evolutionary departure from the typical shelled gastropod and have achieved wide morphological and ecological diversity. Most species are recorded from nearshore rocky habitats where they are primary macrobenthic predators; most species are stenophagous limited to a single, or a few, prey species. Previous investigations had shown that most nudibranch species have evolved to an annual semelparous life history strategy and adopt a planktotrophic larval development strategy which would appear to be the ancestral conditions. Reviews of published data had drawn attention to some species which demonstrated an evolutionary departure from these ancestral modes and had evolved extended life history strategies combined with more advanced larval development strategies. Archidoris pseudoargus and Cadlina laevis are sponge-grazing dorid nudibranchs, and Tritonia hombergi, which feeds on an alyconarean soft coral, appeared to have extended life cycles. The aim of the present study was to investigate the life history and larval development strategies of these species and, via a study their physiological ecology, attempt to determine the selective forces which may have driven any evolutionary departure away from the ancestral condition. Archidoris pseudoargus was found to have a biennial semelparous life history strategy and a planktotrophic larval development strategy; the planktotrophic lai'vae were found to have some characteristics of the more advanced lecithotrophic laeval form. Cadlina laevis was found to have a perennial iteropalous life history strategy and a direct mode of larval development; this was the first observation of perennial iteroparity in the Order Nudibranchia. Archidoris pseudoargus and Cadlina laevis had markedly seasonal growth patterns with a high growth rate in spring and early summer, followed by a period of reduced growth, or even degrowth in late summer and over the winter. An allometric relationship was determined between body size and feeding rate for A. pseudoargus; the exponent was less than unity. For both species, a significant allometric relationship was determined between body size and respiration rate with an exponent less than unity indicating an increase in metabolic efficiency with body size. Both species displayed an acclimatization of respiration rate with increasing temperature; the acclimatization was more pronounced in C. laevis. Fecundity of both species was shown to have a significant allometric relationship body size with an exponent less than unity. Insufficient numbers of T. hombergi were collected and kept alive in the laboratory to undertake any experiments to generate meaningful data to answer these principal questions for this species. Results of the present study together with data published from previous investigations have been interpreted to suggest that the extension of the life cycle is a consequence of seasonal variations in prey quality and quantity. Rather than being an exception to the rule, it is suggested that extended life cycles could be the ancestral condition and the more common annual/sub annual life cycles would be a more adapted condition. These conclusions are based on the results obtained from laboratory investigations which are discussed in their application to the field situation.

Artificial substrata were employed at sites on the east and west coasts of Scotland, in such a manner as to model the habitat found on the undersides of boulders on the shore. Panel treatments were devised to examine the effects of substratum 'age' and larval availability, and the presence or absence of varying densities of herbivorous molluscan grazers on the development of the epifaunal assemblages. The importance of inter- and intraspecific competition in structuring the bryozoan component of the assemblage was also examined. A necessary pre-requisite for the settlement of many marine invertebrate larvae may be the development of a microfouling film, the nature of which may vary depending on its 'age', with corresponding effects on the 'attractiveness', or otherwise, to potentially settling larvae. The lowest numbers of recruits were frequently recorded on the 'youngest' panels, and greater numbers generally occurred on panels immersed for longer periods. Also of overriding significance, however, was the seasonal variability in larval availability. The assemblages were characterized by high levels of post-settlement mortality. The outcomes of the overgrowth interactions between 18 species of encrusting bryozoans were highly variable and complex, each species of an interacting pair won some encounters, and neither consistently overgrew the other. Therefore, the pattern of competitive abilities was neither entirely intransitive or transitive. Variations in outcome were found to be at least partially attributable to differences in the encounter angle between colonies. The competitive ability of a species also varied among sites and between years. The herbivorous grazing gastropod Gibbula cineraria was found to have a markedly deleterious effect on the developing assemblages. Furthermore, individuals and colonies of the epifaunal species were apparently unable to achieve an 'escape-in-size' under the experimental conditions employed.

In this study particular attention has been paid to the foraging biology and ecology of Aeolidia papillosa (L.); an anemone-eating nudibranch mollusc. 'Prey-value', derived in its current biological sense from optimal foraging models, is expected to be an important determinant of prey-selection behaviour. In the present study 'prey-values', or more specifically, food or tissue-values (because intact anemones were not used in the analyses), have been investigated for a range of anemone species. Analysis of the biochemical composition of anemone tissues and their consumption and assimilation by nudibranchs did not reveal any marked interspecific differences. S.troglodytes, however, is indicated as being somewhat different from the other species studied, possibly being assimilated more efficiently and consumed at a faster rate. These factors, in conjunction with the strong preferences shown for S.troglodytes in switching experiments and the apparent preponderance of field associations with this species, do Indicate that S.troglodytes may be a more valuable food item for A.papillosa. Analysis of the canposite estimates of fitness (growth and reproduction), however, did not reveal any contrasts in performance which could be related to diet. This was almost certainly a result of the marked variability in the performance of nudibranchs within each diet group obscuring any dietary effects which may have obtained. In the latter part of this study a series of behavioural experiments were conducted which investigated specific aspects of prey-selection behaviour. Using a variety of multiple-prey species choice experiments, data have been collected which show the effects of previous dietary experience on prey-species selection. The results of these experiments suggest that at least some of the contradictions in previous reports of prey-species preference by A.papillosa may be accounted for by "ingestive conditioning". "Ingestive conditioning" concerns the modification of a predator's behaviour such that it continues to choose that prey species which it has most recently or most frequently encountered. Such alterations of prey preference may exert considerable effects on the control of local anemone prey populations and their relative abundances. A variant of this experiment was ccmpleted during a five week investigation at the Friday Harbor Marine Laboratories, University of Washington, in the summer of 1982. Experiments showed that ingestive conditioning occurs for both U.K. and N.W Pacific A.papillosa. However, the modified experimental design used in Friday Harbor, and in corroborative experiments in St Andrews, also indicate that there is a significant "carry-over" effect from the previous conditioning diet. In the light of these results it was predicted that A.papillosa might exhibit frequency-dependent (i.e. switching) prey selection. Experiments undertaken to test this hypothesis were unable to demonstrate such behaviour due to persistent preference for one particular prey species. Field observations of A.papillosa-anemone associations are discussed in relation to the laboratory investigations.