A study of the biology and behaviour of the copepodid larva of the salmon louse Lepeophtheirus salmonis (Kroyer, 1837) (Copepoda; Caligidae)

The salmon louse Lepeophtheirus salmonis (Krøyer, 1837) (Copepoda; Caligidae) is a serious pathogen of wild and farmed salmonids in the marine environment. A study has been carried out to investigate the biology and behaviour of the infective copepodid stage of this parasite. Copepodids showed a positive phototactic response to a wide spectrum of light wavelengths (300 - 700 nm) with a peak response at 550 nm. Light response was positively correlated to light intensity between 2.4 - 240 lux with immobilsation occurring at 20,000 lux. Copepodids demonstrated a negative geotactic response to increasing pressure. No positive chemotactic or rheotactic behaviour was demonstrated, although copepodids showed a rapid swimming and fast turning response to directed water flow that was considered to be the principal mechanism of host contact. Light and electron microscope studiesshowed copepodids to be equipped with a wide range of sensory organs including integumental organs, a lensed nauplius eye, an extra-optic photoreceptor (Organ of Gicklhorn), an Organ of Bellonci and antennular elements comprising mechanoreceptive and chemoreceptive components. A new receptor termed the "cauliflower organ" was also described and is suggested to function in host recognition. Primary attachment was shown to be accomplished through grappling of the host epidermis by the hooked antennae. The maxillpeds were used to further embed the antennae and anterior cephalothoracic shield. Secondary attachment was accomplished through the use of a frontal fiament. This was formed internally within the copepodid and may be renewed at each moult. Morphological and histochemical studies demonstrated the frontal filament to be a cuticular extension and showed it to comprise an adhesive "basal plate", a fibrous proteinaceous "stem" and an "external lamina" which was continuous with the cuticle of the anterior cephalothoracic shield. An "axial duct" ran through the stem and terminated at the basal plate. An external "filament duct" protruded from the copepodid rostrum and was continuous with the axial duct. Three major organs, A-, B- and C-glands, were described and their role in filament production discussed. A "ventral fiament organ" equating to the "frontal organ" of earlier studies was also present in the copepodid and subsequent stages. The copepodid alimentary canal comprised a cuticle-lined foregut, a midgut comprising anterior midgut caecum, anterior midgut and posterior midgut and a cuticle-lined hindgut terminating in a slit-like anus. The oral cone conformed to the normal caligid pattern although the labium and labrum were less tightly opposed than those of adults. The strigil was covered by a "labial bar". The midgut comprised three cell types corresponding to the B, RIF and E cells of earlier studies. The posterior midgut comprised only RIF cells. Faeces were enclosed in a peritrophic membrane. A number of glandular systems were present which were associated with the oral cone. These were termed "mucoid glands" "proximal gland complex" "labial gland" and "distal labral organ" respectively. No glands were observed to interface to the fore- mid- or hindgut. The free-living copepodid is believed to be lecithotrophic and was demonstrated to carry large vesicles within the epithelial cells of the midgut which are believed to comprise lipid reserves. The attached copepodid was seen to feed on mucus, epidermal I dermal material and other available host components. Host pathology was limited and comprised principally mechanical damage and tissue compression resulting from attachment and feeding. Host tissue response was limited to an increase in mucous cells and occasional influx of eosinophilic granular cells. Maximum survival of copepodids under optimal conditions in vitro was 23 days following moulting from the second nauplius stage. Survival was not increased by provision of fibrous substrates. Experimental host infections demonstrated that copepodids were host specific and failed to attach to non-salmonid species. Copepodids were demonstrated to infect both marine and freshwater host stages. In experimental infections, larvae were attached principally to the fins and gils. Farmed salmon showed attachment mainly to fins. Spatial distribution was suggested to be principally the consequence of local hydrodynamics. The negative correlation of relative density of infection with size in smolts and positive correlation in larger farmed fish was considered to result principally from differences in swimming-speed and surface area. Disadvantaged, mature and diseased hosts showed higher infection levels. Overdispersal of parasites was considered to be principally the result of host heterogeneity in experimental and farm infections. Attempts to determine the spatial and numerical distribution of copepodids in the wild were unsuccessful and this was suggested to result from the temporally discrete "waves" of infection seen on farmed fish. The results of this study were discussed with reference to studies of other parasitic and free-living copepod species and suggestions were made as to the implications of these findings for aquaculture

Production, mortality, and infectivity of planktonic larval sea lice, Lepeophtheirus salmonis (Krøyer, 1837): current knowledge and implications for epidemiological modelling

Current sea louse models attempt to estimate louse burdens on wild and cultured salmon by predicting the production and distribution of lice larvae and estimating the risk of transmission. While physical characteristics of water bodies and weather can be accurately modelled, many aspects of sea lice biology require further parameterization. The aims of this review are (i) to describe current knowledge regarding the production, mortality, and infectivity of planktonic sea lice larvae and (ii) to identify gaps in knowledge and suggest research approaches to filling them. Several major gaps are identified, and those likely to have the greatest impact on infection levels are (i) egg production, viability and hatching success, (ii) predation in plankton and (iii) copepodid infectivity profiles. A key problem identified in current parameter estimates is that they originate from a number of sources and have been determined using a variety of experimental approaches. This is a barrier to the provision of “best” or consensus estimates for use in modelling. Additional and more consistent data collection and experimentation will help to fill these gaps. Furthermore, coordinated international efforts are required to generate a more complete picture of sea louse infections across all regions experiencing problems with sea lice

Elemental analysis of Scottish populations of the ectoparasitic copepod Lepeophtheirus salmonis

Conventional nebulisation ICPMS (Inductively Coupled Plasma Mass Spectrometry), was used to determine the concentration of a broad range of elements in the salmon louse Lepeophtheirus salmonis. Lice samples were collected from Atlantic salmon in seven localities (4 fish farms and 3 wild salmon fisheries) on two separate sampling occasions and prepared for analysis. Sixty six elements were measured, 35 of these were found to be variable and were subjected to univariate and multivariate statistical analysis. The results of the single element comparisons showed that not all individual sites could be discriminated from each other. Sea lice collected from cultured salmonids could be discriminated from those on wild salmonids at the same site using the elements magnesium (less than 0.05%), vanadium (less than 0.01%) and uranium (less than 0.05%). Using discriminant analysis based on 28 elements, the separation of all sampled sea lice localities from each other was clear (100% correct classification) giving each an individual signature. Further analysis examined the effects of sequentially removing elements from the discrimination model in order to determine the minimum number of elements required to obtain satisfactory discrimination of populations. It was found that 16 elements could still provide 100% correct classification, whilst 12 elements still provided 97.30% correct classification. This pilot study has shown elemental analysis to be a potentially successful method for the discrimination of populations of L. salmonis, although the biological basis of the elemental signatures derived remains to be established

Role of kairomones in host location of the pennellid copepod parasite, Lernaeocera branchialis (L. 1767)

The life cycle of the parasitic copepod Lernaeocera branchialis involves 2 hosts, typically a pleuronectiform host upon which development of larvae and mating of adults occurs and a subsequent gadoid host, upon which the adult female feeds and reproduces. Both the copepodid and adult female stages must therefore locate and identify a suitable host to continue the life cycle. Several mechanisms are potentially involved in locating a host and ensuring its suitability for infection. These may include mechano-reception to detect host movement and chemo-reception to recognize host-associated chemical cues, or kairomones. The aim of this study was to identify the role of kairomones in host location by adult L. branchialis, by analysing their behaviour in response to fish-derived chemicals. Experiments demonstrated that water conditioned by immersion of whiting, Merlangius merlangus, elicited host-seeking behaviour in L. branchialis, whereas cod- (Gadus morhua) conditioned water did not. Lernaeocera branchialis are considered a genetically homogeneous population infecting a range of gadoids. However, their differential response to whiting- and cod-derived chemicals in this study suggests that either there are genetically determined subspecies of L. branchialis or there is some form of environmental pre-conditioning that allows the parasite to preferentially recognize the host species from which it originated

Innovative solutions for aquaculture: Assessment of in situ monitoring techniques and life history parameters for monogenean skin and gill parasites

First paragraph: External parasitic flukes that infect the skin and gills of yellowtail kingfish are among the most serious health issues for the culture of this species. Fingerlings grown in land-based hatcheries are free of parasites when transferred to sea-cages for grow out. The skin and gill parasites occur naturally and infect wild yellowtail kingfish stocks. Fluke populations proliferate on captive, seacaged stocks due to the direct lifecycle of the two parasite species. Fluke infections require regular monitoring by farm staff throughout the production cycle of yellowtail kingfish. Infections contribute to reduced growth, morbidity and if fluke populations reach sufficient intensity, the parasites can cause kingfish mortality on farms

Qualitative assessment of initial biofouling on fish nets used in marine cage aquaculture

A short communication which assesses the process of initial development of biofilms on nets used for fish culture in marine systems. Settlement follows a pattern of succession through conditioning, bacteria and diatoms with associated Extracellular Polymeric Substance (EPS) and other microscopic algae and fauna. Initial settlement appears to accelerate further settlement

Comparing the transcriptomes of embryos from domesticated and wild Atlantic salmon (Salmo salar L.) stocks and examining factors that influence heritability of gene expression

Background  Due to selective breeding, domesticated and wild Atlantic salmon are genetically diverged, which raises concerns about farmed escapees having the potential to alter the genetic composition of wild populations and thereby disrupting local adaptation. Documenting transcriptional differences between wild and domesticated stocks under controlled conditions is one way to explore the consequences of domestication and selection. We compared the transcriptomes of wild and domesticated Atlantic salmon embryos, by using a custom 44k oligonucleotide microarray to identify perturbed gene pathways between the two stocks, and to document the inheritance patterns of differentially-expressed genes by examining gene expression in their reciprocal hybrids.  Results  Data from 24 array interrogations were analysed: four reciprocal cross types (W♀×W♂, D♀×W♂; W♀×D♂, D♀×D♂)×six biological replicates. A common set of 31,491 features on the microarrays passed quality control, of which about 62% were assigned a KEGG Orthology number. A total of 6037 distinct genes were identified for gene-set enrichment/pathway analysis. The most highly enriched functional groups that were perturbed between the two stocks were cellular signalling and immune system, ribosome and RNA transport, and focal adhesion and gap junction pathways, relating to cell communication and cell adhesion molecules. Most transcripts that were differentially expressed between the stocks were governed by additive gene interaction (33 to 42%). Maternal dominance and over-dominance were also prevalent modes of inheritance, with no convincing evidence for a stock effect.  Conclusions  Our data indicate that even at this relatively early developmental stage, transcriptional differences exist between the two stocks and affect pathways that are relevant to wild versus domesticated environments. Many of the identified differentially perturbed pathways are involved in organogenesis, which is expected to be an active process at the eyed egg stage. The dominant effects are more largely due to the maternal line than to the origin of the stock. This finding is particularly relevant in the context of potential introgression between farmed and wild fish, since female escapees tend to have a higher spawning success rate compared to males

A comparison of gene transcription profiles of domesticated and wild Atlantic salmon (Salmo salar L.) at early life stages, reared under controlled conditions

Background: Atlantic salmon have been subject to domestication for approximately ten generations, beginning in the early 1970s. This process of artificial selection will have created various genetic differences between wild and farmed stocks. Each year, hundreds of thousands of farmed fish escape into the wild. These escapees may interbreed with wild conspecifics raising concerns for both the fish-farming industry and fisheries managers. Thus, a better understanding of the interactions between domesticated and wild salmon is essential to the continued sustainability of the aquaculture industry and to the maintenance of healthy wild stocks. Results: We compared the transcriptomes of a wild Norwegian Atlantic salmon population (Figgjo) and a Norwegian farmed strain (Mowi) at two life stages: yolk sac fry and post first-feeding fry. The analysis employed 44k oligo-microarrays to analyse gene expression of 36 farmed, wild and hybrid (farmed dam x wild sire) individuals reared under identical hatchery conditions. Although some of the transcriptional differences detected overlapped between sampling points, our results highlighted the importance of studying various life stages. Compared to the wild population, the Mowi strain displayed up-regulation in mRNA translation-related and down regulation in nervous and immune system -related pathways in the sac fry, whereas up-regulation of digestive and endocrine activities, carbohydrate, energy, amino acid and lipid metabolism and down-regulation of environmental information processing and immune system pathways were evident in the feeding fry. Differentially regulated pathways that were common among life stages generally belonged to environmental information processing and immune system functional groups. In addition, we found indications of strong maternal effects, reinforcing the importance of including reciprocal hybrids in the analysis. Conclusions: In agreement with previous studies we showed that domestication has caused changes in the transcriptome of wild Atlantic salmon and that many of the affected pathways are life-stage specific We highlighted the importance of reciprocal hybrids to the deconvolution of maternal/paternal effects and our data support the view that the genetic architecture of the strains studied highly influences the genes differentially expressed between wild and domesticated fish

First record of Aplysia dactylomela Rang, 1828 (Mollusca: Gastropoda) in Libyan coastal waters

The spotted sea hare Aplysia dactylomela Rang, 1828 is reported from four locations in Libyan coastal waters. The direct observation reported here resulted from a field survey of sea snails in the coastal environs of Tripoli, to the west of the country, while the remaining records result from postings made on the Facebook (TM) social media website, made by recreational spear-fishermen who took images and filmed specimens at Abo Fatma beach and Misrata, to the west of the country, and also at sites close to Benghazi and Al Hamamah, to the east of the country. This is the first record of A. dactylomela in Libyan waters

Transcriptomic comparison of communally reared wild, domesticated and hybrid Atlantic salmon fry under stress and control conditions

Background Domestication is the process by which organisms become adapted to the human-controlled environment. Since the selection pressures that act upon cultured and natural populations differ, adaptations that favour life in the domesticated environment are unlikely to be advantageous in the wild. Elucidation of the differences between wild and domesticated Atlantic salmon may provide insights into some of the genomic changes occurring during domestication, and, help to predict the evolutionary consequences of farmed salmon escapees interbreeding with wild conspecifics. In this study the transcriptome of the offspring of wild and domesticated Atlantic salmon were compared using a common-garden experiment under standard hatchery conditions and in response to an applied crowding stressor. Results Transcriptomic differences between wild and domesticated crosses were largely consistent between the control and stress conditions, and included down-regulation of environmental information processing, immune and nervous system pathways and up-regulation of genetic information processing, carbohydrate metabolism, lipid metabolism and digestive and endocrine system pathways in the domesticated fish relative to their wild counterparts, likely reflective of different selection pressures acting in wild and cultured populations. Many stress responsive functions were also shared between crosses and included down-regulation of cellular processes and genetic information processing and up-regulation of some metabolic pathways, lipid and energy in particular. The latter may be indicative of mobilization and reallocation of energy resources in response to stress. However, functional analysis indicated that a number of pathways behave differently between domesticated and wild salmon in response to stress. Reciprocal F1 hybrids permitted investigation of inheritance patterns that govern transcriptomic differences between these genetically divergent crosses. Additivity and maternal dominance accounted for approximately 42 and 25% of all differences under control conditions for both hybrids respectively. However, the inheritance of genes differentially expressed between crosses under stress was less consistent between reciprocal hybrids, potentially reflecting maternal environmental effects. Conclusion We conclude that there are transcriptomic differences between the domesticated and wild salmon strains studied here, reflecting the different selection pressures operating on them. Our results indicate that stress may affect certain biological functions differently in wild, domesticated and hybrid crosses and these should be further investigated