The fine structure of the epidermis of two species of salmonid fish,the Atlantic salmon (Salmo salar L.) and the brown trout (Salmo trutta L.)

The fine structure of epidermal mucous cells of two species of salmonid fish has been described. Mucous cells are, next to filament-containing cells, the most commonly encountered cells in fish epidermis. The development of the cells as they progress to the periphery has been characterised. They are initially difficult to distinguish from filament-containing cells: later, they can be recognised by the presence of much smooth-surfaced E.R. The mucigenesis and the subsequent secretion of mucus has been observed and it is essentially comparable to that which occurs in the mucous cells of the mammalian intestine. The mucous layer of the epidermal surface seems to mainly comprise of the products of these mucous cells and the "cuticle" seen in other species has not yet been observed in the salmonid species investigated here.     

Seasonal changes in the epidermis of the brown trout Salmo trutta (L.)

A sexual dimorphism was found in the epidermis of hatchery-reared brown trout. The epidermis of both male and female fish undergoes rhythmical changes in thickness during successive spawning cycles. For most of the year the male has a significantly thicker epidermis than the female. During the spawning period (December and January) there is a highly significant drop in the concentration of mucous cells in the epidermis of the male brown trout. These findings are discussed in the light of our knowledge of the skin structure in other salmonid fish and in relation to possible hormonal factors influencing the epidermis of teleosts.     

The fine structure of iridosphores in the skin of the Atlantic salmon (Salmo salar L.)

The fine structure of the distinct morphological forms of iridophore cells of the dermis of the Atlantic salmon, Salmo solar L., is described. Three types were found in the pigmented region of the sub-epidermis of the head, dorsal and lateral skin of the fish. A fourth type of iridophore found in great numbers in the hypodermal region of the ventral skin has been identified. The organization and the dimensions of the lacunae of the reflecting platelets and of the other cytoplasmic inclusions of the cell types are compared. Reflecting platelets have been isolated from the skin and are described.     

The distribution of mucous cells in the epidermis of the brown trout Salmo trutta (L.) and the char Salvelinus alpinus (L.)

The mucus of the trout and char was found to contain only one sialic acid, n-acetyl neuraminic acid (NANA). The concentration of NANA in the epidermis is directly proportional to the mucous cell concentration. In both species the highest concentration of mucous cells occurs on the anterior regions of the body and there are significantly fewer cells on the fins. These findings are discussed in the light of our knowledge of the functions of fish mucus.     

Studies on the skin of plaice (Pleuronectes platessa L.)

A histological, histochemical and ultrastructural examination of the skin of wild and cultured plaice was carried out, using fish from each year class from 0+ to 4+. The skin was shown to be similar in general structure to that of other teleosts but a previously undescribed cell, designated the Eosinophilic Granular Cell, a dendritic secretory cell found throughout the basal layers of the epidermis, is described. It was fixed only by formalin or dichromate, and contained numerous acidophilic granules. Melanin-bearing macrophages were observed migrating through the epithelium, but no DOPA or tyrosinase positive cells were observed by the methods used. Mast cells were very common in the dermis but were only demonstrable by special techniques. The melanophore and guano-phore systems are described and although no melanophores or melanocytes were found in the unpigmented areas of partially pigmented hatchery-reared fish, the integrity of the guanophore system was complete in such fish.     

Phagocytosis by B-cells and neutrophils in Atlantic salmon (Salmo salar L.) and Atlantic cod (Gadus morhua L.)

Phagocytosis by fish cells has mostly been studied using adherent leucocytes, excluding suspended cells such as the majority of B-cells and neutrophils, but a recent study describes professional phagocytosis of latex beads and bacteria by B-cells from rainbow trout. In the present study, phagocytosis by B-cells and neutrophils from salmon and cod was studied. Leucocytes were isolated from peripheral blood (PBL) and head kidney (HKL). By flow cytometry analyses, proportions of MAb labelled cell populations with internalized fluorescent beads, as well as the number of beads within each cell, could be determined. Phagocytic capacity and ability were demonstrated in B-cells and neutrophils from salmon and cod. In salmon, B-cells had higher phagocytic ability than neutrophils in HKL, but not in PBL. For cod the phagocytic ability of B-cells were lower than for neutrophils in both HKL and PBL, but the phagocytic capacity of cod B-cells were higher than for neutrophils in both HKL and PBL. For salmon B-cells the phagocytic capacity was lower than or similar to neutrophils in HKL and PBL. The total phagocytic ability of leucocytes was different in the species studied. The highest phagocytic ability was observed in cod, showing similar values for PBL and HKL. Salmon PBL displayed about twice the phagocytic ability of cod PBL. There seemed to be some major differences between the two fish species concerning phagocytosis. In salmon, a rather large proportion of phagocytic leucocytes were phagocytic B-cells, indicating that B-cells may have an important function in particle clearance in this species. In cod, phagocytic leucocytes in HKL and PBL were mostly neutrophils, and only a small proportion of B-cells were phagocytic, supporting the more prominent role of innate immune functions in cod neutrophils.     

Acidophilic granular cells in the epidermis of the brown trout,Salmo trutta L.

Summary Acidophilic cells occur in the epidermis of several species of salmonid fish, although their abundance fluctuates considerably between individuals within the same population and at different times during the life cycle. The histology, histochemistry and ultrastructure of an acidophilic, granular celltype in the epidermis of the brown trout, Salmo trutta L., is described. At the light microscope level this cell type is easily distinguished from the large, mucus-secreting, epidermal goblet cells by its acidophilic, proteinaceous secretion. At the ultrastructural level this secretion consists of membrane-bound granules formed by the very active Golgi region. It is argued that the acidophilic, granular cell is not a transformed blood cell but constitutes a normal epidermal component of the brown trout. Possible roles of this cell in the function(s) of the epidermis are discussed.     

Studies on the skin of plaice (Pleuronectes platessa L.)

Histological and ultrastructural examinations were carried out on the integuments of larval plaice from hatching to 60 days. The skin of the newly hatched larva consisted of a delicate epidermis overlying a fluid filled dermal space. As the fish matured this became thicker, the fibrous dermis developed and the mitochondrion rich 'chloride cells' degenerated. Around the 42nd day the eosinophil granule cell appeared in the basal layers of the epidermis and by the sixtieth day the epidermis could be considered fully developed.     

Fine structural analysis of the basal epidermal receptor cells in the earthworm (Lumbricus terrestris L.)

Summary A fine structural investigation was performed on receptor cells lying at the base of the epidermis in the earthworm, Lumbricus terrestris. Two types of receptor cells with many similarities, but also with major differences, were discriminated.One receptor is of the microvillar receptor type, that appears to be identical with the photoreceptor cell described earlier by Röhlich et al. (1970). Proximal to the nucleus is a large vacuole (phaosome, Binnenkörper) with many daughter cavities containing a large number of microvilli and several cilia with the 9 × 2 + 0 microtubular pattern. The intracellular cavity has no connection with the surface membrane, in contrast to that in hirudineans (White and Walther, 1969) and pogonophores (Nørrevang, 1974).The other receptor is the ciliated receptor type, that is presently described for the first time. This receptor also has a comparatively large uniform cavity, few microvilli and about 20 cilia with the 9 × 2 + 2 microtubular pattern. The cilia leave the cell in the proximal part through a wide opening, make a turn of 180 °, and proceed toward the epidermal surface. Receptors of a similar type have been described by Golding and Whittle (1975) in the cerebral ganglion of four limicole oligochaete annelids; they presumed that these cells have an osmoreceptor function. The new epidermal receptor type described in the present investigation probably has a chemoreceptor function of hitherto unknown kind.This investigation was supported by the Royal Physiographic Society at Lund, Sweden. The author would like to express his thanks to Mrs. Lena Sandell for skilful technical assistance     

Movements of salmon Salmo salar (L.) to and from Irish waters

Fish tagged as kelts at a number of Irish stations have been recaptured outside Irish waters along the coasts of Great Britain, Norway and West Greenland. Salmon tagged as smolts in three Irish rivers have been recaptured off the coast of West Greenland and one off the Faroes. Sixteen fish tagged as smolts in Denmark, Great Britain and Sweden were recaptured in Irish rivers or on the coastline of Ireland. Clean salmon tagged in the open sea have travelled to the coasts of Great Britain and a single fish travelled to southern Sweden. Feeding salmon tagged off West Greenland, in the Labrador sea and off the Faroes have been recaptured in Irish waters.     

Identification of glycoproteins in goblet cells of epidermis and gill of plaice (Pleuronectes platessa L.), flounder (Platichthys flesus (L.)) and rainbow trout (Salmo gairdneri Richardson)

Synopsis A quantitative analysis has been made of the glycoproteins present in the goblet cells of the epidermis, gill filaments and gill lamellae of three species of teleost fish. The glycoproteins have been identified by a combination of techniques, including the use of the enzyme sialidase followed by Alcian Blue staining, at pH 2.6 or I. o, in combination with periodic acid-Schiff. The selected fish were representative of species living in marine, freshwater and estuarine environments.The range of glycoproteins identified in these fish was similar to that found in mammalian tissue in that both neutral and acid glycoproteins were present, the latter included both sialomucins sensitive and resistant to sialidase, and sulphomucin. A single goblet cell contained either neutral or acid glycoproteins alone or in combination. Only the epidermis of the plaice and rainbow trout contained uniform cell populations producing acid glycoproteins, the former sulphomucin and the latter mainly sialomucin. At each site in the flounder and in the gill epithelia of the plaice and rainbow trout, the goblet cell population was mixed, with cells producing each type of glycoprotein. The number of goblet cells producing each type of glycoprotein varied at each tissue site.     

Comparative ionic flux and gill mucous cell histochemistry: effects of salinity and disease status in Atlantic salmon (Salmo salar L.)

Two experiments were conducted to assess the physiological effects of freshwater exposure and amoebic gill disease (AGD) in marine Atlantic salmon (Salmo salar L.). The first experiment monitored marine salmon during a 3 h freshwater exposure, the standard treatment for AGD in Tasmania. The second experiment described the gill mucous cell histochemistry for freshwater adapted and seawater acclimated fish (AGD affected and unaffected) for possible correlations to ionoregulation. When exposed to freshwater, marine Atlantic salmon experienced a minor ionoregulatory dysfunction represented by a net efflux of Cl(-) ions at 3 h. AGD affected fish experienced the net efflux of Cl(-) ions 1 h sooner, and had a significantly greater net efflux of total ammonia. Changes to gill mucous cell populations corresponded to differing salinity and the presence of AGD. In AGD affected fish, these populations significantly differed between lesion and non-lesion associated areas of the gill filament. Our results have shown changes in the ionoregulatory capacity of Atlantic salmon due to freshwater exposure and AGD. Gill mucous cell histochemistry indicates the potential importance of the mucous layer in ionoregulation and disease. In comparison to previous studies on rainbow trout, these results suggest that Atlantic salmon have a greater short-term ionoregulatory capacity.     

Enzyme activities of intestinal triacylglycerol and phosphatidylcholine biosynthesis in Atlantic salmon (Salmo salar L.)

The substitution of fish oil with plant-derived oil in diets for carnivorous fish, such as Atlantic salmon, has previously revealed the potentially deleterious supranuclear accumulation of lipid droplets in intestinal cells (enterocytes) which may compromise gut integrity, and consequently, fish health. This suggests that unfamiliar dietary lipid sources may have a significant impact on intestinal lipid metabolism, however, the mode of lipid resynthesis is largely unknown in teleost fish intestine. The present study aimed at characterising three key lipogenic enzymes involved in the biosynthesis of triacylglycerol (TAG) and phosphatidylcholine (PC) in Atlantic salmon enterocytes: monoacylglycerol acyltransferase (MGAT), diacylglycerol acyltransferase (DGAT), and diacylglycerol cholinephosphotransferase (CPT). Furthermore, to investigate the dietary effect of plant oils on these enzymes, two experimental groups of fish were fed a diet with either capelin (fish oil) or vegetable oil (rapeseed oil:palm oil:linseed oil, 55:30:15 w/w) as the lipid source. The monoacylglycerol (MAG) pathway was highly active in the intestinal mucosa of Atlantic salmon as demonstrated by MGAT activity (7 nmol [1-(14)C]palmitoyl-CoA incorporated min(-1) mg protein(-1)) and DGAT activity (4 nmol [1-(14)C]palmitoyl-CoA incorporated min(-1) mg protein(-1)), with MGAT appearing to also provide adequate production of sn-1,2-diacylglycerol for potential utilisation in PC synthesis via CPT activity (0.4 nmol CDP-[(14)C]choline incorporated min(-1) mg protein(-1)). Both DGAT and CPT specific activity values were comparable to reported mammalian equivalents, although MGAT activity was lower. Nevertheless, MGAT appeared not to be the rate-limiting step in salmon intestinal TAG synthesis. The homology between piscine and mammalian enzymes was established by similar stimulation and inhibition profiles by a variety of tested cofactors and isomeric substrates. The low dietary n-3/n-6 PUFA ratio presented in the vegetable oil diet did not significantly affect the activities of MGAT, DGAT, or CPT under optimised assay conditions, or in vivo intestinal mucosa lipid class composition, when compared to a standard fish oil diet.     

Effects of substrate embeddedness on juvenile brown trout (Salmo trutta L.) growth and behaviour

Young salmonids may use substratum as hiding stations and/or shelter and they depend on invertebrates, which develop on substratum, for their feeding. For several decades, human activities have contributed to increase siltation in streams, and negative consequences on trout production have sometimes been highlighted. In the research devoted to the understanding of that negative effect, most studies have focused on embryo‐larval survival, and consequences of substrate embeddedness on later stages have rarely been investigated. In the present work we attempt at studying the impact of embeddedness on brown trout juveniles. In an experimental channel, trout growth was compared in embedded and non‐embedded sections. Growth was reduced with embeddedness due to change in trophic conditions and/or in habitat. To investigate the direct role of substratum for fish, trouts behaviour was observed from an under water observation room in two cages offering embedded and non‐embedded substrate conditions but similar trophic conditions. Competition appeared heavier in the embedded cage where dominated fishes stayed almost motionless. The effect of substratum quality on intra‐specific competition is discussed in relation with visual isolation and territory size.     

Activity cycles in the brown trout (Salmo trutta Lin.)

Summary The annual and diurnal activity cycle for four naturally feeding brown trout separately confined in netting cages on the bed of Windermere is described. All the fish showed a similar annual cycle of maximum activity during May and June, one fish showing a second activity peak during the autumn. The fish also showed a similar diurnal activity rhythm of low activity during the night and increased activity during the day with a pronounced increase at dawn.The possible influence of light and temperature on the fishes activity is briefly discussed.     

Ligula intestinalis (L.) (Cestoda: Pseudophyllidea): plerocercoid-induced changes in the spleen and pronephros of roach, Rutilus rutilus (L.), and gudgeon, Gobio gobio (L.)

The effects of the plerocercoid of Ligula intestinalis (L.) (Cestoda: Pseudophyllidea) on the major lymphoid organs, the spleen and pronephros, of roach, Rutilus rutilus (L.), and gudgeon, Gobio gobio (L.), are described. The spleen of ligulosed roach showed a significant decrease in weight. Differential cell counts suggested this was due to a reduction in erythrocytes, despite significant increases in macrophages and vacuolated granulocytes. The spleen of gudgeon, which consisted almost entirely of erythrocytes, showed a slight reduction in weight in ligulosed fish. In both roach and gudgeon this decrease was independent of parasite burden. Differential cell counts of the pronephros from ligulosed roach revealed a significant decrease in neutrophils and increase in vacuolated granulocytes. In the pronephros of gudgeon, however, cell counts were unaffected by ligulosis. Ultrastructural observations included an apparent disintegration of vacuolated granulocytes and increased pinocytic activity in specialized endothelial cells in the spleens of ligulosed roach. Also, melano-macrophage centres and melano-macrophages increased in the spleen of ligulosed roach and gudgeon, respectively. The marked changes in spleen weight and differential cell counts in ligulosed roach and lack of such changes in ligulosed gudgeon correlate with the differential response to the parasite by these two fish species.     

Histochemical observations on the salmonids Salmo salar L. and Salmo trutta L. and the ephemeropterans Baetis rhodani (Pict.) and Ecdyonurus venosus (Fabr.) following a simulated episode of acidity in an upland stream

Salmonids (Salmo salar, Salmo trutta) and mayflies (Baetis rhodani, Ecdyonurus venosus) which had been exposed to simulated episodes of low pH, and low pH with elevated aluminium, were examined histochemically for the presence of aluminium and mucus. The control fish and mayflies and those exposed to low pH did not stain for aluminium, although increased mucus production was demonstrated in fish gills. Trout and salmon exposed to aluminium at low pH exhibited extensive aluminium and mucus coating of the secondary gill lamellae. No mucus was produced by mayflies but aluminium was apparent on all parts of the body. Mean aluminium concentrations of digested fish gills were 2950 and 3050 g g^–1 dry wt. for trout and salmon respectively, whilst for whole specimens of B. rhodani and E. venosus values were 1 200 and 3 175 g g^–1 dry wt.     

Temporal variations in the skin of Atlantic salmon Salmo solar L.

The thickness of the skin of Atlantic salmon increases throughout the first two years of life. This increase involves, principally, an increase in the connective tissue of the dermis, and occurs independently of sexual maturity. The concentration of mucous cells in the epidermis changes seasonally, being least during the winter period. No sexual dimorphism is observed in these features among sexually immature fish. The epidermis of precociously mature male parr is thicker and contains more mucous cells than that of sexually immature individuals.     

The hepato-renal syndrome in cultured turbot (Scophthalmus maximus L.)

The histopathology of biliary hyperplasia and renal calcinosis, associated with infections of two parasites, Myxidium incurvatum and Rhabdospora thelohani , is described in turbot, Scophthalmus maximus (L.), from Scottish marine fish farms and wild populations. Liver and kidney lesions were divided into four, and parasite infections into five grades of severity. A few of the fish examined were not parasitized, but every one showed some evidence of pathological change, even the apparently normal individuals from all populations. Although there is a positive association between the parasitic and pathological data, parasitism is not considered to be the principal cause of either the hepatic or renal lesion; the evidence suggests rather a secondary opportunist role for the parasites. The general severity of the condition in captive stocks suggests that the cause may lie in the fundamentals of present techniques of turbot husbandry, but no specific agent has been identified. A case is put forward for considering Rhabdospora to be a parasite rather than a specialized host cell.