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.     

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

Eighty-nine O-group plaice from a natural population were exposed at 15°C to heavy infection by Cryptocotyle lingua cercariae. Subsequently 45 fish were retained at 15°C, whilst 44 were held at 5°C. Both groups were sampled by killing individual fish at intervals of 6,18,42 h and daily thereafter up to 710 h. Entire fish were fixed immediately in formol saline, transversely sectioned and stained by H & E, PAS, PAS-diastase, JSDB 109, Picro-Mallory, Masson's trichrome, Gram-Weigert and Alcian blue. Histopathological observations showed: (a) epidermal lesions associated with encysted metacercariae in adjacent tissues; (b) myofibrillar necrosis associated with bacteria possibly introduced by the parasite; and (c) a reactive swelling of the intermuscular septa. The progressive development of the parasite cyst and host capsule is described. Development of both was markedly inhibited at the lower temperature, but the inflammatory response at either temperature was slight. This may be evidence of a long-standing host-parasite relationship which has evolved to an advanced state of adaptation on the part of the parasite and tolerance on the part of the host.     

The immunohistochemical localization of lysozyme in plaice (Pleuronectes platessa L.) tissues

The location of lysozyme in fresh plaice ( Pleuronectes platessa L.) tissues was studied using an immunofluorescent technique. An antiserum to a homogeneous sample of plaice serum lysozyme was prepared in rabbits and was shown to be monospecific by immunodiffusion and inhibition studies. This preparation, used in conjunction with fluorescent labelled sheep anti-rabbit immunoglobulin, demonstrated lysozyme in plaice neutrophils, peritoneal macrophages, gill cartilage and epithelium, spleen, kidney, skin and mucus. The distribution of lysozyme in the skin and secreted mucus was affected by the method of killing the fish.     

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.     

Monoaminergic nerves in the skin of plaice Pleuronectes platessa (L.)

1. The fluorescent histochemical technique of Falck and Hillarp was applied to plaice skin. The presence of monoaminergic nerve terminals, containing predominantly stores of adrenaline, forming a plexus in and around the melanophore layer was demonstrated. 2. Such stores were enriched by noradrenaline in the presence of monoamine oxidase inhibitor, unaffected by spinal section, depleted by spinal nerve section or ligatures and abolished by reserpine. 3. The observations support the view that teleost sympathetic melanophore aggregating nerves are truly adrenergic.     

The accumulation of zinc by O-group plaice, Pleuronectes platessa (L.), from high concentrations in sea water and food

The uptake of zinc by young O-group plaice, Pleuronectes platessa was investigated at levels of high input from sea water and food ( Artemia nauplii). Zinc concentrations in the fish were strongly weight-specific and accumulation effects were distinguished by regression analysis. Sea water was estimated to contribute >10% of the total input up to levels corresponding to high environmental concentrations ( c. 0.1 μg Zn ml⁻¹) and food was the major source of zinc up to sea water concentrations of c. 0.6 μg Zn ml⁻¹. A high degree of regulation obtained along the food pathway to the fish although, in the system studied, most of this regulation was effected by the food organism. Regulation of food input explained the lack of variation in zinc levels previously observed in wild populations.     

A specific ringer solution for the plaice (Pleuronectes platessa L.)

Following analysis of the serum of the blood of the plaice ( Pleuronectes platessa L.), a Ringer solution specific for this fish has been devised and tested. It has proved a satisfactory physiological saline for use in electrophysiological experiments and in the preparative procedures for electron microscopy.     

The lysozyme of the plaice Pleuronectes platessa L.

Lysozyme from the serum of the plaice, Pleuronectes platessa L., has been purified 78-fold with chitin coated cellulose.

• 1.2. Further purification on CM-cellulose yielded a single band on acrylamide electrophoresis, exhibiting lysozyme activity.
• 2.3. The quantitative amino acid composition of plaice serum lysozyme is reported.
• 3.4. The mol. wt is identical with hen egg white lysozyme.
• 4.5. A method is described for identifying fractions with lysozyme activity in polyacrylamide gels.

     

Determining abundance, distribution, and mortality of 0-group plaice (Pleuronectes platessa L.) in the Wadden Sea

Errors in estimates of year class strength and mortality among 0-group plaice can result from migrations. In order to reliably determine the number present, tidal flats and channels in a large and clearly delimited Wadden Sea area (715 km²) were sampled simultaneously using different fishing methods (by-catch, beam trawls, pushnets, and fyke nets). The numbers of 0-group plaice calculated for each depth stratum reflected changes in the distribution patterns during the complete diurnal cycle and during the course of the season and proved to be most reliable for estimating the total number in the whole area. In contrast to this, the traditional way of calculating population size by multiplying the arithmetic mean of simultaneous catches resulted in very different numbers which would entail an overestimation of mortality.     

Effect of opsonization on oxidative metabolism of plaice (Pleuronectes platessa L.) neutrophils

The effect of serum opsonization on Vibrio alginolyticus (heat-killed)-stimulated chemiluminescence (CL) by plaice kidney- and peritoneal exudate-derived neutrophils was investigated. Peritoneal neutrophils only recognized heat-labile and kidney neutrophils only heat-stable opsonic activity in normal serum. Specific antibody did not show opsonic activity nor any synergism with the normal serum opsonins for either neutrophil population. Evidence was found for the production, by plaice neutrophils, of H2O2, O2-, OH. and two or more, as yet unidentified, reactive oxygen species (ROS).     

Year-class strength regulation in plaice (Pleuronectes platessa L.) on the Swedish west coast

Year-class strength of plaice (Pleuronectes platessa) has been studied over periods of 10 and 35 years, respectively, in two shallow sandy areas on the Swedish west coast. In one area, Gullmar Bay (N 58 ° 19 – W 11 ° 33), 0-group plaice were sampled quantitatively with a drop trap at 0–0.7 m depth and densities between 0.2 and 3.8 ind./m² were recorded in early summer. In the other area, Laholm Bay (N 56 ° 30 – W 12 ° 55), 0-group plaice were sampled semi-quantitatively in August with young-fish trawl at 1.5 m depths. Densities between 0.001 and 0.28 ind./m² were assessed.The effects of temperature and wind (in the winter and early spring) and predation (in early summer), on the recruitment of 0-group plaice were investigated. In Gullmar Bay high recruitment occurred after severe winters and in years when on-shore winds dominated during spring. No such correlations were found in Laholm Bay. In Gullmar Bay a significant inverse relationship was found between the density of O-group plaice in early summer and the biomass of brown shrimp (Crangon crangon) and the shore crab (Carcinus maenas), the main predators in the nursery area.The importance of physical and biological factors regulating recruitment in plaice are discussed.     

A trypanosome from the plaice, Pleuronectes platessa L.

The results of a 12 month study of Trypanosoma platessae Lebailly (1904), a haemoflagellate parasitising the plaice, Pleuronectes platessa L., are presented. The morphological features of the trypanosome are described, with experimental investigations of the host specificity of T, platessae . The flagellate occurred at low parasitaemias in wild plaice with a seasonal variation in the numbers of infected hosts, the percentage of parasitised fish reaching a maximum of 20 % in March and declining in the summer months.
Attempts to establish T. platessae in the laboratory, by means of passage of infected blood, gave inconsistent results. Elevated levels of β-globulins were detected in the sera of T. platessae infected plaice, and it is suggested that this may be associated with antibody secretion to the parasite.     

The ultrastructure of plaice (Pleuronectes platessa) leucocytes

The leucocytes of plaice were examined and, at a morphological level, in addition to their response to the injection of carbon particles, they could be divided into four main types. Unlike previous reports, monocytes were seen. The neutrophil granules resembled the third granule type described in mammalian neutrophils. Monocytes and thrombocytes were the only cells to take up appreciable amounts of the carbon.     

Temperature-dependent development rates of plaice (Pleuronectes platessa L.) eggs from the Irish Sea

Experimentally derived temperature-dependent development ratesfor planktonic fish eggs are required for developing IndividualBased Models (IBMs) of early life history stages and for estimatingdaily egg production in ichthyoplankton surveys. Developmentrates of Irish Sea plaice eggs were experimentally determinedfor temperatures between 4.5 and 12° C. Differences in developmentrates were apparent when these results were compared with literaturedata for eggs from North Sea parents. On average, plaice eggsfrom the Irish Sea developed more rapidly and hatched up to2 days earlier compared with North Sea eggs incubated at thesame temperature. In terms of parental effects, larger femalestended to produce larger eggs but egg size decreased over successivebatches from the same female within a year. Temperature-dependentegg development rates were influenced by egg size in a non-linearmanner for eggs incubated at 6° C, but egg size had no effectat higher temperatures. Since most plaice eggs sampled fromthe Irish Sea during ichthyoplankton surveys in 1995 and 2000were found in waters cooler than 8° C, such effects maymodify egg development rates in the wild.     

Aspects of leucocyte migration in the plaice, Pleuronectes platessa L.

Histological studies on plaice tissues revealed mononuclear phagocytes emigrating through capillary endothelium in response to bloodborne foreign erythrocytes, with a >50% increase in circulating leucocytes within 2 h. Despite this rapid influx of leucocytes, cellular accumulation during peritoneal inflammatory responses was significantly slower than in mammals. In vitro studies with chemotaxis chambers revealed that random leucocyte migration was enhanced in the presence of endogenous chemostimulatory substances (inflammatory exudate fluid and endotoxin-activated plaice serum) although directional migration was not demonstrated. It is suggested that, compared with mammals, the slower accumulation of teleostean leucocytes at sites of injury is not due to a locomotory defect but possibly results from lower levels of endogenous chemoattractants being generated.     

The effects of adrenal hormones, endotoxin and turpentine on serum components of the plaice (Pleuronectes platessa L.)

1. Within 24 hr of injection into plaice, cortisol, deoxycorticosterone, adrenalin or endotoxin cause an increase (P less than 0.001) in circulating C-reactive protein (CRP). Turpentine and soluble dexamethasone have no effect. 2. The increase in CRP with endotoxin is not enhanced with adrenalin or deoxycorticosterone, and in conjunction with cortisol the increase is additive. 3. Changes in CRP are independent of the amounts of serum amyloid P-component or total protein. 4. Turpentine, cortisol and adrenalin cause a rapid increase in circulating glucose. 5. It is concluded that some adrenal hormones stimulate the CRP acute phase response in plaice, without an apparent provoking agent.     

Serum cortisol, glucose and lipids in plaice (Pleuronectes platessa L.) exposed to starvation and aquarium stress

Plaice were maintained in the aquarium (11-12 degrees C) during May for 15 days without feeding. Within 48 hr, there was a decline in serum total lipids (P less than 0.001), phospholipids (P less than 0.01), triglycerides (P less than 0.001), cortisol (P less than 0.01) and glucose (P less than 0.001), but an increase in nonesterified fatty acids (NEFA; P less than 0.01). There was a significant inverse correlation between NEFA and glucose over 15 days (P less than 0.001) and between NEFA and cortisol over the first 5 days (P less than 0.01). Cortisol and glucose showed a significant correlation over 15 days (P less than 0.01). Serum cortisol and glucose were not apparently affected by starvation. Only cortisol provided a sensitive indicator of aquarium disturbance. Exposure of the fish to agitation or reduced O2 for 1 hr significantly elevated cortisol (P less than 0.001) but only the latter treatment elevated glucose (P less than 0.01); neither treatment affected the lipids.     

Low effective population size and evidence for inbreeding in an overexploited flatfish, plaice (Pleuronectes platessa L.)

Overexploitation and subsequent collapses of major worldwide fisheries has made it clear that marine stocks are no inexhaustible. Unfortunately, the perception remains that marine fished are resilient to large population reductions, as even a commercially 'collapsed' stock will still consist of millions of individuals. Coupled with this notion is the idea that fisheries can, therefore, have little effect on the genetic diversity of stocks. We used DNA from archived otoliths collected between 1924 and 1972 together with 2002 juvenile;s tissue to estimate effective population size (Ne) in plaice (Pleuronrctes platessa). Ne was estimated at 20,000 in the North Sea and 2000 in Iceland. These values are five orders of magnitude smaller than the estimated census size foe the two locations. Populations examined between 1924 and 1960 were in Hardy-Weinberg equilibrium, whereas populations examined after 1970 were not. Extensive testing was performed to rule out genotyping artefacts and Wahlund effects. The significant heterozygote deficiencies found from 1970 onward were attributed to inbreeding. The emergence of inbreeding between 1905 and 19070 coincides with the increase in fishing mortality after World War II. Although the biological mechanisms remain speculative, our demonstration of inbreeding signals the need for understanding the social and mating behaviour in commercially important fishes.