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.     

Cold hardiness of the eggs of the plaice,Pleuronectes platessa

Summary The pelagic eggs of the plaice, Pleuronectes platessa, at the stage of first heart beat, can tolerate freezing into solid sea ice at temperatures down to-6°C. A long term freezing period of 5 days, at sublethal temperatures of-3 to-4°C, had only a minor effect on the eggs. When the ice was absent from the medium, the eggs supercooled readily to temperatures around-15° to-20°C. However, no thermal hysteresis agents were present in the eggs at low temperature and the cold hardiness therefore seem to rely on a high tolerance to brine solutions at low temperatures.     

Precipitating activity in the sera of plaice, Pleuronectes platessa L. to a helminth antigen

An antigen abstract of Proleptus obtusus an intestinal nematode of the common dogfish has been shown to precipitate with a serum component of plaice. As the parasite has never been shown to infect plaice it is suggested that perhaps the serum component might be C–reactive protein (CRP). However, characterization of the component revealed that it was specific immunoglobulin analagous to antibody of the IgM class. It is suggested the helminth antigen involved is cross–reacting with a common antigen to which the plaice is normally exposed.     

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.     

Gastric evacuation in plaice, Pleuronectes platessa L.: effects of temperature and fish size

Using a volume dependent model of gastric evacuation, the effects of temperature and fish size were examined. Rates of gastric evacuation were unaffected by fish size but increased with increasing temperature. The relationship between maximum stomach volume and fish weight was found to be a linear one. From information of gastric evacuation rates and stomach volume, the amount of food evacuated from the stomach per day was calculated for different size classes of fish. Daily food evacuation increased in proportion to body weight to the power 0·68. Assuming these methods give a crude estimate of daily food intake, the results are discussed in relation to published work on food intake in fishes.     

Seasonal changes in serum lipid composition of the plaice, Pleuronectes platessa L.

Seasonal changes in the major lipid classes of plaice serum were examined in fish caught off the north-east coast of Scotland. Both male and female plaice were sampled monthly between June 1983 and May 1984. Total lipid, total cholesterol, free cholesterol, triglycerides, phospholipids and non-esterified fatty acids were measured using commercial kits validated against standard reference methods. There were no significant differences in lipid levels between male and female fish and, although there were real differences between months for each lipid class, there was no regular pattern over the 12-month period. All the lipid class values, however, reached a maximum in September, which could be related to the increase in feeding after spawning. The smaller peak in February could be associated with the mobilization of lipid reserves for gonadal development and to overcome the effect of starvation during spawning.     

Seasonal changes in serum glucose and condition of the plaice, Pleuronectes platessa L.

Monthly sampling of plaice caught off the north-east coast of Scotland between February 1983 and May 1984 revealed a seasonal increase in condition factor, hepatosomatic index and serum glucose, which appeared to be related to the period of active feeding. The spleen indices followed a similar seasonal increase, although this was not statistically significant, while the kidney indices showed little change over the 16 months. The peak for the gonadosomatic index was in February/March 1984, but among these plaice (aged 4–5 years) there were still large numbers of immature males and females and there was no increase in 1983. Apart from the hepatosomatic index for January 1984, there was no significant difference between male and female plaice.     

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.     

Seasonal changes in thyroidal status in the plaice, Pleuronectes platessa L.

A chemically specific analysis for plasma thyroid hormones has been used to disclose a seasonal bimodality in the concentrations of thyroxine (T₄) and triiodothyronine (T₃) in the plaice; maxima in both hormone levels occur in winter and in summer. The ratio of T₄ concentrations to T₃ varies seasonally being at a minimum in summer. A highly significant correlation of T₄ plasma levels with the landings per unit fishing effort for the same area has been observed.     

The cell kinetics of teleost fish epidermis: Epidermal mitotic activity in relation to wound healing at varying temperatures in plaice (Pleuronectes platessa)

An autoradiographic study of experimental wounding of the skin of juvenile plaice was performed using tritiated thymidine (20 μCi per gram body weight) injected intraperitoneally, at water temperatures of 5, 10 and 15°C.
Samples of lesion were taken at regular intervals up to 108 hours and autoradiographic preparations made. In addition wounds of less than 12 hours duration, unsuitable for autoradiography were examined and in vitro explant autoradiography studies on similar material used to support the findings for these short term lesions. Results showed that there was a very rapid migration of Malpighian cells into the defect which was not accompanied by any evidence, over the time-scale of the study, of a mitotic burst. Adjacent normal skin was markedly reduced in thickness. Closure of these relatively small wounds was achieved within nine hours at 10°C and by 12 hours at 5°C. The thickness of the migrated epidermal cover was much thicker at 15°C than 5°C.
Numbers of labelled cells were similar in migrating and peripheral epidermis and mucous cells appeared to be randomly distributed except in the periphery of advancing migrating cells, where thev were absent.     

Metabolic and immunological effects of endotoxin in the plaice, Pleuronectes platessa L.

In an effort to identify factors contributing to the resistance of fish to endotoxin toxicity, the metabolic effects of an intraperitoneal injection of a Boivin preparation of E. coli lipopoly-saccharide (LPS) were studied in plaice over 4 days. Significant changes were found in serum concentrations of glucose, cortisol and non-esterified fatty acids but not in total or free cholesterol. The effect of injected LPS on the phagocytic capacity of kidney neutrophils was examined because of the possible use of LPS to promote non-specific immunity. There was no enhancement of the chemiluminescent response to the phagocytosis of unopsonized bacteria by neutrophils from plaice 24 h after LPS treatment.     

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.     

Levels of calcium and iron in the ovaries and eggs of cod Gadus morhua L. and plaice Pleuronectes platessa L

In cod, Gadus morhua L., calcium levels expressed as percentage of tissue dry weight were (mean +/- SD) 0.0042 +/- 0.0010% of ovary before final maturation and 0.0133 +/- 0.0024% of mature eggs. In plaice Pleuronectes platessa L. these values were 0.0029 +/- 0.0006 and 0.0097 +/- 0.0001. In mature eggs of both fish species these values are at least one order of magnitude less than in the yolk of the domestic fowl Gallus domesticus L. (0.14% of dry weight). The corresponding values for iron were (2.31 +/- 0.22) X 10(-3)% of ovary and (3.07 +/- 0.29) X 10(-3)% of mature egg in cod. In plaice these values were (1.57 +/- 0.15) X 10(-3)% and (2.74 +/- 0.94) X 10(-3)%. These levels are 4-7 times less than in the yolk of the domestic fowl (0.012-0.020%). These differences between the mature pelagic eggs of marine teleosts and the eggs of other oviparous vertebrates appear to be related to the availability of these elements in sea water, the different nature of the yolk phosphoprotein and the lower levels of protein phosphorus in the eggs of these fish species.     

Gyrodactylus unicopula Glukhova, 1955, from young plaice Pleuronectes platessa L. with notes on the ecology of the parasite

A population of Gyrodactylus unicopula Glukhova, 1955, is described from the gill filaments of young plaice from a nursery ground on the north-west coast of Scotland. The seasonal incidence of this parasite was followed by sampling the 1965 year class of plaice at intervals of from one to three months from April 1965 to April 1967. The level of intensity is usually low and shows no progressive increase, although the fish are shown to be capable of supporting much higher populations of the parasite, which suggests the existence of some mechanism controlling the level of intensity in the natural population. The distribution of G. unicopula on the filamentous surface of the gills is shown in detail. χ² analyses indicated no significant association or antagonism between this parasite and two ciliates commonly found on the gills of the same population of plaice. Gyrodactyliasis, the disease caused by parasites of the genus Gyrodactylus, is discussed with special reference to marine fish farming.     

Isolation of substances with vascular permeability-increasing activity from the skin of the plaice (Pleuronectes platessa L.)

A methanolic extract of plaice skin, from which lipids had been removed, was chromatographed on alumina, eluted with decreasing concentrations of ethanol. Only the 60% ethanol fraction exhibited smooth muscle activity, with bradykinin-like properties. The 20% ethanol fraction increased vascular permeability in rat skin, as measured by dye-leakage. This was not due to the degranulation of mast cells. Intradermal injection of either fraction into the plaice caused localized erythema.     

Some effects of temperature, feeding and body weight on nitrogenous excretion in young plaice Pleuronectes platessa L.

Ammonium-nitrogen accounted for 75–85% of the nitrogenous excretion of young plaice, Pleuronectes Platessa L., prior to feeding. Rates of excretion increased with increasing temperature and in proportion to the body weight to the power 0.67. Following feeding the rate of excretion increased to between 2and 11 times the pre-fed rate and both the peak rate and as the duration of the effect were increased, the greater the amount of nitrogen absorbed from the food. The magnitude of the effect increased with increasing ration and the relationship was best described by an asymptotic equation. Estimates of endogenous nitrogen excretion and maintenance requirements were made from the results of the feeding experiments and were fond to be in close agreement with published data. Estimats of nitrogen requirements obtained from the experiments on fish in the post-absorptive state yielded values intermediate between endogenous and maintenace requirements.