Morphometric analysis of the gills of the European eel, Anguilla anguilla

The gills of the European eel, Anguilla unguilla L. were analysed morphometrically. Fresh (unfixed) gills and resin-replica casts of the branchial vascular system were examined. The total gill surface area was found to be proportional to (body mas) 0-715 for fish of between 60 and 1 160 g. This relationship between gill surfxe area and body mass was maintained irrespective of a reduction in body mass produced by fasting. Vessel dimensions were obtained from the vascular casts. Calculations made using these values suggest that the major sites of gill vascular resistance lie at the level of the afferent lamellar arterioles and the secondary lamellae.     

Structural characteristics of the epidermal mucosa in yellow and silver European eel, Anguilla anguilla (L.)

Histological observations were made on the distribution and concentration of the superficial mucous cells and on the thickness of the epidermis in immature yellow and in male and female silver eels. In all groups there was a decrease in the concentration of mucous cells from the anterior regions to the posterior regions, while the epidermis was thickest in the median parts of the body. Besides these common characteristics, a sexual dimorphism was shown: in silver eels the epidermis is thicker than in immature yellow eels; female silver eels have both the thickest epidermis and the greatest concentration of superficial mucous cells.
Results are discussed in the light of knowledge on the influence of environmental and endocrine factors on the structure and function of epidermal mucosa in teleosts.     

The influence of body size and temperature on oxygen consumption of the European eel, Anguilla anguilla

The effect of temperature and body size on oxygen uptake of European eels, Anguilla anguilla , was studied. The mass specific oxygen uptake of large eels was lower at all temperatures than that of small eels. The effect of temperature on metabolism was greater on small eels than on large eels. The relationship between oxygen consumption and body size is described by the equation M = a W^b , where M is metabolism (O₂ h^-1 ), W is body weight (g), b is the slope of the function which describes the relationship between body weight change and metabolism, and a is the temperature-dependent constant of the equation. In this study it was found that increased temperature caused an increase in a and a decrease in b .     

Glycolytic fluxes in European silver eel, Anguilla anguilla: sex differences and temperature sensitivity

European silver eels migrate 6000 km to their supposed spawning area in the Sargasso sea. As the eel is fasting, this intense swimming activity is realised only with fat stores, involving mainly red muscle i.e. aerobic metabolism. However, eel migration is performed at depth and thus in cold water, both being known to induce changes in muscle energy metabolism. During migration, white and red muscles can operate together or separately in order to counteract the eventual effects of low temperatures and/or high pressures. We have studied the temperature sensitivity (5, 15, and 25 °C) of aerobic and anaerobic metabolism in both sexes. At the same temperature, migrating eels have a higher basal glycolytic flux. Moreover, there are temperature and sex effects: anaerobic glycolysis (JB) is more sensitive to cold water whereas aerobic (JA) is more affected by warm. Males, which are less sensitive to cold water, also have higher aerobic fluxes than females. As depth corresponds to low temperature, the possibility that males migrate more deeply than females is discussed. In an ecophysiological context, it is interesting to suppose that males and female eels migrate at different depths in order to optimize their energy utilization by aerobic and / or anaerobic pathways.     

Effect of environmental salinity change on osmotic permeability of the isolated gill of the eel, Anguilla anguilla L.

Net water fluxes in the isolated gills of Anguilla anguilla were studied during incubation in fresh water (FW) and in sea water (SW). When incubated in FW, water entry was greater in SW-adapted eels than in FW-adapted eels. In contrast, water loss in SW was less in SW-adapted eels than in FW-adapted eels. Rectification of osmotic water fluxes was observed for both FW and SW-adapted eels, net water fluxes in the mucosal-serosal (m-s) direction being greater than those in the opposite (s-m) direction. These results indicate that adaptation to a given external medium brings about a decrease in the osmotic permeability so that water gain in FW or water loss in SW is minimal.     

Comparative studies on lipid metabolism in various salt transporting organs of the European eel (Anguilla anguilla). Mono-unsaturated phosphatidylethanolamine as a key substance

1. Incorporation in vivo into tissue lipids of (1-14C)acetate added to the water in the incubation tank showed the same relative distribution pattern of 14C-activity among various phospholipids in the gills, the esophagus and the intestine, when the eel was incubated in sea-water; in fresh water this pattern was found only in the intestine, while both the gills and the esophagus showed a relative excess of 14C-label in phosphatidylethanolamine (PE). 2. Similar studies with (32P)phosphate also showed a relative excess of (32P)PE in both the gills and esophagus in fresh water compared to sea-water, and no such difference in the intestine. 3. As long as the labelled precursors were added to the water in the incubation tank both (14C)PE and (32P)PE were not identical to unlabelled PE on thin-layer chromatograms, and the 14C-labelled lipids contained predominantly C16:1 and C18:1 fatty acids. 4. However, when the two precursors were injected directly into the eel there was no longer any marked difference between the distribution patterns of radioactivity among gill phospholipids in fresh water and sea-water; there was no longer any difference between labelled and unlabelled PE on thin-layer chromatograms, and the 14C-labelled gill lipids contained predominantly C16:0 and C18:0 fatty acids. 5. The corresponding liver lipids were affected neither by a change in environmental salinity nor in precursor application.     

Response of prolactin cells to environmental calcium in the eel (Anguilla anguilla L.)

Prolactin (PRL) cell activity was investigated in eels kept in fresh water (FW), deionized water (DW) supplemented or not with Ca (2 mM), in Ca-enriched FW (10 mM), in normal (Ca 3.4 mM) or Ca-free 1/3 sea water (SW), and in SW (Ca 10.2 mM) or Ca-free SW (Ca 0.15 mM). Light-microscopic studies, including measurement of the nuclear area and cell height, showed that PRL cell activity, reduced in DW, is not affected by Ca supplementation. Activity is reduced in Ca-enriched FW, in 1/3 SW and in SW, conditions inducing an increase in the plasma sodium level. The lack of calcium in saline environments partly suppresses the nuclear atrophy occurring in SW. There is no significant correlation between external or total plasma calcium concentration and PRL cell activity. In artificial Ca-free SW, eels show a rapid increase in plasma osmolarity and sodium levels; there is a significant negative correlation between these two plasma values and the nuclear area or cell height of PRL cells. As in some other teleosts, plasma osmolarity and plasma sodium seem to play a more important role than external or internal calcium in controlling PRL secretion. This correlation is not apparent in eels kept in SW, having unstimulated PRL cells but active calcium-sensitive (Ca-s) cells in the pars intermedia.     

Blood morphology of the European eel (Anguilla anguilla). II. Studies on granulopoiesis]

It was found that the granulocytes for Anguilla anguilla mainly form in the kidney. The stem cell of granulocytes is the large hemocytoblast. Sequences of development were established for the heterophil, cosinophil and basophil granulocytes. Myeloblasts, promyelocytes, metamyelocytes, mature red-shaped and segmented forms were described as stages of development of the heterophil granulocytes. In the case of the eosinophil and basophil granulocytes the myeloblastic and myelocytic stages could be demonstrated as well as the mature granulocyte. By the aid of special granular staining, phase contrast observations, supravital and cytochemical investigations the granulocytes could be described. The cytochemical proof for the granulocytes delivered a distribution pattern of the lipids, carbohydrates, RNA (RNS) and enzymes: unspecific esterase, alkaline and acid phosphatases, oxydase and peroxydase. By means of heart puncture a larger loss of blood was caused and the hematopoiesis stimulated. The following blood letting gave indications of how rapidly the granulopoiesis develops and how much time maturing takes. After a larger loss of blood the new-growth of the granulocytes will be completed nine days later.     

Cholinesterase activities and sensitivity to pesticides in different tissues of silver European eel, Anguilla anguilla

Cholinesterase (ChE) activities were characterized in silver European eel, Anguilla anguilla, grown in the brackish lagoon of Comacchio (Italy). All specimens were harvested at the “lavoriero”, a traditional eel trapping weir that captures eels while leaving internal waters at the onset of reproductive migration. To our knowledge, no investigation on ChE was reported in silver eels. Therefore a first characterization of enzyme activity in muscle, brain, liver and plasma of silver eel was carried out, in the presence of different substrates, selective inhibitors, and four pesticides representative of the carbamate and organophosphate classes. Brain and white skeletal muscle showed similar ChE activities, 5- and 10-fold higher than those detected in liver and plasma, respectively. Km values of 0.31 and 0.30 mM, and Vmax values of 40.28 and 35.47 nmol min^–1 mg protein^–1 were obtained in brain and muscle ChE, respectively. Acetycholinesterase was the predominant ChE form in all tissues, as concluded by comparing the effects of BW 284c51, iso-OMPA and eserine. ChE activities in brain and muscle were significantly inhibited by in vitro treatment with pesticides, with the following order of potency: carbofuran > carbaryl > chlorpyrifos ≥ diazinon.     

Effects of temperature on the growth and survival of the European eel, Anguilla anguilla L.

Experiments to determine the growth rate of eels ( Anguilla anguilla L.) at different temperatures are described and show the optimum temperature for growth to be 22–23° C. The ultimate upper lethal temperature was found to be 38° C and the critical thermal maximum varied from 33 to 39° C for fish acclimated at 14 to 29° C. An attempt was also made to determine lower lethal temperatures. Eels enter a state of torpor at temperatures varying from 3° C for fish acclimated at 29° C to less than 1° C for fish acclimated at 23° C or below. The results have been used to estimate the growth rates expected from eels cultured in power station cooling water using different types of temperature control.     

Development of the swimbladder in the European eel (Anguilla anguilla)

The swimbladder of the adult eel, Anguilla anguilla, with its bipolar countercurrent system, the rete mirabile, is a widely used model for swimbladder function, but very little is known about the development of this swimbladder. Our histological studies on the developing swimbladder revealed that during metamorphosis the swimbladder becomes present as a dorsal outgrowth of the esophagus. It is filled with surfactant, and gas was not detected in the swimbladder. In the young glass-eel, the epithelial (gas gland) cells of the swimbladder are columnar, but do not yet have the typical basolateral labyrinth established in adult animals. Few blood vessels are found in the swimbladder tissue, and the submucosa is present as a thick layer of connective tissue, giving a large diffusion distance between blood vessel and swimbladder lumen. Within the next 2 or 3 months of development, gas gland cells develop their typical basolateral labyrinth, and the thickness of the submucosa is significantly reduced, resulting in a short diffusion distance between blood vessels and the swimbladder lumen. The first filling of the swimbladder with gas is observed while the gas gland cells are still in a poorly differentiated status and it appears unlikely that these cells can accomplish their typical role in gas deposition. The presence of small gas bubbles in the swimbladder as well as in the ductus pneumaticus at the time of initial swimbladder inflation suggests that the swimbladder is filled by air gulping or possibly by taking up gas bubbles from the water.