Active chloride cell density in some tissues of rainbow trout (Oncorhynchus mykiss Walbaum, 1792) larvae during the early development stage

In this study, active chloride cell density in some tissues (gill arch epithelium, skin, and yolk-sac membrane) of rainbow trout (Oncorhynchus mykiss Walbaum, 1792) larvae during the early development stage was investigated using a vital fluorescence staining technique. It was found that the numbers of active chloride cells were very variable, depending on the tissue and age of the larvae. Active chloride cells were most abundant in the skin and yolk-sac membrane, but less so in the gill arch epithelium of newly hatched larvae. With larval age, the density of active chloride cells in the gill epithelium increased, while that in the skin and yolk-sac membrane decreased.     

Developmental Sequence of Chloride Cells in the Body Skin and Gills of Japanese Flounder (Paralichthys olivaceus) Larvae

The developmental sequence of chloride cells was examined in both the body skin and gills of Japanese flounder (Paralichthys olivaceus) larvae by whole-mount immunocytochemistry using an antiserum specific for Na(+),K(+)-ATPase. In premetamorphic larvae at 0 and 4 days after hatching (days 0 and 4), immunoreactive chloride cells were distributed only in the yolk-sac membrane and body skin. Premetamorphic larvae at days 8-18 possessed both cutaneous and branchial chloride cells. Large chloride cells in the skin of premetamorphic larvae often formed multicellular complexes, suggestive of their ion-secreting function. Cutaneous chloride cells decreased in size and density at the beginning of metamorphosis (days 21 and 24), and disappeared at the metamorphic climax (days 28 and 33). In contrast, branchial chloride cells first appeared at day 8, and increased during metamorphosis. These results indicate that the site for ion secretion in seawater may shift from cutaneous to branchial chloride cells during metamorphosis. The appearance of branchial chloride cells before the differentiation of gill lamellae suggests that the primary function of the gills during the early development is ion regulation rather than gas exchanges.     

Scanning and transmission electron microscopy of the squamose gill-filament epithelium from fresh- and seawater adaptedTilapia

Synopsis The architecture of the gill structure of variousTilapia species was studied in relation to their adaptability to hypersaline media. Using SEM and EM, it was shown that the squamose epithelial cells of the gills have species-typical patterns of ridges on their outer surfaces. These have previously been misinterpreted by other authors as microvilli or stereocillia. The ridges are more dense and better developed in euryhaline species, likeT. zillii, and less so in stenohaline species likeSarotherodon niloticus. Comparing freshwater and seawater-adapted individuals ofT. zillii, S. niloticus, S. galflaeus, andTristramella sacra, it was shown that in fresh water the surface cells are slightly swollen, extending over the openings of the chloride cells. During adaptation to sea water, these ridges become higher and denser and the cell surface shrinks, exposing the underlying orifices of the apical crypts of the chloride cells. The more euryhaline the species, the less change there is in the ridge pattern of the cells during passage from fresh to sea water. This evidence implicates the gill epithelium, together with the chloride cells, in the process of osmoregulation.     

Mitochondria-rich (chloride) cells in the gill epithelia from four species of stenohaline fresh water teleosts

Summary The mitochondria-rich (chloride) cells have been found to be present in the gill epithelia of four species of stenohaline fresh water teleosts. The cytoplasm of these chloride cells contains an extensive network of cytoplasmic tubules which communicate with intercellular spaces bordering the lateral and basal cell surfaces. Numerous vesicles with fairly electron-dense interiors are also present in the apical cytoplasm of chloride cells. The apical surface of a chloride cell forms an apical pit, but the lumen of the pit does not appear to be in continuity with the interior of the apical vesicles and tubules inside the cell.When Carassius auratus were kept in 100, 200, 300, and 400 mOsm-diluted sea water for a month, no appreciable changes occurred in the number and fine structure of the chloride cells, except for a dilation of the apical vesicles and a slight decrease in diameter of the cytoplasmic tubules in these cells in the fishes kept in 300 and 400 mOsm.These results suggest that chloride cells may be a rather common occurrence in the gill epithelia of stenohaline fresh water teleosts, and may function in ion-transport in these fishes in fresh water environments.     

Ultrastructural features of mitochondria-rich cells in stenohaline freshwater and seawater fishes

In order to elucidate the functional significance of accessory cells in freshwater fishes, such as the rainbow trout, which displays a poor adaptability to seawater life, a search for such cells was performed in two stenohaline freshwater fishes: the loach and the gudgeon. Accessory cells were never encountered in these species; but, in contrast, two types of chloride cells were observed consistently that strikingly resembled the alpha- and beta-cells previously described in the guppy, a freshwater-adapted euryhaline fish. The alpha-cell, a pale and elongated chloride cell, was located at the base of the secondary lamellae in close contact with the arterioarterial pillar capillary. Darker, ovoid chloride cells resembling the beta-cell were found exclusively in the interlamellar region of the primary epithelium facing the central venous sinous. The latter cells frequently formed multicellular complexes linked together by deep, narrow, apical junctions. In another experiment, a stenohaline seawater fish, the turbot, was adapted to diluted 5% saltwater and to fresh water. In seawater, the gill epithelium contained only one type of chloride cell, always associated with accessory cells. Due to numerous cytoplasmic interdigitations between the accessory cells and the apical portion of the chloride cell, there was a noticeable increase in the length of the shallow apical junction, sealing off the intercellular space between the two cell types. In 5% saltwater, there was a decrease in the number of these interdigitations and a concomitant decrease in the length of the shallow apical junction. In fresh water, chloride cells were partially or completely separated from the outside medium by modified accessory cells. It is thus concluded that accessory cells are found exclusively in fish living in seawater or preadapted to seawater and that they probably are involved in the formation and modulation of paracellular pathways for ionic excretion. In contrast, the respective roles of the two types of chloride cells observed in freshwater fishes are still to be determined.     

Why are there so few freshwater fish species in most estuaries?

The freshwater fish assemblage in most estuaries is not as species rich as the marine assemblage in the same systems. Coupled with this differential richness is an apparent inability by most freshwater fish species to penetrate estuarine zones that are mesohaline (salinity: 5·0–17·9), polyhaline (salinity: 18·0–29·9) or euhaline (salinity: 30·0–39·9). The reason why mesohaline waters are avoided by most freshwater fishes is difficult to explain from a physiological perspective as many of these species would be isosmotic within this salinity range. Perhaps, a key to the poor penetration of estuarine waters by freshwater taxa is an inability to develop chloride cells in gill filament epithelia, as well as a lack of other osmoregulatory adaptations present in euryhaline fishes. Only a few freshwater fish species, especially some of those belonging to the family Cichlidae, have become fully euryhaline and have successfully occupied a wide range of estuaries, sometimes even dominating in hyperhaline systems (salinity 40+). Indeed, this review found that there are few fish species that can be termed holohaline (i.e. capable of occupying waters with a salinity range of 0–100+) and, of these taxa, there is a disproportionally high number of freshwater species (e.g. Cyprinodon variegatus, Oreochromis mossambicus and Sarotherodon melanotheron). Factors such as increased competition for food and higher predation rates by piscivorous fishes and birds may also play an important role in the low species richness and abundance of freshwater taxa in estuaries. Added to this is the relatively low species richness of freshwater fishes in river catchments when compared with the normally higher diversity of marine fish species for potential estuarine colonization from the adjacent coastal waters. The almost complete absence of freshwater fish larvae from the estuarine ichthyoplankton further reinforces the poor representation of this guild within these systems. An explanation as to why more freshwater fish species have not become euryhaline and occupied a wide range of estuaries similar to their marine counterparts is probably due to a combination of the above described factors, with physiological restrictions pertaining to limited salinity tolerances probably playing the most important role.     

Short-term low-salinity tolerance by the longhorn sculpin, Myoxocephalus octodecimspinosus

The bottom-dwelling, longhorn sculpin, Myoxocephalus octodecimspinosus, is traditionally viewed as a stenohaline marine fish, but fishermen have described finding this sculpin in estuaries during high tide. Little is known about the salinity tolerance of the longhorn sculpin; thus, the purposes of these experiments were to explore the effects of low environmental salinity on ion transporter expression and distribution in the longhorn sculpin gill. Longhorn sculpin were acclimated to either 100% seawater (SW, sham), 20% SW, or 10% SW for 24 or 72 hr. Plasma osmolality, sodium, potassium, and chloride concentrations were not different between the 20 and 100% treatments; however, they were 20-25% lower with exposure to 10% SW at 24 and 72 hr. In the teleost gill, regulation of Na(+), K(+)-ATPase (NKA), Na(+)-K(+)-2Cl(-) cotransporter (NKCC1), and the chloride channel, cystic fibrosis transmembrane conductance regulator (CFTR) are necessary for ion homeostasis. We immunolocalized these proteins to the mitochondrion-rich cell of the gill and determined that acclimation to low salinity does not affect their localization. Also, there was not a downregulation of gill NKA, NKCC1, and CFTR mRNA or protein during acclimation to low salinities. Collectively, these results suggest that down to 20% SW longhorn sculpin are capable of completely regulating ion levels over a 72-hr period, whereas 10% SW exposure results in a significant loss of ions and no change in ion transporter density or localization in the gill. We conclude that longhorn sculpin can tolerate low-salinity environments for days but, because they cannot regulate ion transporter density, they are unable to tolerate low salinity for longer periods or enter freshwater (FW). The genus Myoxocephalus has three FW species, making this group an excellent model to test evolutionary and physiological mechanisms that allow teleosts to invade new low salinities successfully.     

Physiological and molecular endocrine changes in maturing wild sockeye salmon, <Emphasis Type="Italic">Oncorhynchus nerka</Emphasis>, during ocean and river migration

Maturing adult sockeye salmon Oncorhynchus nerka were intercepted while migrating in the ocean and upstream in freshwater over a combined distance of more than 1,300 km to determine physiological and endocrine changes associated with ionoregulation. Sockeye migrating through seawater and freshwater showed consistent declines in gill Na⁺/K⁺-ATPase (NKA) activity, plasma osmolality and plasma chloride concentration. In contrast, plasma sodium concentration became elevated in seawater as fish approached the river mouth and was then restored after sockeye entered the river. Accompanying the movement from seawater to freshwater was a significant increase in mRNA for the NKA α1a subunit in the gill, with little change in the α1b subunit. Potential endocrine signals stimulating the physiological changes during migration were assessed by measuring plasma cortisol and prolactin (Prl) concentrations and quantifying mRNA extracted from the gill for glucocorticoid receptors 1 and 2 (GR1 and GR2), mineralocorticoid receptor (MR), growth hormone 1 receptor (GH1R), and prolactin receptor (PrlR). Plasma cortisol and prolactin concentrations were high in seawater suggesting a preparatory endocrine signal before freshwater entry. Generally, the mRNA expression for GR1, GR2 and MR declined during migration, most notably after fish entered freshwater. In contrast, PrlR mRNA increased throughout migration, particularly as sockeye approached the spawning grounds. A highly significant association existed between gill PrlR mRNA and gill NKA α1a mRNA. GH1R mRNA also increased significantly, but only after sockeye had migrated beyond tidal influence in the river and then again just before the fish reached the spawning grounds. These findings suggest that cortisol and prolactin stimulate ionoregulation in the gill as sockeye salmon adapt to freshwater.     

Effects of salinity on chloride cells in the euryhaline cyprinodontid fish Rivulus marmoratus

Summary The ultrastructure and density of chloride cells in the gill, opercular epithelium, and opercular skin of the euryhaline self-fertilizing fish Rivulus marmoratus (Cyprinodontidae) were studied with electron and fluorescence microscopy. R. marmoratus raised from birth in 1, 50, 100, and 200% seawater were compared. Chloride cells from fish raised in each of the four salinities exhibited an invaginated pit structure at the apical crypt. Multicellular complexes were present in the 1% seawater group and in those fish raised in higher salinities where elaborate interdigitations were seen between cells. Chloride cells from gills of fish raised in 200% seawater had a significantly higher percentage of their cytoplasmic volume composed of mitochondria than did those from fish raised in 1% seawater (69.9% vs 37.4%). The opercular skin and opercular epithelium had the same density of chloride cells (4.2×10⁴-4.5×10⁴ chloride cells/cm²), and this number did not vary significantly with increased salinity. The opercular skin thus appears far more responsive to environmental salinity than the opercular epithelium. Chloride cells from the opercular epithelium of fish raised in 200% seawater were found to be 39% larger than those from fish raised in 1% seawater, whereas the chloride cells from the opercular skin of the 200% seawater group were 107% larger than those from the 1% seawater group.     

Permeability properties and occludin expression in a primary cultured model gill epithelium from the stenohaline freshwater goldfish

Techniques for the primary culture of fish gill epithelia on permeable supports have provided ‘reconstructed’ gill models appropriate for the study of gill permeability characteristics in vitro. Models developed thus far have been derived from euryhaline fish species that can tolerate a wide range of environmental salinity. This study reports on procedures for the primary culture of a model gill epithelium derived from goldfish, a stenohaline freshwater (FW) fish that cannot tolerate high environmental salt concentrations. The reconstructed goldfish gill epithelium was cultured on permeable filter inserts and using electron microscopy and immunocytochemical techniques, was determined to be composed exclusively of gill pavement cells. When cultured under symmetrical conditions (i.e. with culture medium bathing both apical and basolateral surfaces), epithelial preparations generated appreciable transepithelial resistance (TER) (e.g. 1,150 ± 46 Ωcm²) within 36–42 h post-seeding in inserts. When apical medium was replaced with FW (asymmetrical conditions to mimic conditions that occur in vivo), epithelia exhibited increased TER and elevated paracellular permeability. Changes in permeability occurred in association with altered occludin-immunoreactive band position by western blot and no change in occludin mRNA abundance. We contend that the goldfish gill model will provide a useful in vitro tool for examining the molecular components of a stenohaline fish gill epithelium that participate in the regulation of gill permeability. The model will allow molecular observations to be made together with assessment of changing physiological properties that relate to permeability. Together, this will allow further insight into mechanisms that regulate gill permeability in fishes.     

Structural changes in the zonulae occludentes of the chloride cells of young adult lampreys following acclimation to seawater

Summary Thin sections and freeze-fracture replicas have been used to study the structure of the zonulae occludentes of the branchial chloride cells in young adults of the anadromous lamprey Geotria australis, caught during their downstream migration to the sea and after acclimation to full-strength seawater (35). The chloride cells in the epithelium of the gill filaments of both freshwater- and seawater-acclimated animals form extensive multicellular complexes. In freshwater animals, the majority of chloride cells (64%) are covered by pavement cells and are thus not exposed to the external environment. Most of the other chloride cells are separated from each other by pavement cells or their processes. The zonulae occludentes between chloride cells and pavement cells and between adjacent chloride cells are extensive and characterised by a network of 4 (range 3–7) superimposed strands. In seawater-acclimated animals, the pavement cells cover only 30% of the chloride cells and their processes no longer occur between chloride cells. Whereas the zonulae occludentes between chloride cells and pavement cells are still extensive, those between chloride cells are shallow and comprise only a single strand or two parallel strands. The zonulae occludentes between the chloride cells of lampreys acclimated to seawater are similar to those in the gills of teleosts in seawater, and are thus considered to be leaky and to provide a low-resistance paracellular pathway for the passive transepithelial movement of Na⁺.     

Changes in the ultrastructure of the gills of the river lamprey,Lampetra fluviatilis (L.), during the anadromous spawning migration

Summary Two types of mitochondria-rich cells were found in the interplatelet areas of the gills of the migrating river lamprey. Both cell types are thought to be responsible for ion-transport across the gills. In the fresh-run migrant the gills are dominated by large, flask-shaped cells which show some ultra-structural similarities with the teleost chloride cell and have been tentatively referred to as ion-excretory cells. During the spawning migration the ion-excretory cells are replaced by smaller, mitochondria-rich cells which are similar in structure to the presumed ion-transporting cells in the ammocoete gill. They lack the tubular, smooth-membraned endoplasmic reticulum so characteristic of the lamprey ion-excretory cell and the teleost chloride cell and have been referred to as ion-uptake cells. The ion-uptake cells are found during the stenohaline, freshwater phases of the lamprey's life history. Ion-excretory cells are present during the periods of the life cycle when the animal is euryhaline. The possibility that the ion-excretory cells are also responsible for ion-uptake in fresh water is discussed.     

Embryonic ionocytes in the European sea bass (Dicentrarchus labrax): Structure and functionality

Early ionocytes have been studied in the European sea bass (Dicentrarchus labrax) embryos. Structural and functional aspects were analyzed and compared with those observed in the same conditions (38 ppt) in post hatching stages. Immunolocalization of Na⁺/K⁺‐ATPase (NKA) in embryos revealed the presence of ionocytes on the yolk sac membrane from a stage 12 pair of somites (S), and an original cluster around the first gill slits from stage 14S. Histological investigations suggested that from these cells, close to the future gill chambers, originate the ionocytes observed on gill arches and gill filaments after hatching. Triple immunocytochemical staining, including NKA, various Na⁺/K⁺/2Cl^? cotransporters (NKCCs) and the chloride channel “cystic fibrosis transmembrane regulator” (CFTR), point to the occurrence of immature and mature ionocytes in early and late embryonic stages at different sites. These observations were completed with transmission electronic microscopy. The degree of functionality of ionocytes is discussed according to these results. Yolk sac membrane ionocytes and enteric ionocytes seem to have an early role in embryonic osmoregulation, whereas gill slits tegumentary ionocytes are presumed to be fully efficient after hatching.     

The ultrastructure of chloride cells in the gills of the teleostOreochromis mossambicus during exposure to acidified water

Summary Branchial chloride cells, which actively take up ions in the gills of freshwater fish, were studied in tilapia (Oreochromis mossambicus) exposed to sublethally acidified freshwater. Structural damage of cells, resulting in cell death by necrosis, only occurred transiently, when the reduction of water pH was acute rather than gradual. The most prominent effects of water acidification were the rapid increase in the number of chloride cells and the changes in frequency of the different stages of the chloride cell cycle. In the opercular inner epithelium, a twofold increase in cells occurred 48 h after gradual acidification. Cell density stabilized after 4 weeks at a level 5 times that of control fish. Four transitory stages were distinguished in the chloride cell cycle: accessory or replacement cells, immature, mature, and degenerating (apoptotic) cells. In control fish, mature chloride cells dominated (over 50%) with immature and apoptotic cells totalling about 40%. After 4 weeks in acid water, only 13% of the cells were mature. Immature and apoptotic cells dominated, each representing about 40% of the total number of chloride cells. Mature cells apparently age rapidly under these conditions. Thus, chloride cells turn over quickly in acid water, with a minor increase in ion transport capacity of the gills. This conclusion is supported by the observation that opercular and branchial Na⁺/K⁺ ATPase activities in treated fish are only 40%–50% higher than in controls.     

Ultrastructure and distribution dynamics of chloride cells in tilapia larvae in fresh water and sea water

Integumental and branchial chloride cells of tilapia larvae (Oreochromis mossambicus) were studied at the light-microscopical and ultrastructural level. Total numbers and distribution of chloride cells were quantified after immunostaining of cross sections of the entire larvae with an antibody against the alpha-subunit of Na+/K+-ATPase. The majority (66%) of Na+/K+-ATPase-immunoreactive (ir) cells, i.e. chloride cells, of freshwater tilapia larvae were located extrabranchially up to 48 h after hatching. Five days after hatching, the majority (80%) of chloride cells were found in the buccal cavity. Transfer of 24-h-old larvae to 20% sea water speeded up this process; 24 h after transfer (i.e. 48 h after hatching), the majority (59%) of chloride cells were located in the buccal cavity. The branchial chloride cell population of 24-h- and 120-h-old larvae consisted of immature, mature, apoptotic and necrotic chloride cells. However, relatively more immature chloride cells were observed in freshwater larvae (42-63%) than in (previously studied) freshwater adults (21%), illustrating the developmental state of the gills. After transfer to sea water, the incidence of degenerative chloride cells did not change. Furthermore, the incidence of immature cells had decreased and a new subtype of chloride cells, the "mitochondria-poor" cells, appeared more frequently. These mitochondria-poor chloride cells were characterised by an abundant tubular system and relatively few mitochondria, which were aligned at the border or concentrated in one part of the cytoplasm. Most of these cells did not contact the water. The function of their enhanced appearance after seawater transfer is unknown.     

Functional analysis of the musculo‐skeletal system of the gill apparatus in Heptranchias perlo (Chondrichthyes: Hexanchidae)

Musculo‐skeletal morphology is an indispensable source for understanding functional adaptations. Analysis of morphology of the branchial apparatus of Hexanchiform sharks can provide insight into aspects of their respiration that are difficult to observe directly. In this study, I compare the structure of the musculo‐skeletal system of the gill apparatus of Heptranchias perlo and Squalus acanthias in respect to their adaptation for one of two respiratory mechanisms known in sharks, namely, the active two‐pump (oropharyngeal and parabranchial) ventilation and the ram‐jet ventilation. In both species, the oropharyngeal pump possesses two sets of muscles, one for compression and the other for expansion. The parabranchial pump only has constrictors. Expansion of this pump occurs only due to passive elastic recoil of the extrabranchial cartilages. In Squalus acanthias the parabranchial chambers are large and equipped by powerful superficial constrictors. These muscles and the outer walls of the parabranchial chambers are much reduced in Heptranchias perlo , and thus it likely cannot use this pump. However, this reduction allows for vertical elongation of outer gill slits which, along with greater number of gill pouches, likely decreases branchial resistance and, at the same time, increases the gill surface area, and can be regarded as an adaptation for ram ventilation at lower speeds.     

The adaptations of fish to extremely alkaline environments

The Lake Magadi Tilapia (MT; Oreochromis alcalicus grahami, the Lahontan cutthroat trout (LCT; Oncorhynchus clarki henshawi) and the tarek (Ct; Chalcalburnus tarichi) have evolved unique strategies that allow them to overcome problems associated with ammonia excretion (J_Amm) and acid-base regulation in their alkaline environments. In Lake Magadi, Kenya (pH 10), the MT circumvents problems associated with J_Amm by excreting virtually all (>90%) of its waste-nitrogen as urea. Base excretion appears to be facilitated by modified seawater-type gill chloride cells, through apical Cl⁻/HCO₃⁻ exchangers and an outwardly directed OH⁻/HCO₃⁻/CO₃⁼ excretion system. The LCT avoids potentially toxic increases in internal ammonia by permanently lowering ammonia production rates following transfer into alkaline (pH 9.4) Pyramid Lake, Nevada, from its juvenile freshwater (pH 8.4) environment. Greater apical exposure of LCT gill chloride cells, presumably the freshwater variety, probably facilitates base excretion by elevating Cl⁻/HCO₃⁻ exchange capacity. In Lake Van, Turkey (pH 9.8) high ammonia tolerance enables C. tarichi to withstand the high internal ammonia concentrations that it apparently requires for the facilitation of J_Amm. It also excretes unusually high amounts of urea. We conclude that adjustments to nitrogenous waste metabolism and excretion patterns, as well as modifications to gill functional morphology, are necessary adaptations that permit these animals to thrive in environments considered unsuitable for most fishes.     

Morphological changes induced by acclimation to high pressure in the gill epithelium of the freshwater yellow eel

After acclimation either to high pressure (101 ATA) or to low temperature (9°C), the number of mucous cells within gill epithelium of freshwater eel Anguilla anguilla was significantly decreased and the density of chloride cells was significantly increased when compared to control fish (1 ATA, 19°C).     

Ultrastructural features of chloride cells in the gill epithelium of the Atlantic salmon, Salmo salar, and their modifications during smoltification

To elucidate the ultrastructural modifications of the gill epithelium during smoltification, gills of the Atlantic salmon (Salmo salar) were examined by electron microscopy at three stages of this process, which were defined as follows: "parrs" were freshwater fish that had not yet started their transformation; "freshwater smolts" were freshwater fish that were ready to enter seawater; and "seawater smolts" were smolts that had been transferred from fresh water and maintained for 4 days in seawater (35%). In the gill epithelium of parrs, there were two types of chloride cells. The large chloride cells contained deeply stained mitochondria and numerous apical, irregular, dense, membrane-bound bodies that formed 77% of the chloride cell population and were distinguished easily from small chloride cells that have distinctly paler mitochondria and no dense bodies in their apical cytoplasm. In freshwater smolts, the large chloride cells formed 95% of the chloride-cell population. In contrast to the small chloride cells that were not modified, they almost doubled in size. Their tubular system developed extensively to form a tight network with regular meshes significantly smaller than those observed in parr chloride cells. Forty percent of the large chloride cells were associated with a new type of cell, the accessory cell, to which they were bound by shallow apical junctions. Half of these accessory cells were not seen to be in contact with the external medium. In seawater smolts, 80% of the large chloride cells were associated with accessory cells. Most accessory cells reached the external medium and sent numerous cytoplasmic interdigitations within the apical portion of the adjacent chloride cells. As a result, a section through the apical portion of the chloride cells and their associated accessory cells revealed a mosaic of interlocked cell processes bound together by an extended, shallow apical junction. It was concluded that the Atlantic salmon develops in fresh water most of the ultrastructural modifications of the gill epithelium which in most euryhaline fish are triggered by exposure to seawater. The effective transfer into seawater would act only as a final stimulus to achieve some adequacy between the freshwater smolt and its new environment.     

Growth and development of larvae and galls of Urophora cardui (Diptera,Tephritidae) on Cirsium arvense (Compositae)

Summary The tephritid fly Urophora cardui induces a large multi-chambered gall within the stems of Cirsium arvense. Three distinct phases of gall development have been identified as initiation, growth, and maturation. During initiation the insect gains control of tissue development and during the gall's growth phase parenchyma cells proliferate rapidly surrounding the larvae with thick layers of cells. Patches of primary nutritive cells appear along the surface of larval chambers during the growth phase but few of these cells are consumed. In the gall's maturation phase, thick layers of secondary nutritive cells appear around the surface of larval chambers and the remaining gall parenchyma lignifies. Secondary nutritive cells are the primary food of U. cardui.The gall expands rapidly during the growth phase then abruptly slows at the beginning of the maturation phase. Rate of gall growth is dependent upon the number of larvae per gall but the number of larvae does not affect duration of this phase.Larvae remain in the second instar throughout the growth phase and grow slowly. Once the gall enters the maturation phase and the secondary nutritive cells appear, the larvae moult to the third instar and grow quickly. Larvace attain over 98% of their final weight during the maturation phase and consume all secondary nutritive cells.It is postulated that larvae do not feed extensively on primary nutritive cells since these cells play a key role in gall morphogenesis. The appearance of secondary nutritive cells stimulates larval feeding at a time when gall growth and development is finished.