Responses of gill mitochondria-rich cells in Mozambique tilapia exposed to acidic environments (pH 4.0) in combination with different salinities

On exposure to hyposmotic acidic water, teleost fish suffer from decreases in blood osmolality and pH, and consequently activate osmoregulatory and acid-base regulatory mechanisms to restore disturbed ion and acid-base balances. In Mozambique tilapia Oreochromis mossambicus exposed to acidic (pH 4.0) or neutral (pH 7.4-7.7) freshwater in combination with 0mM or 50mM NaCl, we examined functional and morphological changes in gill mitochondria-rich (MR) cells. We assessed gene expression of Na(+)/H(+) exchanger-3 (NHE3), Na(+)/Cl(-) cotransporter (NCC), vacuolar-type H(+)-ATPase (V-ATPase) and Na(+)/HCO(3)(-) cotransporter-1 (NBC1) in the gills. The mRNA expression of NHE3 and NCC in tilapia gills were higher in acidic freshwater than in that supplemented with 50mM NaCl, while there was no significant difference in mRNA levels of V-ATPase and NBC1. In addition, immunocytochemical observations showed that apical-NHE3 MR cells were enlarged, and frequently formed multicellular complexes with developed deep apical openings in acidic freshwater with 0mM and 50mM NaCl. These findings suggest that gill MR cells respond to external salinity and pH treatments, by parallel manipulation of osmoregulatory and acid-base regulatory mechanisms.     

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.     

Metabolic changes induced during adaptation of Saccharomyces cerevisiae to a water stress

When exponentially growing Saccharomyces cerevisiae was transferred from a normal high water activity growth medium (a_w 0.997) to a medium containing 8% NaCl low water activity growth medium (a_w 0.955), glycerol accumulation during the first eight hours of the adaptation was both retarded and greatly diminished in magnitude. Investigation of the underlying reasons for the slow onset of glycerol accumulation revealed that not only was overall glycerol production reduced by salt transfer, but also the rates of ethanol production and glucose consumption were reduced. Measurement of glycolytic intermediates revealed an accumulation of glucose-6-phosphate, fructose-6-phosphate, fructose 1,6 bisphosphate and phosphoenolpyruvate in S. cerevisiae 3 to 4 h after transfer to salt, suggesting that one or more glycolytic enzymes were inhibited. Potassium ions accumulated in S. cerevisiae after salt transfer and reached a maximum about 6 h after transfer, whereas the sodium ion content increased progressively during the adaptation period. The trehalose content also increased in adapting cells. It is suggested that inhibition of glycerol production during the initial period of adaptation could be due to either the inhibition of glycerol-3-phosphate dehydrogenase by increased cation content or the inhibitin of glycolysis, glycerol being produced glycolytically in S. cerevisiae. The increased accumulation of glycerol towards the end of the 8-h period suggests that the osmoregulatory response of S. cerevisiae involves complex sets of adjustments in which inhibition of glycerol-3-phosphate dehydrogenase must be relieved before glycerol functions as a major osmoregulator.     

Lymphoid apoptosis in Edwardsiella tarda septicemia in tilapia, Oreochromis niloticus

The present study revealed a relationship between the kinetic change of apoptosis and the inflammatory response during experimental intraperitoneal infection with Edwardsiella tarda as a septicemic model. The morphological changes of apoptotic cells including cellular shrinkage, condensed nuclear chromatin, nuclear fragmentation and membrane blebbing were detected by light and transmission electron microscopy. TUNEL and agarose gel electrophoresis confirmed the fragmentation of DNA in the apoptotic cells. Apoptosis was highly detected in lymphoid organs prior to the inflammatory process and gradually decreased after an extensive inflammatory response. Apoptosis in thymus and spleen was extensive and an in vitro study revealed that lymphocytes were the major cell population which underwent apoptosis. The result suggests that E. tarda-induced systemic immunosuppression via lymphocyte apoptosis as determined by suppression of the systemic inflammatory response during an initial step of generalized septicemia.     

Biochemical characterization of isolated branchial mitochondria-rich cells of Oreochromis mossambicus acclimated to fresh water or hyperhaline sea water

Mitochondria-rich cells have been separated from other epithelial cells of tilapia (Oreochromis mossambicus) gills by density gradient centrifugation on Percoll. During centrifugation two main bands of cells formed. The viability of the cells in both bands was high (>90%). In one band, 45–47% of the total cell number was mitochondria-rich cells. The other band contained at least 80% pavement cells, representing the majority of other gill epithelial cell types. A comparison of the activities of four enzymes involved in major metabolic and ion regulatory functions was made between these two different fractions of cells. Furthermore, the separation of gill epithelial cells and determination of enzymatic activity was carried out in tilapia after the fish were acclimated to fresh water or hyperhaline sea water (60 mg·ml^-1 S) to gain an indication of the relative contribution of mitochondria-rich cells and pavement cells to both NaCl excretion and absorption. Regardless of acclimation salinity, the activities of Na⁺/K⁺-ATPase, glutamate dehydrogenase and glucose-6-phosphate dehydrogenase were significantly higher in mitochondria-rich cells than in pavement cells. However, tilapia acclimated to hyperhaline sea water possessed significantly lower carbonic anhydrase activity in mitochondria-rich cells than in pavement cells. In contrast, no significant difference of carbonic anhydrase activity was observed between the two cell fractions in tilapia acclimated to fresh water.Abbreviations ATPase adenosine triphosphatase - CA carbonic anhydrase - DASPMI dimethylaminostyrylmethylpyridinium iodine - FW fresh-water - GIDH glutamate dehydrogenase - G6PDH glucose-6-phosphate dehydrogenase - HSW hyperhaline sea water (60 mg·ml^-1) - MR cells, mitochondria-rich cells - S salinity     

Fish condition in introduced tilapias of Ugandan crater lakes in relation to deforestation and fishing pressure

This study identifies environmental predictors of the condition of two introduced tilapia species (Oreochromis leucostictus and Tilapia zillii) that are known to have divergent trophic niches (planktivore and herbivore, respectively) in 17 crater lakes in western Uganda. We asked whether fish condition differs among lakes characterized by differences in fishing pressure and catchment deforestation; and we related relative condition factor to gradients of environmental variation across lakes. Lakes characterized by severe catchment deforestation tended to be lakes with high fishing pressure, so it was difficult to explore independent and interactive effects. However, mean relative condition factor was higher in populations with high fishing pressure compared to populations with low fishing pressure for both O. leucostictus and T. zillii. The condition of O. leucostictus populations was higher in lakes with severely deforested catchments; but mean relative condition factor of T. zillii did not differ between deforestation categories. Principal components analysis (PCA) was used to describe the major environmental gradients of variation among the lakes; and PCA factor scores were regressed against relative fish condition. The association between fish condition and environmental gradients was stronger for O. leucostictus than for T. zillii. For O. leucostictus, fish condition was related to PC1 (43% of the variance) and factors that loaded most heavily included Chl-a, water transparency, lake area and depth, suggesting higher condition in lakes characterized by higher primary productivity and smaller size. For T. zillii, PC3 (11%) was the only axis related to fish condition; and factors that loaded most heavily included lake area (positive), and conductivity and total nitrogen (negative). Some of the larger lakes are characterized by higher availability of macrophytes that may positively affect the food base for T. zillii.     

Ultrastructural analysis of the developing follicle during early vitellogenesis in tilapia,Oreochromis niloticus,with special reference to the steroid-producing cells

The development and distribution of steroid producing cells (SPCs) in the ovary of tilapia have been studied by light and electron microscopy. At 40–50 d after hatching, these cells are seen only in the vicinity of blood vessels; there are no SPCs in the interstitial region, nor in the thecal layer enclosing young oocytes at the peri-nucleolus stage. By 70–80 d after hatching, the number of SPCs in the area near blood vessels has increased, and the capillaries have spread among the developing peri-nucleolar stage oocytes, and into the ovarian tunica. Clusters of SPCs have also migrated into the interstitial region and into the tunica along with these capillaries. In the ovary 100 d after hatching, some SPCs can be found in the thecal layer enclosing vitellogenic oocytes. Moreover, masses of SPCs can now be observed infiltrating the thecal layer of the oocyte. Serum testosterone (T) and estradiol-17 (E2) levels at 40–70 d after hatching, are low (T, 0.75–1.10 ng/ml, E2, 0.36–1.08 ng/ml), but at 100 d, plasma E2, but not T, is elevated (T, 1.95 ng/ml, E2, 4.65 ng/ml). These results suggest that SPCs appearing in the vicinity of blood vessels move into the interstitial region between oocytes, and finally enclose the oocytes at an early vitellogenic stage. It is interesting to note that the enclosure of oocytes by SPCs coincides with significant increases in E2 production.     

Changes in expression of provasotocin and proisotocin genes during adaptation to hyper- and hypo-osmotic environments in rainbow trout

Summary The physiological roles of neurohypophysial hormones, vasotocin (VT) and isotocin (IT), are not yet clear in teleosts. Since information on responsiveness of hypothalamic neurosecretory neurons to environmental stimuli may contribute to an understanding of their physiological roles, effects of environmental hyper- and hypo-osmotic stimuli on expression of VT and IT precursor (proVT and proIT) genes in rainbow trout were investigated, using an in situ hybridization technique in which 46 mer synthetic oligonucleotides were used as hybridization probes. The probes corresponded to the mRNA loci encoding chum salmon proVT (-5 to 11) and proIT (-5 to 11), and were labeled at the 3-end with ³⁵S. Autoradiographic silver grains which represent the hybridization signals of proVT and proIT mRNAs were localized in both magnocellular and parvocellular neurons in the nucleus preopticus magnocellularis (NPOmg). Localizations of proVT and proIT hybridization signals coincided with those of VT- and IT-immunoreactive neurons in adjacent sections, and showed that proVT and proIT genes are expressed in separate neurons. The intensity of proVT hybridization signals as determined by grain counting in magnocellular neurons in the NPOmg was conspicuously decreased after transfer from fresh water (FW) to 80% seawater (SW). The proVT mRNA levels in SW trout were consistently lower than those of FW trout for up to 2 weeks. After return from 80% SW to FW, the proVT mRNA level increased, attaining the initial FW level. The proIT mRNA levels in SW trout were not statistically different from those in FW trout, except for the 1st day after transfer to SW. These results suggest that synthesis of proVT was elevated by transfer from higher to lower salinity, and that VT may have a physiological role in salmonid osmoregulation, especially in adaptation to a hypo-osmotic environment.Abbreviations ABC avidin-biotin-peroxidase complex - AVP arginine vasopressin - BSA bovine serum albumin - EDTA ethylene diamine tetra-acetic acid - cpm counts per minute - FW fresh water - GFR gromerular filtration rate - ISH in situ hybridization - IT isotocin - mRNA messenger ribonucleic acid - NPOmg nucleus preopticus magnocellularis - OXT oxytocin - PBS phosphate-buffered saline - SSC saline sodium citrate - RT room temperature - SW seawater - VT vasotocin     

Massive fish kills within the Nyanza Gulf of Lake Victoria,Kenya

The morphometry and hydrology of the Nyanza Gulf of Lake Victoria can be greatly affected by violent storms. This can result in a condition in which nutrient-rich bottom mud is mixed with the sediment-laden runoff water from neighbouring marshes and rivers. This situation can lead to massive fish kills. It is such an event in 1984 which will be described that caused the sudden death of over 400 000 fish weighing over 2400 tonnes.The unusually low precipitation in the catchment area during that year resulted in a drop in the lake level from 12.4 to 11.8 m. This low water level, combined with a severe storm created the conditions which caused this large fish kill. The species affected were Lates niloticus and Oreochromis niloticus. Mortality was attributed to several factors which included high levels of suspended material in the water column (detritus and algae) which clogged the gills of the fish, low dissolved oxygen, low pH, and high concentrations of algae.     

Evidence of steroidogenesis in postovulatory follicles of the tilapia,Oreochromis mossambicus

Summary Postovulatory follicles of the tilapia, Oreochromis mossambicus, were examined with electron microscopy and enzyme histochemistry for evidence of steroid-hormone production. Light microscopy was also used to examine changes in the ovary with time after spawning. Electron microscopy detected the presence of smooth endoplasmic reticulum, lipid droplets, and mitochondria with tubular cristae in certain cells of the theca interna. These structures are suggestive of cells that synthesize steroid hormones. Granulosa cells also contained some smooth endoplasmic reticulum, along with an augmentation of Golgi complexes, vesicles, microvilli, and microfilaments within 5–7 days after spawning. Enzyme histochemistry demonstrated an intense reaction of 5, 3-hydroxysteroid dehydrogenase (3-HSD) in variably placed thecal cells up to 7 days after spawning. At this time, the thecal cells of vitellogenic oocyte follicles also began to show strong 3-HSD activity. During the first 7 days after spawning, there was an increase in young primary oocytes and recruitment of some of these to vitellogenic oocytes. By 10 days after spawning, certain thecal cells in the follicles of these vitellogenic oocytes showed an intense 3-HSD reaction, while the postovulatory follicular tissue demonstrated a weak reaction. This arrangement continued for the lifespan of the postovulatory follicular tissue. Postovulatory follicles had a lifespan of up to 25 days after spawning in females that continued to hold the developing fry inside their mouths, i.e., mouthbrooders. At 25 days after spawning, the postovulatory follicular tissue showed signs of degeneration with the presence of vacuoles and lysosomes. In females that ate the zygotes, therefore exhibiting no parental behavior, the postovulatory follicular tissue showed signs of degeneration at l0days after spawning. In these females, the next clutch of eggs also developed at a higher rate than in mouthbrooders.     

The origin of particulate organic matter and the diet of tilapia from an estuarine ecosystem subjected to domestic wastewater discharge: fatty acid analysis approach

The fatty acid composition of suspended particulate organic matter (POM) and tilapia (Oreochromis mossambicus) and potential sources of organic material in the Manko Estuary, Okinawa Island, Japan, were investigated to elucidate the origin of organic matter in suspended particulate matter and the contribution of these inputs to the diet of tilapia. Fatty acid fingerprints of POM revealed that diatoms, bacteria, and possible material input originating from domestic waste discharges contributed a major proportion of the organic matter pool in the estuary. The diet of tilapia is likely influenced by domestic waste-derived organic matter through a POM link. Because of their highly plastic feeding habits, tilapia may develop a feeding strategy of utilizing the most favorable food resources in the ecosystem. Tilapia likely does not prefer mangrove detritus as a food source as indicated by the absence of even-number long-chain fatty acids (LCFAs) in their tissues. Instead of mangrove detritus, tilapia preferentially utilized more palatable organic food sources in the water column, such as phytoplankton and its detrital matter. In addition to phytoplankton and bacterioplankton, the other dietary sources that contributed a minor fraction were possible green macroalgal materials and zooplankton.     

Effects of the organophosphate insecticide,monocrotophos, on acetylcholinesterase activity in the nile tilapia fish (Oreochromis niloticus) brain

The neurotoxic effects of monocrotophos on the brain of the nile tilapia fish (Oreochromis niloticus) were examined, using a static bioassay under laboratory conditions. By probit analysis the 96 h LC₅₀ value of monocrotophos was 4.9 mg/l. After 96 h exposure to acute levels of monocrotophos, the brain acetylcholinesterase (AChE) activity decreased progressively as the concentration of monocrotophos increased. In addition, four weeks following transfer to toxicant-free water after exposure to 1 mg monocrotophos, nile tilapia fish brain regained 95% of control AChE activity. The results indicate that inhibition of AChE activity in fish exposed to monocrotophos may serve as an indicator of hazard due to application of this chemical in the natural environment.Special issue dedicated to Dr. Robert Balazs.     

The blood-testis barrier and its breakdown following immunization to testis material in the Nile tilapia,Oreochromis niloticus

Summary The blood-testis barrier and its changes following immunization to testis material, were investigated by light- and electron microscopy in a teleost fish, the Nile tilapia Oreochromis niloticus, using horseradish peroxidase and bovine serum albumin as tracers. In the normal testis, histochemistry using horseradish peroxidase revealed that a barrier composed of junctional complexes connecting adjacent Sertoli cells existed around the central lumina of the seminal lobules, and also around the germ-cell cysts containing spermatids at the middle or late phase of chromatin condensation. By contrast, bovine serum albumin was prevented from passing through the basement membrane and could not penetrate any of the spermatogenetic cysts, indicating that the basement membrane may be an ion-selective barrier. In tilapia immunized with allogeneic testis homogenate emulsified in Freund's complete adjuvant, bovine serum albumin could penetrate the spermatogenetic cysts, and horseradish peroxidase was able to pass through the intercellular spaces between Sertoli cells to the region nearer the seminal lobule lumen, due to the junctional complexes becoming loosened. The results suggest that the blood-testis barrier, both junctional complexes and the basement membrane, are broken down during immune responses.     

The composition,nutritional status and digestibility of the diets of Sarotherodon mossambicus from nine man-made lakes in Sri Lanka

Synopsis The dietary composition and the nutritional status and the digestibility of the diets of Sarotherodon mossambicus from nine reservoirs in Sri Lanka were evaluated. The feeding habits of S. mossambicus were variable from reservoir to reservoir; they ranged from herbivory to total carnivory. The protein, total lipid, carbohydrate and total organic matter content of the ingested material were related to the dietary composition and ranged from 18.53% to 35.15% (x−24.18%), 5.94% to 9.84% (x−7.91%), 11.6% to 34.7% (x−22.34%) and 34.4% to 64.4% (x−45.71%), respectively. Irrespective of the feeding habits, the diet contained a significant proportion of organic material which cannot be accounted for by protein, total lipid and carbohydrate. As much as the ingested material was related to the feeding habit, the digestibility of the nutrient components was related to the food material devoured. For example, the mean digestibility of the total organic matter in S. mossambicus feeding on detritus, plants and animal were 36.85, 33.5 and 29.5 respectively, and compared well with observations from elsewhere. It is hypothesised that the favourable nutrient quality of the available dietary material in the reservoirs of Sri Lanka, which could be and is effectively utilized by S. mossambicus, may have been, at least partially, responsible for its almost unprecedented success in Sri Lanka.     

The breeding season of tilapia, Oreochromis niloticus L. in Lake Awassa (Ethiopian rift valley)

The breeding season of Oreochromis niloticus in Lake Awassa was studied from fish caught monthly from December 1987 to November 1988 in relation to a number of environmental factors. Fish at various stages of gonad development and spawning were caught in almost all months, but breeding fish were more frequent twice a year: main peak occurred during January–March and a secondary one occurred during July–September. Gonadosomatic index (GSI) values peaked twice in the study year, a major peak being during January–March and a less pronounced one during July–September. Thus, it was concluded that O. niloticus in Lake Awassa breeds intensively in the former and less intensively during the latter period, and that some breeding occurs at any time of the year. Intensive breeding activity during January–March appeared associated with increase in solar radiation and sunshine hours d^–1 whereas that during July–September appeared associated with heavy rainfall. However, intensive breeding in both periods coincided with increase in phytoplankton biomass. Previously, phytoplankton biomass in this lake is shown to increase following increase in nutrients due to mixing- and rainfall-associated changes in the hydrography and hydrology of the lake. Thus, although further studies are required, increase in phytoplankton may be one of the environmental cues to stimulate spawning in O. niloticus in Lake Awassa whereas other factors such as sunshine and rainfall may have indirect roles through their effects on the hydrology and hydrography of the lake ultimately resulting in changes in phytoplankton biomass.     

外来养殖鱼类的入侵风险评估及防控对策——以尼罗罗非鱼为例

[目的]筛选合适的指标建立一套外来养殖鱼类的生物入侵风险评估体系,并对外来养殖鱼类的生物入侵防控提出对策建议。[方法]通过文献资料的收集和整理,对外来养殖鱼类中典型入侵物种的入侵过程、影响危害和入侵生物学特性进行分析和归纳,从适应能力、繁殖能力、扩散能力3方面指示其入侵性;从对生物的影响和对环境的影响2方面指示其生态影响;从自然因素和人为因素2方面指示环境可入侵性,以上述3方面为框架进行评估体系构建。[结果]筛选20个指标构建了外来养殖鱼类的生物入侵风险评估体系,并举例说明该评估体系的应用。从法规政策、科学研究、治理技术和公众参与等4个方面针对性地提出外来养殖鱼类生物入侵风险防控对策。[结论]防范和治理入侵生物是一个系统工程。对于外来养殖鱼类的管理,既不能只考虑经济效益而置生态风险于不顾,也不能片面放大外来养殖良种的入侵风险。科学管控的关键在于完善制度建设、加强风险评估、发展防治手段、促进公众参与,使外来鱼类养殖业在严格受控的前提下发挥其经济效益,将其潜在的生态危害效应降到最低。     

Degeneration and death,by apoptosis and necrosis,of the pavement and chloride cells in the gills of the teleost Oreochromis mossambicus

Summary Degeneration and death of branchial epithelial cells were studied in an African cichlid fish. In both freshwater and seawater fish the superficially located pavement cells are sloughed off at the end of their lifecycle. This process is preceded by degeneration via a process of cytoplasmic shrinkage and condensation related to apoptotic (physiologically controlled) cell death. The chloride cells are pleomorphic, i.e., accessory, mature, and degenerating cells. Degeneration of chloride cells mainly occurs by apoptosis. Degenerating cells show shrinkage and densification of cytoplasm and nuclei, and swelling of the tubular system; these cells are then separated from the ambient water by pavement cells. They are finally phagocytosed and digested by macrophages. Apoptosis of chloride cells, but not of pavement cells, is greatly stimulated when the fish are in seawater; this reflects an increase in cellular turnover of the chloride cells. Accidental cell death (necrosis) of pavement cells or chloride cells is rarely observed in fully adapted freshwater and seawater fish. Its incidence increases in the first few days following transfer of fish from fresh water to seawater.     

The immune response of tilapia Oreochromis mossambicus and its susceptibility to Streptococcus iniae under stress in low and high temperatures

Mozambique tilapia Oreochromis mossambicus acclimated to 27 degrees C were then held at 19, 23, 27 (control), 31 and 35 degrees C, and were examined for non-specific cellular and humoral responses after 12-96 h. Total leucocyte count decreased significantly when fish were transferred to 19 and 23 degrees C after 48 and 96 h, and when transferred to 35 degrees C over 12-96 h, respectively. Respiratory burst decreased significantly when fish were transferred to 19, 31 and 35 degrees C over 24-96 h, whereas phagocytic activity and phagocytic index decreased significantly when fish were transferred to low temperatures (19 and 23 degrees C) and high temperatures (31 and 35 degrees C) over 12-96 h. Lysozyme activity decreased significantly when fish were transferred to 19 degrees C after 12-96 h, but increased significantly when transferred to 31 and 35 degrees C over 48-96 h. Alternative complement pathway (ACH(50)) also decreased significantly when transferred to 19 and 23 degrees C after 12h, but increased significantly when transferred to 31 and 35 degrees C after 24h. In another experiment, tilapia reared at 27 degrees C were injected intraperitoneally with Streptococcus iniae at a dose of 1 x 10(7)colony-forming units (cfu)fish(-1), and then reared onward at water temperatures of 19, 23, 27 (control), 31 and 35 degrees C. Over 48-168 h, the cumulative mortality of S. iniae-injected fish held in 19 and 35 degrees C was significantly higher than that of injected-fish held in 23, 27 and 31 degrees C. It is concluded that transfer of tilapia O. mossambicus from 27 degrees C to low temperatures (19 and 23 degrees C) after 12h, and transfer of fish from 27 degrees C to high temperatures (31 and 35 degrees C) reduced their immune capability. Moreover, tilapia under temperature stress at 19 and 35 degrees C from 27 degrees C decreased its resistance against S. iniae.     

Structural changes in the pars intermedia of the cichlid teleost Sarotherodon mossambicus as a result of background adaptation and illumination

Summary The MSH producing cells in the pars intermedia of Sarotherodon mossambicus have been shown to be involved in background adaptation processes. Reflected light received by the eyes affects the activity of these cells. In the present study the hypothesis has been tested that also the pineal organ, as a second photoreceptor, is involved in regulation of the metabolic activity of the MSH cells. The pineal organ appears to contain photoreceptor cells and is considered to be capable of transferring information about light conditions to the animal. Removal of the pineal organ of fish kept on a black background has no effect on activity of MSH cells, whereas the activity of these cells in fish kept in darkness is increased. Thus it seems that the pineal organ exercises its influence on MSH cells only in darkness and that this influence results in a reduced activity of these cells. It is therefore concluded that the metabolic activity of MSH cells is inhibited not only by reflected light received by the eyes, but also by the action of the pineal organ as a result of the absence of illumination.No structural signs of secretory activity can be observed in the pineal, which might indicate synthesis or release of substances like melatonin. However, administration of melatonin reduces the activity of MSH cells. Neither pinealectomy nor treatment with melatonin has any influence on the second cell type of the pars intermedia, the PAS positive cells.