Flavobacterium psychrophilum, invasion into and shedding by rainbow trout Oncorhynchus mykiss

The infection route of Flavobacterium psychrophilum into rainbow trout Oncorhynchus mykiss was studied using bath and cohabitation challenges as well as oral challenge with live feed as a vector. Additionally, the number of bacterial cells shed by infected fish into the surrounding water was determined in the cohabitation experiment and in challenge experiments at 3 different water temperatures. The experiments showed that skin and skin mucus abrasion dramatically enhanced the invasion of F. psychrophilum into the affected fish in bath and cohabitation challenges. Disruption of the skin is discussed as an important invasion route for F. psychrophilum into the fish. The shedding rate of F. psychrophilum by infected fish was associated with water temperature and the mortality of the infected fish. High numbers of F. psychrophilum cells were released into the water by dead rainbow trout during a long time period compared to the numbers of cells shed by live fish. The results emphasise the importance of removing dead and moribund fish from rearing tanks in order to diminish the infection pressure against uninfected fish in commercial fish farms. In immunohistochemical examinations of organs and tissues of orally infected fish, F. psychrophilum cells were detected in only 1 fish out of 31 studied. Mortality of the orally challenged fish was not observed in the experiment.     

The relations between 0+ fish density,zooplankton size and the vulnerability of pikeperch,Stizostedion lucioperca,to angling in the Frisian lakes

During a five year study, the size and abundance of zooplankton and 0+ fish in September were registered in a number of Frisian lakes. The mean size of Daphnia was related to the abundance of planktivorous 0+ fish in September. Immigration of fish larvae into the Frisian Lake District from the IJsselmeer in June was observed in 1979 and 1980. The abundance of 0+ fish was negatively related and the size of Daphnia was positively related to the vulnerability to angling of predatory pikeperch.     

倪氏复口吸虫在稀有鮈鲫体内的移行及对其耳石微结构的影响

倪氏复口吸虫是我国淡水鱼类的一种常见的危害严重的寄生虫。利用稀有鮈鲫作为寄主,研究了倪氏复口吸虫在其体内的移行途径,并观察了复口吸虫的感染是否在寄主耳石上留下标记轮。倪氏复口吸虫尾蚴从稀有鮈鲫体表的各个部位侵入,但主要集中在头部,侵入体内的尾蚴通过肌肉和结缔组织直接进入眼球。感染倪氏复口吸虫的稀有鮈鲫耳石表面微结构发生变化,部分鱼耳石上出现明显的标记轮。     

Expression and Transmission of Wild-Type Pigmentation in the Skin of Transgenic Orange-Colored Variants of Medaka (Oryzias latipes) Bearing the Gene for Mouse Tyrosinase

Transgenic fish carrying a reconstructed mouse tyrosinase gene, mg-Tyrs-J, were produced by microinjecting the gene into the oocyte nucleus of an orange-colored variant of medaka (Oryzias latipes). Of 64 oocytes microinjected and subsequently inseminated, 13 embryos developed normally beyond hatching and three of them exhibited brown skin pigmentation in the adult as was commonly observed in the wild type of this species. Light and electron microscopic examination disclosed a ubiquitous distribution of typical melanophores in the skin of these transgenic fish. Judging from their population density and distribution pattern, it was presumed that melanogenesis in these fish was elicited in amelanotic melanophores that resided in the skin of the orange-colored fish of this variant. Immunofluorescence with use of the anti-mouse tyrosinase antiserum lacking reactivity to medaka tyrosinase clearly disclosed that the gene introduced was expressed in the melanophores of transgenic fish. Crosses of female transgenic fish and males from an orange-colored variant yielded offspring exhibiting wild-type or orange-colored pigmentation in a ratio of 1:1, thus implying that mg-Tyrs-J integrated into the medaka genome behaves like a dominant gene. Little melanogenesis was observed in xanthophores, leucophores and iridophores in transgenic fish, suggesting possible specificity in recognition of teleostean cell types (i.e., melanophores) by the regulatory region of the mouse tyrosinase gene.     

Introduction history of non-native freshwater fish in Okinawa-jima Island: ornamental aquarium fish pose the greatest risk for future invasions

We examined the introduction history of non-native freshwater fish in Okinawa-jima Island, Japan, to identify the pathway posing the greater risk for future invasions and to measure the strength of ornamental fish introductions as a vector of new invasions using all available records of freshwater fish introductions to date. At least 45 non-native freshwater fish have been introduced to Okinawa-jima Island. Of those, 22 species have successfully established in natural waters and, consequently, non-native fish species represent 76 % of the total freshwater fish fauna. The most important pathway was via the dumping of unwanted ornamental aquarium fish, which accounted for 71 % of the total introductions. The introduction of ornamental fish has occurred extensively in recent decades, and all fish introduced since the 1980s have been ornamental aquarium fish. The overall rate of establishment success was estimated at 51 %, although this is undoubtedly an overestimate because of unreported failed introductions. Establishment rates varied between stocked fish (100 %) and dumped ornamental fish (39 %), which may reflect the low propagule pressure of ornamental fish. From the observed trends in freshwater fish introductions, the dumping of ornamental fish into natural waters is expected to continue if no action is taken to reduce this most important pathway. We recommend regulations against the release of unwanted pet fish into natural waters, better programs for returning unwanted pet fish to stores or local zoos, educational programs for aquarists and retailers, and regulations against selling and trading ornamental fish that are judged to pose a high risk of causing economic or ecological harm.     

Histological observation of the urogenital papillae in the bi-directional sex-changing gobiid fish, Trimma okinawae

The gobiid fish Trimma okinawae changes its sex bi-directionally according to its social status. Morphological changes in the urinogenital papillae (UGP) of this fish have been reported during sex change. However, there have been no detailed observations of such changes. Here, we histologically examined the UGP structure of male- and female-phase fish. UGPs of fish in female and male phase contained both oviducts and sperm ducts. Both ducts were coalesced into one duct within the posterior region of the UGP. Female-phase fish had many longitudinal folds in the hypertrophied tunica mucosa of the oviduct, which was found to be responsible for the transport of eggs and the removal of follicular cells from the oocyte. In contrast, male-phase fish had an immature oviduct and a mature sperm duct in the UGP. In the male-phase fish, the co-existence of spermatozoa and fibrillar secretions was observed in the sperm duct during spermiation.     

Hierarchical status and colour preference in Nile tilapia (<Emphasis Type="Italic">Oreochromis niloticus</Emphasis>)

We studied the colour preference of isolated Nile tilapia (Oreochromis niloticus) and whether previous residence or body size can affect environmental colour choice. In the first phase, a cylindrical tank was divided into five differently coloured compartments (yellow, blue, green, white and red), a single fish was introduced into the tank and the frequency at which this fish visited each compartment was recorded over a 2-day study period. An increasingly larger fish (approx +2 cm in length each time) was then added into the tank on each of days 3, 5 and 7 (=four fish in the tank by day 7), and the frequency at which each fish visited the different compartments of the tank was observed twice a day to obtain visit frequency data on the differently sized fishes. This experiment was replicated six times. In the first phase, the solitary fish established residence inside the yellow compartment on the first and second days. Following the introduction of a larger fish, the smaller fish was displaced from the occupied compartment. Nile tilapia possibly shows this preference for yellow as a function of its visual spectral sensitivity and/or the spectral characteristics of its natural environment. Moreover, body size is an important factor in determining hierarchical dominance and territorial defence, and dominant fish chose the preferred environmental colour compartment as their territory.     

东湖放养鱼类时空分布的水声学研究

2000年,用鱼探仪逐月对东湖鱼类的空间分布进行探测的结果表明：东湖鱼类主要分布在1．5m以下的水层,1．5m以上与1．5m以下的水层的鱼类密度分布存在显差异,此外,东湖中不同区域的鱼类密度分布亦有显性差异,统计分析显示,这种水平分布差异与水深,离岸距离等因素没有明显的相关,可能主要由群聚行为引起,由不同月份群聚程度不一致,推测水温的变化可能会影响鱼类的群聚行为,污水排放对鱼类空间分布也可能有一定的影响。     

Introduction of a foreign gene into medakafish using the particle gun method

We developed a procedure to introduce a foreign gene into fertilized eggs of medakafish (Oryzias latipes) using the particle gun method, which is one of the easiest and most reliable techniques for gene transfer. A plasmid construct with the green fluorescence protein (GFP) gene driven by the madakafish beta-actin gene promoter was successfully introduced into eggs, and the expression of GFP was observed in 20% of the primary transfectant (chimera) fish. In addition, germ line transmission of GFP was observed in 13% of the GFP-positive primary transfectant fish. The new application described here should enable us to investigate gene expression using the fish model on a routine basis without high technical sophistication. J. Exp. Zool. 287:285-293, 2000.     

Attachment and penetration of Centrocestus armatus (Digenea: Heterophyidae) cercariae to gills of secondary intermediate fish hosts

The attachment and penetration of Centrocestus armatus cercariae into the fish host Zacco temmincki are described in this study. Light and scanning electron microscopy (SEM) were used to examine the topographical features and behavior of cercariae. Histochemistry and transmission electron microscopy (TEM) were employed to trace glandular products and secretions released by cercariae during penetration. Cercariae are first carried into the fish gill chambers via the respiratory currents. The frequency of respiratory-current reversals of fish increased when infected with cercariae. The behavior of cercariae, during breaks in current flow that preceded each current reversal, was observed using a specially devised apparatus. Cercariae produce a mucus-like secretion upon attachment, shed their tail, and employ a brief period of leech-like creeping behavior before penetration. In all cases, the site of penetration was via the surface of the primary gill lamellae. SEM revealed a well-developed anterior penetration apparatus, and a highly contractile body region, that created a driving force for penetration. TEM and histochemistry showed that the mucus observed on the surface tegument of cercariae during attachment were glandular secretions from the parasite. The significance of fish respiratory current reversals to the success of cercariae penetration nicely illustrates the exploitation by the parasite of a host response to a stimulus.     

Tool‐like behavior in the sixbar wrasse,Thalassoma hardwicke (Bennett, 1830)

There are only a few documented cases of the use of either tools or substrates (anvils) as pseudotools in fishes. Described here is an anvil behavior of a labrid fish, Thalassoma hardwicke, observed under aquarium conditions. This fish was fed with pellets that are too large to swallow and too hard to break up into manageable bits using jaws only. The observed individual carried a pellet to an anvil to break it up into pieces small enough to be swallowed. This feeding behavior was frequently repeated (observed in detail about 15 times), nearly always successful, and remarkably consistent, suggesting that the rock selected for an anvil is remembered and its functional qualities or other factors may play a part in its choice. These observations agree with evidence for other advanced cognitive abilities in members of the genus Thalassoma and suggest that, for welfare demand, rocks with rough surfaces should be provided to these fish, especially when they receive hard food for variety. Zoo Biol 29:767–773, 2010. © 2010 Wiley‐Liss, Inc.     

Role of cellular response in elimination of the monogenean Neoheterobothrium hirame in Japanese flounder Paralichthys olivaceus

Adult worms of the blood-feeding monogenean parasite Neoheterobothrium hirame, which cause anemia in the Japanese flounder Paralichthys olivaceus, attach to the host fish by embedding their posterior part deeply into the host tissue. To investigate the possibility that cellular responses of the host fish can eliminate N. hirame, flounder were experimentally infected with N. hirame larvae and reared in either fed or starved conditions. Mature parasites were identified on the buccal cavity wall of the fish 33 d post-infection (Day 33). Monocytes/macrophages and granulocytes increased rapidly in the blood and infected sites after the appearance of mature parasites. These cells adhered to the tegument of the parasites. In addition, a few cells with large electron-dense granules (DGCs) were observed in the inflammatory foci. On Day 47, the tegument of some parasites collapsed partially and were phagocytosed by the infiltrated host cells. Some infiltrated cells adhered directly to the inner tissues of the parasites. On Day 54, in the fed fish group, the loss of the tegument led to damage of the parasites' inner tissue by a large number of infiltrated cells. In this group, the elimination of the parasites was noted from Day 47 to 54. These observations probably suggest that the cellular response of the host fish destructed the parasite's posterior part embedded in the tissue, thereby eliminating the parasites. On the other hand, a high mortality was observed in the starved group. The starved fish developed much more severe anemia than the fed fish, and the elimination of the parasites was not observed in this group. The results of the present study suggest that flounder can eliminate N. hirame if they are fed sufficiently.     

Growth and mortality of introduced fish larvae in a newly restored urban river

The potential of a newly restored river as a nursery for endangered rheophilic fish species was estimated by introducing a population of individually marked larvae of nase Chondrostoma nasus into two river sections differing in river morphology and hydrological conditions. After 12 months, clear differences in fish abundance and fish size were observed: in the braided floodplain section C. nasus were more abundant and larger in size than in the straight river channel section. Total mortality rates over the whole investigation period did not differ between the two sections.     

The relationship between fish species richness, abundance and habitat complexity in a range of shallow tropical marine habitats

A simple habitat assessment score (HAS) was designed to assess habitat complexity across several different shallow tropical marine habitats including sandy patches, algal beds, seagrass beds and reefs. It measured rugosity, variety of growth forms, height, refuge size categories, percentage live cover and percentage hard substratum. Multiple regression models using HAS variables as predictors accounted for 71 and 22% of the variation in observed species richness and total fish abundance respectively. The two most important predictors of observed species richness were rugosity and variety of growth forms, while height was the most important predictor of total fish abundance. The HAS method worked consistently across a variety of habitat types and the complexity map closely mirrored the map of observed species richness, reflecting the patchy habitat mosaic of shallow tropical marine areas. Stations at the mouth of an enclosed lagoon, however, had a higher number of species than might have been expected judging from the habitat complexity scores. It is possible that this was linked to the preferential settling of pelagic fish larvae in this area as tidal water exchanges between the bay and the reef were funnelled through one small gap. This study highlights the need for fish biodiversity studies to take habitat complexity into account.     

Electrophysiological abnormalities produced by ammonium in isolated perfused brook trout, Salvelinus fontinalis, hearts

Electrocardiographic (ECG) abnormalities and arrhythmias have been observed in hyperammonemic patients and in animals injected with ammonium salts. Ammonium is excreted directly into the environment by fish, and it is a potent fish toxin, but the effects of ammonium on the cardiovascular system of fish are unknown. This study investigated the cardiotoxic effects of ammonium on isolated perfused fish hearts. These were compared with ammonium-induced cardiac electrophysiological abnormalities observed in humans and other experimental animals. Isolated perfused fish hearts exhibit ammonium-induced cardiac abnormalities similar to the ammonium-induced abnormalities seen in the hearts of higher vertebrates. Therefore, we conclude that the myocardium of ammonotelic species does not possess a specialized ammonium detoxification mechanism, and that ammonium is cardiotoxic to salmonid fish.     

Gene Expression in Tilapia Following Oral Delivery of Chitosan-Encapsulated Plasmid DNA Incorporated into Fish Feeds

DNA delivery into fish is important for transient gene expression, (e.g., DNA vaccination). Previous studies have generally focused on intramuscular injection of DNA vaccines into fish. However, this method is obviously impractical and laborious for injecting large numbers of fishes. This study reports oral delivery of a construct expressing the β-galactosidase reporter gene into fish by encapsulating the DNA in chitosan and incorporating it into fish feeds. We found that β-galactosidase expression could be observed in the stomachs, spleens, and gills of fishes fed with flakes containing the chitosan-DNA complex. These results suggest that DNA vaccines and other constructs can be easily and cheaply delivered into fishes orally by use of carriers and incorporation into fish feeds.     

Investigation of Paradiplozoon homoion (Monogenea, Diplozoidae) life cycle under experimental conditions

Diplozoids (Diplozoidae, Monogenea) are fish ectoparasites with a direct life cycle without intermediate hosts. Their free swimming larva, the oncomiracidium, hatches from eggs, invades a fish host and metamorphoses into a post-oncomiracidial larval stage, the diporpa. Later, two diporpae fuse and live as a pair in cross-copulation during their adult life. An experimental study was designed to investigate the life cycle of Paradiplozoon homoion (Monogenea, Diplozoidae) parasitizing their common fish hosts, gudgeon (Gobio gobio). A total of 35 gudgeon parasitized by diplozoids were collected from their natural environment of the Vlára River, Czech Republic, and kept together in tanks with 41 non-parasitized gudgeons reared in a laboratory environment. In total, 100 adult specimens of P. homoion were collected from the Vlára River gudgeon and a new parasite generation was expected to be observed on fish reared in the laboratory environment. Eight days after the first diplozoid eggs appeared on fish gills, the presence of diporpae with one or two pairs of clamps was noted. The appearance of the first juveniles was recorded at the same time as diporpae. Development of P. homoion from egg to sexually mature adult stage took 33 days at a constant temperature of 20 degrees C. The development of eggs in adults of the second generation was observed 2 days after the first observation of these adults. The behavior of oncomiracidia was also studied and this free swimming stage of diplozoids survived for 22 h in the absence of a host. When host fish were experimentally infected by oncomiracidia, diporpae were found attached to the fish gill apparatus within 2 h of infection.     

Changes in blood cell composition of the brown trout, Salmo trutta L., during the spawning season

Differential blood cell counts were monitored in three different strains of brown trout through three separate spawning seasons. A consistent elevation in the number of circulating erythrocytes was observed in sexually mature male fish (compared with immature fish of either sex) during October-December of each season. A marked lymphocytopenia also occurred during October in mature fish of both sexes with lymphocyte numbers still being significantly depressed as late as March the following year. It is concluded that the use of haematology as a technique for assessing the state of health of salmonid fish must take into account the sex and state of maturation of the fish. The possible consequences of changes in haematology during sexual maturation are discussed in terms of respiratory function and defence against pathogenic challenge.     

Nutrient and fish effects on the morphology of the dinoflagellate

The effects of nutrient loading and fish density on Ceratium hirundinella morphology were investigated during an experimental mesocosm study in Lake Créteil during the summer of 1990. In the presence of zooplanktivorous fish, more than 80 % of C. hirundinella cells had an hypotheca with two horns. In the enclosures without fish, we observed a highly significant increase in the proportion of C. hirundinella with three antapical horns. The proportion of C. hirundinella cells with three antapical horns tended to increase with nutrient loading, but this effect was not significant and no significant interaction effect between nutrients and fish was observed. Temperature profiles were similar in fish and fishless enclosures and could not explain the morphological variability of C. hirundinella. Our results show that biotic factors should be taken into account in polymorphism studies on Ceratium. Frequently invoked hypotheses based only on water temperature, nutrient availability or turbulence level, may not be sufficient to explain this phenomenon.     

Aggressive behaviour and dominance relationships of the dark chub,Zacco temmincki with special reference to their individual recognition

Aggressive behaviour and dominance relationships ofZacco temmincki were observed by introducing fish into an enclosed pond. Chase (-flee), lateral display, parallel swim and butt were the principal behavioural patterns in aggressive encounters between fish, while chase, resulting in lateral display by the chased fish was the most common behavioural sequence. Initially, mutual behavioural patterns such as parallel swim and mutual lateral display were most evident among the total aggressive acts although chase became dominant three days after introduction into the pond. The dominance matrix constructed from chase-flee interactions during all observation periods contained many reverse attacks (336 out of 2,740 chases). These reverse attacks did not concentrate upon a specific period and were not site-dependent. Examination of chase-flee interactions and the subsequent behavioural pattern revealed that a chased fish reacted to the chaser either by attacking in turn, or performing lateral display etc. roughly in relation to the dominance rank of the chaser. This result implies that fish recognized each other to a great extent during aggressive encounters. It seems likely that such individual recognition was initiated during the early period when mutual behaviour was most frequent, and that some attacks against the dominance order were caused as a result of revolts rather than mistakes.