Complete Yolk Absorption Research Articles

Captive maturation, breeding and seed production of the brackishwater ornamental fish silver moony,Monodactylus argenteus(Linnaeus, 1758)

Silver moony, Monodactylus argenteus (Linnaeus, 1758), belonging to the family Monodactylidae, has high significance in the ornamental fish industry. The present study provides the details of captive broodstock maturation, induced breeding and larval rearing of this species for the first time. Juveniles were collected from wild and raised to adults in net cages. Female with oocyte diameter =500 µm and oozing males were selected in the sex ratio of 1:2 (female: male) and induced to spawn using a single dose of LHRHa. Spontaneous spawning and fertilization were observed after 36 to 40 hr of hormonal induction. The average fertilization and hatching rates were 83.50 ± 4.37% and 70.83 ± 5.19% respectively. The incubation period was observed to be 18 to 20 hr at a temperature of 29 ± 1.8°C. Newly hatched larvae measured 1,630 ± 0.20 µm in length. Larval rearing was carried out in 500-L circular fibre reinforced plastic tanks using a green water system. Larvae were stocked at 5 L−1 in larval rearing tanks. Initial mouth opening and complete yolk absorption were observed at 48 and 96 hr post-hatching respectively. Larval feeding was done using live feeds, rotifer and Artemia nauplii. Metamorphosis began on 23 dph, and 100% larvae became juvenile on 32 dph with an average survival rate of 49.6 ± 8.73%. These findings on the development of captive seed production of silver moony would pave the way for mass scale production and reduced exploitation of wild stocks.

Embryonic and larval development of Labeobarbus megastoma and Labeobarbus platydorsus(pisces: cyprinidae) in lake tana (Ethiopia)

The information on the early ontogeny of the endemic Labeobarbus megastoma and Labeobarbus platydorsus fish species of Lake Tana are absent. In this paper we describe for the first time the full developmental sequence from egg to fingerling in controlled conditions. In addition, information on larval behavior, identification and developmental rates are provided. Embryonic development lasted 50:30h (26.3°C) and 61:38 (23.4°C) for the two species L. megastoma and L, platydorsus respectively and larval development to the morphology until the fry became very similar to the adult fish lasted 40 days (24.4°C). The mean total length of the newly hatched larvae of L. megastoma and L. platydorsus were 5.7±0.03 and 6.4±0.03mm respectively. The larvae hatched with closed mouth and anus, unpigmented eyes, large yolk and complete yolk absorption and exogenous feeding started on day 10 (8.9±0.07 and 10.5±0.12mm TL for L. megastoma and L. platydorsus respectively). The first posterior swim bladder aeration completed 5 days and 5 days and 6 h with 8.4±0.06 and 8.2±0.08mm TL for L. megastoma and L. platydorsus respectively and show full juvenile pigmentation and behaviour on the 41st day.

Enhanced osmoregulatory ability marks the smoltification period in developing chum salmon (Oncorhynchus keta)

The freshwater (FW) life of chum salmon is short, as they migrate to the ocean soon after emergence from the substrate gravel of natal waters. The alevins achieve seawater (SW) acclimating ability at an early developmental stage and the details of smoltification are not clear. We examined the stage-dependent SW acclimating ability in chum salmon alevins and found a sharp increase in SW tolerance during development that resembles the physiological parr-smolt transformation seen in other salmonids. Perturbation of plasma Na+ after SW exposure was prominent from the hatched embryo stage to emerged alevins, but the plasma Na+ became highly stable and more resistant to perturbation soon after complete absorption of yolk. Marker gene expression for SW-ionocytes including Na/K-ATPase (NKA α1b), Na-K-Cl cotransporter 1a (NKCC1a), Na/H exchanger 3a (NHE3a), cystic fibrosis transmembrane conductance regulators (CFTR I and CFTR II) were all upregulated profoundly at the same stage when the alevins were challenged by SW, suggesting that the stability of plasma Na+ concentration was partly a result of elevated osmoregulatory capability. FW-ionocyte markers including NKA α1a and NHE3b were consistently downregulated independent of stage by SW exposure, suggesting that embryos at all stages respond to salinity challenge, but the increase in SW osmoregulatory capability is restricted to the developmental stage after emergence. We propose that the “smoltification period” is condensed and integrated into the early development of chum salmon, and our results can be extrapolated to the future studies on hormonal controls and developmental triggers for smoltification in salmonids.

Simple energy-budget model for yolk-feeding stages of Atlantic cod (Gadus morhua)

Atlantic cod (Gadus morhua) is a commercially important species, and therefore, understanding the influence of environmental factors and anthropogenic stressors on its early life stages is of considerable relevance. In this contribution, we apply a simple and generic energy-budget framework (DEBkiss) to data for the yolk-feeding stages of cod. The model is capable of explaining the changes in yolk volume, dry weight, oxygen use and body length, simultaneously with a small number of parameters. The calibrated model was subsequently successfully tested with other data sets. Interestingly, the light conditions after hatching affect growth and respiration rates, which is traced to a change in the maintenance costs (linked to swimming activity). Despite the satisfactory performance of the model, several uncertainties remain. Especially the bioenergetics around the point of complete yolk absorption require further attention, which is complicated by the fact that the behaviour around this point differed between data sets. The presented model can be used for exploring effects of stressors on early-life stages of cod, and likely for other aquatic egg-laying species as well.

Embryonic, larval and juvenile development of the longfin batfish, Platax teira (Forsskål, 1775) under controlled conditions with special regard to mitigate cannibalism for larviculture

Longfin batfish Platax teira is one of the most important finfish species in Asian aquaculture. However, knowledge on early ontogeny and reproduction of this species in captivity are scarce and incomplete. In this study, the embryonic, larval and juvenile development of P. teira are described and illustrated for the first time. The effect of temperature (27, 30 and 33 °C) on hatching rate, yolk-sac volume and oil globule volume was demonstrated. Cannibalism control via aeration adjustment for the enhancement of larval survival was also examined. Fertilized eggs were spherical, buoyant and had a diameter of 1.29 ± 0.05 mm (mean ± SD). Embryonic development lasted 16 h at 26.2 °C. Newly hatched larvae were 2.81 ± 0.24 mm in total length (LT) with 24 myomeres and had an oil globule in the ventroposterior area of the yolk sac. Early larvae had xanthophores appeared all over the body, except for the tip of the notochord. Larvae completed yolk absorption within 3 days post hatching (dph) at 3.41 ± 0.11 mm LT. Larvae were fed initially on rotifers Brachionus ibericus, followed by Artemia nauplii and finally weaned onto an artificial diet. From 8 dph, the larger larvae (over 4 mm LT) began to exhibit cannibalistic behaviour. Transformation of larvae to the juvenile stage was completed at 23.80 ± 1.98 mm LT, all fins had the adult complement of rays and spines. A statistical model: Y = 2.4968e0.0786x, where Y is mean LT (mm) and X represents dph, explained 96.45% of variation in growth (P < 0.001, R2 = 0.9645). The hatching rate, yolk-sac volume and oil globule volume of newly hatched larvae were significantly (P < 0.05) decreased when the incubation temperature increased. Only 2–6% of embryos hatched at 33 °C, while the hatching rate was 63 ± 6% at 27 °C. Larvae reared in aeration rate of 300 mL min−1 attained the highest survival rate at 70.00 ± 7.20%, but remained significantly (P < 0.05) lower in cannibalism: 24.44 ± 4.16% compared with 66.67 ± 5.44% in 20 mL min−1. These results indicate that the optimal incubation temperature recommended is approximately at 27 °C, and aeration at 300 mL min −1 could enhance larval survival by reducing cannibalism.

Oil sands tailings pond sediment toxicity to early life stages of northern pike (Esox lucius)

The Athabasca River in Alberta flows through natural sources of eroding oil sands bitumen and oil sands mining operations that may result in low level contamination of surface waters. Northern pike (Esox lucius) are apex predators and important food and game fish species native to the Athabasca River system. This species has the potential to be exposed to both natural and anthropogenic sources of contamination from oil sands related materials throughout its life cycle. Pike are difficult to rear in the laboratory and little information exists on the toxicity of oil sands related materials to this key indigenous fish species. In this study, the potential effects of two sediment samples collected from different areas of one tailings pond in the Athabasca oil sands area are assessed in a daily renewal bioassay on early life stages of northern pike. Gametes were collected from spawning wild pike captured from a reference site outside of the oil sands area. Fertilized eggs were exposed to control water or increasing concentrations of tailings pond sediments for 21days, coinciding with initiation of exogenous feeding and completion of yolk absorption. Developing fish were examined for survival and changes in body weight, length, and development. Embryos exhibited increased developmental abnormalities and decreased growth and survival with increasing sediment concentration. Both sediment samples had similar levels of naphthenic acids and similar types of PAHs, with alkylated PAHs dominating. However, concentrations of total and alkylated PAHs differed between sediment samples and were related to increasing developmental abnormalities and decreased growth and survival. This is consistent with developmental changes observed with exposure to PAHs in other fish species. These results provide information on the effects of tailings pond sediments comprising mixtures of PAHs and alkylated PAHs on the development and survival of a key species in the northern aquatic ecosystem.

The effect of oil sands tailings pond sediments on embryo-larval walleye (Sander vitreus)

Walleye (Sander vitreus) are a commercially important North American fish species that inhabit the Athabasca River. This river flows through the Athabasca oil sands where natural sources of bitumen erode from the McMurray formation. Little information is available on responses of walleye embryos to oil sands tailings pond sediments in a laboratory setting. The current study describes the design and implementation of a daily-renewal bioassay to assess the potential effects of tailings pond sediments from the Athabasca oil sands area on walleye development. Developing walleye embryos were exposed to increasing concentrations of two tailings pond sediments (collected in the Athabasca oil sands area) until the completion of yolk absorption in control fish. Sediments from the tailings pond represent a mixture of polycyclic aromatic hydrocarbons (PAHs) and alkylated PAHs. During the 31 day exposure, the walleye were examined for mortalities, weight, length and developmental abnormalities to provide an initial evaluation of the effects of the oil sands tailings pond sediments. Walleye embryo survival differed between the tailings pond sediments, and survival decreased with increasing sediment concentration. Alkylated PAH content differed between the two tailings pond sediments and lower embryo survival corresponded to higher total and alkylated PAH content. Tailings pond sediment-exposed walleye exhibited a delay in development, as well as increased percentages of larvae with heart and yolk sac edema, and cranial and spinal malformations. These abnormalities in development are often associated with PAH and alkylated PAH exposure. This study provides an exposure design that can be used to assess sediment toxicity to early developmental stages of a fish species not commonly tested in the lab, and lays the groundwork for future studies with this and other difficult-to-culture species. These results offer information on the potential effects of tailings pond sediments containing PAH/alkylated PAH mixtures on walleye development and survival.

Development of the digestive tract in first feeding larvae ofBetta splendensRegan, 1910

The aim of this study was to analyse the development of the digestive tract of Siamese fighting fish larvae (Betta splendens Regan, 1910), from hatching to 92 hours post-hatching (hph) at 28.0°C, in order to determine the most appropriate time to begin exogenous feeding (live feed) and to investigate the digestive tract and its function during larval development. At hatching (29 hours post-fertilization), the digestive tract was a simple and straight undifferentiated tube lined by a single layer of columnar epithelial cells; the mouth and anus were closed. At 18 hph, the mouth was open. At 32 hph, the tongue was located in the depression of the buccopharyngeal cavity floor and supported by basal cartilage. Goblet cells were present in the epithelium lining the pharynx and oesophagus. At 56 hph, the midgut had four distinct layers: the mucosa with typical villi, the submucosa, the smooth muscle layer, and the serosa. Histological analysis revealed that the larvae retained endogenous yolk reserves until 74 hph. Lipid accumulation was observed in the liver, which coincided with complete yolk absorption. At this time, the digestive tract was fully open and functional. Thus, it can be concluded that live feed should be given to B. splendens larvae from 74 hph onward, when the larvae are able to consume the food provided. This study also provides useful data for the improvement of husbandry techniques and for the formulation of diets specific to ornamental fish larvae.

Thyroxine distribution in the hemolymph, hepatopancreas, ovary, sponge, and larvae of female mud crabs (Scylla serrata) during ovarian maturation

An experiment was conducted to study the profiles and the dynamics of thyroxine in the hemolymph, hepatopancreas, ovary, sponge, and larvae of female mud crabs during ovarian maturation. The experiment consisted of three parts. First, we measured thyroxine concentrations in the hemolymph, hepatopancreas, and ovary. Second, we measured cholesterol, neutral lipid, phospholipid, and protein in the ovary. Third, we determined thyroxine concentrations in the sponge and larvae. During vitellogenesis, the highest thyroxine concentration was found in the hepatopancreas, followed by the hemolymph and ovary. Thyroxine concentrations in the hemolymph and the ovary increased with the increased stage of vitellogenesis. However, thyroxine concentrations in the hepatopancreas decreased with the increased stage of vitellogenesis. Cholesterol and phospholipid concentrations in the ovary increased from vitellogenic stage 1 to stage 2, but began to decline in vitellogenic stage 3. In contrast, neutral lipid concentrations decreased from vitellogenic stage 1 to stage 2, while the concentration of protein in the ovary increased with the advanced of vitellogenic stage. Concentration of thyroxine in the sponges was the highest during the early embryonal stage and continued to decrease with the completion of yolk absorption and during the transformation to the juvenile stage. The results imply that thyroxine plays a critical role in ovarian maturation and embryonic development of mud crabs.

First results of larval rearing and development of the bluestriped angelfishChaetodontoplus septentrionalis(Temminck &amp; Schlegel) from hatching through juvenile stage with notes on its potential for aquaculture

The aim of this study was to report the first successful results on larval rearing of Chaetodontoplus septentrionalis, a description of larval and juvenile development stages, and appropriate first-feed items for pilot-scale larviculture systems. Fertilized eggs were obtained from natural spawnings from communally reared captive wild broodstock. Newly hatched larvae were 2.11 ± 0.06 mm (mean ± SD) in total length (LT) with 27–28 myomeres. Larvae completed yolk absorption within 3 days post hatching (dph) at 2.92 ± 0.11 mm LT. Transformation of larvae to the juvenile stage was completed at 16.51 mm LT, all fins had the adult complement of rays and spines. A statistical model: Y = (X × 0.0112) + (X2 × 0.0086) − (X3 × 0.00004) + 2.8065, where Y is mean LT (mm) and X represents dph, explained 99.8% of variation in growth (P < 0.001, R2 = 0.998). The effects of live feed on survival was significantly (P < 0.05) higher in larvae fed a combination of diet (ciliates, rotifers and copepods) than the others. These results indicate that C. septentrionalis is a potential species for captive breeding programs and the use of a combination of diet may be a suitable first food for the larviculture.

Stages in the early and larval development of the African catfish Clarias gariepinus (Teleostei, Clariidae)

The African catfish Clarias gariepinus Burchell 1822 is a favourite aquaculture fish in many parts of Africa and Asia because of its hardiness and fast growth rate. In this study, early, post-embryonic and larval developmental stages of C. gariepinus were examined chronologically and described. Photomicrographs of unfertilized matured oocytes from 0 min of fertilization through all cell stages to alevin, to complete yolk absorption, to free swimming larval stages are shown and documented live from lateral and top views, with the aid of a light microscope. Extruded oocytes had a mean diameter of 1 ± 0.1 mm, and possessed a thin perivitelline membrane whose space was filled with a protoplasmic layer. Heartbeat was in the range of 115-160/min prior to hatching. Hatchability rate was 85% and hatching occurred at 17 h at a controlled temperature of 28.5 ± 0.5°C, while ontogeny of the eyes and other organs were discernible. At day 4, larvae mean length was 9.3 ± 0.5 mm, exogenous feeding had commenced fully and melanophores spread cephalocaudally but were concentrated significantly on the head parts. This paper, for the first time, presents the significant chronological developmental stages of C. gariepinus embryology that will have significant implications for genetic manipulation and catfish seed production for aquaculture.

First feeding and subsequent growth performances in piscivorous larvae of Chinese perch Siniperca chuatsi (Perciforms: Sinipercidae)

The success of first feeding is influenced by many factors, such as prey availability, and is critical to subsequent larval growth performances. To test the advantages of prey exposure before complete yolk absorption in piscivorous larvae, feeding incidence and specific growth rate were longitudinally measured in Chinese perch Siniperca chuatsi larvae first fed at different ages. The results showed that Chinese perch larvae were able to capture live piscine prey at 4 DAH (days after hatching), 1 day before complete yolk absorption. The feeding incidence and specific growth rate were higher in larvae first fed at 4 DAH than in larvae first fed at 5 or 6 DAH. These results indicate that prey exposure experience can facilitate the onset of first feeding, and that successful exogenous feeding, even before complete yolk absorption, is important for growth performances of piscivorous fish larvae.

Gene expression throughout a vertebrate's embryogenesis

BackgroundDescribing the patterns of gene expression during embryonic development has broadened our understanding of the processes and patterns that define morphogenesis. Yet gene expression patterns have not been described throughout vertebrate embryogenesis. This study presents statistical analyses of gene expression during all 40 developmental stages in the teleost Fundulus heteroclitus using four biological replicates per stage.ResultsPatterns of gene expression for 7,000 genes appear to be important as they recapitulate developmental timing. Among the 45% of genes with significant expression differences between pairs of temporally adjacent stages, significant differences in gene expression vary from as few as five to more than 660. Five adjacent stages have disproportionately more significant changes in gene expression (> 200 genes) relative to other stages: four to eight and eight to sixteen cell stages, onset of circulation, pre and post-hatch, and during complete yolk absorption. The fewest differences among adjacent stages occur during gastrulation. Yet, at stage 16, (pre-mid-gastrulation) the largest number of genes has peak expression. This stage has an over representation of genes in oxidative respiration and protein expression (ribosomes, translational genes and proteases). Unexpectedly, among all ribosomal genes, both strong positive and negative correlations occur. Similar correlated patterns of expression occur among all significant genes.ConclusionsThese data provide statistical support for the temporal dynamics of developmental gene expression during all stages of vertebrate development.

First feeding diet of young brown trout fry in a temperate area: disentangling constraints and food selection

Diet composition of newly emerged brown trout fry in natural areas remains poorly known, and foraging abilities at this early stage, although presumably reduced, are still under discussion. We have studied gut content composition of brown trout fry in a temperate area (Galicia, NW Spain) and compared it to the benthic macroinvertebrate community. Small prey such as chironomid larvae and baetid nymphs were the most important food items for newborns, some of them still presenting yolk remnants. However, the positive selection observed for Polycentropodidae and Simuliidae and the rejection of Elmidae and Leuctricidae suggest that other factors apart from size, such as locomotor abilities of fish or accessibility and antipredator behaviour of prey play an important role in feeding behaviour. Additionally, analysis of diet changes on the studied fry suggests a dramatic shift in niche breadth at the moment of complete yolk absorption, which might be related to the improvement of swimming and handling ability of fry for capturing and ingesting prey. The presence of aerial imagoes only in the stomachs of fry with no yolk provides further support to this hypothesis. Planning of restoration works on spawning grounds should then allow enough time for complete recolonization by benthic macroinvertebrates, including first instars, as searching for food in newborns is limited to the nest area due to mobility constraints.

Growth and morphological development of laboratory-reared larvae and juveniles of the Laotioan indigenous cyprinid Hypsibarbus malcolmi

The morphological development, including the pigmentation, body proportions, fins, and survival rate for 30 days after hatching, of laboratory-reared larval and juvenile Hypsibarbus malcolmi is described. Body lengths (BL) of larvae and juveniles were 2.0 ± 0.2 (mean ± SD) mm at 1 h after hatching (day 0) and 9.2 ± 0.6 mm on day 16, reaching 12.1 ± 0.9 mm on day 30. Yolk volume decreased linearly, with the yolk being completely absorbed by day 3 in all preflexion larvae (all specimens >3.2 mm BL). Feeding was observed on day 2 in fish which had rapidly undergone complete yolk absorption following mouth and anus opening on day 1, and on day 3 in all remaining fish. Myomere numbers were 20–21 + 11–12 = 31–33, although they were not clearly visible in juveniles. Melanophores were few on the body during days 0–2, but increased with growth and covered the entire upper dorsal body surface during the juvenile stage. Body proportions tended to become constant in juveniles. Notochord flexion began in larvae >5.2 mm BL on day 8, and was completed in larvae >8.4 mm BL on day 14. Specimens with full fin ray complements were initially observed on day 22 (10.4 mm BL in juveniles). All specimens >11.5 mm BL had attained the juvenile stage. A high survival rate of 92.7% was estimated on day 30.

Spawning behaviour, early development and first feeding of the bluestriped angelfish [Chaetodontoplus septentrionalis (Temminck &amp; Schlegel, 1844)] in captivity

Successful natural spawning of Chaetodontoplus septentrionalis in captivity from 19 March to 11 May, 2008 is described for the first time. A single male dominates a harem of two females, spawning with each at dusk, from 10 min before to 20 min after sunset. Each female laid an average 119 × 103 eggs during the spawning period. Fertilized eggs were spherical, buoyant and had a diameter of 0.83 ± 0.02 mm (mean ± SD). Embryonic development lasted 15–18 h at 28.1 °C. Newly hatched larvae were 1.60 ± 0.07 mm in total length (TL) with 27 myomeres. Larvae completed yolk absorption within 3 days post hatching (ph) at 3.01 ± 0.08 mm TL. Ten days ph, the larvae had attained 3.95 ± 0.12 mm TL. Larvae were fed either 100% s-type rotifers (Brachionus rotundiformis), 100% copepods (Microsetella sp.), a combination of the two (50%:50%) or without live feed (starved control) to determine the effect of live feed on the survival rate. The survival was significantly (P<0.001) higher in larvae fed a combination of diet than the others. These results indicate that C. septentrionalis is a potential species for captive breeding programs and the use of a combination of diet (s-type rotifers and copepods) may be a suitable first food for the larvae.

Development of testis and digestive tract in longnose gar (Lepisosteus osseus) at the onset of exogenous feeding of larvae and in juveniles

The aim of this study was to describe the ontogenetic development of the testis and the alimentary tract in longnose gar (Lepisosteus osseus) related to fish size and age at the onset of exogenous feeding and late ontogenesis. Using light microscopy, testes were first detected histologically by the appearance of primordial germ cells 9 days after the first exogenous feeding [31-31.5 mm total body length(TL)] and presumptive seminiferous tubules (maleness characteristic) in fish of 107 mm TL. The present histological studies indicated that the alimentary tract of lepisosteids is completely functional at the beginning of exogenous feeding, several days before the completion of yolk absorption. Based on these results, we have concluded that garfish larvae/juveniles can be effectively trained to consume formulated diets at early stages, after an initial feeding of live food for 2-3 days (23.5 mm TL). Our findings provide evidence of the first controlled rearing of longnose gar using live and formulated diets, providing the possibility of experimental work with this non-teleost fish.

Natural spawning, early development and first feeding of the semicircle angelfish [Pomacanthus semicirculatus(Cuvier, 1831)] in captivity

Successful natural spawning of Pomacanthus semicirculatus in captivity from 11 September to 18 October, 2006 is described for the first time. Each female laid an average of 230 000 eggs during the spawning period. Fertilized eggs were spherical, transparent and buoyant and had a mean diameter of 0.61 ± 0.03 mm (mean ± SD). Embryonic development lasted 18–21 h at 28.5 °C. Newly hatched larvae were 1.35 ± 0.02 mm in total length (TL) with 27 (12+15) myomeres and had an oil globule in the ventroposterior area of the yolk sac. Larvae completed yolk absorption within 3 days post hatching at 2.37 ± 0.05 mm TL. Larvae were fed either 100% microalgae (Nannochloropsis sp.), 100% s-type rotifers (Brachionus rotundiformis), 100% dinoflagellates (Gonyaulax sp.) or different combinations of the three (50%:50%:0%, 30%:35%:35%) to determine the effect of live feed on the survival rate. The survival was significantly (P<0.001) better in larvae fed a combination of diets (30%: 35%: 35%) than others. These results indicate that P. semicirculatus is a potential species for captive-breeding programmes and the use of a combination of diets (microalgae plus s-type rotifers and dinoflagellates) may be a suitable first food for fish larvae.

Otolith formation, microstructure and daily increment validation in juvenile perch Perca fluviatilis

The otoliths of laboratory‐reared larval and juvenile perch Perca fluviatilis of known age were analysed to determine the age of otolith formation and validate the formation of daily increments. There was a linear relationship between number of increments and age in days post‐hatching, although by 82 days post‐hatching daily increment counts underestimated actual age by an average of 5 days. Otolith dimensions in relation to standard length indicated allometric growth of otoliths until completion of yolk absorption, and isometric growth thereafter, up to 82 days post‐hatching.

Rhesus glycoprotein and urea transporter genes are expressed in early stages of development of rainbow trout (Oncorhynchus mykiss)

The objective of this study was to determine if the genes for the putative ammonia transporters, Rhesus glycoproteins (Rh) and the facilitated urea transporter (UT) were expressed during early development of rainbow trout, Oncorhynchus mykiss Walbaum. We predicted that the Rh isoforms Rhbg, Rhcg1 and Rhcg2 would be expressed shortly after fertilization but UT expression would be delayed based on the ontogenic pattern of nitrogen excretion. Embryos were collected 3, 14 and 21 days postfertilization (dpf), whereas yolk sac larvae were sampled at 31 dpf and juveniles at 60 dpf (complete yolk absorption). mRNA levels were quantified using quantitative polymerase chain reaction and expressed relative to the control gene, elongation factor 1alpha. All four genes (Rhbg, Rhcg1, Rhcg2, UT) were detected before hatching (25-30 dpf). As predicted, the mRNA levels of the Rh genes, especially Rhcg2, were relatively high early in embryonic development (14 and 21 dpf), but UT mRNA levels remained low until after hatching (31 and 60 dpf). These findings are consistent with the pattern of nitrogen excretion in early stages of trout development. We propose that early expression of Rh genes is critical for the elimination of potentially toxic ammonia from the encapsulated embryo, whereas retention of the comparatively benign urea molecule until after hatch is less problematic for developing tissues and organ systems.