Morphology of unfertilized mature and fertilized developing marine pelagic eggs in four types of multiple spawning flounders

Starry flounder Platichthys stellatus, spotted halibut Verasper variegates, turbot Scophthalmus maximus, and Japanese flounder Paralichthys olivaceus are four commercially cultivated multiple spawning flounders that spawn pelagic eggs. Through appropriate light and scanning electron microscope processing, the shape and surface structures (such as micropyle, pores, pore density, and paten) of unfertilized mature and fertilized developing eggs of the four species were observed and measured. First, individual or intraspecific comparisons of the surface structures of eggs at different developmental stages were made. Second, interspecific differences among the four species at the same developmental stage of unfertilized mature eggs were statistically computed and analyzed through one-way analysis of variance and hierarchical cluster analysis. Eggs of the same species collected at different stages of development tend to be different in morphology. Smoothing of the convoluted egg envelope surface and closure of the micropyle to serve as a final step of the polyspermy-preventing reaction are common after fertilization. Based on detailed morphology of micropyle of just-mature fertilizable eggs, turbot, starry flounder, and Japanese flounder each have a micropyle with a long canal but no distinct micropylar vestibule, type III of Riehl and Götting (Arch Hydrobiol 74:393–402, 1974). In contrast, spotted halibut has a micropyle with a distinct flat micropylar vestibule and a long canal, type II. Envelope surface microstructures, especially those in the micropyle region, are useful characters for egg identification among the four species. Cluster analysis using selected egg characters indicated the highest similarity between turbot and Japanese flounder and that starry flounder is obviously more similar to turbot and Japanese flounder than to spotted halibut.     

Identification and differential expression of microRNAs during metamorphosis of the Japanese flounder (Paralichthys olivaceus)

Background

MicroRNAs (miRNAs) are a class of endogenous small non-coding RNAs of 20–25 nucleotides that play a key role in diverse biological processes. Japanese flounder undergo dramatic metamorphosis in their early development. The metamorphosis is characterized by morphological transformation from a bilaterally symmetrical to an asymmetrical body shape concomitant with extensive morphological and physiological remodeling of organs. So far, only a few miRNAs have been identified in fish and there are very few reports about the Japanese flounder miRNA.

Methodology/Principal Findings

Solexa sequencing technology was used to perform high throughput sequencing of the small RNA library from the metamorphic period of Japanese flounder. Subsequently, aligning these sequencing data with metazoan known miRNAs, we characterized 140 conserved miRNAs and 57 miRNA: miRNA* pairs from the small RNA library. Among these 57 miRNA: miRNA* pairs, twenty flounder miRNA precursors were amplified from genomic DNA. We also demonstrated evolutionary conservation of Japanese flounder miRNAs and miRNA* in the animal evolution process. Using miRNA microarrays, we identified 66 differentially expressed miRNAs at two metamorphic stages (17 and 29 days post hatching) of Japanese flounder. The results show that miRNAs might play a key role in regulating gene expression during Japanese flounder metamorphosis.

Conclusions/Significance

We identified a large number of miRNAs during flounder metamorphosis, some of which are differentially expressed at two different metamorphic stages. The study provides an opportunity for further understanding of miRNA function in the regulation of flounder metamorphosis and gives us clues for further studies of the mechanisms of metamorphosis in Japanese flounder.     

Rearing of Procypris rabaudi during early life-history stages

The early life‐history of Chinese rock carp Procypris rabaudi was investigated during a 56‐day rearing period: 318 artificially propagated P. rabaudi larvae were reared throughout metamorphosis in a small‐scale recirculation system (345 L water volume, 10 × 18 L rearing tanks, 150 L storage and filter compartment with bioballs, 20–30 larvae L^?1) at the Institute of Hydrobiology, Wuhan, Hubei Province, China. The newly hatched larvae had an initial total length of 8.93 ± 0.35 mm SD (n = 10) at 3 days post‐hatch and reached an average total length of 33.29 mm (±1.88 mm SD, n = 10) 56 days after hatching. Length increment averaged 0.45 mm day^?1, resulting in a mean growth of 24.4 mm within the 56‐day period. High mortality rates of up to 92% derived from an introduced fungus infection and subsequent treatment stress with malachite green. Our results indicate that Chinese rock carp can be raised successfully from artificially fertilized eggs. We therefore assume this species to be a candidate for commercial aquaculture.     

Isolation and characterization of microsatellite DNA markers in the rare species barfin flounder (Verasper moseri) and its closely related species spotted halibut (V. variegatus)

Barfin flounder and spotted halibut have been selected in Japan as target species for stock enhancement due to the fact that the number of individuals of these species in the wild has been decreasing in the last three decades. Barfin flounder is now considered to be a rare species as its situation is critical. The first microsatellite DNA markers for barfin flounder and spotted halibut are described in this study. Cross‐amplification of barfin flounder markers was examined in spotted halibut species and vice versa.     

Embryonic and larval staging of summer flounder (Paralichthys dentatus)

Early development of flatfishes such as the summer flounder Paralichthys dentatus (Pleuronectiformes) has not been extensively documented, largely because of a dearth of material; however, the recent expansion of flatfish aquaculture has made embryos of P. dentatus readily available for developmental studies. We divide development of P. dentatus embryos and larvae into two main periods, pre- and posthatching, and assign stages within each of those primary divisions. Stages from fertilization to hatching loosely follow the general teleost staging scheme suggested by Shardo ([1995] J Morphol 225:125-167); stages from hatching through metamorphosis are aligned with the series used for Japanese flounder, P. olivaceus (Minami [1982] Nippon Suisan Gakkaishi 48:1581-1588; Fukuhara [1986] Nippon Suisan Gakkaishi 52:81-91). Although length, width, and age may serve as approximate indicators of developmental progression in summer flounder, these characteristics are too variable to form the sole basis of a staging table. Therefore, we define stages by morphological criteria drawn from the development of the jaw apparatus and digestive system, eye migration, and notochord tip flexion. Examination of these morphological features in hatched larvae allows accurate and consistent assessment of developmental stage despite variation in timing and size. The staging scheme for flounder embryonic and larval development presented here should facilitate both experimental and comparative research on summer flounder and other flatfish species.     

Chemical composition, mineral content and amino acid and lipid profiles in bones from various fish species

Insulin-like growth factor-I (IGF-I) is an important regulator of growth and development in vertebrates. Both the endocrine and paracrine actions of IGF-I are mediated through ligand-binding to a membrane-bound IGF-I receptor (IGF-IR). The characterization of this receptor and subsequent expression studies thus help elucidate the endocrine regulation of developmental processes. As other flatfish species, the Atlantic halibut (Hippoglossus hippoglossus) undergoes a dramatic larval metamorphosis. This process is largely under endocrine control, and data indicate that IGF-I could be a key regulator. IGF-I content increases up to late pre-metamorphosis and decreases during metamorphosis. The IGF-IR has, however, not been studied during flatfish metamorphosis. To examine IGF-IR gene expression, two IGF-IR mRNA were cloned and sequenced. These partial sequences share high identity (>or=95%) and similarity (>or=97%) with other fish IGF-IR and lower identity (>or=77%) and similarity (>or=83.5%) with Japanese flounder insulin receptors. The expression of mRNA for both IGF-IR was analyzed by quantitative real-time RT-PCR during six larval developmental stages from pre- to post-metamorphosis. IGF-IR1 and IGF-IR2 mRNA are differentially expressed during metamorphosis, but if this indicates an isoform-specific regulation of developmental processes by circulating and/or locally-secreted IGF-I is unclear. Both IGF-IR genes are down-regulated in halibut larvae experiencing arrested metamorphosis, suggesting the IGF-I system is critical for metamorphic success in halibut.     

Morphological and morphometric aspects of early life stages of piabanha Brycon gouldingi (Characidae)

Adult specimens of piabanha Brycon gouldingi were collected from Rio das Mortes (Mato Grosso, Brazil), adapted to captivity and induced to spawn at Buriti Fisheries (Nova Mutum, MT, Brazil). The early developmental stages of B. gouldingi were then characterized. Samples were collected at pre‐determined times from oocyte extrusion to total yolk absorption. Oocyte diameter, total larval length (L_T) and yolk‐sac volume were measured. The mean ± s.d . duration of embryo developmental of B. gouldingi was 13·90 ± 0·06 h at 26·40 ± 1·13° C. The mean ± s.d . oocyte diameter was 1·13 ± 0·06 mm with 54% of oocytes ranging from 1·11 to 1·20 mm. Seven stages characterized the early developmental phase of this species: zygote, cleavage, morula, blastula, gastrula, histogenesis–organogenesis and hatching, with unique features related to each stage. At hatching, the larvae measured 3·40 ± 0·07 mm, presented an elongated shape with yolk‐sac volume of 0·46 ± 0·08 µl, non‐pigmented eyes and exhibited swimming ability. When the yolk was completely absorbed at 55 h post‐hatch, mean ± larval L_T was 6·68 ± 0·65 mm, the eyes were highly pigmented and the teeth were visible. These are the first reported findings on the initial developmental stages of B. gouldingi and could be used to improve captive breeding management and conservation practices.     

Changes in growth performance and proximate composition in Japanese flounder during metamorphosis

Morphological changes are described in Japanese flounder Paralichthys olivaceus larvae and juveniles with emphasis on growth during the period of metamorphosis. Ontogenetic changes in condition factor and lipid, glycogen and protein levels were also analysed to determine the utilization of stored energy. Fish grew from 6·6 to 20·3 mm L _T(1·15 to 84·4 mg in mass, M ) during the period from 11 to 40 days after hatching (DAH) at 19·3° C. Per cent specific growth rate per day (% G day⁻¹) for wet mass was lowest during the metamorphic phase (21–30 DAH) compared to pre‐(11–20 DAH) and post‐metamorphic (31–40 DAH) phases. When L _T and M were expressed as     , the b value was highest during the pre‐metamorphic phase and lowest during the metamorphic phase. These findings indicate that the developmental changes that occur during metamorphosis of Japanese flounder are closely related to the growth pattern. Moisture, lipid and glycogen contents were also at the lowest level during metamorphosis compared to pre‐ and post‐metamorphosis, which suggest that Japanese flounder use their energy reserves to accomplish metamorphosis due to an apparent decline in feeding during this period.     

Developmental biology of medaka fish (Oryzias latipes) exposed to alkalinity stress

Alkalinity stress is common in cultured aquatic animals and considered to be one of the major stress factors for fishes when they are transferred to saline‐alkali waters. To evaluate potential effects of alkalinity on the developmental biology of Oryzias latipes, fertilized eggs, larvae and breeding fish were exposed to different carbonate alkalinity concentrations of 1.5–64.5 meq l^?1, for 9, 120, and 60 days, respectively. The mortality of embryos significantly increased when exposed to the high concentrations (16.5–64.5 meq l^?1). Although more than 50% of survived embryos hatched in 16.5 and 31.4 meq l^?1 concentrations of carbonate alkalinity, most were not able to swim up after hatching. Morphological abnormalities such as coagulated embryos, halted embryo development, and hatching failure were observed at stages 15, 29–33 and 38 in high concentrations (31.4, 64.5 meq l^?1). Almost all larvae in 16.5 and 31.4 meq l^?1 treatments died 70 d post‐hatch. Growth of juveniles exposed to carbonate alkalinity of 5.3 and 8.8 meq l^?1 was not significantly different at 70 d and 120 d post‐hatch. The number of eggs released by breeders, the fertilization rate and the hatching rate of eggs were significantly lower in the 31.4 meq l^?1 treatment than in other treatments. Although medaka are capable of surviving in high alkalinities (31.4, 64.5 meq l^?1) for an extended period of time, these conditions are stressful to the fish, especially at the embryonic and reproductive stages.     

Occurrence of skeletal deformities and osteological development in red porgy Pagrus pagrus larvae cultured under different rearing techniques

The present study describes the osteological development and the occurrence of skeletal deformities in red porgy Pagrus pagrus larvae in relation to the intensification of the rearing system. Eggs obtained from natural spawning were cultured under two different rearing systems: intensive (100 eggs l^?1) in 2000 l and semi‐intensive (mesocosm) system (5 eggs l^?1) in 40 000 l conico‐cylindrical tanks. Fish samples were periodically collected along the development from hatching to juveniles at 95 days post hatching (dph). Osteological development, meristic counts and the presence of skeletal deformities were evaluated. Despite the external appearance of the juveniles being similar to wild standards, X‐ray studies revealed a high number of fish (semi‐intensive: 37·8%; intensive: 45·5%) with skeletal deformities. Regardless of the rearing system, no significant interaction was found between the per cent of the most common deformities, axial deviations (lordosis and presence of fused vertebrae). Cranial deformities and kyphosis incidences, however, were significantly higher in intensively cultured P. pagrus. Also, the fused vertebrae in these fish were located mainly in the caudal area instead of pre‐haemal area for semi‐intensively reared P. pagrus. Moreover, a significant interaction was found between the total number of vertebrae and the type of rearing system used; fish from the intensive system showing a higher number of fish with an extra vertebrae (10 abdominal + 15 caudal). Present results suggest a relationship among feeding sequence, osteological development and deformity incidence and location in P. pagrus larvae.     

Cholecystokinin and tryptic activity in the gut and body of developing Atlantic halibut larvae: evidence for participation in the regulation of protein digestion

At 7 days after first feeding (DAFF), the peptide hormone cholecystokinin (CCK) content (fmol individual^?1) and the tryptic activity [μmol arginine‐methyl‐coumarinyl‐7‐amide (MCA) min^?1 individual^?1] per individual gut of Atlantic halibut Hippoglossus hippoglossus larvae were low: 0·2 ± 0·1 and 0·14 ± 0·10, respectively. Thereafter, both parameters increased with the increase in gut mass and reached 19·67 ± 5·58 and 2·71 ± 0·64 at 26 DAFF, respectively. Due to the small sample size, the dry mass (M_G, mg) of the individual gut could not be determined accurately at 7 DAFF. At 13 DAFF M_G represented 5·5% of whole body dry mass (M_w, mg) while at 26 DAFF it had increased to 23%. The mass specific tryptic activity [μmol MCA min^?1 per mg dry mass (M)] in the gut increased from 2·74 ± 1 ± 98 at 13 DAFF to 5·00 ± 0·78 at 26 DAFF. There was more individual variation in the mass specific CCK content (fmol M^?1) but no significant differences were found, although the data indicated an increase (from 23·38 ± 11·26 at 13 DAFF to 36·27 ± 8·96 fmol M^?1 at 26 DAFF). At 7 DAFF the CCK content of the gut represented c. 2% of the whole body CCK content while it increased to c. 62% of the whole body CCK content at 26 DAFF. This demonstrates that it is necessary to separate neural and gastrointestinal sources of CCK in order to determine its alimentary role in fish larvae. Trypsin activity was only found in the gut compartment. In larvae aged 45 DAFF dietary proteins delivery into the gut by tube‐feeding appeared to stimulate post‐prandial secretion of CCK from the gut as well as stimulate pancreatic trypsin secretion, suggesting that both factors contribute to protein digestion.     

Ionregulation in juvenile swordspine snook (Centropomus ensiferus,Poey, 1860) in relation to environmental salinity

Ninety‐nine swordspine snook Centropomus ensiferus (9.80 ± 0.3 g, mean ± SE) were studied in order to evaluate the influence of salinity on physiological properties under rearing conditions. Growth performance, survival rates, and ion concentrations (Na⁺, K⁺, Cl^?) as well proximal composition were measured over 76 days. Fish were exposed to three experimental salinities (0, 10, 20 ‰ , three replicates per treatment) and maintained in plastic tanks with a recirculation system equipped with flow‐through aquaria pumps (533 L per tank). Fish were fed twice daily to apparent satiation; at the end of the experiment the weight of fish kept in 10 ‰ was higher than that of fish kept in 0 and 20 ‰ , however no significant differences (P > 0.05) were observed among the experimental salinities. Survival was significantly lower in 10 ‰ salinity than in fish kept in 20 and 0 ‰ salinities. No significant differences (P > 0.05) were found in the Condition factor (K), specific growth rate (SGR), or in plasma Na⁺, K⁺, or Cl^? concentrations among treatments. Salinities also did not affect body composition (P > 0.05), but were significantly lower (P < 0.05) than at the start of the experiment. However, towards the end of the experiment a large accumulation of visceral fat in fish farmed in the three salinities (VFI > 4%) was observed. Water quality was within the optimum range (T: 28.7 ± 0.1°C; O2: 5.6 ± 0.1 mg L^?1; ammonia: 0.2 mg L^?1) for the growth of swordspine snook. Data indicates that C. ensiferus is an ionoregulator fish and able to cultivate successfully in various osmotic conditions, and in turn, maintain high levels of survival in captivity.     

The effects of light intensity and algae-induced turbidity on feeding behaviour of larval striped trumpeter

Striped trumpeter larvae reared in algal cell‐induced turbid water (greenwater) fed equally well in clearwater in a light intensity range of 1–10 μmol s^‐1 m^‐2, when evaluated in terms of both the proportion of larvae feeding and larval feeding intensity. An ontogenetic improvement in photopic visual sensitivity of larvae was indicated by improved feeding at 0·1 μmol s^‐1 m^‐2, from 26±5% of larvae feeding and 0·027±0·005 rotifers consumed per feeding larva min^‐1 on day 8, to 96±2% and 0·221±0·007 rotifers consumed larva^‐1 min^‐1 on day 23 post‐hatching. Algal cell‐induced turbidity was shown to reduce incident irradiance with depth, indicated by increasing coefficients of attenuation (1·4–33·1) with increasing cell densities (0–2×10⁶ cells ml^‐1), though light intensities in the feeding experiment test chambers, at the algal cell densities tested, were within the optimal range for feeding (1–10 μmol s^‐1 m^‐2). Algae‐induced turbidity had different effects on larval feeding response dependent upon the previous visual environment of the larvae. Young larvae (day 9 post‐hatching) reared in clearwater showed decreased feeding capabilities with increasing turbidity, from 98±1% feeding and 0·153±0·022 rotifers consumed larva^‐1 min^‐1 in clearwater to 61±10% feeding and 0·042±0·004 rotifers consumed larva^‐1 min^‐1 at 56 NTU, while older clearwater reared larvae fed well at all turbidities tested. Likewise, greenwater reared larvae had increased feeding capabilities in the highest algal cell densities tested (32 and 66 NTU) compared with those in low algal cell density (6 NTU), and clearwater (0·7 NTU) to which they were naïve.     

Effect of turbulence on feeding intensity and survival of Japanese flounder Paralichthys olivaceus pelagic larvae

Three‐day rearing experiments were conducted to study the effect of turbulence on the feeding intensity and survival of pelagic larvae of Japanese flounder Paralichthys olivaceus. Four levels of turbulence as control (10^?7·2 m² s^?3), low (10^?6·2 m² s^?3), mid (10^?5·6 m² s^?3) and high (10^?5·0 m² s^?3) were set by changing the flow rate of water pumped through pipes set on the bottom of the tanks. In B‐stage larvae, defined as having buds of elongated dorsal fin rays, the feeding intensity and growth were higher in the low and mid turbulence levels, while survival was highest in the control level. Most of the larvae surviving in the control level, however, were judged to be in a seriously starved condition leading to subsequent high mortality. Because the three‐day span of the rearing experiments was thought to be a little shorter than the periods before starvation‐induced, high mortality occurs. In contrast, for D‐stage larvae, their feeding and growth were optimal in the control and low levels. Feeding was more adversely affected in the high level for D‐stage larvae compared with B‐stage larvae. This is probably due to the compressed body shape and elongated dorsal fin rays of D‐stage larvae, which may be more strongly affected by turbulence and, as a consequence, the larval feeding behaviour such as pursuit and capture of prey organisms becomes less efficient than in lower turbulence. Considering the vertical distribution of B and D‐stage larvae in the oceanic water column, the optimum turbulence level range found in the present study corresponded to a wind speed of 7–10 m s^?1. Therefore, moderate weather conditions of this wind speed range are considered to potentially enhance survival of early larval stages of P. olivaceus.     

Developmental defects in pelagic fish embryos from the western Baltic

In February/March 1983 and 1984 a survey of pelagic fish eggs was conducted in the western Baltic (Kiel Bight), employing a horizontally towed plankton net (1 m Ø and 300 μm mesh). Maximum egg numbers in the upper meter of the S=21×10^?3 salinity layer were 200·100 m^?3. The most abundant eggs were cod (up to 142 eggs·100 m^?3), followed by plaice (up to 74 eggs·100 m^?3) and flounder (20 eggs·100 m^?3). A considerable percentage of embryos of all species displayed aberrant development. In 1983 18% of cod, 22% of flounder and 24% of plaice eggs caught contained defective embryos; in 1984 this number was larger, ranging from 28% in plaice over 32% in cod to 44% in flounder. Early developmental stages showed the highest malformation rates (up to 51% in the case of early flounder embryos). With progressive development, malformations decreased in numbers, being lowest prior to hatching. Highest rates of malformations were recorded in the Mecklenburg Bight in 1983. A second area with high incidence of malformation rates was located south and east of the island of Langeland. Several reasons, including environmental and anthropogenic factors, for the occurrence of malformed embryos in pelagic fish eggs are discussed. The potential of malformation rates in embryos of pelagic fish eggs as a tool for monitoring is considered.     

Effects of low-salinity on the growth and development of spotted halibut Verasper variegatus in the larva-juvenile transformation period with reference to pituitary prolactin and gill chloride cells responses

Low-salinity adaptability was investigated in a flatfish spotted halibut Verasper variegatus during the period from late metamorphic larvae to early juveniles by a 20-day rearing experiment under different salinity regimes (1, 4, 8, 16 and 32 ppt). Effects of low-salinity on growth and development were examined and the changes in the prolactin (PRL) production level in the pituitary and the gill chloride cell morphology were examined as physiological backgrounds for low salinity adaptation. PRL cells and chloride cells were identified by immunocytochemistry with a specific antiserum for PRL₁₈₈ and Na⁺,K⁺-ATPase. Most of the fish exposed to over 4 ppt survived for 20 days, but all the fish exposed to 1 ppt died within 5 days. Fish kept in intermediate salinities (8, 16 ppt) grew significantly better than those in the control group (32 ppt). Fish exposed to 4 ppt attained almost the same body length as the control group at 20 days after transfer, although these fish showed an abnormally dark body color as well as delayed development. These results suggested that spotted halibut has a high-adaptability to low-salinity environments and prefers an intermediate salinity near iso-osmolality (about 12 ppt) from the late metamorphic larval stage, but does not completely adapt to a hypoosmotic of 4 ppt salinity or less than half of the osmolality. The percentage of PRL-cell volume to pituitary volume was significantly higher at 4 ppt than in the control group. The chloride cells in gill filaments were significantly larger at 4 ppt than in the control group. These results suggest that juveniles could adapt to a low-salinity environment due to the activation of PRL production and enlargement of chloride cells. These laboratory findings suggest that late metamorphic larvae and early juveniles of spotted halibut may utilize a low salinity environment such as estuarine tidal flats or very shallow coastal areas as their nursery grounds in the sea.     

Ontogenetic changes in RNA,DNA and protein contents of Chinese loach,Paramisgurnus dabryanus (Dabry de Thiersant, 1872), larvae and juveniles

The changes in nucleic acid‐based indices and protein variables of Chinese loach, Paramisgurnus dabryanus, larvae and juveniles from hatching to 60 days after hatching (DAH) were conducted to assess its growth potential. The nucleic acid contents were analysed using a UV‐based method (n = 3, rearing temperature 24.4 ± 0.4°C, dissolve oxygen 7.1 ± 0.5 mg L^?1, pH 7.9 ± 0.4). Ribonucleic acid (RNA) concentration significantly decreased from 2 to 5 DAH, then increased rapidly until 10 DAH, declining slightly thereafter. Deoxyribonucleic acid (DNA) concentration increased 2–5 DAH, decreased until 9 DAH, slightly increased again around 26 DAH, and then declined to a relatively stable level. Both RNA‐DNA and protein‐DNA ratios showed a statistically obvious relationship with growth rates. A significantly positive relationship was found between RNA‐DNA ratio and growth rates during the early life stage of Chinese loach. According to the results, growth of Chinese loach is characterized by rapid hyperplasia from hatching through completion of the yolk‐sac stage followed by continued rapid hyperplasia combined with increasing hypertrophy after feeding commences. The stage preceding 17 DAH of Chinese loach P. dabryanus is presumed to be critical for its survival and growth at 24°C.     

Acute toxicity of ammonia and nitrite to different ages of Pacific cod (Gadus macrocephalus) larvae

Abstract

This study was conducted to evaluate the acute toxicity of ammonia and nitrite to three developmental stages of Pacific cod (Gadus macrocephalus) larvae (11, 22, and 35 days after hatching, with mean total lengths of 4.63 ± 0.14, 5.83 ± 0.17, and 7.46 ± 0.23 mm, respectively). The results showed for the first time that the acute toxicity of ammonia and nitrite is closely related to the age of Pacific cod larvae, and the acute toxicity of ammonia or nitrite increased with increased Pacific cod larval growth. Lethal concentrations (LC₅₀) of un-ionized ammonia nitrogen (UIAN) for a 48-h exposure in 11-day post-hatch, 22-day post-hatch, and 35-day post-hatch Pacific cod larvae were 1.72, 0.69, and 0.32 mg L^?1, respectively. The 48-h LC₅₀ of nitrite nitrogen to Pacific cod larvae 11-day post-hatch, 22-day post-hatch, and 35-day post-hatch were 831.76, 269.15, and 223.87 mg L^?1, respectively. The present findings demonstrate that the acute toxicity of ammonia for Pacific cod larvae is much higher than that of nitrite.     

Location,size and age at onset of metamorphosis in the Japanese eel Anguilla japonica

This study clarifies the location, size and age at the onset of metamorphosis in Japanese eels Anguilla japonica through oceanic surveys, rearing experiments and analyses of the morphology and otoliths of leptocephali and glass eels. Twenty‐eight metamorphosing leptocephali were collected in the mesoscale eddy region to the east of Taiwan during research expeditions in 2004. Rearing experiments showed that the total length (L_T) of leptocephali decreased by an average of 12·5% during metamorphosis and 13·9% during the 2–12 h after death. Thus, the mean back‐calculated L_T at the onset of metamorphosis for 630 glass eels from Taiwan and Japan was estimated at 67·8 ± 2·7 mm (mean ± S.D.). The estimated mean ante‐mortem size of the fully grown pre‐metamorphic leptocephali collected in 2004 was 64·6 ± 3·4 mm, which was consistent with the L_T estimate for glass eels. Otolith analysis showed that the mean age at the onset of metamorphosis was 137 ± 15 days and indicated that Japanese eels may have a recruitment route through the mesoscale eddies to the east of Taiwan in addition to the direct transfer route from the North Equatorial Current to the Kuroshio Current.