Effect of excess iodide on thyroid function of rainbow trout,Salmo gairdneri

The acute and chronic effects of excess iodide (KI or NaI) were studied on thyroid function of rainbow trout at 11±1°C. No Wolff-Chaikoff effect, characteristic of mammals, was observed and instead plasma L-thyroxine (T₄) levels increased 6 hr after a single iodide injection. Plasma 3,5,3′-triiodo-L-thyronine (T₃) did not change and by 24 hr plasma T₄ returned to normal. This iodide-induced elevation in plasma T₄ was probably not due to toxic effects demonstrated at higher NaI or KI doses. A single iodide injection also decreased the plasma iodide distribution space, decreased the fractional rate of plasma iodide loss and completely blocked thyroidal uptake of radioiodide. Injections of iodide over a 22-day period elevated plasma iodide 200X with no mortality and no influence on plasma T₄ or T₃. It is concluded that: (i) apart from the transient 6h increase in plasma T₄, trout thyroid function, as judged by plasma hormone levels, is insensitive to considerable iodide excess, (ii) non-invasive iodide suppression of thyroidal radioiodide recycling may be useful in kinetic studies of¹²⁵I-labeled thyroid hormones, and (iii) fundamental differences in intrathyroidal iodine metabolism appear to exist between mammals and fish.     

Changes in plasma T3 during fasting/refeeding in tilapia (Oreochromis niloticus) are mainly regulated through changes in hepatic type II iodothyronine deiodinase

Fasting and refeeding have considerable effects on thyroid hormone metabolism. In tilapia (Oreochromis niloticus), fasting results in lower plasma T₃ and T₄ concentrations when compared to the ad libitum fed animals. This is accompanied by a decrease in hepatic type II (D2) and in brain and gill type III (D3) activity. No changes in kidney type I (D1) activity are observed. Refeeding results in a rapid restoration of plasma T₄ values but not of plasma T₃. Plasma T₃ remains low for two days of refeeding before increasing to normal levels. Liver D2 and gill D3 also do not increase until two days after refeeding. Brain D3, on the other hand, rises immediately upon refeeding. These results suggest that the change in hepatic D2 activity is one of the main factors responsible for the changes in plasma T₃ observed during starvation and refeeding in tilapia. This finding supports the hypothesis that, in contrast to mammals and birds, liver D2 is the primary source of plasma T₃ in fish. Although the deiodinases important for the gross regulation of plasma T₃ during fasting/refeeding differ (mammals: D1 and D3, birds: D3, fish: D2), they all occur in the liver, suggesting that the organ itself may play a crucial role. In addition, the changes in brain and gill D3 suggest that these enzymes constitute a fine tuning mechanism for regulation of T₃ availability at the cellular or plasma levels, respectively.     

Does the time of feeding affect the diurnal rhythms of plasma hormone and glucose concentration and hepatic glycogen content of rainbow trout?

The diurnal rhythms of plasma glucose, cortisol, growth hormone (GH) and thyroid hormone (T₄, T₃) concentrations and hepatic glycogen content were measured in rainbow trout that had been entrained to a specific time of daily feeding (post-dawn, midday, pre-dusk); the purpose of the study was to investigate the significance of feeding time on hormones and metabolite patterns. Plasma GH, cortisol and T₄ concentrations all showed evidence of a diurnal rhythm in some treatment groups. There was a significant interaction between the time of feeding and plasma GH and cortisol concentration rhythms; for GH, this appeared to be related to the phase-shifting of the post-prandial increases in plasma GH concentrations, and for cortisol, the rhythms were only evident in fish fed in the post-dawn period [diurnal rhythms were not evident in treatment groups fed in at midday or pre-dusk]. Peak plasma T₄ concentrations were evident during the photophase in all three treatment groups; however, the time of feeding had a negligible effect on the timing of those peaks. There were no apparent diurnal rhythms of plasma T₃ and glucose concentrations, hepatic glycogen content or hepatosomatic index in any of the three treatment groups. To whom correspondence should be addressed     

The relationship between T3 production and energy balance in salmonids and other teleosts

Extrathyroidal T₄ 5′-monodeiodination, demonstrated in several teleost species, generates T₃ which binds more effectively than T₄ to putative nuclear receptors and is probably the active thyroid hormone. T₄ to T₃ conversion is sensitive to the physiological state and provides a pivotal regulatory link between the environment and thyroid hormone action. T₃ generation is enhanced in anabolic states (positive energy balance or conditions favoring somatic growth; food intake or treatment with androgens or growth hormone) and is suppressed in catabolic states (negative energy balance or conditions not favoring somatic growth; starvation, stress, or high estradiol levels associated with vitellogenesis). In fish, as in mammals, thyroidal status may be finely tuned to energy balance and through T₃ production regulate energy-demanding processes, which in fish include somatic growth, development and early gonadal maturation.     

In vivo calcitropic effect of some vitamin D compounds in the marine Antarctic teleost,Pagothenia bernacchii

The Antarctic notothenioid, Pagothenia bernacchii, were found to have plasma total and free calcium levels, plasma inorganic phosphate and whole body calcium efflux rates which were similar to those seen in other teleosts. But total bone calcium was lower than reported for other teleosts. A single injection of vitamin D₃ (5 ng g^–1 fish) increased plasma total and plasma free calcium and these increases were associated with an increase in whole body calcium efflux and bone calcification. Conversely, the same treatment with 1,25-(OH)₂-D₃ reduced plasma free calcium. This seco-steroid also increased the specific activity of ⁴⁵Ca in bone at 40h post-injection but did not significantly effect total bone calcium, plasma total calcium or whole body calcium efflux. 25-OH-D₃ at the same dose had no effect on any of the parameters tested and none of the seco-steroids tested had any effect on plasma total inorganic phosphate. These data show that both D₃ and 1,25-(OH)₃-D₃ can have calcitropic effects in this marine teleost and that these two forms of vitamin D can exert different effects within the same species.     

Effects of thyroid hormone on gonadotropin-induced steroid production in medaka,Oryzias latipes,ovarian follicles

Blood and ovarian samples were collected at intervals of 4h prior to spawning time from medaka (Oryzias latipes) that were maturationally synchronized with artificial photoperiod (14h light: 10h dark). Plasma estradiol-17β (E₂) levels increased rapidly from 16h before spawning and peaked at 8h before spawning. Follicle-enclosed oocytes (ovarian follicles) at different stages of development were isolated from the ovaries and used to study the in vitro effects of thyroid hormone (triiodothyronine; T₃) on pregnant mare serum gonadotropin (GTH)-induced E₂ production. GTH at a concentration of 100 IU/ml stimulated E₂ production by ovarian follicles collected between 32 and 16h before spawning. At 32h before spawning, T₃ (5 ng/ml) administered along with GTH (100 IU/ml) resulted in a 3.5 fold increase in E₂ production, compared with GTH administered alone. These results suggest that T₃ can act on ovarian follicles directly to modulate GTH-stimulated E₂ production in the medaka.     

Intestinal HCO3− secretion in marine teleost fish: evidence for an apical rather than a basolateral Cl−/HCO3− exchanger

Intestinal fluid was collected from 11 marine teleost fish from the Baltic sea and the Pacific ocean. The anterior, mid and posterior segments of the intestine contained 33–110 mM of HCO₃ ^– equivalents (with exception of the Atlantic cod which contained only 5–15 mM). Considering literature values of transepithelial potentials and concentration gradients, these high levels of HCO₃ ^– equivalents are probably the result of active HCO₃ ^– transport. Possible HCO₃ ^– transport mechanisms were studied in the Pacific sanddab (Citharichthys sordidus) in vitro. Measurements of net secretion of HCO₃ ^– equivalents across the intestinal epithelium revealed mucosal DIDS sensitivity (10^–4 M) and Cl^–-dependence of the HCO₃ ^– equivalent net flux, but no serosal DIDS (10^–4 M) sensitivity. Net Na⁺ uptake was abolished in the absence of Cl^–, but some Cl^– uptake persisted in the absence of Na⁺, at a rate similar to that of net HCO₃ ^– secretion. Anterior, mid and posterior segments of the intestine performed similarly. These observations support the presence of an apical rather than a basolateral Cl^–/HCO₃ ^– exchanger and thus contrast the currently accepted model for intestinal HCO₃ ^– secretion. This apical Cl^–/HCO₃ ^– exchanger alone, however, is not sufficient for maintaining the observed HCO₃ ^– equivalents gradient in vivo. We suggest a coupling of cytosolic carbonic anhydrase, a basolateral proton pump and the apical Cl^–/HCO₃ ^– exchanger to explain the intestinal HCO₃ ^– transport.     

Combined effects of a simulated marine heatwave and an algal toxin on a tropical marine aquaculture fish cobia (Rachycentron canadum)

Ongoing global warming is one of the major challenges for the development of aquaculture in the tropical regions where species are already cultured in the water temperature close to their upper physiological thresholds. Furthermore, warming can trigger blooms of toxic algae, yet we do not know how extreme warming such as a marine heatwave (MHW) and algal toxins may affect marine aquaculture species. To address this issue, we investigated the effects of a simulated MHW in combination with exposure to trans‐4‐trans‐decadienal (PUA), a diatom‐derived toxin, on survival, growth, development and biochemical composition of cobia larvae and juveniles. Cobia larvae were exposed for 48 hr to one of two temperatures (29 vs. 34°C) and two PUA treatments (0 vs. 0.5 µM). Surviving larvae from each treatment were divided into two subsets: three replicates were used for the feeding test and five replicates were used for the recovery test in a non‐contaminated environment at the respective temperatures of 29 or 34°C. Survival of cobia larvae was reduced by 16% in either MHW or PUA, but it dropped by 60% when both stressors were present, indicating a synergistic effect. MHW, but not PUA, reduced the feeding of cobia larvae. PUA had no delayed effects on growth rate and biochemical composition of the fish. MHW strongly reduced specific growth rate, body protein and lipid contents in cobia. Our results provide the first empirical evidence of how MHW and toxic algae may interact and challenge cobia and marine aquaculture production in tropical countries.     

Density-dependent habitat selection and the ideal free distribution in marine fish spatial dynamics: considerations and cautions

Current methods and theory used in the study of the spatial dynamics of marine fish are problematic. Positive relationships between population abundance and occupied area are typically interpreted as evidence of density‐dependent habitat selection. However, both abundance and area may co‐vary with an un‐parameterized variable, such as a density‐independent effect. In addition, if density‐dependent habitat selection is present, population growth rates in optimal habitats would be expected to be lower than in marginal habitats. This same pattern can also evolve from a large‐scale, spatially autocorrelated change in a density‐independent factor. The theory underlying density‐dependent habitat selection, the ideal free distribution, can be tautological when no a priori information of how habitat suitability changes with density is known. In this case, an ideal free distribution can be defined for any pattern of habitat‐specific population growth rates. However, these problems are not insurmountable and solutions may be found by considering spatial variation in proxies of fitness and explicitly allowing for the relative importance of habitat selection (density dependent) and environmental (density independent) effects to vary with spatial scale.     

Effects of hypoxia on blood pressure and heart rate in three marine teleosts

Ventral aortic blood pressure (P_va) and heart rate (HR) responses to rapidly (within 1 min) induced hypoxia (P_WO₂=4−5.3 kPa) were investigated in vivo in three species of marine teleosts (shorthorn sculpin, Myoxo-cephalus scorpius; eel-pout, Zoarces viviparus; and five-bearded rockling, Ciliata mustela). Fish were exposed to hypoxia for 4 min (M.scorpius) or 5 min (Z.viviparus and C.mustela). P_va was unaffected in M.scorpius, decreased in Z.viviparus and increased in C.mustela in response to hypoxia. Untreated M.scorpius and Z. viviparus responded with a characteristic bradycardia during hypoxia, whereas C.mustela developed no bradycardia. Injection of atropine followed by the β-adrenoceptor antagonist sotalol in M.scorpius and Z. viviparus, revealed that both the inhibitory (cholinergic) and the excitatory (adrenergic) influence on the heart increase during hypoxia. The inhibitory influence dominates, resulting in the observed bradycardia.     

Thyroid hormone deiodinase systems in salmonids,and their involvement in the regulation of thyroidal status

The trout thyroid secretes L-thyroxine (T₄) which undergoes enzymatic deiodination in liver and other tissues. Based on mammalian studies, T₄ outer-ring deiodination (ORD) or T₄ inner-ring deiodination (IRD) could generate respectively 3,5,3′-triiodo-L-thyronine (T₃) or 3,3′,5′-T₃(rT₃), while subsequent T₃ORD or T₃IRD could generate respectively 3,5-diiodo-L-thyronine (T₂) or 3,3′-T₂, and rT₃ORD or rT₃IRD could generate respectively 3,3′-T₂ or 3′,5′-T₂. In practice, T₄ in trout undergoes hepatic ORD to produce T₃ but negligible IRD to produce rT₃, and T₃ in turn undergoes negligible ORD but modest IRD to produce 3,3′-T₂. T₄ORD, which is particularly important in converting T₄ to the biologically more potent T₃, also occurs in gill, muscle and kidney. At least two isozymes are involved: i) a high-affinity, propylthiouracil (PTU)-sensitive T₄ORD which displays ping-pong kinetics, requires thiol as a cofactor, and is present in liver, gill and muscle, and ii) a low-affinity, PTU-insensitive T₄ORD with sequential kinetics with a thiol cofactor, and is present in liver and kidney. Receptor-bound T₃ is derived primarily from the plasma for kidney, mainly from intracellular sources for gill and about equally from both plasma and intracellular sources for liver. Thus, the high-affinity T₄ORD may produce T₃ for local intracellular use while the low-affinity 5′-monodeiodinase may produce T₃ for systemic use. T₄ORD activity responds to nutritional factors and the physiologic state of the fish. Furthermore, T₃ administered orally for either 6 weeks or 24h reduces the functional level (V_max) of hepatic T₄ORD, and T₃ added to isolated hepatocytes also reduces activity, indicating direct T₃ autoregulation of T₄ORD to maintain hepatocyte T₃ homeostasis. However, T₃ administration also induces T₄IRD to produce biologically inactive rT₃ and induces T₃IRD to produce 3,3′-T₂. Thus, the trout liver has several iodothyronine deiodinase systems which in a coordinated manner regulate tissue T₃ homeostasis in the face of a T₃ challenge. It does this by decreasing formation of T₃ itself, by diverting T₄ substrate to biologically inactive rT₃ and by increasing the degradation of T₃. These deiodinases differ in many respects from any mammalian counterparts.     

Circadian pattern of hepatosomatic index,liver glycogen and lipid content,plasma non-esterified fatty acid,glucose, T3, T4, growth hormone and cortisol concentrations in Oncorhynchus mykiss held under different photoperiod regimes and fed using demand-feeders

The circadian patterns of several tissue and plasma metabolites, and several plasma hormone concentrations are described in rainbow trout (Oncorhynchus mykiss) that were held in groups under three different photoperiod regimes, and given free access to a demand-feeder. Regardless of photoperiod regime, all the measured parameters showed significant diel rhythms that appeared to be synchronized by dawn; dawn was represented by the concomitant onset of both light and feeding. The diel increases in hepatic glycogen content, and plasma T₄ and cortisol concentrations were in phase with the main period of feeding activity, whereas the peaks in plasma T₃ and glucose concentrations that may also be triggered by feeding activity, were delayed by several hours. The peaks in hepatosomatic index, plasma non-esterified fatty acids and plasma growth hormone concentrations were 180° out of phase with the main period of feeding activity, and associated with periods of hypophagia and low activity.     

Seasonal variations of trimethylamine oxide and urea in the blood of a cold-adapted marine teleost,the rainbow smelt

Trimethylamine oxide (TMAO), which has previously not been known to occur in significant amounts in the blood of marine teleosts, rose to concentrations of approximately 50 mM in the blood of winter-acclimatized rainbow smelt, Osmerus mordax. Urea also increased in the blood of cold-acclimatized smelt, and, with TMAO, contributed significantly to the winter freezing point depression. TMAO and urea also varied seasonally in muscle and liver tissues. TMAO and urea appeared to be reabsorbed from the urine. Losses of TMAO and urea from the head region of the fish, where most of the losses appeared to occur, were approximately 9 μmol and 8 μmol 100 g⁻¹ h⁻¹, respectively. Despite the effluxes, TMAO and urea levels in both the blood and muscle either increased or were maintained in starved, cold-acclimated fish, suggesting that they were synthesized in response to cold temperature. TMAO was also found in the blood of some other cold-hardy teleosts.     

Antifreeze proteins in the urine of marine fish

Several species of marine teleosts have evolved blood plasma antifreeze polypeptides which enable them to survive in ice-laden seawater. Four distinct antifreeze protein classes differing in carbohydrate content, amino acid composition, protein sequence and secondary structure are currently known. Although all of these antifreezes are relatively small (2.6–33 kd) it was generally thought that they were excluded from the urine by a variety of glomerular mechanisms. In the present study antifreeze polypeptides were found in the bladder urine of winter flounder (Pseudopleuronectes americanus), sea raven (Hemitripterus americanus), ocean pout (Macrozoarces americanus) and Atlantic cod (Gadus morhua). Since the plasma of each of these fish contains a different antifreeze class it would appear that all four classes of antifreeze can enter the urine. The major antifreeze components in the urine of winter flounder were found to be identical to the major plasma components in terms of high performance liquid chromatography retention times and amino acid composition. It is concluded that plasma antifreeze peptides need not be chemically modified before they can enter the urine.     

Effects of dietary thyroid hormones on the red drum (Sciaenops ocellatus)

Four separate 8-week feeding trials were conducted to assess the effects of supplementing semipurified diets with either triiodothyronine (T₃) or thyroxine (T₄) at 0, 2, 10, and 50 mg/kg on growth and body composition of juvenile red drum (Sciaenops ocellatus) held in artificial brackish water (6‰) and artificial seawater (32‰). At both levels of salinity, increasing doses of T₃ resulted in fish with reduced weight gain, feed efficiency, condition factor (weight × 100/length³), and muscle ratio (muscle weight × 100/body weight), as well as a lighter body color. Significant (p < 0.05) effects of T₃ on the proximate composition of whole body, liver, and muscle were variable, generally reflecting decreased lipid and protein storage in liver and muscle, respectively. The two highest doses of T₃ given to seawater adapted fish increased survival. Dietary T₄ supplementation had no distinctive effects on appearance, growth or proximate body composition. These results indicate that whereas T₃ may function to regulate protein and lipid metabolism in red drum, dietary supplementation with T₃ leads to a hyperthyroidism-induced catabolic state. The elevated endogenous thyroid hormone levels found in fish fed optimal diets may thus adequately supply tissue needs during juvenile growth.     

Determination of 3,5,3′-triiodo-L-thyronine (T3) levels in tissues of rainbow trout (Salmo gairdneri) and the effect of low ambient pH and aluminum

Tissue T₃ (3,5,3′-triiodo-L-thyronine) concentrations were measured in rainbow trout, Salmo gairdneri, after digestion by Pronase or collagenase and extraction with ethanolic ammonia (99:1, v/v) followed by 2N NH₄OH and chloroform. Recoveries of [¹²⁵I]T₃ administered in vivo or in vitro were high and consistent and there was close parallelism between sample dilutions and the radioimmunoassay curve, but recoveries of unlabeled T₃ administered in vitro were low and variable. Alternatively, trout were brought to isotopic equilibrium by [¹²⁵I]T₃ infusion for 96 h, the extracted [¹²⁵I]T₃ determined by gel filtration and the tissue T₃ content calculated from the specific activity of plasma [¹²⁵I]T₃. By the latter method, tissue T₃ concentrations were: intestine (4.2 ng/g), kidney (2.5), liver (2.8), stomach (1.5), heart (1.0), muscle (0.7), gill (0.6) and skin (0.3). Muscle (67% of body weight) comprised the largest tissue T₃ pool (82% of all tissues examined). Seven days exposure of trout to water acidified with H₂SO₄ (pH 4.8) or acidified water containing aluminum (21.6 mM), decreased tissue T₃ content generally and particularly in muscle (14% of controls). In conclusion, skeletal muscle is the largest T₃ tissue pool and seems highly responsive to altered physiologic state.     

The influence of bottom type and shelf position on biodiversity of tropical fish inside a recently enlarged marine reserve

• 1. A necessary component of implementing a successful marine reserve is the quantification of the biological resources that fall under its protection. Without such an initial assessment, the future effects of the reserve on the local habitat and biotic community cannot be quantified and will remain the subject of debate.
• 2. This study provides such a baseline assessment of fish diversity and habitat types within a recently enlarged marine reserve. Buck Island Reef National Monument, US Virgin Islands, was recently enlarged from approximately 4 km² to over 76 km². Areas of sand, seagrass, and hard‐bottom under protection were increased from 0.29 km², 0.47 km², and 1.96 km² to 2.70 km², 2.89 km², and 18.30 km² respectively when the Monument was expanded. A 53 km² area of pelagic/deep‐water habitat with unknown bottom type is now also protected by the Monument.
• 3. Visual counts of fish within 25×4 m² transects conducted during the day were used to assess fish community structure and habitat utilization patterns. Species richness, diversity, assemblage structure, and fish density were evaluated and compared among sand, seagrass, and hard‐bottom habitats. Hard‐bottom sites had over twice the mean species richness and diversity as sand and seagrass sites, and several times greater mean fish density.
• 4. Quantification of the fish community in pelagic and deep‐water habitats within the reserve is recommended to provide a more comprehensive assessment of the offshore areas of the reserve. Fish numbers, size, and diversity outside the reserve boundaries must also be evaluated to allow quantification of the effects of the marine reserve on the adjacent fish communities.

Copyright © 2004 John Wiley & Sons, Ltd.     

Using patterns of reef fish assemblages to refine a Habitat Classification System for marine parks in NSW,Australia

• 1. The Solitary Islands Marine Park (SIMP) in New South Wales (NSW), Australia, is located in a tropical‐temperate biotone, with variable influence of the tropical East Australian Current (EAC) from inshore to offshore. Marine communities on the extensive, subtidal, rocky reefs are poorly described. As a result, the current Habitat Classification System (HCS) used as part of the process to determine the marine park zoning arrangements, which places all shallow reef (<25 m) in the same category, is unlikely to represent real biotic patterns.
• 2. To evaluate the influence of five key factors (distance from shore, reef type, dominant benthos, latitude, and depth range) fish assemblages were widely surveyed, using 30‐minute timed counts, at 68 sites across the extent of shallow reef within the park. Relationships between assemblage patterns and levels of the different factors were subsequently examined using multivariate analyses.
• 3. Patterns of reef fish assemblages were most strongly correlated with distance from shore. Three distinct assemblages occurred on inshore (<1.5 km), mid‐shelf (1.5–6 km), and offshore (>6 km) reefs. Differences in assemblage structure by reef type were also apparent on inshore and offshore reefs, but not on mid‐shelf reefs. Correlations with the other factors were weak. The cross‐shelf pattern was persistent over the scale of years.
• 4. The results of the study provide strong support for incorporating distance‐from‐shore categories into a refined HCS. This will improve its ability to represent biological diversity as reflected by patterns of reef fish assemblages. Further research is required to determine the wider application of the HCS to other marine parks in NSW and to determine if it also, effectively, represents other components of biodiversity. Copyright © 2009 John Wiley & Sons, Ltd.

     

Effects of replacement of dietary fish oil with soybean oil on growth performance and tissue fatty acid composition in marine herbivorous teleost Siganus canaliculatus

Fish oil (FO) substitution has been studied in many marine carnivorous fish, but seldom in marine herbivorous or omnivorous species. To evaluate the feasibility of using soybean oil (SO) as a dietary lipid and confirm its capability of converting C₁₈ polyunsaturated fatty acid (PUFA) into long chain polyunsaturated fatty acid (LC‐PUFA) in the marine herbivorous teleost Siganus canaliculatus, juvenile fish were fed with four formulated diets differing in lipid composition, with SO accounting for 0.76% (SO0), 23% (SO23), 45% (SO45) and 67% (SO67) of total dietary lipid respectively. After feeding for 8 weeks, growth performance including weight gain, specific growth rate, feed conversion ratio and protein efficiency rate were better in the SO23 and, especially, SO45 groups than in the SO0 and SO67 groups (P < 0.05). Tissue fatty acid compositions were affected by diet, with the liver contents of eicosapentaenoic (EPA), docosapentaenoic (DPA), docosahexaenoic (DHA) acids and total n‐3 PUFA displaying parallel changes with the corresponding dietary fatty acids. While the muscle contents of EPA, DPA and total n‐3 PUFA between SO0 and SO23 groups, and the liver contents of arachidonic acid (ARA) and 20:4n‐3, as well as the muscle content of 20:3n‐6 between SO0 and SO45 groups showed no difference, confirming the biosynthesis of LC‐PUFA from C₁₈ precursors in vivo as the contents of corresponding fatty acids in diets SO23/SO45 were much lower than those in diet SO0 (P < 0.05). The results indicate that SO may be a suitable dietary lipid source for S. canaliculatus, and can replace up to 67% or 45% of total dietary FO without negatively compromising growth performance or nutritional quality of fish respectively. Moreover, the study increases our knowledge of FO substitution in marine herbivorous fish.     

Molecular cloning of the cDNA encoding the β subunit of thyrotropin and regulation of its gene expression by thyroid hormones in the goldfish, Carassius auratus

A cDNA encoding the subunit of thyrotropin (TSH) was isolated from a goldfish (Carassius auratus) pituitary gland cDNA library. By comparing the sequence with other teleost TSHs, a signal peptide of 19 amino acids and a mature hormone of 131 amino acids were predicted for goldfish TSH subunits. The resulting putative mature hormone of 131 amino acids had well-conserved cysteine positions and a putative N-linked glycosylation site; homology was 51–67% with TSHs from other teleosts, 38–43% with tetrapod TSHs, but only 27 and 29% with goldfish GTH-I and -II, respectively. We also examined the effects of thyroid hormones (TH) and thiourea (TU, an inhibitor of TH production) treatments on TSH and GTH subunit gene expressions in the goldfish pituitary gland. After thyroxine (T₄) treatment, circulating T₄ concentration increased and TSH mRNA level decreased. Supressing the amount of circulating T₄ and triiodothyronine (T₃) by TU treatment increased the TSH mRNA level. Moreover, T₄ replacement therapy (simultaneous treatment of both TU and T₄) caused a high level of circulating T₄ and a low level of circulating T₃, and a decrease in the TSH mRNA level. Thus, changing levels of circulating TH exert a negative feedback on the level of TSH subunit mRNA in goldfish in vivo. On the other hand, GTH subunit mRNA levels were not affected by changes in the levels of circulating TH.