Björn Thrandur Björnsson

Thyroid hormones in growth and development of fish

The thyroid hormones (THs), thyroxine (T(4)) and triiodothyronine (T(3)) are products of the thyroid gland in all vertebrates. Their role in early development and metamorphosis is well established in mammals and amphibians, respectively, and recently several studies in fish have highlighted the importance of THs during flatfish metamorphosis. THs are present in high quantities in fish eggs and are presumably of maternal origin. During embryogenesis the concentration of T(4) and T(3) in the eggs decrease until endogenous production starts. Thyroid hormone receptors (TR) have been isolated from several teleosts and in common with tetrapods two receptor isoforms have been identified, TR alpha and TR beta. Both the receptors are expressed in early embryos and larvae of the Japanese flounder (Paralichthys olivaceus), zebrafish (Danio rerio) and seabream (Sparus aurata) although a different temporal pattern is apparent. The role of THs and TRs in fish embryogenesis, larval development and during metamorphosis will be discussed.

Growth Hormone Endocrinology of Salmonids: Regulatory Mechanisms and Mode of Action

The focus of this review is on the regulatory mechanisms and the mode of action of GH in salmonids. To stimulate further research, it aims at highlighting areas where numerous important breakthroughs have recently been made, as well as where data are currently lacking. The regulation of GH secretion is under complex hypothalamic control, as well as under negative feedback control by GH and IGF-I. Further, the recently characterized ghrelin is a potent GH secretagogue, and may prove to be a link between feed intake and growth regulation. GH plasma profiles show indications of diurnal changes, but whether salmonids have true pulsatile GH secretion remains to be elucidated. The recent cloning and characterization of the salmon GH receptor (GHR) is a major research break-through which will give new insights into the mechanisms of GH action. It should also stimulate research into circulating GH-binding proteins (GHBPs), as they appear to be a soluble form of the GHR. The salmonid GHR sequences show evolutionary divergence from other fish species, but with a high degree of identity within the salmonid group. Radioreceptorassay studies have found GHR present in all tissues examined, which is in line with the highly pleiotropic action of GH. Data are currently scarce on the plasma dynamics of GH in salmonids, and further studies on GHR and GHBPs dynamics coupled to assessments of GH clearance rates and pathways are needed. The direct versus indirect nature of GH action remains to be clarified, but GH appears to act both locally at the target tissue level to stimulate the autocrine/paracrine action of IGF-I, as well as on the liver to increase plasma IGF-I levels. In addition, GH interacts with other hormones such as cortisol, thyroid hormones, insulin, and reproductive hormones, generating a wide range of physiological effects. GH may act both peripherally and directly at the level of the central nervous system to modify behavior, probably by altering the dopaminergic activity in the brain.

Towards Fish Lipid Nutrigenomics: Current State and Prospects for Fin-Fish Aquaculture

Lipids are the predominant source of energy for fish. The mechanisms by which fish allocate energy from lipids for metabolism, development, growth, and reproduction are critical for understanding key life-history strategies and transitions. Currently, the major lipid component in aquaculture diets is fish oil (FO), derived from wild capture fisheries that are exploited at their maximum sustainable limit. The increasing demand from aquaculture for FO will soon exceed supply and threaten the viability of aquaculture. Thus, it is essential to minimize FO use in aquaculture diets. This might be achieved by a greater understanding of lipid storage and muscle growth, or the identification of alternatives to FO in feeds. This review focuses on recent research applying molecular and genomic techniques to the study of fin-fish lipid metabolism from an aquaculture perspective. Accordingly, particular emphasis will be given to fatty acid metabolism and to highly unsaturated fatty acid (HUFA) biosynthesis and to the transcriptional mechanisms and endocrine factors that regulate these processes in fish. Comparative studies of gene function and distribution are described, which, when integrated with recent fish genome sequence information, provide insights into lipid homeostasis and the outcomes associated with the replacement of FO in fish diets.

Differential expression of gill Na + ,K + -ATPase - and -subunits, Na + ,K + ,2Cl - cotransporter and CFTR anion channel in juvenile anadromous and landlocked Atlantic salmon Salmo salar

SUMMARY This study examines changes in gill Na+,K+-ATPase (NKA) α- and β-subunit isoforms, Na+,K+,2Cl- cotransporter (NKCC) and cystic fibrosis transmembrane conductance regulator (CFTR I and II) in anadromous and landlocked strains of Atlantic salmon during parr-smolt transformation, and after seawater (SW) transfer in May/June. Gill NKA activity increased from February through April, May and June among both strains in freshwater (FW), with peak enzyme activity in the landlocked salmon being 50% below that of the anadromous fish in May and June. Gill NKA-α1b, -α3, -β1 and NKCC mRNA levels in anadromous salmon increased transiently, reaching peak levels in smolts in April/May, whereas no similar smolt-related upregulation of these transcripts occurred in juvenile landlocked salmon. Gill NKA-α1a mRNA decreased significantly in anadromous salmon from February through June, whereas α1a levels in landlocked salmon, after an initial decrease in April, remained significantly higher than those of the anadromous smolts in May and June. Following SW transfer, gill NKA-α1b and NKCC mRNA increased in both strains, whereas NKA-α1a decreased. Both strains exhibited a transient increase in gill NKA α-protein abundance, with peak levels in May. Gill α-protein abundance was lower in SW than corresponding FW values in June. Gill NKCC protein abundance increased transiently in anadromous fish, with peak levels in May, whereas a slight increase was observed in landlocked salmon in May, increasing to peak levels in June. Gill CFTR I mRNA levels increased significantly from February to April in both strains, followed by a slight, though not significant increase in May and June. CFTR I mRNA levels were significantly lower in landlocked than anadromous salmon in April/June. Gill CFTR II mRNA levels did not change significantly in either strain. Our findings demonstrates that differential expression of gill NKA-α1a, -α1b and -α3 isoforms may be important for potential functional differences in NKA, both during preparatory development and during salinity adjustments in salmon. Furthermore, landlocked salmon have lost some of the unique preparatory upregulation of gill NKA, NKCC and, to some extent, CFTR anion channel associated with the development of hypo-osmoregulatory ability in anadromous salmon.

Humoral immune parameters in Atlantic cod (Gadus morhua L.) I. The effects of environmental temperature

The effects of environmental temperature on certain humoral immune parameters in Atlantic cod (Gadus morhua L.) were studied. Serum samples were collected from captive cod, of wild origin, kept at different temperatures for 12 months. It was found that immunoglobulin and natural antibody levels increased with increasing temperature whereas the total serum protein concentration, anti-protease activity, iron concentration, unsaturated and total iron binding capacity decreased with increasing temperature. Haemolytic activity and percentage iron saturation also tended to decrease with increasing temperature although this was not statistically significant.

Leptin and leptin receptor genes in Atlantic salmon: Cloning, phylogeny, tissue distribution and expression correlated to long-term feeding status.

The present study reports the complete coding sequences for two paralogues for leptin (sLepA1 and sLepA2) and leptin receptor (sLepR) in Atlantic salmon. The deduced 171-amino acid (aa) sequence of sLepA1 and 175 aa sequence for sLepA2 shows 71.6% identity to each other and clusters phylogenetically with teleost Lep type A, with 22.4% and 24.1% identity to human Lep. Both sLep proteins are predicted to consist of four helixes showing strong conservation of tertiary structure with other vertebrates. The highest mRNA levels for sLepA1 in fed fish (satiation ration=100%) were observed in the brain, white muscle, liver, and ovaries. In most tissues sLepA2 generally had a lower expression than sLepA1 except for the gastrointestinal tract (stomach and mid-gut) and kidney. Only one leptin receptor ortholog was identified and it shares 24.2% aa sequence similarity with human LepR, with stretches of highest sequence similarity corresponding to domains considered important for LepR signaling. The sLepR was abundantly expressed in the ovary, and was also high in the brain, pituitary, eye, gill, skin, visceral adipose tissue, belly flap, red muscle, kidney, and testis. Fish reared on a rationed feeding regime (60% of satiation) for 10 months grew less than control (100%) and tended to have a lower sLepA1 mRNA expression in the fat-depositing tissues visceral adipose tissue (p<0.05) and white muscle (n.s.). sLepA2 mRNA levels was very low in these tissues and feeding regime tended to affect its expression in an opposite manner. Expression in liver differed from that of the other tissues with a higher sLepA2 mRNA in the feed-rationed group (p<0.01). Plasma levels of sLep did not differ between fish fed restricted and full feeding regimes. No difference in brain sLepR mRNA levels was observed between fish fed reduced and full feeding regimes. This study in part supports that sLepA1 is involved in signaling the energy status in fat-depositing tissues in line with the mammalian model, whereas sLepA2 may possibly play important roles in the digestive tract and liver. At present, data on Lep in teleosts are too scarce to allow generalization about how the Lep system is influenced by tissue-specific energy status and, in turn, may regulate functions related to feed intake, growth, and adiposity in fish. In tetraploid species like Atlantic salmon, different Lep paralogues seems to serve different physiological roles.

Abrupt changes in photoperiod affect age at maturity, timing of ovulation and plasma testosterone and oestradiol-17β profiles in Atlantic salmon, Salmo salar

Abstract Atlantic salmon ( Salmo salar L.), reared in sea cages for 18 months (age 36 months from hatching), were exposed to natural light (NL, 61°N), or continuous additional light from January (ALJ) or March (ALM) until July. On July 13, the fish were moved to indoor raceways with brackish water (2–19‰) and ambient temperature (declining from 13.0 to 5.6°C). Fish from each treatment were subjected to either simulated natural photoperiod (SNP), continuous light (24L), or short photoperiod (8L=8 L:16D), creating a total of nine experimental groups with approx. 50 fish in each. The proportion of sexually maturing females was reduced from 91% in the NL groups, to 67% and 9% in the ALM and ALJ groups, respectively ( p ≤0.005). A similar reduction was observed among the males, from 74% in the NL groups, to 57% and 16% in the ALM and ALJ groups, respectively ( p ≤0.001). Ovulation commenced in late October in the control group (NL-SNP). Compared with control, median ovulation time was advanced by 5, 4 and 3 weeks in the ALM-8L, NL-8L and ALM-SNP groups, respectively, whereas ovulation was delayed by 1 and 6 weeks in the ALM-24L and NL-24L groups, respectively. The altered timing of ovulation among the groups was paralleled by similar shifts in the seasonal plasma oestradiol-17 β and testosterone profiles. Survival of eggs to the eyed stage was lower in the ALM-8L group (mean=64.2%) compared with the NL-SNP group (mean=92.5%), indicating a negative effect on egg quality in the most advanced group. Although abrupt changes in photoperiod can be used to control timing of ovulation in Atlantic salmon to obtain off-season eggs, the decrease in egg survival and proportion of maturing fish may set constraints on how much maturation can be advanced by use of continuous light during winter and spring. However, the effect on age at maturity may also be exploited to reduce the problem with unwanted early maturation in salmon farming.

Increased daylength stimulates plasma growth hormone and gill Na+, K+-ATPase in Atlantic salmon (Salmo salar)

Atlantic salmon juveniles reared at constant temperature (9–10°C) were exposed to four photoperiod treatment and sampled every 2 weeks from January through May. Fish reared under normal photoperiod exhibited eight-and three fold increases in plasma growth hormone and gill Na+, K+-ATPase activity, respectively, between January and April. Fish exposed to abrupt increases in daylength (LD 15:9) in February or March responded with earlier increases in plasma growth hormone and gill Na+, K+-ATPase activity, and earlier decreases in condition factor relative to fish in the normal photoperiod group. Fish maintained under short daylength (LD 9:15) from January to May exhibited delayed and muted increases in plasma growth hormone and gill Na+, K+-ATPase activity. Plasma thyroxine exhibited a 2.5-fold increase from February to late March in the normal photoperiod group, was generally lower in the LD 9:15 group, but exhibited no obvious response to abrupt increases in daylength. There was an increase in plasma 3,5,3′-triiodo-l-thyronine with time in all groups (43–80%) but no significant response to photoperiod. Plasma levels of somatostatin-25 were highest in the LD 9:15 group, but there was no detectable response to increased daylength in any of the photoperiod treatments. The results indicate that plasma growth hormone is responsive to increased daylength and may be causally related to subsequent increases in gill Na+, K+-ATPase.

Effects of stocking density on growth rate of halibut (Hippoglossus hippoglossus L.) reared in large circular tanks for three years

Abstract The aim of this study was to estimate optimal stocking density for halibut. Two size-classes of halibut of initial mean weight 1.8 and 3.2 kg respectively were stocked at three different densities: 11, 22 and 33 kg/m2 and reared in six large circular tanks (8 m) for 3 years at 7°C. The fish were fed to satiation 6 days a week with frozen fish (capelin and herring). Stocking density increased as the fish grew, and for controlling the density fish were removed four times during the experiment. The average stocking densities for the two size-classes were 18, 43 and 63 kg/m2 corresponding to 50, 100 and 160% coverage of the tank bottom by fish. The maximum observed stocking density, 95 kg/m2, corresponded to 215% coverage. There was no significant difference in growth rate (kg/year) between the two size-classes. The mean weight at the end of the experiment was 5–7 kg for the males and 9–14 kg for the females. For the two size-classes combined the growth rate was significantly lower at the highest than at intermediate or low stocking density, but there was no significant difference between the groups at intermediate and low density. It is concluded that stocking density affects growth rate of halibut only above a certain threshold level corresponding to 100% coverage of the tank bottom. The results indicate that the optimal stocking density is somewhere between one and two layers of fish on the tank bottom. Thus, for a 2 kg halibut the optimal stocking density is between 25 and 50 kg/m2 and for a 10 kg halibut between 50 and 100 kg/m2.

Environmental endocrinology of salmon smoltification.

Smolting is a hormone-driven developmental process that is adaptive for downstream migration and ocean survival and growth in anadromous salmonids. Smolting includes increased salinity tolerance, increased metabolism, downstream migratory and schooling behavior, silvering and darkened fin margins, and olfactory imprinting. These changes are promoted by growth hormone, insulin-like growth factor I, cortisol, thyroid hormones, whereas prolactin is inhibitory. Photoperiod and temperature are critical environmental cues for smolt development, and their relative importance will be critical in determining responses to future climate change. Most of our knowledge of the environmental control and endocrine mediation of smolting is based on laboratory and hatchery studies, yet there is emerging information on fish in the wild that indicates substantial differences. Such differences may arise from differences in environmental stimuli in artificial rearing environments, and may be critical to ocean survival and population sustainability. Endocrine disruptors, acidification and other contaminants can perturb smolt development, resulting in poor survival after seawater entry.