This Review explores the impact of environmental factors, with temperature as a starting point, on gut motility and digestive function in non-mammalian vertebrates, with a focus on species that are likely to be affected by climate change. Understanding gut physiology, particularly motility, is crucial in allowing us to predict how animals will respond to changing environmental conditions, as it plays a key role in nutrient absorption, immune defence and overall health. Rising temperatures and heatwaves pose significant challenges, especially for ectothermic species, the gut functions of which may be compromised under conditions outside their thermal tolerance. Here, we provide examples of how temperature-induced changes in gut motility affect gut transit time and digestive efficiency, and discuss their effects on the balance of energetic cost and gain. Although higher temperatures generally accelerate motility, further research is needed to assess how these changes impact digestion across species and under fluctuating environmental conditions. This Review emphasizes the need for integrated studies on motility patterns, digestive efficiency and energetic costs - along with the neural and cellular mechanisms controlling motility - to better predict the resilience of species in a warming world.

Gut motility is involved in breakdown and transport of food. It includes mixing and propulsive activity achieved when the smooth muscle in the gut wall contracts and relaxes. Motility is a well-coordinated process that involves intrinsic and extrinsic factors, including the innate properties of the smooth muscles as well as nerves and hormones. Although lack of in vivo studies has hampered our understanding of gut motility patterns in fish, the effect of many diverse signaling molecules are known and motility patterns depend on the integration of these signals.

This article gives an overview of how gut hormones regulate digestion in fishes. Hormones such as gastrin, histamine, and somatostatin produced in the stomach mucosa modulate gastric acid secretion. Cholecystokinin is a key hormone in the subsequent digestion process. The arrival of acid chyme and nutrients trigger the release of cholecystokinin from endocrine cells in the intestine and subsequently, cholecystokinin inhibits acid secretion, and delays gastric emptying rate while stimulating gallbladder and pancreas contraction and secretion. Somatostatin and serotonin may also influence smooth muscle contractions of the gut. Data indicate that ghrelin and motilin influence intestinal motility, while ghrelin may also stimulate digestive enzymes and glucose transporters in the intestinal mucosa.

ABSTRACT Coronary arteriosclerosis is a common feature of both wild and farmed salmonid fishes and may be linked to stress-induced cardiac pathologies. Yet, the plasticity and capacity for long-term myocardial restructuring and recovery following a restriction in coronary blood supply are unknown. Here, we analyzed the consequences of acute (3 days) and chronic (from 33 to 62 days) coronary occlusion (i.e. coronary artery ligation) on cardiac morphological characteristics and in vivo function in juvenile rainbow trout, Oncorhynchus mykiss. Acute coronary artery occlusion resulted in elevated resting heart rate and decreased inter-beat variability, which are both markers of autonomic dysfunction following acute myocardial ischemia, along with severely reduced heart rate scope (maximum−resting heart rate) relative to sham-operated trout. We also observed a loss of myocardial interstitial collagen and compact myocardium. Following long-term coronary artery ligation, resting heart rate and heart rate scope normalized relative to sham-operated trout. Moreover, a distinct fibrous collagen layer separating the compact myocardium into two layers had formed. This may contribute to maintain ventricular integrity across the cardiac cycle or, alternatively, demark a region of the compact myocardium that continues to receive oxygen from the luminal venous blood. Taken together, we demonstrate that rainbow trout may cope with the aversive effects caused by coronary artery obstruction through plastic ventricular remodeling, which, at least in part, restores cardiac performance and myocardium oxygenation.

Cardiovascular disease may pose a major threat to the health and welfare of farmed fish. By investigating a range of established cardiovascular disease indicators, we aimed to determine the prevalence, severity and consequences of this affliction in farmed rainbow trout (Oncorhynchus mykiss) from an open cage farm in the Baltic Sea, an open cage farm in a freshwater lake, and a land-based recirculating aquaculture system. We also aimed to identify environmental, anthropogenic and physiological factors contributing towards the development of the disease. The majority of trout possessed enlarged hearts with rounded ventricles (mean height:width ratios of 1.0-1.1 c.f. similar to 1.3 in wild fish) and a high degree of vessel misalignment (mean angles between the longitudinal ventricular axis and the axis of the bulbus arteriosus of 28-31 degrees c.f. similar to 23 degrees in wild fish). The prevalence and severity of coronary arteriosclerosis was also high, as 92-100% of fish from the different aquaculture facilities exhibited coronary lesions. Mean lesion incidence and severity indices were 67-95% and 3.1-3.9, respectively, which resulted in mean coronary arterial blockages of 19-32%. To evaluate the functional significance of these findings, we modelled the effects of arterial blockages on coronary blood flow and experimentally tested the effects of coronary occlusion in a sub-sample of fish. The observed coronary blockages were estimated to reduce coronary blood flow by 34-54% while experimental coronary occlusion adversely affected the electrocardiogram of trout. Across a range of environmental (water current, predation), anthropogenic (boat traffic intensity, hatchery of origin, brand of feed pellets) and physiological factors (condition factor, haematological and plasma indices), the hatchery of origin was the main factor contributing towards the observed variation in the development of cardiovascular disease. Therefore, further research on the effects of selective breeding programs and rearing strategies on the development of cardiovascular disease is needed to improve the welfare and health of farmed fish.

The enteric nervous system is one of the main actors involved in control of gut motility and secretion. It constitutes a part of the autonomic nervous system, but may act independently of central input. This chapter attempts to summarise the current knowledge about the development of the enteric nervous system in fish, with emphasis on its role in controlling gut motility. A proper development of the enteric nervous system is dependent on the development of the gut itself. Enteric reflexes underlie distinct motility patterns like peristalsis, the aboral propulsion of luminal content. Enteric neurones in fish, as in other vertebrates, are derived from the neural crest. Enteric neurones contain combinations of neurotransmitters of different types including acetylcholine, amines, neuropeptides and dissolved gaseous molecules.

Anadromy is a distinctive life-history strategy in fishes that has evolved independently many times. In an evolutionary context, the benefits of anadromy for a species or population must outweigh the costs and risks associated with the habitat switch. The migration of fish across the freshwater-ocean boundary coincides with potentially energetically costly osmoregulatory modifications occurring at numerous levels of biological organization. By integrating whole animal and sub-cellular metabolic measurements, this study presents significant findings demonstrating how an anadromous salmonid ( i.e. rainbow trout, Oncorhynchus mykiss ) is able to transform from a hyper- to hypo-osmoregulatory state without incurring significant increases in whole animal oxygen consumption rate. Instead, underlying metabolic mechanisms that fuel the osmoregulatory machinery at the organ level ( i.e. intestine) are modulated, as mitochondrial coupling and anaerobic metabolism are increased to satisfy the elevated energetic demands. This may have positive implications for the relative fitness of the migrating individual, as aerobic capacity may be maintained for locomotion ( i.e. foraging and predator avoidance) and growth. Furthermore, the ability to modulate mitochondrial metabolism in order to maintain osmotic balance suggests that mitochondria of anadromous fish may have been a key target for natural selection, driving species adaptations to different aquatic environments.

Rhythmic contractions of the mammalian gastrointestinal tract can occur in the absence of neuronal or hormonal stimulation owing to the generation of spontaneous electrical activity by interstitial cells of Cajal (ICC) that are electrically coupled to smooth muscle cells. The myogenically driven component of gastrointestinal motility patterns in fish probably also involves ICC; however, little is known of their presence, distribution and function in any fish species. In the present study, we combined immunohistochemistry and in vivo recordings of intestinal motility to investigate the involvement of ICC in the motility of the proximal intestine in adult shorthorn sculpin (Myoxocephalus scorpius). Antibodies against anoctamin 1 (Ano1, a Ca2+-activated Cl- channel), revealed a dense network of multipolar, repeatedly branching cells in the myenteric region of the proximal intestine, similar in many regards to the mammalian ICC-MY network. The addition of benzbromarone, a potent blocker of Ano1, altered the motility patterns seen in vivo after neural blockade with TTX. The results indicate that ICC are integral for the generation and propagation of the majority of rhythmic contractile patterns in fish, although their frequency and amplitude can be modulated via neural activity.

Upon exposure to seawater, euryhaline teleosts need to imbibe and desalinate seawater to allow for intestinal ion and water absorption, as this is essential for maintaining osmotic homeostasis. Despite the potential benefits of increased mixing and transport of imbibed water for increasing the efficiency of absorptive processes, the effect of water salinity on intestinal motility in teleosts remains unexplored. By qualitatively and quantitatively describing in vivo intestinal motility of euryhaline rainbow trout (Oncorhynchus mykiss), this study demonstrates that in freshwater, the most common motility pattern consisted of clusters of rhythmic, posteriorly propagating contractions that lasted ∼1-2 minutes followed by a period of quiescence lasting ∼4-5 minutes. This pattern closely resembles mammalian migrating motor complexes (MMCs). Following a transition to seawater, imbibed seawater resulted in a significant distension of the intestine and the frequency of MMCs increased two to three-fold with a concomitant reduction in the periods of quiescence. The increased frequency of MMCs was also accompanied by ripple-type contractions occuring every 12 to 60 seconds. These findings demonstrate that intestinal contractile activity of euryhaline teleosts is dramatically increased upon exposure to seawater, which is likely part of the overall response for maintaining osmotic homeostasis as increased drinking and mechanical perturbation of fluids is necessary to optimize intestinal ion and water absorption. Finally, the temporal response of intestinal motility in rainbow trout transitioning from freshwater to seawater coincides with previously documented physiological modifications associated with osmoregulation and may provide further insight on the underlying reasons shaping the migration patterns of salmonids.

The histology of putative light organs in the parapodia of five species of Tomopteris (pelagic annelids) is examined and compared using light, epifluorescence and scanning electron microscopy. The structural homology of rosette glands in the parapodial pinnae of the tail-bearing species T. helgolandica and T. pacifica, and hyaline glands of the tail-less species T. carpenteri, T. planktonis and T. septentrionalis is highlighted. However, the rosette glands point towards the ramus of the coelomic cavity inside the parapodia, whereas the hyaline glands point towards the surrounding water and penetrate the pinnal surface on the posterior side of the parapodia. Further, in order to assess the photogenic properties of rosette glands from T. helgolandica, we analysed the distribution and the temporal dynamics of their endogenous fluorescence in isolated parapodia in response to light emission induced by KCl and carbachol. The gradual extinction of bioluminescence was combined to the centrifugal spread of fluorescence from the core of the gland. We suggest this fluorescence to be produced by a “breakdown” product of the chemiluminescent reaction. Finally, both gland types probably evolved from a common light-emitting structure and differentiated along a functional and migrational axis extending from endocrine secretion close to the coelomic ramus to exocrine secretion close to the lateral margin of the pinna.

Increased gastrointestinal blood flow is essential for euryhaline fishes to maintain osmotic homeostasis during the initial phase of a transition from freshwater to seawater. However, the cardiorespiratory responses and hemodynamic changes required for a successful long-term transition to seawater remain largely unknown. In the present study, we simultaneously measured oxygen consumption rate ( Ṁ O2 ), cardiac output (CO), heart rate (HR), and gastrointestinal blood flow (GBF) in rainbow trout ( Oncorhynchus mykiss) acclimated to either freshwater or seawater for at least 6 wk. Seawater-acclimated trout displayed significantly elevated Ṁ O2 (day: 18%, night: 19%), CO (day: 22%, night: 48%), and GBF (day: 96%, night: 147%), demonstrating that an overall cardiorespiratory upregulation occurs during seawater acclimation. The elevated GBF was achieved via a combination of increased CO, mediated through elevated stroke volume (SV), and a redistribution of blood flow to the gastrointestinal tract. Interestingly, virtually all of the increase in CO of seawater-acclimated trout was directed to the gastrointestinal tract. Although unfed seawater-acclimated trout displayed substantially elevated cardiorespiratory activity, the ingestion of a meal resulted in a similar specific dynamic action (SDA) and postprandial GBF response as in freshwater-acclimated fish. This indicates that the capacity for the transportation of absorbed nutrients, gastrointestinal tissue oxygen delivery, and acid-base regulation is maintained during digestion in seawater. The novel findings presented in this study clearly demonstrate that euryhaline fish upregulate cardiovascular function when in seawater, while retaining sufficient capacity for the metabolic and cardiovascular changes associated with the postprandial response.

BACKGROUND:Neurons in lumbar and sacral dorsal root ganglia (DRG) comprise extrinsic sensory pathways to the distal colon and rectum, but their relative contributions are unclear. In this study, sensory innervation of the rectum and distal colon in the guinea pig was directly compared using retrograde labeling combined with immunohistochemistry.METHODS:The lipophilic tracer, DiI, was injected in either the rectum or distal colon of anesthetized guinea pigs, then DRG (T6 to S5) and nodose ganglia were harvested and labeled using antisera for calcitonin gene-related peptide (CGRP) and transient receptor potential vanilloid 1(TRPV1).KEY RESULTS:More primary afferent cell bodies were labeled from the rectum than from the distal colon. Vagal sensory neurons, with cell bodies in the nodose ganglia comprised fewer than 0.5% of labeled sensory neurons. Spinal afferents to the distal colon were nearly all located in thoracolumbar DRG, in a skewed unimodal distribution (peak at L2); fewer than 1% were located in sacral ganglia. In contrast, spinal afferents retrogradely labeled from the rectum had a bimodal distribution, with one peak at L3 and another at S2. Fewer than half of all retrogradely labeled spinal afferent neurons were immunoreactive for CGRP or TRPV1 and these included the larger traced neurons, especially in thoracolumbar ganglia.CONCLUSIONS & INFERENCES:In the guinea pig, both the distal colon and the rectum receive a sensory innervation from thoracolumbar ganglia. Sacral afferents innervate the rectum but not the distal colon. Calcitonin gene-related peptide immunoreactivity was detectable in fewer than half of afferent neurons in both pathways.

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of catecholamines and TH immunoreactivity is indicative of cells synthesising either adrenaline/noradrenaline or dopamine. In this study, the distribution of TH immunoreactivity was examined in two distantly related teleost species, zebrafish (Danio rerio) and shorthorn sculpin (Myoxocephalus scorpius). In both species, TH-immunoreactive nerve cell bodies and varicose nerve fibres were common in the myenteric plexus of the intestine. However, no TH-immunoreactive nerve cell bodies were seen in the sculpin stomach. The TH-immunoreactive nerve cell bodies seemed to constitute a larger proportion of the total enteric population in shorthorn sculpin (50 ± 5 %, n = 3067 cells) compared with zebrafish (14 ± 2 %, n = 10,163 cells). In contrast, in sculpin, the TH-immunoreactive cells were smaller than the average enteric nerve cell bodies, whereas in zebrafish, the relationship was the opposite. In developing zebrafish larvae, TH-immunoreactive nerve cell bodies were common (approx. 75 % of the total population) at 3 days post-fertilization (dpf), but decreased in numbers between 3 and 7 dpf. In conclusion, in contrast to previous studies, TH-immunoreactive intrinsic neurons are common in the fish gut. Their role and function need to be further characterized in order to understand the potential importance of this enteric subpopulation in controlling various gut functions.

The large-scale migrations of anadromous fish species from freshwater to seawater have long been considered particularly enigmatic, as this life history necessitates potentially energetically costly changes in behaviour and physiology. A significant knowledge gap concerns the integral role of cardiovascular responses, which directly link many of the well-documented adaptations (i.e. through oxygen delivery, water and ion transport) allowing fish to maintain osmotic homeostasis in the sea. Using long-term recordings of cardiorespiratory variables and a novel method for examining drinking dynamics, we show that euryhaline rainbow trout (Oncorhynchus mykiss) initiate drinking long before the surrounding environment reaches full seawater salinity (30-33 ppt), suggesting the presence of an external osmo-sensing mechanism. Onset of drinking was followed by a delayed, yet substantial increase in gastrointestinal blood flow through increased pulse volume exclusively, as heart rate remained unchanged. While seawater entry did not affect whole animal energy expenditure, enhanced gastrointestinal perfusion represents a mechanism crucial for ion and water absorption, as well as possibly increasing local gastrointestinal oxygen supply. Collectively, these modifications are essential for anadromous fish to maintain homeostasis at sea, whilst conserving cardiac and metabolic scope for activities directly contributing to fitness and reproductive success.

As a consequence of increasing atmospheric CO2, the world's oceans are becoming warmer and more acidic. Whilst the ecological effects of these changes are poorly understood, it has been suggested that fish performance including growth will be reduced mainly as a result of limitations in oxygen transport capacity. Contrary to the predictions given by the oxygen- and capacity-limited thermal tolerance hypothesis, we show that aerobic scope and cardiac performance of Atlantic halibut (Hippoglossus hippoglossus) increase following 14–16 weeks exposure to elevated temperatures and even more so in combination with CO2-acidified seawater. However, the increase does not translate into improved growth, demonstrating that oxygen uptake is not the limiting factor for growth performance at high temperatures. Instead, long-term exposure to CO2-acidified seawater reduces growth at temperatures that are frequently encountered by this species in nature, indicating that elevated atmospheric CO2 levels may have serious implications on fish populations in the future.

We appreciate the on-going discussion and healthy evaluation of the hypothesis of oxygen and capacity limitation of thermal tolerance (OCLTT). However, we think it is unfortunate that Pörtner (Pörtner, 2104) sees little value in our study (Gräns et al., 2014), which currently represents the largest long-term experimental test of OCLTT.The OCLTT hypothesis emphasises the importance of oxygen delivery to aerobic processes as the major evolutionary constraint shaping organisms, their physiology and ecosystems. However, such a broad view does not easily produce testable predictions, and we argue that the value of a scientific idea lies in its ability to predict future observations. Therefore, we focused on a testable prediction that OCLTT is founded upon; that reduced aerobic scope is the physiological limitation that impairs other organismal performances, such as growth at high temperatures and high PCO2 (Gräns et al., 2014; Pörtner and Farrell, 2008; Pörtner and Knust, 2007). We found that aerobic scope increased continuously with acclimation temperature, and even more so in CO2-acidified seawater, whereas growth plateaued at the three intermediate temperatures and declined at the highest acclimation temperatures [fig. 1A and fig. 3 in original article (Gräns et al., 2014)]. This clear mismatch in thermal profiles for aerobic scope and growth indicates that oxygen delivery capacity does not decrease at high temperatures and cannot have limited growth, as OCLTT would have predicted.Pörtner (Pörtner, 2014) suggests that we should change the definition of aerobic scope to include the scope for all oxygen-requiring performances: 'use the term aerobic scope for all routine performances that draw on aerobic energy such as growth, reproduction or steady-state swimming'. We, however, use the widely adopted definition of aerobic scope: the difference between standard metabolic rate (SMR) in resting unfed animals, and maximum metabolic rate (MMR) (Fry and Hart, 1948; Pörtner and Farrell, 2008). Growth rate, reproductive output and aerobic scope are all commonly used terms with clear definitions, and we fail to see how redefining and mixing of terminology can improve our understanding of thermal biology. If anything, such a move would risk confusing the debate further with semantic misunderstandings.Pörtner (Pörtner, 2014) proposes that the thermal mismatch between aerobic scope and growth is due to growth occurring in unstressed fish in 'steady state', whereas we measured MMR (and thus aerobic scope) during non-steady state recovery from exhaustive exercise. We suspect the misunderstanding may lie in our differing definitions of aerobic scope. MMR can, in most animals, only be quantified during or immediately after exercise when the fish are using, or recovering from, partly anaerobic white muscle activity. Moreover, Pörtner suggests that oxygen limitation at high temperature can occur for growth at rest, but that oxygen transport capacity can increase greatly during exercise because of catecholamine release and shifts in blood chemistry, which seems unlikely. The stimulatory effects of catecholamines on cardiac performance and oxygen transport also typically decrease with increasing temperature, as a result of blunted β-adrenergic stimulation of the myocardium (Keen et al., 1993). Therefore, our experimental protocol would be expected to have fewer stimulatory effects on aerobic performance at the higher temperatures and yet we still observed the highest aerobic scope at these temperatures.The positive effect of CO2 on aerobic scope that we reported (Gräns et al., 2014) is also questioned. Pörtner claims that aerobic scope could be protected by bicarbonate accumulation in fish exposed to high PCO2, which may be possible. However, this is not a relevant argument against the positive effects of CO2 on aerobic scope, because this would also occur in nature. Although the effect size on aerobic scope by CO2 was arguably small, it was nonetheless confirmed by statistical tests across temperatures in the opposite direction to what OCLTT predicts and the increase in aerobic scope from CO2 was not matched by increased growth.We feel that Pörtner's critique of our statistical analysis might be due to misunderstandings. As stated in the original article, the trend lines added to the figures are for visual aid, and not based on the statistical models we used. The experimental design and statistical models were developed together with a mathematical statistician, and we are confident that our statistical analyses are of the highest standard.Pörtner (Pörtner, 2014) highlights two examples 'of how to investigate OCLTT, performance and aerobic scope more successfully' (Eliason et al., 2011; Pörtner and Knust, 2007). They are, however, like any study (including ours) not without limitations. In Eliason et al. (Eliason et al., 2011), aerobic and cardiac scope was measured in instrumented sockeye salmon using swim tunnels. Although this is an impressive experimental endeavour, we do not understand why these animals should be considered to be in 'steady state'. First, SMR was obtained from highly instrumented salmon after an overnight recovery from surgery and during intermittent blood samplings. Second, the thermal challenges were acute for fish at the upper and lower thermal extremes (4°C h–1 and left for 1 h), whereas the intermediate temperatures were tested after short-term thermal acclimation (5°C day–1 and left for 1 day), presumably leaving the fish in different stages of the thermal acclimation process. For MMR measurements, a Ucrit swim protocol was used with an electric motivator grid. Thus, whereas the MMR measured in Eliason et al. (Eliason et al., 2011), and our study probably consisted of a combination of aerobic and anaerobic metabolism because white muscle is increasingly recruited with increasing swimming speed (Clark et al., 2013a; Jayne and Lauder, 1994), the metabolic measurements in our study were not affected by surgery and variable thermal test protocols.The other suggested example, Pörtner and Knust (Pörtner and Knust, 2007), combines results from many earlier publications, none of which measured aerobic scope according to conventional definitions. The study reports, in our view, rather weak thermal associations and claims a causal link to oxygen limitation. For example, the authors conclude that oxygen transport limitation was the reason for the drop in field abundance at 19°C, yet in the same paper, growth rate remained high at 20°C (∼80% of max) and markers of anaerobic metabolism only became elevated after 72 h at 24°C (in liver, but not heart muscle). In addition, arterial blood flow was reported from only one individual during thermal ramping in a NMR setup with the fish confined in a space half the length of the fish. It is not clear to us how this can represent 'a similar physiological status as organisms experiencing thermal limitation in their natural environment'.Curiously, both the suggested examples of how to correctly investigate OCLTT include fish with zero oxygen transport in their datasets, representing either dead fish or measurement errors [no arterial blood flow in Pörtner and Knust, fig. 1C (Pörtner and Knust, 2007); zero MMR in Eliason et al., fig. S2A (Eliason et al., 2011)].For these reasons, we fail to see how these two studies can be considered to represent aerobic scope measurements under 'routine steady-state as in the field', whereas our study is not, as proposed by Pörtner. If these publications represent the best empirical evidence for OCLTT, then the hypothesis is standing on loose ground, and it may not be surprising that a growing number of studies are questioning the generality of OCLTT (Clark et al., 2013a; Clark et al., 2013b; Ern et al., 2014; Norin et al., 2014; Overgaard et al., 2012).In our paper (Gräns et al. 2014) we aimed to test the core proposition of the OCLTT hypothesis; that tissue oxygen limitation is the mechanism behind reductions in other performances at high temperatures. We demonstrated that the thermal windows for aerobic scope and growth differ, and that reduced aerobic scope was not associated with the decline in growth at high temperatures. This forced us to question the tissue oxygen limitation mechanism that is central to the OCLTT hypothesis. We agree with Pörtner that the ecophysiological community needs to find out at which conditions a species becomes thermally limited. However, we argue that it is unlikely that we will ever identify a single physiological mechanism explaining the complex subject of thermal tolerance and climate change vulnerability in ectothermic animals. Instead, we suggest that the physiological cause of limitation can vary and will depend on a number of biological and environmental factors including the rate of temperature change, species, lifestyle and physiological state of the organism. We therefore encourage other researchers to look for thermal limitation mechanisms beyond oxygen supply and not to feel obligated to fit their experimental findings into the framework of OCLTT.

This is the first study to catalogue the diverse array of in vivo motility patterns in a teleost fish and how they are affected by feeding. Video recordings of exteriorised proximal intestine from fasted and fed shorthorn sculpin (Myoxocephalus scorpius) were used to generate spatio-temporal maps to portray and quantify motility patterns. Propagating and non-propagating contractions were observed to occur at different frequencies and durations. The most apparent difference between the feeding states was that bands of relatively high amplitude contractions propagating slowly in the anal direction were observed in all fasted fish (N=10) but in only 35% of fed fish (N=11). Additionally, fed fish displayed a reduced frequency (0.21±0.03 versus 0.32±0.06 contractions min−1) and rhythmicity of these contractions compared with fasted fish. Although the underlying mechanisms of these slow anally propagating contractions differ from those of mammalian migrating motor complexes, we believe that they may play a similar role in shorthorn sculpin during the interdigestive period, to potentially remove food remnants and prevent the establishment of pathogens. ‘Ripples’ were the most prevalent contraction type in shorthorn sculpin and may be important during mixing and absorption. The persistence of shallow ripples and pendular movements of longitudinal muscle after tetrodotoxin (1 μmol l−1) treatment suggests these contractions were myogenic in origin. The present study highlights both similarities and differences in motility patterns between shorthorn sculpin and other vertebrates, as well as providing a platform to examine other aspects of gastrointestinal functions in fish, including the impact of environmental changes.

Ongoing climate change has led to an increase in sea surface temperatures of 2-4°C on the west coast of Greenland. Since fish are ectothermic, metabolic rate increases with ambient temperature. This makes these animals particularly sensitive to changes in temperature; subsequently any change may influence their metabolic scope, i.e. the physiological capacity to undertake aerobically challenging activities. Any temperature increase may thus disrupt species-specific temperature adaptations, at both the molecular level as well as in behavior, and concomitant species differences in the temperature sensitivity may shift the competitive balance among coexisting species. We investigated the influence of temperature on metabolic scope and competitive ability in three species of marine sculpin that coexist in Greenland coastal waters. Since these species have different distribution ranges, we hypothesized that there should be a difference in their physiological response to temperature; hence we compared their metabolic scope at three temperatures (4, 9 and 14°C). Their competitive ability at the ambient temperature of 9°C was also tested in an attempt to link physiological capacity with behaviour. The Arctic staghorn sculpin, the species with the northernmost distribution range, had a lower metabolic scope in the higher temperature range compared to the other two species, which had similar metabolic scope at the three temperatures. The Arctic staghorn sculpin also had reduced competitive ability at 9°C and may thus already be negatively affected by the current ocean warming. Our results suggest that climate change can have effects on fish physiology and interspecific competition, which may alter the species composition of the Arctic fish fauna.

For a fish to thrive, the gut must function efficiently. This is achieved through a range of processes, including controlled patterns of gut motility and modifications in gut blood flow. The knowledge of how gut functions in fish are affected by environmental temperature is sparse, and in order to understand how changes in climate may affect fish populations, we need to understand how gut blood flow and gut motility are affected by changes in temperature. By simultaneous recording of gut blood flow, gut motility, cardiac output, heart rate and cardiac stroke volume, in vivo at 4, 9 and 14 °C, the acute thermal sensitivity of a thermal generalist (shorthorn sculpin Myoxocephalu scorpius) was compared to the more strictly Arctic species (Arctic sculpin M. scorpioides). Temperature effects on gut motility were further explored in vitro, using isolated smooth muscles. Elevated water temperatures increased gut blood flow and contractile activity. Contraction frequency increased nearly threefold and gut blood flow almost doubled with the 10 °C increase. Both cardiac output and heart rate increased with temperature, while cardiac stroke volume decreased. The cholinergic agonist carbachol was most potent on smooth muscles at 9 °C. There were no differences between the two species, suggesting that the gastrointestinal and cardiovascular systems of Arctic sculpin, although a more pronounced Arctic species, have similar abilities to cope with acute fluctuations in water temperature as shorthorn sculpin. The impact of increased gut activity at higher temperatures needs further investigation before the effects of climate change can be predicted.