In situ observation of a macrourid fish at 7259 m in the Japan Trench: swimbladder buoyancy at extreme depth

A macrourid, Coryphaenoides yaquinae sp. inc., was observed to be attracted to bait and exhibiting normal foraging behaviour during a period of 80 min within view of a baited video camera on the sea floor at 7259 m - the deepest ever observation of a fish species with a swim bladder. The buoyancy provided by an oxygen-filled swim bladder at 74.4 MPa pressure was estimated to be 0.164 N, at a theoretical energy cost of 20 kJ, 200 times less than the cost of equivalent lipid buoyancy. During normal metabolism, 192 days would be required to fill the swimbladder. At these depths, oxygen is very incompressible, so changes in volume during ascent or descent are small. However, swimbladder function is crucially dependent on a very low rate of diffusion of oxygen across the swimbladder wall. The oxygen in the swimbladder could theoretically sustain aerobic metabolism for over 1 year but is unlikely to be used as a reserve.

Near-equal compressibility of liver oil and seawater minimises buoyancy changes in deep-sea sharks and chimaeras

Whereas upper ocean pelagic sharks are negatively buoyant and must swim continuously to generate lift from their fins, deep-sea sharks float or swim slowly buoyed up by large volumes of low-density oils in their livers. Investigation of the pressure, volume, temperature (PVT) relationships for liver oils of 10 species of deep-sea Chondrichthyes shows that the density difference between oil and seawater, Δρ, remains almost constant with pressure down to full ocean depth (11 km, 1100 bar), theoretically providing buoyancy far beyond the maximum depth of occurrence (3700 m) of sharks. However, Δρ does change significantly with temperature and we show that the combined effects of pressure and temperature can decrease buoyancy of oil by up to 10% between the surface and 3500 m depth across interfaces between warm southern and cold polar waters in the Rockall Trough in the NE Atlantic. This increases drag more than 10-fold compared with neutral buoyancy during horizontal slow swimming (0.1 m s-1), but the effect becomes negligible at high speeds. Chondrichthyes generally experience positive buoyancy change during ascent and negative buoyancy change during descent, but contrary effects can occur at interfaces between waters of different densities. During normal vertical migrations buoyancy changes are small, increasing slow-speed drag no more than 2- to 3-fold. Equations and tables of density, pressure and temperature are provided for squalene and liver oils of Chimaeriformes (Harriotta raleighana, Chimaera monstrosa, Hydrolagus affinis), Squaliformes (Centrophorus squamosus, Deania calcea, Centroscymnus coelolepis, Centroscyllium fabricii, Etmopterus spinax) and Carcharhiniformes (Apristurus laurussonii, Galeus murinus).

New High-Tech Flexible Networks for the Monitoring of Deep-Sea Ecosystems

Increasing interest in the acquisition of biotic and abiotic resources from within the deep sea (e.g., fisheries, oil-gas extraction, and mining) urgently imposes the development of novel monitoring technologies, beyond the traditional vessel-assisted, time-consuming, high-cost sampling surveys. The implementation of permanent networks of seabed and water-column-cabled (fixed) and docked mobile platforms is presently enforced, to cooperatively measure biological features and environmental (physicochemical) parameters. Video and acoustic (i.e., optoacoustic) imaging are becoming central approaches for studying benthic fauna (e.g., quantifying species presence, behavior, and trophic interactions) in a remote, continuous, and prolonged fashion. Imaging is also being complemented by in situ environmental-DNA sequencing technologies, allowing the traceability of a wide range of organisms (including prokaryotes) beyond the reach of optoacoustic tools. Here, we describe the different fixed and mobile platforms of those benthic and pelagic monitoring networks, proposing at the same time an innovative roadmap for the automated computing of hierarchical ecological information on deep-sea ecosystems (i.e., from single species' abundance and life traits to community composition, and overall biodiversity).

Bathyal and abyssal demersal bait-attending fauna of the Eastern Mediterranean Sea

Baited cameras were deployed over a depth range of 532-5111 m in the Ionian Sea to characterise the large mobile fauna. The planned installation of a neutrino telescope also offers the potential for biological observatories. The current study was intended to aid observatory placement. At increasing depths, sediment was observed to become more uniform and animal burrows and tracks reduced. A total of 10 species of deep-sea fishes were identified from images; four elasmobranchs, which were not recorded deeper than 1841 m, and six teleosts. At depths > 3000 m, including Calypso Deep, the deepest point in the Mediterranean, only one fish species was observed; the Mediterranean grenadier, Coryphaenoides mediterraneus (3400-5111 m), extending this species' maximum recorded depth to 5111 m. Four species of decapod crustacea could be identified from images. The dressed deep-sea shrimp, Acanthephyra eximia (1346-5111 m) was the only invertebrate recorded at abyssal depths, including the deepest point. A faunal change was detected at ~ 1000 m depth. Incorporating other studies from the Eastern Mediterranean identified additional faunal boundaries at ~ 1500 m and ~ 2500 m. The time from landing the observation equipment to the arrival of the first fish increased exponentially with depth at a slower rate to that observed in the Atlantic Ocean. The estimated density of bait-attending deep-sea fish was, therefore, significantly impoverished compared to the Atlantic Ocean at equivalent depth. Barriers to colonisation, low resource input, and high temperature at depth relative to the Atlantic Ocean are probable causes of the impoverished fauna.

Colonization of the deep sea by fishes

Analysis of maximum depth of occurrence of 11 952 marine fish species shows a global decrease in species number (N) with depth (x; m): log10 N = -0·000422x + 3·610000 (r(2)  = 0·948). The rate of decrease is close to global estimates for change in pelagic and benthic biomass with depth (-0·000430), indicating that species richness of fishes may be limited by food energy availability in the deep sea. The slopes for the Classes Myxini (-0·000488) and Actinopterygii (-0·000413) follow this trend but Chondrichthyes decrease more rapidly (-0·000731) implying deficiency in ability to colonize the deep sea. Maximum depths attained are 2743, 4156 and 8370 m for Myxini, Chondrichthyes and Actinopterygii, respectively. Endemic species occur in abundance at 7-7800 m depth in hadal trenches but appear to be absent from the deepest parts of the oceans, >9000 m deep. There have been six global oceanic anoxic events (OAE) since the origin of the major fish taxa in the Devonian c. 400 million years ago (mya). Colonization of the deep sea has taken place largely since the most recent OAE in the Cretaceous 94 mya when the Atlantic Ocean opened up. Patterns of global oceanic circulation oxygenating the deep ocean basins became established coinciding with a period of teleost diversification and appearance of the Acanthopterygii. Within the Actinopterygii, there is a trend for greater invasion of the deep sea by the lower taxa in accordance with the Andriashev paradigm. Here, 31 deep-sea families of Actinopterygii were identified with mean maximum depth >1000 m and with >10 species. Those with most of their constituent species living shallower than 1000 m are proposed as invasive, with extinctions in the deep being continuously balanced by export of species from shallow seas. Specialized families with most species deeper than 1000 m are termed deep-sea endemics in this study; these appear to persist in the deep by virtue of global distribution enabling recovery from regional extinctions. Deep-sea invasive families such as Ophidiidae and Liparidae make the greatest contribution to fish fauna at depths >6000 m.

Does presence of a mid-ocean ridge enhance biomass and biodiversity?

In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers, trawls, traps, and a remotely operated vehicle to survey habitat, biomass, and biodiversity. Satellite remote sensing provided information on flow patterns, thermal fronts, and primary production, while sediment traps measured export flux during 2007-2010. The MAR, 3,704,404 km(2) in area, accounts for 44.7% lower bathyal habitat (800-3500 m depth) in the North Atlantic and is dominated by fine soft sediment substrate (95% of area) on a series of flat terraces with intervening slopes either side of the ridge axis contributing to habitat heterogeneity. The MAR fauna comprises mainly species known from continental margins with no evidence of greater biodiversity. Primary production and export flux over the MAR were not enhanced compared with a nearby reference station over the Porcupine Abyssal Plain. Biomasses of benthic macrofauna and megafauna were similar to global averages at the same depths totalling an estimated 258.9 kt C over the entire lower bathyal north MAR. A hypothetical flat plain at 3500 m depth in place of the MAR would contain 85.6 kt C, implying an increase of 173.3 kt C attributable to the presence of the Ridge. This is approximately equal to 167 kt C of estimated pelagic biomass displaced by the volume of the MAR. There is no enhancement of biological productivity over the MAR; oceanic bathypelagic species are replaced by benthic fauna otherwise unable to survive in the mid ocean. We propose that globally sea floor elevation has no effect on deep sea biomass; pelagic plus benthic biomass is constant within a given surface productivity regime.

Locomotory activity and feeding strategy of the hadal munnopsid isopod Rectisura cf. herculea (Crustacea: Asellota) in the Japan Trench

Benthic fauna in the hadal zone (6500-11,000 m) rely on maintaining sufficient locomotory activity to exploit a low, patchy and uniquely distributed food supply while exposed to high pressure, low temperatures and responding to predator-prey interactions. Very little is currently known about the locomotory capabilities of hadal fauna. In situ video footage of the isopod Rectisura cf. herculea (Birstein 1957) (Asellota, Munnopsidae) was obtained from 6945 and 7703 m deep in the Japan Trench (NW Pacific Ocean). Measurements of locomotion revealed routine walking speeds of 0.19±0.04 BL s-1 (mean ± s.d.), increasing to 0.33±0.04 BL s-1 if naturally perturbed by larger organisms. When immediately threatened by the presence of predators (decapod crustaceans), the isopods are capable of eliciting backward escape jumps and burst swimming escape responses of 2.6±1.5 BL s-1 and 4.63±0.9 BL s-1, respectively. These data suggest no significant reduction in locomotory capability despite the extreme depths in which they inhabit. These observations also revealed the isopod to be a bait-attending and aggregative species and suggest that it may not be obligatorily selecting infaunal food sources as previously thought.

Diversification of acorn worms (Hemichordata, Enteropneusta) revealed in the deep sea

Enteropneusts (phylum Hemichordata), although studied extensively because of their close relationship to chordates, have long been considered shallow-water, burrowing animals. The present paper more than doubles the number of enteropneust species recorded in the deep sea based on high-resolution imaging and sampling with remotely operated vehicles. We provide direct evidence that some enteropneusts are highly mobile-using changes in posture and currents to drift between feeding sites-and are prominent members of deep, epibenthic communities. In addition, we provide ecological information for each species. We also show that despite their great morphological diversity, most deep-living enteropneusts form a single clade (the rediagnosed family Torquaratoridae) on the basis of rDNA sequences and morphology of the proboscis skeleton and stomochord. The phylogenetic position of the torquaratorids indicates that the group, after evolving from near-shore ancestors, radiated extensively in the deep sea.

Diet and feeding niches of juvenile Gadus morhua, Melanogrammus aeglefinus and Merlangius merlangus during the settlement transition in the northern North Sea

A study on the feeding ecology of juvenile cod Gadus morhua, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus during the pelagic to demersal transition was carried out on fishes sampled throughout their settlement season at a local nursery ground in the north-western North Sea, off the Scottish east coast. A comprehensive quantitative taxonomic analysis of the diets, as described in the paper, showed the emergence of distinctive feeding niches, minimizing the potential for competition between species and size categories. The diet of the juveniles changed with fish size, water depth, time of year and distance offshore. Small G. morhua were present in the study area earlier in the season, settled further inshore and ate a higher proportion of pelagic prey (copepods) and as size increased they moved into deeper waters and targeted larger, more benthic prey. As M. aeglefinus grew larger and moved into deeper waters, a diet of largely copepods, amphipods, pelagic Ammodytes spp., cyprids and pelagic gastropods evolved to one dominated predominantly by fishes and benthic invertebrates. In the case of M. merlangus, widespread ages and sizes throughout the sampling season, a consequence of their more protracted spawning season, resulted in dietary changes which were more likely to be influenced by seasonal changes in the prey field, in addition to developmental (size) changes, than the diets of the other two species.

Scavenging interactions between the arrow tooth eel Synaphobranchus kaupii and the Portuguese dogfish Centroscymnus coelolepis

A scavenging interaction between the arrow tooth eel Synaphobranchus kaupii and the Portuguese dogfish Centroscymnus coelolepis, both ubiquitous components of fish assemblages at bathyal depths, was observed. Using a baited camera between 1297 and 2453 m in the eastern Atlantic Ocean continental slope, it was shown that despite consistently rapid arrival times of S. kaupii (<5 min), their feeding bouts (indicated by acute peak in numbers) did not take place until shortly after C. coelolepis arrived and removed the exterior surface of the bait (skipjack tuna Katsuwonus pelamis carcass). Change in the numbers of S. kaupii was hence dependent on the arrival of a more powerful scavenger throughout the study site, and at the deeper stations where the population of C. coelolepis declined, S. kaupii was observed to be present but waited for >2 h before feeding, thus contradicting conventional scavenging assumptions in the presence of a food fall.

Hadal trenches: the ecology of the deepest places on Earth

Hadal trenches account for the deepest 45% of the oceanic depth range and host active and diverse biological communities. Advances in our understanding of hadal community structure and function have, until recently, relied on technologies that were unable to document ecological information. Renewed international interest in exploring the deepest marine environment on Earth provides impetus to re-evaluate hadal community ecology. We review the abiotic and biotic characteristics of trenches and offer a contemporary perspective of trench ecology. The application of existing, rather than the generation of novel, ecological theory offers the best prospect of understanding deep ocean ecology.

Liparid and macrourid fishes of the hadal zone: in situ observations of activity and feeding behaviour

Using baited camera landers, the first images of living fishes were recorded in the hadal zone (6000-11000 m) in the Pacific Ocean. The widespread abyssal macrourid Coryphaenoides yaquinae was observed at a new depth record of approximately 7000 m in the Japan Trench. Two endemic species of liparid were observed at similar depths: Pseudoliparis amblystomopsis in the Japan Trench and Notoliparis kermadecensis in the Kermadec Trench. From these observations, we have documented swimming and feeding behaviour of these species and derived the first estimates of hadal fish abundance. The liparids intercepted bait within 100-200 min but were observed to preferentially feed on scavenging amphipods. Notoliparis kermadecensis act as top predators in the hadal food web, exhibiting up to nine suction-feeding events per minute. Both species showed distinctive swimming gaits: P. amblystomopsis (mean length 22.5 cm) displayed a mean tail-beat frequency of 0.47 Hz and mean caudal:pectoral frequency ratio of 0.76, whereas N. kermadecensis (mean length 31.5 cm) displayed respective values of 1.04 and 2.08 Hz. Despite living at extreme depths, these endemic liparids exhibit similar activity levels compared with shallow-water liparids.

Long-term changes in deep-water fish populations in the northeast Atlantic: a deeper reaching effect of fisheries?

A severe scarcity of life history and population data for deep-water fishes is a major impediment to successful fisheries management. Long-term data for non-target species and those living deeper than the fishing grounds are particularly rare. We analysed a unique dataset of scientific trawls made from 1977 to 1989 and from 1997 to 2002, at depths from 800 to 4800 m. Over this time, overall fish abundance fell significantly at all depths from 800 to 2500 m, considerably deeper than the maximum depth of commercial fishing (approx. 1600 m). Changes in abundance were significantly larger in species whose ranges fell at least partly within fished depths and did not appear to be consistent with any natural factors such as changes in fluxes from the surface or the abundance of potential prey. If the observed decreases in abundance are due to fishing, then its effects now extend into the lower bathyal zone, resulting in declines in areas that have been previously thought to be unaffected. A possible mechanism is impacts on the shallow parts of the ranges of fish species, resulting in declines in abundance in the lower parts of their ranges. This unexpected phenomenon has important consequences for fisheries and marine reserve management, as this would indicate that the impacts of fisheries can be transmitted into deep offshore areas that are neither routinely monitored nor considered as part of the managed fishery areas.

The absence of sharks from abyssal regions of the world's oceans

The oceanic abyss (depths greater than 3000 m), one of the largest environments on the planet, is characterized by absence of solar light, high pressures and remoteness from surface food supply necessitating special molecular, physiological, behavioural and ecological adaptations of organisms that live there. Sampling by trawl, baited hooks and cameras we show that the Chondrichthyes (sharks, rays and chimaeras) are absent from, or very rare in this region. Analysis of a global data set shows a trend of rapid disappearance of chondrichthyan species with depth when compared with bony fishes. Sharks, apparently well adapted to life at high pressures are conspicuous on slopes down to 2000 m including scavenging at food falls such as dead whales. We propose that they are excluded from the abyss by high-energy demand, including an oil-rich liver for buoyancy, which cannot be sustained in extreme oligotrophic conditions. Sharks are apparently confined to ca 30% of the total ocean and distribution of many species is fragmented around sea mounts, ocean ridges and ocean margins. All populations are therefore within reach of human fisheries, and there is no hidden reserve of chondrichthyan biomass or biodiversity in the deep sea. Sharks may be more vulnerable to over-exploitation than previously thought.

Trends in body size across an environmental gradient: a differential response in scavenging and non-scavenging demersal deep-sea fish

Body size trends across environmental gradients are widely reported but poorly understood. Here, we investigate contrasting relationships between size (body mass) and depth in the scavenging and predatory demersal ichthyofauna (800-4800 m) of the North-east Atlantic. The mean size of scavenging fish, identified as those regularly attracted to baited cameras, increased significantly with depth, while in non-scavengers there was a significant decline in size. The increase in scavenger size is a consequence of both intra and inter-specific effects. The observation of opposing relationships, in different functional groups, across the same environmental gradient indicates ecological rather than physiological causes. Simple energetic models indicate that the dissimilarity can be explained by different patterns of food distribution. While food availability declines with depth for both groups, the food is likely to be in large, randomly distributed packages for scavengers and as smaller but more evenly distributed items for predators. Larger size in scavengers permits higher swimming speeds, greater endurance as a consequence of larger energy reserves and lower mass specific metabolic rate, factors that are critical to survival on sporadic food items.

High Swimming and Metabolic Activity in the Deep‐Sea EelSynaphobranchus kaupiiRevealed by Integrated In Situ and In Vitro Measurements

Several complementary studies were undertaken on a single species of deep-sea fish (the eel Synaphobranchus kaupii) within a small temporal and spatial range. In situ experiments on swimming and foraging behaviour, muscle performance, and metabolic rate were performed in the Porcupine Seabight, northeast Atlantic, alongside measurements of temperature and current regime. Deep-water trawling was used to collect eels for studies of animal distribution and for anatomical and biochemical analyses, including white muscle citrate synthase (CS), lactate dehydrogenase (LDH), malate dehydrogenase (MDH), and pyruvate kinase (PK) activities. Synaphobranchus kaupii demonstrated whole-animal swimming speeds similar to those of other active deep-sea fish such as Antimora rostrata. Metabolic rates were an order of magnitude higher (31.6 mL kg(-1) h(-1)) than those recorded in other deep-sea scavenging fish. Activities of CS, LDH, MDH, and PK were higher than expected, and all scaled negatively with body mass, indicating a general decrease in muscle energy supply with fish growth. Despite this apparent constraint, observed in situ burst or routine swimming performances scaled in a similar fashion to other studied species. The higher-than-expected metabolic rates and activity levels, and the unusual scaling relationships of both aerobic and anaerobic metabolism enzymes in white muscle, probably reflect the changes in habitat and feeding ecology experienced during ontogeny in this bathyal species.

Sexually dimorphic expression of glutamate decarboxylase mRNA in the hypothalamus of the deep sea armed grenadier, Coryphaenoides (Nematonurus) armatus

Glutamate decarboxylase (GAD), is a key enzyme in the central nervous system (CNS) that synthesizes the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) from glutamate. Our previous phylogenetic studies on the evolution of this enzyme indicates that there are at least two distinct forms: GAD65 and GAD67. They are the products of separate genes and probably derive from a common ancestral GAD gene following gene duplication prior to the emergence of the teleosts more than 200 Myr ago. Furthermore, a third GAD-like molecule, GAD3, discovered in the armed grenadier, Coryphaenoides (Nematonurus) armatus, is equally divergent from both GAD65 and GAD67. Specimens of C. (N.) armatus were collected by trawl at a depth of 4,000 m in the Porcupine Seabight (Northeastern Atlantic), and brains dissected and frozen for RNA extraction. All three GAD forms are found in the cerebellum, telencephalon and hypothalamus. Semiquantitative PCR analysis showed that males and females have similar levels of expression of GAD67 and GAD3 in the tissues studied. Independent of the sex examined, the levels of expression of GAD65 and GAD67 in the cerebellum were approximately half that in the telencephalon. GAD3 levels were approximately 30% higher in the cerebellum than in either the telencephalon or hypothalamus. In contrast to GAD67 and GAD3, hypothalamic expression of GAD65 mRNA is 1.8 times higher (p < 0.05) in males than in females. These data indicate that the expression of GAD65, a key enzyme for the synthesis of GABA is sexually dimorphic in females and males of C. (N.) armatus.

Multiplicity of glutamic acid decarboxylases (GAD) in vertebrates: molecular phylogeny and evidence for a new GAD paralog

The evolution of chordate glutamic acid decarboxylase (GAD; EC 4.1.1.15), a key enzyme in the central nervous system synthesizing the neurotransmitter gamma-amino-butyric acid (GABA) from glutamate, was studied. Prior to this study, molecular data of GAD had been restricted to mammals, which express two distinct forms, GAD65 and GAD67. These are the products of separate genes and probably are derived from a common ancestral GAD following gene duplication at some point during vertebrate evolution. To enable a comprehensive phylogenetic analysis, molecular information of GAD forms in other vertebrate classes was essential. By reverse transcriptase-polymerase chain reaction (RT-PCR), partial nucleotide sequences of GAD were cloned from brains of zebra finch (Taeniopygia guttata), turtle (Trachemys scripta), goldfish (Carassius auratus), zebrafish (Danio rerio), and armoured grenadier (Coryphaenoides (Nematonurus) armatus, a deep-sea fish), and from the cerebral ganglion plus neural gland of Ciona intestinalis, a protochordate. Whereas GAD65 and GAD67 homologs were expressed in birds, reptiles, and fish, only a single GAD cDNA with equal similarities to both vertebrate GAD forms was found in the protochordate. This indicates that the duplication of the vertebrate GAD gene occurred between 400 and 560 million years ago. For both GAD65 and GAD67, the generated phylogenetic tree followed the general tree topology for the major vertebrate classes. In turtle, an alternative spliced form of GAD65, putatively encoding a truncated, nonactive GAD, was found. Furthermore, a third GAD form, which is equally divergent from both GAD65 and GAD67, is expressed in C. (N.) armatus. This third form might have originated from an ancient genome duplication specific to modern ray-finned fishes.

Melatonin receptors are present in non-optic regions of the brain of a deep-sea fish living in the absence of solar light

Pineal melatonin hormonally transduces photoperiod to influence daily and seasonal cycles in most vertebrates (1, 2). Evidence of melatonin receptors throughout the brain of several fish species (3-5), particularly in retinorecipient structures, also indicates a role in visual processing. Despite the absence of solar light many deepsea organisms show seasonality (6-8). The presence of central melatonin receptors was investigated by quantitative in vitro autoradiography in the deep-sea fish Coryphaenoides (Nematonurus) armatus. Specific, time-dependent, saturable, high affinity and guanine nucleotide sensitive, 2-[125I]iodomelatonin binding was found over the mid-brain tegmentum and hindbrain. Competing ligand potency was iodomelatonin > melatonin >> 5-HT. Although C.(N.) armatus has well developed eyes no 2-[125I]iodomelatonin binding occurred in optic tectum, cerebellum or hypothalamus. Thus melatonin involvement in processing of visual information and control of seasonal physiology via hypothalamic areas appears to be absent in this species. The presence of central G-protein coupled receptors indicates a function for melatonin unrelated to solar light.

Role of autoantibodies in the autoimmune response to testis in rainbow trout (Salmo gairdneri)

Rainbow trout, Salmo gairdneri, passively immunized with anti-sperm antibodies developed lesions in the testis characteristic of autoimmune attack. Only animals pre-treated with Freund's complete adjuvant developed these lesions. Investigations into the possible role of autoantibodies during autoimmune attack, carried out in vitro, showed that sperm antibodies were not cytotoxic but did have an opsonic effect on macrophage phagocytosis of sperm in the presence of complement.

The Effect of Swimming Activity and Section of the Vagus Nerves on Heart Rate in Rainbow Trout

1. Heart rates associated with swimming activity were measured in intact and vagotomized fish at 6.5 and 15 °C.

2. Low swimming speeds had no effect on heart rate but above a threshold speed it increased logarithmically with swimming speed up to the critical speed and maximum heart rate.

3. Times for recovery after exercise increased rapidly above the critical speed.

4. Bilaterally vagotomized fish at 6.5 °C showed high resting heart rates and erratic cardiac responses to exercise.

5. In bilaterally vagotomized fish at 15 °C heart rates were normal except for a low maximum rate.

6. It is concluded that the vagus nerve can function differently at different temperatures.