Under pressure: biomechanical limitations of developing pneumatocysts in the bull kelp (Nereocystis luetkeana,Phaeophyceae)

Maintaining buoyancy with gas‐filled floats (pneumatocysts) is essential for some subtidal kelps to achieve an upright stature and compete for light . However, as these kelps grow up through the water column, pneumatocysts are exposed to substantial changes in hydrostatic pressure, which could cause complications as internal gases may expand or contract, potentially causing them to rupture, flood, and lose buoyancy. In this study, we investigate how pneumatocysts of Nereocystis luetkeana resist biomechanical stress and maintain buoyancy as they develop across a hydrostatic gradient. We measured internal pressure, material properties, and pneumatocyst geometry across a range of thallus sizes and collection depths to identify strategies used to resist pressure‐induced mechanical failure. Contrary to expectations, all pneumatocysts had internal pressures less than atmospheric pressure, ensuring that thalli are always exposed to a positive pressure gradient and compressional loads, indicating that they are more likely to buckle than rupture at all depths. Small pneumatocysts collected from depths between 1 and 9 m (inner radius = 0.4–1.0 cm) were demonstrated to have elevated wall stresses under high compressive loads and are at greatest risk of buckling. Although small kelps do not adjust pneumatocyst material properties or geometry to reduce wall stress as they grow, they are ~3.4 times stronger than they need to be to resist hydrostatic buckling. When tested, pneumatocysts buckled around 35 m depth, which agrees with previous measures of lower limits due to light attenuation, suggesting that hydrostatic pressure may also define the lower limit of Nereocystis in the field.     

Foraging behaviors and the interaction of alewife,Alosa pseudoharengus,and bloater,Coregonus hoyi

Synopsis Alewife, Alosa pseudoharengus, and bloater, Coregonus hoyi, are common planktivores in Lake Michigan. Both alewife and bloater use a variety of feeding modes. Alewives can filter, gulp and particulate feed; bloaters can only gulp and particulate feed. We examined handling time per prey and probability of capture for alewife and bloater particulate feeding on Mysis relicta. Using these estimates and available data for filtering alewives, cost curves were derived for alewife and bloater particulate feeding and for alewife using all three modes of feeding. Alewives filter small prey relative to their own body size and particulate feed on larger prey. Feeding mode appears to be dependent on prey size and density and shifts in feeding mode are apparently based on maximizing biomass eaten per time. The ability to filter confers a competitive advantage on alewife when small prey are abundant as they were in the mid 1960s in Lake Michigan. If the zooplankton are large, bloater young-of-year do not suffer this relative disadvantage. In fact, large bloaters can consume prey on the bottom not available to alewife. This shifting competitive balance may explain, in part, the observed dynamics of alewife and bloater.     

Post-stocking movement and survival of hatchery-reared bloater (Coregonus hoyi) reintroduced to Lake Ontario

1. Determining the movement and fate of fishes post-stocking is challenging due to the difficulty in monitoring them, particularly immediately after release. Bloater (Coregonus hoyi; Salmonidae) is a deepwater cisco that has been extirpated from Lake Ontario for several decades and is presently the focus of binational restoration stocking efforts; however, there is limited information to evaluate the efficacy of these efforts. The aim of this study was to examine the initial post-release survival, 3D movement, and behaviour of hatchery-reared bloater stocked in Lake Ontario to expand knowledge of post-stocking ecology of fish and inform stocking practices for deepwater ciscoes.
2. In total, 74 hatchery-reared bloater were tagged with acoustic transmitters with depth and temperature sensors in 2016, 2017, and 2018 and passively monitored on an array of 105 69-kHz acoustic receivers deployed in north-eastern Lake Ontario. Several spatial metrics analysed movements after release to investigate immediate post-stocking survival and behaviour for the first time in a pelagic freshwater forage fish.
3. Estimated survival for tagged bloater was low (≤42%) and detection periods of live bloater ranged from 0.2 to 12.1 days (mean ± SD: 2.9 ± 2.9 days). Following release, tagged bloater dispersed quickly and exhibited an association with deeper water (>40 m). Despite overlap in space use for some bloater, there was no evidence of schooling behaviour. Bloater underwent extensive diel vertical migration from near bottom to within metres of the surface. These results demonstrated that, despite high initial mortality, some hatchery-reared bloater survived the initial stress of release and displayed characteristic behaviour of the species.
4. This study demonstrated the value of acoustic telemetry in restoration efforts and revealed survival and behaviour of bloater that has never been observed at this resolution, providing novel information for the management of reintroduced species. Establishment of a self-sustaining population of bloater will help restore fish native to Lake Ontario thus increasing prey fish diversity, improving ecological integrity and resilience, and serving as a model for the reintroduction and management of other native species throughout the Great Lakes.

     

Intra- and inter-specific variation in buoyancy of some estuarine fishes

Synopsis Buoyancy was measured on eight species of estuarine fishes that were caught in 1 m depth or less. Mean buoyancies of the physoclists Fundulus heteroclitus, F. majalis, Cyprinodon variegatus and Leiostomus xanthurus were similar and ranged from –6.5 to –18.0 kiloPascals below atmospheric pressure at sea level. Menidia menidia and Pomatomus saltatrix measured –36.6 and –46.1 kPa, respectively. Two physostomes, Brevoortia tyrannus and Anchoa mitchilli, measured + 2.9 and –23.5 kPa, respectively, but the latter probably releases air when handled.The four most buoyant physoclist species live near the bottom in areas that receive daily tide induced currents. Negative buoyancy probably functions in them as in stream dwelling minnows and salmonids, which respond to currents by decreasing their buoyancy. The pronounced negative buoyancy of M. menidia may be a response to a preference for habitat where the currents are stronger, P. saltatrix, which can secrete gas into the swim bladder at the fastest rate known for any fish, combines high secretion (and resorption) rates with marked negative buoyancy. This enables it to quickly change depths over a wide vertical range, without overexpanding the swim bladder to cause positive buoyancy.     

Patterns of genetic diversity in Great Lakes bloaters (Coregonus hoyi) with a view to future reintroduction in Lake Ontario

The originally diverse ciscoe fish fauna of the Laurentian Great Lakes has suffered many extinctions and local extirpations. Bloaters (Coregonus hoyi) are presumed extirpated from Lake Ontario and the reintroduction of this deepwater fish is under consideration. Given the demographic fluctuations of this species in the other Great Lakes and its recent intralacustrine origin, we sought to identify a genetically diverse and similar source of C. hoyi via an analysis of genetic diversity and population structure using 10 microsatellite loci. Despite well-documented demographic declines, we found no genetic evidence of bottlenecks in 12 C. hoyi samples from the four potential donor lakes (Huron, Michigan, Superior and Nipigon). By contrast, evidence of bottlenecks in historical samples of C. artedi from Lake Ontario suggested that standard genetic methods frequently used to identify population bottlenecks can only detect very severe and long-lasting demographic declines in naturally large populations. Patterns of genetic differentiation and assignment tests indicated that C. hoyi from Lake Huron and Lake Michigan, which are not differentiated, are genetically most similar to Lake Ontario ciscoes. The small available sample of deepwater ciscoes recently caught in Lake Ontario did not allow determining if these represent a small undetected C. hoyi population or a recent invasion of the deep section by C. artedi. On the basis of genetic criteria, we conclude that C. hoyi from any location within Lake Huron or Lake Michigan would be judicious sources of breeders for reintroducing C. hoyi in Lake Ontario.     

Planktothrix populations in subalpine lakes: selection for strains with strong gas vesicles as a function of lake depth,morphometry and circulation

1. The genus Planktothrix (Cyanobacteria) usually produces concentrated populations of filaments in the summer metalimnion of thermally stratifying lakes. This has been associated with the action of gas vesicles, cellular structures providing positive buoyancy. At the end of the summer, filaments are carried by convective mixing deeper into the water column where some gas vesicles collapse as a result of high hydrostatic pressure. They then lose their buoyancy, sink and are lost from the euphotic zone. 2. The resistance of gas vesicles to hydrostatic pressures is critical for the survival of Planktothrix in deep lakes. However, comparative observations on populations from lakes of a range of depths and hydrodynamic regimes are still needed to examine the relationships between the adaptive trait (i.e. the ‘critical’ pressure at which each gas vesicle collapses) with the environmental factor (i.e. the maximum hydrostatic pressure). 3. To explore the adaptation of Planktothrix populations to the depth of winter circulation in different systems, we collected 276 strains of P. cf. rubescens from eight lakes (z_max = 24–410 m) in Northern Italy during summer 2009 and we analysed the multicopy gene gvpC coding for a protein that crucially influences the critical pressure. 4. The strains analysed clustered into two main groups having gas vesicles with a mean critical pressure of 1.1 and 0.9 MPa, respectively. The proportion of the stronger strains was generally positively related to lake depth, although the overall pattern was complicated by individual lake morphology and hydrology. The relative frequency of stronger filaments was (i) greatest in deep basins with concave slopes and (ii) least in one deep, but permanently stratified lake. 5. The simultaneous presence of ‘weaker’ and ‘stronger’ filaments could allow for a rapid adaptive response to changes in hydrostatic pressures, related to changes in the amplitude of vertical circulation characterising deep lakes.     

Vulnerability of early life intervals of Coregonus hoyi to predation by a freshwater mysid,Mysis relicta

Synopsis The bloater, Coregonus hoyi, deposits its eggs on deep sediments (70–100 m in Lake Michigan), where its eggs and embryos can be exposed to epibenthic predators. We investigated the vulnerability of early life intervals of bloaters to predation by mysids, Mysis relicta, which are mostly epibenthic by day and planktonic at night. Bloaters were raised from spawn in the laboratory and presented to field-collected mysids in laboratory predation trials. Eggs were not ingested by the mysids. Embryonic bloaters were vulnerable to predation by mysids only during the interval between hatching and swim up, usually 1–24 h under laboratory conditions. The mysids required about a day of exposure to this novel prey before they were able to kill significant numbers of the bloater embryos by making successive attacks with their thoracic legs. In experiments with experienced (2 and 3 days with bloater embryos) mysids, a functional response between embryo density and mysid predation rates was apparent. Temperature and the presence of alternative prey (zooplankton) did not alter the ‘kill rate’ (about 2.5 embryos mysid^-1d^-1) of experienced mysids at high bloater densities (>4 bloaters/mysid). However, more embryos were partially, rather than completely, ingested at 4 versus 9° C and in the presence of zooplankton.     

Application of machine learning to identify predators of stocked fish in Lake Ontario: using acoustic telemetry predation tags to inform management

Understanding predator–prey interactions and food web dynamics is important for ecosystem-based management in aquatic environments, as they experience increasing rates of human-induced changes, such as the addition and removal of fishes. To quantify the post-stocking survival and predation of a prey fish in Lake Ontario, 48 bloater Coregonus hoyi were tagged with acoustic telemetry predation tags and were tracked on an array of 105 acoustic receivers from November 2018 to June 2019. Putative predators of tagged bloater were identified by comparing movement patterns of six species of salmonids (i.e., predators) in Lake Ontario with the post-predated movements of bloater (i.e., prey) using a random forests algorithm, a type of supervised machine learning. A total of 25 bloater (53% of all detected) were consumed by predators on average (± S.D. ) 3.1 ± 2.1 days after release. Post-predation detections of predators occurred for an average (± S.D. ) of 78.9 ± 76.9 days, providing sufficient detection data to classify movement patterns. Tagged lake trout Salvelinus namaycush provided the most reliable classification from behavioural predictor variables (89% success rate) and was identified as the main consumer of bloater (consumed 50%). Movement networks between predicted and tagged lake trout were significantly correlated over a 6 month period, supporting the classification of lake trout as a common bloater predator. This study demonstrated the ability of supervised learning techniques to provide greater insight into the fate of stocked fishes and predator–prey dynamics, and this technique is widely applicable to inform future stocking and other management efforts.     

Unexpected Positive Buoyancy in Deep Sea Sharks,Hexanchus griseus,and a Echinorhinus cookei

We do not expect non air-breathing aquatic animals to exhibit positive buoyancy. Sharks, for example, rely on oil-filled livers instead of gas-filled swim bladders to increase their buoyancy, but are nonetheless ubiquitously regarded as either negatively or neutrally buoyant. Deep-sea sharks have particularly large, oil-filled livers, and are believed to be neutrally buoyant in their natural habitat, but this has never been confirmed. To empirically determine the buoyancy status of two species of deep-sea sharks (bluntnose sixgill sharks, Hexanchus griseus, and a prickly shark, Echinorhinus cookei) in their natural habitat, we used accelerometer-magnetometer data loggers to measure their swimming performance. Both species of deep-sea sharks showed similar diel vertical migrations: they swam at depths of 200–300 m at night and deeper than 500 m during the day. Ambient water temperature was around 15°C at 200–300 m but below 7°C at depths greater than 500 m. During vertical movements, all deep-sea sharks showed higher swimming efforts during descent than ascent to maintain a given swimming speed, and were able to glide uphill for extended periods (several minutes), indicating that these deep-sea sharks are in fact positively buoyant in their natural habitats. This positive buoyancy may adaptive for stealthy hunting (i.e. upward gliding to surprise prey from underneath) or may facilitate evening upward migrations when muscle temperatures are coolest, and swimming most sluggish, after spending the day in deep, cold water. Positive buoyancy could potentially be widespread in fish conducting daily vertical migration in deep-sea habitats.     

Gas secretion and resorption in the swimbladder of the codGadus morhua

Summary The codGadus morhua has a closed, compliant swimbladder which occupies 5% of its volume. Pressure changes caused by vertical movements lead to the expansion and compression of the swimbladder gas, and the fish responds to the accompanying changes in density with compensatory swimming movements and the resorption or secretion of gas. A simple physiological model, based on estimates of cardiac output, the blood supply to the swimbladder and the oxygen-carrying capacity of the blood, is developed to set limits to these processes.An apparatus is described for making observations on the rate of buoyancy adaptation by cod subjected to pressure changes within the range 1.15 to 7.50 ATA at temperatures from 0 to 17°C. One hundred and twenty eight experiments were made with 38 cod ranging in length from 25 to 50 cm and in weight from 138 to 1440 g.The results showed that the rate of gas resorption increased markedly with the pressure to which the fish were adapted (from 0.14 ml kg^–1 min^–1 at 1.5 ATA to 1.54 ml kg^–1 min^–1 at 7.0 ATA), but not significantly with the weight of the fish or with temperature. In contrast, the rate of gas secretion increased markedly with temperature (from 0.02 ml kg^–1 min^–1 at 0°C to 0.11 ml kg^–1 min^–1 at 17°C), increased slightly with pressure, and decreased with the weight of the fish.The rate of gas resorption was much faster than that of gas secretion, and the difference between the two rates increased with pressure. The difference in performance is discussed in relation to the restriction that the swimbladder might impose to the speed and extent of vertical movements. It is suggested that when a closed swimbladder has a hydrostatic function it may be advantageous if neutral buoyancy is maintained only at the upper limit to the diurnal vertical range.     

Food-web manipulations influence grazer control of phytoplankton growth rates in Lake Michigan

Stocking piscivorous salmonids in Lake Michigan produced dramaticalterations in food-web structure, including higher numbersof large-bodied zooplankton (especially Daphnia pulicaria),lower summer chlorophyll concentrations and increased watertransparency. Experimental determinations of epilimnetic phytoplanktongrowth rates and of zooplankton grazing rates indicate thatherbivorous zooplankton controlled algal dynamics during thesummer of 1983 because grazers occupied the surface waters throughoutthe day. In 1985, however, both large- and small-bodied Daphniamade approximately equal contributions to total grazer biomass,and all grazers displayed pronounced diel vertical migrations,visiting epilimnetic waters only at night. This prohibited zooplanktonfrom controlling algal dynamics because grazing losses did notexceed phytoplankton growth rates. The changes in zooplanktoncommunity composition and behavior observed in summer 1985 probablyresulted from increased predation by visually orienting planktivorousfish, especially bloater chub (Coregonus hoyi). Effects of food-webmanipulations on phytoplankton dynamics were evident only duringJuly and August. During spring and early summer copepods dominateLake Michigan's zooplankton community. Owing to their smallbody size, copepods are less susceptible to fish predation andexhibit much lower filtering rates than Daphnia. Variabilityin zooplanktivorous fish abundance probably has little effecton phytoplankton dynamics during spring and early summer.     

Swimming behavior in relation to buoyancy in an open swimbladder fish, the Chinese sturgeon

The swimbladder of fishes is readily compressed by hydrostatic pressure with depth, causing changes in buoyancy. While modern fishes can regulate buoyancy by secreting gases from the blood into the swimbladder, primitive fishes, such as sturgeons, lack this secretion mechanism and rely entirely on air gulped at the surface to inflate the swimbladder. Therefore, sturgeons may experience changes in buoyancy that will affect their behavior at different depths. To test this prediction, we attached data loggers to seven free-ranging Chinese sturgeons Acipenser sinensis in the Yangtze River, China, to monitor their depth utilization, tail-beating activity, swim speed and body inclination. Two distinct, individual-specific, behavioral patterns were observed. Four fish swam at shallow depths (7–31 m), at speeds of 0.5–0.6 m s⁻¹, with ascending and descending movements of 1.0–2.4 m in amplitude. They beat their tails continuously, indicating that their buoyancy was close to neutral with their inflated swimbladders. In addition, their occasional visits to the surface suggest that they gulped air to inflate their swimbladders. The other three fish spent most of their time (88–94%) on the river bottom at a depth of 106–122 m with minimum activity. They occasionally swam upwards at speeds of 0.6–0.8 m s⁻¹ with intense tailbeats before gliding back passively to the bottom, in a manner similar to fishes that lack a swimbladder. Their bladders were probably collapsed by hydrostatic pressure, resulting in negative buoyancy. We conclude that Chinese sturgeons behave according to their buoyancy, which varies with depth due to hydrostatic compression of the swimbladder.     

Synchronous Effects of Temperature, Hydrostatic Pressure, and Salinity on Growth, Phospholipid Profiles, and Protein Patterns of Four Halomonas Species Isolated from Deep-Sea Hydrothermal- Vent and Sea Surface Environments

Four strains of euryhaline bacteria belonging to the genus Halomonas were tested for their response to a range of temperatures (2, 13, and 30°C), hydrostatic pressures (0.1, 7.5, 15, 25, 35, 45, and 55 MPa), and salinities (4, 11, and 17% total salts). The isolates were psychrotolerant, halophilic to moderately halophilic, and piezotolerant, growing fastest at 30°C, 0.1 MPa, and 4% total salts. Little or no growth occurred at the highest hydrostatic pressures tested, an effect that was more pronounced with decreasing temperatures. Growth curves suggested that the Halomonas strains tested would grow well in cool to warm hydrothermal-vent and associated subseafloor habitats, but poorly or not at all under cold deep-sea conditions. The intermediate salinity tested enhanced growth under certain high-hydrostatic-pressure and low-temperature conditions, highlighting a synergistic effect on growth for these combined stresses. Phospholipid profiles obtained at 30°C indicated that hydrostatic pressure exerted the dominant control on the degree of lipid saturation, although elevated salinity slightly mitigated the increased degree of lipid unsaturation caused by increased hydrostatic pressure. Profiles of cytosolic and membrane proteins of Halomonas axialensis and H. hydrothermalis performed at 30°C under various salinities and hydrostatic pressure conditions indicated several hydrostatic pressure and salinity effects, including proteins whose expression was induced by either an elevated salinity or hydrostatic pressure, but not by a combination of the two. The interplay between salinity and hydrostatic pressure on microbial growth and physiology suggests that adaptations to hydrostatic pressure and possibly other stresses may partially explain the euryhaline phenotype of members of the genus Halomonas living in deep-sea environments.     

The properties and buoyancy-providing role of gas vacuoles in Trichodesmium Ehrenberg

All three species of the marine blue-green alga Trichodesmium collected in the Sargasso and Caribbean seas were found to possess gas vacuoles. The constituent gas vesicles were much stronger than those found in any freshwater blue-green alga, the mean critical collapse pressures being 12 bars in T. erythraeum, 34 bars in T. contortum and 37 bars in T. thiebautii. This great strength is obviously an adaptation to the hydrostatic pressures at the depths to which these organisms occur in the ocean. In each case the gas vesicles are far too strong to be collapsed by rising cell turgor pressure, though gas-vacuolation could be slowly regulated by the differential growth of gas vesicles and cells. Since the vesicles are of a similar shape and size to those in other species, the vesicle wall material must be stronger. The majority of Trichodesmium colonies collected were positively buoyant, and in all cases tested the buoyancy was dependent on the presence of gas vacuoles. The buoyancy is important in increasing the residence time of these slowly growing algae in the euphotic zone and it is responsible for the surface water-blooms which they form.     

Compensatory swimming in the kokanee and sockeye salmon Oncorhynchus nerka (Walbaum)

When negatively buoyant, such as by increased pressure or loss of swimbladder gas, kokanee and sockeye salmon ( Oncorhynchus nerka ) attempt to swim upward by increased use of the pectoral fins. This response is termed compensatory swimming. Prior to initial filling of the swimbladder, sockeye fry showed no behavioural response to pressures above atmospheric. Following air-gulping at the surface and bladder inflation, kokanee and sockeye fry responded to increased pressure by assuming a more vertical position and by beating the pectoral fins more rapidly. In young sockeye this response occurred over the pressure range of atmospheric to 20 lb/in², and the effect of this behaviour would be to distribute these fish in the upper 14 m of the lacustrine environment. Fingerling kokanee showed a more gradual increase in compensatory swimming over the range of pressure equivalent to depths of 0–50 m. The behaviour of yearling kokanee would tend to concentrate these fish in the upper 30 m. Sockeye older than 1 year responded to negative buoyancy with increased horizontal swimming whilst planing upward on the pectoral fins. Depth distribution postulated on the basis of pressure-induced compensatory swimming is consistent with the known vertical distribution of kokanee and sockeye salmon.     

Buoyancy in Marine Fishes: Direct and Indirect Role of Lipids

SYNOPSIS. The major lipids that have a direct role in buoyancyof marine fish are wax esters, squalene, and alkyldiacylglycerols.Wax esters are stored extracellularly in certain fishes, suchas the orange roughy (Hoplostethus atlanticus), and thereforebuoyancy appears to be their sole function. Some myctophid fisheshave wax-invested swimbladders, where the non-compressible waxesters may aid in diurnal vertical migration, by replacing compressibleswimbladder gases. Squalene is metabolically inert in the liversof certain sharks, and therefore probably has buoyancy as itsonly function. Alkyldiacylglycerols (DAGE) are abundant componentsof liver oil of certain deep sea sharks and holocephalans, wherethey may have an important role in buoyancy. Triacylglycerolsand cholesterol are lipids that have an indirect role in buoyancyof marine fish. Many fishes in the ocean have oil-filled bones(mostly triacylglycerols). Although this oil aids buoyancy,its major function is as an energy storage lipid which can beutilized during starvation. Cholesterol, which is found in highamounts in the lipid-rich membranes of the swimbladder of deepsea fishes, may aid buoyancy by combining with oxygen gas inthe swimbladder membranes to facilitate gas secretion in fishat great depths in the ocean. Further research is needed tounderstand the physical state of lipids, such as wax estersat deep sea temperatures and pressures, and more evidence isneeded to clarify the role of cholesterol-rich membranes inswimbladders of deep sea fishes.     

The distribution of benthic invertebrates along a natural turbidity gradient in Lake Temiskaming,Ontario-Quebec

Lake Temiskaming, a rift valley lake on the Ontario-Quebec border, exhibits a permanent gradient of turbidity due to tributary streams which cut through clay deposits to the north of the lake. Concentrations of total phosphorus (TP) also decreased from north to south, with values suggesting mesotrophic conditions. Concentrations of chlorophyll a were characteristic of oligotrophic lakes and showed little relationship to either turbidity or TP. Large numbers of Tubificidae were found at our northernmost sampling station at a depth of 50 m, probably reflecting the localized impact of allochthonous organic matter introduced by a tributary stream. Numerical abundance of the benthic fauna was much lower and did not vary significantly among the six more southerly 50 m stations, but biomass declined from north to south as Heterotrissocladius oliveri relaced Pontoporeia hoyi. Numerical abundance did not differ significantly among stations at depths of 10 m, but biomass decreased from north to south reflecting the distributions of the largest species, Hexagenia sp. and P. hoyi. Intensive sampling on two transects showed that maximum numbers of invertebrates occurred in the profundal zone. While these results are consistent with the correlation between TP and zoobenthic biomass reported by other investigators, size selective predation by fish may also be important in controlling the distribution of benthic invertebrates in Lake Temiskaming.     

Lipid content and composition of three species of Antarctic fish in relation to buoyancy

Summary The lipip content and composition of various tissues from three species of nototheniid fish from McMurdo Sound, Antarctic have been examined in relation to their habitat and buoyancy. The pelagic midwater Dissostichus mawsoni is neutrally buoyant. It is rich in lipid which is located subcutaneously, as adipose tissue associated intimately with white muscle, and as lipid droplets within the cells of various tissues. White muscle, red muscle and liver are particularly lipid-rich, although the liver is not positively buoyant. The amount of lipid stored in the white muscle increases towards the centre of buoyancy of the fish. These deposits are documented at the anatomical, histological and ultrastructural levels. Tissues of Pagothenia borchgrevinki contain less lipid than D. mawsoni, but liver, red muscle and white muscle are still very rich in lipid. This species is cryopelagic, that is it spends most of the time in the water column just beneath the surface ice layer. It is not neutrally buoyant, but has a low weight in seawater. The tissues of the benthic Trematomus bernacchii contain only normal levels of lipid. The lipid class compositions of all three species are dominated by triacylglycerol, particularly when lipid contents are high. Serum lipids are an exception in containing high levels of the transport lipid sterol ester. The reason why Antarctic fish use triacylglycerols for buoyancy rather than was esters (as used by many myctophids) or squalene (as used by some sharks) is unclear.     

Pressure inactivation of tetrameric lactate dehydrogenase homologues of confamilial deep-living fishes

Summary The susceptibility to inactivation by hydrostatic pressure of the tetrameric (Fig. 1) muscletype (M₄) lactate dehydrogenase homologues (LDH, EC 1.1.1.27;l-lactate: NAD⁺ oxidoreductase) from six confamilial macrourid fishes was compared at 4 °C. These marine teleost fishes occur over depths of 260 to 4815 m. The pressures necessary to half-inactivate the LDH homologues are related to the pressures which the enzymes are exposed to in vivo (Table 1); higher hydrostatic pressures are required to inactivate the LDH homologues of the deeper-occurring macrourids. The resistance of the LDH homologues to inactivation by pressure is affected by protein concentration (Fig. 3). After an hour of incubation at pressure, the percent remaining activity approaches an asymptotic value (Fig. 2). The inactivation of the macrourid LDH homologues by pressure was not fully reversible. Assuming that inactivation by pressure was due to dissociation of the native tetramer to monomers, apparent equilibrium constants (K _eq) were calculated. Volume changes (V) were calculated over the range of pressures for which plots inK _eq versus pressure were linear (Fig. 4). The V of dissociation values of the macrourid homologues range from –219 to –439 ml mol^–1 (Table 1). Although the hydrostatic pressures required to inactivate the LDH homologues of the macrourid fishes are greater than those which the enzymes are exposed to in vivo, the pressure-stability of these enzymes may reflect the resistance of these enzymes to pressure-enhanced proteolysis in vivo.     

Ontogenetic changes in behavioural and histological measures of visual acuity in three species of fish

Synopsis Vision plays an important role in the early life history of fishes. We investigated the ontogenetic changes in visual acuity of early life history stages of alewife,Alosa pseudoharengus, yellow perch,Perca flavescens and bloater,Coregonus hoyi, across a range of sizes. Acuities were determined through histological examination of the retinae of larvae. Reactive distances of larvae to prey were estimated through videophotography of their response to prey and were then converted to measurements of visual angle. Both measures of visual ability improved with size (age) for all species. When behavioural and anatomical measures of ability were compared as a function of size, the data indicate that fish are anatomically more capable of seeing objects than the behavioural response suggests. In two of the three species, the relationship between histological acuity and visual angle was not constant. These results may indicate that while vision may limit initial rates of encounter and feeding, increases in visual acuity mean that in older stages limitations on encounter and feeding are more likely to be behavioural. Furthermore, these results indicate that encounter rates based upon histological estimates of visual acuity will be greater than comparable estimates based upon reactive distances. We recommend calculation of encounter rates based upon reactive distances. Present address: Department of Biology, McGill University, 1205 Ave, Dr. Penfield, Montreal, PQ H3A 1B1, Canada