Buoyancy Range, Gas Bladder Volume, and Lipid Content of Adult Bloater, Coregonus hoyi Gill, in the Laurentian Great Lakes

We analyzed buoyancy characteristics (buoyancy range, gas bladder volume, and lipid content) of adult bloater, Coregonus hoyi Gill, in the Laurentian Great Lakes. Buoyancy was measured directly in individual bloater and compared with the hydrostatic pressures at the depths of capture. The results of the buoyancy comparisons suggest that C. hoyi has a buoyancy range comparable to the 'free vertical range' of movement that has been described for other fishes. The buoyancy range of bloater does not support the argument that they undergo diel vertical migrations, as suggested in earlier studies. Rather, the buoyancy range for bloater reflects the depths of neutral buoyancy and the distance above these depths at which they can maintain vertical position in the water column. The available data suggest that adult bloater are neutrally buoyant near the lake bottom, and C. hoyi with 50% positive buoyancy are at the top of their depth range. Large bloater have relatively small gas bladder volumes and a high lipid content. Based on the observed vertical distribution of large and small C. hoyi and our results, we deduced that large fish are more adept than small fish at regulating their buoyancy for prolonged stays at higher hydrostatic pressures. Lipid content for bloater was not significantly different with respect to sex or origin (wild vs. captive-raised).     

Buoyancy of sub-Antarctic notothenioids including the sister lineage of all other notothenioids (Bovichtidae)

The radiation of notothenioid fishes (Perciformes) in Antarctic waters was likely the result of an absence of competition in the isolated Antarctic waters and key traits such as the production of antifreeze glycoprotein and buoyancy modifications. Although notothenioids lack a swim bladder, the buoyancy of Antarctic species, ranging from neutrally buoyant to relatively heavy, corresponds to diverse life styles. The buoyancy of South American notothenioids has not been studied. Static buoyancy was measured in adult notothenioids (n = 263, from six species of the sub-order Notothenioidei, families Bovichtidae, Eleginopidae, Nototheniidae, and Harpagiferidae) from the Beagle Channel. Measurements were expressed as percentage buoyancy (%B). Buoyancy ranged from 3.88 to 6.96% (median, 4.0–6.7%), and therefore, all species could be considered benthic consistent with previous studies that found that neutral buoyancy in notothenioids is rare. Harpagifer bispinis, Patagonotothen cornucola, and Cottoperca gobio were significantly less buoyant than Paranotothenia magellanica. The buoyancy values of most species were concordant with known habitat preferences. These data, especially the data of C. gobio (sister lineage of all other nototehnioids) and E. maclovinus (sister lineage of the Antarctic clade of notothenioids), could be useful for understanding the diversification of this feature during the notothenioid radiation.     

Whole-body lift and ground effect during pectoral fin locomotion in the northern spearnose poacher (Agonopsis vulsa)

The northern spearnose poacher, Agonopsis vulsa, is a benthic, heavily armored fish that swims primarily using pectoral fins. High-speed kinematics, whole-body lift measurements, and flow visualization were used to study how A. vulsa overcomes substantial negative buoyancy while generating forward thrust. Kinematics for five freely swimming poachers indicate that individuals tend to swim near the bottom (within 1 cm) with a consistently small (less than 1°) pitch angle of the body. When the poachers swam more than 1 cm above the bottom, however, body pitch angles were higher and varied inversely with speed, suggesting that lift may help overcome negative buoyancy. To determine the contribution of the body to total lift, fins were removed from euthanized fish (n=3) and the lift and drag from the body were measured in a flume. Lift and drag were found to increase with increasing flow velocity and angle of attack (ANCOVA, p<0.0001 for both effects). Lift force from the body was found to supply approximately half of the force necessary to overcome negative buoyancy when the fish were swimming more than 1 cm above the bottom. Lastly, flow visualization experiments were performed to examine the mechanism of lift generation for near-bottom swimming. A vortex in the wake of the pectoral fins was observed to interact strongly with the substratum when the animals approached the bottom. These flow patterns suggest that, when swimming within 1 cm of the bottom, poachers may use hydrodynamic ground effect to augment lift, thereby counteracting negative buoyancy.     

Confirmation of neutral buoyancy in <Emphasis Type="Italic">Aethotaxis mitopteryx</Emphasis> DeWitt (Notothenioidei: Nototheniidae)

Aethotaxis mitopteryx is a nototheniid species with a circum-Antarctic distribution in the Southern Ocean. We present new locality records, buoyancy measurements, and morphometric data on five A. mitopteryx specimens collected from the Bransfield Strait. Our analyses demonstrate that A. mitopteryx is neutrally buoyant. This result lends additional support to the hypothesis that neutral buoyancy has a single evolutionary origin in Notothenioidei. Food items in these specimens consisted entirely of krill (Euphausia sp.), and all specimens were sexually immature. We present morphometric data from these five specimens and compare to the holotype specimen, which is the only A. mitopteryx specimen collected in the Ross Sea.     

Buoyancy of some Palaeozoic ammonoids and their hydrostatic properties based on empirical 3D‐models

The interpretation of the function of the ammonoid phragmocone as a buoyancy device is now widely accepted among ammonoid researchers. During the 20^th century, several theoretical models were proposed for the role of the chambered shell (phragmocone); accordingly, the phragmocone had hydrostatic properties, which enabled it to attain neutral buoyancy, presuming it was partially filled with gas. With new three‐dimensional reconstructions of ammonoid shells, we are now able to test these hypothetical models using empirical volume data of actual ammonoid shells. We investigated three Palaeozoic ammonoids (Devonian and Carboniferous), namely Fidelites clariondi, Diallagites lenticulifer and Goniatites multiliratus, to reconstruct their hydrostatic properties, their syn vivo shell orientation and their buoyancy. According to our models, measurements and calculations, these specimens had aperture orientations of 19°, 64° and 125° during their lives. Although none of our results coincide with the aperture orientation of the living Nautilus, they do verify the predictions for shell orientations based on published theoretical models. Our calculations also show that the shorter the body chamber, the poorer was the hydrodynamic stability of the animal. This finding corroborates the results of theoretical models from the 1990s. With these results, which are based on actual specimens, we favour the rejection of hypotheses suggesting a purely benthonic mode of life of ammonoids. Additionally, it is now possible to assess hydrodynamic properties of the shells through ontogeny and phylogeny, leading to insights to validate theoretical modes of life and habitat through the animal's life.     

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.     

Energy cost and putative benefits of cellular mechanisms modulating buoyancy in aflagellate marine phytoplankton

Little information is available on the energetics of buoyancy modulation in aflagellate phytoplankton, which comprises the majority of autotrophic cells found in the ocean. Here, we computed for three aflagellate species of marine phytoplankton (Emiliania huxleyi, Thalassiosira pseudonana, and Ethmodiscus rex) the theoretical minimum energy cost as photons absorbed and nitrogen resource required of the key physiological mechanisms (i.e., replacement of quaternary ammonium by dimethyl‐sulfoniopropionate, storage of polysaccharides, and cell wall biosynthesis) affecting the cell's vertical movement as a function of nitrogen (N) availability. These energy costs were also normalized to the capacity of each buoyancy mechanism to modulate sinking or rising rates based on Stokes' law. The three physiological mechanisms could act as ballast in the three species tested in conditions of low N availability at a low fraction (<12%) of the total photon energy cost for growth. Cell wall formation in E. huxleyi was the least costly ballast strategy, whereas in T. pseudonana, the photon energy cost of the three ballast strategies was similar. In E. rex, carbohydrate storage and mobilization appear to be energetically cheaper than modulations in organic solute synthesis to achieve vertical migration. This supports the carbohydrate‐ballast strategy for vertical migration for this species, but argues against the theory of replacement of low‐ or high‐density organic solutes. This study brings new insights into the energy cost and potential selective advantages of several strategies modulating the buoyancy of aflagellate marine phytoplankton.     

Diurnal buoyancy changes of Microcystis in an artificially mixed storage reservoir

In a storage reservoir, which is artificially mixed in order to reduce algal and especially cyanobacterial growth, the cyanobacterium Microcystis is still present. The aim of the research was to investigate why Microcystis was able to grow in the artificially mixed reservoir. From the results it could be concluded that the large shallow area in the reservoir allows this growth. The loss of buoyancy during the day was much higher in this shallow part than in the deep part. Assuming that the loss of buoyancy was the result of a higher carbohydrate content, a higher growth rate in the shallow part may be expected. A higher received light dose by the phytoplankton in the shallow mixed part of the reservoir than in the deep mixed part explains the difference in buoyancy loss. A significant correlation between the received light dose (calculated for homogeneously mixed phytoplankton) and the buoyancy loss was found. Apparently, the Microcystis colonies were entrained in the turbulent flow in both the shallow and the deep part of the reservoir. With a little higher stability on one sampling day, due to the late start of the artificial mixing, the loss of buoyancy at the deep site was higher than on the other days and almost comparable to the loss at the shallow site. Although the vertical biomass distribution and the temperature profiles showed homogeneous mixing, the colonies in the upper layers apparently received a higher light dose than those deeper in the water column. Determination of the buoyancy state of cyanobacteria appeared to be a valuable method to investigate the light history and hence their entrainment in the turbulent flow in the water column.     

Buoyancy control in diving behavior of the loggerhead turtle,Caretta caretta

Neutral buoyancy at the stationary depth is advantageous for diving animals. The adjustment of the air inspiration before diving can be a mechanism of buoyancy control for diving animals with lungs. The stationary depth of neutral buoyancy becomes deeper with larger inspiration. Our aim was to examine whether the loggerhead sea turtle,Caretta caretta regulates the buoyancy to be neutral at the stationary depth of the dive. During an internesting period of the breeding season, we recorded the diving pattern of an adult female using a time-depth recorder and a time-swim distance recorder. The dives were classified into four types (Types 1 to 4) based on the time-depth profile. Types-3 and 4 (66% of the total dive duration) have three phases in each dive: (1) first descent, (2) gradual ascent (stationary period), and (3) final ascent. In the gradual ascent phase, the turtle stayed at a certain depth without swimming. This means that the turtle was neutrally buoyant during the gradual ascent phase. The depth of the gradual ascent phase was positively correlated with the dive duration, supporting the hypothesis that neutral buoyancy of the loggerhead turtle is achieved by the air in their lungs.     

Influence of solar ultraviolet-B on pelagic fish embryos: osmolality, mortality and viable hatch

Eggs of dab (Limanda limanda) and plaice (Pleuronectes platessa) were experimentally exposed to ultraviolet-B (UV-B) radiation in a solar radiation simulator. The experimental design tried to simulate present and future conditions with reference to increased UV-B exposure due to northern hemisphere ozone loss, employing mainly two scenarios, a reduction to 270 (S1) and to 180 (S2) Dobson units (DU) in single or repetitive exposures of 2, 4 or 6 h. Depending on the total dose of UV-B irradiation and the developmental stage, exposed eggs displayed loss of buoyancy as a sublethal effect, as well as increased embryo mortality and reduced viable hatch. In the single exposure experiments only under conditions of 180 DU for 6 h were effects apparent. Double exposure under conditions of 270 DU did not lead to lasting effects. At the sublethal effect level, i.e. loss of buoyancy, considerable photorepair was observed. It was concluded, that under the present general weather conditions in spring and at the present levels of environmental ozone, allowing for a reduction to 180 DU, the embryonic development of North Sea spring spawning fish is not endangered by UV-B radiation. Received in revised form: 19 June 2000 Electronic Publication     

The Biogeographic Importance of Buoyancy in Macroalgae: A Case Study of the Southern Bull-Kelp Genus Durvillaea (Phaeophyceae), Including Descriptions of Two New Species1

Long-distance dispersal plays a key role in evolution, facilitating allopatric divergence, range expansions, and species movement in response to environmental change. Even species that seem poorly suited to dispersal can sometimes travel long distances, for example via hitchhiking with other, more intrinsically dispersive species. In marine macroalgae, buoyancy can enable adults—and diverse hitchhikers—to drift long distances, but the evolution and role of this trait are poorly understood. The southern bull-kelp genus Durvillaea includes several non-buoyant and buoyant species, including some that have only recently been recognized. In revising the genus, we not only provide updated identification tools and describe two new species (D. incurvata comb. nov. from Chile and D. fenestrata sp. nov. from the Antipodes Islands), but also carry out biogeographic analyses to determine the evolutionary history of buoyancy in the genus. Although the ancestral state was resolved as non-buoyant, the distribution of species suggests that this trait has been both gained and lost, possibly more than once. We conclude that although buoyancy is a trait that can be useful for dispersal (creating evolutionary pressure for its gain), there is also evolutionary pressure for its loss as it restricts species to narrow environmental ranges (i.e., shallow depths).     

Regulation of blue-green algal buoyancy and bloom formation by light,inorganic nitrogen,C02, and trophic level interactions

In highly eutrophic ponds, buoyancy of the gas-vacuolate blue-green alga Anabaenopsis Elenkinii (Miller) was regulated by complex interactions between chemical and physical parameters, as well as by biological interactions between various trophic levels. Algal buoyancy and surface bloom formation were enhanced markedly by decreased light intensity, and to a lesser extent by decreased CO₂ availability and increased availability of inorganic nitrogen. In the absence of dense populations of large-bodied Cladocera, early season blooms of diatoms and green algae reduced light availability in the ponds thus creating conditions favorable for increased buoyancy and bloom formation by A. Elenkinii. The appearance of blue-green algal blooms could be prevented by a reduced density of planktivorous fish, which allowed development of dense cladoceran populations. The cladocerans limited the growth of precursory blooms of diatoms and green algae, and given the resulting clear-water conditions, buoyancy of A. Elenkinii was reduced, and blue-green algal blooms never appeared.     

CYANOBACTERIAL BUOYANCY REGULATION: THE PARADOXICAL ROLES OF CARBON1

In stratified lakes, dominance of the phytoplankton by cyanobacteria is largely the result of their buoyancy and depth regulation. Bloom-forming cyanobacteria regulate the gas vesicle and storage polymer contents of their cells in response to interactive environmental factors, especially light and nutrients. While research on the roles of nitrogen and phosphorus in cyanobacterial buoyancy regulation has reached a consensus, evaluations of the roles of carbon have remained open to dispute. We investigated the various effects of changes in carbon availability on cyanobacterial buoyancy with continuous cultures of Microcystis aeruginosa Kuetz. emend. Elenkin (1924), a notorious bloom-former. Although CO₂ limitation of photosynthesis can promote buoyancy in the short term by preventing the collapse of turgor-sensitive gas vesicles and/or by limiting polysaccharide accumulation, we found that sustained carbon limitation restricts buoyancy regulation by limiting gas vesicle as well as polysaccharide synthesis. These results provide an explanation for the positive effects of bicarbonate enrichment on cyanobacterial nitrogen uptake and bloom formation in lake experiments and may help to explain the pattern of cyanobacterial dominance in phosphorus-enriched, low-carbon lakes.     

Studies in African Cyperaceae 33. A new species of Fuirena from Somalia

A new very rare species, Fuirena somaliensis, is described from the lowlands of South Somalia below 150 m. The new species belongs to the F. ciliaris complex and has bulliform inner perianth‐segments and thus a similar adaptation to buoyancy and dispersal by water as four other African species.     

Density regulation inSarsia tubulosa (Hydrozoa)

The majority of wild-caughtSarsia tubulosa M. Sars medusae are less dense than the surrounding water. The bell ofS. tubulosa is the buoyant structure; the tentacles and manubrium sink if cut off from the bell.S. tubulosa individuals placed in dilute seawater sink initially but recover positive or neutral buoyancy and normal activity within a couple of hours. In all cases observed animals were able to achieve positive buoyancy in seawater of 20.25 S and some individuals were able to adjust to lower salinities. In most cases where positive buoyancy was not attained within two hours the animal did not achieve positive buoyancy within twelve hours and died within that period. While the mechanism of regulation is not known, ionic pumping, possibly involving the extrusion of sulphate ion, has been suggested to be responsible for the buoyancy of mesoglea in other jellyfishes.     

Interrelations of photosynthetic behaviour and buoyancy regulation in a natural population of a blue-green alga

The photosynthetic activity of Anabaena cirdnalis and associated changes in buoyancy were determined from prepared suspensions exposed in the natural light field of Crose Mere. The observations are related to variations in subsurface irradiance and temperature. Parallel experiments, aimed at trapping algal colonies undertaking controlled vertical movements within the lake system, are also described. Buoyancy loss and downward migration are clearly associated with specific photosynthetic rates: rates as low as 1.8 mg O₂ (mg chlorophyll a) h⁻¹ are shown to be sufficient to effect buoyancy loss, while movements in the lake tend towards a depth where rates of 5–7 mg O₂ (mg chlorophyll a)⁻¹ h⁻¹ are possible. These rates are significantly less than those possible at light saturation. The effect of increasing temperature is to depress the population in the light-gradient. The significance of this response is discussed in relation to the growth of natural populations of blue-green algae.     

Ontogenetic shift in buoyancy and habitat in the Antarctic toothfish, <Emphasis Type="Italic">Dissostichus mawsoni</Emphasis> (Perciformes: Nototheniidae)

Buoyancy measurements and depth of capture were taken on 70 individuals of Dissostichus mawsoni collected from the Southern Scotia Arc and McMurdo Sound, Antarctica, to examine the effect of age on buoyancy and habitat use. Standard lengths (SL) ranged from 10.4 to 138.0 cm. Juveniles were not buoyant (heavy in water), whereas adults were neutrally buoyant. The slope of the relationship between buoyancy and SL was significantly negative for juveniles (individuals less than 81 cm SL), but there was no significant relationship for adults (individuals greater than 81 cm SL). These results demonstrate an ontogenetic shift in buoyancy. For juveniles, depth of capture and SL had a significantly positive relationship. As individuals reach adulthood they achieve neutral buoyancy and appear to use deeper water habitats. This interpretation is supported by a significant positive correlation between buoyancy and depth of capture for juveniles. Changes in buoyancy with maturation of juveniles may also be associated with a shift in habitat use. Juveniles appear to exploit benthic habitats, whereas adults use the entire water column over deeper water. Given the differences in prey species available in these habitats and based on our results, we predict that diets of juveniles and adults should differ significantly. We also hypothesize that accumulation of lipid deposits from the diet during maturation of juveniles may account for the ontogenetic shift in buoyancy and allow neutral buoyancy to be achieved in adulthood.     

BUOYANCY REGULATION IN THE COLONIAL DIAZOTROPHIC CYANOBACTERIUM TRICHODESMIUM TENUE: ULTRASTRUCTURE AND STORAGE OF CARBOHYDRATE,POLYPHOSPHATE, AND NITROGEN1

Trichodesmium tenue Wille (1904) was examined using transmission electron microscopy to determine the role of carbohydrate, phosphorus, and nitrogen storage in buoyancy regulation. Carbohydrate storage area (mean = 2.06 ± 0.61 [SE] μm²; 6.62% of total cell area) in negatively buoyant colonies (NBCs) was significantly higher (P < 0.001) than in positively buoyant colonies (PBCs) (mean = 0.38 ± 0.06 μm²; 0.73%). Distinct diel periodicity of carbohydrate content was found in NBCs demonstrated by an increase from darkness to afternoon. Polyphosphate content was significantly higher (P < 0.001) in NBCs, with a mean of 0.44± 0.10 μm² (1.54%), as compared to PBCs, with a mean of 0.14 ± 0.05 μm² (0.24%). Polyphosphate content increased in NBCs from morning to evening, and PBCs had a 10% decrease from morning to afternoon. Calculations indicated that averaged effects of polyphosphate on increased cell density is approximately 20% of that from carbohydrate accumulation. Density contribution due to ballast weight of carbohydrate and polyphosphate indicated that NBCs were 12 times more dense than PBCs. Mean area of cyanophycin granules (N storage) was not significantly different between PBCs and NBCs. In conclusion, Trichodesmium tenue can regulate buoyancy by carbohydrate ballasting similar to that noted in limnetic cyanobacteria. Polyphosphate storage and possibly nitrogen storage products play a significant role in buoyancy regulation.     

Functional significance of bone ballastin in the evolution of buoyancy control strategies by aquatic tetrapods

The primary function of pachyostosis, pachyosteo‐sclerosis, and osteosclerosis may be to act as ballast, not so much (as previously suggested) to neutralise the buoyancy of existing lungs, but to allow enlargement of the lungs. Enlarged lungs cause an animal to lose buoyancy more rapidly with depth. They also provide a larger oxygen store. These features are useful for slow swimmers and shallow divers, such as feeders on benthic plants and invertebrates. Examples are sirenians, primitive sauropterygians ("not‐hosaurs"), placodonts, and the sea otter Enhydra. These last two show convergent evolution of adaptations to feeding on hard‐shelled invertebrate prey in shallow water. Mesosaurids are problematical. Bone ballast uses body mass and volume less efficiently than other buoyancy control strategies. Theoretical analysis predicts that bone ballast should not occur in semiaquatic forms, fast swimmers or deep divers. It does not usually occur in such organisms. Marine iguanas of the Galápagos, desmostylians, and the aquatic sloth Thalassocnus are all littoral feeders and all lack bone ballast as predicted. Plesiosaurs adopted varied strategies: some used bone ballast, and others used gastroliths. Biomechanical considerations lead to the prediction that a new marine tetrapod clade will typically evolve bone ballast as part of its adaptation to life in water. Slow swimmers and grazers on sessile food, like sirenians and placodonts, develop it more strongly, but active predators like ichthyosaurs and cetaceans secondarily lose this character.