Buoyancy problems

Most protoplasm, cell walls, skeletons and shells have a density greater than seawater, and therefore tend to sink. The specific gravity of seawater is usually within the range 1.024-1.028. The overall density of much of the zooplankton is around 1.04 and of fish tissues about 1.07. Within a floating body the distribution of weight determines its orientation in the water. Therefore one of the problems facing pelagic organisms is how to keep afloat in a suitable attitude between whatever levels are suitable for their life.

The phytoplankton must obviously remain floating quite close to the surface because only here is there sufficient illumination for photosynthesis (Smayda, 1970). Plants which sink below the euphotic zone die once their food reserves are exhausted. Although animals are not directly dependent on light, the most numerous pelagic fauna are small herbivorous planktonts feeding on phytoplankton, and these must also remain fairly close to the surface to be within easy reach of their food. However, many of these herbivores do not remain constantly in the illuminated levels but move up and down in the water, ascending during darkness for feeding and retiring to deeper levels during daylight. Many carnivorous animals, too, both planktonic and nectonic, make upward and downward movements, often of considerable extent (see pages 140 ff.), and traversing ranges of temperature and pressure. These creatures must therefore be able to control their level in the water, and make the necessary adjustments to suit the conditions at different depths.

There are broadly two ways in which pelagic organisms can keep afloat and regulate their orientation and depth: by swimming, or by buoyancy control. In many cases the two methods function together.

A wide variety of marine creatures, both small and large, swim more or less continuously and control their level chiefly by this means. For example, dinoflagellates are said to maintain themselves near the surface by repeated bursts of upward swimming, alternating with short intervals of rest during which they slowly sink. In the laboratory, copepods seem generally to swim almost continuously, mainly with a smooth, steady motion but with occasional darting movements. When there are brief resting periods they usually appear to sink slowly; but Bainbridge (1952), using aqualung equipment, observed copepods in the sea and reported Calanus apparently hanging motionless in the water as if suspended from its long outstretched antennae, drifting without any obvious tendency to rise or sink. Among the larger crustacean zooplankton are many forms which swim almost ceaselessly, and seem to sink fairly quickly if swimming ceases, for example mysids, euphausids and sergestids. Some of the most abundant pelagic fish also keep afloat by swimming; for example, almost all the cartilaginous fish, and certain teleosts including mackerel and some tunnies.

Apart from flotation, swimming movements also fulfil several other important functions. They are obviously important for pursuit or escape, and in many forms they serve to set up water currents for feeding and respiration.

The alternative to swimming is to float by means of some type of buoyancy device. Gas vacuoles sometimes form in planktonic algae. Some pelagic animals have a gas-filled float, which in a few cases is so large that the animal has positive buoyancy and floats at the surface, i.e. pleuston. Examples are the large pneumatophores of some species of siphonophora (Physalia, Velella), or the inflated pedal disc of the pelagic actinians (Minyas). The pelagic gastropod, lanthina, floats at the surface attached to a raft of air bubbles enclosed in a viscid secretion. If detached from its raft, the animal sinks and apparently cannot regain the surface. In some of the attached brown algae of the seashore (Fucus vesiculosus, Ascophyllum nodosum, Sargassum spp.) the fronds gain buoyancy from air bladders within the thallus.

Many of the pelagic organisms which dwell below the surface have some method of controlling their overall density to match that of the water, in this way achieving neutral buoyancy so as to remain suspended. This is commonly done by means of some form of adjustable gas cavity. Some siphonophoran colonies (Nanomia and Forskalia) float below the surface suspended beneath a gas-filled pneumatophore. Nautilus obtains buoyancy from gas secreted into the shell, and in Sepia the cuttlebone is partly filled with gas. In numerous families of teleost fish there is a gas-filled swimbladder.

The volume of a gas varies inversely with pressure, so the pressure gradient presents special problems to those animals which rely for buoyancy upon gas-filled cavities. When a fish with a gas bladder moves upwards there is a tendency for the bladder to expand as the hydrostatic pressure falls. If this should occur, the fish would rapidly gain positive buoyancy and bob up to the surface, and the distension of the bladder would be likely to cause damage. Alternatively, during downward movement the bladder must tend to contract under increasing pressure, and this would cause loss of buoyancy and force the fish downward. Normally, of course, such changes of buoyancy are prevented by the ability of the animal to maintain the volume of the float virtually constant despite changes of external pressure. A fish with a closed swimbladder does this by absorbing gas during ascent, or secreting more gas during descent, thereby counteracting the tendency of the float to change volume as the pressure alters. However, these processes of absorption and secretion cannot take place very quickly, and creatures which rely for flotation on this mechanism are therefore unable to make very rapid changes of depth without danger. This effect shows dramatically in fish brought quickly to the surface in trawls, when the swimbladder often expands so greatly that it extrudes through the mouth like a balloon.

With increase of depth and pressure the density of gas in a swimbladder increases and the lift it gives decreases. For epipelagic marine fish to achieve neutral buoyancy by means of a swimbladder, the volume of the bladder must be about 5 per cent of total body volume. At deeper levels the density of gas is greater and the bladder must therefore occupy a greater proportion of body volume. Also, the greater the density of fish tissues, the larger the bladder necessary for neutral buoyancy. But the larger the bladder, the more difficult to control its size with rise or fall of pressure because of the large volume of gas that must be absorbed or secreted. Small bladders have the advantage that the fish can rapidly adjust its buoyancy as it changes level.

The lift provided by a swimbladder is equivalent to the force the fish would otherwise have to exert continually in upward swimming to maintain level if it had no swimbladder. As this force is approximately 5 per cent of body weight in air, neutral buoyancy clearly effects a great economy of energy. Also, because fish with neutral buoyancy do not need to use their lateral paired fins for dynamic lift while swimming, these fins are available for other functions. Used as paddles they give very fine control of movement and hovering. The evolution of teleosts from their heavily armoured ancestors has involved a progressive lightening of structure, giving greater ease of buoyancy control together with more efficient and precise locomotion. In some species the fins have also been modified and adapted to function as feelers, suckers, defensive spines, or for various roles in signalling and courtship.

Some creatures with gas-filled floats can release gas through an opening; for example, the siphonophoran Nanomia controls the buoyancy of its pneumatophore by secreting gas into it, or by allowing gas to escape through a sphincter-controlled pore. An interesting feature of the gas in this and several other siphonophora is that it contains carbon monoxide, a possible advantage being its low solubility in water. There are some fish which have a connection between the swimbladder and the exterior via the gut. This is commoner in freshwater fish than in marine species but is found in the Clupeids (herring, sprat, pilchard, etc.) and in some others. Some of these fish can release pressure during ascent by allowing gas to escape from the bladder through the mouth or anus; at the surface, air can be swallowed and forced into the bladder. However, in most marine teleosts, if a swimbladder is present, it is closed. In those that live at great depths the gas is at high pressure and gas-secreting structures are enlarged; for example in Myctophidae (lantern fish), Sternoptychidae (hatchet fish) and some Gonostomatidae. Below 1000 m few of the pelagic species have a swimbladder, although in some cases it is present during the larval stages, which are spent nearer the surface. In certain bottom-dwelling fish highly developed swimbladders are found down to about 7000 m (in some Macrurids, Brotulids and Halosaurs), probably associated with a need for neutral buoyancy to enable the fish to hover just above the sea floor, i.e. benthopelagic forms.

Apart from acting as a buoyancy device, the swimbladder has additional functions. It is a pressure-sensing organ whereby buoyancy is automatically adjusted to changes of level. Also, it probably has a role as a detector of vibrations in the water, and in some species has connections with the ear, indicating a hydrophonic function. In certain fish it is evident that the swimbladder can itself be set in vibration to function as a sound-producing mechanism, perhaps for communication, defence of territory, courtship or echo location. Examples of fish producing sounds in this way include the Drums (Sciaenidae), Gurnards (Triglidae), toad-fishes (Batrachoidiformes) and some gadoids such as haddock and cod.

Some cephalopods obtain flotation from gas but have a different method of regulating buoyancy from that of fish. In the cephalopods the problems of pressure change are avoided by enclosing the gas in a rigid-walled container which cannot appreciably change volume as depth alters. For example, Sepia has gas in its cuttlebone, which contains spaces filled partly with gas and partly with liquid. The overall density of the cuttlebone varies between about 0.5 and 0.7, but the mass of gas within it seems to remain constant, and the density of the bone is controlled by regulating the amount of liquid it contains. The siphuncular membrane on the underside of the cuttlebone apparently acts as a salt pump, controlling osmotically the quantity of liquid in the bone. By maintaining the concentration of the cuttlebone liquid below that of the blood, sufficient osmotic pressure is generated to balance the hydrostatic pressure of the surrounding water. The distribution of gas within the cuttlebone also assists in maintaining the normal orientation of the animal in the water. There is a similar mechanism in

Nautilus, which also has gas and liquid in its shell and adjusts its buoyancy by controlling the quantity of liquid.

Instead of gas-filled floats with their attendant difficulties of control, many pelagic organisms obtain buoyancy from liquids of lower specific gravity than seawater in a way similar to the bathyscaphe (see page 86). Liquid-filled floats have the advantage of being virtually incompressible; but because of their higher density they must comprise a much greater proportion of the organism's overall volume than is necessary with gas-filled floats if they are to give equivalent lift. We have referred earlier to the probability that the large central vacuole of diatoms contains cell sap of low specific gravity, conferring some buoyancy (see page 28). In young, fast-growing cultures, diatom cells often remain suspended, or sink only very slowly, although in older cultures they usually sink more rapidly. Studies of the distribution of diatoms in the sea suggest that some species undergo diurnal changes of depth, usually rising nearer the surface during daylight and sinking lower in darkness, possibly due to slight alterations of their overall density effected by changes in specific gravity of the cell sap, or in some cases by formation or disappearance of gas or oil vacuoles in the cytoplasm. Pelagic fish eggs during the later stages of their maturation gain buoyancy by the follicle cells secreting into them dilute fluids containing smaller quantities of salts than in seawater. In radiolaria, the vacuolated outer protoplasm is probably of lower specific gravity than the water, giving buoyancy to the whole. Changes in the number of vacuoles cause the cells to rise or sink. Some of these animals possess myonemes with which the outer protoplasm can be expanded or contracted, and this presumably alters the overall density and may provide a means of changing depth.

Many pelagic organisms contain considerable quantities of oil. This may well serve a dual purpose, acting as both a food reserve and a buoyancy device. The specific gravity of most of these oils averages about 0.91 but some, e.g. squalene (see page 134) are considerably lighter. Oil droplets are common inclusions in the cytoplasm of phytoplankton, and the thermal expansion of these oils may be of some significance in effecting diurnal depth changes. Many zooplanktonts also contain oil vacuoles, or oil-filled cavities; for example, radiolaria, some siphonophora, many copepods and many pelagic eggs. The body fluids of marine teleosts are more dilute and less dense than seawater, and therefore confer some buoyancy, and many species derive additional buoyancy from accumulations of fat. In the abundant oceanic fish, Cyclothone, there is fat in the swimbladder and tissues amounting to about 15 per cent of the total volume of the fish.

The thick fat layer in the skin of marine mammals provides both heat insulation and buoyancy. In sperm whales Physeter macrocephalus, which often weigh some 30 tonnes or more in air, additional buoyancy is obtained from the large mass of clear white spermaceti wax enclosed in a reservoir in the snout (Clarke, 1978). A 30-tonne whale probably contains about 2-3 tonnes of this oil. During deep dives these whales seem to have fine control of neutral buoyancy, enabling them to lie virtually still in the water. It is most likely that this control is effected by regulating the temperature of the oil. It seems that water is drawn into the huge nasal passages (the right one is 5 m long and up to 1 m in diameter) and that this cools the surrounding spermaceti wax just enough to increase its density such that neutral buoyancy is achieved. By controlling the amount or distribution of water of different temperature from different depths in these passages, it can change the density of the wax to suit its buoyancy needs.

The gelatinous tissues which are a feature of many pelagic organisms, for example medusae, siphonophora, ctenophora, salps, doliolids, heteropod and pteropod molluscs, have a slight positive buoyancy which affords flotation to the denser parts. The tissue fluid is isosmotic with seawater but of a lower specific gravity due to the replacement of sulphate ions with lighter chloride ions. The small deep-sea squids of the family Cranchidae, mainly 1-2 cm in length, gain buoyancy from coelomic fluid of slightly lower density than seawater contained in their capacious coelomic cavities. About two-thirds of their weight is made up of coelomic fluid, isosmotic with seawater but of low density due to its high concentration of ammonium ions. Some other families of squids also attain near neutral buoyancy in this way (Clarke et al., 1979). The dinoflagellate Noctiluca also gains buoyancy from a high concentration of ammonium ions, exclusion of relatively heavy divalent ions, especially sulphate, and a high intracellular content of sodium ions relative to potassium.

In those mesopelagic fish which lack swimbladders there are several ways in which body weight is reduced. Compared with species having swimbladders the body fluids are more dilute, the haematocrit is lower, the blood is of lower viscosity and the tissues contain more fat and less protein. Apart from the jaw structures much of the skeleton is lightly developed and relatively poorly ossified, and the associated muscles correspondingly reduced. The heart is smaller and the red part of the myotomes which functions mainly for sustained swimming is less developed than the white musculature used for the brief, rapid movements of escape or attack. Denton and Marshall (1958) have drawn up a 'buoyancy balance sheet' comparing a coastal fish, Ctenolabrus rupestris, which has a gas bladder, with a bathypelagic fish, Gonostoma elongatum, in which the swimbladder is degenerate and filled with fat (Figure 4.9 and Table 4.4). According to this reckoning, the deep-water fish is only slightly heavier than the weight of water it displaces, and presumably does not need to make much effort to maintain its level. Gonostoma elongatum migrates daily through some 400-500 m, and it is possible that such fish may exert some control of buoyancy by alterations in the ionic content of their body fluids.

In elasmobranchs, none of which has swimbladders, the overall density varies considerably in different species in ways related to their different shapes and modes of life. Some species approach or attain neutral buoyancy due to the relative lightness of their skin, skeleton and muscles. Lift is provided by

Figure 4.9 Diagram of the ebuoyancy balance sheet3 for a bathypelagic fish Gonostoma elongatum without a swimbladder (below) and a coastal fish Ctenolabrus rupestris with a swimbladder (above). Positive values are given for those components of the fish which are heavier than the seawater which they displace and thus tend to 'sink3 the fish, whilst negative values are given for those components which displace more seawater than their own weight and thus tend to 'float' the fish. Weights given per 100 g offish. Dil. Flu., dilute body fluids; Sk + C, skeleton and other components.

(From Denton and Marshall (1958), published by Cambridge University Press.)

Figure 4.9 Diagram of the ebuoyancy balance sheet3 for a bathypelagic fish Gonostoma elongatum without a swimbladder (below) and a coastal fish Ctenolabrus rupestris with a swimbladder (above). Positive values are given for those components of the fish which are heavier than the seawater which they displace and thus tend to 'sink3 the fish, whilst negative values are given for those components which displace more seawater than their own weight and thus tend to 'float' the fish. Weights given per 100 g offish. Dil. Flu., dilute body fluids; Sk + C, skeleton and other components.

(From Denton and Marshall (1958), published by Cambridge University Press.)

low-density oil, especially in the liver, and also from other non-fatty tissues of low density, notably the subcutaneous layer or masses of gelatinous tissue. Some elasmobranchs may be able to change buoyancy by regulating their lipid content. When elasmobranchs are swimming they gain dynamic lift from the plane surfaces of their pectoral fins, which function also for changing direction or braking.

It appears that many of the bottom-dwelling elasmobranchs of shallow water have a relatively high density, probably associated with a fairly inactive mode of life and a habit of resting on the bottom. But other bottom species are close to neutral buoyancy, especially those of deeper water; these are probably more active benthopelagic fish which cruise above the bottom in search of food, which at deep levels is more scarce.

The pelagic predatory sharks, swimming rapidly in pursuit of prey, have densities which result in nearly neutral buoyancy. This enables them to be fully manoeuvrable with fins which are quite small and therefore add little frictional

Table 4.4

(a) Balance Sheet for the goldsinny wrasse Ctenolabrus rupestris

Component Percentage Weight in wet weight seawater/100 g of fish

Protein 16.6 +3.8

Body fluids 73.3 -0.9

Other components including bone 9.2a +2.6!

Buoyancy. This fish without its swimbladder has a weight in seawater of +5.4 per cent of its weight in air.

aThese values are given by difference.

(b) Balance Sheet for the deep-sea fish Gonostoma elongatum

Component Percentage Specific gravity Weight in seawater/

(b) Balance Sheet for the deep-sea fish Gonostoma elongatum

Component Percentage Specific gravity Weight in seawater/

wet weight

100 g of fish

Fat

3.7

0.91a

-0.5

Protein

5.0

1.33b

+ 1.1

Body fluids (water + dissolved salts)

87.6

1.013

-1.2

Other components including bone

3.2c

+1.1c

Buoyancy. These fish had no gas-filled swimbladder and their average weight in seawater was approximately +0.5 per cent of their weight in air (wet weight).

Buoyancy. These fish had no gas-filled swimbladder and their average weight in seawater was approximately +0.5 per cent of their weight in air (wet weight).

aHandbook of Biological Data (1956). Ed. W.S. Spector. Ohio, USA.: Wright Air Development Center. bHober, (1954). Physical Chemistry of Cells and Tissues. London; Churchill. Specific gravity taken as the reciprocal of the partial specific volume. cThese values are given by difference.

drag to their well-streamlined forms. There are other pelagic species which are slow-moving and feed by filtering plankton, and these are of two shapes with different densities. The shark-shaped form Cetorhinus (basking shark) gains little lift from its small fins but can remain afloat while swimming slowly because it has almost neutral buoyancy. This is largely due to the low-density oil squalene (specific gravity 0.86 at 20°C), found in the huge liver, and a mass of gelatinous tissue in the nose. On the other hand, the manta rays (Mobula) have a relatively high density but gain such enormous dynamic lift from their large plane surfaces that they can leap out of the water.

The buoyancy afforded by the water is of course important in reducing the skeletal needs of aquatic organisms, which can be very lightly built compared with terrestrial forms. This has made possible the evolution of some extremely large marine creatures. The enormous present-day whalebone whales, for example the blue whale Balaenoptera musculus and the fin whale B. physalus, may reach weights of the order of 100 tonnes. In comparison, the largest known terrestrial animals, the huge Mesozoic reptiles, probably did not exceed some 30 tonnes. The rapid death of large whales on stranding is often caused by the collapse of the rib cage when the massive weight is unsupported by the water.

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