BIOLOGY 457/657 PHYSIOLOGY OF MARINE & ESTUARINE ANIMALS March 29, 2004 MOVEMENT & BUOYANCY

MUSCLE FUNCTION: Vertebrate Swimming Muscle Energy reserves are usually creatine phosphate. Red muscle: aerobic, slow-twitch (tonic); and usually with unineuronal innervation. Used in “cruising”-type swimming. White muscle: anaerobic (glycolytic), fast-twitch; usually with polyneuronal innervation. Used for fast bursts. In tunas, it is the red muscle that is kept warm by the countercurrent heat exchanger. In sharks, on the other hand, the red muscle is exterior to the white muscle (see next slide).

Vertebrate Swimming Muscle (2) From Prosser (1991) From Schmidt-Nielsen (1990)

Crustacean Muscle Receives multiple innervation with different types of motor neurons: (1) Slow excitatory (2) Fast excitatory (both use acetylcholine as a neurotransmitter) (3)Inhibitory (uses GABA as a neurotransmitter) From Schmidt-Nielsen (1990)

Crustacean Muscle (2) Energy reserves are usually arginine phosphate. Arrangement is often pinnate for great strength. From Schmidt-Nielsen (1990)

LOCOMOTION IN WATER: Buoyancy Swimming in water is much like flying in air, although the density and viscosity of water limit maximum speeds. However, the density of water also permits animals to float, reducing metabolic requirements at rest. To optimize the ability to float, animal density should be equal to that of water. But since most animals contain some dense material (bone, shell, etc), the rest of the tissues must have a density lower than that of seawater. How can this be done?

Creating Buoyancy (1) Reduce the amount of heavy substance like calcium carbonate or calcium phosphate. Examples: pelagic medusae, nudibranchs (2) Replace heavy ions (Mg 2+, SO 4 2- ) with light ones (e.g. Na +, Cl -, H +, NH 4 + ). Examples: Noctiluca, some squids (3) Reduce concentrations of ions. Examples: marine vertebrates (4) Increase the quantity of light substances, like fats, oils, waxes. Examples: many animals, including copepods and sharks. (5) Use gas floats or swimbladders. Examples: jellyfish, cephalopods, teleost fishes

Buoyancy: Advantages and Disadvantages From Schmidt-Nielsen (1990)

Buoyancy: Gas Floats Problems of a gas float in water: (1)If flexible, the float is very unstable with any vertical movement. (2)If rigid, it must be able to withstand substantial pressure – this can make the casing heavier than the buoyancy gained by the gas inside! (2)The gas must be contained at partial pressures equal to the total hydrostatic pressure of the surrounding water; 1 atm/10 m depth. But the bases in solution in the body are normally in equilibrium with partial pressure of gases in water, at <1 atm total. This creates serious problems both with inflation of the float and keeping the gas from diffusing away. From Schmidt-Nielsen (1990)

Buoyancy: Flexible floats in siphonophores Here, the float is used simply to keep the organism (a colony of hydroids) at the surface of the water. It contains CO, produced from serine. CO is also used by some planktonic (midwater) siphonophores. (Photo shows the Portuguese Man-of- War, Physalia physalia)

Buoyancy: Rigid floats in cephalopods About 9% of the volume of the cuttlefish (Sepia) is made of cuttlebone, with a density of about 0.6 g/cm. The hollow cuttlebone is supported by a framework of calcium carbonate with columns to hold the layers in place.

Buoyancy: Rigid floats in cephalopods In Nautilus, the float is the shell of the animal. Water is removed from each chamber by the central tube, called a siphuncle.

Buoyancy: Rigid floats in cephalopods The gas in the float is N 2 with a trace of O 2. The gas is formed by removing water by osmosis; nitrogen then diffuses into the vacuum so formed. Since the cuttlefish is a marine osmoconformer, it maximum osmotic gradient (hemolymph vs. fresh water) is about 22.4 atm (equivalent to 224 m depth). The cuttlebone itself can withstand about 25 atm of pressure! Interestingly, Nautilis frequently lives deeper than 250 m, so something fishy is going on. sh-cuttlefish.jpg oceanlink.island.net/pictures/ nautilus.gif

Buoyancy: Swim bladders in fish Generally is found just below the vertebral column, occupying about 5% of the body volume. This type of float achieves perfect buoyance at only one depth, but has been found in fish to 4000 m depth! The major gas in the swimbladder is O 2 ; N 2 and CO 2 are also present. The swimbladder is lined with thin sheets of guanine, which reduces its permeability to the enclosed gases. Nevertheless, the interior gas is in contact with blood. Why doesn’t the gas simply diffuse into the blood? How does the gas get there, anyway?

Buoyancy: Swim bladders in fish Gas retention: The blood supply to the swimbladder is double. A dorsal vessel serves only to remove gas from the swimbladder when required. The major supply is at the gas gland (gas-secreting gland, where the rete mirabile is found. The blood supply in the capillaries of the rete are (you guessed it!) countercurrent in flow. Thus, gas is retained passively. Gas secretion: The major obstacle is the secretion of gas from blood, with a total partial pressure of 100 atm in some cases.

Buoyancy: Swim bladders in fish Oxygen: At the gas gland, lactic acid is secreted, reducing the pH in the blood in the vicinity of the gland. Due to the Root effect, the ability of hemoglobin to bind O 2 is reduced, and the excess O 2 moves into solution and eventually diffuses into the spaces in the swimbladder. The Root effect reduces the solubility of O 2 in the blood, while simultaneously allowing for a great increase in blood PO 2. The rete as a whole acts as a countercurrent multiplier. Gas secretion: As the lactic acid dissolves in the plasma, the solubility of all gases decreases. Thus, N 2, CO 2, and any other gas present will tend to enter the swimbladder. This is called “salting out”.