Introduction

Not all animals glide or fly in the air. Many marine animals are masters of "flight" and speed under the water. The ocean environment brings its own set of adaptations and specializations for the animals that move through it.

Underwater Dynamics

Archimede's principle states that an object immersed in water is buoyed by an upward force equal to the weight of the water the object displaces. A fish, for example, will float if it weighs less than the amount of water it displaces. Most fish, however, weigh more than the sea water and will therefore sink to the ocean floor. For ground-dwelling fish this is not a problem but most fish spend a large part of their life up in the water and therefore need to be able to get off the ocean floor. These fish, especially the slower swimmers, have an organ called a swim-bladder, a built-in gas filled chamber, that increases the volume of the fish without increasing the weight. This increases the volume of water displaced by the fish so that the water weight is greater than the fish's weight, thus bouying the fish upward. Some fast-moving fish and sharks do not have a swim bladder. Therefore, in order to stay up in the water, they need to keep moving. Sharks do have an oil-rich liver that helps to reduce their body weight.

In addition to gravity and buoyancy, fish also have to deal with the fact that water is about 800 times denser than air and has a high viscosity. Viscosity is the internal friction of a fluid and combined with the density, makes water much more difficult to move through than air. Salty sea water has an even higher density and viscosity than freshwater so ocean animals have to work harder than their freshwater counterparts. When a fish moves through the water it experiences drag, a function of the fish's speed, shape, body surface and the type of water flow across the body. Drag comes in two different forms: pressure drag which is a direct function of body design and streamlining and friction drag which depends on the surface of the skin.

To combat pressure drag, fish have streamlined bodies that move through the water with the least possible resistance. Fish bodies come in a variety of shapes and sizes depending on where they live, what they eat, how they eat, how fast they swim, etc. Fish bodies can be fusiform (torpedo shaped) like sharks and barracuda, laterally compressed (flattened side to side) like angelfish and butterfly fish, dorsoventrally compressed (flattened top to bottom) like skates and rays, attenuated like the moray eel and any combination of the above. Some of the fastest fish in the ocean (tuna, mackerel and jacks) have a somewhat flattened fusiform shape with a reinforced, narrow tail base. Many laterally compressed coral reef fish have extreme maneuverability which helps them move in and around the coral. Dorsoventrally flattened fish are well suited for life on the ocean floor.

Fast swimming fish often have special features to improve the streamlining including fins that can be tucked away in special grooves when moving quickly. The fastest swimmers often have fins positioned behind the deepest part of the body to reduce drag when not in use.

As the fish moves, water flows over its body. This flow can be classified as laminar (smooth), turbulent (irregular) and transitional. Laminar flow causes the least amount of drag and turbulent flow causes the most drag. Streamlining of the bodyshape promotes laminar flow. Turbulence occurs when the thin layer of water right along the body (the boundary layer) becomes unstable.

Fish have several different ways of dealing with friction drag and thus achieving more laminar flow over their bodies while swimming. Many fast swimmers are covered with a slimy mucous layer secreted from glands beneath their scales. The long-chained slime molecules help stabilize the water into laminar flow. In barracuda, for example, the slime layer reduces drag by at least 60%! Sharks have denticles, or small teeth, on their skin that are arranged in the direction of the water flow over the body. These denticles may trap a thin film of water along the skin to form a stable boundary layer with little drag.

Dolphins have confused scientists for years because in order to reach speeds of thirty miles per hour, their bodies must be generating pure laminar flow. However pure laminar flow had never been observed in an object that size going at that speed. Recent studies have shown that marine mammals such as dolphins and whales have a pressure-sensitive, flexible skin that dampens out turbulence in the boundary layer. When the boundary layer is thickened by turbulent eddies the skin is depressed and vica versa. This requires no energy from the dolphin and greatly improves its swimming performance.

The fins of the fish are crucial for swimming through the water. Fins consist of a fan of skin supported by skeletal rays. Muscles attached to these rays allow the fins to be moved side to side as well as along the body causing waves to travel the length of the fin. Reverse these waves and the fish can also swim backwards.

Fish fins can be placed into two categories. Median (vertical) fins originate along the midline of the body and include the dorsal fins on the back, the caudal or tail fin, and the anal fin on the bottom, just behind the vent or anus. Paired fins are on the sides of the body and include the pectoral fins just behind the head and the pelvic or ventral fins located below and behind the pectorals. Fish use this combination of fins for stability, propulsion, steering and braking.

While swimming, the fish is subjected to various forces other than gravity, buoyancy, and drag. These forces include yaw, pitch and roll. If the fish yaws, or turns on its vertical axis, the dorsal fin is erected. The pectoral fins are extended to resist pitching, when the fish rotates up and down on its transverse axis. The pectoral fins also act as hydroplanes to raise the front end of the fish while the paired pelvic fins bring up the rest of the body so the fish swims horizontally. If the fish rolls on its longitudinal axis, all fins are extended.

The caudal or tail fin is the main source of propulsion for most fish. Fish tails come in different forms depending on the lifestyle of the fish. The main job of the tail is displacing the necessary amount of water needed with each tail stroke in order to move the fish. A broad tail is useful for fish who need to make quick starts from a stand-still. Long narrow tails are more efficient for fast, long-distance swimmers. Some fish have an uneven (heterocercal) tail where the spinal column extends into the larger lobe of the fin. This is often found in sharks and other fish without a swim bladder because the upper lobe gives an upward thrust to keep the fish horizontal in addition to forward thrust. Many fish have an even (homocercal) tail with both lobes symmetrical. This type of tail produces only forward thrust without the added upward thrust. The broad tail of the grouper allows it to accelerate quickly while the round tail of the blackfish creates cavitation and a sonic boom. The fast swimming tuna has a crescent shaped tail with a narrow base that acts as a hydrofoil, creating little drag. (*A 180 millimeter trout can go from zero to 1.33 meters per second with one double-flick of its tail!)

When fish swim, they undulate their bodies in an S-shaped wave. This wave motion begins at the head and travels down the body toward the tail. The body and tail movements create a backward directed jet of water which propels the fish forwards. Attenuated fish, like the sea snake and moray eel, have a traveling wave that increases in size and decreases in length as it progesses down the body. Because their small pectoral fins are useless for swimming, the eel uses its body movements to "push" against the water and move forward. Rigid-bodied fish like the cowfish and trunkfish are not able to bend their bodies so they rely on their short tails to move them. Most other fish fall in the middle, combining the waving of the tail with subtle body waves.

The fish also uses its muscles as a major source of propulsion power. The muscles along the fish's side are its strongest and are what contracts the body in undulations as it swims. The W-shaped muscle segments, called myomeres, contract on one side and relax on the other, allowing the fish to push against the water as it bends to one side and then the other. The muscles have different colors depending on where they are located and how they are used. The white muscles are the larger group, found throughout most of the body. These fast muscles are used for powerful bursts of speed but they exhaust quickly. The red muscles are usually found just beneath the skin and are slow and virtually inexhaustible. These muscles are used to provide long-term power for slow, cruising speeds. Red muscles are aerobic which means they demand more oxygen. Therefore, a fast-moving fish like a tuna has a high oxygen demand. This is met by swimming with its mouth open so that large quantities of oxygen-carrying water can pass its gills.

The fish's skeleton is quite strong and flexible and provides a strong foundation for the many contracting muscles. The backbone extends from the head to the tail and consists of many interlocking vertebrae that are jointed to allow side to side movement. Some fish even have a bony keel at the base of the tail that smoothly cuts through the water as the tail moves from side to side.

Cetaceans (Dolphins and Whales)

Dolphins and whales, like fish, spend their whole lives in the water. However, unlike fish, they are mammals. This means that they are warm-blooded, they have live births, they nurse their young and they need to breathe air. These animals used to live on land but returned to the seas about 70 million years ago. They re-adapted to their ocean environment by assuming a fusiform body shape, losing any narrowing between the head and the body and becoming round or cylindrical to reduce drag. Their front limbs evolved into flat, paddlelike pectoral flippers and their hind limbs disappeared. The tail has become the flukes which, combined with the powerful trunk muscles, provides the main propulsive power that allows these animals to swim and dive. A big difference between fish and dolphins is that a fish's tail moves from side to side and a dolphin's tail moves up and down. The dolphin's flukes and trunk muscles are so powerful that they can even push the dolphin straight up out of the water.

As mentioned earlier, dolphins and whales are mammals and therefore need to breathe air. Their nose is located on top of the head and is called a blowhole. When the animal needs to breathe, it swims up to the surface of the water, opens its blowhole and takes a breath. When the dolphin breaks the surface, it creates a wake that acts as a strong drag force and slows it down. The dolphin faces much less drag when it is completely submerged. Therefore, when a dolphin is traveling at high speeds, it will jump clear out of the water in order to get the necessary breath of air. This greatly reduces the amount of time spent fighting the pull of the wake and does not slow the dolphin down.

Flying Fish

The flying fish is specially adapted to get up and out of the water in order to escape from predators. Some fish have been seen at heights of 36 feet above the water's surface. A flying fish may soar at speeds of up to 35 miles an hour for distances of 250 yards or more in a single glide! In order to get out of the water, the fish gathers speed swimming toward the surface. As it leaves the water, the pectoral and pelvic fins are extended, providing enough lift for the fish to completely clear the water. Flying fish do not actually fly, instead they glide across the surface of the water. As the fish returns to the surface, its tail moves quickly from side to side, allowing the fish to take-off again.

Rays

Animals like rays and skates use their pectoral fins to move through the water but in a very different way. Instead of moving them horizontally (sideways) like a fish, a ray moves its pectoral fins vertically in an S-shaped wave, beginning at the side of the head and moving toward the base of the tail. They are able to change the pitch of the fin in order to get the most efficiency during the upbeats as well as the downbeats of the fin. Rays do not have a swim bladder and spend most of their time lying on the ocean floor.

Hydrofoils

Many animals swim through the water using their flippers as hydrofoils. A cross-section of a sea turtle's flipper resembles an airplane wing and is subjected to the same types of forces when the animal is swimming. As the turtle's flipper strokes downward, the leading edge is tilted slightly forward about 25 degrees, creating lift and propulsion. When the flipper comes back up, the lift is cancelled and the turtle is propelled forward. Sea lions also use their flippers as hydrofoils. Their long front flippers are used to pull them through the water, while their smaller hind flippers are used for steering. Seals, on the other hand, do not have large, well-developed front flippers and use vertical movement of their hind flippers and tail to propel themselves through the water.

Jet Propulsion

Another, very different form of locomotion in the water is jet propulsion. The simplest example of this can be seen in jellyfish. These animals fill their umbrella section with water and then push the water out, sending the jellyfish in the opposite direction. This type of movement does not allow much control over direction. More complex relatives of the jellyfish are the colonial organisms such as the Portugese man-of-war. Some of the individuals in this colony are specialized for movement, some serve as gas-filled floats for buoyancy control, and others serve for prey capture, digestion, or reproduction. Salps are another simple animal that uses jet propulsion to move. These animals are related to sea squirts and live in large chains, some as long as 100 feet. Each individual salp has two siphons, one for taking water in and one for expelling water. They have bands of muscles easily seen in their transparent bodies that contract and expand, forcing water in and out of the siphons. This propels the animals.

A more complex animal using jet propulsion is the squid. Some squid are able to reach speeds high enough to shoot them out of the water and onto the decks of passing ships! The squid has a muscular mantle which, when expanded, fills with water. When these muscles contract, water is expelled through a single siphon and the squid is propelled the opposite direction. Unlike jellyfish, the squid can control its direction by rotating the siphon. Often the expulsion of water is accompanied by a puff of dark ink from the squid's ink sac in order to deter predators from following.

Several species of bivalves (two-shelled animals) such as scallops and clams also use jet propulsion. These animals do not normally move about but when a predatory sea star approaches, the scallop can sense its presence and make its escape. This is accomplished by rapidly clapping the two shells together with contractions of the scallop's adductor muscles. Water is forced out between the two shells in two jets, sending the scallop to safety. This form of propulsion is very exhausting and cannot be maintained for any length of time. Fortunately, the scallop's predators are slow movers.

Click on the picture to see the clam move!

Explore Space ... Not Drugs!
Hear what astronauts have to say about staying drug-free.
Last modified: Sat Dec 20 16:30:07 PST 1997

Copyright © 1997 by Cislunar Aerospace, Inc. All Rights Reserved.