How do animals live in water? by Dr. Adriana Aquino

Life began in the ocean, and many characteristics of land animals reflect our watery past. Yet it’s nearly impossible to imagine living in the ocean given how much paraphernalia we need to venture beneath its surface, and how quickly we drown if that equipment fails — not to mention trying to eat or sleep there!

How do marine organisms cope? Water is about 800 times denser than air, and 50 times thicker and more viscous (resistant to flow). How can a jellyfish keep from sinking into the depths? How can an anemone fixed to a reef get enough to eat? How can a blue whale locate a mate on the other side of the ocean? How does a marlin, one of the fastest animals on Earth, slip through water so fast? Water also contains about 95 percent less available oxygen than air. How do marine animals get enough of it to survive? How do organisms carry out the fundamental activities of life — breathing, moving, eating, and reproducing — in this dense, oxygen-poor environment?

A striped bass has several features that aid in its movement: Gas bladder: controls buoyancy; helps fishes move up and down or hover in place (illustration shows the relative position of the gas bladder within the body cavity) Spiny dorsal fin: controls roll and provides protection. Soft dorsal fin: controls roll (horizontal rotation) Pectoral fins: provide steering and braking Pelvic fins: controls pitch (up and down tilt) Anal fin: controls roll (horizontal rotation) Caudal fin: provides power and thrust; curved shapes minimize drag ©AMNH

How do marine organisms keep from sinking — and get where they want to go?

Whether a microscopic larva or a barracuda lunging into a school of herring, every ocean-dweller is subject to water’s physical and chemical characteristics. Two universal principles affect how things move through water.

• Water is dense and viscous, and it impedes motion more effectively the slower the organism. Water’s density generates resistance to movement, but also keeps small particles from sinking.
• Objects of different shapes but similar weights move at different rates through water.

If they can’t swim

Because living things are usually denser than seawater, they tend to sink. This is a particularly big problem for very small organisms that drift and cannot propel themselves upward. Some rely on the movement of ocean water. Surface waters are heated during the day and rise, and cool at night and sink, gentle motions that propel vast bodies of plankton up and down in the water column.

Smallness has its advantages. The smaller an object, the greater its surface area relative to its volume, so by remaining small, planktonic organisms offer far more surface area with resistance to water. Shape characteristics can also increase the surface of resistance. Drop a ball and a coin of the same weight into water and the coin will sink more slowly. Similarly, various planktonic organisms have evolved flattened body shapes, spines, and appendages to increase their surface of resistance (and perhaps to keep predators at bay as well).

Microscopic algae like the dinoflagellate Noctiluca replace heavy chemicals in their cells with lighter ones. Other algae, like diatoms, store food as oils to increase buoyancy (the capacity to remain afloat in a liquid). Zooplankton like the crustacean copepods also use oil for buoyancy, storing excess food in the form of oil droplets under their shells.

If they can propel themselves (swimmers)

Animals that are powerful enough to move at will in the water column, from sardines to whales, have an obvious advantage over drifters. But they still have to keep from sinking. Some animals have evolved ways to regulate their density. For example, many fish have developed gas or swim bladders, hollow body organs filled with gas. By regulating the pressure in these organs, fish control their buoyancy and move up or down in the water column at will. In a similar way, marine mammals use their hollow lungs for buoyancy control. Fish like bonito and mackerel swim too fast to have time to regulate their buoyancy with a swim bladder, relying instead on lift from their body surface and/or fins. Large amounts of lipids (fats) are present in the tissues of these and other fast-swimming fishes, primarily those that lack swim bladders like sharks. (Fat or oil is lighter than seawater — imagine an oil spill.) In marine mammals this fat takes the form of layers of blubber.

Sea turtles swim in a completely different way from land turtles that are able to swim, paddling through rivers and ponds with webbed feet. Sea turtles flap their long front flippers up and down, not forward and back, to fly through the water like birds. ©AMNH

How can animals move through water efficiently?

Moving around in water requires a lot of energy. But while water's resistance slows organisms down, it also makes possible certain types of locomotion that are unusual or absent on land.

If they can’t swim

Invertebrates — a huge group of animals that includes literally anything without a backbone, among them sponges (Porifera), corals, jellyfish and anemones (cnidarians), clams and snails (mollusks), sea-stars and sea urchins (echinoderms), crabs, lobsters and shrimps (crustaceans), and tube worms (annelids) — cannot generate much forward thrust. Consequently, they’ve evolved many different ways of moving through water. Some mollusks, such as scallops, squids and octopuses, move by jet propulsion, sucking in water and squirting it out again to generate thrust. Leeches swim by undulating their bodies vertically, unlike fishes, which move from side to side (except for specialized fish like moray eels). Many other invertebrates crawl and burrow in the mud; others simply float or drift wherever the currents take them; and some anchor themselves to the ocean floor and do not move around at all. Clams use hydrostatic pressure to move through mud and sand: the blood in their bodies flows from one large open space to another and can be used to inflate and deflate parts of their bodies, which serve as anchors. Although they belong to a different animal group, starfish also move using hydrostatic pressure to extend and retract suction cups on the bottom of their arms.

If they can propel themselves (swimmers)

Moving through water creates a lot of drag (the resistance to movement caused by the fluid through which an organism swims). It’s almost as though the water sticks to whatever is in it. The amount of drag depends on the viscosity of the water, and the speed, shape, and size of the moving organism. Since most of a fish’s energy goes to overcoming drag, swimming efficiently and having a shape that reduces drag are very, very important. Most fishes swim by pushing off against the water around them. Using the large muscles along the back half of their bodies, they press water backward and to the sides, creating a powerful forward thrust. These rhythmic tail thrusts propel them forward in a wonderfully efficient motion.

Viscosity creates another problem for large animals: it increases the turbulence both around the swimmer and in its trailing wake, thus intensifying drag. Some fish species reduce this effect through streamlining, which helps them “slice” through water. The classic streamlined shape is long and thin (like the teardrop shape of the typical fish), which gives the lowest resistance for the largest volume. It’s no coincidence that whales, porpoises, and dolphins, which have land ancestors that re-colonized the sea, share that streamlined shape. It reduces the energy it takes to get around in this dense, viscous medium. Other streamlining adaptations in marine mammals include very short hair or hairlessness, flattened mammary glands, and genitalia that protrude from the skin only when used.

A fish's body reflects its lifestyle. Some have broad, flexible tails and muscle tissue designed for sudden bursts of speed to attack prey or escape danger. Others have stiff, sickle-shaped tails and muscle made for long-distance cruising.

How can marine organisms get enough oxygen to breathe?

Two physical facts greatly affect how breathing happens in water:

• Water contains much less oxygen than air. Although the chemical formula of water is H₂0, the oxygen within the water molecule (the O in H₂0) is not available for organisms to use. Instead, organisms must use dissolved oxygen within the water. That’s why water actually contains about 95% less available oxygen than air: about 1% of its volume, compared to air’s 21%. To extract this small amount of oxygen, fish must force large volumes of water through their bodies, using up to 10% of that oxygen in the process itself!
• The larger the surface area of the body used for the exchange of gases, the more oxygen that can be extracted. Breathing, or getting oxygen, involves the exchange of gases through either the body wall, the membranes of the gills or the lungs. Water is always necessary for this process, and marine organisms have the advantage of being immersed in water. (Although in their evolution terrestrial animals stepped out from water, their lungs still have to be moist to function properly).

In very small organisms, the exchange of gases that takes place simply through the skin is enough to meet their needs. As organisms become larger and need more oxygen, they must increase the surface area over which the exchange of gases can occur. One way to do this without increasing body size is through the development of gills: respiratory organs with many folds of skin and thus enlarged surface area. In fish, gills contain many filaments, each with thousands of tiny folds called lamellae, which greatly increase the surface area that comes into contact with water. Dissolved oxygen in seawater passes through thin membranes in the lamellae and enters the fish's blood, eliminating carbon dioxide to the seawater in the process. Inside the lamellae, blood flows in the opposite direction to the moving water, a counter-current system that makes gas exchange extremely efficient. About 75% of the oxygen in the water passing through the gills is extracted, twice as much as our lungs remove from a breath of air, which is how marine vertebrates compensate for its relatively low concentration.

Differences in gill development reflect the habitat of the organism. For example, amphibious fish that live in mangrove swamps and spend as much time out of the water as in it have fairly basic gills because they can also extract oxygen through their skin. In tidal pools, for example, organisms such as barnacles and bivalves may enclose respiratory organs in various ways to protect them from drying when the tide is out.

Recent research is showing that different species of jellyfish have diverse diets: from krill and plankton to 'sea snow' to fish. ©Norbert Wu

How do marine organisms eat?

The same properties that make water hard to move through (density and viscosity) also create options for feeding and reproduction that aren’t possible on land or in air. Imagine bits of food floating in the air available for the taking! That’s what happens in seawater, which is filled with small particles: microscopic algae called phytoplankton: microscopic animals called zooplankton; numerous eggs and decaying bits of plants, animals, and fecal matter. Many invertebrates and a good number of fishes are able to collect these tiny food particles.

Some, like invertebrates anchored to the bottom, are suspension feeders: they survive by catching whatever particles drift by. Others actively collect them. Jellyfish pulse their bodies to create currents that draw particles under their bell, where the food can be collected and eaten. These, and related anemones and corals, use stinging tentacles to capture prey; some have hundreds of limbs to increase their catch. Others, such as crinoids, collect food with enormous nets.

Imagine being able to suck in your food from a distance! Suction feeding is a very important mechanism for fish, and enables them to take advantage of many food sources. Most fishes have expandable heads with over 30 moveable bony parts (compare this number with the 22 bones that form our entire skull!). They rapidly expand the space in their heads, creating a low-pressure area that water rushes into when their mouths open. Everything in that mouthful is captured. They swallow the food and the water flows out over the gills. Fishes can expand their mouth volume from six to 40 times in a fraction of a second, with some generating pressures so high they can suck a limpet off a rock. The champion among weird eaters is the deep-sea gulper eel, which can unhinge its enormous jaws and stretch its stomach to consume a fish as big as itself. Sea horses are masters of suction feeding; they can suck in shrimp larger than their mouths, crushing them as they enter.

Another feeding method is filter-feeding, which is an efficient way to process a large volume of water without much effort. Clams, tunicates, sponges and other filter feeders pump water through their bodies, filtering out food according to size. The sea fan, a kind of soft coral, accomplishes this with a huge fan of tentacles that pick up particles that float by (though their main source of energy is provided by the symbiotic bacteria that live in its body wall). Marine vertebrates, such as whale sharks and manta rays, swim with their huge mouths wide open. As water flows out over their gills, rows of finger-like projections sieve the food particles from the water and pass it into the esophagus to be swallowed. Other big filter feeders are some types of whales, which suck up massive amounts of seawater and strain it through fringed plates called baleen, extracting all the krill (shrimp-like plankton).

Nudibranchs are simultaneous hermaphrodites: each individual is simultaneously male and female and can release either sperm or eggs while spawning. ©Ian Skipworth

How do marine organisms reproduce?

When it comes to reproduction, living in the ocean presents three major challenges: how to find a mate in the vast sea, how to synchronize the breeding cycle so that fertilization takes place, and how to guarantee dispersal of eggs and/or the young larvae. This is especially problematic for species that cannot move (are sessile) or move very little. One strategy for dispersal is to develop free-floating fertilized eggs or larvae, allowing for random dispersal. But being surrounded by water also offers some logistical advantages. Unfertilized eggs can stay suspended for long periods outside of their mothers’ bodies without drying out, so they can be fertilized by the more autonomous sperm. External fertilization allows many species to reproduce without ever having to seek out a mate, or even moving at all.

The latter brings another strategic problem: coordination in the release of eggs and sperm. Some invertebrates employ a strategy known as broadcast spawning, in which millions of eggs and sperm are released into the water at precisely the same time. The brief abundance greatly increases the odds of fertilization, and ensures that at least some will escape hungry predators. Many species time their breeding to cues in the environment. Seven days after the 12th or 13th full moon of the year, palolo worms (a kind of segmented worm) release the back ends of their bodies, which wriggle through the water releasing eggs or sperm. Many other animals, including corals and fishes, also engage in synchronous spawning. For example, most intertidal organisms, such as some bivalves, synchronize their breeding cycles with the occurrence of specific tides.

When it comes to finding mates, organisms living in the dark zones of the ocean are at a particular disadvantage. Many twilight species use bioluminescent light to attract mates. Among lanternfish, the arrangement of bioluminescent organs on the body differs between males and females, which has obvious advantages. Deeper down, where there are fewer animals spaced much farther apart, other mating strategies have developed. In many deep-sea species, males are much smaller than females, but are strong swimmers with a well-developed sense of smell for tracking down egg-laden females. Reproduction may involve changing sex. Some hermaphroditic fish begin life as small, mobile males, and change to females when they grow bigger. In other species, each fish has both male and female organs. This way, it can mate with any individual of the same species that it comes across in the dark, increasing the odds for reproduction. Among the weirdest cases of reproduction, the winners are some species of anglerfish, in which the males live as tiny parasites on the females. Males seem to have evolved only to find a mate and deliver sperm.

Traces of a watery past

From our perspective, the challenges of living in water seem enormous, from struggling to breathe to enduring the continuous thrashing of currents and tides. But since life (vertebrates included) originated in water, it’s worth remembering that we evolved from sea-dwellers and had to overcome the tremendous challenges involved in becoming terrestrial. How did our ancestors learn to cope with such an abundance of oxygen? How did they adapt to the burden of gravity? In some ways we still remain aquatic: humans are around 80% water; our senses of smell, sight, and taste all require water; all our membranes must be kept moist. In effect, we only survive on dry land by avoiding dehydration — protecting the aquatic environment within us.

Online Resources:

• AMNH: OLogy - Journey to the Bottom of the Sea
  Did you know sound moves five times faster in water than in air? Deepen your knowledge with this ocean life challenge.
  http://ology.amnh.org/marinebiology/journeytothebottom/
  
  
• AMNH: Hall of Ocean Life - Invertebrates
  Explore the form, function, and behavior of marine invertebrates.
  http://www.amnh.org/exhibitions/permanent/ocean/03_oceanlife/ai_eating.php
  
  
• AMNH: Hall of Ocean Life - Vertebrates
  Explore the diverse ways marine vertebrates live underwater.
  http://www.amnh.org/exhibitions/permanent/ocean/03_oceanlife/bi_eating.php
  
  
• AMNH: Resources for Learning - Water vs. Land
  An activity to imagine how your life would change if you lived in the water. What would you eat? And how would you get aroundespecially if, like a jellyfish, you didn't have a skeleton to support you?
  http://www.amnh.org/education/resources/rfl/web/oceanguide/activities/land.html