Living in Water
Chapter Video Introduction
All aquatic species, including fish and other aquatic animals, are uniquely adapted to living in water or near water.
Chapter 4 - Living in Water
You walk beside a pond. A mallard duck flies overhead. A bluegill swims in the water. Humans, mallards, and bluegills are distinct species. A species is a group of individuals sharing some common characteristics or qualities, and whose offspring also share those characteristics or qualities. All species are specially suited for the lives they lead. Humans are walkers with legs, allowing for effective movement on land. Mallards are fliers with wings, which allow them to move through the air. Bluegills are swimmers with fins enabling them to swim in water. An adaptation is a behavioral, structural, or physiological trait that increases a species’ chance of survival in a specific environment. Every living thing has adapted to fit with where it lives. That’s what it takes for life to survive. Aquatic organisms live in water and have adaptations to do so.
Fish are ectothermic. This means their body temperature changes as the surrounding water temperature changes. For most fish their body temperature remains very close to the temperature of their water habitat. Because temperature has such a great effect on fish, different species have different water temperature preferences. Some species prefer warmer water than others. As a result most Texas species are warm water fish, because most of Texas’ waters are warm. As you go even farther south toward the equator, fish that live there prefer even warmer water.
Fish have many of the same internal organs as humans and other mammals. They have a heart to pump blood, intestines and stomach to digest food, kidneys, a liver, a gall bladder, and a spleen. (Fig. 4.1) A physiological adaptation found in fish that enables some fish species to live in freshwater and others to live in saltwater is called osmoregulation. This is where fish regulate their intake of saltwater or freshwater to keep their fluids, such as blood, from becoming too salty or too dilute. Fish living in saltwater have internal fluids lower in salts than the water in which they live. These fish must drink large amounts of saltwater and excrete small amounts of fluids while they actively secrete high amounts of salts through their gills. Fish in freshwater have higher body fluid salt concentrations than the surrounding water causing them to excrete large volumes of low salt content fluids and take up salts through their gills. Some species are able to adapt to a wide range of salinities. Fish that can live in freshwater, brackish, and saltwater are called euryhaline. Examples of common euryhaline aquatic species in Texas are red drum and blue crab.
Fish have some organs that we don’t have that allow them to live in water. Instead of lungs, fish have gills. Gills contain capillaries (fine blood vessels) that take up dissolved oxygen and release carbon dioxide. This occurs as water enters through the fish’s mouth, passes over the gills, and exits from the fish. In clean, moving water, a fish can absorb up to 85% of the dissolved oxygen available in well-oxygenated water. Fish and amphibians are the only vertebrates (animals with a backbone) that are able to live their entire lives completely submerged in water.
How fish swim
Up to 80% of a fish’s body is made of muscle. In comparison, only 30-40% of human body weight is muscle. Fish muscles are packed along its sides. That’s where a fish gets most of its swimming power. When a largemouth bass wants to move forward, it begins a side-to-side wiggle that starts at its head and moves backward along its body. The wiggle pushes water behind the fish, which propels it forward. Fish also use their many fins to move about in the water. They have two sets of paired fins (pectoral and pelvic) along the side. They also possess a single caudal and anal fin. Some fish species have a single dorsal fin, while others have two. Certain fish (such as freshwater trout and catfish) also have an adipose fin which is located on their back, behind the dorsal fin. (Fig. 4.2)
The dorsal fin, located along the back of a fish, helps keep the fish upright and stable. Some species of fish, such as sunfish, have sharp spines in their fins. These help discourage other fish from eating them. Located underneath the fish near the anus, the anal fin also helps with stabilization. The caudal fin or tail fin can be rounded, forked, or crescent shaped. This helps with speed and movement. Most fish use their pectoral and pelvic fins, which are located along their sides, to steer or maneuver. These fins can be moved independently, giving the fish the ability to move quickly in any direction. Fins can be used as brakes or rudders to help the fish stop, turn, go up or down, or even go backward.
The fish’s body shape also affects how they are able to maneuver through water. Certain body shapes may help a fish survive by allowing it to move in and out of tight places to catch food or to escape from predators. Fish with a flat body shape mostly live and feed on the bottom. Torpedo-shaped fish are built for speed. Fish with a tall and thin shape can easily slip in and out of tight places.
Sink or swim
Many freshwater and saltwater fish have swim bladders. Most of the time, the fish uses its swim bladder to keep from sinking. Being able to float or rise in water is referred to as buoyancy. The swim bladder works a little like a hot air balloon. The more gas (oxygen) it contains, the higher a fish will suspend or float in the water. Some species of fish can also use their swim bladder to make sounds to communicate during courtship, to defend its territory, or as an alarm when it has been disturbed.
Many species of fish are covered with scales that protect them like roof shingles protect a house. There are four main types of scales with many variations to these. Each type can vary greatly in size. Some fish have scales modified into special purposes, such as the tail spines of stingrays. Fish don’t grow more scales as they get older, the scales just get bigger. Fish biologists can estimate a the age of some fish by counting rings on a scale, similar to the way foresters can tell a tree’s age by counting its growth rings. Fish skin is often coated with slime, which helps reduce friction as fish swim through the water. The slime also helps protect them from disease.
Many fish species have counter-shading, often being dark-colored on top and light-colored underneath. This helps them blend in with the dark bottom when seen from above, and with the bright surface when seen from below. Structural adaptations such as color or patterns allow fish to blend in to their surroundings. For example, you must look very closely to see a darter sitting on a gravel streambed (the bottom of the stream) because of its ability to blend into its multicolored surroundings.
Fish have senses like we do to see, hear, smell, taste, and feel. The senses of some fish are better developed than those of others. Some fish use their sense of smell or taste to find food. Others feed primarily by sight. The placement and shape of eyes of some fish species allow them to see almost all the way around their bodies. Although fish are nearsighted (cannot see far away) it is difficult to sneak up on a fish because they see in all directions. Fish can see colors. Fish that feed at night or live on the bottom in very deep or turbid water rely heavily on their senses of smell and taste, instead of on sight.
Fish can also hear, plus they have a special row of sense organs called the lateral line that give them the ability to hear low-frequency sounds. A fish’s ears are located beneath the skin on either side of the head. The lateral line is used to detect movement and vibration in the water. Special hair-like cells along the lateral line, either free standing or located inside very small fluid canals, are very sensitive to vibrations in the water. This allows fish to sense the watery world around it. The lateral line can be seen on many fish as a faint line running along its side. Lateral lines serve an important role in detecting prey or predators, schooling and orienting to objects in the water. “Keep quiet or you’ll scare away the fish” is good advice when you are fishing.
Life in the water
Fish have been on Earth for more than 400 million years. Today there are more than 36,000 species identified worldwide. They are divided into three major classes: (1) jawless fish which include lampreys; (2) cartilaginous fish which include sharks and rays, and the largest group; (3) bony fish.
In Texas, there are over 250 species of freshwater fish and 1,500 species of saltwater fish in the Gulf of Mexico. Each fish species has its own way of surviving. The diversity of fish and their many adaptations allow them to live in a variety of environments. They thrive in the cypress swamps of the Sabine River and Caddo Lake, in complete darkness in the Hill Country’s many caves, and in the brackish water bays and estuaries along Texas’ 367-mile long coastline.
Different species of aquatic life are adapted to play different roles in the aquatic environment. Just like fish, many species of invertebrates, amphibians, reptiles, mammals, and birds have adaptations that allow them to survive in aquatic ecosystems.
There are many kinds of aquatic invertebrates, ranging from giant squid and clams, to squirmy insect larva that live in the mud, to tiny free swimming zooplankton. Invertebrates make up much of the food larger aquatic organisms eat. In addition to being part of the food web, aquatic invertebrates help breakdown organic matter. They are excellent indicators of the health of an aquatic habitat. Species can range from pollution tolerant to extremely sensitive to pollution.
Most aquatic invertebrates require oxygen to live, which they get various ways, depending on the species. For example, some aquatic insects have small gills along the body or tail, others have little holes along their body to absorb dissolved oxygen from the water, some use breathing tubes they raise from the water into the air, while others can carry a small bubble of air underwater with them from the surface.
Molluscs are species that live in freshwater and saltwater and are extremely varied in form. Some mollusks, including clams, oysters, scallops, mussels, and snails, have hard shells (Fig. 4.3). Others like the octopus have soft bodies and tend to live in cavities for protection, while squid which also have soft bodies are free swimming species.
Aquatic insects are found only in freshwaters and the shallow brackish waters of estuaries and bays. These invertebrates have at least two phases of life. Insects change form through a process called metamorphosis. For insects such as many mayflies, stoneflies, dragonflies, and damselflies the larvae or nymph phase is usually spent entirely in water, while the adult phase may be spent in water, on land, or in the air. Many adult aquatic insects have large wings that allow them to fly about. (Fig 4.4) While there are no aquatic insects that can live entirely in saltwater, there are many that inhabit brackish water estuaries and bays. An example is saltwater mosquitoes.
Crustaceans also live in both freshwater and saltwater. They have an exoskeleton, which is an outer covering that supports and protects the animal’s body. Familiar crustaceans are crayfish in freshwater, and shrimp and lobster in saltwater.
Plankton are tiny invertebrates and photosynthetic organisms that are carried about by flowing waters or ocean currents. Animal plankton are called zooplankton and are made up of tiny crustaceans and even tinier animals called rotifers. Some species can be as large as your fingernail, but most are so small you need a microscope to see them. While many are capable of very quick movement on their own, they are so small that the movements they make are tiny. Zooplankton are important food for small fish. (Fig 4.5)
Aquatic plants and algae
Many species of plants have special adaptations for living submerged in water or at the water’s surface. Aquatic and wetland plants do not belong to any one particular plant family. They come from several land plant families and have acquired similar special adaptations to allow for life in water. The most common adaptation is large air spaces or channels running through the leaves, stems, and roots. These air channels allow an exchange of gases between the parts of the plant that remain submerged in water, such as the root, and the parts that reach above the water’s surface. Other adaptations are floating leaves and leaves divided into many deep, narrow segments. Aquatic plants fully adapted for a life in water can only grow in water or in soil that is saturated with water. (Fig. 4.6)
A few aquatic plants are able to survive in brackish or saltwater, but only in shallow areas. Several species of seagrass are found in Texas’ bays and estuaries. Seagrass beds are important to the estuarine food chain. Decomposing seagrass leaves provide nutrients for small shrimp, crabs, and fish. Seagrass leaves also provide protective cover for these small animals. Waterfowl, such as redhead ducks also feed on seagrass leaves and roots.
Seaweed in the ocean and estuaries is often confused with aquatic plants, but seaweed is not a vascular plant. It is really multicellular algae. There are many forms of algae some of which are so small they can only be seen with a microscope. These algae are part of the phytoplankton in freshwater and saltwater.
Photosynthetic organisms in the plankton are called phytoplankton, and sometimes are also called microalgae because they are so small and are a form of algae. While most are far too small to be seen without a microscope, they can grow together in very large groups. In fact, the dark green color of many ponds is due to the presence of very large numbers of tiny algae. Phytoplankton contain chlorophyll and require sunlight to live and grow. Many phytoplankton species are buoyant and remain near the water’s surface during day where sunlight penetrates the water. Phytoplankton also require nutrients such as nitrates, phosphates, and sulfur which they convert into proteins, fats, and carbohydrates. There are many kinds of phytoplankton. One group, called dinoflagellates use a whip-like tail, called a flagella to move about in the water. Their bodies are covered with a shell. Another group, called diatoms, are a type of algae encased within a cell wall of interlocking parts made of silica and can form colonies. (Fig 4.7)
Phytoplankton may be eaten by small fish, but it is the zooplankton that eat most of the phytoplankton. Phytoplankton play an even bigger role in aquatic systems than just as food for aquatic animals. Phytoplankton photosynthesize when exposed to sunlight so they are important “primary producers” in the aquatic food chain. They produce organic compounds from carbon dioxide dissolved in the water. They also produce oxygen that can either remain dissolved in the water or can rise into the atmosphere. To do this the phytoplankton obtain energy through the process of photosynthesis by living in the well-lit surface layer (termed the euphotic zone, where “eu” means true and “photic” means light) of the Earth’s oceans, lakes, and other bodies of water. Phytoplankton account for half of all photosynthesis that occurs on Earth.
Learn About Aquatic Science Careers
Click on photos below for enlarged view.
Fig. 4.1 — Fish have many of the same organs as humans, but also have gills, a swim bladder, lateral lines, and fins that allow them to live in water. Credit: Missouri Department of Conservation
Fig. 4.2 — Fish fins. Credit: Texas Parks and Wildlife Department/Rudolph Rosen
Read About Adaptation to Aquatic Ecosystems, from Headwaters to Ocean
Fig. 4.3 — Aquatic invertebrates: Mollusk — The lightning whelk, shown with a strand of egg capsules from which tiny whelk juveniles emerge, is an aquatic invertebrate. Along with clams and oysters, the lightening whelk (shell shown) is a mollusk. Credit: Texas Parks and Wildlife Department
Fig. 4.4 — Aquatic invertebrates: Insect — Mayfly nymph (top) and adult (bottom). Credit: Valerie Bugh, larvalbug.com>
Fig 4.5 — Aquatic invertebrates: Crustacean zooplankton — Daphnia. Credit: Public Library of Science; PLoS Biol 3(7):e253.doi:10.1371/journal.pbio.0030253.g001>
Fig. 4.6 — Aquatic plants: Water lilies and duckweed — Many aquatic plant species inhabit the waters of Caddo Lake in East Texas. Credit: Texas Parks and Wildlife Department
Fig. 4.7 — Phytoplankton: diatom (top), dinoflagellate (bottom). Credit: Texas Parks and Wildlife Department