14.1: Fish Characteristics

Learning Objectives

By the end of this section, you will be able to do the following:

• Describe the general physiological and anatomical features of fish.

 

Modern fishes include an estimated 31,000 species, by far the most of all clades within the Vertebrata. Fishes were the earliest vertebrates, with jawless species being the earliest forms and jawed species evolving later. They are active feeders, rather than sessile, suspension feeders. The Agnatha (jawless fishes)—the hagfishes and lampreys—have a distinct cranium and complex sense organs including eyes, that distinguish them from the invertebrate chordates, the urochordates and cephalochordates.

 

Fish Thermoregulation

Animals can be divided into two groups: some maintain a constant body temperature in the face of differing environmental temperatures, while others have a body temperature that is the same as their environment and thus varies with the environment. Poikilotherms are animals with constantly varying internal temperatures. An animal that maintains a constant body temperature in the face of environmental changes is called a homeotherm. Additionally, animals can be divided into two groups based on how they maintain body temperature. Animals that rely on external sources to set their body temperature are ectotherms. This group has been called cold-blooded, but the term may not apply to an animal in the desert with a very warm body temperature. In contrast to ectotherms, endotherms are animals that rely on internal, metabolically-generated sources of energy to set their body temperature. Because they generate their own heat and do not rely on external heat sources, endotherms are generally able to maintain relatively constant body temperature in varying environmental temperatures. These animals are therefore able to maintain a level of metabolic activity at cooler ambient temperatures, which an ectotherm cannot due to differing enzyme levels of activity. That said, ectothermy is energetically less expensive than endothermy so they can go longer without eating. Ectotherms are often poikilotherms, and endotherms are often homeotherms. It is worth mentioning that some ectotherms are technically homeotherms because they have a relatively constant body temperature due to the constant environmental temperatures in their habitats. Similarly, some endotherms have such slow metabolisms that their body temperatures vary with the ambient temperature and therefore verge on being poikilothermic.

In general, fish are ectotherms and poikilotherms. This means that their body temperature will match that of the water they are in. If fish need to adjust their body temperature, they need to move to a different location where the temperature of the water is different. Deeper water is colder, so fish can move throughout the water column and/or move in and out of shelter to regulate body temperature. Some deep sea fish are ectothermic homeotherms because they live in regions where the water temperature does not change much. Some sharks and bony fish (like tuna) verge on endothermy. They accomplish this via countercurrent heat exchange: they have adaptations to their circulatory system that enable them to transfer heat generated by muscle contraction from arteries to veins, warming blood returning to the heart. This prevents the cold venous blood from cooling the heart and other internal organs and helps to maintain a higher body temperature.

 

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Fish Sensory Systems

Humans share some homologous organs and body parts with fish. However, characteristics of water exert evolutionary pressures on fish to enhance their sensory capabilities in water. Water is dense, colorless, and odorless and can refract and reflect light waves in such a way that some colors are absorbed, particularly at deeper depths. Consequently, sound waves travel fast, scents are rapidly dissolved and detected in low concentrations, and vision is keen in fish that are active during the daytime. It’s not just the presence but also the location of sensory organs that reflects these evolutionary pressures. Fish smell with nares, far forward on the head, in front of the eyes, so that new scents are detected as the fish swims forward. Taste buds in fish are not restricted to the mouth but are distributed throughout parts of the body to allow the fish to taste its environment. Eyes are typically above the midline and on either side of the head, allowing fish a wide field of vision in front and along the sides and above—locations of typical predator threat. Some fish, such as sharks, also have specialized electroreceptor organs in their snouts.

Virtually all fish, together with most aquatic and larval amphibians, have a row of sensory structures called the lateral line, which is used to detect movement and vibration in the surrounding water, and is often considered to be functionally similar to the sense of “hearing” in terrestrial vertebrates. The lateral line is visible as a darker stripe that runs along the length of a fish’s body. The lateral line is an organ of microscopic pores primarily used to sense vibrations and pressure in the water. The pores are lined with structures containing sensory hair cells (Figure 3.4). Each hair cell has bundles of cilia embedded in a gelatinous structure, called the cupula. Water movements deflect the cupula and cilia bundles, creating a change in membrane potential that is transmitted to the sensory neuron. Fish utilize the lateral line to detect movements of prey, predators, currents, and objects in the water. If there is any difference between the relative movements of the body of the fish and the movements of the surrounding water, it will be sensed by the lateral line.

Figure 3.4: Schematic of the lateral line system of fish. Movements of water in the lateral line canal cause the cupula to move, thereby stimulating sensory hair cells connected to nerves. Long description.

 

Some fish have evolved a reduced or negative capacity for some senses to match their environment. Fish in muddy water habitats often have very small eyes because vision is less important. Some fish that live in dark caves have totally lost the sense of vision. Blind cavefish use the flow-sensing capabilities of their lateral line system rather than vision to avoid swimming into obstacles.

 

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Fish Respiration

Organisms that live in water need to obtain oxygen from the water. Oxygen dissolves in water but at a lower concentration than in the atmosphere. The atmosphere has roughly 21 percent oxygen. In water, the oxygen concentration is much lower than that. Fish and many other aquatic organisms have evolved gills to take up the dissolved oxygen from water (Figure 39.4). Gills are thin tissue filaments that are highly branched and folded. When water passes over the gills, the dissolved oxygen in water rapidly diffuses across the gills into the bloodstream. The circulatory system can then carry the oxygenated blood to the other parts of the body. 

Figure 39.4 This common carp, like many other aquatic organisms, has gills that allow it to obtain oxygen from water. (credit: "Guitardude012"/Wikimedia Commons)

 

The folded surfaces of the gills provide a large surface area to ensure that the fish gets sufficient oxygen. Diffusion is a process in which material travels from regions of high concentration to low concentration until equilibrium is reached. In this case, blood with a low concentration of oxygen molecules circulates through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules in gills. As a result, oxygen molecules diffuse from water (high concentration) to blood (low concentration), as shown in Figure 39.5. Similarly, carbon dioxide molecules in the blood diffuse from the blood (high concentration) to water (low concentration).

Figure 39.5 As water flows over the gills, oxygen is transferred to blood via the veins. (credit "fish": modification of work by Duane Raver, NOAA)

 

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Fish Osmoregulation

Persons lost at sea without any freshwater to drink are at risk of severe dehydration because the human body cannot adapt to drinking seawater, which is hypertonic in comparison to body fluids. Organisms such as goldfish that can tolerate only a relatively narrow range of salinity are referred to as stenohaline. About 90 percent of all bony fish are restricted to either freshwater or seawater. They are incapable of osmotic regulation in the opposite environment. It is possible, however, for a few fishes like salmon to spend part of their life in freshwater and part in seawater. Organisms like the salmon and molly that can tolerate a relatively wide range of salinity are referred to as euryhaline organisms. This is possible because some fish have evolved osmoregulatory mechanisms to survive in all kinds of aquatic environments. When they live in freshwater, their bodies tend to take up water because the environment is relatively hypotonic, as illustrated in Figure 41.3a. In such hypotonic environments, these fish do not drink much water. Instead, they pass a lot of very dilute urine, and they achieve electrolyte balance by active transport of salts through the gills. When they move to a hypertonic marine environment, these fish start drinking seawater; they excrete the excess salts through their gills and their urine, as illustrated in Figure 41.3b. Most marine invertebrates, on the other hand, may be isotonic with seawater (osmoconformers). Their body fluid concentrations conform to changes in seawater concentration. Cartilaginous fishes’ salt composition of the blood is similar to bony fishes; however, the blood of sharks contains the organic compounds urea and trimethylamine oxide (TMAO). This does not mean that their electrolyte composition is similar to that of seawater. They achieve isotonicity with the sea by storing large concentrations of urea. These animals that secrete urea are called ureotelic animals. TMAO stabilizes proteins in the presence of high urea levels, preventing the disruption of peptide bonds that would occur in other animals exposed to similar levels of urea. Sharks are cartilaginous fish with a rectal gland to secrete salt and assist in osmoregulation.

Figure 41.3 Fish are osmoregulators, but must use different mechanisms to survive in (a) freshwater or (b) saltwater environments. (credit: modification of work by Duane Raver, NOAA)

 

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This page titled 14.1: Fish Characteristics is a derivative of Biology 2e by OpenStax that is licensed under a CC BY 4.0 license and Fish, Fishing, and Conservation by Donald J. Orth that is licensed under a CC BY 4.0 license.