Structure and Function – Fish | manoa.hawaii.edu/ExploringOurFluidEarth

External Anatomy of Fishes

Anatomy is the study of an organisms structures. Fishes come in a diverse array of forms, many with special modifications. The shape, size, and structure of body parts permit different fishes to live in different environments or in different parts of the same environment. The external anatomy of a fish can reveal a great deal about where and how it lives.

When describing the basic anatomy of an organism, it is useful to have some common terms to help with orientation. Just as a map uses north, south, east, or west to help determine the location, orientation words are useful in describing anatomy. Table 4.3 defines common anatomy terms, Fig. 4.18 shows their orientation on three different animals.

Scientists measure and describe the external features of fishes to identify species, assess age and health, and learn about structure and function. Scientists work with a variety of types of fishes to do this. They might use a fresh fish, or they may use photographs, scientific drawings, or other kinds of detailed imageseven fish fossils.

One way to document details about a fish is gyotaku. Gyotaku (pronounced gee yo TAH koo) is a traditional Japanese method of printmaking, which uses the whole fish. This method can produce an accurate image of a fish (Fig. 4.19).

Gyotaku is a relatively new art form that developed in Japan, probably in the early- to mid-nineteenth century. Gyotaku means fish rubbing. Gyotaku is valued from both a scientific and artistic perspective. The detail captured in gyotaku, especially in historical prints, is an important source of information for scientists who want to know the size and external features of fish in the past. The color and artistic arrangement of gyotaku prints made by skilled artists also make them valuable pieces of art. The oldest known gyotaku print, made in 1862, is owned by the Homma Museum in Sakata, Japan.

Activity

Use your observation and investigation skills to investigate fish form and function by experimenting with ways of making gyotaku fish prints.

Perches are the most common type of bony fishes. As a result, people often use the words perch-like to describe a generic fish shape. (Fig. 4.21 A). Fusiform is the scientific term used to describe the perchs streamlined, torpedo shaped body. Compressiform means laterally flattened (Fig. 4.21 B). Depressiform means dorso-ventrally flattened (Fig. 4.21 C). Anguilliform means eel-like (Fig. 4.21 D). See Table 4.4 for additional descriptions of fish body shapes.

Table 4.4. Fish form and function: body shape

Images by Byron Inouye

The first anatomical structures many people identify on a fish are the fins. In fact, appendages, when present, as fins is part of one of the scientific definitions of a fish. Most fish have two kinds of fins: median and paired.

Median fins are single fins that run down the midline of the body. The dorsal fin is a median fin located on the dorsal side of the fish. The anal fin and caudal fin are also median fins. Paired fins are arranged in pairs, like human arms and legs. The pelvic and pectoral fins are both paired fins. (Table 4.5).

Table 4.5. Fish form and function: dorsal fin features

Images by Byron Inouye

Median Fins

Median fins, like the dorsal, anal, and caudal fins, can function like the keel of a boat and aid in stabilization (Fig. 4.22 A). Median fins can also serve other purposes, like protection in the lion fish (Fig. 4.22 B).

Caudal (Tail) Fin

The caudal fin is known commonly as the tail fin (Table 4.6). It is the primary appendage used for locomotion in many fishes. The caudal fin is also a median fin (Fig. 4.22 A).

The caudal peduncle is the base of the caudal fin. Peduncle means stem, and the caudal peduncle is where the strong swimming muscles of the tail are found. Together, the caudal fin acts like a propeller for the fish, and the caudal peduncle acts like a motor.

Table 4.6. Fish form and function: Caudal fin features

Images by Byron Inouye

Paired Fins

Fish have two sets of paired fins: pectoral and pelvic (Fig 4.25). The pectoral fins are vertical and are located on the sides of the fish, usually just past the operculum (Table 4.7). Pectoral fins are similar to human arms, which are found near the pectoral muscles. Many fish, such as reef fish like wrasses (Fig. 4.25 B), use their pectoral fins for locomotion.

Table 4.7. Fish form and function: Pectoral fin features

Images by Byron Inouye

The pelvic fins sit horizontally on the ventral side of the fish, past the pectoral fins (Table 4.8). Pelvic fins are similar to legs. Just like human legs, pelvic fins are associated with the pelvis of the fish.

Table 4.8. Fish form and function: Pelvic Fin Features

Unique and Specialized Fins

Paired fins are most commonly used for maneuvering, like the oars on a rowboat. However, both the pectoral and pelvic fins can also be highly specialized like those of the flying fish (Fig. 4.26 A). Unique combinations of other fins can also help fish to be even more specialized, like the pectoral and anal fins of a box fish (Fig. 4.26 B; see Table 4.9) .

Table 4.9. Fish form and function: Combinations of Fins

Scientists use fins to help identify and classify fish species. In more evolutionarily advanced fish, the fins are supported by bony structures: spines and soft rays. Spines are simple, unbranched, structures. Soft rays are compound, segmented, and branched structures (Fig. 4.27).

The mouth is at the front, or anterior end, of the fish. The mouth can reveal a lot about the fishs feeding habits (Table 4.10). The size, shape, and placement of the mouth, combined with the type of teeth, provide critical information about the feeding habits of a fish (Table 4.11).

For example, a fish with a mouth on the bottom of its head often feeds by digging in the bottom sediment (Fig. 4.28 A). A fish with a mouth oriented upward usually feeds in the water column, or even above the water (Fig. 4.28 B). When a fish has its mouth open, the front lip may slide down and out from the mouth. This sliding action of the mouth can help the fish create a vacuum and quickly suck in a big mouthful of water, which hopefully also includes prey!

Fig. 4.28. (A) A bottom facing mouth indicates bottom feeding preferences in the sturgeon. (B) An upward facing mouth shows the surface feeding adaptation of the arowana.

Table 4.10. Fish form and function: Mouth Features

Table 4.11. Fish form and function: Teeth Features

The eyes of fish resemble human eyes (Fig. 4.29). At the front of each eye is a lens, held in place by a suspensory ligament. The lens focuses images of objects on the retina. To bring near and far objects into focus, the lens retractor muscle moves the lens back and forth.

The retina is a light-sensitive membrane rich in nerves that connect to the optic lobes of the brain by optic nerves. When light shines on the nerves of the retina, the optic nerves send impulses to the optic lobes. Because fish have no eyelids, their eyes are always open.

Some elasmobranchs, and most teleost fishes, have color vision. Some fishes can also see in ultraviolet (UV) light. UV vision is especially useful for reef fishes. UV vision helps fishes in foraging, communication, and mate selection.

Elasmobranchs, and some teleosts, also have a tapetum lucidum. The tapetum lucidum is a shiny, reflective structure that reflects light and helps vision in low light situations. The tapetum lucidum is what makes the eyes of sharks and deep sea fish, as well as land mammals like cats and cows, shine at night.

Fish eyes are usually placed just dorsal of and above the mouth. Just like the mouth of a fish, the size, shape, and position of the eyes can provide information about where a fish lives and what it feeds on. For example, fish predators often have eyes facing forward in order to provide better depth perception. Prey fish, on the other hand, often have eyes on the sides of their bodies. This gives them a larger field of view for avoiding predators. (Table 4.12).

Table 4.12. Fish form and function: Eye Features

The sense of smell is well developed in some fishes. Water circulates through openings in the head called nostrils. Unlike humans, fish nostrils are not connected to any air passages. Fish nostrils serve no role in respiration. They are completely sensory.

The largest part of a fishs brain is the olfactory lobe, which is responsible for the sense of smell. Smell is the response to chemical molecules by nerve endings in the nostrils. Chemoreception is the scientific term for what nerve cells do to help an organism smell (see Table 4.13).

Taste ReceptorsTaste is another form of chemoreception. Fish can taste inside their mouth. Many fishes, like goatfish and catfish, also have fleshy structures called barbels around the chin, mouth, and nostrils (see Table 4.13 and Fig. 4.30). In some fishes, these barbels are used for touch and chemoreception.

Fig. 4.30.

Not all barbels have chemoreception. The barbels of some fish, like catfishes, are not equipped for chemical reception (Fig. 4.30 B). Some fish also have fleshy tabs called cirri on the head (Fig. 4.30 C). Cirri are not sensory organs.

Table 4.13. Fish form and function: Chemosensory Adaptation and Camouflage

Lateral lineMost fish have a structure called the lateral line that runs the length of the bodyfrom just behind the head to the caudal peduncle (Fig. 4.31). The lateral line is used to help fishes sense vibrations in the water. Vibrations can come from prey, predators, other fishes in a school, or environmental obstacles.

Fig. 4.31.

The lateral line is actually a row of small pits that contain special sensory hair cells (Fig. 4.32). These hair cells move in response to motion near the fish. The lateral line sense is useful in hunting prey, escaping predators, and schooling.

Fig. 4.32.

Ampullary receptors are sense organs made of jelly-filled pores that detect electricity. They can detect low frequency alternating current (AC) and direct current (DC). Ampullae detect electricity emitted by prey as well as the small electrical fields generated by a fishs own movement through the earths magnetic fields. Researchers think that this may help fishes use the Earths magnetic field for navigation. Fishes that have ampullae include sharks, sturgeon, lungfish, and elephant fish. The ampullae of sharks are known as Ampullae of Lorenzininamed for Stefano Lorenzini, who first described them in 1678(Fig. 4.33).

Fig 4.33. (A) Ampullae of Lorenzini in a sharks head (B) Ampullae of Lorenzini pores on the snout of a tiger shark

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Structure and Function - Fish | manoa.hawaii.edu/ExploringOurFluidEarth