EXTERNAL  ANATOMY

Perch (Perca flavenscens)

Introduction

            The most numerous of all vertebrates are the bony fishes.  They occupy every marine environment and also comprise the vast majority of the freshwater fish population. The perch belongs to this group of fish and is a good example of an aquatic vertebrae.

General Features

            Place a preserved perch in a dissecting pan. Observe the fusiform body shape of the perch.  It is laterally flattened and streamlined to minimize resistance as the fish moves through its aquatic environment.  The body is covered by small overlapping scales.  Note the direction in which the scales overlap – another mechanism to minimize resistance to movement. A layer of mucus covers the scales externally to form a protective layer.  Remove a scale and observe it under a dissecting microscope.  Scales of modern fish are the remnants of bony armour, which covered the bodies of ancient vertebrates.

     On the ventral surface of the fish, locate two openings.  The larger more anterior opening is the anus (Figure 1).  The smaller more posterior opening is the urogenital  opening.  On each lateral surface of the perch is a large crescent-shaped opening  to the gill chamber.  Each gill chamber is covered laterally by a bony operculum.

            Locate the pigmented line extending from the dorsal border of the operculum posteriorly to the tail.  This is the lateral line system and is formed of modified scales containing sensory pits.  The function of the lateral line system is thought to be detection of vibrations of low frequency or movements in the water.

            The body of the perch is divided into three parts:  head, trunk and tail, which are discussed in more detail in the following paragraphs (Figure 1).

Head

Note the terminal mouth of the perch (i.e. at the anterior, leading tip of the head).  The position of the mouth on the head is a modification reflecting the mode of feeding.  The perch feeds by overtaking prey while swimming.  Pull down the lower jaw and note the many sharp uniform teeth of the perch.  Just above the mouth are four nasal apertures (nares), two on each side. Push a fine probe through one of the anterior nasal apertures and note that it emerges through a posterior nasal aperture.  These nares are not connected with the mouth.

The large eyes have no eyelids, but are covered with a layer of transparent integument.  The lateral location of the eyes prevents binocular vision. 

Bend the operculum backward and note four gill arches.  Note gills which are attached to each of the the gill arches. 

Trunk

The two sets of paired appendages (fins) located on the trunk of the perch are homologous to the paired appendages of tetrapods.  Just posterior to the opercula are the two pectoral fins.  They are attached to the pectoral girdle which can be felt immediately beneath the skin.  Posterior and ventral to the pectoral fins are the pelvic fins.

Tail

Tapering from the trunk, the tail ends as a laterally compressed paddle.  The symmetrical caudal fin surrounds the fleshy end of the tail.  This type of symmetrical caudal fin is correlated with the presence of a swim bladder.

The Median Fins

There are four median, unpaired fins.  These are:  the caudal fin, just seen; the anal fin, located posterior to the anus; the anterior dorsal fin, beginning at the anterior end of the trunk; and the posterior dorsal fin, located posterior to the anterior dorsal fin.

Figure 1: External Anatomy of the Perch (Lateral View).