Section II—External Anatomy
The integument of sharks is relatively thin. It is subdivided into dermis and epidermis, but the latter is not keratinized and comprises a layer of live cells, which, in life, is covered by mucus.
As noted earlier, bone is almost entirely absent in sharks, and so they lack extensive bony coverings. However, placoid scales or denticles are embedded in the skin (Figure 3.12a). A scale is built like a tooth, formed from dentine, covered by enamel, and containing a pulp cavity (Figure 3.12b). Such scales, which reduce the drag of water passing over the skin during swimming, are uniformly distributed over the skin and can be seen under low magnification, but their presence can be felt by running your hand over the skin. The integument also contains various specialized cells, such as melanophores that control pigmentation of the skin and secretory cells, but these cannot be seen grossly.
FIGURE 3.12 Skin of the shark. a.-d. Successive blowups of the surface of the skin to show structure of scales. e.-f. Vertical section through skin and scales. g. SEM image of skin showing scales. Source: Dr Steven Campana, Bedford Institute of Oceanography, Canada.
The body regions (Figure 3.13) of the shark, and fishes in general, are not as neatly differentiated as in tetrapods. This is due to the elongated and fusiform body, which facilitates movement through the water. The head extends posteriorly to the end of the pharyngeal slits and includes the laterally placed eyes. The nares (sing. naris) lie Ventrolaterally on the tapered snout. Examine a naris closely, and note that it is incompletely subdivided by a flap of skin into incurrent and excurrent apertures (see Figure 3.18). The naris leads into the blind-ended olfactory sac (see Figure 3.41).
The ventrally located mouth is supported by upper and lower jaws that have rows of sharp teeth. A labial pocket lies on either side of the mouth (Figures 3.13 and 3.18). Mouth and pocket are separated by a flap, the labial fold, which is supported by the labial cartilage. The spiracle is a relatively large opening into the pharynx and lies directly posterior to each eye. Its anterior wall contains a fold of tissue, the spiracular valve, that can be folded over the opening to close the spiracle. The valve bears a pseudobranch, a reduced gill, on its posterior surface. A row of five pharyngeal slits, separated by four interbranchial septa (Figure 3.13), lie posterolaterally on either side and lead into the pharynx. The slits are the means by which water leaves the pharynx as it passes over the respiratory structures or gills. Manipulate the interbranchial septa and note that on its anterior and posterior surfaces each carries gill lamellae (see Figure 3.23), which are injected with red latex. Note that lamellae are present on the anterior wall of the first slit, but not on the posterior wall of the last slit.The trunk follows the head and extends posteriorly to the midventrally located cloaca (Figure 3.14), a large chamber into which the urinary, reproductive, and digestive tracts exit. Pull the pelvic fins laterally to expose the cloaca more clearly. In the male (Figure 3.14a), the genital ducts and urinary ducts open at the tip of the urogenital papilla, a large cone-like structure lying in the cloaca. A similar structure occurs in females (Figure 3.14b), but as only the urinary ducts open at its tip, it is termed the urinary papilla. The anus, the posterior opening of the digestive tract, opens into the cloaca anterior to the papilla; you will see this presently. Locate the abdominal pores, one each on the posterolateral side of the cloaca (Figure 3.33). The pores extend into the pleuroperitoneal cavity and may allow removal of excess fluid from the cavity.
The median anterior dorsal fin (Figure 3.13) lies mid- dorsally on the trunk and carries a sharp spine anteriorly. The paired large pectoral fins lie anteriorly, just behind the gill slits, while the paired pelvic fins lie at the end of the trunk, one on either side of the cloaca.
In the male an elongated, cylindrical, and dorsally grooved clasper, a copulatory organ, lies between the cloaca and pelvic fin (Figure 3.14a).The tail (Figure 3.13) extends posteriorly from the cloaca and carries the well-developed caudal fin. Lateral undulations of the tail and caudal fin produce the propulsive force that moves the shark forward. The posterior dorsal fin, another medial fin, lies at the anterior part of the tail and also has an anterior spine. The spiny dogfish, the common name of Squalus acanthias, is derived from the presence of the spines on the anterior and posterior dorsal fins.
Examine the lateral surface of the body, just dorsal to the midlateral plane, and note the faint, pale stripe extending anteroposteriorly. It marks the position of the lateral line canal (Figure 3.13), a cutaneous tube that contains sensory nerve endings and opens to the surface by way of tiny pores. The lateral line canal is the predominant part of the lateral line system, a system of sensory cells responsive to pressure changes caused by vibrations and movements in the water. The distribution of this system into the head and other parts of the body will be considered later. The lateral line system is modified to provide two other functions. The balancing apparatus is contained within the otic capsule of the chondrocranium. It is mentioned here because this system is connected to the surface by a pair of endolymphatic ducts (recall the endolymphatic foramina, page 28) that open through the endolymphatic pores (Figure 3.15). These lie mid- dorsally on the head between the levels of the eyes and spiracles. The second modification consists of the ampullae of Lorenzini, which function in electroreception. The ampullae open in numerous pores on the dorsal and lateral surface of the head and may be noted by gently squeezing these regions to extrude a gel-like substance.
Key Terms: External Anatomy
| abdominal pores | naris (plur., nares) |
| ampullae of Lorenzini | olfactory sac |
| anterior dorsal fin | pectoral fins |
| anus | pelvic fins |
| caudal fin | pharyngeal slits |
| clasper | placoid scales |
| cloaca | posterior dorsal fin |
| endolymphatic pores | pseudobranch |
| eyes | snout |
| gill lamellae | spiracle |
| head | spiracular valve |
| interbranchial septa | tail |
| labial fold | trunk |
| labial pocket | urinary papilla |
| lateral line canal | urogenital papilla |
| mouth |
FIGURE 3.13 External features of the shark in left lateral view.
FIGURE 3.14 Ventral view of posterior portion of the shark to show cloaca and pelvic fin.
a. Right side of the male. b. Left side of the female.