Jawless Fishes

The only existing jawless fishes are the 80 species of hagfishes and lampreys. These fish were formerly grouped together in the class Agnatha. They are now divided into two classes: Myxini (hag-fishes) and Cephalaspidomorphi (lampreys). Their skin has neither plates nor scales. Hagfishes and lampreys have an eel-like body, a cartilaginous skeleton, and unpaired fins. The notochord remains throughout life. Hagfishes live only in the oceans. Many lampreys live permanently in fresh water, and all lamprey species reproduce in fresh water.

Hagfishes

Hagfishes, shown in Figure 39-4, are bottom dwellers in cold marine waters. Hagfishes are unique in that they do not have vertebrae. Hagfishes are also unique because they are isotonic, which means that their body fluids have nearly the same ion concentration as sea water. They feed on small invertebrates or on dead and dying fish. Because the hagfish lacks jaws, it cannot bite, but within its mouth are two movable plates and a rough tonguelike structure that it uses to pinch off chunks of flesh. Hagfishes often burrow into the body of a dead fish through the gills, skin, or anus. Once inside, they eat the internal organs.

figure 39-4

Hagfishes are modern jawless fishes. Hagfishes lack paired fins, which allows them to burrow into the bodies of the dead fish on which they feed.

figure 39-5

A lamprey's mouth is adapted for latching onto prey and feeding on body fluids of other fishes.

figure 39-5

A lamprey's mouth is adapted for latching onto prey and feeding on body fluids of other fishes.

Lampreys

About half the species of lampreys are free-living (non-parasitic). The other half are parasites as adults and feed on the blood and body fluids of other fishes. Once a suitable host is located, a lamprey uses its disk-shaped mouth, shown in Figure 39-5, to attach to the host. Then, it scrapes a hole in the host with its rough tongue and secretes a chemical that keeps the host's blood from clotting. After feeding, the lamprey drops off. The host may recover, bleed to death, or die from an infection.

Some lamprey species spend most of their adult lives in the ocean. Others live in rivers or lakes and never enter salt water. All lampreys breed in fresh water. Fertilization occurs outside the body of either parent, a process known as external fertilization. The eggs hatch into larvae that resemble an amphioxus, an invertebrate chordate. The larvae eventually transform into adults.

ca figure 39-6

These teethlike placoid scales are found on the skin of cartilaginous fishes. What advantage might they give a shark in swimming?

These teethlike placoid scales are found on the skin of cartilaginous fishes. What advantage might they give a shark in swimming?

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