Early Fish Evolution

• Phylum Chordata• Subphylum Urochordata

• Phylum Chordata• Subphylum Cephalochordata

Fish History

• Phylum Chordata• Subphylum Vertebrata

Jawless FishesClass: Myxini - Hagfishes

Class: Cephalaspidomorphi – Lampreys

Class: Ostracodermi – Ostracoderms, jawless fishes with plate covered skin

Hagfish are very primitive vertebrates.

Primarily scavengers, they live near the ocean floor where they consume dead fish and marine worms.

Hagfish have no paired appendages or backbone yet they are considered vertebrates. They have a cartilaginous “shelf” upon which their brain rests. Biologists consider this the beginnings of the skull.

Hagfish have no jaws. Their tongue has two rows of rasp-like structures that enable them to scrape at their food.

A row of sensory tentacles surround the hagfish’s mouth.

Hagfish are sometimes called “knot fish” because they are able to tie themselves into knots. They do this to rid their skin of excess slime and to gain leverage so that they can enter the anus of dead fish upon which they scavenge.

Agnathan Fish

Class: Cephalaspidomorphi

Lampreys

Brook Lamprey – an Ohio native.

This species only attains a length of about six inches. It spends several years as a larva burrowed in the sediment at the bottom of a stream where it filter feeds.

After metamorphosis, the adults do not feed. They simply migrate to a suitable breeding site in their stream, reproduce, then die.

The Silver Lamprey is another Ohio native. Adults of this species are parasitic upon fish. It is uncommon but does occur in the Ohio River and some of its larger tributaries.

Scientists very rarely are able to collect this fish. Only seven have been collected in Ohio in the past 30 years.

Evidence of its presence is occasionally seen when a fisherman catches a fish with deep wounds caused by the silver lamprey. Adults attain a maximum length of about 8 to 10 inches.

The Sea Lamprey is not native to Ohio, but it has been accidentally introduced by ocean going ships. In the Atlantic Ocean, this species may attain a length of 3 feet, but in Lake Erie it seldom grows to more than 2 feet.

It has caused tremendous damage to the commercial fishing industry. It has all but eliminated the Lake Trout, a large fish that was harvested from Lake Erie by the thousands of tons as recently as the 1930’s. By the 1960’s only about 400 pounds of Lake Trout were harvested from the same waters. Government agencies spend tens of thousands of dollars annually in an attempt to control this species.

The lamprey’s jawless mouth is able to form a suction on the body wall of its host fish enabling it to use the rasps on its tongue to scrape open wounds.

The blood, and fluids that ooze from the wound are swallowed by the lamprey as its source of food. When full, it releases its hold. The host fish usually heals, but in Lake Erie, where sufficiently large fish are rare, scores of Sea Lampreys may attached to a single fish. Lake Trout populations have suffered.

This experiment was conducted in an aquarium. Newly metamorphosed Sea Lampreys were released with Lake Trout. They instantly attached to the trout and started feeding.

The grasp of these small Sea Lampreys on this Lake Trout are so strong that they don’t release, even when removed from the water.

This Lake Trout has been stuffed and mounted with a Sea Lamprey parasitizing it. Lake Trout from the great lakes often have up to ten Sea Lampreys on them and hundreds of old wounds. After being so heavily parasitized, most of the trout will die.

This Lake Trout has just had a Sea Lamprey pulled from its body wall.

The wound created by the lamprey is quite evident in this photograph. Fish usually recover from one or two wounds, but oozing blood attracts more lampreys. If they find the wounded fish, more lampreys will attach.

This wildlife officer is holding two Lake Trout that were severely wounded by Sea Lampreys in Lake Erie.

Efforts to eliminate Sea Lampreys have failed but measures to limit their populations have been successful. Unfortunately they are controlled by chemical means. TFM, the most successful lampricide has grim effects on other species of aquatic animals. TFM causes no harm to adult lampreys. It kills the filter feeding larvae.

A lamprey larvae looks remarkably similar to a Lancelet (Amphioxus). When first discovered, scientists considered the lamprey larvae to be a new species of Lancelet and they gave them the genus nameAmmocoetes. Eventually they learned that these were lamprey larvae. Today Lancelets are the evolutionary ancestors of lampreys based on developmental, ecological, and anatomical similarities between larval lampreys and Lancelets. Like Lancelets, ammocoetes live burrowed in the mud, filter feeding on protozoans and organic matter. Today the word ammocoete is used as the term for any lamprey larva.

Fish History

• Phylum Chordata• Subphylum Vertebrata• Class Ostracodermi

Class: Ostracodermi - The Ostracoderms

These jawless fishes had paired fins, a bony spine, and heavy, protective, bony armor that covered their skin.

Ostracoderms are only known as fossils today. However they once ruled the seas for nearly 100 million years.

They probably got their food by sucking up the sediments from the sea floor and digesting any organic material from it.

Scientists speculate that the Ostracoderms’ heavy armor made them too slow to compete as more advanced fishes evolved. Consequently they eventually became extinct.

Fish History

• Phylum Chordata• Subphylum Vertebrata• Class Placodermi

Class: Placodermi - this fossilized fish represents the first vertebrate animal to have evolved jaws.

Similar to Ostracoderms, the Placoderms were large, heavy, and covered with bony plates. Their jaws, however allowed them to hunt, and consequently they pushed the Ostracoderms into extinction.

This Placoderm fish fossil was discovered in northern Ohio and is on exhibit at the Cleveland Museum of Natural History.

The word Agnatha used to be a Class that included lampreys and hagfishes, boneless, jawless fishes. However they are divided into the Classes: Myxini and Cephalaspidomorphi today.

Although it isn’t a taxonomic term, the word Agnatha is still used by biologists to describe any jawless vertebrates.

A – without gnath – jaws

Gnathostomes are vertebrate animals that do have jaws.

Gnath – jaw

Stome – hole or opening

Gnathostome means opening (mouth) with jaws.