Class Cyclostomata (Agnatha)
Introduction
The Cyclostomata are the modified and degenerate offshoot of the primitive vertebrate stalk. Due to their circular mouth, they are named Cyclostomata.
They are parasitic, usually feeding on fish in their adult stage. Morphologically, they resemble eels. They are known to be the only living vertebrates without true jaws, hence called Agnatha. Cyclostomata includes hagfishes and lampreys.
Characteristics of Cyclostomata
1.The body is round and elongated like an eel.
2.The paired fins are absent.
3.Median fins with cartilaginous fin rays.
4.No paired appendages.
5.The skin is soft and smooth, devoid of any scales.
6.Spleen is absent.
7.The exoskeleton is absent. The endoskeleton is cartilaginous with no bones.
8.The notochord is present throughout their lives.
9.The digestive system is devoid of any stomach.
10.The nostril is single and median.
11.The gills are five to sixteen in pairs.
12.The heart is two-chambered.
13.The brain is visible.
14.The lateral line acts as a sense organ.
15.About ten pairs of cranial nerves are present.
16.The sexes are separate. Some hagfish species are believed to be hermaphrodite.
17.A pair of mesonephric kidneys make up the excretory system.
18.Development may be direct or indirect.
Eg., Petromyzon and Maxine.
Sub-Divisions of Cyclostomata
The cyclostomes are sub-divided into two major orders.
Petromyzontiformes
Lampreys or lamper eels belong to this order.
They are found in both marine and freshwater.
They have a ventral mouth with many horny teeth.
The nostril is present dorsally.
They possess a well-developed dorsal fin.
The dorsal and ventral roots of spinal nerves are separate.
The development is indirect.
Eg., Petromyzon, Lampetra
Myxiniformes
Hagfishes represent this order.
They are found exclusively in the marine environment.
They have a terminal mouth with few teeth.
They have no buccal cavity.
The nostril is terminal.
They possess 6-14 pairs of gill slits.
The dorsal and ventral roots of the spinal nerves are not separate.
Eggs are large and few in number.
The dorsal fin is usually absent, or weak.
Eg., Myxine, Paramyxine.
Sea Lamprey
Classification
The sea lamprey (Petromyzon marinus) is one of 31 species of lamprey found throughout the world and one of four lamprey species found in the Lake Champlain Basin. Lamprey are eel-shaped fish with a skeleton made of cartilage, not bone. They belong to a relic (primitive) group of jawless fishes called Agnathans.
Juvenile parasitic sea lamprey are 6 to 24 inches in length with smooth, scaleless skin that is mottled grey/blue to black, darker on top and fading to a lighter colored belly. Adult sea lamprey, preparing to spawn, are 14 to 24 inches in length and exhibit mottled dark brown/black pigmentation. Sea lamprey have two separated fins on their back (dorsal fins) and suction disk mouth filled with small sharp, rasping teeth and a file-like tongue. The sea lamprey is a jawless parasite that feeds on the body fluids of fish.
Sea lamprey, like many salmon, are "diadromous". They spend the early stages of their life in streams and rivers. The middle stage of their life is spent in the saltwater of the ocean or in a large freshwater lake. Then they return as breeding adults to spawn in the freshwater streams and rivers, and die shortly after spawning. Sea lamprey in Lake Champlain take about six years to complete this life cycle.
Life Cycle
Ammocoetes
The blind worm-like larval lamprey, known as ammocoetes [am-mah-seats], can grow up to 5 inches long. They hatch from eggs in gravel nests in tributaries and drift downstream with the current. When they locate suitable habitat - usually silt/sand stream bottoms and banks in slower moving stretches of water - they burrow in and take up residence, filter-feeding on algae, detritus and microscopic organisms and materials. In the Lake Champlain Basin this stage of the sea lamprey's life cycle usually lasts 3 to 4 years; in other waters lamprey spend up to 10 years in their larval form.
Transformers
Sometime in mid to late summer of their third or fourth year the ammocoetes undergo a dramatic change in both form and function. They develop eyes and a suction disk mouth, and become a smaller version of the adult sea lamprey. Also during this stage their kidneys change to allow them to live in saltwater. Once the ammocoetes´ change is complete, the newly transformed sea lamprey, known as a transformer, leaves its burrow and moves downstream towards Lake Champlain. The sea lamprey is then ready to begin the next stage in its life as a parasite of fish. The juvenile sea lamprey move into deeper water and begin to seek host fish on which to feed.
Parasitic Juveniles
The juvenile sea lamprey uses its suction disk mouth which is filled with small sharp, rasping teeth and a file-like tongue to attach to fish, puncture the skin, and drain the fish's body fluids. An anticoagulant in their saliva ensures that the blood of the host fish does not clot while the sea lamprey feed. Often the host fish die from loss of blood, or infections resulting from stress. Fish that survive sea lamprey attacks will have decreased reproduction. Sea lamprey in Lake Champlain prefer landlocked Atlantic salmon (salmon), lake trout and other trout species, due to their small scales and thin skin. The same native fish species prized by anglers, and that are such an important part of the natural ecosystem of the lake.
Sea lamprey also feed on other fish species, including lake whitefish, walleye, northern pike, burbot, and lake sturgeon. The lake sturgeon is listed as a threatened species in New York and an endangered species in Vermont and it is likely that sea lamprey are affecting their survival. Most sea lamprey hosts are native fish species that have been part of the Lake Champlain Basin ecosystem for thousands of years.
Mortality and Wounding of Host Fish
Studies on the Great Lakes show a 40 to 60 percent mortality rate for fish attacked by sea lamprey. Other studies have found that a single sea lamprey can kill 40 or more pounds of fish during its life. Even when fish survive the attacks, the fish populations will decline as the fish expend more energy on healing than on producing eggs and mating.
During periods when sea lamprey are abundant in Lake Champlain, anglers often catch salmon and trout with wounds or lamprey attached. Frequently these fish have multiple wounds, multiple scars and/or multiple lamprey attached to them. These high wounding rates indicate that sea lamprey are having a significant impact on the lake trout and salmon populations. Angler catches of lake trout and salmon in Lake Champlain were found to be just a fraction of catches in similar lakes, despite intensive stocking efforts by fishery agencies. Sea lamprey were preventing the restoration of these native fish species to Lake Champlain.
Spawning
In the spring, sexually mature adult sea lamprey migrate up tributaries to spawn. They locate spawning streams by following pheromones (naturally produced chemical attractants) released by ammocoetes living in those waters. A pair of male and female sea lamprey build a nest, called a redd, in a gravel stream bottom in section of flowing water. The female lays tens of thousands of eggs and the male fertilizes them, then having completed this act the sea lamprey die. The eggs lie in the small spaces between the gravel, and are provided oxygen by the flowing water. Weeks later the eggs hatch and the complex life cycle of the sea lamprey begins again.
Other lamprey species
Three other lamprey species are found in the Lake Champlain Basin. Two species - the northern brook lamprey and the American brook lamprey - are non-parasitic filter feeders similar in size and habits to sea lamprey ammocoetes. The silver lamprey is parasitic, but does not have the negative impact on the Lake Champlain fish community that the sea lamprey does, due to its smaller size and fewer numbers.
Conclusion
Sea lamprey are an ancient fish, with a complex life cycle and mouth parts that are well adapted for their parasitic life. The elimination of this species from Lake Champlain is neither desired nor possible. However, their population must be reduced to lessen their negative impacts on the Lake Champlain fishery to an acceptable level, to balance the Lake Champlain Basin ecosystem and its world class fishery.
