The Biology of the Sea lamprey
Diogo Ferreira-Martins and Stephen D. McCormick
The sea lamprey, Petromyzon marinus Linnaeus 1758 is an Agnathan, and along with hagfish represent the most basal living vertebrates. They have a complex life cycle that includes metamorphosis, migration to the ocean (anadromy) and return to freshwater to spawn and then die. After hatching the young sea lamprey, known as an ammocoete (larvae stage) lives buried in the silty muddy substrate for 3 and up to 10 years. During this stage they resemble a worm like creature without eyes and a mouth apparatus adapted for filter feeding. After the ammocoetes grow and accumulate sufficient energy reserves they cease feeding and undergo metamorphosis. During metamorphosis eyes begin to emerge and develop, skin transitions from a brownish color to a silvery ventral and dark bluish dorsal color, fins begin to develop for swimming and the mouth apparatus transforms into a disc shape suction cup with teeth and a piston like tongue with teeth to rasp their host’s skin and create a wound from which they feed from. In addition to the exterior morphological changes, some internal organs such as the kidney, gill, and gut undergo reconfiguration. At the physiological level, metamorphosis is accompanied by the development of mechanisms for salt secretion that allows them to move from freshwater to seawater.
After completing metamorphosis, the postmetamorphic juveniles migrate downstream and enter the ocean where they begin their marine trophic phase, and parasitically feed on the body fluids of other fishes. After a period of nearly 18 months in the ocean where rapid growth occurs, the fully grown sea lampreys detach from their hosts, migrate into rivers guided by pheromones released by ammocoetes, Upon reaching a proper spawning site, the male of sea lamprey builds a nest by moving rocks on the stream bottom using its mouth and attracts a female to lay the eggs to be fertilized. When spawning is complete, both progenitors die. In addition to the anadromous ecotype, sea lampreys have also invaded the Great Lakes in North America and developed landlocked populations. In this ecotype, after metamorphosis the sea lamprey migrates from freshwater streams to freshwater lakes where they parasitize fish and after fully grown, they migrate back to freshwater streams in order to spawn.
Sea lampreys play important roles in their multiple ecosystems. During the ammocoetes stage, the sea lampreys are a significant portion of a freshwater stream biomass and provide a food source for predatory fishes, birds and some mammals. When migrating downstream sea lamprey juveniles also provide an important food source for river, estuary and ocean predatory fishes. Nevertheless, during the adult parasitic stages, particularly in the Great Lakes, they can pose a threat to fish stocks. As a result, large sums of money are spent every year for control of these freshwater population in North America. On the other hand, anadromous populations are threatened or endangered in the western coast of Europe due to loss of habitat and overfishing. Sea lamprey are the earliest known vertebrate that have adopted an osmoregulatory strategy in which they maintain constant internal salt concentrations irrespective of their environmental salinity. Their fellow agnathans the hagfish are osmoconfomers (keep the same internal salt concentration as their external environment) and are restricted to marine habitats. The evolution of an osmoregulatory strategy by lampreys has allowed them to utilize both freshwater and ocean habitats. While in freshwater sea lamprey must counteract the passive loss of ions to the environment, which they do by actively taking up salts across the gill using specialized cells known as ionocytes, while the kidney is responsible for the creation of a dilute urine in order to remove passive water gains. While in seawater, the sea lamprey passively loses water to the environment and gains ions. To overcome water loss the sea lamprey drinks seawater, and both water and salt are taken up by the intestine. The excess sodium and chloride is then actively secreted by the seawater type ionocytes in the gill and divalent ions removed via defecation and renal secretion. In other fishes such as the Atlantic salmon, Growth Hormone (GH) and Insulin-like factor 1 (IGF1) are known to play a role in the development of seawater tolerance, while prolactin (PRL) promotes freshwater adaptation. Cortisol has a role in both freshwater and seawater and may interact with the GH/IGF-I and PRL axes. Our group is currently working to understand the underlying physiological mechanisms of the sea lampreys osmoregulatory strategy and how hormones controls their capacity to move between freshwater and seawater.