The sea lamprey (Petromyzon marinus) is an ancient, jawless fish recognized for its eel-like body and predatory, parasitic lifestyle. This organism has a complex life cycle involving dramatic size changes, which directly influence its impact on aquatic ecosystems. The sea lamprey’s dimensions are directly tied to its feeding capacity and its reputation, particularly as an invasive species in landlocked systems like the Great Lakes.
Adult Size and Maximum Dimensions
The size of a mature sea lamprey is highly variable, but it is one of the largest lamprey species in the world. Adult individuals in landlocked populations, such as the Great Lakes, typically range from 14 to 24 inches (35 to 60 centimeters) in length. These freshwater parasitic adults rarely exceed 25 inches (64 centimeters) and a weight of 1.3 pounds (0.6 kilograms).
The largest specimens are found in their native Atlantic marine environment. In these oceanic habitats, where host fish are larger and more abundant, anadromous adults can reach exceptional lengths, approaching 47 inches (120 centimeters) and a weight of up to 5.5 pounds (2.5 kilograms). This considerable size difference underscores the influence of the trophic environment on the lamprey’s final adult dimensions.
Growth Through Distinct Life Stages
The sea lamprey’s growth is characterized by three distinct life stages, each with a specific size and function. The life cycle begins in freshwater streams with the larval phase, where the organism is known as an ammocoete. Ammocoetes are blind filter feeders that burrow into the sediment, consuming detritus.
This larval stage is prolonged, often lasting from three to ten years, and sometimes up to 17 years. Ammocoetes remain relatively small during this time, typically growing to a maximum length of about 5 inches (12-13 centimeters). This long, non-parasitic phase is followed by a dramatic metamorphosis where the lamprey develops eyes, a sucker mouth, and sharp teeth, transforming into a juvenile parasite.
Upon transformation, the juvenile lamprey migrates downstream to a large body of water, such as the ocean or a great lake, to begin its parasitic existence. During this 12 to 20-month feeding phase, the lamprey experiences rapid growth by consuming the blood and fluids of host fish. Once sexually mature, the adults cease feeding, migrate back upstream to spawn, and die shortly after reproduction.
Anatomical Features Related to Feeding Capacity
The final size of the parasitic adult is linked to the unique anatomical features that enable its feeding capacity. The most notable feature is the jawless, circular oral disc, which functions as a powerful suction cup for attaching firmly to a host fish. This disc is armed with numerous concentric rows of sharp, keratinous teeth that help secure the attachment.
Once attached, the lamprey employs a piston-like tongue, also covered in rasping teeth, to bore a hole through the host’s scales and skin. A larger adult size means a larger oral disc, which allows for attachment to bigger, faster-moving host fish, increasing the potential food source. The strong suction power needed for attachment is generated by the greater muscle mass found in larger individuals.
The size of the lamprey determines the diameter of the wound and the volume of fluids it can consume. Larger individuals are more lethal to their hosts, which is why their rapid growth during the parasitic phase is detrimental to fish populations. The secretion from the buccal glands contains an anticoagulant that further enhances the feeding process.
Geographic Factors Influencing Overall Size
The overall size attained by a sea lamprey is influenced by the geographic location and the availability of suitable hosts. In the native Atlantic Ocean, the anadromous life cycle provides access to a massive and diverse population of large marine fish, including sharks and salmon. This rich trophic resource supports the growth of the largest individuals, which can reach nearly 4 feet in length.
In contrast, landlocked populations, such as those in the North American Great Lakes, are constrained by the size and abundance of available freshwater host species. Due to these limitations and ongoing control efforts, landlocked sea lamprey populations are consistently smaller than their oceanic counterparts. For example, anadromous sea lampreys in Europe average 85 centimeters in length, while landlocked adults in the Great Lakes rarely exceed 64 centimeters.