A sea lamprey suctioning itself to a rock. Photo credit: Andrew Muir, Great Lakes Fishery Commission.
The great migrations are wonders of the natural world. Seeing thousands or millions of creatures moving across the Earth or through the sky is truly amazing. Many of these animals can home in on their final location using an internal compass or the Earth's magnetic field, but not every animal that migrates has this ability. Such is the case for the sea lamprey (Petromyzon marinus). These prehistoric and sometimes invasive animals spend the first portion of their lives in rivers and streams before heading downstream to the ocean. Once they become adults, they return to freshwater to breed. Without the capabilities of some other migrating creatures, how do lampreys make it back home without getting lost and still have enough energy to reproduce when they finally arrive? Kandace Griffin, Christopher Cahill and Michael Wagner of Michigan State University, USA, think that they might have found the answer: the lampreys are using the riverbed.
To test their theory, Griffin and colleagues teamed up with Christopher Holbrook of the United States Geological Survey, USA and Daniel Zielinski of the Great Lakes Fishery Commission, USA, to find out how sea lampreys were migrating up the White River in Michigan and what they were using to quickly find their way. This meant first mapping the bottom of the river by paddling up and down it with a sonar unit mounted to the kayak. Then the researchers had to set up an array of sensors across the riverbed and another 0.2 km upstream. Afterwards, the team collected ∼60 sea lampreys and fitted them with transmitters that would record when they went through each set of sensors and their location in the river.
From May to June 2021, the researchers released the lampreys back into the river so that they could swim upstream and through the sensors laid out in the river. Griffin and the team noticed that the lampreys were shying away from the shallow waters near the shoreline, preferring to stay in the deepest part of the river. Also, when given a choice at forks in the river, the lampreys chose the path with the deepest water. This is different to the way that migrating fish move upstream: they generally stick to shallower waters. But the researchers think that the lampreys have a reason for sticking to the deep waters. Being in the depths of the river makes them more difficult for night-time predators to catch. This can come with a trade-off though. The deepest parts of the river are usually where the water is moving fastest.
Surprisingly, when Griffin and colleagues looked at how much energy it cost the lampreys to move through the deep waters, they found that the lampreys used 5.8% less energy than they did when swimming close to the surface. This saved some energy for the lampreys to use for breeding when they finally arrived at their destination.
While no one knows how exactly the lampreys detect the deepest part of the river, knowing where to find these aquatic parasites is incredibly important. Sea lampreys are native to the east coast of the United States and the Atlantic Coast of Europe but are also highly invasive in areas such as the Great Lakes in the northern United States. Given that sea lampreys are also parasitic, they are generally not welcome guests in the habitats they've invaded. Knowing that they tend to travel in the deepest part of the river will help conservationists more easily trap and remove the unwanted lampreys, leaving the native fish in relative peace.
