Blind, worm-like plankton eater: The worm-shaped larvae of the fish-like lampreys were previously considered relics from the prehistoric times of vertebrate evolution. But now fossils show that the seemingly primitive creatures are not an archetype, but a new development. Accordingly, the larvae of the early forms of the lamprey still resembled the adult animals. Apparently they did not bring forth the worm-like juveniles until later as an adaptation to life in rivers. The lampreys aren’t quite the floating time capsules we thought they were, the researchers say.
Ancient and downright bizarre: The lampreys are fish-like vertebrates that, according to fossil finds, have barely changed their primitive-looking physique since prehistoric times. They have an eel-like body, but instead of jaws only a rounded mouth that is covered with teeth. They attach themselves to fish and draw their blood from them. Contrary to the German name, the lampreys only have two eyes. The name goes back to seven gill slits, a nostril and the eye, which when viewed from the side appear like nine organs of vision. The lampreys are counted among the vertebrates because they have various basic characteristics in common with them. Above all, they have a skeleton made of cartilage.
In addition to their primeval-looking properties, the unusual life cycle of the lampreys has so far been regarded as a greeting from the early days of the vertebrate evolution. After hatching, the eyeless larvae (Ammocoetes) of today’s lampreys bury themselves in the river bed and use bristle-like structures to filter food particles out of the water. Only later do they turn into the fish-like bloodsuckers. The larvae are so different from the adults that scientists originally thought they were different living things. The apparently primitive larvae were later viewed as a relic from evolutionary history.
Apparently primeval larvae
“They seemed so primitive, comparable to worm-like invertebrates, that their characteristics perfectly matched ideas about the evolution of vertebrates. The modern lamprey larvae were therefore used as a model for the original state from which the vertebrate lines arose, ”says Tetsuto Miyashita from the University of Chicago. “So far, however, there has been no evidence that such a rudimentary form goes back to the beginning of the vertebrate evolution.” As Miyashita and his colleagues have now been able to show, this is apparently not the case either.
Their results are based on the investigation of newly discovered lamprey fossils including young animals that belonged to four extinct species. They were discovered at sites in South Africa and the USA and dated to an age between 310 and 360 million years. As part of their investigations, the researchers were able to show that the smallest individuals still carried a yolk sac, which indicates that the young animals had just hatched. The detailed investigations then revealed that these hatchlings already had large eyes and the typical toothed suction cup. They already resembled the adult animals. This is in clear contrast to today’s lampreys, which hatch as blind, worm-like larvae.
Innovation instead of relic
“We found specimens that map the path from hatching to adult,” says co-author Michael Coates. As the researchers point out, the species were representatives of different lines of development of the lampreys. “They all show the same pattern: the larval shape was like a miniature adult,” says Coates. It is therefore clear: the larval stage of today’s lampreys is nothing ancient, but evidently a further development in the course of the later evolution of these living beings. The strange life cycle of the lampreys cannot therefore offer any insight into the development history of vertebrates, say the researchers.
But why did the lampreys produce this strange larval stage? The paleontologists suspect that the filtering juveniles may have been an adaptation that enabled the lampreys to colonize rivers and lakes. The fossil lampreys reported in the new study all come from marine sediments. But many modern lampreys live in fresh water or only migrate into the sea as adults. For the marine lampreys of the past, rivers and lakes would have been a rather problematic habitat because of the limited and unpredictable supply of prey, the researchers suspect. “The lampreys solved this problem by producing larvae that buried themselves in the sand and snatched whatever food particles were available until they were ripe enough to start looking for prey with blood,” explains Miyashita. “But for this kind of lifestyle the larvae didn’t have to be built too complicated. So although lampreys have reinvented this filtering larval phase, the larvae themselves looked originally. So far we have therefore misinterpreted this simplicity as primordial, ”says the paleontologist.
His colleague Coates concludes: “So lampreys are not quite the floating time capsules we thought they were. But they still remain important to understand the deep history of vertebrate evolution. However, we have to take into account that the lampreys have also developed and specialized in their own special way, ”says the scientist.
Source: University of Chicago Medical Center, Article: Nature, doi: 10.1038 / s41586-021-03305-9