By Brittney G. Borowiec, CSZ-SCZ Blog Managing Editor & PDF, Wilfrid Laurier University
Lampreys are strange creatures for many reasons. They can sense electricity, regrow a severed spine, and may have even sparked a civil war by poisoning an English King. But perhaps their most fascinating feature is that they metamorphose from a small, filter feeding larvae into a specialized predatory adult.
The evolutionary history of lampreys and how they fit in with other vertebrates is somewhat unsettled. One influential theory argues that the lamprey lifecycle mirrors the evolutionary path followed by our ancient backboned ancestors. The theory largely rests on the striking similarities between extant lamprey larvae and lancelets (also called amphioxus), an early diverging group of chordates that last shared an ancestor with modern vertebrates over 650 million years ago. But this idea has had recent pushback.
“If you put a modern lamprey larva and an amphioxus side by side in an exam, many university biology students won’t be able to tell them apart” notes Tetsuto Miyashita, research scientist at the Canadian Museum of Nature, via email. Miyashita is the co-lead author of a new study in Nature that argues that lamprey larvae are not ancient holdovers from the very early days of vertebrate evolution.
“The idea that modern lamprey larvae retain the image of our distant ancestors – the earliest vertebrate animals – has been the bedrock of our field for 150 years”
Baked in with this assumption is that lamprey larvae are a convenient model to test predictions about vertebrate ancestry by mimicking how our extinct ancestors looked, lived, and behaved. Though recapitulation theory, which proposes that an animal’s embryonic development reflects its evolutionary path, isn’t mainstream science now, it remains prevalent in vertebrate paleontology. It keeps getting cited largely because the fossils needed to disprove it – soft-bodied lamprey larvae or juveniles – are so rare.
“We went and found evidence to the contrary [of recapitulation theory], that the modern lamprey larvae represent a newly evolved condition, that our distant common ancestors didn’t look like that half a billion years back,” says Miyashita.
Miyashita credits his co-lead author Rob Gess, a researcher at Rhodes University, South Africa, as being instrumental in collecting the elusive fossils.
“In the 1990’s, [Gess] heard that a road construction project was being planned near the city of Makhanda, and it would cut through his best [fossil] site. He drove up in his pickup truck to rescue more than 100 tonnes of shale from the site and built a shed in his farm to store these samples. He has been going through this rescued material ever since, and making amazing discoveries, including this lamprey study.”
In the paper, Miyashita, Gess, and colleagues report detailed descriptions of larval and juvenile forms of four ancient lampreys from the Palaeozoic era. Crucially, the samples include a hatchling-to-adult growth series of Priscomyzon, an extinct 360-million-year-old lamprey with many of features that are familiar to researchers of living lampreys, including a large oral disc, circumoral teeth, and a branchial basket.
By meticulously recording every feature of their samples, some of which were only a few millimetres in length and little more than stains on a rock, the researchers “were able to reconstruct the growth series in four different species of fossil lampreys from 360 to 310 million years ago. Nowhere in these growth series did a fossil lamprey undergo a blind, filter-feeding stage like modern lampreys.”
The adult-like larvae occur in at least three independent lineages of ancient lamprey. This suggests that instead of being a hold over from a shared ancestor between lamprey and other vertebrate lineages, the larval stage was an evolutionary innovation specific to lamprey, perhaps an adaptation to the ecological challenges of new freshwater habitats.
“The larval phase of modern lampreys that everyone thought was a distant echo of the vertebrate ancestry turned out to be a recent innovation”
Evidence that points to the larvae not being a 500+ million-year-old time capsule would rewrite a lot of textbooks. Miyashita jokes he’s “made everyone’s job a bit more difficult by saying that lampreys are not swimming time capsules.” But he still thinks larval lamprey are instructive as he reflects upon his experiences at CSZ, and how the community has shaped his scientific career so far.
“If I were to make an analogy of lamprey life history to academic conferences, you start out as a helpless blind larva that buries its head in the sand and needs to take up its nutrients from the surrounding water, before you metamorphose into a bloodsucking monster of a full-fledged scientist.”
“CSZ has been critical to some of the intellectual metamorphosis I went through, because we have [a] keen but comfortable audience that are genuinely curious about and supportive of your work.”
Header image provided courtesy of Tetsuto Miyashita.