Beneath the waves are strange, almost alien creatures that raise questions about the evolution of life on Earth and our own earliest origins. The answers may be hidden in tunicates.
Tunicates are filter-feeding invertebrates, including sea squirts and salps. The most common ascidiacean species are sessile and attach to rocks or the sea floor, while the appendicular species are free-swimming. Yet they all spawn as larvae that vaguely resemble tadpoles. Motile tunicates tend to grow into something that resembles a larger version of the larva. The others eventually plant themselves face down on a surface and absorb their own tails as they transform into a sessile, tube-like form with two siphons.
Despite all this weirdness, there is now strong evidence that tunicates are the closest relatives of vertebrates, but a mystery still hangs around them. How did they evolve and what did they evolve from? A 500 million year old fossil now tells us more about the evolution of these peculiar life forms.
An evolutionary puzzle
Discovered by Harvard University evolutionary biologist Karma Nanglu, what is now known as Megasiphon thylakos has started answering some unknowns about tunicates. Although they have been around since the early Cambrian, tunicates are largely absent from the fossil record. The immaculately preserved M. thylakos specimen finally provides more insight into their evolution and their relationship to vertebrates such as humans.
“[The fossil] indicates a difference between [free swimming] cecums and all other tunicates appeared 50 million years earlier than currently estimated,” said Nanglu and his research team in a study recently published in Nature. “Finally M. thylakos shows that fundamental components of the modern tunicate body plan were already established shortly after the Cambrian explosion.
Because tunicates are almost absent from the fossil record, it was not known whether they began as sessile organisms similar to ascidiaceans, or motile organisms such as cecums. Unfortunately, the other extant tunicate fossil, Shankouclava reports, is unclear. It does have some ascidiacean features, but lacks other features that define ascidiaceans, such as the nutrient siphons.
There are two hypotheses for the evolution of tunicates. The first suggests that their ancestral form was motile, like cecum, and that the sessile species evolved from motile ancestors. The second hypothesis states that the genetics of modern tunicates make it unclear whether their ancestral form was motile or sessile.
New evidence of ancient rocks
Due to its observable features, especially the tube structure and siphons so similar to modern ascidiaceans, the M. thylakos fossil could indicate that ancient tunicates started out as sessile creatures.
Also noticeable were the obvious dark bands running up and down his body. Nanglu and his team took hi-res images of the fossil and compared them to specimens of the extant tunicates Ciona intestinalis. It turned out that the bands were very similar to the muscles C. intestinalis used to open and close its siphons when it feeds. The M. thylakos body plan now believed to have first emerged after the Cambrian explosion, which was perhaps the largest burst of new organisms Earth has ever seen.
What exactly the basis of M. thylakos’ body attached to a rock or the sea floor is not visible due to deterioration. Despite that, its striking resemblance to extant ascidia-like tunicates C. intestinalis has convinced Nanglu that early tunicates were most likely sessile, like their descendants.
So how might tunicates be related to us? Like humans, tunicates belong to the phylum Chordata. Chordates have several common features, but the most notable is a flexible, supportive rod structure, or notochord, that runs the length of the animal. There are only two chordate groups that are not vertebrates: tunicates and cephalochordates. Cephalochordates, like lancets, used to be considered the closest relatives of vertebrates because they appeared more complex and morphologically similar. This was the assumption until a 2006 study found that tunicates are genetically closer to vertebrates than cephalochordates.
“Tunicans are an evolutionarily significant subphylum of marine chordates, with their phylogenetic position as the sister group to Vertebrata making them key to unraveling our own deep time origins,” Nanglu also said in the study.
While tunicates still hold secrets, there may be more fossils waiting to reach the surface of what was once a prehistoric ocean. They may have even more to say about where they – and we – come from.
Nature, 2023. DOI: 10.1038/s41467-023-39012-4 (About DOIs).
Elizabeth Rayne is a creature that writes. Her work has appeared on SYFY WIRE, Space.com, Live Science, Grunge, Den of Geek, and Forbidden Futures. When she’s not writing, she’s altering, drawing, or cosplaying as a character she’s never heard of before. Follow her on Twitter @quothravenrayne.