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The quetognatos , whose name means “jaws with bristles”, can be found all over the world, swimming in brackish estuaries, tropical seas and in the depths of the dark and deep ocean. Also known as arrowworms, creatures have existed since the Cambrian Period, but their precise place in evolutionary history has been difficult to pin down.
Now, researchers from the Okinawa Graduate Institute of Science and Technology (OIST) in Japan have learned where the arrow worms are located on the tree of life, and their results could reveal important trends in the evolution of bilateral agencies.
The researchers tried to verify the relationship of the predatory arrow worm with other spiralia: members of a diverse group of organisms, or clade, believed to share a common ancestor. The spiralia clade includes mollusks, segmented worms and flat worms. Surprisingly, the scientists discovered that the arrow worms do not belong to spiralia, but to a new group of animals that form a brother group of the clade.
The results of this research, detailed in an article published in ‘Current Biology’ , challenge the classical view that complex organisms evolved from simple ancestors by acquiring new traits over time.
“Arrow worms are predators, they have nervous systems, they have developed sensory organs, but the other organisms with which they are grouped are much simpler – says the first author, Ferdinand Marlétaz, postdoctoral scientist in the OIST Molecular Genetics Unit, directed by Daniel Rokhsar – if you put the arrow worms here, it means there’s probably a lot of independent simplification, rather than the independent emergence of complexity . “
Although they are very different in appearance, arrow worms and their phylogenetic relatives, such as microscopic animals known as rotifers, seem to share a unique jaw structure. Composed of a dense matrix of proteins and a fibrous substance called chitin, these jaws are located near the mouths of organisms and allow them to grab their prey.
“The arrow worms are grouped with a rather dark collection of small marine animals: they are not animals that most people are familiar with,” says Professor Daniel Rokhsar, lead author of the study and principal investigator of the research unit. . “The fact that these different animals had jaws that were probably related to each other was not clear until this document,” he adds.
ARROW WORMS FROM ALL OVER THE WORLD
The approximately 200 species of arrow worms resemble small spears and range from only one millimeter in length to 12 centimeters in length. Predators feed mainly on small crustaceans called copepods, which use their sharp vibratory sense to hunt prey and swallow them whole. The strange, wormlike creatures actually share many morphological and developmental traits with other organisms, which makes their evolutionary timeline difficult to trace.
“The different animals that share the same early development are often related to each other,” says Rokhsar. One reason why researchers have worked to characterize arrow worms is that their early developmental patterns are ambiguous; they resemble the patterns observed in two large groups of animals. “We really did not have a way to accurately classify arrow worms in one way or another,” he says.
The two animal supergroups are known as deuterostomados and protostomados. Both sets of organisms have a single intestine running through them, from their mouths to the other end. In early development, the intestine of the deuterostomate is formed from the bottom up, while the intestine formation of the protostomes begins in the mouth. Although the arrow worms develop from the bottom up like the deuterostomes, they look a lot like the protostomads both morphologically and genetically.
To clarify this discrepancy, the researchers collected data from ten species of arrowworms and compared them with publicly available data from other animals. They examined the transcriptome species, which serve as a snapshot of all the genes that are expressed in a given cell.
Scientists originally drew their sample arrow worms from the Atlantic Ocean, the Gullmar Fjord in Sweden, the Amakusa Islands in Japan and Marseille in France. Dr. Katja Peijnenburg, from the University of Amsterdam (Netherlands), Dr. Taichiro Goto, from the University of Mie (Japan) and Professor Noriyuki Satoh, from the OIST Marine Genomics Unit, helped Marlétaz to collect and prepare the samples.
The comparison placed the arrow worms solidly among the protostomes within a subgroup that includes microscopic organisms known as rotifers, gnathostomulids and micrognatozoans. “It surprised me a little,” says Marlétaz. “We still do not fully understand this association with rotifers and others, which will be the focus of future research,” he says .