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Evolutionary mystery: New species of marine worms provide first evidence of male dwarfism

An evolutionary mystery lurked in the Kumano Sea, off the southeast coast of Japan. Researchers collected samples from the muddy seafloor, including hermit crabs, mollusks and discarded shells.

Here, in and on these shells, they found scales that usually lived in pairs, with a striking difference from the nearly 900 already known types of scales: one was a quarter the size of its partner.

The discovery was published as the cover of the Journal of Zoological Systematics and Evolutionary Research.

“The species is characterized by males that are dwarfs, with their tiny bodies always on the dorsal side of females,” said author Naoto Jimi, a postdoctoral researcher at the National Institute of Polar Research, Research Organization of Information and Systems.

“It is the first case of extreme sexual dimorphism in scale worms.” Scale worms can be found in every ocean, from low tide areas to deep seas.

They are characterized by the shell-like structures on their backs, and nearly half of them are symbiotic, depending on a host organism for survival.

Named after Issun Boushi, a Japanese fairytale character only three centimeters tall, the recently discovered scaleworm is the first to exhibit male dwarfism.

“Extreme sexual size dimorphism is one of the most striking phenomena in evolutionary biology,” Jimi said.

“While the origin has been well debated and some causes have been suggested, the evolutionary history remains unclear.

” The researchers conducted extensive observations, as well as morphological and genetic analyses.

They found that the scale worms always lived in shells, occupied or not, and males were never found without a female counterpart, but lone females were occasionally found.

In more than 200 marine collections, they have never found this shell worm that lives without a shell. However, their ancestral analysis found that the species likely descended from a free-living organism.

“Our data strongly suggest that, based on the ecological and behavioral characteristics, the development of male dwarfism in E.

issunboushi is more likely related to their symbiotic lifestyle in gastropod shells ingested by a hermit crab,” Jimi said, noting that other symbiotic species live in the same habitat, but have not developed male dwarfism.

“Its unique ecological and ecological features may have led to the development of the dwarf male, but this needs to be tested in the future based on additional cases.” Reference: