It turns out that starfish don't have arms – they're "just a head crawling along the seafloor"
Starfish are known for their adorable and symmetrical arms that seem to hug everything they touch. But it turns out that they may not be hugs after all – because starfish, researchers found, are basically "just a head."
Having radially symmetrical arms is an iconic trait of starfish, but in new research published in the journal Nature last week, scientists from England's University of Southampton say that the creatures are significantly lacking the genetic codes for animal torsos and tails. They also found that the genetic codes that are generally linked to heads were found in the middle of starfish arms, meaning that the seemingly headless creatures are actually anything but.
"It's as if the sea star is completely missing a trunk, and is best described as just a head crawling along the seafloor," the study's lead author Laurent Formery said in a news release. "It's not at all what scientists have assumed about these animals."
The University of Victoria's Thurston Lacalli offered a similar comparison to the team's findings.
"One could think of the body of a starfish ... as a disembodied head walking about the sea floor on its lips – the lips having sprouted a fringe of tube feet, co-opted from their original function of sorting food particles, to do the walking," Lacalli said. " ... This is truly a radical transformation of the ancestral bilaterian body plan."
Starfish are echinoderms, a form of invertebrate marine animals known for radial symmetry and having spiny skin. Researchers said most animal species have similar genetic structures, prompting them to investigate how echinoderms' unique composition came to be in "one of the most enduring zoological puzzles."
"If you strip away the skin of an animal and look at the genes involved in defining a head from a tail, the same genes code for these body regions across all groups of animals," study author Christopher Lowe said in ScienceDaily. "So we ignored the anatomy and asked: Is there a molecular axis hidden under all this weird anatomy and what is its role in a starfish forming a pentaradial body plan?"
To figure it out, they used a new form of genetic sequencing called HiFi sequencing, which, according to a news release, "can pull highly accurate data from intact, gene-sized DNA strands, making the process much faster and cheaper."
Study co-author David Rank said this process allows them to do months' worth of work "in a matter of hours."
"These advances meant we could start essentially from scratch in an organism that's not typically studied in the lab and put together the kind of detailed study that would have been impossible 10 years ago," he said.
What they found is that starfish don't have a head-to-tail axis that runs from the center to arms, from its top to its belly or from one side of its arms to the other. Instead, the press release says, "they saw that gene expression corresponding to the forebrain in humans and other bilaterally symmetrical animals was located along the midline of the sea stars' arms, with genetic expression corresponding to that of the human midbrain towards the arms' outer edges."
The only place in starfish where scientists found genes similar to that of animal trunks was at the edges of starfish arms.
"These results suggest that the echinoderms, and sea stars in particular, have the most dramatic example of decoupling of the ahead and the trunk regions that we are aware of today," Formery said. "It just opens a ton of new questions that we can now start to explore."