Starfish are the world’s strangest sea creatures. They have no blood or brains. They feed by vomiting their stomachs into their mouths and swallowing their prey inside. They move around on thousands of tiny, tube-like fingers, and many can regenerate lost body parts—some can even form an entirely new sea monster from a single “arm” and part of the central disk.
The anatomy of the heard Ocean Explorers has been, thus far, a disturbing riddle for zoologists. For a while now, starfish have been suggested as not having a head because nothing else can be visually ascertained as a head. “It’s as if sea stars were missing their proboscis entirely and were defined simply as a head dragging along the seafloor,” said lead researcher Laurent Formery, a biologist at Stanford University and the University of California, Berkeley, in a statement from the Chan Zuckerberg Biohub, which funded the study. “It’s not what scientists have thought about these animals.”
Starfish are the world’s strangest sea creatures
Sea stars, also known as starfish, are part of a group of animals called echinoderms, which include sea urchins and curlews. These creatures are typically characterized by traditional fivefold symmetry, meaning they can divide themselves into five equal parts. Humans and most other animals, on the other hand, have bilateral symmetry, meaning we can divide ourselves in half into equal parts. Interestingly, though, sea stars begin their lives as larvae with bilateral symmetry, before adopting their adult radial form.
A new study published last week in Nature absolutely puts this conclusion on its head and shows that starfish are-all head and no tail. “This has been a zoological mystery for hundreds of years,” said Christopher Lowe, a marine and developmental biologist at Stanford University. “How do they manage to morph from a bilateral body plan to pentaradial body plan? How could any part of the sea star ever be compared to ourselves?”
To answer those questions, scientists might want to examine a sea star’s DNA and take note of where on its body the genes that code for its head or trunk are expressed. However conventional methods used to locate gene expression in other animals don’t work adequately on the tissue of young sea stars, according to the statement. Those traditional methods involve cutting up genetic tissue into small pieces. But now, using new technology, the team was able to extract data from intact, gene-sized DNA strands and create a three-dimensional map to pinpoint which genes are active at precise locations in a sea star.
The team compared gene expression in the giant sea star species Patiria miniata to a closely related species of acorn virus. They observed that gene expression similar to head development in acorn worms and other vertebrates was distributed around the sea stars, especially in the center and middle of each “arm,” but genes associated with the trunk (or torso) in other animals were largely absent.
Because the ancestors of sea stars have trunks, this shows that at some point in their evolutionary journey, sea stars lost those genes — though when that happened remains a mystery. “This is an elegant and sound study that carries very important implications for the evolutionary history of sea stars and other echinoderms,” said Ilham Rahman, a paleontologist at the Natural History Museum in London, who did not take part in this research. “Ultimately, this could allow us to uncover the sequence of evolutionary changes that gave impetus to the head-like body design.”
Next, the research group plans to reexamine the fossil record to determine exactly when sea stars lost their torsos, writes Clinical American’s Lori Youmshajekian. They also plan to see if the “all heads” pattern holds for sea urchins and sea cucumbers.