Starfish, sea urchins, sea cucumbers and their relatives are echinoderms. They have unique bodies that have long puzzled evolutionary biologists. These animals are descended from bilateral organisms that are symmetrical on both sides. But they have somehow evolved a radial body shape, meaning their parts are located around a central axis. Adult starfish have five (or more) “arms” extending from the central structure.

The biggest secret of starfish

In a new study published November 1 in the journal Nature, scientists claim that none of the previous hypotheses about the structure of starfish (and echinoderms in general) are correct. They say these animals are They have no body and all their mass is a big head.

To determine the position of the head in the star’s overall structure, the researchers performed genetic tests on Patiria miniata (sea bat or membranous star). They wanted to know how genes were expressed in each part of the animal’s body.

We prepared slices of starfish and sequenced each one to create a 3D model of the template RNA. [мРНК] “all over the body”
said lead author Laurent Formery, an evolutionary biologist at Stanford University’s Hopkins Marine Station.

The researchers then used a fluorescent dye that highlighted specific types of mRNA; This allowed them to view the mRNA under a fluorescence microscope and determine where it was expressed in the starfish’s body. It turned out that the same genes are expressed throughout the entire path of the star – from its center to the ends of the “arms”. This would not be possible if these were different parts of the body because different organs are coded by different genes.

The situation is similar with sea cucumber. Although the sea cucumber body plan typically appears bilateral from the outside, it also exhibits a radial body plan. According to the researchers, although it looks like a chubby worm, it is most likely not a head with a trunk, but an elongated head lying on its side.

Although somewhat simplified, the results of the study suggest that the starfish’s body can be thought of as a disembodied head walking on its lips on the seafloor (at least in terms of the anterior-posterior identity of its surface tissues). growing fringes from the tube feet, which replaced their original function of separating food particles to enable walking.
– Thurston Lacalli, a biologist at the University of Saskatchewan in Canada, wrote that he was not involved in the research.

It is unclear why echinoderms evolved this new body plan. The bilateral body plan has been incredibly successful throughout evolutionary history. Echinoderms first appeared at the beginning of the Cambrian (541 – 485.4 million years ago). A wide variety of organisms emerged during this period. “If you look at the animals that have survived to the present day, all types of organization in their bodies emerged at almost the same time.”, – says Laurent Formery. So while evolution was leading us humans towards our own bilateral symmetry, something strange happened to the echinoderms, and they turned into running heads on the sea floor.