When it comes to starfish, scientists have finally answered Basement Jaxx’s most famous question – their head is all over their body, to the point where their body is essentially, well, a head.
“It’s as if the sea star is completely missing a trunk, and is best described as just a head crawling along the seafloor,” said Laurent Formery, lead author of the new starfish study, in a statement. “It’s not at all what scientists have assumed about these animals.”
The vast majority of animals have bilateral symmetry, meaning they can be split into two mirrored halves along a head-to-tail axis. This body plan is carefully controlled by a series of developmental genes. Starfish, however, have five-fold symmetry and appear to have no clear head or tail, which throws a spanner in the works for our understanding of animal evolution.
In the study led by Formery, a multi-disciplinary team of researchers used a combination of imaging and genetic techniques to create a 3D atlas of starfish gene expression. This included micro-CT scanning, which illustrated the skeleton, muscles, and digestive, nervous, and water vascular systems of a starfish, combined with RNA tomography and in situ hybridization – staining genetic material with fluorescent labels – to show which genes were being expressed and where.
Their efforts revealed that the “head” of a starfish is, in fact, composed of headlike regions distributed in the center of its “body” and the center of each of its five limbs. Thus, what we think of as a starfish’s body is actually more like a head.
How did this happen? By losing some of those body plan-related developmental genes, according to the researchers’ gene expression atlas.
“When we compared the expression of genes in a starfish to other groups of animals, like vertebrates, it appeared that a crucial part of the body plan was missing,” explained study co-author Jeff Thompson in a separate statement. “The genes that are typically involved in the patterning of the trunk of the animal weren’t expressed in the ectoderm. It seems the whole echinoderm body plan is roughly equivalent to the head in other groups of animals.”
This indicates that the starfish body plan evolved in a much more complex way than scientists previously thought, said Thompson. Starfish and other echinoderms – like sea urchins and sea cucumbers – share a common ancestor with bilateral animals. The study’s findings suggest that they may have evolved their five-fold symmetry by losing the trunk region they shared with their bilateral cousins, allowing them to move and feed differently.
“These results suggest that the echinoderms, and sea stars in particular, have the most dramatic example of decoupling of the head and the trunk regions that we are aware of today,” said Formery. “It just opens a ton of new questions that we can now start to explore.”
Perhaps the most critical of these questions: How do starfish wear hats if they are just…head? In the middle? On whatever limb they fancy that day? Over to you, science.
The study is published in Nature.