A starfish’s body is actually a HEAD, suggests new research.
Scientists say their findings help to answer the mystery of how they evolved their distinctive star-shaped bodies.
Echinoderms are a group of animals that includes starfish – also known as sea stars, sea urchins, and sand dollars.
They have a unique “five-fold symmetric” body plan, which means that their body parts are arranged in five equal sections.
That is completely different from their bilateral ancestors, which have a left- and right-hand side that mirror one another, as in humans and many other animals.
Study co-author Dr. Jeff Thompson, from the University of Southampton, said: “How the different body parts of the echinoderms relate to those we see in other animal groups has been a mystery to scientists for as long as we’ve been studying them.
“In their bilateral relatives, the body is divided into a head, trunk, and tail.
“But just looking at a starfish, it’s impossible to see how these sections relate to the bodies of bilateral animals.”
In the new study, led by scientists at Stanford University and published in the journal Nature, researchers compared the molecular markers of a starfish to other deuterostomes – a wider animal group including echinoderms and bilateral animals, such as vertebrates.
The research team said they share a common ancestor, so by comparing their development, they could learn more about how echinoderms evolved their unique body plan.
The team at Southampton used state-of-the-art micro-CT scanning to understand the shape and structure of the animal in unprecedented detail.
Then, their colleagues in the United States used ‘RNA tomography’ and ‘in situ hybridization’ to create a three-dimensional map of gene expression in the starfish and find out where specific genes are being expressed during development.
(University of Southampton via SWNS)
They mapped the expression of genes that control the development of the ectoderm, which includes the nervous system and the skin.
This is known to mark the anterior-posterior (front-to-back) patterning in the bodies of other deuterostomes.
The American researchers found this patterning was correlated with the midline-to-lateral axis of the starfish arms – with the midline of the arm representing the front and the outmost lateral parts more like the back.
In deuterostomes, there is a distinct set of genes expressed in the ectoderm of the trunk.
However, the researchers found that, in the starfish, many of these genes are not expressed in the ectoderm at all.
Dr. Thompson said: “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.
“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.”
He said that suggests that starfish and other echinoderms may have evolved their five-section body plan by losing the trunk region of their bilateral ancestors.
Dr. Thompson says that would have allowed the echinoderms to move and feed differently than bilaterally symmetrical animals.
He added: “Our research tells us the echinoderm body plan evolved in a more complex way than previously thought and there is still much to learn about these intriguing creatures.
“As someone who has studied them for the last 10 years, these findings have radically changed how I think about this group of animals.”
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