Where would you place a miniature hat on a starfish?
Would you pop it right in the centre, or, on top of one of the points as hilariously demonstrated by Patrick in the iconic cartoon SpongeBob SquarePants. If so, which one?
do starfish wear hats like
— slate (@PleaseBeGneiss) September 20, 2020
this or this pic.twitter.com/7V2E5s7FjW
It’s a question that’s puzzled zoologists and developmental biologists alike. Where is the head on a sea star? Perhaps they have no head at all.
Not so. Now, a new study in Nature has mapped the body regions of starfish and found that, actually, starfish are all 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,” says Dr Laurent Formery of Stanford University in the US, lead author of the new study.
“It’s not at all what scientists have assumed about these animals.”
Unlike humans and other bilaterally symmetrical animals, starfish and other related echinoderms – such as sea urchins and sea cucumbers – have a five-fold axis of symmetry. This means that their body parts are arranged in five equal sections.
You can’t make heads nor tails of them and, until now, scientists haven’t been able to figure out how genetic programming drives this.
“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,” says Dr Jeff Thompson, a co-author on the study from the University of Southampton, UK.
“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.”
The team created a 3D map to understand where different genes were expressed during the development and growth of sea stars.
They found sea stars have a headlike region in the centre of each “arm” and a tail-like region along the perimeter. In an unexpected twist, no part of the sea star ectoderm – the outermost layer of cells – expresses a “trunk” genetic patterning program, suggesting that sea stars are mostly, and completely, headlike.
“The echinoderm body plan evolved in a more complex way than previously thought and there is still much to learn about these intriguing creatures,” says Thompson.
“As someone who has studied them for the last ten years, these findings have radically changed how I think about this group of animals.”
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