Starfish Bodies Aren’t Bodies at All, Study Finds

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Echinoderms (starfish, sea urchins, sea cucumbers, etc.) are part of the deuterostomes and all descend from a bilateral ancestor. This ancestor’s anatomical configuration, which divides the right side from the left, is similar to that of humans and many other animals in that it has two directions. It also has a head, a trunk, and a tail (or lower limbs).

However, adult starfish have a pentaradial organization and lack distinct right and left sides, as well as front and back. Although they retain a bilateral configuration as larvae, their anatomy radically reorganizes as they grow and eventually spreads in five distinct directions. This configuration is fundamentally different from their bilateral ancestor, puzzling scientists for decades. Furthermore, it’s particularly challenging to identify the body parts in starfish that are equivalent to those in bilateral animals. Thus, until now, the exact location of the head and the rest of the body in starfish remained a mystery.

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,” explains Jeffrey Thompson from the University of Southampton. “In their bilateral ancestors, the body was divided into the head, trunk, and tail. But just by looking at a starfish, it’s impossible to see how these sections are related to the bodies of bilateral animals,” he adds.

To decipher their astonishing morphology, Thompson and his colleagues compared the genetic markers that translate the anatomical configuration of a starfish to those of bilateral deuterostomes. Given their common bilateral ancestor, this comparative study improves our understanding of how echinoderms evolved their unique morphology.

Lack of specific trunk development genes

In bilateral animals, genes that translate morphological development are expressed in bands of tissue in the outer embryonic layer (ectoderm). The deployment of this genetic regulatory network translates the anteroposterior polarity of the ectoderm, meaning its development into the bidirectional distribution of different body parts. In the context of the new study, highly precise molecular and genomic techniques were used to understand where the equivalents of these genes are expressed during the embryonic development and growth of starfish. Subsequently, RNA tomography and in situ hybridization were used to create a three-dimensional map of the expression of these genes.

Scientists were surprised to find that in starfish, the equivalents of head-specific genes are expressed along a central line of symmetry on each arm. Genes expressing what appear to be the front parts of the body are located on the sides of this line, while those expressing the rear parts are farther away.

Even more astonishingly, the genes translating trunk morphology into bilateral deuterostomes had no equivalent in starfish. This suggests that starfish have abandoned their trunk region and released their ectoderm to flourish in new directions. This also means that their entire body is analogous to the head in other animals, allowing them to move and feed differently during their evolution. “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,” explains Laurent Formery, the study’s lead author, as published in the journal Nature.

Other experts suggest that bilateral animals likely retained their trunks to escape predators by swimming. In contrast, echinoderms defend themselves by curling up, which seems to be sufficient for their overall survival. “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,” concludes Thompson.