Karen Boswarva considers the echinoderms, an ancient group of marine animals with extraordinary healing abilities
OCEANSCIENCE
The regeneration game
Karen Boswarva considers the echinoderms, an ancient group of marine animals with extraordinary healing abilities
How many arms does a starfish have? Hint, it’ s a trick question. Starfish belong to a big group of invertebrates called the echinoderms, which translates from Ancient Greek as‘ spiny skin’. Makes sense, if you’ ve felt one, they’ re rather hard and can feel quite prickly, rather like an aquatic hedgehog.
The echinoderms are a fascinating group of animals that also includes brittlestars, sea urchins, sand dollars, sea cucumbers, and crinoids( feather stars, and sea lilies). Three key traits make them fundamentally distinct from other animal groups( phyla):
■ A spiny outer layer that protects an internal skeleton, made of plates of calcium carbonate.
■ A five-point radial symmetry.
■ A water-vascular system that pumps seawater around their body into thousands of tube feet used for walking, breathing, and detecting food
Echinoderms also possess an additional superpower – regenerative genes, the ability to grow back limbs. Regeneration is not unfamiliar in marine organisms; who hasn’ t chuckled, mask filling with water at the sight of a bruiser crab with one MONSTER claw and one
not so matching itsybitsy teeny tiny claw. Or a starfish reduced to one or two arms.
A clade of species is a group that contains a common ancestor and all its descendants. As it happens, the clades that evolved before vertebrates, particularly the echinoderms show the greatest regenerative capabilities. Yet each of the echinoderm classes varies in their ability to regenerate. Sea urchins are at the bottom, capable of regenerating only the test( shell), spines, and pedicellariae( claw
“ Who hasn’ t chuckled, mask filling with water at the sight of a bruiser crab with one MONSTER claw”
A common sunstar, Crossaster papposus. The urchins in the background are also echinoderms shaped appendages). At the top sit the sea cucumbers, capable of multiple organ muscle and tissue regeneration, given that these species can expel their internal organs to startle predators, and regenerate from remnant body parts!
Starfish and brittlestars sit somewhere in the middle but have an extra trick to avoid predation. They can self-amputate their arms in a process called autotomy by releasing a stressinduced hormone called neurohormone. When attacked, the starfish releases a peptide causing violent contractions in specialised muscles at the base of the arm, snapping off the limb. The affected arms will then regenerate, or a whole organism may grow from a single arm.
The process of regeneration is fascinating. Soon after the arm is severed, the wound is covered by a membrane which forms a protective barrier. This is followed shortly after by a blastema-like structure( a bud of immune cells and local cells from the tissue layers). As the blastema-like structure grows, it generates new tissue and organs, with distinctive arm segments becoming visible. Each new segment grows from the end tip until a full-length arm is formed.
Delving further to understand which genes are associated with the regeneration of the various body parts in echinoderms has kept scientists busy for decades, and still does. The sequencing of echinoderm genomes, achieved now for several species has enabled the visualisation of key factors involved in regeneration processes. Recent genome sequencing of brittle stars( 2024) identified not only that brittle star genomes are more dynamic than other echinoderms, but when compared with other animals it highlighted a set of conserved genes of ancient origin that control regeneration across diverse animal groups, from crustaceans to vertebrates.
Next time you see some starfish, you’ ll have to count how many arms they have. Better yet, play a game to find a starfish with the least( or most) arms. �
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