If you see a starfish on the beach, where is its head? It seems logical to point to the core of the star shape, where the five arms come together. But according to American and British biologists, you are also correct if you indicate an arm of the star, any arm. Because from the perspective of developmental biology, starfish only have a head and a tail, but no torso. And the head and tail are located almost everywhere in the starfish body, both in the core and in the arms. The biologists concluded this from their research in which they studied the formation of, among other things, the nervous system during the development of a starfish, from larva to adult. The research is described this week in the trade journal Nature.
Starfish and other echinoderms, such as sea urchins and sea cucumbers, have a different symmetry than vertebrates. Where our bodies are bilaterally symmetrical, with a left and a right side that are almost mirrors of each other, starfish have a radial or rotational symmetry. They stay the same if you rotate them about an axis perpendicular to their center. Yet only fully grown starfish are radially symmetrical, because during their early development as larvae, they have a left and a right side, just like us and the vast majority of animals. Only when the larvae settle on the seabed after floating around in the seawater for months does rotational symmetry arise.
To better understand this remarkable transformation, the biologists studied starfish at the cellular and genetic level. They focused on the ectoderm, an embryonic structure that grows and differentiates during development into, among other things, the skin, nerves and the brain. In vertebrates, the formation of the ectoderm neatly follows the bilaterally symmetrical building plan, with a clear front and back – a head and a tail. This is also initially the case with starfish larvae, but later in embryonic development this building plan disappears into the background. As a result, a fully grown starfish appears at first glance to have no head and no tail.
No ‘trunk cells’
Although the head-tail pattern can still be found at the molecular biological level, the researchers discovered. They saw it reappear when they made active developmental genes (genes in which an organism’s building plan is encoded) light up with a clever trick. The pattern was mainly visible in the starfish arms. The ‘head cells’ appeared to be located along the midline (lengthwise) of the arms, the ‘tail cells’ more on the outside. In the center of the starfish, mainly head cells lit up. What struck the researchers most was that they found no ‘trunk cells’. The ectoderm of starfish appears to consist mainly of head and tail cells.
It therefore appears that starfish, and echinoderms in general, lost their torso in their early evolution, giving them a different body symmetry than their ancestors, who were bilaterally symmetrical.