A typical starfish has five-sided symmetry. With no clear head, the starfish can move in any direction, led by any one of its five arms. If you were feeling particularly cruel, you could fold one up in five different ways, so each half fitted exactly on top of the other. We humans, like many other animals, have only two-sided symmetry. We’re ‘bilateral’ – our right half mirrors our left, and we have an obvious head.
These two body plans might look radically different, but looks can be deceiving. Chengcheng Ji and Liang Wu from the China Agricultural University have found that starfish have hidden bilateral tendencies, which reveal themselves under times of stress.
Starfish belong to a group of animals called echinoderms, which also include sea urchins, sea cucumbers, brittlestars. As adults, most of the group have five-sided symmetry. As larvae, they’re very different. A baby starfish looks entirely unlike a star and rather like an alien spacecraft, with tentacle-like prongs sticking out from a central bell. The whole structure has a head and is very clearly bilateral. Their five-sided symmetry only emerges when they grow up, but Ji and Wu think that starfish never forget their two-sided beginnings.
The duo studied over a thousand starfish, and exposed them to various challenges to see how they would react. First, they dropped the animals in a new tank of water, waited for them to crawl away, and noted which arms they led with. Next, they turned the animals over. An upside-down starfish pushes two arms against the ground for support and stamps down with the opposite one to flip itself back up. Ji and Wu recorded which arms they led with. Finally, they put the starfish in a shallow tank and dropped an irritating liquid on their backs. Again, as the animals fled, Ji and Wu noted the arms that led the way.
These three challenges revealed that starfish have a hidden bilateral symmetry, and move in a preferred direction. That’s especially obvious when they face stressful situations, such as fleeing or having to turn themselves over.
They tend to lead with the fifth arm. There are many ways of numbering the arms, but here’s what Ji and Wu used. Starfish have a small wart-like valve called a madreporite, which allows water into their bodies. It sits in the central disc, but off to one side. The arm opposite the madreporite is arm number 1, and the rest are numbered clockwise. And it’s the fifth one that usually leads the way. It’s as close to a head as the starfish has.
Ji and Wu think that this hint of bilateral symmetry is a faint vestige of the body that starfish have as larvae. If they have a preferred direction, they could potentially make faster decisions in times of danger.
Ji and Wu now want to see if other aspects of a starfish’s body also conform to this hidden symmetry. For example, the starfish’s brain is spread throughout its body. It has a ring of nerves in its central disc, with spokes branching off into each of the arms. Perhaps these nerves are slightly more concentrated towards the fifth arm, just as our nerves are more concentrated inside our skulls.
Reference: Ji, C., Wu, L., Zhao, W., Wang, S., & Lv, J. (2012). Echinoderms Have Bilateral Tendencies PLoS ONE, 7 (1) DOI: 10.1371/journal.pone.0028978
Photos by Nick Hobgood and authors.
More on echinoderms:
- Fishing bans protect coral reefs from devastating predatory starfish
- Sand dollars avoid predators by cloning themselves
- Sea urchins use their entire body as an eye
- Pocket Science: Stealth mode in the sea