Chitons roll into a ball not because it’s their primary form of defense, but because it makes them more mobile. This allows the animal to roll to a safer position. And for chitons specifically, it is more safe when they are attached to the surface of a rock or even the soil floor.
Chitons are flat, marine mollusks that have a hard shell on their back and a soft belly. It is a common defense mechanism among animals with similar physiological features to roll up into a ball; this ability is known as conglobation or enrolment. This behavior protects the soft belly by touching the head and tail ends together and only exposing the hard shell. This behavior can be commonly observed among hedgehogs, pill millipedes, and also chitions.
Julia Sigward and her colleagues conducted an experiment to figure out why chitons curl into a ball, and how the influence of a predator affects their behavior when put into a vulnerable and unideal position. They set up an experiment by placing a single chiton up-side-down in the middle of an aquarium. Seawater was continuously pumped through the experimental aquarium from either an aquarium with other chitons, which was the control, or an aquarium with a common predator to chiton, a sea star, which was the predator treatment (see figure above). The researchers measured how much time chitons spent in each of three positions with and without the predator cue: rolled, arching, or otherwise exposed, meaning any position that wasn’t rolled or arching. Rolled looks like the animal is curled into a ball, and arching is when the animal is balancing on the head and tail ends and pushing it’s stomach upward (sort of like a backbend).
This study found that in the absence of predator cues, the chiton spent an average of 46% of their time exposed. When predator cues were present, they spent 52% of their time exposed. This biggest difference observed was the chiton’s time spent rolled and arching in the presence and absence of predator cues. When there was no predatory cues, the chiton spent 42% of their time rolled and 12% of their time arching. Where there were predator cues present, the chiton spent 21% of their time rolled and 27% of their time arching.
It’s super interesting that in the presence of a predator, the chiton spent more time in an exposed and vulnerable position as opposed to curling into a ball and protecting themselves. This is because chitons are much more likely to get eaten when they are unattached, and are they safer from predators when they are attached to a rock, the floor, or in this case, the aquarium walls. By arching their bodies, it allows them to reattach to an object. Even though it is very risky for chitons to expose their stomachs when they are arching, the benefit of reattaching outweighs the costs.
Because this study found that chitons don’t roll up when they sense a predator, a probable explanation for this behavior is to roll away to safety. Other animals, like caterpillars and spiders, roll into a ball and use their legs to launch themselves to safety. The results from this study suggest that chitons roll themselves into a ball in order to move to a more advantageous situation, whether there is the threat of a predator or not.
The behavior of rolling into a ball is not a passive defense behavior because research found that they do this less when a predator cue was present. For chitons, rolling into a ball doesn’t help defend itself against predators because they could just consume them whole, but it is an advantageous behavior because it would improve their circumstances by relocating. Chitons prioritize reattaching themselves when danger is present.
Sigward JD, Vermeij GJ, Hoyer P. 2019 Why do chiton curl into a ball? Biol. Lett. 15: 20190429. http://dx.doi.org/10.1098/rsbl.2019.0429
