Brittle Stars: Brainless Echinoderms Can Also Learn Through Experience [Study]

Brittle stars, unlike starfish, spend most of their lives lurking under rocks and burrowing in the sand of the world's oceans. Some consider these headless and brainless echinoderms to be less sentient compared with humans and other larger animals, especially those with heads. However, scientists recently found that these cousins of starfish, also called serpent stars or ophiuroids, can also learn through experience using their nerve cords.

The findings are based on a study led by researchers in the United States, which confirmed that learning without a brain is possible despite the lack of centralized nervous systems, such as in the case of brittle stars and other echinoderms. The researchers also explored an alternative method of learning in a process called classical conditioning. The said process has been demonstrated in previous studies involving starfish.

Brainless Learning

In a study published online in the journal Behavioral Ecology and Sociobiology on November 21, researchers the brainless learning ability of brittle stars, where just like other echinoderms have five nerve cords joined by a central nerve ring, despite the absence of a central nervous system. The scientific paper also acknowledged that although a limiter number of research before have dealt with operant and classical conditioning in sea stars, other echinoderm members remain relatively untested.

The research team examined whether members of the species Ophiocoma echinate can learn an association between a period darkness and the presentation of food reward. The team found that trained individuals of the brittle star species learned to associate the two stimuli by constantly emerging during the dark period, even when no food was presented.

The experiment determined that brainless learning is achievable among ophiuroids by associating two stimuli (darkness and food). Despite the breakthrough discovery, researchers only found relatively limited evidence of this type of learning across the echinoderm group. It is also unclear if the brittle stars can learn different stimuli in other experimental scenarios or real-life situations in the wild.

Earlier this year, a separate yet relevant study showed that a certain jellyfish species, despite also lacking a centralized brain, is capable of associative learning. This September 2023 study was published in the journal Current Biology, where researchers found that the box jellyfish (Tripedalia cystophora) was capable of such a feat.

What are Echinoderms?

Echinoderms are a group of varying invertebrate marine animals that consist of around 7,000 species, which include not only brittle stars and starfish but also sea urchins and sea cucumbers. This group is characterized by its hard and spiny skin covering.

According to scientists, echinoderms can be found in every ocean of the world, even in the waters off the coast of Antarctica. They can be found at depths of over 3 miles (5,000 meters). Some members of the phylum Echinodermata are stealthy predators, who pursue their prey with cunning agility and speed, while others feed on drifting detritus, according to the Smithsonian National Museum of Natural History.