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Brittle stars have no brain but still manage to learn

In the quest to understand intelligence and learning, humans have often focused on brain size as a key factor. However, a fascinating study on brainless brittle stars challenges this notion, revealing that these creatures are capable of learning through experience, despite lacking a centralized brain.

Enigmatic brittle stars

Brittle stars, related to starfish, are fascinating marine creatures known for their secretive nature. They spend most of their time hidden under rocks or buried in sand, with their bodies consisting of five wiggly arms connected to a nerve ring near their mouth, instead of a traditional brain.

Julia Notar, the lead author of the study conducted at Duke University under Professor Sönke Johnsen, emphasizes the uniqueness of the brittle star’s nervous system. “There’s no processing center,” explains Notar. “Each of the nerve cords can act independently, functioning more like a committee than under a central boss.”

Learning by association

Notar’s research posits that brittle stars can learn through classical conditioning, a learning process where an organism learns to associate different stimuli.

One example of classical conditioning is demonstrated by Pavlov’s famous experiments with dogs. When repeatedly fed at the ringing of a bell, they eventually start drooling at just the sound of the bell, even without food present.

Similarly, humans experience this often. When you repeatedly hear the “ding” of a smartphone alert, the sound eventually takes on a special meaning. The mere ping or buzz of someone’s phone, if it matches your phone’s chime, can prompt you to reflexively reach for your phone, expecting the next text, email, or Instagram post.

Researchers have demonstrated classical conditioning in starfish in a few previous studies. However, most echinoderms, a diverse group encompassing around 7,000 species including brittle stars, brainless starfish, sea urchins, and sea cucumbers, remain untested in this area.

How the study was conducted

To determine if brittle stars can learn, researchers placed 16 black brittle stars (Ophiocoma echinata) in individual water tanks and recorded their behavior with a video camera.

They trained half of the brittle stars by dimming the lights for 30 minutes at each feeding time. Whenever the lights went out, the researchers placed a morsel of shrimp — a favorite of the brittle stars — just out of reach in the tanks.

The researchers fed the other half the same amount of shrimp but did so under lit conditions, never coinciding with the 30-minute dark periods.

Watching brittle stars learn

Regardless of light conditions, the animals mostly hid behind the filters in their tanks, emerging only for meals. However, only the brittle stars that received training learned to associate darkness with food.

In the early stages of the 10-month experiment, all the animals remained hidden when the lights dimmed. Over time, the trained brittle stars formed a connection between darkness and feeding time. They began to anticipate food and emerged from hiding whenever the lights went out, even before shrimp was introduced to the tanks.

These brittle stars had learned to associate the dimming lights with the likelihood of food. They responded without needing to smell or taste the shrimp. The mere dimming of the lights was sufficient to prompt them to emerge for dinner.

Even after a 13-day break without training, which involved repeatedly dimming the lights without feeding, the brittle stars still remembered this association.

Implications and future research

Notar finds these results exciting, as they demonstrate classical conditioning in a group of animals previously not known for such learning capabilities.

“Knowing that brittle stars can learn means they’re not just robotic scavengers like little Roombas cleaning up the ocean floor,” Notar said. “They’re potentially able to expect and avoid predators or anticipate food because they’re learning about their environment.”

The study opens doors to further explore how creatures with non-standard nervous systems learn and remember. Notar plans to delve deeper into the mechanisms behind the brittle stars’ learning abilities, aiming to answer the intriguing question: “How do they do it?”

This research fundamentally challenges our understanding of learning and intelligence in the animal kingdom. It suggests that brain size or structure may not be the sole determinants of an organism’s ability to learn and adapt to its environment, paving the way for a broader and more inclusive study of intelligence in nature.

More about brittle stars

As discussed above, brittle stars, fascinating and unique echinoderms, exhibit remarkable features and behaviors in marine ecosystems. Belonging to the class Ophiuroidea, these creatures are close relatives of sea stars, but they stand out due to their distinct physical characteristics and ecological roles.

Physical description and diversity

Brittle stars typically possess a central disc and five slender, highly flexible arms. Unlike sea stars, their arms do not contain vital organs and are sharply demarcated from the central disc. This structure enables them to move swiftly through the water and across the seabed. They exhibit a wide array of colors and patterns, enhancing their appeal.

There are over 2,000 known species of brittle stars, varying greatly in size and form. Some species have smooth arms, while others have spiny or adorned ones. Their size can range from a few centimeters to nearly a meter in arm span, with the central disc usually being relatively small.

Habitat and distribution

Brittle stars inhabit diverse marine environments, from shallow coral reefs to the deep sea. They often hide under rocks, in coral crevices, or within seagrass beds during the day. Some species even live in symbiotic relationships with other marine organisms, such as corals.

Their global distribution is impressive, with species found in every ocean and at various depths, some reaching as deep as 6,000 meters. This wide distribution highlights their adaptability to different environmental conditions.

Feeding and nutrition

Brittle stars are mostly scavengers and detritivores, feeding on dead organic material and small organisms. They use their flexible arms to capture food particles, which they then transport to their mouths. Some species are also known to be suspension feeders, using their arms to catch plankton and other small particles from the water column.

Reproduction and lifecycle

Brittle stars reproduce both sexually and asexually. In sexual reproduction, they release eggs and sperm into the water, where fertilization occurs. The larvae then go through several planktonic stages before settling on the seafloor and metamorphosing into juveniles.

Asexual reproduction occurs through fragmentation, where a part of an arm breaks off and regenerates into a new individual. This ability also aids in their survival, as they can readily regenerate lost limbs.

Ecological role and importance

Brittle stars play a crucial role in marine ecosystems. As scavengers, they aid in the decomposition process and nutrient cycling. They also serve as prey for various marine animals, including fish and crabs.

Their presence in various habitats makes them good indicators of environmental health. Some species are particularly sensitive to changes in water quality, making them useful in monitoring the impact of pollution and climate change on marine ecosystems.

Conservation status

While many brittle star species are abundant, some face threats from habitat destruction, pollution, and climate change. Their conservation is essential for maintaining the health and diversity of marine ecosystems.

In summary, brittle stars are not just captivating creatures but also vital components of the ocean’s biodiversity. Their unique adaptations and ecological roles make them an important subject of study in marine biology and conservation efforts.

The full study was published in the journal Behavioral Ecology and Sociobiology.

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