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Answer by Kirk Janowiak, wildlife biologist, fish-fancier for more than 50 years:
Your fish has some cool adaptations that allow it to know when you put food into the aquarium.
Bettas have good eyes for short distances. They have pretty good color vision and see shapes really well. They are, however, a bit near-sighted.
Bettas have excellent senses of taste and smell, which in fish are essentially a single sense. In only a second or two from dropping pellets or flakes into a betta's aquarium, the chemicals in the food rapidly diffuse (often aided by a filtration system or bubbler that moves the water), and the betta detects those chemicals. Most fish have chemical sensors not only in their mouths, like humans, but also on their heads and faces and, in many species, along the sides of their bodies.
Bettas have internal “ears” and can hear things dropped into the aquarium. They have a set of bone structures in their heads that are similar to the hammer, anvil, and stirrup bones you have inside your head for hearing. Unfortunately, when we set up a filter system and bubblers, we make it difficult for our fish to hear above all the backgound noise.
Bettas have a series of very sensitive pressure sensors embedded along the length of their bodies. You can see a faint line that makes a gentle curve from the side of the head all the way to the tail. This is known as the lateral line. It is often a little darker than the color of the scales and skin. This line is actually a set of tiny pits that contain exquisitely tuned motion- and pressure-sensitive cells that can detect the slightest changes in the pressure of the water. When you drop in the food, it creates a pressure wave. Even a small disturbance in the water can be felt by the lateral line of a betta.
Think about a betta's natural habitat, and you can see how nature has provided a push for the development of these adaptations in this fish. Bettas live in mostly still waters in inlets, coves, and backwaters of slow-moving rivers in Southeast Asia (especially in Thailand). The water in which they live is often murky, if not downright muddy, so vision is limited.
These fish establish and protect a small territory around them in which they feed and breed. They often feed on small insects that land on or fall into the water, as well as small invertebrates (for example, insect larvae, aquatic worms, and tiny swimming crustaceans) that live in the shallow waters of their habitats. Being able to locate food and avoid predators without relying on vision would be a big advantage. Chemical “smells” and small disturbances felt by their lateral lines help them find food. Big disturbances or large pulse waves from a large fish would help them avoid aquatic predators. Being able to tell a male from a female betta, once they got close enough to see, would be a good thing, too.
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