The freshwater Mexican tetra fish has two conspecific forms: a form with eyes that lives near the surface and an eyeless form that lives in caves.
The eyeless Mexican tetra develops an optic primordium—the precursor to the eye—as an embryo, but it degenerates and becomes covered by an overgrowth of skin during the larval stage. It had been thought that this fish cannot sense light, but a study published in 2008 in The Journal of Experimental Biology by scientists from the University of Maryland found otherwise.
While the fish’s eyes are not functional, the researchers found that the fish can detect light with its pineal gland, a pinecone-shaped endocrine gland near the center of the brain. Despite being buried deeper in the flesh than the eyes and thus less likely to receive light, this sensory organ is known as the pineal or “third” eye in some vertebrates.
Cavefish from two populations, Pachón cavefish and Tinaja cavefish, and surface fish were used in the experiment. During the experiment, both surface and cavefish larvae were exposed to light in a plastic chamber for three minutes. Then the researchers shaded the chambers and counted the number of fish that swam upward toward the surface. This shadow response is a behavior that can help young larvae avoid predators by hiding under objects floating on the surface.
Interestingly, at 1.5 days after fertilization, 60–70 percent of both types of cavefish showed the shadow response, while only about 50 percent of surface fish did so. The experiment was repeated once a day for seven days. Tinaja cavefish continued to exhibit more shadow response than surface fish in all but two trials. This result shows that the ability to sense light was present in both surface fish and cavefish.
To determine what the larvae relied on to sense the shadow, the researchers removed the fish’s pineal gland or one or both bilateral eyes and repeated the experiment. Both surface fish and cavefish that had their bilateral eyes removed exhibited similar behavior as before, but among the fish with their pineal glands removed, only about 10 percent retained the shadow response.
The researchers concluded that the pineal gland is critical to the shadow response behavior, and that not only is the pineal gland able to sense light, but also there is a neural connection between the pineal gland and the motor system.
The Epoch Times publishes in 35 countries and in 19 languages. Subscribe to our e-newsletter.