Cavefish Don’t Need to See to Find Food

Though they lack eyes, cavefish have other adaptations that help them to survive in their dark habitats. (Photo credit: Martin Shields / Photo Researchers, Inc.)

The fish species Astyanax mexicanus is interesting in that it includes both cave-dwelling and surface-dwelling populations. In Mexico there are 30 separate populations of cavefish. Many of these populations evolved in isolation, which means each population evolved independently of the others. The populations that live in caves lack eyes and body pigment, while the populations that live aboveground have large eyes and are pigmented. Due to these obvious phenotypic differences within the same species, cavefish are a popular subject for evolutionary biologists.

Though in early stages of development cavefish have eyes that begin to grow, at a certain stage programmed cell death, or apoptosis, occurs in the lens and the eyes stop growing. The surrounding skin tissues around the eyes continue to grow, covering over the space where eyes would typically be found. The remains of the undeveloped eye can be found buried within the eyes orbital socket.

However, even without eyes, cavefish still retain the ability to detect changes in light due to the functions of the pineal gland. If a shadow occurs above the fish, they will swim upward to investigate, as it may be a source of food, and without predators in the cave system, they do not fear being eaten. (In direct contrast, surface-dwelling fish typically seek shelter in the presence of a shadow.)

Compared to surface-dwelling fish, cavefish have a larger mouth and jaws and a greater number of tastebuds. Cavefish also have larger and more neuromasts than surface-dwelling fish. Neuromasts are specialized nerve cells that are a part of a fish’s lateral line. In cavefish, these cells are more densely distributed on the fish’s head, particularly in the area where its eyes would be. Cavefish use these sensory organs to detect movement and vibration in their watery environment. The response to vibrations in the water, called vibration attraction behavior, or VAB, is an adaptive behavior. Vibration detection helps cavefish find sources of food in the water, which, without eyes, they would not be able to see. Recent cavefish research conducted by evolutionary biologists indicated that VAB and neuromast abundance coevolved to make up for the loss of vision in cavefish and help the blind fish find food in darkness.

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Loggerhead Sea Turtles Perceive Longitude Magnetically

The ability to navigate is essential to sea turtle survival. (Photo credit: Michael Patrick O’Neill / Photo Researchers, Inc.)

A sea turtle doesn’t have it easy. Immediately after hatching from its egg, the hatchling sea turtle digs itself out of its beach nest and heads toward the ocean. Those that make the treacherous journey from the beach to the surf swim–and are swept through–the surf zone out to the open ocean, where they continue swimming for several days. The turtles will stay at sea for five to ten years before returning to their breeding grounds to find a mate. The turtles migration is truly astounding–sea turtles that enter the Atlantic Ocean will travel over 15,000 kilometers (9,000 miles) before returning to the coast of North America to breed. During their journey, they will travel via the Gulf Stream to the North Atlantic gyre, a circular, counterclockwise current that flows around the Sargasso Sea and helps transport the turtles east past the Azores, Canary, and Cape Verde Islands, before returning them to the North American coast.

Scientists have known for some time that some animals are able to use Earths magnetic field to navigate from one part of their habitat to another. The term magnetoreception refers to an animals ability to sense magnetic fields. Through laboratory and field experiments, scientists were able to show that, when exposed to certain magnetic fields, animals such as homing pigeons and sea turtles change their orientation or navigation behavior accordingly. Earths magnetic field provides animals with two types of information. Directional information helps a migrating animal to maintain a consistent directional heading, such as traveling north to south. This information is sometimes referred to as the animals magnetic compass. Some animals are also able to use positional information. This type of information lets animals use magnetic cues to figure out their approximate geographical position, or where they are at a certain time in comparison to their final destination. This type of information is sometimes referred to as a magnetic map. This magnetic map is determined by two factors: the strength of the magnetic field and the inclination, or angle at which magnetic field lines intersect with Earths surface. Both of these factors vary in a predictable manner across the surface of Earth.

According to Dr. Kenneth Lohmann, a sea turtle researcher and professor of biology at the University of North Carolina-Chapel Hill, this magnetic map can be thought of as a low-resolution biological equivalent of the Global Positioning System, but one that is based on Earths field instead of satellite signals.

Though it has been well-documented that animals use magnetoreception to determine their location in terms of latitude (that is, the north-south orientation), it was not known whether animals can also use magnetoreception to determine their location in terms of longitude (this is, the east-west orientation). Recent research conducted by scientists in Lohmanns lab at UNC has for the first time documented that loggerhead sea turtles are able to do just that. The results of their research indicate that these sea turtles are able to extract both longitudinal and latitudinal information from Earths magnetic field.

In their lab experiment, the scientists placed hatchling sea turtles in a round, water-filled orientation arena. This tank was surrounded by a computerized coil system, which the researchers used to manipulate the magnetic field. Each hatchling was harnessed to an electronic tracking unit that sent data about each turtles swimming direction to a computer. The scientists subjected the hatchlings to magnetic fields found in two locations along their migratory pathway, each found on opposite sides of the Atlantic Ocean. Both locations have the same latitude, but have differing longitudes. When subjected to a magnetic field similar to that found near the Cape Verde Islands, the hatchlings swam in a southwesterly orientation. When subjected to a magnetic field similar to that found near Puerto Rico, the hatchlings swam in a northeasterly orientation. Both of these swimming orientations coincided with the directions scientists would expect them to swim during their actual migration.

This work not only solves a long-standing mystery of animal behavior but may also be useful in sea turtle conservation,” Lohmann said in a press release about the research. “Understanding the sensory cues that turtles rely on to guide their migrations is an important part of safeguarding their environment.

Scientists who contributed to the research include Nathan Putman, Courtney Endres, Catherine Lohmann, and Kenneth Lohmann. The results of their research were published in the February 24, 2011 issue of the journal Current Biology.

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Researchers Learn How Brain Represents Meaning

In the past, scientists who study brain function have used functional magnetic resonance imaging (fMRI) to determine which areas of the brain are activated when an individual is instructed to think about a certain word. Now, researchers at Carnegie Mellon University in Pittsburgh, Pennsylvania have developed a computer model that can predict the unique brain activation connected with concrete nouns–words for things that you sense through sight, sound, touch, taste, or odor. This computer model is helping brain scientists understand how the brain codes the meanings of certain words.

The computer model was developed by a team of researchers led by Tom M. Mitchell, a computer scientist, and Marcel Just, a cognitive neuroscientist. The results of their research were recently published in the journal Science.

We believe we have identified a number of basic building blocks that the brain uses to represent meaning, said Mitchell. Coupled with computational methods that capture the meaning of a word by how it is used in text files, those building blocks can be assembled to predict neural activation patterns for any concrete noun. And we have found that these predictions are quite accurate for words where fMRI data is available to test them.

Through their research, the team of scientists found that the brain represents the meaning of a concrete noun in places in the brain connected with how it is sensed or used.

The meaning of an apple, for instance, is represented in brain areas responsible for tasting, for smelling, for chewing, said Just. An apple is what you do with it.

The researchers also discovered that some words are connected to areas of the brain associated with planning and long-term memory. For example, thinking about an apple may trigger a persons memory of going to an orchard to pick apples.

The scientists are excited to continue their research. In the future, they plan to study the brain activation patterns for adjective-noun combinations, prepositional phrases, and simple nouns and concepts. The research team also plans to further their study of noun-connections by researching how the brain represents abstract nouns and concepts.

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