URI professor discovers fish that can feed without vision

04/29/09 - A University of Rhode Island professor has made a new discovery in the world of marine sciences - a fish that can feed without using its vision.According to Jacqueline Webb, biological sciences professor and coordinator of the URI Marine Biology Program, most fishes detect their prey visually.

She determined that a species of cichlids found in Lake Malawi of East Africa has a widened lateral line, improving their feeding abilities.

According to Webb, the mechanic-sensory lateral line system is a series of sensory organs called neuromasts that is composed of hair cells that respond to water flow and vibrations, but not sound.

The widened lateral line canal changes the physics of the water flow, which is taking place in the canal. Webb said she believes that it is an adaptation allowing cichlids to feed without using their sense of sight.

"We demonstrated that they can attack prey using their lateral line system and more importantly, they do that at night," Webb said. "In literature on these fishes, there are no records of cichlids feeding at night."

According to Webb, this is the first documentation of nocturnal and non-visual feeding in cichlids.

Daniel Bassett, a marine biologist from New Zealand, determined this cichlid species feeds at night and without the use of its vision through a preliminary experiment. Six Petri dishes were placed in a tank with sand on the bottom, an environment similar to the cichlid's natural habitat. In each of the Petri dishes, either a dead or living adult brine shrimp was tethered down. The cichlids were then released into the tank and their prey preferences were recorded during conditions of daylight, nighttime, intact and lacking of lateral-line capabilities. To deactivate their lateral lines, some cichlids were placed in a separate tank where a chemical solution of cobalt-chloride was administered.

"We asked the fishes to detect their prey," Webb said.

Cichlids feed by swimming over their food with their chin and detecting movements and vibrations in the water. When they sense a disturbance - in this case the flapping tails of the tethered live brine shrimp - they back up and attack their prey. Webb said the results of this experiment matched her original hypothesis.

During the daylight conditions tests, the cichlids were observed eating both live and dead brine shrimp. However, when the lights were turned off, only live brine shrimp were consumed.

"Because they couldn't see anything, they had to use another sense to detect their prey," Webb said. "They were using their lateral line system, which we had hypothesized they were using."

"Different fishes that live in Lake Malawi are evolutionarily unique in the sense that they all evolved from a common ancestor," Webb said. "The cichlids I studied in the genus of fishes Aulonocara have a particular lateral line morphology that is relatively unusual among fishes, most of these live in the deep sea."

Throughout the next three years, Webb will be studying the convergent evolution and development of the widened canal.

Fishes in the genus Aulonocara are commonly found in pet stores. All 16 species of fishes in this genus have widened lateral lines, although according to Webb, there are 12 families of fishes found worldwide with wide canals.

Cichlids are small, brilliantly-colored fish that live in sandy bottoms and occasionally in caves. In Lake Malawi they have been found habituating depths of 70 meters (210 feet,) a depth at which light cannot penetrate. Their diets consist of sand-dwelling invertebrates such as aquatic insects and other soft-bodied animals like crustaceans.