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Loggerhead Turtles Use Visual Cues To Spot Gelatinous Prey

June 13, 2013
Image Caption: This is a loggerhead turtle in the study equipped with a 3-D logger. Credit: Tomoko Narazaki (Atmosphere and Ocean Research Institute, The University of Tokyo)

Brett Smith for redOrbit.com – Your Universe Online

New research from a team of American and Japanese researchers suggests that loggerhead turtles rely heavily on visual cues to spot gelatinous prey as they swim through the ocean.

Using National Geographic Crittercams, which were mounted on the turtles, the researchers were able to watch video captured from just behind the turtle´s head that showed the animals heading for or turning toward potential prey before devouring it.

“Foraging events with a turning point tended to occur during the daytime, suggesting that turtles primarily used visual cues to locate prey,” the researchers wrote in their report, which was recently published in the journal PLoS ONE.

The researchers also observed the loggerheads swimming toward a floating plastic bag, a reminder of how human trash can impact creatures out in the open ocean. An online video of the event showed the plastic bag ℠hanging´ in the water in much the same way that a jellyfish does. After approaching the bag and performing a closer inspection, the turtle is seen moving on.

“The fact that the turtle´s movements while approaching the plastic bag were analogous to those of a true foraging event, having a turning point and deceleration phase, also support the use of vision in mid-water foraging,” the researchers noted.

The research team also used 3D loggers to record observations of the turtles foraging habits. The data loggers “enable researchers to reconstruct 3D underwater movements by dead-reckoning using locomotion vectors: heading, depth (or pitch angle), and swim speed,” according to the report.

Turtles were observed foraging for gelatinous foods about twice every hour, indicating that they may rely on this type of prey more than previously believed, the scientists said.

Previous research has shown that a turtle´s diet can vary according to a variety of factors, but adult turtles tend to prey on deep-sea mollusks for food. The soft, slow-moving prey observed in the study represent low-energy, easily digestible foods that may not replace these other prey, but may benefit loggerhead turtles during oceanic migrations, when deep-sea prey is more difficult to reach, the researchers posited.

Loggerhead turtles can weigh more than 300 pounds and grow to about 3 feet long, meaning the average animal wastes a significant amount of energy chasing down an empty plastic bag. The endangered animals are also regularly ensnared and killed in fishing gear. Human development of their nesting sites and the unintended introduction of exotic creatures to their habitat has been known to negatively impact the slow-moving creatures.

Many steps have been taken to reduce unintended turtle deaths. Some cities have laws in place aimed at limited plastic bag use and Hawaii is planning to eliminate plastic grocery bags from their stores altogether. Commercial fishermen are also being encouraged to use ℠turtle excluder devices,´ which are designed to help turtles escape from fishing gear.

Loggerheads are currently classified as endangered by the International Union for the Conservation of Nature (IUCN), making their international trade illegal. Many countries have classified the sea turtles as threatened or endangered in an effort to protect them within their own jurisdiction. The official conservation effort is supported by many volunteers groups, which inspect nesting turtles and their eggs for signs of distress.


Source: Brett Smith for redOrbit.com - Your Universe Online



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