Newswise — KINGSTON, R.I. – August 2016 — Little is known about most of the fishes that live deep in the oceans, where the pressures are extreme, light is nearly absent, and the water temperatures are especially low. These fishes are difficult to observe in their natural environment, and it is equally difficult to bring undamaged specimens to the surface or conduct experiments on them.

Yet a University of Rhode Island graduate student working with Professor Jacqueline Webb has made great strides in learning about the sensory system of a prominent order of deep-sea fishes called dragonfishes, a wide-ranging group of fishes with needle-like teeth and bioluminescent organs.

“I was interested in seeing how fishes in the deep sea are adapted to the lower light conditions of their environment,” said Ashley Marranzino, a Denver native who earned an undergraduate degree in biology from Regis University. “If there is so little light, you would expect that the fish aren’t using much of their vision to guide their behaviors. Instead, maybe their lateral line system has evolved to become more sensitive.”

According to Marranzino, fish use a sensory system called the mechanosensory lateral line system that detects water flows and low frequency vibrations in the water. Working somewhat like the human ear, the system uses tiny organs called neuromasts located on their skin and in canals on their head and body to detect slight movements in the water, which helps them avoid predators and find prey. It also aids fishes in swimming upstream, helps detect obstacles, and aids schooling fish in sensing their nearest neighbor.

“Having more of these lateral line sensory organs means you’re more sensitive to these flows,” Marranzino said.

The only previous research on the lateral line system of dragonfishes was published in German in 1901 and concluded that they have 24 neuromasts. But by using microCT imaging and scanning electron microscopy to examine specimens collected by former URI scientist Brad Seibel, as well as specimens from museum collections, Marranzino conservatively estimates that dragonfishes have more than 500 neuromasts on one side of their head and body.

“My findings highlight the fact that these fishes adapted to their environment by evolving and relying upon this non-visual sensory system,” she said.

Despite the century-old research on the subject, Marranzino expected deep-sea fishes to have a large number of neuromasts. That’s because shallow-water fish that are nocturnal or live in murky environments, and those that live in caves – all environments where light is limited – have similarly high numbers of neuromasts.

“But while I’ve shown that their lateral line system is really important, we still don’t know much about the behaviors they exhibit using this system,” she added.

Marranzino said that dragonfishes are among the most numerous fishes on Earth, so it is important to understand how they live and behave and how they contribute to deep-sea ecology.

“Some of them make a huge nightly migration to shallow waters, where they are eaten by tunas and other predatory fishes that are important to fisheries, so it’s important that we understand their biology and their ecological role in the world’s oceans,” she said.

In addition, Marranzino said that research on the lateral line system is playing a crucial role in the development of small sensing devices that may soon be used on autonomous underwater vehicles to help them navigate and detect objects in the water.

Marranzino presented her research at the International Congress of Vertebrate Morphology in Washington, D.C., in June and at the Joint Meeting of Ichthyologists and Herpetologists in New Orleans in July. At the latter meeting, she earned the annual Frederick H. Stoye Award given by the American Society of Ichtyologists and Herpetologists for the best student presentation in genetics, development and morphology.

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