Taking the Pulse of the Ocean’s Comb Jellies

The enormity of the global ocean over centuries of exploration has turned indigenous assessments into rough sketches rather than detailed portraits.

Now scientists have developed a precise method for discovering one of the ocean’s more exotic creatures. Estimates of its global occurrence are likely to skyrocket.

The organisms are called ctenophores. Although they look like jellyfish on the surface, they have no spines and none of the usual body pulsations and rhythms that drive jellyfish on. What moves them instead through the sea water are pulsating rows of feathery cilia. The tiny hair-like bundles resemble the teeth of a comb, giving the creatures their other name: comb jellies.

Waves of cilia let the creatures slide forward to sweep up prey and fine dust. Adults range in size from a few centimeters to a few meters. Ctenophores live in the world’s oceans, from the abyss to the sunlit zone. About 200 species have been identified. Most are bioluminescent. Typically, the colors of their lights are bluish or greenish, often shimmering or iridescent.

Four scientists introduced a new method for identifying ctenophores in an article published online last month and soon to appear in Molecular Ecology Resources, a monthly journal. Steven HD Haddock, co-author at the Monterey Bay Aquarium Research Institute in California, said the team worked on the problem for about five years, drawing on a wide range of specimens collected over decades. He said progress will give “much-needed precision” to biologists who want to learn the true dimensions of oceanic life.

If used widely, the method could, according to Dr. Haddock, cause the number of known ctenophore species to increase from 200 to around 600 and possibly up to 800.

“It’s like fingerprints,” said Dr. Haddock in an interview about the technology. “It’s one of the next big things in judging who lives in the ocean.”

The new method applies a powerful new tool to animal identification in the world of ctenophore research. It is known as environmental DNA sampling. Instead of directly observing or testing an organism, it collects and analyzes DNA snippets that are given off by all living things in their environment. Scientists compare the environmental samples of the genetic code of shipwrecked people such as hair, skin and slime with DNA libraries and look for matches and identifications.

The procedure has already been used for other identifications. For example, it helped uncover the hidden presence of endangered organisms, including an aquatic insect known as the rare yellow stonefly. Researchers also used it to show that Scotland’s famous Loch Ness was filled with more eel DNA than anything monstrous.

But before these analytical tools could be applied to ctenophores, advances were required. Dr. Haddock’s team designed a battery of new molecular probes that made it possible to conduct deeper DNA studies.

“It’s like being able to read a new language,” he said.

In a series of tests, they identified 72 species of ctenophores based on their genetic signatures – about five times more than reported in previous databases and GenBank, a library of genetic codes from thousands of organisms maintained by the National Institutes of Health.

The precise tools, say the scientists, will enable researchers to study the DNA sequences they recover from the wild with new precision and better understand the true diversity of marine life. And this, in turn, will help global conservation, fisheries management, and the assessment of things like the effects of climate change on ocean biodiversity.

“Ctenophores are largely overlooked in diversity studies because most are too sensitive to be sampled with trawls,” said Dr. Haddock. “With this study, we are trying to overcome this and give people the opportunity to appreciate how special and diverse these creatures are.”

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