Jellies Down Deep

Welcome to the Subfamily

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Traditionally, biologists distinguished and classified jellies strictly according to physical shape: the number of tentacles, or a certain shape of the stomach. But this approach can be misleading, especially with jellies. “They can be very close cousins and look very different,” Raskoff says. “And things that look very, very similar can actually be very far apart, evolutionarily speaking.” It was years before scientists realized that polyps, small organisms that grow on surfaces such as rocks, are actually the immature form of free-swimming jellies. Matsumoto and Raskoff found that the number of arms on Big Red—often a reliable guide for distinguishing a species—varies anywhere from four to seven, depending on the individual.

The advance of molecular biology has greatly aided scientists in their ability to identify and classify organisms. Ultimately, the taxonomy of organisms—how they are grouped in relation to one another—should reflect a common evolutionary ancestry. By examining and comparing DNA, which organisms inherit through reproduction, taxonomists have gained a much clearer picture of how organisms are related to one another across all taxonomic levels.

Recently, Raskoff and another MBARI scientist, Steve Haddock, have been taking a close look at the order Narcomedusae (phylum Cnidaria), a group of jellies that outwardly look very similar to one another. Based on physical appearance, scientists had previously declared certain Narcomedusae to be more closely related than others. But, says Raskoff, “the genetic evidence supports a very, very different linkage between these different groups. We found that the traditional taxonomy that has been accepted for hundreds of years for a large order of jellies turns out to basically be completely false.”

Through genetic analysis, biologists are slowly gaining a better understanding of how and when the jellies evolved. Needless to say, fossils of jellies are few and far between. The evidence now suggests that jellies are an ancient life-form, hundreds of millions of years old, and probably predate most of the more familiar, complex animals. But many questions remain. For example, the comb jellies are typically classified into two types, those with tentacles and those without. Which type is older? Did the tentacleless kind appear first and the tentacled kind evolve later? Or did tentacles come first and then, in some comb jellies, disappear over time? Only further study and exploration will tell. What marine researchers know for certain is that the jellies they’ve discovered so far represent only a small fraction of what’s out there.

“We’ve been doing this for 15 years now, and yet we still see new stuff almost every dive,” says MBARI scientist Bruce Robison. “And that’s just in Monterey Bay. Our knowledge about the deep sea is still so poor that even after all these years of diving in this one spot, we still see new things all the time.”

Meanwhile, Matsumoto and Raskoff do their best to keep up. In addition to the several new jellies they’ve discovered in the past couple of years, they have perhaps ten more that look like solid candidates for new species. They’ll officially describe them, Raskoff says, just as soon as they discover something else: some spare time.