Jellies Down Deep
A Simple Plan for Supremacy
Only in recent years have marine biologists come to grasp the astonishing abundance of gelatinous animals in the world’s waters. By some estimates, transparent jellies make up as much as 40 percent of the biomass in the open ocean. Now, with an improved ability to detect and study these creatures, scientists are slowly coming to a more complete understanding of how ocean food webs work.
“Jellies were always relegated to an interesting but fringe category of strange, snotty animals in the water,” says Kevin Raskoff, a jelly scientist at the Monterey Bay Aquarium Research Institute. “But once we saw how prevalent they are and the diverse habitats they’re found in, it caused us to rethink their role in ecosystems as a whole.”
Jellies share a remarkably basic construction. The “jelly” in jellies is little more than a mixture of saltwater and some carbon-containing sugars. True jellyfish (phylum Cnidaria, class Scyphozoa) are made of two transparent layers, an outer one for protection, and an inner one that handles digestion. In between, a small amount of fibrous jelly called mesoglea serves as the scaffolding for everything else—what little there is. Ctenophores, or comb jellies, have a similar construction. As a general group, jellies possess a large percentage of watery, transparent tissue.
Being gelatinous has its disadvantages. Jellies are slow and vulnerable to some predators like sea turtles. But having a gelatinous body also provides many advantages. Because jellies are made mostly of water, they are neutrally buoyant, so they waste no energy maintaining their position in the water. Their body material is “cheap to build,” says MBARI scientist Bruce Robison, so a jelly can easily repair most damage it sustains. And jellies can respond quickly to changes in their habitat. When food becomes plentiful, they can grow and reproduce rapidly. When food is scarce, a jelly can actually shrink, or “de-grow.”
“Jellies are perfectly adapted to a three-dimensional watery habitat,” Robison says. “The fact that we see so many different kinds of them reflects the fact that they have a fundamentally successful body plan and way of making a living.”
With a better estimate of how many and what kinds of gelatinous animals exist, scientists are filling gaps in their understanding of oceanic food webs. Using the same basic material, jellies have assumed diverse roles. Some graze on krill and plankton, and will even actively migrate up to surface waters at night to eat their fill. Others snack on “marine snow,” an omnipresent mist of food particles that drifts down from the sea surface to the seabed and includes tiny plankton living and dead, fish feces, and exoskeletons shed by their former occupants. Some jellies prey only on fish, others only on crustaceans. Still others have specialized features with which to prey on their gelatinous cousins. Some jellies of the genus Beroe , for example, are little more than a mouthlike sac that ingests other jellies.
