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 elsewhat 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.

Marine biologists have long struggled to understand how creatures living on the deep-sea bottom acquire enough organic material to support their growth. Larvaceans are an important part of the answer. Species in this class of chordates spin bubblelike webs of mucus around themselves to gather food. When the web becomes clogged, the animal discards it, swims off, and builds a new one. Meanwhile, the old web, thick with organic material, falls toward the seafloor and becomes a free lunch for another animal below. Thanks to larvaceans, the marine snowfall becomes a marine avalanche near the seafloor.

“In recent years we’ve learned that larvaceans account for a quarter to maybe a third of all the organic carbon that gets from the upper layers of the oceanin Monterey Bay, at leastdown to the deep-seafloor community,” Robison says. “They play a critical role in the transfer of energy from the sunlit layers to the deep seafloor.” 

The immense number of jellies, and the many roles they play in food webs, could explain a larger mystery about Earth’s carbon cycle. To better understand the global climate and changes in the biosphere, scientists need an accurate measure of the total amount of carbon that is cycling between the planet’s living inhabitants, atmosphere, oceans, and solid earth. Consistently, however, they have faced a “budget gap” in their accounting. About 25 percent of the carbon that should be out there seems to be missing. Where is it?

Many marine biologists suspect that much of the missing carbon has been in front of their noses the whole timein the transparent, gelatinous bodies of jellies. “Jellies are major players in the ocean’s carbon biomass,” Robison says. “They may be an overlooked part of the equation.”

Jellies may also be important indicators of the health of ocean ecosystems. Some biologists have speculated that jelly populations thrive as increasing numbers of shrimps, fishes, and squids are harvested from the oceans, leaving behind vast amounts of uneaten small prey. A rise in jellies may signal drastic changes underway elsewhere in the ocean. “There is evidence,” Robison says. “But while it’s compelling evidence, it’s not yet convincing evidence.”

What is clear to jelly scientists is how much of the deep sea remains unexplored, and how much there is still to learn about its gelatinous inhabitants. “You can’t really understand what’s going on in there until you know who the players are,” says MBARI’s George Matsumoto. “That’s where we are right now. We’re still trying to understand who all the different players are in the deep sea.”

Even in the deep sea, however, scientists are finding evidence of human impact. On one dive to the bottom of the ocean, MBARI scientists found numerous unusual jellyfish, as well as a beer can. “We know so little about these deep-sea environments and their inhabitants, yet we’re impacting them on a daily basis,” Raskoff says. “It makes us all a little bit nervous, because the environments are changing while we’re studying them.”


Related Links

What is a gelatinous animal?


Jellies: Living Art


Amazing Jellies: From the New England Aquarium