(Click here if you would like to listen to an audio version of this blog!)
A jellyfish is a very simple animal. It is basically two, single-cell-thick sheets that are folded into each other to make an inside tissue (the gastrodermis) and an outside tissue (the epidermis) with varying amounts of a watery gel material called mesoglea in between. Add some long, thin tentacles that have specialized cells that can expel poison-packed nematocysts, and you have it! No true limbs or body parts, no head, no organs, and only one opening into the digestive cavity through which food enters and out of which wastes are expelled. Package all of these parts into a radially symmetrical body and you have what could quite possibly be the first type of animal that ever existed on Earth. Jellyfish fossils are quite rare, but some have been dated back to possibly over 700 million years.
Jellyfish have long been thought to be evolutionary relics at best. They are organisms seemingly stuck in the “gastrula” stage of embryonic development (the nearly universal, in-folded, radially symmetrical life stage through which all animals pass early on in their embryological developments). They are mostly water (most of their body mass is made up of mesoglea) and seem to play no significant role in their ecosystems. They were not thought to be particularly important as predators, and they were not thought to be particularly important as prey.
Biologists attempting to directly observe the behaviors of many species of fish, sea mammals, sea birds and sea turtles saw very few of these potential predators eating jellyfish. Examination of the gut contents and feces of these predaceous species also revealed very little digestive residues attributable to jellyfish. Further, the very light caloric content of jellyfish tissue (one cup of fresh jellyfish contains only 5 calories of food energy (about a third of the calories in an equivalent amount of celery!). The consensus was, then, that jellyfish are interesting from an evolutionary perspective, but they have almost no role in the energy transformations or food chains in the oceans.
Two things, though, have happened in recent decades that have changed this classic biological picture of jellyfish. The first is a very common impetus for transforming scientific ideas: new ways of observation! The second concerned a paper published seventeen years ago that speculated about what will happen to the oceans under the dual influences of over-fishing and climate change.
The new ways of observation involve both new laboratory procedures and also new technologies. It is now possible to analyze the gut contents or the feces of a predator for stable isotope signatures and DNA sequences that are characteristic for specific prey organisms. Although previous visual analysis may not have indicated the presence of jelly fish tissue in a fish’s gut or a sea bird’s feces, the stable isotopes or the specific DNA patterns in these gut or fecal materials just might be able to!
Jellyfish as prey are slow moving (and, so, are very easy to catch), and they are digested amazingly rapidly and thoroughly (hence the absence of jellyfish tissue in the guts or feces of predators!). Further, certain parts of a jellyfish (the reproductive cells, for example) are quite rich in protein and are high in food value. Many jellyfish predators feed almost exclusively on the reproductive cells and tissues!
Eel larvae (according to gut DNA analysis) primarily feed on jellyfish (76% of their gut DNA was from jellyfish tissues). Analysis of albatross feces indicated that 20% of the DNA in the fecal samples were from jellyfish.
Development of durable, micro-miniaturized cameras have allowed the direct observation of of hunting and feeding patterns in a wide variety of species. Cameras attached to penguins have clearly shown that fully 40% of their prey were jellyfish!
Jellyfish are an important component of almost all marine food chains! For some predators, jellyfish represent a prey choice of last resort during times of poor food availability. For other predators, though, like the one ton, ocean sunfish (Mola mola) and the thousand pound, leatherback turtle, jellyfish are prey choice #1!
The second transformational stimulus for our concept of jellyfish involved some predictions in a scientific paper. The paper, “Historical overfishing and the recent collapse of coastal ecosystems,” was written by J. Jackson of the Scripps Institution of Oceanography (U.C. San Diego) and eighteen colleagues from an international array of marine labs and universities. It was published in the journal Science in 2001. One of the ideas in the paper concerned what would happen to marine food chains after humans had removed most of the large (economically desirable) fish species and after the oceans had become warmer due to human-induced climate change. These new oceans, the authors predicted, would contain algae that would increasingly grow in unequilibrated, roller coaster cycles generating frequent Harmful Algae Blooms (“HABs”). These ocean would also, the authors predicted, contain an increasingly large number of jellyfish. The authors called these exaggerated ecological components of the new oceans the “slime blobs,” and this predicted future ocean has been referred to as the “slimy ocean” model.
Over the past two Ecologist’s Notebook posts we have described and discussed HAB’s from a variety of “algal points of view.” These increasingly frequent and increasingly exaggerated HABs fit the slimy ocean model very well, and, apparently, along with these increasingly frequent, out-of-control algae blooms, jellyfish are also becoming more abundant.
It is hard to accurately count jellyfish in the open ocean. You cannot easily see jellyfish from the deck of a research boat or from a survey airplane. You can, however, count some of the larger fish that are known to feed nearly exclusively on jellyfish, and that is what D. Gremillet of the Center for Functional and Evolutionary Ecology in Montpellier, France and his team decided to do.
Recreational divers in the western Mediterranean have observed an increase in ocean sunfish in recent years. The ocean sunfish are impossible to not notice! They can weigh up to 1000 kg (2200 pounds) and can be just under two meters long. They are huge, slow moving fish, and, as previously mentioned, they subsist almost exclusively on jellyfish! It is quite easy to see and count ocean sunfish from survey planes, so Gremillet and his team decided to do just that as a proxy for a very broad-based jellyfish count!
Gremillet determined that the western Mediterranean had 475 ocean sunfish per 100 square kilometers. This density was ten times the ocean sunfish densities measured in other seas! It was much higher than expected! The average size of these sunfish was 120 kg, and the team calculated that each of these average sized sunfish would have to eat 71 kg of jellyfish each day in order to survive. Expanding these numbers to encompass the entire western Mediterranean, the team estimated that every day in the summer this population of ocean sunfish would consume 20,774 tons of jellyfish! The actual population of jellyfish in this sea, then, was staggering!
So, the jellyfish part of the slimy ocean prediction seems to be supported, too!
Slimy oceans may have other drawbacks. They maybe unable to support any of the fish species we value for human food. They may also accelerate species extinctions by shunting food energy away from other higher order consumers. They might also be very unhealthy to be near and quite unpleasant to interact with. I hope that we can keep our beaches and open ocean in their more familiar states of existence! They were perfect to begin with!