There's no such thing as a jellyfish

Everyone is familiar with the basic idea of a jellyfish, but what do we mean when we use that term? The most commonly seen are things like sea nettles, which can bother swimmers and be stranded ashore as amorphous brown blobs. These are some members of the Scyphozoans. Their life cycles typically include a polyp stage, attached to the bottom, which produces baby medusae. When conditions are right, these babies can grow up to form vast blooms of adult jellies. But for every moon jelly, pulsing in coastal waters, pYthere are deep-sea relatives which defy generalizations -- they can be a meter across, 20 meters long, and lack tentacles. The branch of the tree of life called Scyphozoa also includes another lineage: the Coronate medusae. Typically found in the deep sea, they can be brightly bioluminescent and many lack a polyp stage in their life cycle. All of these scyphozoans are Cnidarians (the C is silent), relatives of sea anemones and corals. This group is defined in part based on the presence of stinging cells. The most infamous stingers are the sometimes lethal box jellies, found in tropical waters, but most Cnidarians pose no danger to humans. Most species of swimming jellies are actually in another group called the Hydromedusae. These are often small and transparent. They may have very few tentacles.... Or very many. They might or might not have a small polyp stage called a hydroid. One group of hydromedusae has tentacles which point ahead of them instead of trailing behind. These eat other gelatinous organisms, rather than the crustaceans favored by many other Cnidarians. But even with all this diversity we haven't yet encountered the most unusual Hydrozoans. Would you consider a siphonophore a jellyfish? From the Portuguese man-o-war floating on the surface, to dozens of species found in the deep sea, these are among the strangest of marine animals. They have divided up the tasks of living among different subunits. Some parts pulse to move the animal through the water, but can't feed. Others, connected by a thin tube that runs the length of the body, can feed, or reproduce, but not swim. Although siphonophores can grow to be 30 meters long, the most common are small (rocket-shaped) species, found throughout the ocean. Those are the Cnidarians, including some of the most familiar and unfamiliar animals that might be called jellyfish. They all have in common the presence of stinging cells called cnidae. Not all jellies sting, though. Another deep branch in the tree of life leads to the Ctenophores, also known as comb jellies. Their defining trait is 8 rows of ciliary plates, which flutter like eyelashes to move them through the water. Some species use sticky tentacles to capture prey, much like a spider web. Others lack tentacles, but use tooth-like cilia in the mouths to bite and swallow gelatinous prey. Many deep-sea species are so fragile that they are new to science, or have only been described since the advent of submarines. Nearly all are bioluminescent, and many have red or black pigmentation which is thought to mask the bioluminescence of ingested prey. The next group that is often lumped with jellyfish are the Urochordates, including salps, doliolids, and larvaceans. These are the most vertebrate-like of invertebrates, with many traits shared with chordates. Salps are typically colonial, forming units that can be tens of meters long. Each unit within the colony pumps water through their body, filtering out phytoplankton and even smaller particles. As such, they are the jellyfish that subsist directly on plant material. Because they grow and reproduce rapidly, salps and doliolids can form dense blooms in many regions. Larvaceans are relatives of salps which, during development, do not change their body form from their tadpole-like larval stage. Larvaceans filter particles using a mucus house that they secrete and inflate, sometimes many times each day. The mesh is so fine that they are able to retain and ingest even bacteria-sized particles. So the factors that lead to changes in salp populations, and the effects that salps have on ocean ecosystems, are very different from the causes and effects of changes in Cnidarian and ctenophore populations. We know of slugs and snails that crawl on the ground, but a variety of molluscs live in the water column, flapping their feet to swim. Many have even retained their shells, although typically in a reduced form. Many kinds of worms also live a gelatinous life. This includes polychaetes (distant relatives of earthworms). One of the most abundant deep-sea animals below 2500 meters depth is a polychaete with fans of long setae -- the bristles that characterize polychaetes. Other segmented worms,- transparent and beautifully bioluminescent lack these bristles, and swim around preying on gelatinous organisms. Some worms, with two beady red eyes, are parasites and predators on jellies. So is there really such a thing as a "jellyfish". Are they only circular Cnidarians or also elongated ones? Do they sting people or eat microscopic plants? Are they rare and elusive, or poised to take over the world? All across the tree of life, organisms have evolved jelly-like adaptations, and each has its own way of thriving and surviving. Understanding their diversity is an important first step to understanding life in the largest habitat on earth. this is Steve Haddock from the Monterey Bay Aquarium Research Institute. If you see any jellyfish-like creatures, be sure to report them at jellywatch.org�