The jellyfish: more than mindless protoplasm?
Jellyfish have drifted along the ocean currents for millions of years, even before dinosaurs lived on the Earth. They are found in every ocean, from the surface to the deep. Jellyfish belong to the Cnidaria phylum that also contains the soft and stony corals and sea anemones, collectively known as Anthozoa. The jellies start their lives as polyps with tentacles on top, like coral polyps and sea anemones, but then end up as free swimming adults with a medusa form, the reason why they are named Medusozoa. Adult jellyfish are gelatinous and generally transparent or translucent, with gelatinous umbrella-shaped bell and trailing tentacles. Despite their respectable antiquity, jellyfish have long been ignored or misunderstood by mainstream science, dismissed as so much ‘mindless protoplasm with a mouth’.
Varieties Jellyfish consist of around 4000 different species belonging to various families, that are subdivided in the following four big classes:
-Hydrozoa are a mixed class that contains sessile (nonmoving) members, such as fire coral (Millepora) and the freshwater polyps Hydra, but also the marine hydrozoan Portuguese man 'o war (Physalia physalis), nicknamed the ‘floating terror’ (see picture left).
-Scyphozoa are the most familiar jellyfish, including most of the bigger and more colorful jellies, also called "true jellyfish". The at least 200 species of Scyphozoa spend most of their lives in the medusa body form. The remarkable species of the Cassiopeidae family, known as the upside-down-jellyfish, have their tentacles on top. The upside-down jellyfish Cassiopea xamachana which lives in the Carribbean, Hawaii and Florida, appears as a flower on the seafloor and tends to aggregate in large groups. Another beautiful Scyphozoan species is the Mediterrean jellyfish (Cotylorhiza tuberculata) from the Cepheidae family. Its dome is surrounded by a colourful gutter-like ring, often carrying small horse mackerels. The large barrel jellyfish (Rhizostoma pulmo) from the Rhizostomatidae family, can also be found in the Mediterranean and Adriatic. This jelly can become so large that young fish and small crabs seek shelter in its mildly stinging tentacles
-Cubozoa -or box fellyfish jellyfish- have a more developed nervous system than other jellyfish, including complex eyes with lenses, corneas and retinas. Some species, such as the feared sea wasp (Chironex fleckeri), produce one of the most potent venoms known.
-Staurazoa or stalked jellyfish don't float through the water like other jellies, but rather live attached to rocks or seaweed. They are trumpet-shaped, and mostly live in cold water. There are around 50 staurozoan species, many notable for their unique combination of beauty and camouflage.
Comb jellies Comb jellies are not Cnidaria but belong to another phylum called Ctenophora.Though they have much in common, jellyfish and comb jellies have very different histories in the tree of live. Comb jellies are named for their unique feature: the presence of cilia, known as combs, which run in eight rows up and down their bodies. The combrows of cilia look like small paddles lining their bodies that propel them through the ocean. They can also produce a rainbow effect. This is not bioluminescence, but occurs when light is scattered in different directions by the tissue of their skin. Most comb jellie havt two, often branched, tentacles.
Anatomy and reproduction Jellyfish vary greatly in size depending on the species. Most jellies range from less than half an inch (1 cm) wide to about 16 inches (40 cm), though the smallest are just one millimeter wide. On of the largest jellies is the Lion’s Mane Jellyfish (Cyanea capillata). They are highly distinguishable by their mass of long, thin and hair-like tentacles which can be almost 6 feet wide (1.8 m) with tentacles over 49 feet (15 m) long.
Most jellyfish do not have specialized digestive, osmoregulatory, respiratory or circulatory systems. The mouth/anus at the base of the bell opens into the gastrovascular cavity where digestion takes place and nutrients are absorbed. The mouth is surrounded by tentacles that can stick, tangle or sting and are also meant to bring food to the mouth. As jellyfish squirt water from their mouths they are propelled forward. Some jellyfish are colourless, but others are in vibrant colors such as pink, yellow, blue, and purple, and often are luminescent.
Polyps can live and reproduce asexually for several years, or even decades. Medusa jellyfish reproduce sexually by spawning—the mass release of eggs and sperm into the open ocean—with entire populations sometimes spawning all together. Adult jellyfish typically live for a few months, depending on the species, although some species can live for 2-3 years in captivity. One jellyfish species is almost immortal: Turritopsis dohrnii, a small hydrozoan can revert back to the polyp stage after reaching adult medusa stage through a process called transdifferentiation, meaning a process by which the state of cells differentiates and transforms into new types of cells.
Behavior With their modest oxygen requirements, jellies can grow in post-algal “dead zones” and other polluted waters where most marine life can’t — not surprising perhaps for species that has survived over so many ages. Polyps feed on feed on zooplankton. All adult jellies are carnivorous, feeding on plankton, crustaceans, fish eggs, small fish and other jellyfish, ingesting and voiding through the same hole in the middle of the bell. Jellies do not actively hunt, but instead use their tentacles as drift nets.
In turn, jellies are the favorite food of seabirds, the ocean sunfish (Mola mola) and endangered leatherback turtle (Dermochelys coriacea). The abundance of plastic floating in the ocean is often the cause of death of these animals, mistaking plastic particles for their favorite prey. Humans have also been eating jellyfish, for at least 1700 years. Some 425,000 tons (more than 900 million pounds) of jellyfish are caught each year by fisheries in 15 countries, and most are consumed in Southeast Asia.
Nervous system Are jellyfish more that a lump of mindless protoplasm with a mouth and venom? Many experts now say: Yes. Jellyfish belong to the earliest known animals to have organized tissues—their epidermis and gastrodermis—and a nervous system. Jellyfish don’t have a brain like another famous invertebrate of the sea, the octopus (mte that the octopus does not have tentacles but arms. The jelly's ‘brain' is in fact a network of nerve cells, that allows it to sense their environments, such as changes in water chemistry or the touch of another animal. The nerve net has some specialized structures such as statocysts, which are balance sensors that help jellies know whether they are facing up or down, and light-sensing organs called ocelli, which can sense the presence and absence of light. The nervous system of box jelly fish is more complex. In its visual system there is an interactive matrix of 24 eyes of four distinct types — two of them very similar to our own eyes — allowing the jellies to navigate through rubbish or the mangrove swamps they inhabit.
Some however might say that jellies are intelligent, despite their lack of a brain (i.e. cortex), because of their refined adaptations to various conditions in the environment*. Instead of a brain some jellies have a central circuitry of giant motorneurons controlling movement of tentacles and the bell**. Their behaviors include swimming up in response to somatosensory stimulation, swimming down in response to low salinity, diving in response to turbulence, avoiding rock walls, forming aggregations, and horizontal directional swimming. In short, behaviors that go beyond simple reflexes and require sensory feedback during their execution.
Toxicity Jellyfish and comb jellies both have tentacles with specialized cells to capture prey: nematocysts and colloblasts, respectively. Jellyfishes' nematocysts are organelles within special cells (cnidocytes) that contain venom-bearing harpoons. The cell is activated upon touch or chemical cue, causing the harpoon to shoot out of the cell and spear the prey or enemy, releasing toxin—a process that takes only 700 nanoseconds. The same nematocysts are active on the hydrozoan firecoral. A small number of jellyfish are very toxic to humans, such as the box jellyfish (Chironex fleckeri) mentioned earlier, and the Irukandji jellyfish (Carukia barnesi), which can cause severe reactions and even death in some people.
Jelly bloomings Some beaches can be temporarily flooded by masses of jellyfish, often species of Scyphozoa like the Pelagia Noctiluca (nicknamed the mauve stinger) in the Mediterranean. Occasionally with strong Westerly winds lots of blueish barrel jellyfish from the Northsea strand on Dutch beaches. Some believe that environmental factors like warming and pollution of seawater, and the decline of natural predators are responsible for the rapid growth of certain species. Massive aggregations of jellies, known as "jellyfish blooms" or "jellyfish outbreaks," can cause a wide array of problems. Too many jellies in the water can be a danger to swimmers, forcing towns to close their beaches. Jellies have clogged up machinery at coastal power plants, causing power outages. They can interfere with fisheries by eating fish larvae, and fisherman catching jellies instead of the fish they want.
Sources and links:
Albert, D.J.(2011)*. What's on the mind of a jellyfish? A review of behavioural observations on Aurelia sp. jellyfish. Neurosci Biobehav Rev. 2011 Jan;35(3):474-82.
Satterlie, R.A. (2011). Do jellyfish have central nervous systems? J Exp Biol. 2011 Apr 15;214(Pt 8):1215-23
Kramp, P.L. (1961): Synopsis of the Medusae of the World. Order Rhizostomeae. Journal of the Marine Biological Association of the United Kingdom 40: 348–382.
Mackie, G. O. and Meech, R. W. (1995a)**. Central circuitry in the jellyfish Aglantha digitale. I. The relay system. J. Exp. Biol. 198, 2261–2270
Jellyfish Take Over an Over-Fished Area". 21 July 2006