Many Miami environmental science graduate students take the Ecology Field Courses each year.
Very few sea creatures are as interesting and intimidating as the jellyfish. While these animals are easily identifiable, many misconceptions are held about jellyfish. Jellyfish are found in all the oceans of the world and even in some freshwater lakes and rivers. In the summer, you might see some washed up on a beach or floating in a harbor. The summertime in the Bahamas in considered Jellyfish season.
Jellyfish belong to the phylum Cnidaria, which is a marine group of both fixed and mobile animals. For example, sea anemones, sea whips, corals and hydroids are polyps that grow attached to rocks or other hard surfaces and jellyfish and colonial siphonophores like the Portuguese man-of-war are mobile (Whitaker, King and Knott, 2006). The first Cnidaria appear
in the fossil record 600 million years ago, preceding the Cambrian explosion (Coates, 2003). It is estimated that there are more than 9,000 species of Cnidaria, all of which share the common characteristics: jellylike bodies, tentacles and stinging cells (Gowell, 1993). The phylum Cnidaria has been divided into three different subgroups: the Scyphozoans, Anthozoans and the Hydrozoans. The Scyphozoans are the most familiar free swimming, bell-shaped animals best known as “jellyfish” (Gowell, 1993). There are over 200 species of jellyfish worldwide, 70 of which are known to sting. Scientist prefer to call jellyfish “medusae”.
Jellyfish are simple animals that are 95% water with less than 5% solid mass. Most have a hollow body shaped like a bell or an umbrella (Gowell, 1993). Most jellyfish are nearly clear or a light blue color, ranging in size from one inch to over a foot in diameter. However, some jellyfish species can grow up to seven feet long (Sharth, 2002).The jellyfish body is made of two thin layers known as the epidermis and the gastrodermis that surrounds the gut. Between the epidermis and gastrodermis is a layer of thick elastic jellylike substance called mesoglea (Whitaker et. al, 2006). They have long threadlike tentacles that hang down below the part shaped like a bell. These tentacles capture worms and small ocean life and carry them into the jellyfish’s mouth (Ocean Life, 2009). Jellyfish have a simple digestive system with one opening for both the mouth and the anus. Jellyfish have radial symmetry with their body parts radiating from their central axis and this allows them to detect and respond to food or danger from any direction. Instead of a brain, jellyfish possess an elementary nervous system consisting of receptors capable of detecting light, odor and coordinating appropriate responses (Whitaker et al., 2006). Jellyfish have limited senses; they have touch receptors on their tentacles and around their mouths to help them capture food (Gowell, 1993). They do not have a nose or tongue, but they have special cells that smell and taste scattered all over their bodies (Gowell, 1993). Unlike many animals that use vision as their primary sense, Cnidarians do not use eyes (Coates, 2003). Instead, sensing organs tell them whether they are heading up or down, into the light or away from it (Weston, 2003).
Jellyfish drift in the water with little control over their movement. They have muscles that allow it to contract its bell, reducing the space under it and forcing water out through the opening. This pulsating rhythm allows for regulation of vertical movement. As water is pushed in one direction, the jellyfish moves in the opposite direction (Weston, 2003). Because jellyfish are sensitive to light, this vertical movement can be important (Whitaker et al., 2006). Squids and octopuses use the same technique to make fast getaways (Gowell, 1993). Jellyfish move very well up in down in the water but their swimming technique is no match for strong ocean currents, wind and waves. At certain times of the year, beaches all over the world are covered with jellyfish that have been washed up on shore by storms or strong winds. Once they are on shore, they have no way to move back into the sea (Gowell, 1993).
The life cycle of a typical jellyfish is complex and involves an alteration of generations in which the animal passes through two different body forms (Whitaker et al., 2006). Young jellyfish do not look anything like their parents. Jellyfish reproduce sexually; the female will produce eggs, while the male will produce sperm. The male produces sperm within their gut and release it into the water through their mouths and then the female sweeps sperm into her mouth where fertilization occurs (Whitaker et al., 2006). The fertilized eggs grows into a planula larva which is released into the ocean. The planula larva ride the currents and waves until they reach a rock or other hard surface where they settle. Once the planula larva has settled, it transforms into a polyp. The polyp uses its tiny tentacles to capture the zooplankton for food (Gowell, 1993). Polyps are able to produce buds that will eventually break off and develop into new polyps. In the right conditions, these polyps will develop constrictions in their body that eventually produce a larval stage. Grooves are formed and create a stack of tiny disks in the polyp, each of these disks develop into a tiny jellyfish (ephyra stage) (Whitaker et al., 2006). After a few weeks, the ephyra will grow into an adult medusa, bringing the jellyfish life cycle to completion. Jellyfish normally live for only a few months.
Jellyfish are known for their stinging capability and therefore swimmers often avoid them. Most humans believe that the jellyfish are attacking them as they are swimming, which is not the case. Jellyfish sting humans when humans swim into their tentacles or simply brush the tentacles in passing. Since jellyfish are carnivores, meaning they kill and eat other animals, they need to have a defense mechanism. Jellyfish are equipped with a specialized venom apparatus, a cnidoblast, for defense and feeding. A capsule, nematocyst, inside the cnidoblast contains a trigger and a stinging structure (Whitaker et al., 2006). The nematocyst is responsible for the pain that the snorkelers and swimmers experience if stung. The first response for any jellyfish sting should be to minimize the number of nematocysts discharging into the skin and to reduce the harmful effects of the venom (Whitaker et al., 2006). Immediately rinse the wound with salt water, fresh water would trigger the nematocysts to fire again (Gowell, 1993). Urine is similar to freshwater and can cause the nematocysts to fire. The tentacles should be carefully removed from the skin. Nematocysts are inactivated by vinegar (or dilute acetic acid 5-10%). A cold pack can be used to help eliminate the skin pain. Avoid jellyfish washed up onto the shore because nematocysts of dead jellyfish can still sting.
Three common jellyfish to keep an eye out for in the Florida Keys are the Portuguese man-of-war (Physalia physalis), moon jelly (Aurelia aurita) and the upside-down jellyfish (Cassiopea xamachana) (Away, 2009). The Portuguese man-of-war is the most dangerous jelly-like fish and the sting can be harmful, if not deadly to humans with allergies. This jelly-like invertebrate has a gas-filled gut that allows it to ride high on the ocean waves (Gowell, 1993). The Portuguese man-of-war is a pinkish purple or blue color. Although it is thought of as a jellyfish, it belongs to the Hydrozoa subgroup. The moon jelly is the most common jellyfish and can be found in all the oceans of the world. Their tentacles produce a mild sting or burning. The upside-down jellyfish is often found in the warm mangrove islands in Florida and the Bahamas. These jellyfish sit on their dome with their tentacles extended upward toward the sunlight.
The increase in abundance of jellyfish cannot be blamed solely on global warming but a mixture of man changes in our ocean environment has caused a rapid influx of jellyfish. There are many factors that attribute to the abundance of jellyfish worldwide. Jellyfish take advantage of warming waters to expand their range and accelerate their rates of reproduction (Marlow, 2009). Jellyfish serve as a food source for larger sea creatures, along with preventing over population of organisms they feed on (Stidworth, 1990). Luckily jellyfish have the ability to use their stinging cells in defense from being eaten. The ocean sunfish eats jellyfish and weighs 1,5oo pounds eating almost only jellyfish (Gowell, 1993). Inter-annual variations in jellyfish abundance in oceanic areas are related to zooplankton abundance and temperature changes (Gibbons, 2009). Researchers have found that dense aggregations of jellyfish do not form within five meters of rock walls and jellyfish do not swim into water with salinity less than twenty parts per thousand (Albert, 2008). Researchers have also jellyfish may display centers of preferred distribution with the ocean. (Doyle & Houghton & Buckley & Hays and Davenport, 2007).
Asian cultures have been indulging in cannonball jellyfish for over a thousand years. Cannonball jellyfish contains collagen, which is good for humans. Our bodies need collagen to build cell tissue, cartilage, teeth and bones. Scientific research continues on collagen and its medical potential. Jellyfish has been eaten for medical reasons to treat high blood pressure, arthritis, bronchitis and other diseases. The cannonball jellyfish is an ideal diet food because it is low in fat, cholesterol and calories. Jellyfish must be dehydrated before the nutrients are able to be obtained from the animal (Bronson, 2004). Jellyfish play an important part in the marine community. Jellyfish consume large amounts of zooplankton, which are tiny organisms on the surface of the water, which increases the clarity of our ocean water.
I have always been interested in jellyfish and this paper gave me the chance to research them in depth. I truly enjoyed learning about this fascinating animal and I am excited to pass along the information to my classmates. I hope that we see many jellyfish throughout the course, which lead to strong class discussions.
Albert, D. J. (2008). Adaptive behaviours of the jellyfish Aurelia labiata in Roscoe Bay on the west coast of Canada. Journal of Sea Research, 59(3), 198-201.
Coates, M. M. (2003). Visual Ecology and Functional Morphology of Cubozoa (Cnidaria). Integrative and Comparative Biology, 43(4), 542-548.
Doyle, T. K., Houghton, J. D. R., Buckley, S. M., Hays, G. C., & Davenport, J. (2007). The broad-scale distribution of five jellyfish species across a temperate coastal environment. Hydrobiologia, 579(1), 29 - 39.
Gibbons, M. J., & Richardson, J. (2009). Patterns of jellyfish abundance in the North Atlantic. Hydrobiologia, 616(1), 51 - 65.
Gowell, E. T. (1993). Sea Jellies Rainbows in the Sea. New York: Franklin Watts.
Marlow, J. (2009, Spring2009). Beautiful but Deadly. (Cover story). Earth Island Journal, 24(1), 34-39. Retrieved May 14, 2009, from Academic Search Complete database.
Sharth, S. (2002). Sea Jellies from Corals to Jellyfish. New York: Franklin Watts
Stidworth, J. (1990). Simple Animals. New York: Facts on File.
Weston, Paula. Jellyfish: A clever hunter for a creature with no brain. Creation. Volume 25,
Issue 4. September 2003.
Whitaker, J. D. & King, R. & Knott, D. (2006). Marine Resource Division. Jellyfish.
http://www.fl-seafood.com/species/jellyfish.htm Florida Department of Agriculture and Consumer Services Charles H. Bronson, Commissioner
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