Few marine creatures are as mysterious and intimidating as jellyfish. Though easily recognized, these animals are often misunderstood. Some bathers and beachcombers react with fear upon encountering these invertebrates, but, in fact, most jellyfish in South Carolina waters are harmless. This article was prepared to help coastal residents and vacationers learn the difference between the jellyfish to avoid and the ones you can safely ignore.
Jellyfish are members of the phylum Cnidaria, a structurally simple marine group of both fixed and mobile animals: sea anemones, sea whips, corals and hydroids are polyps that grow attached to rocks or other hard surfaces; jellyfish and colonial siphonophores like the Portuguese man-of-war are mobile (either actively swimming or subject to winds and currents). Inherent to both types of life history is their radial symmetry (body parts radiating from a central axis). This symmetry allows jellyfish to detect and respond to food or danger from any direction.
Instead of a brain, jellyfish possess an elementary nervous system, or nerve net, which consists of receptors capable of detecting light, odor and other stimuli and coordinating appropriate responses.
General Medusa Body Plan of Jelly fish
Jellyfish are composed of an outer layer (epidermis), which covers the external body surface, and an inner layer (gastrodermis), which lines the gut. Between the epidermis and gastrodermis is a layer of thick elastic jellylike substance called mesoglea (“middle jelly”). Jellyfish have a simple digestive cavity, (coelenteron), which acts as a gullet, stomach and intestine, with one opening for both the mouth and anus. Four to eight oral arms are located near the mouth and are used to transport food that has been captured by the tentacles to the mouth.
Jellyfish come in a wide variety of sizes, shapes and colors. Most are semi-transparent or glassy and bell-shaped, measuring less than an inch to more than a foot across the bell, although some may reach seven feet in diameter. The tentacles of some jellyfish can reach lengths greater than 100 feet. Regardless of their size or shape, most jellyfish are very fragile, often containing less than 5% solid organic matter.
Jellyfish inhabit every major oceanic area of the world and are capable of withstanding a wide range of temperatures and salinities. Most live in shallow coastal waters, but a few inhabit depths of 12,000 feet!
Jelly fish Life Cycle
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. The dominant and conspicuous medusa is the familiar form, while the smaller polyp form is restricted to the larval stage. Jellyfish reproduce sexually, and individuals are either male or female. The reproductive organs (gonads) develop in the lining of the gut. During reproduction, the male releases sperm through its mouth into the water column. Some of the swimming sperm are swept into the mouth of the female, where fertilization occurs. Early embryonic development begins either inside the female or in brood pouches along the oral arms. Small swimming larvae (planulae) leave the mouth or brood pouches and enter the water column. After several days the larvae attach themselves to something firm on the sea floor (rocks, shells, etc.) and gradually transform into flower-like polyps (scyphistoma). These polyps use tentacles to feed on microscopic organisms in the water column. Polyps can multiply by producing buds or cysts that separate from the first polyp and develop into new polyps. When conditions are right, fully developed polyps develop constrictions in their bodies that eventually produce a larval stage (the strobila), which resembles a stack of saucers. Each individual saucer develops into a tiny jellyfish (ephyra stage), which separates itself from the stack and becomes free swimming. In a few weeks, the ephyra will grow into an adult jellyfish (medusa), thus completing the complex life cycle. Jellyfish normally live for a few months; however, the polyp stage may be perennial.
The adult jellyfish drifts in the water with limited control over its horizontal movement. It is, however, endowed with 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. Because jellyfish are sensitive to light, this vertical movement can be important. Some jellyfish, like the sea wasp, descend to deeper waters during the bright sun of the midday and surface during early morning, late afternoon and evening. Despite this ability to move vertically, jellyfish largely depend upon ocean currents, tides and wind for horizontal movement.
To some, jellyfish may appear to have no apparent value, but they are, in fact, a very important part of the marine food web. Jellyfish are carnivorous, feeding mostly on a variety of zooplankton, comb jellies and occasionally other jellyfish. Larger species, however, are capable of capturing and devouring large crustaceans and other marine organisms. Jellyfish are themselves preyed upon by spadefish, sunfish, sea turtles and other marine organisms. Some species, including the mushroom and cannonball jellyfish, are even considered a delicacy by humans. Pickled or semi-dried mushroom jellyfish are consumed in large quantities in Asia, where they constitute a multimillion-dollar part of the seafood business.
Jellyfish are equipped with a specialized venom apparatus (cnidoblast) for defense and feeding. A capsule (nematocyst) inside the cnidoblast contains a trigger and a stinging structure. The stinging structure varies according to species but generally consists of a hollow coiled thread with barbs lining its length. Nematocysts are concentrated on the tentacles or oral arms. A single tentacle can have hundreds or thousands of nematocysts embedded in the epidermis. Nematocysts are activated when tentacles make contact with an object. Pressure within the nematocyst forces the stinging thread to rapidly uncoil. The thousands of nematocysts act as small harpoons, firing into prey, injecting paralyzing toxins.
Stings usually paralyze or kill only small creatures (fish, small crustaceans), but some jellyfish are harmful to humans. Although jellyfish do not “attack” humans, swimmers and beachcombers can be stung when they come into contact with the jellyfish tentacles with functional nematocysts. The severity of the sting depends on the species of jellyfish, the penetrating power of the nematocyst, the thickness of exposed skin of the victim and the sensitivity of the victim to the venom. The majority of stings from jellyfish occur in tropical and warm temperate waters. Most species off the southeastern coast are capable of inflicting only mild stings that result in minor discomfort.
Treatment of Sting
Primary first aid 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.
If stung by a jellyfish, the victim should carefully remove the tentacles that adhere to the skin by using sand, clothing, towels, seaweed or other available materials. As long as tentacles remain on the skin, they will continue to discharge venom.
A variety of substances have been used to reduce the effects of jellyfish stings. Meat tenderizer, sugar, vinegar, plant juices and sodium bicarbonate have all been used with varying degrees of success. Methylated spirits and other forms of alcohol formerly recommended for inhibiting stinging cells actually stimulate them and may increase pain and cause severe skin reactions. Picric acid and human urine also cause a discharge of nematocysts and should not be used.
Victims of serious stings should make every effort to get out of the water as soon as possible to avoid drowning. If swelling and pain from more serious stings persist, prompt medical attention should be sought. Recovery periods can vary from several minutes to several weeks.
Care should be taken when swimming in areas where dangerous jellies are known to exist or when an abundance of jellies of any type is present. Keep in mind that tentacles of some species may trail a great distance from the body of the organism and should be given lots of room. Stings, resulting from remnants of damaged tentacles, can occur in waters after heavy storms. Rubber skin-diving suits offer protection against most contact.
Be careful when investigating jellyfish that have washed ashore. Although they may be dead, they may still be capable of inflicting stings. Remember to take precautions when removing tentacles after contact or additional stings may result.