Jellyfish Facts

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Updated: September 22, 2012
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By: Dr. Jennifer E. Purcell

What types of jellyfish occur in the Chesapeake Bay and nearby waters?

In the summertime in ocean waters, for example, off Ocean City, two types of large jellyfish often occur. The Moon Jelly (Aurelia aurita) is a clear, flattened disk, with numerous small tentacles around the edge, and a pink four-leaf clover design in the middle. This species has a very mild sting and poses no threat to swimmers. The Lion’s Mane jellyfish (Cyanea capillata) is brown in color, and has longer tentacles hanging from the edge of its swimming disk and from the middle. This species has a potent sting, and while it is not dangerous to swimmers, it is very unpleasant to encounter. Some other jellyfish are seen less often in the coastal waters. In the autumn, the Mushroom Cap Jellyfish (Rhopilema verrilli) may enter the Bay. It has a deep swimming bell without tentacles, and is creamy white with darker markings on the sturdy central tentacle structures. The Cabbage Head or Cannonball Jellyfish (Stomolophus meleaqris) is like a white softball, or larger, and has a short, hard tentacle structure in the middle. Neither of these jellyfish sting swimmers. The Portuguese man of war (Physalia physalis) very seldom is seen off beaches north of Cape Hatteras. It is a relative of jellyfish that floats at the water’s surface by means of a gas-filled blue float. Beneath the float are extremely long tentacles which have a powerful sting. They should be avoided by swimmers, because a serious sting would cause extreme pain, and could result in hospitalization. The Moon Jelly also occurs in southern Chesapeake Bay during the summer. The Lion’s Mane, or Winter Jellyfish, is found in Chesapeake Bay during the winter (January – April). The jellyfish for which Chesapeake Bay is widely known in the summer is the Sea Nettle (Chrysaora auinquecirrha). It occurs from Cape Cod south along the U.S. East Coast, Caribbean and Gulf of Mexico, yet it abounds in Chesapeake Bay in numbers unequaled elsewhere. It occurs most abundantly in the tributaries of the middle Bay (salinities 10 – 20 ppt), where it is white in color. In the southern Bay, it often has red/maroon markings on the long central tentacles and on the swimming bell. It has an annoying sting, but is not dangerous to swimmers.

Jellyfish stings: How to prevent them and how to treat them

Lightweight protective clothing, like a lycra “swim skin” or panty hose, or a layer of petroleum jelly spread on unprotected skin, will protect a swimmers against stings. If you are stung by a jellyfish, liberally sprinkling a meat tenderizer or baking soda (or vinegar for PhYsalia) on the sting may reduce the irritation. Severe allergic reactions (anaphylaxis) are uncommon to jellyfish in U.S. waters, but emergency treatment is essential in such cases.

Where do the sea nettles come from each year?

The swimming jellyfish are either male or female. They produce eggs or sperm, which are shed daily into the water during the summer. Fertilized eggs form larvae that attach to hard surfaces, like oyster shells, and grow into tiny polyps. The bottom-dwelling polyps live through the winter in a dormant state. During May through August, the polyps bud off tiny sea nettles about 1/25 of an inch in diameter, that grow rapidly into the visible jellyfish.

How do sea nettles feed and what do they eat?

Sea nettles capture prey that contact the tentacles trailing behind the swimming bell. These tentacles have millions of microscopic stinging cells that inject toxins to stun or kill tiny animals, and which are responsible for the stings swimmers feel. The prey are transported up the central tentacles to the heart-shaped gastric pouches in the swimming bell where digestion occurs. Jellyfish are prodigious predators because they swim and feed continuously. They do not have eyes, and so do not need light to feed. They feed without interruption because the many tentacles function independently of the others. The tentacles provide a very large surface area for prey capture. Sea nettles feed mostly on microscopic crustaceans called copepods that are very abundant in Bay waters. They also eat young minnows, bay anchovy eggs, worms, mosquito larvae, and comb jellies, so they would seldom be without something to eat.

Why are there so many sea nettles in Chesapeake Bay?

Sea nettles are made up mostly of water and salts, with organic materials totalling only about 0.2k of their entire live weight. For this reason, very little food can provide enough organic materials to result in a lot of growth. The jellyfish can get bigger very rapidly, and the amount of food they catch increases directly with their increasing size. Sea nettles are geared for high reproduction. They begin producing eggs when the swimming bell is only about 1 1/2 inches in diameter. The number of eggs increases tremendously as they grow, with a nettle about 4 inches in diameter shedding about 40,000 eggs into the water daily. The polyps can bud to produce more polyps. Each polyp produces up to 45 jellyfish each summer The Sea Nettle is unusual in its ability to live in water of low salinity (salt content). Most jellyfish species live at ocean water salinity, about 35 ppt (35 parts salt to 965 parts water). The Sea Nettle prefers waters having as little as 12 ppt salinity, and may have estuaries like Chesapeake Bay, to itself without serious competition from most other jellyfish. In fact, sea nettles eat their most abundant competitors in the Bay, the comb jellies. Populations of plants and animals often are controlled by other organisms that feed on them. However, adult sea nettles may have few natural predators in the middle reaches of Chesapeake Bay. Sea turtles, which are known to eat Portuguese men of war and some other jellyfish, rarely come far into the Bay. And fish species (harvestfish, butterfish) observed feeding on sea nettles prefer waters of higher salinity.

Are there more sea nettles now than in earlier years?

It is possible that the changes in Chesapeake Bay caused by human activities have led to larger populations of the Sea Nettle in recent times. The waters of the Bay have been enriched with organic materials and nutrients from waste products and fertilizers due to heavy human settlement around the Bay. This kind of enrichment can change the species of microscopic plants suspended in the waters from large to small types. The small plants are more suitable as food for small grazers (copepods), which in turn are more suitable for predators like jellyfish, and less suitable for visual predators like fishes. Dr. Roger Newell proposed that removal of most of the oysters that were once so abundant in the Bay also may leave much uneaten plant material suspended in the water to be consumed by copepods, which in turn could be used by jellyfish and increase their populations. Unfortunately, no early records document the abundance of the Sea Nettle before Man began to seriously change Chesapeake Bay, so there is little with which to compare the present abundance. Mr. David Cargo kept daily records of the numbers of sea nettles at the Chesapeake Biological Laboratory in Solomons, MD for nearly 30 years. He found great annual variation in jellyfish numbers: more occurred in dry years than in rainy years, but no overall increase in abundance was apparent. Such records would need to go back more than 100 years to reveal if human-caused changes in the Bay have affected the abundance of sea nettles.

Is it possible to reduce the numbers of jellyfish?

A lot of effort was spent on jellyfish control in the 1960s, but no method has been very successful. Nets and bubble screens were used to keep them away from swimming areas. The jellyfish tended to clog the nets and to break into pieces that continued to sting. The bottom-living polyp stage also was targeted. Chemicals that killed the polyps also killed many other organisms, and so were unsuitable. A small species of sea slug was found that ate the polyps, but culture methods to produce large numbers of the sea slugs were unsuccessful, and they did not live well at the low salinities favored by the polyps. So far, the greatest reduction of the jellyfish populations occurred after Hurricane Agnes in 1972, but this method of control cannot be recommended.

Can anything good be said about sea nettles?

Research in my laboratory has shown that sea nettles may help oysters. Oysters have a larval stage that spends about two weeks swimming in the Bay before they settle to grow into large oysters. During this swimming stage, they are vulnerable to predators such as sea nettles and comb jellies. Although sea nettles can catch the larval oysters, they spit them out undigested and unharmed. In contrast, comb jellies catch and digest the larvae readily. Comb jellies are a favorite food of sea nettles, and they reduce comb jelly populations to zero in the tributaries during the summer when oyster larvae are most abundant. Therefore, sea nettles appear to protect oyster larvae from a major predator.

What recent research is there on Sea Nettles?

In my laboratory, we have been working to determine the importance of sea nettles as predators of copepods. This is of interest because if sea nettles reduce copepod populations, then less food would be available for planktivorous fish, for example bay anchovy and menhaden, which then are food for piscivorous fish like striped bass and bluefish. We have found that sea nettles eat at most 5% of the copepods daily in mid Chesapeake Bay, which is small compared with the growth rate of the copepod population. In the tributaries, however, Sea Nettles can consume 50 – 90% of the copepods daily, and copepod abundances seem to decline over the summer, possibly due to this predation. Sea Nettles consume up to 50% daily of the bay anchovy eggs and larvae during the peak spawning in July. This research was in collaboration with Dr. Ed Houde of the Chesapeake Biological Laboratory. We also are studying which environmental factors have the greatest effects on the population size of sea nettles. We have done experiments to measure the numbers of sea nettles produced by polyps, and the survival and growth of the baby jellyfish at varlous comblnatlons of salinity, temperature, and food concentrations. Few sea nettles are produced at low salinities (< 7 ppt) or at high salinities (> 25 ppt). I am beginning a large-scale project to map the distributions of jellies throughout Chesapeake Bay in April, July, and October. In collaboration with several other scientists, I will examine environmental and biological (e.g. food) factors that may determine the distribution and abundance of jellies in the Bay.

Will this be a heavy or light year for Sea Nettles in the Bay?

Because this has been a dry year, I expect that sea nettles will be numerous this year.

Prepared by: Dr. Jennifer E. Purcell, Associate Professor
University of Maryland System
Center for Environmental and Estuarine Studies
Horn Point Environmental Laboratory
P.O. Box 775
Cambridge, MD 21613

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