Understanding Jellyfish
By: Jessica Warren, ANR Agent Camden County
Jellyfish, or jellies, are some of nature’s strangest creatures and one that you’re bound to encounter on the Georgia Coast. Jellyfish aren’t fish at all. They are marine invertebrates that are a type of gelatinous zooplankton. Jellies have no bones, brain, blood, teeth, or fins. Their bodies are more than 95% water. Lacking a brain, jellies have a simple nerve net that detects light and odor, and coordinates responses to stimuli. They are closely related to corals, sea whips, hydrozoans, and sea anemones.
Jellies are very adaptable generalists. They live in every ocean and are plentiful in cold and warm waters. Jellyfish can tolerate a wide range of temperatures and salinities. While most live in shallower coastal waters, some species inhabit depths up to 12,000 feet. Jellyfish pre-date the dinosaurs. Scientists have discovered fossilized evidence suggesting that jellies have been around for at least 600 million years. This makes them one of the oldest multicellular lifeforms on the planet.
Jellyfish serve an important role in the food web as both predator and prey. Jellies use specialized stinging cells on their arms, called nematocysts, to subdue and capture prey. The arms then move prey to the mouth which is located on the underside of their bell-shaped body and is used for both eating and excreting waste. Some jellies employ mucus nets to help capture prey. These nets are produced by a specialized organ, and range in size from one inch to over six feet. Jellies feed on a variety of marine organisms including plankton, crustaceans, comb jellies (which aren’t a true jellyfish and don’t sting), and sometimes other jellyfish. Jellies are an important prey item in the food web as well. More than 150 animal species are known to prey upon jellyfish – including humans. The Chinese have harvested jellyfish for consumption for 1,700 years. There is a cannonball jellyfish (often referred to as jellyball) industry in Coastal Georgia, with most of the product being exported to other countries.
Jellyfish drift with the current and have little control over horizontal movement. They can create vertical movement by contracting their bell. Some jellyfish species will descend into deeper waters during the midday sun and return to surface waters during the cooler portions of the day. Due to their limited locomotive abilities, jellies often appear in masses called a bloom, a swarm, or a smack. This may occur due to currents, tides, wind, or weather patterns.
Jellyfish species are very diverse. They range greatly in color, size, and luminescence. One of the largest species has a bell that can reach eight feet across, with tentacles that can reach more than 100 feet in length. The jellyfish species that you are most likely to see on the Georgia Coast are moon jellies, cannonball jellies, sea nettles, box jellyfish (although not the lethal kind found in Australia) and lion’s manes. Many jelly species, and comb jellies, possess bioluminescence – the ability to produce light. Jellies contain proteins in certain tissues that undergo a chemical reaction in response to stimuli, such as touch, that produces a blue or green light. It is not currently known exactly why jellies are bioluminescent, but it is most likely a defense tactic. A bright flash could startle a predator or attract an even larger predator that would feed on the jellyfish’s predator.
Most human interest in jellyfish revolves around their ability to sting. Stings are a physical reaction, not a conscious choice. When you brush against a tentacle, tiny triggers on the tentacle’s surface release the stingers which shoot out like a microscopic, barbed dart and inject venom. It’s important to note that jellyfish that have washed up dead on the beach may still release venom if their stingers are touched. Most jellyfish stings in Georgia do not require medical treatment and are similar in pain and severity to a bee sting. If stung, the general recommendation is to rub the area with a wet towel or sand to remove any remaining nematocysts from the skin and apply topical lidocaine to numb the pain. Only rinse the area with salt water, as fresh water (or urine which is mostly fresh water) will only make the sting worse.
Another common question is why jellies seem to be growing in number and becoming more common around the world. There are four factors that seem to be contributing to this trend. Overfishing of fish and marine invertebrates that feed on zooplankton and other jellyfish prey, leads to an overabundance of these prey species which leads to growth in jellyfish populations – due to a loss of competition for food resources. Nutrient runoff into waterways from the overuse of fertilizers causes dead zones in the ocean where few life forms can survive. Jellyfish can tolerate low-oxygen environments that many other species cannot, allowing them to thrive in dead zones where other predators have died off. As the ocean warms due to climate change, jellyfish embryos can develop more quickly allowing their populations to grow quickly. Lastly, many industries build docks, platforms, and other structures – referred to as ocean sprawl – in the water which can serve as nurseries for jellyfish and increase their reproductive rate.