This Month in Science: Jellyfish

If you see a purple or blue flag flying while at the beach, you might want to think twice before getting in the water. This colored flag indicates a warning that dangerous marine life, like jellyfish, are present in the water. Some of the most common jellyfish found around the North Carolina coast include cannonballs, moon jellies, purple sails, sea nettles, and mushroom jellyfish1. There are over 1,500 species of jellyfish drifting on the ocean currents, varying in size, shape, toxicity, color, and reproductive habits2. How is something made up of 95% water so dangerous it prompts beaches to fly the warning flag3? What makes up the other 5%? How do they function without a heart, lungs, or brain? Read on to find out!

Jellyfish aren’t actually fish2. Jellyfish are a form of plankton, most closely related to corals and anemones4. These marine creatures all have one feature in common: they have specialized cells that provide a painful, incapacitating sting to protect them against predators and assist in obtaining food. This adaptation is important because corals and anemones can’t move and jellyfish don’t control their movement (some species are capable of limited movement by expelling water from the top bell portion of their body, but most species are at the mercy of ocean currents).

The stinging cells are called cnidocytes (pronounced: ny-doe-sites) and every tentacle has thousands of cnidocytes on it5. Each cell contains a tiny venom-tipped harpoon called a nematocyst6. When a creature brushes past a jellyfish tentacle, the cell is triggered to release the nematocyst, spearing the prey that touched it with neurotoxin. This process takes just 700 nanoseconds to occur. The response is automatic and indiscriminate. Jellyfish will release the nematocyst anytime it is triggered, whether by floating debris, actual food, or an unsuspecting human.

The body structure of a jellyfish has two main cell layers—an external epidermis and an internal gastrodermis. The space between these two layers is made of a gelatinous material called the mesoglea. The outer layer consists of a neural network of cells that serves as a very basic nervous system. Instead of a brain, this neural network contains cells that have primitive sensory functions. Different cells have specialized abilities to detect light, smell, and orientation (whether the jellyfish is facing towards the surface or the bottom of the ocean)6. Jellyfish don’t need a heart or lungs because the thin cell layers of the gastrodermis allow oxygen to diffuse in and carbon dioxide waste to diffuse out6.

The gastrodermis surrounds the multi-purpose gastrovascular cavity. The jellyfish has one hole that leads into this cavity and is both the mouth and the anus. Jellyfish are carnivores, consuming zooplankton, algae, crustaceans, and sometimes other jellyfish. The food enters through the mouth and is broken down by enzymes into nutrients and waste. The nutrients diffuse through the gastrodermis and the remaining waste is expelled through the hole. This cavity also contains the gonads7.

Jellyfish reproduce both asexually and sexually depending on what phase of the life cycle they’re in. Adult jellyfish, called medusa, reproduce sexually by spawning—male and female jellies release eggs and sperm into the ocean. In some species, the fertilization of eggs occurs in the ocean, while in others, the sperm swim into the mouth of a female jellyfish and fertilize the eggs inside. Fertilized eggs grow into a planula—an egg-shaped form with tiny hair-like projections called cilia that allow them to move in the water. The planula swims to the ocean bottom seeking a surface to attach to and develop into a polyp8.

The polyp feeds on plankton and grows until it begins reproducing asexually through budding. Budding is a process where the polyp divides to produce “buds” that are genetically identical clones. These buds, called ephyra, are released from the polyp as immature jellyfish. Many ephyra will bud from one polyp8. The ephyra feeds and matures until it develops into an adult medusa. Medusa jellyfish tend to live for only a few months, while polyps can live for years to decades6. Despite their simplistic structure, jellyfish have evolved protective strategies that have allowed them to drift in the ocean for over 500 million years2.

While it may be common to see a jellyfish or its tentacles washed up on shore, you should never touch them. They can still sting despite being dead9. If you are ever stung by a jellyfish, the first thing to do is make sure you remove the tentacles from the affected area. Running a plastic ID card or credit card along the skin should remove the jellyfish barbs. Following removal, clean the sting with vinegar to neutralize the jellyfish venom. Hot water is the most effective way to ease the burning following a sting. Doctors suggest water at a temperature of 110°F or hotter10. If you’d like to get up close and personal with some jellyfish in a safe way, check out the jellyfish tank down at the North Carolina Aquarium at Fort Fisher.




Previous Columns

July 2019: What’s in Your Childhood Chemistry Set?
June 2019: What Happened to the Dinosaurs?
May 2019: Hurricane Season
April 2019: Quantum Levitation
March 2019: Venus Flytrap
February 2019: A Shifting Magnetic Field
January 2019: Giant Ground Sloth
December: Snowflakes
November: Citizen Science
October: Parker Solar Probe Voyaging to the Sun
August: Sea Turtle Season
July: Plastic Free July
June 2018: All About Alligators

Cannonball jellyfish on a wooden pier likely at Wrightsville Beach, about 1916.
Gift of Arthur Bluethenthal.

The anatomy of a cnidocyte, the cells containing the stinging harpoons in jellyfish.
Source: Pearson Education.

Basic anatomy of a jellyfish.
Source: HowStuffWorks.

The lifecycle of a jellyfish.
Source: Smithsonian Ocean Portal.
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