Each summer, a small creature silently haunts the Chesapeake Bay. So intimidating is this little animal that even the hardiest souls won’t enter the water.
No, it’s not a shark or some other fearsome predator. Chesapeake Bay summers belong to the sea nettle (Chrysaora quinquecirrha), whose graceful, gelatinous body keeps us onshore not wanting to risk its sting.
A member of the jellyfish family, sea nettles are actually year-round residents. Before taking on the distinctive umbrella shape, sea nettles go through a variety of growth stages, which are not recognizable or cannot be seen with the naked eye. It is only the adult “medusa” form that we are all too familiar with.
From late summer to early fall, adult sea nettles spawn, then die. Males release sperm into the water and females pump the sperm-laden water through their bodies to fertilize their eggs. Females then release minute larvae known as planulae. The planulae float through the water at the mercy of the currents and many predators, including other sea nettles. After a few days, the surviving larvae settle to the bottom.
Finding a hard surface to settle on, the planulae secrete a type of body glue and attach themselves. Rocks, oyster shells and even bottles may serve as habitat for this stationary phase of the sea nettle life cycle. After attaching, the larvae swell and develop into flower-shaped “polyps.” At one time, scientists considered controlling sea nettles by eliminating the sedentary polyps. But nettle polyps share the same habitat as oysters, and disturbing this shared habitat would likely harm oysters as well.
For most animals, the young or larval phase is the most vulnerable period. Sea nettle polyps, though, are able to ball up or “encyst” during unfavorable or dangerous conditions. This allows the polyps to overwinter and survive other harsh conditions for months or years. In 1972, when Tropical Storm Agnes raged up the Chesapeake, the deluge of freshwater dumped into the Bay caused the majority of sea nettle polyps to encyst that year and wait for more favorable salinities to return.
In the spring, as temperatures rise, the polyps change shape. Each polyp transforms into a series of small stacked discs. Each disc then separates from the base of the polyp as tiny “ephyra.” The ephyra, microscopic in size, are actually miniatures of the adult sea nettle. They float in small creeks and streams during April and May. As they grow, the immature nettles move out toward bigger tributaries and by early summer, into the open waters of the Bay. Here, they continue to grow and take on their most familiar and fearsome form.
Nettles’ movements are predominately driven by wind, tides and currents. By contracting and relaxing its body, the sea nettle can, however, produce weak swimming motions.
Because of their preference for brackish to salty water, sea nettles are usually found in greater numbers in the mid or lower Bay regions. But droughts can increase their reach up the Bay They cannot survive in the fresher reaches of the Bay or freshwater rivers. Adult sea nettles favor warmer water temperatures, too. Unusually hot, dry summers will produce larger nettle populations and increase their range into the upper Bay.
The body of an adult nettle consists of a white umbrella-shaped head, oral arms that can digest and move prey toward the mouth, and eight to 24 tentacles, which may be 4–5 feet long. Each tentacle is armed with stinging cells called nematocysts. Each nematocyst contains a trigger and a venomous barb, similar to a hypodermic needle.
Sea nettles are carnivorous and consume small fish, shellfish larvae, worms, zooplankton and even other types of jellyfish. Prey become entangled in the sea nettle’s tentacles.
When touched, the stinging cells shoot the barb to sting the unwary with a potent poison. Once stung, prey is stunned and can be easily consumed by the nettle. The mere design of the sea nettle makes it an extremely efficient hunter. Tentacles that are broken off are readily replaced by regeneration and even amputated tentacles retain their stinging abilities. Luckily for humans, a sea nettle sting causes only minor irritation to the skin.
Sea nettles have few enemies. Because their body mass consists mostly of water, nettles are not much of a meal. Some species of sea turtles, though, do include sea nettles in their diet. One turtle in particular, the loggerhead turtle, feeds voraciously on sea nettles in the Bay.
Sea nettles and their relatives have existed on earth for nearly a quarter billion years. Their life cycle is perfectly adapted for the estuarine environment of the Bay. Looking beyond the minor inconvenience of sea nettle stings to swimmers, water skiers, and boaters, it’s hard not to appreciate the prehistoric beauty of these translucent creatures as a measure of simple and efficient, evolutionary success.
For a short period in the summer, sea nettle medusae rule the waters. They have been in the Chesapeake long before human habitation and will probably remain here long after. Whether you admire or despise them, sea nettles are here to stay. To coexist, we need only to stay out of their way.