Chesapeake summers. They’re hot and humid. We are drawn to the Chesapeake Bay and its many rivers for a cooling dip.

Whether you are along a beach, dock or on a boat, there is that one animal that makes you leery of entering the water. But this intimidating creature is not a shark. The animal that rules Chesapeake Bay summer waters is the simple sea nettle (Chrysaora quinquecirrha) whose graceful, gelatinous body keeps us onshore not wanting to risk its inevitable sting.

A member of the jellyfish family, sea nettles inhabit the Bay year-round. Before taking on the distinctive umbrella shape, sea nettles go through a variety of growth stages, neither recognizable nor seen with the naked eye. It is only the fearsome adult “medusa” form we are all too familiar with.

Adult sea nettles spawn from late summer to early fall, after which they 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 at the mercy of the currents and many predators, including other sea nettles. After a few days, the surviving larvae settle to the bottom.

After 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. After attaching, the larvae swell and develop into flower-shaped polyps.

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 them to overwinter and survive other harsh conditions for months or years.

In the spring, as temperatures rise, the polyps change again. Each polyp transforms into a series of small stacked discs. Each disc then separates as tiny versions of adult sea nettles known as ephyra. These float in small creeks 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 Chesapeake Bay. Here, they continue to grow and take on their familiar and formidable form.

Nettles’ movements are predominantly 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. Drought conditions can increase their reach up the Chesapeake Bay.

Sea nettles cannot survive in the fresher reaches of the Bay or freshwater rivers. Adults 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 digest and move prey toward the mouth, and eight to 24 tentacles that can grow to 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 becomes entangled in the sea nettle’s tentacles. When touched, the stinging cells shoot a 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. Broken tentacles are readily replaced by regeneration and even amputated tentacles retain their stinging abilities. Luckily for us, a sea nettle sting causes only minor irritation to the skin.

Because their body consists mostly of water, nettles are not much of a meal, and thus have few enemies. Some sea turtle species, though, 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 of a billion years. Their life cycle is perfectly adapted for the estuarine environment of the Chesapeake Bay. Looking beyond the minor inconvenience of sea nettle stings to swimmers, it’s hard not to appreciate the prehistoric beauty of these translucent creatures as a measure of simple and efficient evolutionary success.