I
n the summer months, jellyfish mysteriously arrive by the thousands in the Chesapeake Bay and its tribu- taries. These lucid creatures appear to be drifting along aimlessly, a mass infiltration of jelly-filled blobs. From May through September they are present in the waters of the lower and middle Bay, but seem to surge during July and August. Why are they so abundant and is there something unique about their nature that sets them apart from other species in our waters? The appearance of jellyfish makes us
second guess ourselves before diving in on a hot, summer day. Local residents share tales of skiing in turtlenecks and jeans or wearing lycra hose under swimsuits just to avoid a potential sting. There have even been attempts to diminish their populations in certain areas. Consequently, other marine life suffered. The painful truth about jellyfish is that they are most likely here to stay based on how well-adapted they are for their way of life. In recent years, “fossil snapshots” have been discovered in rocks over 500 million years old that exhibit characteristics relating to modern day jellies. The more we study them, the more we will understand why they are one of nature’s great survival stories.
Getting Around
The word “jellyfish” is a misnomer, for they are not fish at all. Moving primarily by drifting, they are the largest type of floating zooplankton, or animal plankton. Though they are not strong swimmers, they can propel themselves by contracting and expanding their umbrella-shaped bell. Mostly they rely on the tides and currents for transportation.
Jellyfish have no brain. Instead they have an elementary nervous system with receptors for detecting light, vibrations, and certain chemicals in the water. These senses, combined with a feeling of gravity, give the animal the ability to guide itself.
The Stinging Variety
Jellyfish that sting are members of the phylum Cnidaria (from the Greek word for “stinging nettle”), and of the class Scyphozoa. The mature form of the jellyfish, with its bell and tentacles, is known as “medusae,” appropriately named for Neptune’s serpent-haired
The House & Home Magazine 23 Mnemiopsis leidyi (comb jelly/ctenophore)
wife. Covering the tentacles are specialized stinging cells. Inside the cells (cnidoblasts) are capsules, or nematocysts, each containing a coiled up thread lined with barbs. A single tentacle can have hundreds or thousands of nematocysts. These cells are like “spring- loaded harpoons” that release venom once the jellyfish senses touch. Simple animals, jellyfish are made up of two cell layers, the epidermis (“outer skin”) and the gastrodermis which lines the gut. In between is a thick, non-living non-cellular layer called the mesoglea
Stinging cells of a Lion’s Mane (Cyanea Nematocysts), Photo courtesy of Dr. Ron Condon, Virginia Institute of Marine Science
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