But perhaps some viewers still harbor nervous feelings about jellies because these bloodless, boneless, brainless, eyeless (and the -less list could go on and on) do possess tiny venomous stinging cells that they release in volleys at prey, predators and sometimes passing swimmers.

Let’s look at jellies’ exquisitely evolved mechanism for bringing home the plankton and keeping hungry, more solid-bodied animals at bay.

Nematocysts—stinging cells—are, after all, one of the defining characteristics of the nearly 2,000 species of sea jellies and about 8,000 other mainly marine species that include the corals, anemones, Portuguese man-of-wars and even stranger creatures that collectively are classified as cnidarians (the c is silent).

Anchored or planktonic, cnidarians are a soft, squishy lot, and their stinging cells help enforce a hands- and mouth-off policy. The merest brush against a jelly’s tentacles, oral arms, and in some, even the bell, will loose a barrage of tiny harpoonlike cells that inject venom on contact.

Jellies don’t seem to distinguish between food and foe in this initial action. After envenomation, size can be a determinant. Zooplankton, such the eggs and larvae of assorted invertebrates and fishes, tiny crustaceans, smaller jellies and even small fishes are fair game: They can be ensnared in the sticky tentacles and oral arms and moved into one or more mouths.

By immobilizing their prey with venom, jellies minimize the risk that a struggling food item will tear their tissue-thin membranes.

Anything that can escape from a jelly is probably too large to eat; if it is also undeterred by the stinging cells, it might also be a predator. Interestingly, special exhibit aquarist Maureen Koneval says that jellies may have the ability to sense chemical distinctions—kind of like a taste test—between what is edible and what isn’t. Most jellies appear to be able to tell the difference between themselves and other jellies, and between jellies of their species and prey species, such as moons.

Mark Schick, special exhibits collection manager, points out that we can look at jellies the same way many people look at spiders. People may not want to encounter them up close and personal, but jellies do provide services in the ocean ecosystem, consuming massive quantities of tiny marine creatures including parasitic copepods and even other jellies. Several jelly species are a staple food for leatherback sea turtles and also figure in the diets of loggerhead sea turtles, fishes such as blue rockfish and ocean sunfish, and sea lions, all of which seem to be unfazed by the stinging cells.

As for jellies’ effects on humans, that depends on the toxicity of the venom, which varies by species, and on a person’s sensitivity. Jelly venoms range in potency from negligible to not quite lethal. Among people who are not allergic to jelly stings, usually the worst reaction to nematocysts is an irritating rash.

For swimmers and snorkelers, avoidance is the best prevention. Here’s a heads-up: Don’t swim over a colony of upside-down jellies. These sedentary shallow-water jellies are found in many of the world’s premium snorkeling and diving destinations, blanketing lagoon and reef floors and waving their algae-filled tentacles and oral arms (and therefore stinging cells) up toward the sunlit surface. Snorkelers whose swim fins disturb these jellies might trigger a defensive release of a nearly invisible sheet of nematocyst-embedded mucus that drifts up, swaddles the swimmers and causes an itchy red rash as the stinging cells penetrate their skin.

Mark says that once the nematocysts fire, you’re stung. But you can neutralize any unfired stinging cells by rinsing the exposed area with vinegar or, lacking that, salt water, usually in abundance around jellies.

He points out that for jellies, stinging cells are all about getting a meal and not becoming one. For those of us nearly hypnotized by the jellies’ gentle orbitings, stinging cells are a tantalizing contradiction and another stunning wonder of nature.

Posted by Karen Furnweger, web editor