Jellyfish

There are some creatures that seem too strange to be true. Chief among them are the ghostly, alien – and sometimes deadly – jellyfish.

“Jellyfish,” although widely used, is considered a misleading name, as they aren’t related to fish. Instead, researchers prefer the terms jellies and medusas. Medusas belong to the phylum Cnidaria (pronounced “nidaria”) along with corals, anemones, and certain parasites. The incredibly venomous Portuguese Man o’ War is also among this group, but – despite a striking resemblance – it’s a siphonophore, not a medusa.

Medusas’ bodies consist of a dome-shaped bell and down-hanging, thin tentacles. Jelly bodies are more than 90% water, giving them the gelatinous consistency for which they’re named. Among jellies, there is a huge size range: from the lion’s mane jellyfish (up to 120ft long) and the Nomaru’s jellyfish (up to 440lbs), to the Irukandji jellyfish (no larger than a cubic centimeter).

Medusas lack specialized digestive, circulatory, respiratory, and central nervous systems. Their skin is thin enough to simply draw oxygen from surrounding water through diffusion, and they break down food and absorb nutrients through digestive cells that line the bell’s inner cavity. A loose network of nerves allow jellies to swim by contracting and expanding their bell bodies; some medusas get around mainly by swimming, while others rely mostly on ocean currents. Several species of jellies also have light-detecting cells, allowing them to tell up from down.

How jellies breed may be one of their oddest traits. Mature medusas mainly reproduce sexually, with males fertilizing eggs outside of the body. These eggs eventually drift to the ocean floor – but they don’t hatch jellies. Most medusas have two life stages, so their eggs hatch into sea anemone-like polyps, with tube bodies and upward-facing tentacles surrounding a mouth-like cavity. Mature polyps reproduce by budding: sprouting clones on their bodies that eventually break off and become separate organisms. Polyps either produce more polyps or ephyrae – immature jellies.

Finally, let’s explore medusas’ most infamous feature: stinging cells. Like all Cnidarians, jellies possess venomous cells (or cnidocytes). In jellies, cnidocytes are mainly found on the tentacles and contain special organelles called nematocysts. When a jelly is touched, incredible pressure builds inside the nematocysts to launch venom-filled, harpoon-like structures into predator or prey.

Medusa venom has varied effects on humans, from slight itch to excruciating pain and death. The box jellyfish Class boasts some of the most poisonous jellies. With over 50,000 nematocysts per tentacle, a sea wasp’s sting causes incredible pain and can kill within four minutes, attacking the heart, skin, and nervous system simultaneously. The above-mentioned, tiny Irukandji jellyfish are also in this class; they have nematocysts on every part of their body (not just the tentacles) and a single sting can cause agony and even death.

Despite these last examples, most jellies aren’t dangerous to humans. Like many things that seem strange at first glance, jellies are actually quite interesting and beautiful – you just have to make the effort to learn a little more about them.

Source: 1, 2