A single cell from the tentacle of a Hydra can fire a harpoon-like structure in as little as 2 milliseconds. This event generates pressures comparable to those found inside scuba air tanks, reaching up to 140 atmospheres. Such microscopic violence defines the phylum Cnidaria, which contains over 11,000 species of aquatic invertebrates. These creatures range from tiny freshwater polyps to massive lion's mane jellyfish that exceed two meters in diameter. Their bodies consist of mesoglea, a non-living jelly-like substance sandwiched between two layers of epithelium. Each layer is mostly one cell thick and lacks complex organs like lungs or hearts. Instead, they rely on a decentralized nerve net distributed throughout their gelatinous body. Most cnidarians possess only a single opening for both ingestion and excretion, functioning as both mouth and anus. This simple architecture supports radial symmetry, resembling a wheel or a tube rather than the bilateral symmetry seen in humans. Some species have developed mineralized skeletons made of calcium carbonate, while others remain soft and flexible. The mesoglea acts as a hydrostatic skeleton in many forms, allowing them to move by squeezing water out of their cavities. In medusae, this springy middle layer returns to its original shape after muscle contraction, enabling jet propulsion swimming.
Life Cycle And Reproductive Strategies
A larva swims until it finds a suitable site and then transforms into a sessile polyp stage. This polyp grows normally before absorbing its tentacles and splitting horizontally into a series of disks. These disks become juvenile medusae in a process called strobilation. The juveniles swim off to mature while the parent polyp re-grows and may continue strobilating periodically. Many species exhibit an alternation between these asexual polyp stages and sexual medusa stages. Some oceanic scyphozoans omit the polyp stage completely, while cubozoan polyps produce only one medusa. Hydrozoa display a variety of life cycles with some having no polyp stages and others like Hydra lacking medusae entirely. Life cycle reversal has been observed where polyps form directly from medusae without sexual reproduction. Spawning is generally driven by environmental factors such as changes in water temperature or lighting conditions like sunrise. At the Great Barrier Reef, at least 110 coral species release enough gametes to turn the water cloudy during mass spawnings. Fertilized eggs develop into planulae larvae that are cigar-shaped but slightly broader at the aboral end. These larvae swim or crawl using cilia until they attach to a substrate if the species has a polyp stage. Asexual reproduction allows daughter cnidarians to be clones of the adult, ensuring greater numbers of mature individuals.