Most hydrozoan species have a planktonic larval stage called a planula. Planulae are radially symmetric ovoids, often covered with flagellate cells for swimming. They may be very simple embryos or have cells differentiated into several types. Planulae most often settle onto a benthic substrate and develop into a polyp.
Polyps are radially symmetric, and may be urn-shaped, conical, cylindrical, or club-shaped. In most species they are only a few millimeters tall, though the largest grow up to many centimeters, and one, Branchiocerianthus imperator can be 2 meters tall. At their base hydrozoan polyps have basal disks or elongate processes for attaching to substrate, or they may be attached to other polyps. Often there will also be connections here to hollow tubes (called stolons) that connect the polyp to others in its colony, and allow the exchange of food between polyps. Above the base is a ring of contractile cells called the sphincter. These can contract to isolate the contents of the polyp from the stolons, preventing undigested food from entering the stolons. Above this is the gastric column, which usually contains a digestive chamber with a single opening, a mouth at the apex of the column. A ring of tentacles is attached to the column below the apex and above the sphincter. The number, shape and size of tentacles varies greatly, but there are usually between and 8 and 50 on a single polyp (some have many more, and some specialized polyps may have fewer). Most colonial hydrozoans are polymorphic, with different structures reflecting different functions. Some are armed with large spines tentacles for defense but have no mouth, some have tentacles and functional mouths for feeding, and some are only reproductive, with no tentacles or mouth, and produce medusae (see below) or gametes.
Like all cnidarians, hydrozoans have special ectodermal cells called cnidocytes, each containing a single intracellular structure called a cnida (aka nematocyst). Cnidae are unique to the Cnidaria. Each cnida, when triggered by a mechanical or chemical stimulus, shoots out a tiny hollow tube at high speed. Some cnidae are is equipped with sharp spines, and/or venomous or acidic compounds, but some are adhesive and have neither spines nor toxins. Hydrozoans use different types of cnidae to capture prey, to repel predators, and to attach to substrate.
Most hydrozoan species are colonial. A founding polyp produces new polyps by budding, and these grow a network of interconnecting hollow tubes (stolons) formed of living tissue, collectively called the coenosarc. Colony growth forms vary between species, some may form a single layer of polyps spreading across the substrate, others growing as erect stems, with polyps growing off the stems. Polyps and the coenosarc may secrete chitinous sheaths, or stems, or calcareous coatings (the latter forming structures similar to the anthozoan Scleractinia, the stony corals). In many colonies, polyps are polymorphic, with different structures reflecting different functions. Some have no mouth, but are armed with large spines or cnidae-equipped tentacles for defense, some have tentacles and functional mouths for feeding, and some, with neither mouth nor tentacles, are strictly reproductive, and produce medusae (see below) or gametes.
The medusa is the sexually reproducing stage in most hydrozoans. They are often formed by budding from polyps, and are usually solitary free-swimming organisms. They are similar in structure to an inverted polyp, radially symmetric, and often have four-fold symmetry. Their main body part is the umbrella, a bell or cone shaped gelatin-filled structure, which floats with the opening down. Medusa are usually small, usually 1-50 mm in diameter, though a few are larger, the largest (genus g. Rhacostoma) grow to 400 mm in diameter. Around the inside of the opening is a muscular ring of tissue called the velum. The velum can contract and relax, changing the diameter of the opening, and playing an important role in swimming The presence of the velum is a diagnostic character for Hydrozoa, only one genus, Obelia, has lost it. Around the outside of the opening of the umbrella is a ring of tentacles, which vary greatly among species in number, shape, and degree of arming with cnidocytes. Inside the umbrella, suspended like the clapper of a bell, is the manubrium, which contains the gastric cavity, and ends in a mouth. Structures that produce gametes form on the sides of the manubrium. Most species have dioecious medusae, each individual producing only eggs or sperm. Some are monoecious, but usually not simultaneously hermaphrodite. In some species sex is determined by environmental conditions, mainly temperature.
Both polyps and medusae have networks of nerves, but no brain or central ganglion. Some have light-sensitive structures called ocelli, and many have statocysts that allow them to detect gravity and their orientation.
These structural patterns are common, but there is great variation in the life cycles of hydrozoans. Some have suppressed or reduced one or more stages. In the Siphonophora and a few other groups of hydrozoans, colonies of polyps are pelagic, and float at the surface by means of a gas-filled tissue. They often retain medusae as part of the colony.
Other Physical Features: ectothermic ; heterothermic ; radial symmetry ; polymorphic ; venomous