Sea-thread Hydroid (Obelia sp.)
Hydroids are conspicuous members of the fauna that grows on floating docks, and that is where they are most easily seen by anyone wishing to lie down on such a dock and look beneath the surface at the profusion of life below. Look for pinkish or tan fuzzy or threadlike masses, then look closer to see the structure of the colony of polyps.
Most cnidarians exhibit an alternation of generations, a sessile polyp stage alternated with a swimming medusa stage, and Obelia is no exception. What you see on the dock is the hydroid stage, a colony that is anchored to the substrate by a rootlike structure called a hydrorhiza. The stalk of the colony branches and branches again, at the end of each branch a polyp. The polyp is like a tiny sea anemone, with a body and radiating tentacles. Most polyps will be feeding polyps called gastrozooids enclosed in a transparent bell-like covering called a hydrotheca.
Just as in a jellyfish or anemone, the feeding tentacles surround the mouth, in this case in a single whorl. They contain nematocysts that paralyze tiny drifting prey animals, which are then placed in the mouth by the tentacle. The digestive cavity extends down the stalk, where it is continuous with those of the other gastrozooids. Nutrients are absorbed directly into the body. Remember that cnidarians have only two cell layers, one covering the outer surface of the body and the other lining the digestive cavity. The rest of the body, between those two layers, consists of a jellylike mesoglea.
Along with the gastrozooids are clublike structures called gonophores. Tiny medusae grow on these structures by budding (asexual reproduction). They escape into the water through an opening in the gonophore when about a half millimeter in diameter. They float around with the currents and grow to about 5 mm in diameter, all the while developing gonads in the two separate sexes. These then shed sperm or eggs into the water in sexual reproduction, and fertilization takes place.
The larva that develops is called a planula, and it is covered with cilia that allow it to move actively. The planula settles to the bottom and metamorphoses into a polyp, which grows and develops new branches that form new polyps. Thus the cycle is completed.