Hydroids are actually colonies of individuals called polyps, which grow on a common stalk. Polyps in a typical colony each have specific jobs and look different from each other. These jobs include defense, feeding, and reproduction. In order to build up their own colony the hydroids reproduce by budding; all polyp types can bud. New colonies in Tubularia are formed when reproductive polyps make little polyps that stay attached until they develop into mature polyps. The mature polyps drop away from the "parent" and settle on the ground, then grow a new colony.
There are four major groups of cnidarians:
- Anthozoa, which includes true corals, anemones, and sea pens;
- Cubozoa, the amazing box jellies with complex eyes and potent toxins;
- Hydrozoa, the most diverse group with siphonophores, hydroids, fire corals, and many medusae; and
- Scyphozoa, the true jellyfish.
Hydras (class Hydrozoa) are a bit like elongated sea anemones with long, waving tentacles. They are quite common in fresh water, where they can be found clinging to plants and feeding on any small water animals that happen to blunder into their arms.
Hydroids are colonies of polyps. Each polyp is similar to a small sea anemone i.e. has its own jelly like body and a mouth surrounded by tentacles. The difference is that individuals are organised to benefit the whole colony. They do this, by being interconnected via a common tube the stolon. The stolon which can be tough and horny, allows the transfer of food between the polyps.
Hydroids have interesting life cycles, which vary depending on the species, but generally they go through three stages.
1. Free swimming planktonic larvae that settles and forms sessile polyps.
2. The polyps produce free swimming medusa (small jellyfish like stage).
3. The medusa in turn produce planktonic larva.
The picture depicts an example of what a hydroid colony can look like. It is also possible to see the different stages of the hydroids life cycle. Notice how the colony consists of several genetically identical individuals called polyps When the planula larvae have attached themselves to the substrate, the first individual grows out and starts catching food with its tentacles. The colony grows when new polyps bud off. Polyps that comprise the colony can share nutrition via the stems, and it is possible for different polyps to have different functions within the colony. In the picture above it is possible to see the feeding polyps that catch food with their poisonousness tentacles and the reproductive polyps that produce the free-swimming medusae that build either sperms or eggs. The colony that is depicted in the picture above is about 3 cm high and the medusae are about 5 mm wide. The egg, sperm and planula larvae are smaller than 1 mm. Hydroid colonies can vary greatly in composition. Many colonies can for example contain defensive polyps. The type of life cycle can vary between different colonies. There are hydroids that lack a colony building polyp stage, while with other specie the medusae stage is lacking. It is the leptomedusae Obelia geniculata depicted above.
To find hydroids one has to look on the lower shore particularly during spring tides when the kelp is exposed. They will be found attached to seaweeds and rocks. They are not always easy to find as many are small or form seaweed like colonies. Many species do not possess a non scientific name The species found in Cornwall include the Sea Fir Obelia geniculata, Sea Oak Dynameita pionila, and Clava multicornis. Included within the hydroids are jellyfish like creatures that are easily confused with true jellyfish.
Two species commonly confused are the Portuguese Man of War, Physalia physalis and the By-the-wind sailor Velella velella. Both species are not true jellyfish but free floating hydroids. Both are regularly stranded on Cornish shores, particularly the By-the-wind sailor which can be stranded in the thousands.