The hydroid colony and its life cycle

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.


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Bo Johannesson | Martin Larsvik | Lars-Ove Loo | Helena Samuelsson