Hydroids

These delicate, plant-like organisms are actually colonies of very small sessile animals. They are sedentary and look more like delicate seaweeds. Each tiny individual is a small polyp like a sea anemone, with tentacles around a small bag. These are formed together on a stalk. The arrangement on the stalk varies between the species.
Clava is exquisite forming pink clusters of polyps on a soft extensible stalk. Dynamena has a stiff stalk and is a short hair like colony with the polyps coming off in pairs, opposite each other on the stalk. Obelia has polyps alternating down the stalk.

Like jellyfish they tend to have two phases in the life cycle: a polyp stage and a medusa stage. The latter is the dispersal and sexual phase whilst the first is the growth and asexual phase. As the hydroid colony grows so some of the polyps produce a medusa - a minute jellyfish which swims off and produces sperm and eggs. The resulting larvae hatching from the fertile egg, lands on the shore and, stimulated by a positive response to light, it establishes a new colony of polyps. New polyps are added by asexual reproduction. The feeding polyps collect plankton and after digestion disperse the food amongst the colony.

Tubularia sp. up close

They are usually attached to rock, seaweeds or even animals. Clava is commonly found on Knotted and Serrated Wrack and is limited to the western Atlantic coast of Europe. Dynamena and Obelia can be very abundant on just about any suitable substrate and are the most widespread species including the Mediterranean

Dynamene on Serrated Wrack