Hydroids

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.

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.

The hydroid pictured here was found on a floating plastic crate.

The crate had been at sea for some time, as there were many mature goose barnacles attached to it.

The crate was covered by hundreds of 'fine hairs'. This is a magnified image of two of the 'hairs'.

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.

Species of Hydroids Found In Cornwall

     By-the-wind-sailor Velella velella

  Portuguese Man of War Physalis physalis

By-the-wind-sailor Velella velella

They are under four inches long and are a distinctive deep blue. They possess a float with small blue tentacles on the underside, used to catch their prey.

To aid transportation they have a transparent sail, which can be either left or right handed. Winds tend to distribute them into groups comprised of the same handedness, so strandings tend to be made up of one group.

Portuguese Man of War Physalis physalis

This is the most well known (infamous) hydroid. It has a float which is up to a foot across. On its top is a sail which stands high out of the water, catching the wind so enabling transportation.

This jellyfish should not be touched as its sting can be fatal, or approached, as its stinging tentacles can exceed 60 feet.

The diagram shown here is only a rough diagram, but is included as this is an animal of which one must stay clear .