Hydroid     Obelia longissima



Looking like small white trees in an aquatic forest, this is really a colonial animal called a hydroid that is closely related to sea jellies and sea anemones.  Hydroids are commonly seen hanging from floats as well as on the fronds of the large brown seaweed called Sugar Kelp.


If the picture was taken closer to the animal you would see a series of small cup-shaped structures on each small branch.  These structures house a individual feeding device called a polyp.  Looking like a sea anemone, the hydroid polyp has a small mouth ringed with tentacles armed with stinging cells or nematocysts with which they capture their animal prey.  If you were to put the branch of a colony under a microscope you would see that the mouth leads to a digestive cavity which continues on to connect by way of the branch and stem to all the other polyps as well as the asexual reproductive structures.


Hydroids reproduce both sexually and asexually.  Small club-shaped structures on the stalks bud off tiny jellyfish.  These jellyfish swim off to feed and then produce sperm and eggs.  The fertilized eggs develop into larvae which swim about looking for a good location to settle. After settling they transform themselves into a polyp which grows and branches resulting in the hydroid.