OBELIA

Defining Physical Characteristics: The polyps live in colonies of a large numbers of polyps called zooids, which are interconnected and share a continuous gastrovascular cavity. The zooids are connected to each other and to the substrate by a root-like stolon. Each zooid possesses an oral end called a hydranth, which bears a mouth. There are usually two types of polyp in the colony. Those specialized for feeding, called
gastrozooids,possess a ring of retractable tentacles. Each tentacle is covered with nematocysts (stinging cells) and once they catch the microscopic prey consisting of protozoa and zooplankton, they withdraw into the hydrotheca ,the transparent bell-shaped part of the perisarc, a transparent protective tube. They then ingest the prey through the single opening into the gastrovascular cavity. Digestion is extracellular and then becomes intracellular as the food is distributed throughout the colony.Polyps specialized for reproduction are gonozoids, and often lack tentacles since they do not feed, but depend upon the colony for nutrition.They are responsible for producing the motile adult stage of life, the medusa, which is the jelly fish. The medusae swim off and reproduce sexually with other medusae Their eggs grow to form ciliated planula larvae (non-feeding) that attach themselves to a substrate and metamorphose into a colony of polyps, some of which are gonozoids and produce the medusa.

Habitat: on algae, especially Desmarestia, found along shallow coastal shores, on top of rocks and attached to pilings, docks, submerged lines, just below waterline.

Niche: Carnivore: feeding on zooplankton.

Range:Atlantic Coast.

Personal Observations: Obelia are found living on the brown algae Desmarestia. When viewing them under the microscope you can see the tentacles of Obelia which are housed in transparent cups, hydrotheca, for protection. When touched or exposed to the microscope's light the tentacles will withdraw inside the cups. They then shoot out again to capture food and spring back into the cups when food is caught.The individual polyps are all connected by a branching stalk. Under the microscope you can see a fluid running through the stalk - this is how they share food. The stalk is lined on the inside with cilia, which beat in order to create a current to move the food and fluid throughout the colony. This is fascinating to see.

MEDUSA STAGE: