Wednesday, 28 December 2016

‘Flowers’ beneath the surface

Bryozoans - commonly called moss animals - form plant-like colonies. There are about 4000 species of bryozoans, which occur in both fresh and salt water, from the tropics to the polar regions. In this case the video shows a fresh water variety. They attach themselves to every conceivable surface providing grip. The colonies of bryozoans are formed by a large number of individual animals, which are also called zooids. Any zooid, which usually is not yet a millimeter in length, is locked up in a ‘shell’, which can be box-shaped, oval, vase-shaped or tube-shaped, depending on the shape of the colony.

The ‘shell’ consists of an outer layer of horn-like chitin. Below this is a thick layer of lime. The entire skeleton resembles that of a barnacle or a coral. On one side of the tube there is an opening, through which opening the food grasping organ comes out. This organ is part of the digestive system and consists of a ring of fine tentacles. The number of tentacles, which lie like a fan around the mouth and which are covered with fine cilia or hairs, can amount from eight up to thirty-four. When the bryozoan feeds, the tentacles come out. By moving their numerous small hairs, the tentacles cause water flow. Small plants and animals are driven towards the tentacles, trapped in mucus and brought to the mouth. In times of danger, the tentacles may withdraw at lightning speed.

Bryozoans reproduce mostly asexually, by budding on the body wall. In this way, a colony can develop rapidly. But sexual reproduction also occurs. Most bryozoans are hermaphrodites, meaning they have male and female organs. The eggs and the seeds are released in the body cavity. Once the eggs are fertilized, they stay there until they hatch. Often, the fertilized eggs are discharged into the water. The tiny larvae that come out, swim around for a while, looking for a suitable place to settle. When found, they secrete a sticky fluid in order to attach themselves to a suitable surface.

Source: Joost van de Sande, Dutch Society For Microscopy

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