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Brachionus rotundiformis, commonly known as the S-type rotifer, is a microscopic animal that occurs in both freshwater and marine environments and serve as prey for both copepods and fish. The term “rotifer” is derived from the Latin word for “wheel-bearer” due to their rotating hair-like protrusions on the anterior end that move like wheels. Currently there are roughly 1700 species of rotifers.
In general, rotifers are small zooplankton with simple body forms. The word “plankton” comes from the Greek word “planktos,” meaning “drifting.” This ecological term refers to the community of plants (phytoplankton) and animals (zooplankton) that drift near the surface of both fresh and marine bodies of water; currents and tides carry plankton through the water. Some plankton are fairly large, but most are microscopic in size (not visible by the naked eye). Although rotifers are capable of swimming freely, they are still considered planktonic due to their small size and the force that tides and currents have on them. Rotifers have a disc shaped anterior end and they do not have legs, although some have a single foot at the end of the body. Rotifers possess an organ called the mastax which consists of a complicated arrangement of muscles that activate a set of translucent jaws used to seize, tear and grind food. This organ is unique to the digestive system of this group and no comparable device is known elsewhere in the animal kingdom.
Brachionus rotundiformis is a cyclical parthenogen, meaning that it alternates between parthenogenesis (asexual reproduction) and sexual reproduction. Parthenogenesis allows it to rapidly colonize an environment given favorable environmental conditions. Although certain species of rotifers have lost the ability to reproduce sexually, B. rotundiformis is still capable of sexual reproduction when population density reaches a certain threshold, accompanied by a limited availability in food and increased levels of ammonia. Sexual reproduction results in the production of resting eggs that settle into the sediment and are triggered to develop into parthenogenic females once the environmental cues are optimal.
Used with great success in the larval culture of more than 60 species of marine finfish and 18 varieties of crustaceans, B. rotundiformis provides an excellent food source for hatchlings due to the following attributes:
- Easily captured due to their slow motility.
- Uniform distribution in the water column.
- Readily swallowed by larvae due to their small size (90-150µm).
- Their content can be manipulated to meet hatchling nutritional and survival requirements and are enriched with fatty acids, antibiotics, etc. to transfer to the developing fish larvae.