Rotifers are firmly established as the preferred feed for larvae of many aquacultured fish and crustaceans, because rotifers are the only live zooplankton that can be reliably cultured in mass quantities, thanks to the application of culture protocols developed by the aquaculture industry in recent years.
by Eric Henry PhD, Research Scientist, Reed Mariculture Inc.
otifers are a distinct group of small aquatic animals with about 1500 known species, found in fresh or salt water, either planktonic or attached to surfaces. They both swim and feed by the beating of cilia arranged in a “corona” encircling the mouth. The beating pattern of these cilia creates the illusion that the corona rotates like a wheel, inspiring the name of the group. The rotifers used in aquaculture are almost invariably saltwater strains of the genus Brachionus, most often called B. plicatilis (the larger, “L-type”) or B. rotundiformis (the smaller, “S-type”). In recent years molecular genetic analyses have revealed that aquacultured Brachionus include an array of genetically distinct isolates, including some that should not be lumped into either of these species. Nevertheless, it is a useful generalisation that “L-types” are larger (typically a maximum lorica length ca. 250-350 µm) with a temperature optimum of around 26 °C, whereas “S-types” are smaller (typically a maximum lorica length ca. 100-200 µm) with a temperature optimum around 32-35 °C. These Brachionus strains possess certain attributes that make them particularly suitable as live feeds in aquaculture: • They are not truly marine organisms, instead they occur in estuarine and inland saline habitats, and consequently tolerate a wide range of salinities. They can live for several days in fresh water, and remain reproductively active in salinities as low as 5 psu, but can also be easily cultured at seawater salinity, remaining reproductive up to 45 psu. So the same rotifers can be used for freshwater or marine larviculture. • In the domesticated strains used in aquaculture, rotifer cultures normally consist exclusively of females that reproduce by parthenogenesis, whereby females produce young asexually. Asexual eggs hatch as juvenile females that develop directly into reproductive adults, with no larval stages that slow
development (in contrast to Artemia and copepods). This is one reason why rotifers can double their numbers in less than a day. • Despite their small size and very simple body plan, rotifers possess a unique chewing organ (the mastax) that enables them to mechanically disrupt food particles such as toughwalled algae cells. Rotifer cultures can therefore be fed effectively with a variety of feeds, including diets that contain “enrichment” components (omega-3 and omega-6 fatty acids, vitamins, etc.) that are taken up by the rotifers to provide optimal nutrition for fish and shellfish larvae. • Rotifers can be routinely cultured at very high densities, as high as 5-10 million/L. In part this is made possible by their tolerance of vigorous aeration—contrary to the common misconception that strong aeration can strip the eggs from the females. Even if this could happen, it has been demonstrated many times that eggs removed from the females develop normally. • *Because they tolerate a wide range of conditions (temperature, pH, salinity, oxygen concentration — they even grow in sewage treatment plants!), Brachionus cultures are robust, and with due care are not subject to unexplained crashes.
Optimised production protocols
Effective and economical rotifer production is now routine practice in many hatcheries, thanks to the development of production protocols designed to provide optimal growth conditions for rotifers while making hatchery operations simpler, easier, and more economical. Such protocols are based on these fundamental factors:
Stability promotes rotifer health and consistently high productivity
All fluctuations in culture conditions (temperature, pH, feed dosing, harvest rate, etc.) should be minimized. To this end, “continuous” culture provides a significant advantage over “batch” culture.
30 | September | October 2016 - International Aquafeed