Reliable Rotifers

By Eric Henry PhD, Research Scientist, Reed Mariculture Inc.

 

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.

 

Rotifer Basics

 

Rotifers 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 generalization 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 tough-walled 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.

 

Optimized 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.

Batch cultures are grown to a maximum target density and harvested completely. The culture tank is then cleaned, sanitized, and a new culture is started. With “continuous” cultures, once the culture reaches the target density only a fraction (typically 20–50%) is harvested, and this harvest can be repeated every day, indefinitely. Continuous culture offers several advantages:

 

• Rotifers are most productive under stable conditions. The consistent feeding and harvest regimes that prevail in continuous culture promote rotifer health, supporting high productivity and nutritional quality.
• The rotifers have a younger age distribution, due to the high daily harvest rate. Younger rotifers feed more actively, are more fecund, and are more vigorous swimmers.
• Once daily harvesting begins, the culture requires the same feeding and harvest every day, simplifying the management of culture operations and so minimizing the opportunities for costly mistakes.
• There is no interruption of production while a new culture grows to harvest density.
• Labor inputs are reduced because culture tanks do not require frequent sanitizing and re-inoculation.

 

Intensive cultures save space and water — “Small is beautiful”

 

• “Intensive,” high-density (3 –10 million rotifers per liter) cultures require smaller culture volumes and so less space in hatcheries. They are more readily enclosed, allowing better control of culture conditions and exclusion of contaminants. They require less water, and more concentrated rotifers are easier to harvest.
• High-density cultures are only possible using high-quality, high-density feeds with much higher biomass concentrations than can be provided by live algae cultures. Liquid algae concentrates such as Reed Mariculture’s RotiGrow feeds provide high and consistent feed concentrations and enable precise, automated feed dosing to the culture by metering pump.

 

Grow-out enrichment vs. supplemental enrichment — No more “Too much, too late”

 

The nutritional value of rotifers depends on what they are fed. In conventional practice, a batch culture may be grown to harvest density using a low-cost, yeast-based feed of low nutritional value, and then switched to a high lipid content “enrichment” feed a few hours before harvesting. This “gut loading” strategy fills the digestive tracts of the rotifers with the lipid-rich feed, to be delivered to the larvae when the rotifers are consumed. However, the extreme lipid content of conventional enrichment feeds is stressful to the rotifers, harming their health and motility. Lipid emulsions foul rotifers, requiring them to undergo a washing procedure before feeding to larvae. Harvesting on screens, washing procedures, and temperature shocks when enriched rotifers are “cold banked” before feeding to larvae can cause the rotifers to eject their gut contents (and enrichment) before they are fed to larvae.

 

A more effective enrichment strategy is to grow the rotifer culture on a more moderate enrichment feed, so that the entire body of the rotifer is enriched. The rotifers are not stressed, so they show good motility. They require no washing, and can be fed directly into the larval tank, eliminating the labor of harvesting and washing, and avoiding shocks that cause ejection of the gut contents. Rotifers enriched during grow-out retain their enrichment in the larval tank and can deliver more enrichment to the larvae. If the levels of “grow-out” enrichment are below desired levels, a less extreme (less stressful, less fouling) supplemental enrichment is then sufficient.

 

Reed Mariculture has developed algae-based RotiGrow® liquid concentrate enrichment feeds for both rotifer grow-out and supplemental enrichment. Intact, whole algal cells ensure that rotifers receive the maximum nutritional benefit from our exceptional feeds, which in turn provides maximum nourishment to larvae. Our feeds also produce clean rotifers that ordinarily do not require washing, so they can be fed directly into the larval tank by automated pumps, avoiding stress to the rotifers, greatly reducing labor inputs, and allowing continuous feeding of larvae.

 

How do I start a rotifers culture? How many do I need?

 

It all depends on how patient you are. Because they can reproduce asexually, in principle you can start with a single rotifer! Knowing that they can double every day, you can use one of the online compound interest calculators to predict how long it will take to generate the desired rotifer population from a given starting population. Most hatcheries will want to start with at least a few million rotifers, since their requirements may be as great as billions per day.

 

Possible sources of rotifer cultures are academic institutions, other hatcheries, or aquaculture feed suppliers. Consideration must be given to the characteristics of the rotifer strain (size, temperature requirements, culture performance) and biosecurity—are the rotifers free of contaminating organisms such as ciliates, and is the production facility free of pathogen vectors? Reed Mariculture Inc. routinely supplies orders of up to 2 billion rotifers, produced in a biosecure facility free of pathogen vectors. 

 

It is possible to start Brachionus cultures from cysts (resting eggs). But unfortunately, commercially available cysts are derived from strains that are prone to sexual reproduction and consequent cyst formation, displacing production of asexual eggs. Cysts remain dormant and do not quickly generate more rotifers, drastically limiting culture productivity. By contrast, highly productive domesticated strains suitable for commercial aquaculture reproduce only asexually under normal culture conditions. Asexual eggs hatch while still attached to the swimming female, and additional eggs are often produced before the first egg can hatch. Females may be seen carrying multiple asexual eggs, all of which hatch within a few hours of being produced, ensuring very rapid growth of the culture.

 

Contaminating organisms

 

Rotifer cultures easily become invaded by protozoa from their surroundings; most conspicuous are large ciliates such as Euplotes and Vorticella. Fortunately, only very rarely do protozoa appear to cause any harm to rotifers. In healthy, well-managed rotifer cultures the protozoa usually remain at low levels. Their proliferation is generally a sign that the rotifers are under stress, or especially that the feed is not being assimilated efficiently by the rotifers. Feeds such as yeast and dry particulates that rapidly leach nutrients are easily exploited by protozoa as well as bacteria that protozoa feed upon. Oil emulsions are prone to stick to surfaces, making them unavailable to rotifers while feeding bacteria and protozoa. Feeding with algae strongly favors rotifers, and Nannochloropsis has even been shown to suppress Euplotes.

 

Getting started with the Compact Culture System

 

It is now possible for rotifer culturists to trial new culture protocols at a small-scale using Reed Mariculture’s Compact Culture System. The CCS combines a conveniently sized 5-gallon (~20 L) bucket with a custom-made fitting that incorporates an aerator and a particulate waste trap. Using only hand feeding 2 times per day with RotiGrow feeds, a rotifer density of 1 million/L can be achieved, and use of a feed pump to dose feed hourly allows densities in excess of 3 million/L. At a fill level of 14 L and a conservative harvest rate of 35%/day the CCS can provide 15 million rotifers/day.

 

With the adoption of optimized protocols, mass production of rotifers for larviculture is no longer a daunting prospect. Reed Mariculture has been serving aquaculture customers for over 20 years, providing rotifers, rotifer feeds, and technical support to aquaculture facilities in over 80 countries.