Reef Ramblings

The second in the series of articles on breeding marine fishes continues on the subject of larval nutrition. This article looks at rotifers, describing what they are, their role in larviculture, and presents a basic protocol to allow you to culture them yourself.

Rotifers are small herbivorous animals belonging to the phylum Rotifera, typically less than 400 microns long, they feed on phyto and in turn make a great first food for marine fish larvae and for corals. They can be used both for raising marine larvae (they’re almost essential for this purpose), and for general feeding of the reef tank. Live foods are useful in the reef tank, but growing them can be time consuming, if you don’t have the time to devote to culturing rotifers you can purchase them in a preserved state but they will not be suitable for raising newly hatched marine larvae. We’ve already had a look at phytoplankton but we need to touch on it again – live phyto is essential when cultivating rotifers and artemia.

Rotifers are available as cysts, effectively embryonic rotifers in a state of suspended animation that hatch within a short time given the right environment. Once hatched they rapidly start reproducing asexually, one thousand rotifers can multiply to become one million in as little as ten days. Cysts and live cultures are available from some marine outlets, and from specialist distributors such as Midland Reefs.

A Bit of History.

Until the late fifties rotifers had actually been considered pests by the aquaculture industry. They were behind a phenomenon known as “Mizukawari!” that caused devastation in the eel culture ponds of Japan. What was happening was that phytoplankton populations in these ponds were apparently crashing, leading to dangerously low oxygen levels and the subsequent death of the eels. On investigation it was discovered that the cause was a rotifer bloom consuming the phytoplankton. As aquaculture expanded and attempts made to rear more species it was realised that a problem existed concerning suitable foods for larval animals. Dr Ito, who had been involved in the research into Mizukawari, saw a possible solution and experimented using rotifers with various larval animals. We have Dr Ito to thank for his work in the early sixties establishing the importance of rotifers for larval culture, leading to the successes we now have in raising marine fishes and invertebrates.

Uses.

As usual the base of the food chain when raising marine larvae is phytoplankton. Next up from phytoplankton are rotifers, then after rotifers there’s newly hatched artemia or brineshrimp. Rotifers are probably the most important link in the chain, marine fish larvae need a small particle size – far smaller than artemia nauplii – and they need something that’s moving and that’s easily digestible. Live rotifers are also very useful in the reef where they simulate natural zooplankton. A good way to think about the relationship between these species is that the phyto actually supplies the nutrition a larva actually requires while the rotifers and artemia (older than twelve hours) are more the delivery system. By feeding rotifers and artemia with different strains of phyto we can fine-tune the selection of nutrients being delivered to the larvae according to that particular species requirements. This is because the different species of phyto all have different nutrient profiles i.e. different proportions of protein, fat, and carbohydrate; as adult fish of different species have different dietary requirements, so do their larval forms.

The commonly available marine rotifer, Brachionus plicatilis, is typically 150 to 350 microns (depending on strain), smaller than artemia nauplii at 400 microns. The strain chosen can be critical when raising larval animals but is of little importance for general reef feeding.

Rotifers reproduce best at lower salinities but are intolerant of changes in salinity of more than 10 ppt, say 20 ppt to 30 ppt. For general usage try keeping them at around 26 ppt (1.019 s.g.) so that they can survive the transition to full strength seawater (1.026 s.g. / 35 ppt salinity).

A Practical Guide to Culturing Rotifers.

The process is very similar to the one I’ve described previously for phytoplankton. You can culture rotifers in 2 litre drinks bottles but my preference would to be use something a little larger, say 15 to 20 litre plastic aquaria. You shouldn’t need more than two cultures so a larger vessel makes harvesting easier. A commercially manufactured reactor can also be used; usually this is the same as a phyto reactor but without a light. Follow a similar method

To start off your rotifer culture you’ll need a couple of 2-litre clear lemonade bottles, rigid airline tube, standard airline tube, air pump, a couple of air valves, 250ml live rotifers as a starter culture, and of course you’ll need either a phytoplankton culture or frozen / preserved phytoplankton for feeding the rotifers. In addition you need a fine net for rotifer collection, if you can’t find a dedicated rotifer net a reusable coffee filter can be used as long as the mesh size is fine enough (50 to 100 microns).

Split your starter culture between two 2-litre clear lemonade bottles, at room temperature and aerate briskly. There’s no need to use an air stone, rigid airline tubing will give the best results.

Make up 1- litre of water to a specific gravity of 1.015 or 1.019 – 20 25 ppt (depending on application – larval feeding or reef). Dechlorinated tap water is OK for this application. To help prevent contamination it’s good practise to sterilise all containers before use, and to sterilise make-up water in the microwave, allowing it to cool before using. This may not be strictly necessary for hobby use but for aquaculture it’s considered essential. Add enough phytoplankton to the mix to tint the water green while still allowing you to see through the culture bottle and then split this evenly between the two bottles. Wrap a strip of filter floss around the airline to block the mouth of the bottle; this helps to limit the possibility of rotifers contaminating any phytoplankton being cultured in the near vicinity. Add 500ml phyto daily, maintaining a light green colour until both the bottles are full. Once a 2 litre bottle is full it can be used to start a culture in a larger vessel if required. The second bottle is a back up in case the first culture crashes.

Note: more is not better! Too dense an algal culture will slow rotifer reproduction and raise the pH, possibly to a lethal level.

Rotifers do need light but as long as they’re maintained in an area that’s lit for around twelve hours a day, say next to your aquarium or on a windowsill, no special lighting’s needed.

Temperature should be between 20 and 30 ˚c. Higher temperatures will accelerate reproduction but the downside is more feeding and cleaning will be needed.

Harvesting.

Start using the culture when the density reaches 50 rotifers per ml, not the easiest of things to determine without specialised equipment. As a quick and easy hobby solution I find Joyce Wilkerson’s suggestion in her book, Clownfishes, of drawing a 3/8 inch circle on the outside of a dip hydrometer very useful. You then fill the hydrometer from your rotifer culture and then estimate the number of rotifers you see inside the circle, this will give a reasonable approximation of rotifers per ml. When you want to start feeding pour about 200ml of the culture through your sieve. Return this water to the culture if everything seems healthy or replace the same volume with phyto. As a rule of thumb you should be able to sustainably harvest one tenth of the culture’s volume daily. I rinse the collected rotifers with a little tank water before using and avoid getting culture water into larval raising tanks.

If the productivity starts to decline pour the culture through your sieve, being careful to just collect rotifers, leaving any accumulated muck behind. Use these rotifers to start off a new culture in clean bottles.

Periodically you’ll need to either clean your culture bottles with a bottlebrush to remove green slime build up or replace them with fresh ones, restart your culture as above.

You can control rotifer density depending on your needs, by manipulating salinity, temperature, and feeding you can slow down or increase the productivity of a culture as required.

A Question of Salinity.

If using your rotifers for raising larval animals I’d like to draw your attention to the question of what salinity to use when embarking on a breeding project. It’s accepted practise these days to maintain salinity at natural sea water levels – 35 parts per thousand (ppt) or roughly a specific gravity (S.G.) of 1.026 at 26˚c. When raising larvae and their associated live foods it becomes advantageous to work at lower salinities, the reasoning behind this is that rotifers grow and reproduce at a faster rate when kept at lower salinities than that usually used to maintain marine animals. Rotifers are intolerant of a change in salinity greater than 10 ppt (a difference of .007 s.g); they grow best in a range of salinity between 10 and 20 ppt (1.007 – 1.014sg). From this we can determine that to raise marine fishes most effectively we can use a rotifer culture of 20 ppt (1.014 s.g) along with a larval tank salinity of 29 ppt (1.021 s.g), phyto is not so fussy so I grow mine somewhere between the two.

As a result of all this some breeders maintain their brood stock at a lower S.G. As an aquarist who’s more interested in learning about and researching the captive marine environment, rather than breeding regular quantities of fish, I maintain all my tanks at 35 ppt but then gradually reduce the salinity of any larval tank prior to hatching by the addition of R.O. water. The choice is up to you.

Recommended reading

As with the previous article on phytoplankton I’d recommend:

Clownfishes by Joyce D. Wilkerson published by Microcosm.

Plankton Culture Manual by F.H. Hoff published by Florida Aqua Farms.

If you’re interested in raising larval fishes I’d also suggest:

Breeding the Orchid Dottyback by Martin Moe.

This is a diary of trying to breed one of the more difficult species of fish by one of the pioneers of marine captive breeding. It’ll give you a real insight into the frustrating business of maintaining live food cultures.

Tim Hayes

Midland Reefs

©2006 – 2009