Wednesday, April 29, 2015

29/04/2015: Rotifers

They often evoke a love-hate relationship, but you just can’t get away from them.    
by Rohan Mak, ZM Systems

First published in International Aquafeed, March-April 2015
The green water encouraged by the traditional carp farmers in the Far East and then Europe would be rich with live infusoria including cilates and freshwater rotifers for first feeding. The marine fish industry has had to look at marine rotifers and recreating the plankton soup as many larval species are too small to take newly hatched Artemia. The leading research labs using fish in medical and ecotoxicology projects that have relied upon lab-grown Paramecia cultures are revisiting rotifer culture to maximise fry survival rates.

Whilst attempts have been made to replace livefoods with artificial diets, the protocol of co-feeding live and processed diets cannot be ignored. Through careful enrichment, rotifers can be used as a smart nutritional package to aid the development of gut bacteria, boost health and support early larval development.
Do your research and plan ahead
The Plankton Culture Manual by Frank Hoff of Florida Aqua Farms is an invaluable introduction to live food culture including microalgae, rotifers, and Artemia culture. The practical examples shown in this book are based on the experience gained from the creation and development of Instant Ocean Hatcheries and operating a commercially viable marine fish hatchery. The biggest mistake we have seen customers make is not planning ahead and not having well-managed cultures in place.

Where to get rotifers from

Live resting rotifer cysts are available to establish cultures and can be shipped internationally by courier or airmail service. Two rotifer species are normally available: Brachionus plicatilis (L-strain) for brackish-marine work and Brachionus calyciflorus, used for freshwater cultures. The resting cysts are stored in vials and may be frozen for long-term storage and may be stored until you are ready to inoculate a starter culture. The dehydrated cysts will first need to hydrate in a Petri dish before completing the incubation process and hatching 48 plus hours later. 

For the UK market we can supply live B.plicatilis starter cultures at different salinities for different applications. The smaller marine S-strain Brachionus rotundiformis is sometimes available for specialist projects where a smaller prey item is required.

How to view rotifers
A basic binocular dissecting microscope with at least 20x magnification is an essential tool in managing rotifer cultures and even monitoring the density of any live microalgae present. Right from resting cysts to adult rotifers the behaviour, feeding and breeding condition of a culture can be observed. Once the rotifer culture has been transferred to a larger vessel, a simple torch can often be used to illuminate and monitor culture densities.
Rotifer management and harvesting
Under optimal conditions rotifer cultures breed asexually with daughter cells produced. The life cycle is normally 6-8 days; L-strain rotifers are most prolific with a salinity at 22ppt and at optimal temperatures of 25 degrees Celsius. Only light aeration is normally applied to avoid fast water movement that may strip the daughter cells off the adult rotifers.

Batch culturing rotifers in four to five vessels has proved a straightforward protocol and allows a culture to develop over four to five days with one vessel harvested each day. Normally two thirds of a mature culture is harvested for feeding and one third retained to establish a new culture. This culture setup limits water quality becoming an issue, the vessels are cleaned between uses, and any background build-up in ciliate populations that may compromise the rotifers can be kept in check.

Single vessel culture setups with pH, ammonia control and biological filtration can operate within a compact area, yield high rotifer densities and allow daily harvesting. Careful management of single vessel cultures is still required to avoid ciliate population build up and water parameters being exceeded through laziness. This setup will represent ‘all your eggs in one basket’ so your hatchery will be at risk if the culture does crash and need replacing.

Food options for rotifers:
Live microalgae have traditionally been cultured for feeding and enriching rotifer cultures.  Starter cultures can be established from microalgae plates that can be stored in the fridge and shipped internationally by courier. Normally Nannochloropsis and/or Tetraselmis is used for a day-to-day rotifer feed and Isochrysis used as a DHA enrichment just prior to using the rotifers as a larval feed. Microalgae culture technique remains a useful skill to know and may be applied to other specialist livefood cultures including copepods.

The use of marine algae concentrates and prepared solutions have proved convenient and have aided the design of high-density rotifer rearing systems. The ability to cold-store concentrates and feed by peristaltic pump on a timer leads to regular feeding, a more stable culture and a major labour-saver. 

The development of formulated dry yeast invert feeds was the next progressive step and these can be mixed daily and fed in the same way as algae solutions. In their nature prepared algae solutions have a lower percent dry matter nutritional content, making it easier for the new dry diets to offer a higher contribution and cost savings. 

INVE Aquaculture’s latest S.parkle product is an evolution of the Culture Selco range based on deactivated yeast. As a separate development S.presso is the latest HUAF emulsion/suspension enrichment product to evolve from the Easy DHA Selco range and now has protocols for both Artemia and rotifers.
Harvesting rotifers:
Rotifers are normally harvested with 53-micron nylon mesh strainers, smaller than the standard 120-micron used for newly hatched Artemia. For breeding projects requiring specifically small rotifers a 25-micron mesh strainer can be used to grade out the smallest individuals.

When harvesting rotifers it is important to limit physical damage; for example, when using a strainer to harvest rotifers, aim to have the mesh submerged as long as possible. When siphoning a rotifer culture from one tank to another, limit the head difference so water velocity is reduced.
Rotifers, like Artemia cysts, can get everywhere and cross contaminate cultures including microalgae cultures if you are not careful. Ideally rotifers and microalgae should be maintained in separate rooms and staff should thoroughly wash and dry their hands in between any maintenance work. 

If you wish to be ultra-careful start the day with water quality work on reservoir water stocks, follow with microalgae work, and then follow with live rotifer or copepod work with hand-washing in between each session. When working with both S-strain and L-strain rotifer cultures cross contamination may be reduced by running S-strain cultures at higher salinity and temperatures to L-strain cultures.

About the author
Rohan Mak has 27 years’ experience in aquaculture, aquatics, biotechnology, research holding systems, specialising in early feeding, live-food and microalgae culture. He trained at Sparsholt College, Hampshire and the University of Plymouth for his M.Sc. in Applied Fish Biology. He was subsequently employed at the University of Southampton on transgenic Nile tilapia and at Kings College, London and University College, London on zebrafish for developmental biology and medical research studies.

ZM Systems are ZM fish-food and fishroom equipment UK distributors for INVE products including Sep-Art Polarised Artemia, S.parkle, S.presso enrichments, Florida Aqua Farms microalgae culture plates and fertilisers, resting rotifer cysts and the Plankton Culture Manual. 

Their customers include developmental biology, biomedical and ecotoxicology research laboratories, government and commercial fish hatcheries and public aquaria.
In 2015 they are due to sponsor the first UK Artemia Workshop at the Plymouth Marine Laboratory, currently being planned by John Rundle.

Read the magazine HERE.

The Aquaculturists
This blog is maintained by The Aquaculturists staff and is supported by the
magazine International Aquafeed which is published by
Perendale Publishers Ltd

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