Saltcorner: Article: Q&A

Although rotifers are a small and generally uninteresting group to almost everyone else, they are quite well known among marine aquarists as the food of choice for breeding most saltwater fishes and many invertebrates. The topic of breeding marine fish or invertebrates is far too complicated to cover here, but there are some excellent books available that cover breeding of various marine fishes and invertebrates in detail if you look through the reference book section at the back of this magazine.

The phylum Rotifera (the “wheel animals”) includes about 1,800 described species, of which only about 50 are marine or brackish. The remaining species are primarily freshwater, although some live in damp soils, rainforests, or even in the water film on some mosses! Rotifers are all tiny: a few species reach a respective 2-3 mm, but most are less than 1 mm long. The body is typically divided into a head, which bears a ciliary organ called the corona, a trunk, and a foot. The coronal cilia, when the animal is actively swimming/feeding, resemble two spinning wheels -- hence the name "wheel animals" from which the phylum name is derived. The animals have a small pair of internal jaws (called trophi). These jaws may be of several types depending on whether the animal is a predator or a suspension feeder. Suspension feeding rotifers basically use the coronal cilia to generate feeding currents (this is technically incorrect, but the mechanism of ciliary feeding is quite complex, and I will not bore you with the details here) which bring tiny particles (such as organic detritus, marine snow, or phytoplankton) to the mouth. Among predatory rotifers, the coronal cilia are used entirely for locomotion. These animals feed by grasping their prey, which usually consists of smaller rotifers, ciliates and other protozoans (although they often ingest phytoplankton as well when given the opportunity), with a set of protrusible jaws. Predatory rotifers ingest their prey whole and “chew it up” internally or pierce the body of their prey and feed by sucking out the fluid contents.

The body of rotifers is covered in a tough coating called a cuticle (actually this isn’t exactly accurate, either, but is close enough for our purposes here), and ends in a telescoping foot which generally bears spines or a pair of toes. Sticky secretions from the toes allow the animals to attach temporarily to hard surfaces. The foot may be modified for permanent attachment in some sessile forms (e.g., Floscularia) or completely absent from some obligately swimming forms (e.g., Asplanchna). This cuticular covering allows the animals to withstand some extreme environmental conditions by entering a metabolically dormant state. For example, the resting zygotes (explained below) of some rotifers have survived freezing in liquid helium (-272ΕC!!), while others have been dried out and kept on a shelf for years, only to spring back to life when water was added (some rotifers have been successfully hatched from resting zygotes in storage for as long as 20 years at room temperature).

Rotifers are unusual in that they always have separate sexes, but males are either reduced (in size, complexity and abundance) or may even be completely absent. That’s right - in some species there are only female rotifers - no males! If you're wondering how these animals can reproduce when the entire species is composed of only females, you are not alone. For many years the absence of males in some species of rotifers puzzled scientists, and many people assumed that we simply had not looked hard enough to find the males. However, it is now generally accepted that males appear to indeed be entirely absent from the group of rotifers belonging to the Class Bdelloidea. Bdelloid rotifer females continue to reproduce despite the complete absence of males, however via obligate parthenogenesis. Parthenogenesis is a mode of 'asexual' reproduction (for lack of a better term), in which females produce eggs that develop into female offspring without fertilization. Even in the species which have males, the males are often very short-lived (perhaps because they have a highly reduced gut), and only occasionally present in the population. The males are typically produced in response to various environmental stresses (such as high temperature, high salinity, starvation or overcrowding) or as part of a regular seasonal cycle. Males produced in response to stressful conditions then fertilize females who produce special resistant ‘eggs’(really resting zygotes) that are able to withstand the unsuitable conditions and wait until favorable conditions return before they hatch to begin the cycle again.

The reproductive rate of these parthenogenetic females is very high. Under ideal conditions, each female typically produces between 2 and 8 offspring per day, and those offspring reach reproductive maturity within a day themselves. Thus, if each rotifer has an average of 6 daughters per day and all of them survive to reproduce at the same rate, a single rotifer would give rise to 134,455 offspring in only 7 days! Obviously, it does not take many rotifers to start and maintain a viable culture from which to feed your aquarium or larval culture project. The tiny size of rotifers, together with the relative ease with which they are cultured, and the very high reproductive rate together make these animals a popular choice for feeding many tiny and obligately suspension-feeding marine animals.

Sadly, rotifers themselves are not particularly nutritious, and it is only by stuffing them full of nutritious food that they make a reasonable food for juvenile fishes and suspension-feeding marine invertebrates. I think that Martin Moe (1992) has one of the best analogies I have seen: rotifers are the equivalent of a grocery bag, you could swallow it, but if it is empty, it is not very nutritious at all. If rotifers are cultured on pea-flour and yeast-based meals, they may well grow, but they do not provide much nutrition to juvenile fishes. In fact, in most cases larvae of coral reef fishes fed exclusively on rotifers cultured in this way fail to complete their larval stage. These rotifers would be the human equivalent of filling that grocery bag with potato chips, soda and candies: it could well be very tasty, but it would not be very nutritious, and if it were the only food available to you, you would not remain healthy for long. On the other hand, rotifers cultured on phytoplankton and enriched with a highly-unsaturated fatty acid (HUFA) supplement (I discussed phytoplankton and fatty acid supplements in greater detail in my are potentially very nutritious. For example, fish breeders discovered many years ago that rotifers cultured on pea-flour and yeast based invertebrate foods grow quite well, but perform poorly (in terms of growth and/or survival of larvae) when fed to juvenile fishes. In contrast, rotifers enriched with phytoplankton and HUFA provide a suitable food for the juveniles of a number of marine fish species (e.g., Brinkmeyer and Holt 1995; Craig et al. 1994; Holt 1993; Moe 1997; Wilkerson 2001).

The most common ‘species’ of rotifer used for feeding animals in the aquarium trade is the ‘marine’ rotifer Brachionus plicatilis (although frequently sold as a marine species, this rotifer is really a brackish/saline lake species, more similar in habitat preference to the brine shrimp Artemia salina). I use ‘species’ lightly here, because it turns out that there are at least a few species of identical looking (at least to our eye) rotifers that have always been thought to be a single species named B. plicatilis that was distributed throughout the entire world. Joyce Wilkerson gives the standard view of rotifer taxonomy in her book Clownfishes (2001, pg 167): “The worldwide abundance of rotifers can be attributed to their ability to survive adverse conditions because of their secondary mode of reproduction [the highly resistant resting zygote stage I described above]. These resistant eggs, or cysts, are about as durable as cement; they can withstand adverse conditions and remain in a state of dormancy for years. Cysts are transported by winds, birds, and currents, which distribute them worldwide. Once a cyst reaches a hospitable environment, it ‘wakes up,’ hatches into a female rotifer, and almost immediately begins asexual reproduction of identical daughter rotifers.” I do not mean to belittle Joyce’s work here at all; this really is the standard view of how a single species of rotifers manages to be distributed worldwide (even today), despite their small size and the fact that they are restricted to only a few rare habitats (appropriate saline lakes) throughout the world. However, it turns out that this view is simply wrong.

It has long been recognized in the aquaculture industry that there are at least a couple of different “strains” of rotifers available, and commercially available size strains, commonly referred to as the large or L-strain and the small or S-strain), have been available in the industry for many years. Based on a detailed examination of both the body shapes and the genetics of these different size strains by Fu et al. (1991a;b), these researchers suggested that the two strains may, in fact, be different species. Additional evidence, including chromosome number and arrangement, as well as individual mating preferences, resulted in the two strains being formally recognized as different species a few years later (Segers 1995). More recently, Gómez and colleagues (Gómez and Snell 1996; Serra et al. 1997) have argued that break into two species is still not sufficient to describe the full degree of biological diversity across the globe within the animals previously named B. plicatilis. A couple of recent studies by this same group have demonstrated that there are at least three cryptic species (researchers use this phrase to describe morphologically similar species that have previously been considered a single entity) occurring within a single coastal lagoon in Spain. These three species (L, S, and SS) have consistent morphological (size in this case), ecological (feeding and preferred location) and mating behavioral differences that prevent the animals from breeding in the wild (Carmona et al. 1995; Gómez et al. 1995; Serra et al. 1998). This discovery makes L, S, and SS ‘strains’ of B. plicatilis good biological species (they have now been redescribed as the true Brachionus plicatilis, B. rotundiformis, and B. ibericus, respectively) rather than simply size strains (Ciros-Perez et al. 2001). Given that these studies are the first to examine the taxonomy of this important and apparently world-wide group, I expect that there will be further revisions and more new species discovered in the future.

Well, I imagine that I have just covered far more of the biology of rotifers than you probably ever wanted to know, but if you have been reading my column for any length of time, you’ll know that I like to provide the background biology for any group before I discuss the specifics of how to keep them in the aquarium (or in this case, how to culture them). I will continue on with a very brief introduction on how to culture rotifers at home in the next column, but I’ll end here with how to locate a starter culture of these animals.

The simplest way in which to start a culture of rotifers will be to track down a friend or fellow hobbyist who keeps the animals locally. If you happen to belong to an active marine aquarium society (there are several such clubs within an hour’s drive of where I live, for example), then there is a good chance that you’ll be able to get a starter culture from another member of the club. If you are not able to get a starter culture from a friend or fellow hobbyist nearby, then there are a number of options for buying a starter culture of rotifers from which to start your own. In some of the more advanced pet shops you may be able to find these live foods, or starter cultures, available for sale locally. Perhaps the best known among suppliers for live rotifer cultures are Florida Aqua Farms (www.florida-aqua-farms.com) and Reed Mariculture (www.rotifer.com). If you are buying rotifers to feed to a batch of juvenile fishes, you will need a much larger initial supply than if you simply plan to start a culture in anticipation of feeding rotifers to something in the future. Obviously, if you have baby fish to feed, you don’t have the luxury of waiting a week or two to develop a dense culture from which to feed, and you’ll need to buy a bulk culture from which you can begin feeding immediately. Other options include Mountain Corals (www.mountaincorals.com), L.F.S. Cultures (www.lfscultures.com), Sach’s Aquaculture (www.aquaculturestore.com), CellPharm (www.biosynthesis.co.uk), and Carolina Biological Supply (www.carolina.com), among others. Suppliers such as these ones provide small and easily shipped rotifer cultures that are ideal for starting your own culture at home, but are not really economical if you intend to buy enough rotifers to feed directly without culturing your own rotifers.

One advantage of going with a supplier such as Florida Aqua Farms, Mountain Corals, CellPharm, or Reed Mariculture is that they can supply you with not only the starter culture, but also with food in the form of microalgae disks or liquid cultures. In most cases, they will also provide you with detailed instructions (such as The Plankton Culture Manual by Hoff and Snell 1999, which remains one of the most detailed guides to raising plankton at home) that will go well beyond what I have the space to cover in this column. So, hopefully that should explain to you what rotifers are, and why you might want to know something about them. If after reading all this, you still want to try culturing your own rotifers at home (it’s a fair bit of work, but not really very difficult), I’ll explain how to go about it in the next article.

Brinkmeyer, R. L., and G. J. Holt. 1995. Response of red drum larvae to graded levels of menhaden oil in semipurified microparticulate diets. Progressive Fish Culturist 57:30-36.

Carmona, M. J., A. Gómez, and M. Serra. 1995. Mictic patterns of Brachionus plicatilis in small ponds. Hydrobiologia 313/314:365-371.

Ciros-Perez, J., A. Gómez, and M. Serra. 2001. On the taxonomy of three sympatric species of the Brachionus plicatilis (Rotifera) complex for Spain, with a description of B. ibericus n.sp. Journal of Plankton Research 23:1311-1328.

Craig, S. R., C. R. Arnold, and G. J. Holt. 1994. The effects of enriching live foods with highly unsaturated fatty acids on the growth and fatty acid composition of larval red drum Sciaenops ocellatus. Journal of the World Aquaculture Society 25:424-431.

Fu, Y., K. Hirayama, and Y. Natsukari. 1991a. Genetic divergence between S and L type strains of the rotifer Brachionus plicatilis O.F. Müller. Journal of Experimental Marine Biology and Ecology 151:43-56.

Fu, Y., K. Hirayama, and Y. Natsukari. 1991b. Morphological differences between two types of the rotifer Branchionus plicatilis O.F. Müller. Journal of Experimental Marine Biology and Ecology 151:29-41.

Gómez, A., and T. W. Snell. 1996. Sibling species and cryptic speciation in the Brachionus plicatilis species complex (Rotifera). Journal of Evolutionary Biology 9:953-964.

Gómez, A., M. Temprano, and M. Serra. 1995. Ecological genetics of a cyclical parthenogen in temporary habitats. Journal of Evolutionary Biology 8:601-622.

Hoff, F., and T. Snell. 1999. Plankton Culture Manual. Florida AquaFarms, Inc., Dade City, FL.

Holt, G. J. 1993. Feeding larval red drum on microparticulate diets in a closed recirculating water system. Journal of the World Aquaculture Society 24:225-230.

Moe, M. 1992. The Marine Aquarium Reference: Systems and Invertebrates. Green Turtle Publications, Plantation, FL.

Moe, M. 1997. Breeding the Orchid Dottyback, Pseudochromis fridmani: An Aquarist's Journal. Green Turtle Publications, Plantation, FL.

Segers, H. 1995. Nomenclatural consequences of some recent studies on Brachionus plicatilis (Rotifera, Brachionidae). Hydrobiologia 313/314:121-122.

Serra, M., A. Galiana, and A. Gómez. 1997. Speciation in monogonont rotifers. Hydrobiologia 358:63-70.

Serra, M., A. Gómez, and M. J. Carmona. 1998. Ecological genetics of Brachionus sibling species. Hydrobiologia 387/388:373-384.

Toonen, R. 2001. Invert Insights: Phytoplankton. Tropical Fish Hobbyist #543:50-54.

Wilkerson, J. D. 2001. Clownfishes: A Guide to Their Captive Care, Breeding & Natural History. Microcosm, T.F.H. Publications Professional Series, Neptune City, NJ.

This month I wanted to finish up the discussion of rotifers that I started in the last column. As I mentioned in that article, if you are really interested in trying to culture rotifers at home, I suggest that you pick up a copy of Joyce Wilkerson’s excellent book Clownfishes: A Guide to Their Captive Care, Breeding & Natural History (Wilkerson 2001), rather than rely entirely on the sketchy account I am about to provide. I don’t have the space in this article to give you a detailed account of culturing rotifers at home, so I am not going to try. Instead, my purpose here is to introduce you to what is involved in rotifer culture so that you can make an informed decision about whether you want to attempt this at home, or whether you prefer to pass on the whole thing and purchase these animals from a supplier, instead.

So, after the introduction to the biology of the animals last month, I’ll jump directly into the mechanics of how to culture these animals in this article. The most important factor in successfully raising rotifers is to have a good food supply. Although many commercial outfits are successful in raising rotifers on a variety of pea flour and/or yeast-based invertebrate foods, I do not generally recommend that these be used for the small-scale culture of rotifers at home. The reason that I generally advise beginners to avoid such foods is that, these foods will foul the culture water in which rotifers are grown much more quickly than live phytoplankton foods. Fouled water typically results in a crash of the cultures (I will come back to culture crashes later). Personally, I prefer a good healthy algae culture for keeping the animals growing well -- I find that the rotifers reproduce faster and are of higher nutritional quality when grown on live phytoplankton cultures (Moe 1992 reported the same thing). Most people will find that getting a "good healthy algae culture" going is more work and harder to maintain than are their rotifer cultures. Fortunately, there are now a wide variety of economical phytoplankton foods for feeding to reef aquaria, and many of these are perfectly suitable for culturing rotifers. In general, buying live phytoplankton to feed your rotifers is more expensive than some of the alternatives, but also results in more nutritious rotifers that grow in a more stable and consistent manner. Freezer pastes of phytoplankton provide a more economical option, but also are much easier to overdose and foul your cultures. Pea flour and yeast-based invertebrate foods are generally the cheapest option of all, but also contribute to the fastest decline in water conditions in your cultures if not dosed carefully.

In practice, B. plicatilis appears to be able to survive a wide range of water conditions (Wilkerson 2001 reports a range of specific gravity from 1.005 to 1.040, pH of roughly 5 to 10, and temperature of 40 to 98ΕF!), but in reality the reproductive rate of the animals is noticeably slower outside their ideal range. As with virtually everything in this hobby, the ideal range for you may be somewhat different than the ideal range for someone else depending on the exact goals you are trying to accomplish because there are a variety of trade-offs involved in determining the ideal range.

To get a good growth rate from your rotifer cultures, the temperature ought to be somewhere between 70 and 80ΕF. All other things being equal, rotifers reproduce more quickly at higher temperatures. However, the higher the temperature, the higher the metabolism of the rotifers in the culture, and therefore more often the cultures need to be fed and cleaned. In general, the higher the temperature, the more productive, variable and unstable the culture is likely to be, so you need to find a balance between how many rotifers you need (and how quickly you need them), and how often you’re willing to clean and possibly restart your cultures due to crashes.

Although rotifers are usually not particularly demanding of the pH at which they are cultured, Wilkerson (2001) makes a convincing argument for keeping the pH somewhat lower than natural seawater: depressed pH reduces the toxicity of free ammonia in the cultures. Rotifer cultures are likely to accumulate wastes because they do not benefit from the filtration and frequent cleaning that we typically do for our aquaria. One of the main products of waste decomposition is ammonia, and the higher the pH of the culture, the more of that ammonia will exist in the toxic non-ionized form. Maintaining a pH of between 7.5 and 7.9 will provide a little more of a safety zone in terms of regular culture cleaning to avoid problems with ammonia toxicity in the cultures. Obviously, the best solution to this problem is regular maintenance and a good routine of cleaning to eliminate the wastes that could decompose to ammonia. However, in practice, when faced with rearing a batch of larval fish, the care of your rotifer cultures is very likely to take a back seat to the care of your fish. Keeping a slightly decreased pH in your rotifer cultures will give you a little more leeway in terms of your culture maintenance routine when this happens.

Finally, in terms of the salinity of your culture, you have to balance a higher growth rate at decreased salinities with the need to avoid osmotic shock when feeding rotifers to your animals. In general, rotifer growth rates are highest at salinities ranging from about 11 – 20 ppt (or a specific gravity of roughly 1.008 and 1.014), so if you’re trying to get the maximum number of rotifers in the shortest period of time, you’ll want to use about half-strength seawater (natural seawater is about 35 ppt, so ½ strength seawater is roughly 17 - 18 ppt, or approximately 1.013 S.G.). However, you also have to keep in mind that a sudden change of more than about 8-10 ppt (0.006-0.007 S.G.) sends rotifers into shock: shocked rotifers are not an attractive food item to larval fishes, and rather than swimming about as active prey, they will promptly settle out on the bottom of your larval rearing tanks. For this reason, I tend to keep my cultures at a higher salinity at the expense of a slightly depressed growth rate of the cultures. Many successful breeders maintain their larval clownfish rearing tanks at a slightly depressed salinity (say 30 ppt or so) to help with this change. By meeting both your rotifer cultures and your larval fish tank closer to an intermediate salinity value, the change in salinity is reduced and neither animal suffers particularly from the minor deviation from ‘ideal’ conditions. Another strategy is to select the rotifers that grow fastest at higher salinities and develop a culture for natural seawater conditions. This is accomplished by starting a large number of small cultures at higher salinity from a sample of animals from your currently cultures. After a week or so, pick the densest culture and again start a number of cultures from a sample of those animals, and so on until you have found a strain of rotifers that thrive at higher salinities and will reproduce well enough to supply your needs without requiring half-strength sea water.

Having covered the basics of the culture conditions, we can turn to foods for the rotifer culture. As I explained in the last article, rotifers themselves are not very good food for larval fishes, and it is only by stuffing them full of nutritious food that they become a good food source for rearing larval marine fishes. There are a variety of dry and liquid commercial rotifer feeds that can be used, but these all suffer from the drawback that the rate of feeding much be carefully matched to the ever-changing feeding requirements of your cultures, because they are easily overdosed. As with adding flake-food to a tropical aquarium, any excess food that is not consumed quickly begins to pollute the water, but unlike this analogy, we cannot actually see the rotifers eating the food, and so it is much harder to judge the correct amount to feed.

However, despite the fact that live phytoplankton makes a much more reliable food source for culturing rotifers, it usually turns out to be more difficult to culture phytoplankton than it does to culture rotifers. The basics of phytoplankton culture is to obtain a sterile starter from some reliable source, to provide those unicellular algae with "sterile" seawater supplemented with the nutrients essential to growth, and supply those cultures with a lot of light. There are a wide variety of phytoplankton that prove suitable for culturing rotifers, including Dunaliella, Isochrysis, Monochrysis, Rhodomonas, and Nannochloropsis (Moe 1992; Wilkerson 2001). In fact, rotifers will survive and reproduce on a wide range of foods other than phytoplankton, including yeast, bacteria, vegetable juice, cooking oil, baby pabulum, and even chicken droppings (Wilkerson 2001), however that does not mean that they will have any nutritional value when cultured on such foods. Numerous studies on the growth and survival of larvae under aquaculture conditions have shown that certain highly unsaturated fatty acids (HUFA) are necessary for the growth, health and normal development of juvenile marine fishes and invertebrates (e.g., Craig et al. 1994; reviewed in last June’s column Toonen 2001). These essential fatty acids are found almost exclusively in marine phytoplankton, and by filling the guts of rotifers with nutritious phytoplankton before you feed them to something else, you essentially turn them into little vitamin pills for your animals.

OK, so with all those reasons to use phytoplankton to culture rotifers, you’ll probably want to do just that. But now we come to the question of how do you get a phytoplankton culture to feed to your rotifers. The simplest way will be to buy the phytoplankton from a commercial supplier such as DT’s, Mountain Corals, LiquidLife USA, Kent Marine, or Two Little Fishies (I supplied URLs for most of these companies in the last issue). Although this is the simplest solution, it also turns out to be the most expensive, and many people soon decide to try culturing their own phytoplankton as an alternative to buying it. I don’t have the space to go into phytoplankton culture techniques here, but I have a detailed article on how to do this on the web at http://www.garf.org/news13p2.html#green, and the primary authority on this subject is the Provasoli-Guillard National Center for Culture of Marine Phytoplankton (http://ccmp.bigelow.org/GI/GI_00.html).

Although rotifers do not care about lighting at all, if you decide to feed live phytoplankton to your rotifer cultures, it is a good idea to keep these cultures well-lit. There are a couple of reasons for this. First, individual phytoplankton cells can continue to grow and incorporate nutrients until the moment they are eaten if there is sufficient light, rather than consuming their energy stores and being a non-nutritive food. Second, although the rotifers are voracious and efficient feeders, they may always miss a few cells by chance, and the reproduction of those missed phytoplankton cells will stretch out the feeding for the culture over a longer time. Finally, because rotifer cultures do not tend to be cleaned on a regular basis, they are often elevated in nitrogenous wastes, and the growth of phytoplankton in these well-lit cultures will help to reduce the concentration of wastes in the culture water. Both the algal and rotifer cultures are very sensitive entities, and as I have mentioned before, are subject to crashes.

Crashes can happen remarkably quickly, and typically occur because of two factors: poor water quality and high population density. If you remember from the last article, I mentioned that male rotifers are typically produced in response to various environmental stresses (such as overcrowding or poor water conditions). Males are typically smaller and faster moving than females, and can provide an ‘early warning’ system that your cultures are in need of immediate attention to prevent a crash (provided that you are attentive and have access to a microscope to regularly check your culture). Often overcrowding and a decline in water quality are intimately linked, and so the simplest way to avoid problems with either is to regularly thin your culture by removing a portion of the culture and replacing it with clean water and phytoplankton.

Even if your culture does crash, if you are diligent and attentive, you can save it by quickly cleaning and feeding the culture - transferring even a couple of healthy females into the new culture is enough to start again. However, if you miss a crashed culture of for any prolonged period of time (i.e., more than a few days), it will often become an anoxic bacterial soup; these not only tend to smell horribly, but are often lifeless and cannot often be resurrected so easily. As I mentioned last month, the resting zygotes of rotifers are extremely resistant to adverse environmental conditions. Carefully removing the sediment at the bottom of a crashed rotifer culture and transferring it to clean, good-quality water with adequate live phytoplankton food should cause those resting zygotes to hatch into new amictic females which can rapidly populate your cultures once again. If you fail at hatching the resting zygotes, then you will need to obtain a new starter batch of rotifers in order to get your culture going again.

Good water conditions can be maintained by a combination of not overfeeding (this is always important, but it is ESSENTIAL with yeast/pea flour-based foods), and regular water changes. Water changes can be done by periodically filtering the entire culture through a 50 micrometer mesh screen (available from companies such as Florida AquaFarms, or Aquatic EcoSystems), or by removing a large portion of the culture and replacing it with clean water. Cultures should be fed regularly to keep a constant green tint to the culture medium if you intend to harvest your cultures heavily. If you're using live phytoplankton, I'd aim for a constant haze due to algae in the culture water; if you're using artificial food, I'd aim for a haze for an hour or two after feeding, but ensure that the water clears up before ever feeding with artificial food a second time. You can keep the foods in suspension and prevent stagnation by gently aerating your culture. Another strategy for culturing rotifers is to set up a series of phytoplankton cultures (say in 2 L soda bottles), and cycle through them on a regular schedule. In this way, the oldest culture is fed, cleaned and then restarted while others are growing from previous feedings. Obviously, if you decide to go this route, you’ll need at least one culture for each time you plan to feed, and it would be better to have a backup in case of emergency as well. This is perhaps the most space and labor intensive method of culturing rotifers, but also typically the most productive.

Depending on how large a culture you are maintaining recommend a 10 gallon culture to rear a batch of clownfish), you can add more or less aeration to keep the food from settling out. In a 10-gallon tank, a single long airstone with low to moderate air output will keep the culture well suspended. In small (1 gallon) culture vessels, I use a glass pipette releasing 1-2 bubbles per second to keep the culture aerated. The correct amount of aeration and food to add to a culture is a bit of an art, and a little experimentation on your part will likely produce a huge increase in harvest as you gain some experience and discover what works best for you. You may be wondering how you can do water changes on a rotifer culture. The answer is simple: you harvest the animals frequently and replace the harvested water with fresh "sterile" seawater. Joyce Wilkerson (1995) recommends 25% per day, whereas Mark Rosenquist (1993) suggests 10% per day. Both of these will work, and in my experience some portion of the culture in this range of roughly 10-25% per day is about right. You can do a quick cleaning of the culture tank with a siphon at the same time as removing the rotifers if you siphon the culture water into a bucket and allow it to settle prior to filtering the rotifers out of the culture. I underline quick cleaning here because production seems most consistent in older, well-established and "unclean" cultures rather than new, virtually sterile cultures. I am not the only person to have noticed this (e.g., Wilkerson 2001 makes the same comment), and other authors go so far as to encourage letting your tank become coated with a thick algal mat in order to culture rotifers successfully (e.g., Rosenquist 1993). You can either strain rotifers through a fine-mesh cheesecloth, or a home- made Nitex© filter (see Toonen 1996-1997 for further details). Alternatively, you could add the culture water with rotifers directly to your rearing tank. The primary advantage of adding your rotifer culture to the tank directly is that it is simple. The primary drawback to this method is that water conditions within your rotifer culture are unlikely to be as high as those in your larval rearing tank, and you risk seriously degrading your larval tank rearing conditions by continually adding large volumes of rotifer culture.

Whatever you decide to do, you should never let rotifers removed from the culture sit for extended periods prior to feeding; rotifers kept alive without algae on which to feed for any length of time decline greatly in nutritional value. Rosenquist (1993) cites research by Dr. Elizabeth Clarke (Division of Marine Biology & Fisheries at the University of Miami) that showed simply waiting an hour or two between filtering your rotifers and feeding them to juvenile fishes reduced the nutritional value to the point that the rotifers were “useless as food” for juvenile fishes. Therefore, do not harvest your rotifers in advance: filter them from the culture immediately before you plan to feed them to your tank. Likewise, rotifers that are not eaten immediately in the aquarium will likewise decline in nutritional value. Thus it is always a good idea to include some live phytoplankton in your larval rearing tanks as well, so that the rotifers can continue to feed (and maintain a high nutritional value), and this provides the added benefit of some nutrient processing by the phytoplankton as well.

If you do not plan on harvesting your rotifer culture regularly, decrease the feeding from daily additions of microalgae to a periodic or weekly additions to decrease the likelihood of high population densities leading to culture crashes. I have managed to keep cultures alive for extended periods by simply placing a flask near a window with a few rotifers and some greenwater in it (but make sure it is out of direct light that would overheat the cultures). I cover the flask to reduce evaporation, and add DI water as necessary to keep the salinity roughly constant. These cultures typically survive without additional food for months at a time, and although the densities are always quite low, when I begin feeding again the culture usually blooms back to it's former density within a week or so.

Well, as I said at the outset, my goal in this article was not so much to give you a detailed account of how to culture rotifers at home, but rather a general overview of some of the techniques and commitment involved in setting up and maintaining a rotifer culture at home. If you have plans to try this at home, I strongly suggest that you locate all the references mentioned herein, that you read them all, and then distill the disparate view points into a technique that is likely to work best for you. Good luck, and I hope that this discussion of rotifer biology and methods of culture has been useful to you.

Moe, M. 1992. The Marine Aquarium Reference: Systems and Invertebrates. Green Turtle Publications, Plantation, FL.

Rosenquist, M. 1993. Rotifer Culture. Journal of Maquaculture 1(3):1-4.

Toonen, R. 1996-1997. Invertebrate Culture, Parts 1-4. Journal of Maquaculture 4(4):6-31, 5(1):4-13, 5(2):31-37, 5(3):41-51

Toonen, R. 2001. Invert Insights: Phytoplankton. Tropical Fish Hobbyist #543:50-54.

Wilkerson, J. D. 1995. The Captive Food Chain. Journal of Maquaculture 3(4):1-4.

Wilkerson, J. D. 2001. Clownfishes: A Guide to Their Captive Care, Breeding & Natural History. Microcosm, T.F.H. Publications Professional Series, Neptune City, NJ.

Q&A - Rotifers

Authored by: Rob Toonen