Freshwater Prawns as Pets
Introducing Macrobrachium rosenbergii
This article originally published in
Freshwater and Marine Aquarium:
1981,4(7):32-34 & 74-75.
For the tropical fresh-water aquarist there are precious few opportunities to enjoy the challenges and rewards of keeping invertebrates. Except for some snails one can only look with envy at the fanworms, the multi-hued sea anemones, or the banded coral shrimp in a marine aquarium.
All that may change soon. Within the past 15 years the giant Malaysian prawn Macrobrachium rosenbergii has been the subject of intensive study for aquaculture. It grows rapidly, reaching a body size of 13 centimeters (5") in 10 months or less in aquaculture ponds. But beyond its enormous potential as an animal grown food, Macrobrachium rosenbergii may become a very popular addition to the home aquarium.
Fresh-water prawns are a member of the phylum Arthopoda. They are decapod crustaceans related to crabs and marine shrimp. But in their native land of Malaysia and Southeast Asia, M. rosenbergii has evolved to survive in the brackish water of the estuaries and the fresh-water rivers. In fact, modern day M. rosenbergii grow so much better in fresh water that they rarely venture into water which is more than one half seawater.
As with most arthropods, prawns have an exoskeleton. Instead of muscles connected over a bony internal skeleton, prawn muscles attach underneath, on to the rigid, calcium-impregnated carapace. While viewing a Macrobrachium in an aquarium, one cannot help but be in awe of how well designed it is. Like an armored knight, virtually the entire body is protected. But, unlike a knight, prawns show excellent coordination and agility. I have seen as many as six of their ten legs active, cleaning different parts of their body all at the same time!
In an aquarium Macrobrachium seem to spend most of their time cleaning. Six long feelers are covered with chemoreceptor cells which allow the prawns to detect the smell of food in the water. These are continually scrubbed to remove pieces of dirt which may affect their function. The one long pair of claws (chelae) may also serve a sensory function and are kept spotlessly clean. Even the eyeballs, each one sitting atop a short stalk, are scrubbed clean of dirt.
Macrobrachium are also highly social animals, especially during courtship. The male will use his long arms to tap, grab, pinch and even embrace the female (just like people!). in short, these animals are almost continually up to something, providing endless hours of watching enjoyment.
The fresh-water prawn will eat just about anything. In nature it will eat pieces of worms, snails, clams, fish, pieces of rice, wheat, beans, nuts, aquatic plants and some fruits (Ling and Merican, 1961). These can also be used in the aquarium. I have found Macrobrachium to be most fond of shrimp pellets, pieces of squid and fish, live Tubifex worms (or freeze-dried), frozen adult brine shrimp, certain seaweeds and water sprite. Another excellent food is "egg custard" (Ling, 1969). Scrambled eggs are heated over steam, enough to coagulate yet not so hot that much of the nutritional value is lost. Eggs are good in that they supply some vitamins, carotenoid pigments, and, most importantly, cholesterol. Cholesterol is critical to a prawn's life functions, yet prawns are not able to make it (New, 1976). Prawns will also eat fish flake foods. However, alone the flake foods do not provide adequate nutrition. As with some fish, Macrobrachium will not always eat a new food at first. But be persistent (without fouling the water).
Not having to deal with a finicky eater makes keeping a pet a lot easier -- sometimes. Adult prawns, especially females, can be voracious. Even zebras and young neons are neither too small nor too fast to evade a hungry prawn, even in large (50 gallon or larger) aquaria. Plants can also be uprooted or devoured: the nibble marks on my plastic plants will attest to how far a prawn will go. However, I have raised post-larvae and young juveniles in planted aquaria with some assorted livebearers and tetras with no problems -- prawn attacks on fish or vice versa. So in general. I think experienced aquarists will notice that the care and keeping of prawns is not much different than, for example, cichlids.
Prawns cannot be overcrowded and expected to do well. But how many is too many? For young Macrobrachium, one can almost cover the bottom with them. But once they reach a 5 centimeter (2") body size, their claws are strong enough to defend a territory. At this age prawns should be given at least 600 square centimeters of bottom area each, or about 120 square centimeters for each centimeter of body length. Sexually mature adults, 12 centimeters (4.5") in body length or larger, need at least a 20 gallon tank. They should be kept as a pair or a male and two females. Overcrowding, poor water quality, or a poor diet all seem to make prawns more aggressive, leading to injury or death, especially during molting.
Molting is the process by which Macrobrachium, and all crustaceans, grow. Although their thick exoskeleton is great protection, it limits growth. So every once in awhile prawns must shed their carapace. Young prawns molt much more frequently, once every two or three days. But as they age their growth rate slows. An adult female will molt every 20-40 days while a male may only molt once every six months. Molting is truly one of the remarkable things in the animal kingdom to watch. Before molting, prawns grow a new shell below their old one. But this new carapace is extremely soft and somewhat folded, like and empty balloon. During this time prawns start to reabsorb some of the calcium in the carapace, weakening parts of it.
One can often tell a prawn s about to molt because it will stop eating, especially adults. At molting the new carapace inflates with water, splitting the old one at the weakened points. The old carapace splits in half where the head portion attaches to the tail. The entire head section -- eyes, claws, legs and all -- slides out of the old shell. A sharp flick of the tail frees the rest of the prawn, leaving the entire old exoskeleton lying on the bottom of the aquarium.
The prawn will continue to inflate with water, which will later be replaced by new tissue. For a short time though (about 15 minutes), a prawn will be unable to walk or use his claws since the flexible new carapace does not yet offer enough leverage for the muscles to act. Within half an hour the body will be rigid enough to appear normal and permit movement. But the carapace will not be fully hardened for as much as a day afterwards. During this time after molting, prawns are extremely susceptible to attack. The aquarist should provide places for soft prawns to hide. But in my experience the most important things in minimizing cannibalism are good water quality and good nutrition.
Macrobrachium rosenbergii do best in water of 26 to 30 degrees C. (79 to 86 F.). They can survive from 22 to 32 degrees C. (71 to 90 degrees F.), but growth and activity becomes at best sluggish at the ends of their range.
Prawns need somewhat alkaline water, pH 7.2 to 8.4. At pH's below 7.0, prawns have a difficulty hardening properly after a molt. The carapace of intermolt prawns also becomes weak providing a site for infection. Normally, I add a small amount of dolomite or Florida crushed coral to the gravel. or to the filter, which helps to keep the water alkaline and provides some calcium to the water. It is debatable whether calcium dissolved in the water is of much benefit to a prawn, but the value of pH maintenance is important enough.
Regular water changes should be made, just as with tropical fish. However, prawns are very sensitive to chlorine. "Cheating" and adding tap water directly to a Macrobrachium tank will usually prove to be a fatal mistake.
Good aeration is important. Although there are indications that M. rosenbergii is tolerant of low oxygen for short periods of time, extended hypoxia seems to decrease their appetite. Overall, they become less active, but often more aggressive towards their fellow prawn, resulting in cannibalism in their effort perhaps to "weed out the competition"
Lighting and Substrate
Prawns do not like direct light but can adapt to it after awhile. Even so, dark hiding places or a covering of floating plants such as duckweed or water sprite would be appreciated.
To a certain extent the color of the prawns will be darker or lighter depending on the gravel substrate. Very young post-larvae are completely transparent, revealing all the body organs at work. Juveniles are also somewhat transparent, but may show blue stripes (Ling, 1962). In nature, adult females are dark brown with jet black claws and orange antennae. Adult males will be dark blue or brown in color. Actually a number of pigments are involved including black, neon blue, and yellow or orange carotenoids. On light gravel some of the colors may appear to fade, primarily due to less black pigment. This is not always bad since the almost electric shades of blue and orange become much more visible.
Normally prawns are grown in mud-bottom ponds. In an aquarium any small or medium size gravel is fine and much easier to keep clean. Macrobrachium will not disrupt rock or gravel formations seriously so be creative!
Prawns are subject to a variety of diseases in ponds (Johnson, 1980) or in closed systems such as aquaria (Delves-Broughton and Poupard, 1976). The most frequent is damage to the exoskeleton due to fighting or mishandling, sometimes leading to infection. This damage can be repaired when the old exoskeleton is shed during molting. Likewise broken chelae, legs, or antennae will eventually be regenerated at the next molt.
A second major disease I have observed is loss of pigment. All body pigments including those in the eyes fade away leaving a white, translucent prawn. This can happen even in a dark aquarium. It now appears to be directly related to diet since such animals were maintained solely on shrimp pellets. A varied diet including plants is important in providing essential nutrients and vitamins. Plants seem to be helpful for this condition.
The third problem is "molting death" syndrome. During molting the prawn is unable to extract his claws, legs or other parts of the body from the old carapace. The new exoskeleton will either not harden or harden in the contorted shape the trapped prawn may be in while shackled to the old carapace. Death follows shortly thereafter. Recent work on lobsters (Conklin et al, 1980) indicates that this syndrome may be the result of low levels of soy lecithin in the diet. The active ingredient of lecithin has not been isolated but these results underscore the importance of good nutrition in keeping prawns.
Anyone who has had the pleasure of keeping Macrobrachium rosenbergii can easily understand how it won Class V Freshwater Invertebrates at the second annual Florida Tropical Fish Farmers Convention. It certainly won a place in my home aquarium.
Conklin, D.E., L.R. D'Abramo, C.E. Bordner and N.A. Baum, 1980. A successful purified diet for the culture of juvenile lobsters: the effect of lecithin. Aquaculture, 21:243-249.
Delves-Broughton, J. and C.W. Poupard, 1976. Disease problems of prawns in recirculation systems in the U.K. Aquaculture, 7:201-217.
Johnson, S.K., 1980. Diseases of Macrobrachium. Texas Agri. Ext. Service: Texas A & M, 11 pp.
Ling, S.W., 1962. Studies on the rearing of larvae and juveniles and culturing of adults of Macrobrachium rosenbergii (de Man). Tech Pap. #57, 10th Session Indo-Pac. Fish. Council, 15 pp.
---, 1969. The general biology and development of Macrobrachium rosenbergii (de Man). FAO Fisheries Reports, 3(57):589-606.
---, and A.B.O. Merican, 1961. Notes on the life and habitats of the adults and larval stages of Macrobrachium rosenbergii (de Man). Proc. 9th IPFC Meeting, 9(2):55-61.
New, M.B., 1976. A review of dietary studies with shrimp and prawns. Aquaculture, 9:101-144.