Name: Macrobrachium rosenbergii
Origin: SE Asia to Oceania
Macrobrachium rosenbergii, commonly known as the long-armed prawn, is a large shrimp-like crustacean with deadly claws. Long-armed prawns also have 6 long antenna-like feelers, 1 more set of shorter clawed arms, 1 short set of arms without claws, 6 legs for walking, and paddles under the tail for swimming. They like living in tropical temperature freshwater with a high pH, but they will survive in a variety of conditions. When kept in acidic water, crustaceans have trouble forming their exoskeletons. Hard water and a diet with iodine and calcium is essential for exoskeleton growth. Frozen krill are a great food for these prawns, but they will eat most anything, even their own shell after they molt. When they molt, their shells are so complete that it looks like there is two prawns in the tank. After a few days the prawn will eat its old shell and it will be noticeably larger than before it molted.
Caution must be used when keeping these prawns in an aquarium. In an aquarium setting they can be expected to grow over 15 cm in body length, with another 25+ cm of claws. As the males grow, their claw arms grow increasingly larger in proportion to the rest of the body. Very mature males can have large blue claw arms twice the length of their body. A mature prawn would also have six antenna, each over or nearly a foot long. A full-grown prawn requires lots of space to move about easily and it will kill anything else in the tank. Do not try to keep any fish or invertebrates with this M. rosenbergii. My prawn has eaten four angelfish, a firemouth cichlid, 10 zebra danios, and four gold barbs. This was all in one night, before I realized how much of a predator he was. Needless to say, he now has his own tank. No fish is small enough or fast enough to save itself from a hungry prawn. Sometimes mine gets feeder fish but not often because usually when he attacks them instead of just grabbing them he slices them in half and each half floats to the top of the tank. A diet of invertebrate pellets, bottom feeder pellets (such as Hikari crab cuisine or HBH crab and lobster bites), and frozen krill will keep a prawn very happy and healthy.
With a good diet, these prawns can live for a long time and provide endless entertainment. At any given time, mine is moving at least two of its limbs, whether he is searching for food, climbing a rock wall, or cleaning himself. When I drop a food pellet into the tank he uses his flippers to shoot himself towards it and then he wildly swings all 12 of his arms and legs until he grabs it. A prawn is a very interesting pet and if you have the space, you should consider one.
My girlfriend and I had two of these in a 280 liter tank. The man at the fishstore we bought them from said they are very mild tempered and shouldn't touch other fish. He was so wrong. They ate the 5 fish in the tank in about a week. They have an aggressive attitude and seem to be very territorial. A 280 L tank was barely big enough for two. They became very scared when it came to cleaning the tank though. So no worries on them attacking your hands. We bought them measuring around 10 cm. The day we brought them back to the store they'd grown at least another 10 cm. The claws were unbelievable. The fishstore employees were in awe at them. Including the claws they were about 35 cm long. Very intimidating looking things, but they were extremely interesting to look at, given you want to dedicate a generous size tank to them. The tank was kept in a bedroom and at night we could hear them swimming to the top and tapping the glass lid. They'd swim backwards and hit the lids thinking they could escape. Make sure to close the glass lids after feeding!
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