By Nancy King
"Ollie", about 9 months old -- Photo by Nancy King
O. bimaculoidesis a medium sized octopus, reaching a mantle size of 7 inches (17.5cm) and arms to 23 inches (58cm). Some remain smaller than this. The bimac is not usually heavily textured and has several common colors, such as grey with yellow splotches. O. bimaculoides can be recognized by the false eyespots on its mantle below its eyes. Other species have these ocelli but there is an unbroken blue chain as part of these dark eyespots on a bimac.
Conditions for Keeping a Bimac
A bimac should be kept by itself in a 50-gallon or larger species tank. The tank needs to be well cycled and mature, which can take up to three months to prepare. The tank should be covered and well sealed to prevent escape. Bimacs like a sandy substrate and caves of rock or several lengths of PVC pipe to hide in. Bimacs tolerate a wide temperature range, ideally around 65-72 degrees F. (18 22 degrees C) in the home aquarium. They don't need a lot of light a 30-watt daylight spectrum lamp for 8-10 hours/day should be enough. All overflow holes and powerhead intakes should be covered with mesh or netting. A sump could be used to house all filtration and other equipment to keep them safe from the octopus as they can interfere with fittings. The water must be RO or RO/DI water and the tank must be provided with an over-spec filter system. Most people use a wet/dry filter, powerhead, and also a good quality protein skimmer (very useful for the heavy waste of an adult octopus, and also to remove ink).
Salinity - 1.026, pH- 8 8.4, NO3 0, NO2- 0, NH3 - <30 ppm, Copper - 0 Octopuses are very sensitive to Copper (Cu), ammonia (NH3) and nitrite (N02), but they can tolerate small amounts of nitrates. Regular partial water changes are recommended, 10% weekly or 20% biweekly.
"Inklet", around 3 months old -- Photo by Carol Sauer
Hatchlings are fed amphipods or mysid shrimp. Some are then fed baby clams and small crabs. (The clams can be placed in a small shell or shallow bowl.) Young octos may be given ghost shrimp, which are easier for them to catch than shore shrimp but are not suitable for long-term use. When they are small, they can also eat hermit crabs. Most will accept only live food until they are several months old. Then they can be offered thawed frozen shrimp, fresh scallops, live fiddler crabs, live shore shrimp and eventually larger crabs (crabs should be smaller than the octo's mantle). Some will take pieces of fresh fish (never use goldfish) All seem to like crayfish. Never add more food than the octo can eat at one sitting as uneaten food may die or rot and pollute the tank. Bimacs three months old or less should be fed more than once a day.
Bimacs are friendly octopuses and will respond to your attempts to make 'friends' with them. Using a feeding stick to offer food is one way to get their attention. Individual bimacs have different personalities and many like toys such as Lego blocks. They may bite you out of curiosity, and the bite is somewhat like a bee sting (provided you are not allergic to the venom).
Lifespan and Reproduction
O. bimaculoides has a natural lifespan of 1 to 1 ½ years. The approach of the end is signaled by egg laying in the female or senility in the male. In the ocean, the female encounters a male who gives her a packet of sperm, which she keeps until she is ready to lay eggs. Egg laying takes place toward the end of her life. The first sign of egg laying is that the female bimac builds a very secure den by piling up rocks and shells and she usually does not leave it until the eggs hatch. Even when a female has never mated, she can still lay infertile eggs, which may come as a surprise to the bimac's owner. There are a number of instances of female bimacs continuing to eat while holed up in the den with their eggs, so food should be offered. The eggs will hatch within about two months, depending on water temperature. Although the female may die shortly after the eggs hatch, on occasion she may live on for many weeks afterwards. Nancy King
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Octopus bimaculoides Care Sheet
Bimac/Two-spot Octopus Care Sheet by Nancy King