Q. Can an octopus and/or nudibranchs be kept in a single tank in the 50 to 75 gallon size range? I understand that nudibranchs eat anemones. Because we also want to keep anemones and clownfish, should we not consider including nudies in our setup? If we do have nudies, would we have to replace the anemones constantly or would they just regenerate what the nudies gobble up? Would the nudie live long enough to do any of this or would the octopus eat it first (maybe this food-chain thing is getting out of hand)? About the octopus — should we look for a particular species so we don’t have to deal with it outgrowing the tank or overwhelming the tank’s ecosystem? (I’d like to avoid having to set up our swimming pool as a viable marine environment for this lovely creature!)
A. Let’s start with nudibranchs. There are numerous species in this order — known scientifically as the Nudibranchia — that make their living in different ways. A great number of these mollusks have very specialized diets. For example, the beautiful Spanish dancer (Hexabranchus sanguineus) is an obligatory sponge feeder. Others eat only hydroids, anemones (as you mentioned), soft corals, bryozoans or barnacles.
For the most part, the ornately colored species available in aquarium stores — most of which belong to the family Chromodorididae — do poorly in captivity unless they are provided with their natural bill of fare. For example, Kunie’s (Chromodoris kuniei), Bullock’s (C. bullocki), Loch’s (C. lochi) and Elizabeth’s (C. elizabethina) nudibranchs, which are the species most commonly encountered by hobbyists, all feed on sponges. If they are not provided with encrusting sponges to browse on they will become inactive and die.
Most of the anemone-eating nudibranchs belong to the family Aeolidiidae. These nudibranchs have cerrata (finger-like projections) on their backs in which they store the ingested stinging cells (nematocysts) of their anemone and hydroid prey. They then use this recycled weaponry for their own defense.
In an aquarium the feeding activity of one of these nudibranchs will typically end up indirectly killing larger anemones. This may result from the increased susceptibility of the anemone to infections, as well as its propensity to remain contracted — preventing it from feeding. Because of the difficulty in meeting the dietary needs of carnivorous nudibranchs, I think it best to leave them on the coral reefs, where they belong.
If there is someone at your local university studying these animals, it is possible they have a species that is easier to keep to facilitate their research endeavors. For example, some species feed on algae, a food source that is abundant in most aquariums. But, most of the algae-eating species do not possess the striking coloration of their predatory relatives, and therefore are considered less desirable as display animals. The carnivorous nudibranchs are more ornately colored to blend in with the colorful substrates on which they live, or in some species their striking chromatic attire warns potential predators of the fact that they are unpalatable.
Another thing that makes nudibranchs undesirable aquarium animals is that many species are short lived in nature. For example, there are species whose total life cycle is only one month long, and most species live no more than one year!
The octopuses are another group of interesting mollusks that have special care requirements. Most are voracious predators that will make short work of fish, like anemonefish, or for that matter, anything else that moves! Octopuses have sharp beaks, and in most species there is a venom gland associated with this structure. Although in the majority of species bites are not lethal, they can be quite painful. The blue-ringed octopus (Hapalochlaena faciata) is very venomous, and, for this reason, should not be kept by marine hobbyists.
Because of their predatory nature, octopuses are best kept by themselves. They will not only feed on fish that are small enough to subdue, they will also eat crabs, shrimp, snails and some worms. Octopuses are great at escaping. Therefore, it is important that the aquarium is tightly covered, or you may find a hair-covered, amorphous blob that was once a cephalopod on your living room floor. Be warned — they can slip through unbelievably small holes in the aquarium back strip. Because the beak is the only body part that is not malleable, if it can fit through an opening the rest of the octopus can too!
You should provide your octopus with some decor to hide in or under. This can be live rock, pieces of artificial coral or even PVC pipe. They also appreciate small pieces of coral rubble, which they will use to line the openings of their hideouts. Like their nudibranch relatives, most octopuses live short lives. Many of the species available in fish stores complete their life cycle in a year or less. It is best to buy a small octopus, because your chances of getting a young specimen are increased. However, there is not always a correlation between small size and greater life expectancy, because there are some species that just stay small.
Most octopuses could be successfully housed in a 75-gallon aquarium. In fact, there are several pygmy species that can be kept in tanks as small as 10 gallons. The problem is telling the various forms apart! I think you would be much better off forgoing the mollusks and sticking with the anemonefishes and the anemones.