The phylum Mollusca contains a diverse array of life forms, including animals as different as the clam and the giant squid. Other mollusks include the oysters, snails, nudibranchs and octopuses. This is one of the largest invertebrate phyla, containing over 10,000 described species (it is second only to the Arthropods).
Although it may be difficult to see, members of this assemblage share some common characteristics. They have a muscular foot, a shell (composed of calcium) that is secreted by the mantle and a feeding organ called a radula. Although most of the mollusks possess all of these characteristics, some lack at least one of these (for example, not all mollusks have shells).
There are many species available to aquarists, some of which serve a vital role in helping us keep algae under control or our sand beds stirred. In this article, we will look at two groups within this species-rich family of animals: the giant clams (class Bivalvia; genus Tridacna), which are coveted by reef aquarists, and the Einsteins of the mollusk clan, the octopuses (order Octopoda).
The tridacnids are members of the class Bivalvia. These animals have two valves (the two sections that comprise the shell), which protect the animals’ internal organs. These are hinged by ligaments and completely enclose the body of the animal. The shell is pulled closed by adductor muscles. This group includes oysters and mussels, as well as the clams.
The tridacnid clams are some of the most highly sought-after invertebrates. The fleshy mantle of the clam harbors zooxanthellae, and between the chlorophyll of the algae and protective pigments present in the tissue, they can exhibit spectacular colors. They are best viewed from above; as a result, some “clam nerds” set up shallow aquariums on a shorter stand that have lights hanging above it. In this way, you can view your tridacnid charges in their full glory.
Tank Size For Tridacnid Clams
When it comes to keeping tridacnid clams, tank size needs to be taken into consideration. Tridacna gigas (47 inches) can easily outgrow a large reef tank, and if conditions are optimal, it will grow very quickly. Adults of this species will eventually need a huge tank (in the 500-gallon range). The medium-to-large sized species are Tridacna maxima (15 inches), T. squamosa, Hippopus hippopus, Hippopus porcellanus (all three to about 16 inches), and T. derasa (to at least 20 inches). Large adults of these species are best-kept in a tank of 100 gallons or more. The “pygmy” in the group is T. crocea (about 7.5 inches). This clam can be kept in tanks as small as 30 gallons as an adult. Of course, juveniles of all the Tridacna can be housed in small tanks, as long as enough light and calcium are provided. Some aquarists have even successfully housed young specimens in well-lit nano-reefs (watch the calcium levels if you attempt this).
Keeping them Healthy
It is prudent to avoid small individuals (under 1.5 inches in length), as their nutritional needs are more difficult to meet. Small T. maxima are particularly tricky to keep. A healthy clam will clap its valves shut when a shadow passes over it. It will not have a gaping appearance — this is often indicative of disease or low oxygen levels (larger clams are more prone to “oxygen starvation” than smaller individuals/species). Look for parasites between the scutes or ridges on the valves (see the sidebar “Snails on Clams” for more). The color of the mantle should not have lighter patches, which may indicate localized bleaching. If a clam has had bleaching occur as a result of shading or being kept under inadequate lighting, slowly increase the intensity and duration of illumination. This bleached tissue needs time to build up the protective pigments that will block the ultraviolet rays. Avoid individuals that exhibit small, white spots on the mantle, which is thought to be caused by a lethal and contagious protozoan. If you need to relocate a clam, never rip it from hard substrate, which can cause irreparable damage to the byssal gland. Use a sharp knife to carefully cut the byssal threads from the attachment site (cut close to the latter to prevent damage to the byssal gland).
As with the zooxanthellae-harboring corals, adequate lighting is essential to successfully keep clams. The more colorful T. crocea and T. maxima should be kept under stronger lights and may also benefit from being placed higher up in the aquarium closer to the illumination source. In most cases, these two tridacnids do best if kept under metal halides (a good rule of thumb is approximately 175 to 250 watts of light for a tank 20 inches deep). The less light-demanding clams include Tridacna derasa, T. gigas and T. squamosa. These species can be successfully kept under T-5 lights or even power compacts, if they are placed close to the light source (say, within 10 inches from the lamps).
A newly acquired clam should not be immediately placed under an intense light, unless it was under strong illumination in the aquarium you acquired it from. If a clam does not remain upright, or “jumps” off of its perch, it may be signaling it has not adapted to strong illumination. You can place the clam in an area of the tank where light levels are more subdued and gradually move it up the “reef” closer to the main light source. Another way to gradually acclimate a tridacnid to a new, brighter light source is to place a piece of frosted glass between the clam and the light, remove it after several weeks and see how the clam behaves.
While lighting is of paramount importance to ensure success with clams, not all their nutrients are provided by zooxanthellae. These clams also filter-feed by straining nanoplankton out of the water as it passes over their gills. In the aquarium, it is beneficial to feed them phytoplankton. Because of their great metabolic needs, small clams will do much better if regularly fed.
The giant clam tank should be supplemented with calcium so that levels remain between 400 and 480 mg/L. The alkalinity level should be 2.5 to 4.5 meq/L. Additions of strontium and iodine can also facilitate good clam health. In order to maintain appropriate trace element concentrations, it is a good idea to conduct frequent, partial water changes (10 percent of water volume every two weeks). Water temperatures of 76 to 82 degrees Fahrenheit are optimal, with a pH of 8.0 to 8.4 and a specific gravity of 1.023 to 1.025. It has been suggested to wait six months to a year after setting it up before adding a clam to your reef tank.
One thing to consider when placing the clam in the tank is that larger individuals that are placed near the water’s surface or kept in a shallow tank may ejaculate water out of the tank when they suddenly slam their valves close. The water may be ejected onto overhanging lights, which can cause damage to lamps and fixtures, or onto power cords and electrical outlets. A glass top can help keep water in the tank, but may deleteriously impact gas exchange and light penetration (especially if it is not cleaned often).
Do not place clams near stinging invertebrates (e.g., sea anemones, large-polyped stony corals), as their mantles will retract due to the irritation they can cause. Fish have been known to accidentally become trapped between the clam’s valves when it slams shut. Some potential victims include morays, anemonefishes, gobies, surgeonfishes and rabbitfishes.
The mantles of tridacnid clams are sometimes nipped by butterflyfishes (including the copperband butterflyfish, Chelmon rostratus), angelfishes, larger wrasses, cleaner wrasses, clingfishes, boxfishes, filefishes, triggerfishes, puffers and porcupinefishes. Herbivores, such as blennies, surgeonfishes and rabbitfishes, have been known to occasionally feed on the mantle mucus, which may lead to actual tissue ingestion. If you have a fish that persistently nips at a clam, the piscine perpetrator should be removed. On rare occasions, tridacnid clams fall prey to large, predatory polychaete worms and crabs (both can remove chunks of mantle).
Octopuses (Class Cephalopoda)
The cephalopods contain some diver favorites, including the nautiluses, cuttlefishes, the squids and the octopuses. The Latin name means “head-footed,” and most members of this group are simply that: one big head and many long “feet.” The head of these animals is surrounded by large, prehensile tentacles (i.e., arms). The arms are used in locomotion in those species that spend more time on the sea floor (such as the octopuses) and in prey capture. They have highly developed sensory organs (e.g., their eyes are similar to our own). The tentacles are rich in chemoreceptor and tactile cells — octopus can use their arms to discriminate between different textures and chemicals. They are well-known for their ability to learn and problem-solve. Octopuses have a horny beak used for biting and ripping up prey, while some produce a toxic saliva, which is most highly developed in the deadly blue-ringed octopuses (Hapalochlaena spp.). The enzyme-rich saliva of the octopuses is injected onto its prey to help digest the flesh before and as it is consumed. A radula is also present and is used to pull bits of food into the “mouth.”
Octopuses can expel clouds of ink when attacked, which may distract a would-be predator. It is thought that the ink may also affect a predatory fish’s sense of smell and taste. Many octopuses can change their color in an instant. Some can even change the texture of their skin, developing dermal flaps that help break up their outline against the sea floor. This helps them avoid detection by predators and prey. While they have special needs, octopuses can make wonderful aquarium pets if these requirements are met. But it is important that you select the species you keep carefully, as some do better in the aquarium than others. One of the best octopus species for the home aquarium is the California two-spot octopus (Octopus bimaculoides).
One of the most important considerations when setting up a tank for an octopus is that it has a securely fitting, weighted top, as they are escape artists. Octopuses will also slither over overflow boxes or into stand pipes that have not been protected with fiberglass screening. Powerheads should never be accessible to an octopus, as they are likely to damage their arm tips in the pump impellers. It is important to create suitable, secure hiding places (e.g., caves, crevices, burrows); they will interact with the aquascaping materials in their environment, pulling rocks about, digging under reef walls, flipping corals, etc. You should also introduce plastic toys, PVC elbows, coral rubble, etc., to provide sensory stimulation.
The aquarist should be aware that a rise in ammonia or nitrite can be lethal, while an increase in nitrate (e.g., over 100 ppm nitrate/nitrogen) can cause stress, including more regular inking behavior. Keep the specific gravity between 1.023 to 1.025. Octopuses are also sensitive to low concentrations of dissolved oxygen, so make sure that the tank is well-aerated, and keep the temperature for tropical species between 76 and 80 degrees Fahrenheit. Incorporate a protein skimmer and an external filter with good-quality carbon to deal with excessive waste; clean mechanical filters often, as they will clog with the shed octopus skin. Regular water changes are also useful in keeping dissolved organics and nitrogenous waste in check. Make sure you always use deionized or reverse-osmosis water in the cephalopod aquarium. Also, you should not place octopuses in a tank with stinging corals.
As mentioned, octopuses are well-known for their ability to expel ink when they are threatened or stressed. If they do this in captivity, it can be lethal. Therefore, it is important to intervene immediately by performing a partial water change. Provide your octopus with as varied a diet as possible, feeding every other day. Fresh or frozen table shrimp, scallops or marine fish flesh are good staples; add live food (e.g., ghost shrimp, fiddler crabs) to stimulate a finicky octopus to feed.
Octopuses are best kept on their own. They are voracious opportunists that will eat many different types of marine animals. There are also larger fish that will eat cephalopods (e.g., morays, groupers) that should never be kept in the octopus tank. Only one octopus should be kept per tank, unless you can acquire a heterosexual pair. If you can acquire a pair, they may spawn.
That ends our brief look at two of the most renowned mollusk groups. While these animals can make beautiful, fascinating inhabitants in the home aquarium, they do have special care requirements that must be met if they are going to thrive in captivity. If you are not willing or able to meet these requirements, it is best to stick with less demanding aquarium inhabitants. Happy mollusk watching!
Scott W. Michael is the author of Reef Sharks and Rays of the World, Reef Fishes: A Guide to Their Identification, Behavior and Captive Care; and more. His photos have appeared in publications around the world.