Derasa Clam: Tridacna derasa

The Derasa Clam is very pretty and quite durable, but it is also one of the largest Giant Clams!

The beautiful Derasa Clam Tridacna derasa has warm tonal colors with iridescent accents. It is one of the most widely available giant clams for the aquarium. Along with its cousin, the Gigas ClamTridacna gigas, it was one of the of the first aquacultured species of Tridacna clams.

Popular food item, these two giant clams have been hunted extensively throughout their natural habitats. But today both of these clams are cultivated in captivity. The Derasa Clams you purchase today are the result of aquaculture projects, not wild collecting. They are supplied as a food source, but are also readily available to the aquarist. Clams obtained from aquaculture systems are very hardy in reef aquariums.

The Derasa Clam is quite attractive and has proven to be one of the hardiest giant clams. They usually have a mantle with a striped pattern of wavy lines or a spotted pattern. They sport various color combinations of orange, yellow, black, blue, and white, some can even have brilliant blue or green accents.

Another claim to fame of the Derasa Clam, again shared with the Gigas Clam, is that it is one of the largest of the giant clams. The Derasa Clam grows fast and can double or even triple in size in a year. They will generally reach up to about 20″ in length in captivity, though specimens can reached up to 24″ in the wild. Still, Its adult cousin the Gigas Clam is much larger, reaching over 3 feet in length.

A giant clam just sitting in still may seem like a rather unlikely choice for the aquarium. But after one look at these beautiful creatures it is easy to understand why enthusiasts are eager to include them in their tanks. The Derasa clam, being one of the hardiest of the giant clams, is a great selection for an intermediate aquarist. With their large size, they will need an aquarium of at least 100 gallons, and with a particular intolerance of sediment in the water they require diligent aquarium care.

For more about keeping Tridacna Clams, see:
Giant Clam Care: Caring For Tridacnid Clams

Habitat: Distribution / Background

The Derasa Clam Tridacna derasa was described by Röding in 1798. They are the second largest clam in their genus reaching 18 to 24†in length. Derasa Clams are found throughout the Indo-West Pacific. They are listed on the IUCN Red List as vulnerable (VU) because they were hunted primarily for food in the wild. Today plenty of aqua-cultured Derasa Clams are available, so there is little pressure put on collecting wild specimens. Other common names they are known by are Smooth Giant Clam and Southern Giant Clam.

Derasa Clams primarily inhabit the outer edges of the reef. They can occur at depths between 1 to 65 feet (0 to 20 meters), but they are typically found only as deep as 1 to 49 feet (0 to 15 meters). One reference, however, claimed finding these clams in waters as deep as 65.6 feet (20 meters).

Unlike Crocea Clams, Derasa Clams are found in the substrate or on top of corals, as they do not burrow. They can be found in the intertidal areas of corals, attached by their foot, but as these clams lose their byssus glands early in their lives, they can often be found lying free on the substrate in lagoons as well. Juveniles like shallow waters, attaching themselves to the tops and sides of coral outcrops. It is thought that adults possibly detach their foot to settle in the deeper, clearer ocean waters.

They are often found alone, but they can be found in groups as well. In protected areas, such as on the Great Barrier Reef in Australia, they are sometimes found in densities of up to 30 clams to a hectare (2.47 acres). Predators are small parasitic pyramidellid snails that bore into their tissue and feed on their bodily fluids. While this is less of a problem in the wild, it is more of an issue in enclosed systems.

Giant clams are filter feeders. They take advantage of the phytoplankton, microscopic organisms that inhabit the sunny upper waters such as dissolved organic matter like ammonium nitrate. The Derasa Clams, however, rely much more on phytoplankton than other in their genus, and they seem to handle less intense lighting in captivity.

• Scientific Name: Tridacna derasa
• IUCN Red List: VU – Vulnerable – They had been hunted for food, but as they are now aquacultured, there is little pressure put on collecting wild specimens.

Description

Derasa Clams are the second largest of the giant clams. They usually grow to a maximum length of 20″ (50 cm) in captivity, though they can reach up to 24″ (60 cm) in the wild. They can grow at a rate 3″ plus per year, becoming sexually mature around 12″ (30 cm), which is around the time they are 3 to 5 years old. They are all born males then turn female as needed. Many aquarists have kept Derasa Clams for 15 to 20 years, however this genus may live over 200 years in the ocean.

Giant clams have a soft, laterally compressed body that is enclosed in an elongated shell. The shell consists of two hinged parts, with the hinge usually being longer than half the shell’s length. The hinge has an opening called the “byssal opening,” where a muscular foot attaches to a hard surface.

They have no head, but their soft body consists of a mantle forming an outer wall that encloses a visceral mass containing the body’s organs. The mantle protrudes in the form of flaps that are usually quite colorful. The siphon, also called the intake siphon, is a fleshy tube-like structure that is part of the mantle. The intake siphon is used to direct water flow into the mantle cavity and across the gills.

The Derasa Clam is also called the Smooth Giant Clam because of its thick smooth textured shell. This clam’s shell has six to seven vertical folds, but lacks much in the way of ribbing or scales. Derasa’s may develop scutes when grown in the aquarium. This may be a result of the artificial light and its effect on the expansion of the mantle. Some specimens are said to form scutes as a result of a genetic trait.

Some characteristics of Tridacna derasa:

• The shell is heavy and very plain.
• The shell may not have scutes, or the scutes may be tiny, or sparser but larger.
• The inhalant siphon has clearly visible tentacles.
• Small, narrow byssus gland opening.
• Loses its byssus gland as it grows.
• The hinge is usually longer than half the shell’s length.

Comparing Derasa Clams to other species of Giant Clams:

The Derasa Clam T. derasa is sometimes confused with the Gigas ClamT. gigas. Although as an adult the Gigas Clam reaches a much larger overall length than the Derasa, there are some other ways to tell the difference between these two:

• The Gigas Clam has a different mantle coloration.
• The Derasa Clam’s mantle extends further over the shell.
• The top edges of the Gigas Clam shell have triangle shaped projections, which extend inwards.

• Size of organism – inches: 23.6 inches (59.99 cm) – Sexually mature at 12″ (30 cm), around 3 to 5 years of age.
• Lifespan: 200 years – They can live over 200 years in the wild, and may live for decades in captivity.

Difficulty of Care

The Derasa Clams are the most widely available, and the hardiest, of the Tridacna clams. Although these attractive clams are easier to keep than the Crocea ClamsTridacna crocea, they are still best kept by intermediate aquarists. These clams need diligent attention paid to their water parameters, water flow and salinity, and the lighting needs to be stable. They also need sediment-free water, so will need good filtration, and will do best if not kept with tankmates who need frequent feedings as this can pollute the water.

If a healthy clam is obtained, these clams are hardy in captivity and relatively easy to keep. With proper lighting, good filtration, and careful attention the Derasa Clams require require little else in the way of care. It is important to make sure they are not being irritated, not being fed upon by other organisms, and good water quality must be maintained.

• If you have a healthy specimen, it will generally attach itself to the substrate in less than a day. Keep this in mind when placing your clam in the tank. Make sure you put it where you want it to stay.
• Keep a watch for predators as this clam can be easy prey with its large and wide byssus gland opening.
• Derasa Clams can do well under a variety of lighting intensities and occasional stronger currents will be tolerated. They are intolerant of changing salinity and they need sediment-free water.

They are much more forgiving than some clams, but they do need to be placed correctly in the tank. They can be shocked by a very bright aquarium. You can acclimate them to a bright aquarium that has Metal Halides or intense LED lighting by using layers of plastic screening between them and the lights. Remove one layer of screening every couple of days until the clam is fully adjusted to the light.

If the clam is predominately brown and showing little of its normal coloration and patterning, it should be placed lower in the tank away from strong lighting. The brown coloration is the clam’s zooxanthellae showing and is probably due to the clam losing its protective coloration during transport. They will adjust to their new surroundings in time, and will become a little more hardy. But to keep them healthy, avoid wide fluctuations in light, water flow, reef water parameters (calcium, magnesium, etc.), and salinity.

• Aquarium Hardiness: Moderately hardy
• Aquarist Experience Level: Intermediate

Foods and Feeding

Most clams fulfill their nutritional requirements by filter feeding and absorbing dissolved organic compounds from the water. The Tridacna clams have gone even further than this, using zooxanthellae to manufacture food for themselves. They receive the majority of their nutrition from their zooxanthellae, yet research has shown that all Giant Clams that are under 4″ do not have enough mantle tissue to provide enough space for zooxanthellae to keep the clam alive. Derasa Clams also eat more food, via filter feeding, than some of the other clams, such as the Crocea Clams.

It is generally believed that mature giant clams do not require feeding in the aquarium. Whether additional feeding is required is still debated. Some hobbyists believe they should be fed, going on the assumption that they are filter feeders like other clams. Basic nutrients in the aquarium that giant clams need are calcium, strontium, iodine, and a minute amount of nitrate that is at least 2 ppm. They will not thrive at a level of 0 nitrates.

Four ways clams make food for themselves:

• These clams have large amounts of zooxanthellae that live in their tissues. With plenty of light this algae will make too much food for themselves and the extra carbon and glucose is given to the clam (similar to most reef corals).
• The actual zooxanthellae themselves can be eaten by amoeboid cells within the host clam if needed.
• Giant clams have the ability to absorb nutrients like ammonia, nitrate and phosphates from the water.
• Giant clams are filter-feeders, straining fine particulates like phytoplankton, zooplankton and detritus from surrounding waters with their specialized gills.

Since larger Derasa Clams are more dependent on phytoplankton from various sources, they need to be spot fed several times per week. If they are under 2″ to 4″ they may need daily feedings. Feed them micro-foods designed for filter feeders such as a yeast-based suspension that has been mechanically whisked, live phytoplankton, or commercially prepared micro-foods like ‘marine snow’ or ‘reef snow’.

Once they are older they will still benefit from phytoplankton foods fed on a regular schedule in an aggressively skimmed tank. If there is a healthy population of fish in the tank, and the clam is over 4†long, feed a little less. For more information about the feeding process of Derasa Clams see What Do Clams Eat.

• Diet Type: Omnivore – Nutrition is obtained through filter-feeding of phytoplankton, zooplankton and detritus along with lighting and the marine algae, zooxanthellae.
• Flake Food: No – Will not eat this food
• Tablet / Pellet: No – Will not eat this food
• Live foods (fishes, shrimps, worms): Some of Diet – Clams under 4″ will need live phytoplankton to thrive.
• Liquid Foods: Some of Diet – Marine snow or other phytoplankton substitutes, especially if there are no fish. Lighting and the marine algae, zooxanthellae, make up the rest of the food.
• Meaty Food: Some of Diet – The rest comes from lighting and the marine algae, zooxanthellae.
• Feeding Frequency: Daily – Feed clams daily when they under 4″ and several times per week when over 4.”

Aquarium Care

Good water quality must be maintained, but if a healthy clam is obtained and proper light provided, these clams are relatively easy to keep. With careful attention paid to water parameters and proper lighting, Derasa Clams require require little else in the way of care.

Stable tank conditions are required to keep the Derasa Clam healthy. Do typical water changes of 10% biweekly, 20% a month, or 5% weekly. It has been noted that 5% weekly water changes replenish many of the needed additives.

Basic nutrients in the aquarium that giant clams need are calcium, strontium, iodine, and a minute amount of nitrate. Along with regular water changes the following parameters are important in keeping your Derasa Clam long term. Adding large doses weekly is not recommended. Daily doses or adding to top off water is best:

• Calcium: Calcium is the main building block for clams and should be present in the water at levels of at least 280 mg/L for growth to occur. Seachem’s calcium additive works at 385. More rapid, natural growth is seen when calcium is in the range of 400-480 mg/L.
• Strontium: Strontium is incorporated in the shell along with calcium and should also be provided for optimum growth.
• Iodine:The addition of iodine to the aquarium will also enhance growth and color. Use caution, as iodine spikes have been known to kill them. Add iodine with top off water or dose daily, but not at one time in a big weekly dose.
• Nitrate: They require some nitrogen for proper growth. They will not thrive at a level of 0 nitrates. Nitrate can be added if levels are extremely low, but be careful as nitrates should never exceed 2 mg/L. Provide a minute amount of nitrate that is at least 2 ppm.

Derasa Clams do not like linear water flow, but occasional stronger currents will be tolerated. Moderate to strong water movement is acceptable if water is turbulent, but you really don’t want your water parameters to fluctuate too much. It is also important to make sure they are not being irritated and not being fed upon by other organisms, so keep a watch for predators. For more in depth information on caring for Derasa Clams see, Caring For Tridacnid Clams.

• Water Changes: Bi-weekly – To keep water stable, test weekly.
• Calcium Levels: 400.0 – 450.0 ppm – If using Seachem’s calcium, 385 should suffice.
• Alkalinity Levels: 8.0 – 12.0 dKH – Best at 9 dKH
• Magnesium Levels: 1,250.0 – 1,350.0 ppm – Adjust magnesium levels before checking calcium levels.
• Strontium Levels: 5.0 – 15.0 ppm
• Iodine Levels: – .030 to .060 ppm: Control is not recommended.

Aquarium Setup

Derasa Clams can be kept in a reef environment with live rock and you can place them almost anywhere in the tank. Keep fluctuations in water parameters to a minimum. They are not very tolerate of sediment or pollutants in the water, nor do they like changes in salinity. A high pH and high temperatures can cause problems.

The size of the tank should be at least 100 gallons for your Derasa Clam (though larger is better) as this will help keep water parameters stable. Live rock is necessary and sand is preferred if they will be on the substrate. A mature tank is also important. The tank should be at least 6 months old from the time of adding that last piece of live rock. Once you see Coralline algae growing (that cool pink and purple hard algae) you know your tank is doing well and ready for your clam.

Put a flat rock on the bottom glass of the tank and the clam on top of that. Stabilize the clam by filling in the area half way up the clam with sand. This will keep their large foot opening blocked off, which helps prevents pests from attacking them through the opening. They will attach in about a day. Derasa Clams can also be placed in a larger hole in the rockwork (large enough for them to open fully), but you should periodically use a turkey baster or power head to remove collected detritus. Where ever they are placed, make sure the eventual 18″ to 24″ of room they will occupy is free of shadowing.

Coming from shallower waters in the wild, giant clams typically need intense lighting. Derasa Clams will do well under a variety of lighting intensities. Of course, the more light you supply, the faster they will grow. It is not unheard of for a 2.5 inch (6 cm) clam to double or triple their size in less than a year as long as they are given plenty of calcium (more than 400 mg/L).

They are tolerant of tanks that have T5s or other fluorescent lighting, and some will also do well with Metal Halides or intense LED lighting once acclimated. Using plastic sheeting can help them adjust to brighter tanks, and help to not shock the darker specimens. If keeping them close to the top of the tank, and the lights, be sure to protect the lights from their inevitable squirt of water if they become startled.

Some care should be taken with placement of the clam depending upon the clam’s color. Bright blue Derasa Clams will need stronger light to keep their coloring. Clams showing the iridescent gold color will typically handle the higher light intensities as well. However, if the clam is not showing the gold pigmentation and the mantle is mostly brown, it is better to put the clam in the lower part of your tank. Definitely avoid placing these brown colored Derasa’s directly under strong metal halides. Once they adjust to the lighting, the light must stay constant.

Salinity is also important, too high or low a salinity can cause the death of a clam. Aim for a specific gravity of 1.024 to prevent swinging out of the 1.023 and 1.025 range. Be careful when replacing evaporated water, Derasa’s do not like fluctuating salinity. Automatic fresh water top offs are highly suggested to keep salinity stable. Keep pH at 8.1 to 8.3, do not let the aquarium exceed a pH above 8.4. For hardness, maintain a dKH of 7.9-12, with the optimal being 9. A temperature between 74 to 83ËšF (23 to 28ËšC) is best, but be sure to keep the temperature you choose stable.

• Minimum Tank Size: 100 gal (379 L) – Tank should be at least 6 months old, preferably with coralline algae growth.
• Suitable for Nano Tank: No
• Live Rock Requirement: Typical Amount
• Substrate Type: Sand
• Lighting Needs: Moderate – normal lighting – Bright blue Derasa Clams will need stronger light, Kelvin’s of 6K to 10K, to keep their coloring. Clams showing the iridescent gold color can also be kept under stronger light.
• Temperature: 74.0 to 83.0° F (23.3 to 28.3&deg C) – Keep the temperature stable.
• Specific gravity: 1.023-1.025 SG – Salinity is important, too high or low a salinity can cause the death of a clam. Stay within this range, aiming for 1.024. Be careful when replacing evaporated water, Derasa’s do not like fluctuating salinity. No higher
• Water Movement: Moderate – Derasa Clams do not like a linear water flow, but occasional stronger currents will be tolerated. Moderate to strong water movement is acceptable if the water is turbulent.
• Water Region: Bottom – Dependent on lighting.

Social Behaviors

Clams are very stationary and peaceful, so they are not aggressive towards other aquarium inhabitants. In the wild Derasa Clams are often associated with hard corals and they may be kept in an aquarium with them. Yet even though they have the ability to close their shell, they will need protection from anemones and some corals. They should not be kept near any stinging cell creatures and must be kept away from any sweeper tentacles. Anemones need to be watched, as they can move close to a clam and sting or eat it.

Derasa Clams commonly have small shrimp and crabs that live in the mantle cavity. They are considered ectoparasites or commensals, and they will not harm a healthy clam. Be cautious with tank inhabitants that may pick at the clam or eat its mantle such as Trigger Fish and Puffers. Blennies, Butterfly Fish, and Clown Gobies. Angelfish and some shrimp may also disturb Tridacna clams.

• Venomous: No
• Temperament: Peaceful
• Compatible with:
  • Same species – conspecifics: Yes
  • Anemones: Monitor – Protect the clam from anemones that wander.
  • Mushroom Anemones – Corallimorphs: Monitor – Safe as long as they do not come in contact with the clam.
  • Leather Corals: Monitor – Should be okay if spaced apart.
  • Zoanthids – Button Polyps, Sea Mats: Monitor – Should be okay if spaced apart.
  • Sponges, Tunicates: Safe
  • Shrimps, Crabs, Snails: Monitor – Small commensal shrimp are fine if the clam is healthy. Remove any small, rice-sized snails found on the clam’s surface with a toothbrush..
  • Starfish: Monitor – Starfish usually only eat dying organisms.
  • Feather Dusters, Bristle Worms, Flatworms: Safe
  • Clams, Scallops, Oysters: Safe
  • Crabs: Safe – Commensal crabs are okay on a healthy clam.
  • Snails: Monitor – Remove parasitic pyramidellid snails, other snails are safe.
  • Sea Apples, Cucumbers: Threat – The clam will die if these species expel their toxins. Giant clams are much more sensitive to these toxins than other invertebrates, even if the system is large.
  • Urchins, Sand Dollars: Safe
  • Nudibranch, Sea Slugs: Monitor – May be safe, but it is unknown if any or all species are safe.
  • Copepods, Amphipods, Mini Brittle Stars: Safe
  • Stony Corals: May be aggressive – Stony corals can sting your clam.
  • Soft Corals: May be aggressive – Give clam plenty of space.

Sex: Sexual differences

There are no discernible sexual differences.

Breeding / Reproduction

The giant clams are protandry, meaning they are born male and change to female. Clams will release eggs and sperm that can number into the tens of thousands of eggs. This event tends to happen around sunset. One aquarist noted a spawning clam pulsing out eggs every two minutes. Once the eggs are externally fertilized, the embryos develop into trocophore larva, which are free-swimming. The next stage is the bivalve veliger, which sort of looks like a tiny free swimming clam. Once they are 4 to 20 mm long, they will find a shallow area of the reef to settle into, often perching on top or on the side of coral outcrops.

Derasa Clams have been propagated in captivity, and the demand from aquarists has raised interest in producing colorful varieties of all the species. Typically, there is some sort of outside stimuli that causes spawning, which can be a change in temperature, salinity, or other parameter. However this should be left to the experts, as clams spawning in a small system can cause serious pollution issues. For detailed information of tridacnid propagation, see Giant Clam Breeding and Reproduction

• Ease of Breeding: Difficult

Ailments / Diseases

Keep a watch out for predators! The Giant Clams can be easy prey with their large and wide byssal gland opening. The most dangerous predators are the highly prolific pyramidellid snails of the Tathrella, Pyrgiscus, and Turbonilla genera. These are parasitic snails about the size of a grain of rice or smaller, rarely reaching a maximum size of about 7mm in length. These snails attack tridacnids with a trunk-like snout called a “proboscis.” They punch holes into the clam’s soft tissue and then feed on its bodily fluids.

While in nature, giant clams can deal with a few of these parasitic snails, in captivity these snails tend to multiply to dangerous numbers. They may hide in the scutes of the clam or in the substrate by day, but will often will be found around the edges of the clam’s mantle tissue or byssal gape (large foot hole) after the lights go out. They can produce numerous small, gelatinous, egg masses on the clam’s shell. These masses are transparent, so difficult to spot. Before putting it in the tank, scrub your clam’s shell vigorously with a toothbrush, or other brush with firm bristles, to remove the snails and their eggs. Be careful not to scrub the clam’s soft tissues, as that can cause injury and lead to infection. Inspect your clam regularly for several months and remove any snails you discover.

Some other common ailments of Derasa Clams:

• Pinched Mantle: Another common affliction of giant clams is called the Pinched Mantle disease. The edges of the mantle become pinched and contorted, and the margins won’t extend fully. This condition almost always results in death unless treated, and it can spread to other clams. The actual cause of this ailment is unknown, but it may possibly be an attack by some sort of protozoan. An easy treatment, that is is stressful but effective, is to submerse the clam in a freshwater dip for a few minutes. Signs of recovery can take a day or two.
• Bubble Mantle: Caused by laminar flow directed at the clam or pouring water directly into the aquarium in a way that causes bubbles to form (use a siphon and water pump and/or pour very slowly and carefully or pour into sump only). These bubbles can get stuck in the clam’s mantle with can cause illness and death.
• Gaping: The larger of the two holes is gaping open, larger than normal. This is usually due to poor water quality. Stability of your water parameters need to be checked quickly and addressed.

Giant clams are also quite sensitive to chemicals or toxic substances dissolved in the water, so be sure to maintain good water quality. As mentioned under aquarium care and aquarium parameters above, high pH, high salinity, and high temperatures can also cause problems. Avoid large weekly doses of anything, especially iodine, which has been known to kill them. For in-depth information on potential Tridacna clam problems, see: Tridacnid Clams: Friends, Enemies & Ailments.

Availability

Derasa Clams obtained from aquaculture systems are readily available and are usually from 1.25″ to 5″ in size. They can be found online and in stores, ranging in price from moderately expensive to expensive, depending on size and color. The Derasa Clams available today are the result of aquaculture projects, and are very hardy in reef aquariums. Both T. derasa and T. gigas were among the first clams to be commercially bred.

References

• Animal-World References: Marine and Reef
• Tridacna derasa (Röding 1798) smooth giant clam, SeaLifeBase
• Tridacna derasa, IUNC Red List, International Union for Conservation of Nature and Natural Resources
• Scott W. Michael, The 101 Best Marine Invertebrates, TFH Publications, 2008
• Anthony Calfo, Book of Coral Propagation, Volume 1 Edition 2: Reef Gardening for Aquarists, Reading Trees; 2 edition, 2007
• Ronald L. Shimek, Guide to Marine Invertebrates: 500+ Essential-to-Know Aquarium Species, Microcosm, 2005
• Anthony Calfo and Robert M. Fenner, Reef Invertebrates: An Essential Guide to Selection, Care and Compatibility , Reading Trees, 2003
• Alf Jacob Nilsen and Svein A. Fossa, Reef Secrets: Starting Right, Selecting Fishes & Invertebrates, Advanced Biotope Techniques , T.F.H Publications inc., 2003
• John H. Tullock, Natural Reef Aquariums: Simplified Approaches to Creating Living Saltwater Microcosms, 2001
• Julian Spring and Daniel Ramirez, Invertebrates: A Quick Reference Guide, Ricordea Publishing, 2001
• Daniel Knop, Giant Clams: A Comprehensive Guide to the Identification and Care of Tridacnid Clams , Ricordea Publishing, 1996
• Julian Spring and J. Charles Delbeek, The Reef Aquarium: A Comprehensive Guide to the Identification and Care of Tropical Marine Invertebrates (Volume 1), Ricordea Publishing, 1994
• Dersa Clam, Aquacultured (Tridacna derasa), LiveAquaria, 2015

Featured Image Credit: Tridacna derasa. Aquarium Finisterrae. Galiza by =mc2, Wikimedia Commons CC SA 2.0 Generic