A Close-up Look at Tridacna crocea

Several species of giant clams are available to hobbyists, but Tridacna crocea is one of the most popular, if not number one. So, I’d like to give you a close-up look at this species, including information on how to identify them, some of their basic biology and how to care for them. As you’ll see, these are some very interesting animals, despite the fact that, for the most part, they seem to just sit around and look good.

Note that this species is simply called “crocea” by most, but is also known as the crocus clam, boring giant clam and saffron-colored giant clam in different places and crowds. And they’re also sometimes sold under the trade names super crocea, ultra crocea or something similar, because certain specimens have a more colorful and/or unusual appearance than others. It should be no surprise that they’re highly desirable, because they often come in wonderful blues and greens, with just about every other color mixed in at times, as well. Purple, orange and gold are often seen, although I've seen some that look pretty much solid brown, too. On top of their wonderful colors, the patterns that typically cover their upper mantle tissue may include stripes, waves, spots, rings, etc., making them look even more beautiful.

Tridacna crocea in all its splendor and variety.

Identification


After looking at enough T. croceas it’s usually pretty easy to spot them by the appearance of their mantle alone, but not everyone has enough experience with them to do so, and some specimens are harder to figure out than most. Fortunately, several other characteristics can be used to make a sound identification, though, so we’ll run through them here. Note that, for most hobbyists, taking a good look at the shell is usually the best way to make an identification, and a figure is provided below that shows the shell parts, in case you aren’t familiar with the terminology that follows.

Rosewater (1965) reported that a shell length of 15cm is the maximum size for T. crocea, making it the smallest of the giant clams. The shell is also typically grayish-white, but is sometimes tinted with yellow, orange or pinkish-orange, too. These colors may also form an obvious band at the shell’s upper margin, particularly on its inner surface.

The shell is often slightly to moderately elongate, and the hinge is typically less than 1/2 to 1/3 of the shell’s length. They’re typically strongly inflated, being rather fat near the hinge, and the valves (each half of the shell) may be heavy/thick for their size, too. Note that due to T. crocea's habit of burrowing into hard substrates, deformed shells are sometimes seen. At times the substrate may be harder in some areas than in others, and the clam must comply by growing into a form to fit the available space.

Each valve typically has six to ten mostly flat folds, with four or five of them usually being only slightly more pronounced than the others. In some cases the shell is almost smooth, though, aside from the small, undulating lines/growth-banding that covers them. Even these are commonly completely absent near the bottom of the shells of specimens that have burrowed into hard substrates, as the shells often become increasingly smoothed near the umbo in the process.

The valves often have no large scutes, either because they never form or get scoured away during burrowing. Occasionally, however, specimens may have scutes that are seen as light scales near the upper part of the shell. Of course, in the case of aquacultured specimens that aren’t allowed to burrow, the whole shell may be covered with prominent scutes. So, it can be easy to confuse an aquacultured T. crocea with other species that don’t burrow and that usually have scutes, if you go strictly by the presence or absence of scutes.

On the top side, the upper margin of each valve is symmetrical to the other, and they are usually topped by four or five inter-digitating projections. These allow croceas to close up quite tightly in most cases. On the bottom side, croceas typically have the largest/longest byssal openings of all the giant clams, usually being 1/3 to 1/2 the shell’s width and 1/3 of the shell’s total length, or even more. The large opening, in combination with an appropriately-sized byssus, allows croceas to form an exceptionally strong attachment to substrates, and also allows for the complete extension and withdrawal of their oversized pedal mantle tissue (this will be covered below).

As far as soft parts go, croceas typically extend their mantles well beyond the upper margin of their shell, enough to completely hide the shell from sight when viewed from above. The mantle also commonly has numerous tubercles/protrusions on its upper surface, and often has a row of “eyes” near its outer edge, too. The inhalent siphon (the slit-like opening in the mantle where water is drawn into the body) is ringed with very small but numerous tentacles, which may be extremely finely branched or oddly shaped; and the exhalent siphon (where water exits the clam’s body) often takes on a tall, tubular/chimney-like form.

Top and bottom view of typical crocea shells.
Side view of typical crocea valves.

Exhalent siphon of a typical crocea.
Inhalent siphon of a typical crocea, which is lined by fine tentacles.

Geographic Range and Natural Habitat


Croceas can be found in the northeastern Indian Ocean and throughout the western and central Pacific Ocean, ranging from the west coast of the Malaysian Peninsula and northwestern Australia eastward to the Marianas and Solomon Islands (Rosewater, 1965). Rosewater actually reported that they were found all the way east to Fiji, but Lewis, et al (1988) reported that none had been found there despite what Rosewater had said. However, the species has been introduced there since then, according to Raymakers, et al (2003). They’re also found as far north as the southern islands of Japan and south to the Great Barrier Reef and New Caledonia.

Still, despite having such a broad geographic range, croceas have been heavily overfished in many areas within their natural range, and are even extinct in a few areas such as Guam and the northern Marianas (Munro, 1989 and Wells, 1997). This is due to collection for the aquarium hobby to some degree, but collection for consumption by locals and for Asian seafood markets is the primary culprit. The species is still abundant enough throughout most of its range, however, that it’s not considered to be at risk of endangerment/extinction (IUCN 2006).

Throughout much of their range, croceas can be found in large numbers in the intertidal zone where they are periodically exposed by low tides, and in shallow subtidal waters. I haven’t found a reference that provides their exact maximum depth of habitation, though, with the exception of Rosewater (1965), who stated that they live at depths up to "several fathoms." One fathom is 6’, so this likely means something like 18 to 24 feet, which would mean it lives at the shallowest depth of all the Tridacna species. Additionally, Hamner (1978) and Hamner & Jones (1976) reported that they are "regularly" found at densities of 100 clams per square meter in some locations, and sometimes at 200 per square meter on some interior reef flats of the Great Barrier Reef.

Geographic range of crocea.

Many croceas living on or near reefs in the intertidal zone can be exposed at extremely low tides. Temporary exposure to air and sometimes rain does not harm them.

Croceas are also called boring clams because they literally bore a hole into whatever substrate they live on, which they enlarge and deepen as they grow. For this reason, they’re typically almost entirely enclosed in live or dead coral heads, or in solid limestone substrates with their upper shell margin approximately even with the surface of their burrows. Still, I’ve occasionally seen  individuals whose shell protruded from their burrow, with up to about one-third of it sticking out.

Most croceas are deeply encased in the substrate, with only the mantle and upper margin of the shell protruding.

Burrowing starts when they’re a miniscule 4 to 20mm long (Kawaguti, 1983 and Suzuki, 1998), and is done primarily through the action of a thin sheet of specialized mantle tissue that protrudes from the byssal opening at the bottom of the shell. This pedal mantle tissue secretes a weak acid that can soften/dissolve the calcium carbonate material under and around a clam, and it can actually extend all the way up the sides of the shell (Hedley, 1921). It has also been suggested that the clams can grind back and forth in their burrow and use the fine corrugations on the surface of their valves to mechanically wear away the burrow's walls (Yonge, 1936 and Hamner & Jones, 1976).

Here you can see the pedal mantle tissue partially extended from this crocea’s byssal opening.

Additionally, I’ve observed several croceas in the wild that have used the underside of their upper mantle tissue to dissolve carbonate substrates. In such cases the substrate was dissolved and smoothed away where it obstructed the upper mantle’s extension, and the form of the dissolved areas made it clear that the upper mantle tissue, not the pedal mantle tissue, was responsible. This is not something unique to croceas, though; there are other instances of clams using their mantle tissue to dissolve, rather than precipitate, carbonate material (e.g., de Zwaan, 1977 and Lutz & Rhodes, 1980).

Here you can see that the depressions made in this dead coral skeleton match the mantle’s form perfectly when it is extended fully.

It’s interesting to note that, oddly enough, croceas don’t form burrows when placed in aquariums. Apparently, if they don’t start making a burrow at a very small size, they won’t do it at all. In their natural habitat, they start to burrow into substrates when they’re only a few millimeters long, so I’m guessing that for some unknown reason, they won’t make a new burrow once they reach a certain size, which is much smaller than the specimens sold in the hobby.

Growth Rates


Croceas are the slowest growing of the giant clams, but their exact growth rates are highly variable due to differences in environmental conditions and individual genetics. Light intensity, nutrient availability, water temperature, wave activity, salinity, etc. can all affect growth rates, but even when living under identical conditions (i.e., in the same tank or on the same reef), some individuals can still grow much faster or slower than others. In fact, “enormous differences” in individual growth rates of clams from the same parents have been observed and reported by numerous researchers (e.g. Beckvar, 1981; Gwyther & Munro, 1981; Munro & Heslinga, 1983).  Additionally, Dor (2002) observed that on one clam farm 5% of each species were super fast growers, 10% were fast growers, 70% were average growers, 10% were slow growers and 5% were super slow growers. Note that individual clams, even of the same species, also reach sexual maturity at different times, which slows their growth as more energy is put into making gonads, sperm and eggs. So, once they reach reproductive age after a few years, growth rates can slow drastically.

Still, to give you at least a ballpark estimate of how fast juvenile croceas grow, I’ll tell you about a study done by Hart, et al (1998). They divided several hundred croceas (and other species, too) into several protective cages and distributed them to 11 different areas that had different environmental conditions around the Solomon Islands (e.g., different depths, water clarity, wave activity, etc.). Then, after being monitored for about two years, it was reported that specimens in some locations had much higher average growth rates than those in others. For example, the croceas in one location grew at an average of 2.02mm per month, but only 1mm per month in another. It’s important to remember that these are only averages, though, so the fastest growing individual could have been significantly higher than 2.02mm/month and the slowest could have been much less than 1mm/month.

However, this range of growth rates correlates well with my personal observations of numerous croceas living in aquariums, which are typically no more than a couple of millimeters per month – at best, with some growing almost none. Yes, slow. By comparison, T. gigas may grow more than 10mm/month!

These croceas are the same age and from the same parents, and have been raised under the same conditions at an aquaculture facility, but they still have significantly different growth rates.

Aquarium Care


As you’d probably expect, water quality requirements for keeping croceas fall right in with what’s generally considered “standard” for reef aquariums. Temperatures between 25° and 28°C are optimal, as is a pH of 8.1 to 8.3. Alkalinity should optimally be kept in the range of 9 to 12dKH , and calcium should be maintained at 380 to 450ppm. About the only thing in particular to note is that as a clam grows, it adds new shell material to the entire inner surface of its valves, not just their upper margin; so even a slow growing crocea can use more calcium than you might expect, and having several in an aquarium can deplete calcium and alkalinity surprisingly quickly.

Other than that, sufficient lighting is really the key to keeping them healthy. Light falling upon the mantle and the zooxanthellae kept inside it is the primary means by which croceas get their energy in the wild, and the same goes for their life in aquariums. So, you absolutely must give them sufficient lighting if you expect to keep them alive.

Croceas live at shallower depths than other Tridacna species, where they receive intense light; so they require more light than the other species when kept in aquaria. And, on top of species-level differences, there’s also variability between individuals. Genetic differences can make one clam more fit than another under the same conditions; two individuals may be carrying different strains of zooxanthellae, and so on. This is very important to remember, as you always want to provide at least enough light to keep the average clam of a given species alive, not the minimum that you think an individual of the species could possibly live under.

In the case of crocea, fluorescent lighting will suffice only in very shallow tanks, or if a specimen is placed near the water’s surface in a deeper tank. I highly recommend squeezing as many bulbs into the canopy/fixture as possible, mounting the bulbs so that they are as close to the water as possible (without causing heat problems), and then placing the specimen within 20 or 30cm of the water’s surface, preferably less. Some specimens may be able to get by at times with less light, or further down in deeper tanks, but I implore you not to take chances. Metal halide lighting is really the way to go, preferably a combination system comprised of metal halide and fluorescent lighting.

Using a metal halide system allows you to place a crocea deeper into a tank, which also makes it easier to view. Most of them look their best when viewed from a high angle, so they actually look much better when placed near or on the bottom of a tank. A standard 175-watt metal halide bulb should be sufficient for keeping a crocea on the bottom of any small- to medium-sized tanks, as in anything less than 45 or 50cm deep (or no deeper than this in a larger tank). But, I’d go ahead and move up to 250-watt, or even 400-watt, bulbs if a specimen will be any further than that from the surface.

Next, we get to feeding (or not feeding). All tridacnids are filter-feeders that ingest a variety of particulates they strip from surrounding waters. However, their zooxanthellae can provide a great deal of their nutritional needs, and they also can absorb nutrients directly from seawater. In fact, if provided with enough light, croceas of any size can completely forgo filter feeding and can thrive in particulate-free water as long as enough dissolved nutrients are present.

I dedicated an entire chapter to tridacnid nutrition in Fatherree (2006), which covers how they “work” in great detail, but I’ll keep it simple here by saying that, in aquariums, basically everything is taken care of by having good lighting and simply feeding the fishes. Some fish food is left uneaten and becomes detritus, which croceas can filter out, and detritus also releases other nutrients into the water as it decomposes. But, most food is eaten by the fishes, which then give off dissolved nutrients (ammonia, in particular) and excrete solid wastes that can also become detritus. So, when you feed the fishes, you’re feeding the clams as well.

The real question is whether or not there are enough fishes in your aquarium, and/or enough fish food going into your aquarium, to support one or more tridacnids. It is, indeed, possible to have too low a fish load (or too high a clam load, depending on how you look at it) in a tank, which means that the amount of fish waste produced is not enough to support the needs of the clam(s). My advice, then, is to refrain from taking any chances and to use a quality phytoplankton product if you have any doubts. Again, though, the vast majority of hobbyists do not need to do so.

Then there’s water flow to consider, because croceas live in shallow waters on reefs and near-reef environments, and therefore are regularly exposed to strong currents and wave activity. Water movement in aquariums, however, is typically nothing like that on the reef or nearby, as the flow in most aquariums tends to be quite linear and constant. In most tanks a pump outlet might blast water in one particular spot day and night at about the same volume per minute, and rarely creates any real surge or turbulence. Although it’s perfectly fine to expose croceas to a low velocity surge, or to turbulent flow, putting them in a position where a pump just blasts them with a strong, non-stop linear current is not recommended. Basically, any sort of current that causes the mantle to fold upward too much, or over onto itself all the time, is bad, as is any current that makes a specimen chronically retract its mantle. Thus, you can put a clam anywhere you like with respect to current, as long as it doesn’t elicit either of these reactions. Yes, a crocea can take an occasional blasting that folds it up or makes it retract, but if this happens all the time, the clam can suffer from stress, or may even begin to starve from lack of light due to lessened mantle extension.

Finally, we get to placement. I think it’s best to place any species on the same sort of substrate that you’d find it living on in its natural habitat, and croceas are never found on sand. I’ve never found one living on rubble, either; they live on (in) hard substrates only. So, a crocea should be placed on a solid substrate, if at all possible, where it can attach itself firmly to one spot as it would do in the wild. A flat piece of live rock or coral skeleton works very well, but a piece of tile can also be used. It may take anywhere from a few hours up to a couple of weeks, but a healthy clam will usually attach itself with at least a few byssal threads, or maybe many. I’m not suggesting that you absolutely must to do this by any means, but I do recommend it.

Croceas normally reach out of the bottom of their shell and form an attachment using a number of tough byssal threads. Here you can see the foot (left), which contains the byssal organ, reaching out from a crocea and forming some fresh byssal threads (on the shell of a neighboring clam, in this case).

Keep in mind that if you don’t like the way it looks to have your clam attached to and sitting on top of a rock, you can always wait for it to attach to whatever you put it onto, then bury that piece. It’s not a problem to bury a rock, or whatever else it attaches to, just under the surface by covering it with a bit of sand or gravel. However, you shouldn’t overdo it in such a way that the bottom part of a clam is down in the sand, as it can be irritated when it opens its shell if any of the grains manage to get inside.

Okay, with all that stuff covered, now let me tell you about some of the things that you should not do when it comes to the placement of a specimen. Don’t place a crocea in a tight crevice between rocks and such; this may restrict its ability to open fully, and also increases the risk of it falling down into the rockwork if it moves around too much. Don’t put one in a hole in a rock that might restrict its ability to open fully, either. And, if you do place one in a large enough hole, never let a lot of detritus settle into the hole around the clam. You should blow out any buildup of crud by using a turkey baster, powerhead, etc.

If a specimen has attached to a piece of rock, shell, etc., never try to move the clam and the piece by grabbing the clam. That’s a good way to injure it; you should always pick up the clam and the piece together, and be very careful when handling the two. Likewise, you should never try to pull a crocea off anything that it’s attached to because you can rip the byssal organ out of it. Finally, don’t move one repeatedly over a short period of time. It can be stressful enough trying to adapt to changes in lighting and current when its moved into an aquarium, and quickly moving it from one place to another to another can sometimes be too stressful. Such activities can lead to slowed growth or a greater susceptibility to disease due to stress, or can even outright kill them. If you must move one around/up, be sure to give it plenty of time between each move, as in a week or two at the least.

Photos copyright James Fatherree.



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References


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Dor, P. 2002. Memoirs of a clam farmer. Reefs.org.

Fatherree, J.W. 2006. Giant Clams in the Sea and the Aquarium. Liquid Medium. Tampa, FL. 227pp.

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A Close-up Look at Tridacna crocea by James Fatherree - Reefkeeping.com