The Reefer: Why do Tridacnids Look the Way They Look?

Author: James W. Fatherree, MSc

The tridacnid clams are usually quite beautiful, often covered in unique patterns and a broad range of colors, and I’ve wondered for a long time why they look as fancy as they do. After all, they’re just clams, albeit very unusual ones.

So, after a whole lot of digging around for answers, I found some pretty interesting information about tridacnid coloration. I’ll cover some basics first, in case you don’t know much (or anything) about tridacnids, and then we’ll get to the other stuff, some of which will be news to just about everybody.

Zooxanthellae

First of all, tridacnids contain populations of single-celled algae, called zooxanthellae, within some of their tissues. These are the same things that reef-dwelling corals harbor, and when a coral or tridacnid is exposed to bright light, the zooxanthellae can make more food than they need for themselves and actually “feed” their host. Excess nutrients made by the zooxanthellae via photosynthesis are donated to the coral or clam host, helping them meet their daily needs.

In the case of tridacnids, the bulk of the zooxanthellae are kept in a thin sheet of soft tissue, called the mantle, which can be extended well beyond the edge of the shell by most species. A tridacnid shell is made to sit up facing the sun when it’s open, rather than lying flat on its side. So during the day, a tridacnid simply has to open its shell and extend its zooxanthellae-packed mantle tissue toward the sun in order to feed, and it’s this mantle tissue that we notice when we look at a tridacnid clam.

Mantle Colors

Tridacnids can produce numerous pigments, and the zooxanthellae make their own pigments, as well, making the appearance of tridacnid mantles a collaboration of sorts between the host and the hosted. The zooxanthellae make pigments used in the process of photosynthesis, like chlorophyll and peridinin, as well as a few others, and the pigments created by the clams are used primarily as sunscreens that protect both of them from excessive light (Yonge 1975). That’s not all, though, as in different situations some pigments may also serve other functions.

Photosynthetic Pigments

The primary photosynthetic pigments in the zooxanthellae absorb visible light in order to perform photosynthesis, turning solar energy into the chemical energy trapped in simple sugars. However, these pigments can absorb and use some parts of the spectrum of visible light better than others, and they don’t absorb much green or red light. That’s why zooxanthellae tend to be rather brownish, or reddish-brown in color. Their color, as we see it, is a mixture of the green and red light that is reflected by their photosynthetic pigments.

On top of those, there can be some other pigments that also help to convert some unusable (or just less useable) colors of light coming from the sun into readily usable colors like blue. For example, a specific pigment may absorb less-suitable violet light and then give off (reflect or fluoresce) blue light, which can then be used by the zooxanthellae to produce more food. Likewise, one pigment may actually absorb unusable near-UV light and emit violet light, which is then absorbed by another pigment that emits usable blue light, and so on. Thus, the net effect is that the zooxanthellae and the tridacnid host may be able to get more food out of the same quantity of light in some situations, due to the presence of these “accessory” pigments.

Non-Photosynthetic Pigments

Conversely, some non-photosynthetic pigments are produced in order to block damaging ultraviolet light. Excessive amounts of UV light isn’t just bad for people, it’s also bad for tridacnids and zooxanthellae, and when tridacnids are living in shallow tropical waters, they obviously need something to help block out the great amounts of UV light that reaches them there. So some pigments can be produced to function as natural sunscreens, which act as UV shields by reflecting and/or absorbing light in the UV part of the spectrum.

It’s likely that some pigments can also help when there is simply too much visible light, rather than only too much UV light. Although you’d think more is always better, it’s possible to have too much usable light, which can actually decrease photosynthetic rates. This occurs when there is so much light that the photosynthetic process is overloaded, and the rate of food production can actually start to go down rather than up. Thus, some pigments may actually help on bright days in shallow waters when things get out of hand.

Color Range

Unfortunately, I have yet to find any book, article, or scientific paper that covers all of the pigments that can be made, but Yonge (1975) wrote that they are mainly in the color range from blue to green and brown to yellow, which can apparently mix to create many of the other colors that we see. But there are also other colors that simply can’t be produced by combining these, so there must be some number of other pigments, as well. Of course this means I haven’t been able to find the exact functions of all the tridacnid and zooxanthellal pigments and how they work, either, but you should have the basic idea now.

Additional Notes

On another note, it’s also interesting that not all tridacnids living in really shallow water are brightly colored, while some deeper-living ones are. And there are other colorful sorts of non-tridacnid clams that live deep enough that they certainly don’t need photo-protective pigments, and don’t carry zooxanthellae either. Thus, it would seem that the roles of various clam pigments aren’t quite as straightforward as I’ve made it sound; again, these are just the basics.

Iridophores

In addition to pigments, the mantle also contains some little structures called iridophores, which are made of small groups of cells (called iridocytes). These cells contain stacks of tiny reflective platelets, and they can act as sunscreens, too (Griffiths et al. 1992). It’s these little reflectors that are also responsible for the iridescent appearance of some tridacnids and some of the colors of the mantle.

I’ll add that tridacnid mantles also contain some other things called mycosporine-like amino acids (MAAs), which are also known to be very effective, naturally produced UV sunscreens. These are also used by both tridacnids and reef corals, and a few other creatures (ex. Ishikura et al. 1997 and Dunlap & Shick 1998). Thus, the job of UV screening can fall at least partially upon the MAAs rather than just the iridophores and/or the pigments. MAAs are clear, though, and don’t have anything to do with how a clam looks.

Mantle Patterns

Regardless of exactly what all of the pigments and iridophores are doing, as best as I can tell, nobody really knows why tridacnid mantles look the way they do when it comes to putting all these colors into mantle patterns. Tridacnids can be covered by a seemingly endless array of not just colors, but patterns, and sometimes it seems like every tridacnid of a given species that I see on a dive looks different. Likewise, many tridacnids are aquacultured on “clam farms,” and if you ever visit one you’ll see that hundreds of juveniles, which you know came from the same parents and were raised in the same tanks, can all look at least a little different. Many may look similar overall, but some will look nothing like the others. They can all have different patterns, different colors, or both.

Random Patterns

Thus, at least to some to some degree, the colors and the patterns seem to be rather random, as each species has a range of colors/pigments it seems to be able to make, and then uses them in many different quantities and arrangements of spots, stripes, blotches, borders, etc. with no obvious reason why.

Rosewater (1965) thought that these differences were somehow related to the zooxanthellae, but McMichael (1974) suggested it was due to some genetic variability in clams. However, apparently neither of these ideas has been thoroughly investigated. Ellis (1998) did report that extracting zooxanthellae from colorful clams and giving it to other clams did not cause the recipients to develop the same coloration, though, and Laurent et al. (2002) looked for a genetic link to color in tridacnids, but didn’t find anything at all. Burton (1992) also reported the same, as he found no apparent genetic link to color, either.

Genetic Control

Still, it’s obvious that there’s at least some genetic control over appearance, as each tridacnid species can have some mantle patterns that are unique to that species. For example, many specimens of Tridacna maxima have a mantle pattern that is composed of small, rather teardrop-shaped spots (these are known as “teardrop maximas”), but no other species of tridacnid has this same look. There’s something about some maximas that produces this species-specific pattern at times.

Additionally, a particular pattern seen on a particular species may be more or less prevalent, or even absent, in various geographic localities. Species X from location Y may have a lot of different mantle patterns, but in a large enough population sample of them, a few will look essentially alike, and those may look unlike specimens of species X found in other areas.

Color Changes

It’s also worth noting that tridacnids may or may not get darker and/or change color as a response to long-term changes in lighting. Here’s a good example of what I mean: In a series of mantle color experiments performed by ICLARM (2002), several predominantly brown-colored aquacultured specimens of Tridacna maxima known to have brightly colored parents were collected and used in a series of tests.

Some specimens were kept in tanks under different light intensities, and some of them were kept at three different depths in protective aquaculturing cages in the sea, along with some colorful specimens of the same species, to see what effect the changes in lighting would have. Some were given zooxanthellae collected from other brightly colored specimens to see what that would do, and some were provided with extra nutrients to see what that might do.

It was found that darkness and/or color were indeed affected by the different lighting regimens in the tanks, but not consistently. Those kept under reduced lighting experienced an “intensification of baseline colors,” while the brown clams kept in the ocean cages stayed brown, the initially blue clams kept with them turned green, and the initially purple clams turned blue! Providing the brown specimens with the zooxanthellae from brightly colored specimens had no effect, and adding the extra nutrients had no effect, either. It’s all pretty hard to figure out.

Eye Spots

Oh, and there are eyes to talk about, too. The mantles of the Tridacna species also have a number of dark-colored simple eyes on their surface. In fact, a single clam may have several thousand eyes. They aren’t very fancy, and they can’t produce a real picture like our eyes do, but they do allow tridacnids to sense in which direction the sun is.

To some degree tridacnids can also detect shadows and different visible colors and UV light, and movement too. So mantles often have dark spots on them in addition to everything else, and the distribution of the eyes can be quite random at times. In some cases the eyes are found relatively widely spaced around the edge of the mantle, but in other cases they may be very tightly spaced, actually touching each other. Then again, they may also be scattered randomly all over the mantle in high numbers or low. Again, I found no real answers as to why there’s so much variability.

Camouflage

Aside from all this, it has also been suggested that not so much the colors, but rather the patterns on the mantle have something of a camouflage effect, as they can break up the outline of the exposed fleshy mantle (Knop 1996). This supposedly makes the mantle less obvious and more difficult for predators to see.

While diving I’ve found numerous relatively drably colored tridacnids with mottled mantle patterns that were pretty well camouflaged. However, I could obviously still see them anyway. Likewise, if you move too close to a tridacnid it can definitely see you coming and will usually retract its mantle into its shell as you approach, consistently giving away their own position. I’d miss many of them if they just sat still, but my eyes regularly pick up the jerking motion of the mantle much more easily than their actual mantle color or pattern.

Thus, some tridacnids are rather well hidden at a distance and may not be seen by something quite far away, but others are either very obvious and/or give their positions away themselves. So, for at least some of them, I can’t see that the color/pattern has anything to do with camouflage. Besides, it would seem to me that one pattern, or at least a few, would work better than the others and those patterns would eventually dominate the rest as the less-camouflaged clams got eaten by predators. Instead, it seems that particular colors and/or patterns may not provide any advantage or disadvantage at all.

Viewing Angle

All right, one more bit of info before we’re done. In addition to everything covered above, you should also note that the perceived color/pattern of many tridacnids’ mantle can change drastically depending on the viewing and/or lighting angle, particularly in the case of Tridacna crocea and T. maxima. In other words, if you change the angle that you view a clam from, or change the angle of the light source, or both, you will often see different colors being reflected off the mantle. Sometimes this isn’t very pronounced or doesn’t occur at all, but in some cases a specimen can look completely different when viewed from the front instead of from above.

Some specimens can also have a brilliant reflective sheen when looked at from certain angles, as well. This isn’t caused by the pigments, but is instead due to the reflective iridophores in the mantle.

Unanswered Questions

Well, that’s about all I can say on the matter. As you can see, there are still a number of unanswered questions when it comes to why tridacnids look the way they do, and I can assure you that I’ve looked in a lot of places for those answers. Some information is better than none, though, and we’ll just have to keep wondering about, and researching, the rest.

References

Burton, C. 1992. Mantle color variation and genetic diversity in Lizard Island giant clams (Tridacna gigas): http://www.uoguelph.ca/zoology/courses/ZOO4600/Copy92.pdf

Dunlap, W. C. and J. M. Shick. 1998. “Ultraviolet radiation-absorbing mycosporine-like amino acids in coral reef organisms: a biochemical and environmental perspective.” Journal of Phycology 34:418-430.

Ellis, S. 1998. Spawning and Early Larval Rearing of Giant Clams. Center for Tropical and Subtropical Aquaculture Publication 130. 55 pp.

Griffiths, D. J., H. Winsor, and T. Luongvan. 1992. “Iridophores in the mantle of giant clams.” Australian Journal of Zoology 40(3):319-326.

International Center for Living Aquatic Resources Management (ICLARM). 2002. Coastal and Marine Resources Research Program operational plan: http://www.worldfishcenter.org/operational%20plan/op2001/pdf/15-42.pdf (pg.16)

Ishikura, M., C. Kato, and T. Maruyama. 1997. “UV-absorbing substances in zooxanthellate and azooxanthellate clams.” Marine Biology 128:649-655.

Knop, D. 1996. Giant Clams: A Comprehensive Guide to the Identification and Care of Tridacnid Clams. Dahne Verlag, Ettlingen, Germany. 255 pp.

Laurent, V., S. Planes, and B. Salvat. 2002. “High variability of generic pattern in giant clam (Tridacna maxima) populations within French Polynesia.” Biological Journal of the Linnean Society 77:221-231.

McMichael, D. F. 1974. “Growth rate, population size and mantle coloration of the small giant clam Tridacna maxima (Roding), at One Tree Island, Capricorn Group, Queensland.” Proceedings of the Second International Coral Reef Symposium 1:241-254.

Rosewater, J. 1965. “The family Tridacnidae in the Indo-Pacific.” Indo-Pacific Mollusca 1:347-396.

Yonge, C. M. 1975. “Giant clams.” Scientific American 232(4):96-105.