(Does that sound like the title of a Krista Tippett show?). I’m on the move today, and posting will be light, which means that if I report on science or natural history, I won’t have time to read the original papers. But this finding is cool anyway, and there’s a video, so I’ll use a secondary source. In fact, checking the Nat. Geo. article and the Wikipedia piece on the clam, I don’t think this phenomenon has yet been published in a reviewed journal (it’s been presented at a meeting).
National Geographic reports on the phenomenon of “disco clams” off Indonesia. The species (Ctenoides ales) is also called the “electric clam” and the “electric flame scallop,” for it has a habit that (as far as I know) is unique among bivalves: it produces flashing lights. You can see that in this video:
Why the light show? First of all, it’s not really a light show: the clams are not bioluminescent. Rather, they have a cute trick to make it look as though they’re flashing. From Wikipedia:
Research by graduate student Lindsey Dougherty showed that the apparent flashing-light display of this clam is not a bioluminescence phenomenon, but is instead coming from reflection of the ambient light (sun or diving light). A staff member of the Lembeh Resort in Indonesia, where Dougherty was working with Dimpy Jacobs in August 2013, wrote, “The clams have a highly reflective tissue on the very outer edge of their mantle that is exposed and then hidden very quickly, so the change back and forth from the white reflective tissue to the red tissue creates the appearance of flashing”.
Dougherty went on to discover that the brightly reflective edge of the mantle of these clams contains nano-spheres made of silica, which are very reflective.
So why do they flash? Presumably not to attract John Travolta, but there are three other hypotheses:
1. Attracting a mate
2. Attracting prey (plankton in this case)
3. Warning predators to stay away (what biologist call an aposematic trait)
#3 would only work, of course, if the clam was either toxic or distasteful to predators. In fact, that turns out to be the case, but read about the research supporting #3 as the answer on the National Geographic site.
In other words, the trait evolved for Staying Alive:
h/t: M(C)squared
Are you aware of the joke whose punch line is “I left my harp in Sam Clam’s disco”?
That joke is the only memorable thing I ever heard from a priest at mass.
No, but I think I just read the best part of it. 😀
Or…
4. Relaxing (it feels good to jiggle some silica)
5. It’s funny and pretty cool (clams are know for their sense of humour and fashion sense)
6. To attract a human or a curious squid
7. Communication (jokes, how their day has been, sharing ideas about why the water is tasting funny, feelings or whether they saw any interesting sea life recently)
Do these things always have to be about surviving and mating? Has anything evolved just for the fun of it (e.g. relatively external testicles)?
8. An ad for neighboring seafood joints (no creature has yet to evolve an “air dancer”, Statue of Liberty costume or “sign spinner” to my knowledge).
Do you mean one of those cleaning stations? Where big fish and sharks come to get massages and have their teeth cleaned by the little fish?
What’s in it for the clam?
FILL UP ON THE FISH NOT ME
Come for the lights, stay for the communication. That “clam up” idiomatic expression is clearly false.
Is this now going to be the Scientology claim.
My first guess was iridescence with waves of cilia. In any case very interesting.
10,000 pun points for the Stayin’ Alive joke.
At first I thought the joke was that Travolta was displaying a sexual aposematic trait. Any person displaying such a move would keep em away!
Couldn’t some clams have reflective tissue without being toxic or distateful? Some may have it as an aposematic character couldn’t they?
The flashing is like those wiggling adds on the side bar. To prey fish they may be irresistible. I’d go with #2, attracting dinner.
Aren’t clams filter feeders?
Right. It wouldn’t be fish. Do plankton take an interest in flashy come-ons?
I think that at least some zooplankton do. That’s IIRC supposed to be the reason that the megamouth shark has bioluminescent lips. But that would require the clam to eat zooplankton instead of phytoplankton, and I don’t know if they do that.
Mother of pearl, that’s an interesting clam. I swear The Electric Clam was the name of a beachside dance club in So Cal back in the 70’s, but that must be a Brian Williams on my part – there were so many clubs that were “Electric” this or that.
Is there such a thing as an incidental trait? That is, in this case hypothetically, the reflective silica is just a property the way the pearlescence inside a shell is, and the dappled light of the tide pool just happens to yield that particular effect for no reason? It seems like an expensive and rare trait so probably not. I wonder how that looks exactly to a potential predator: given the diminished and blue-shifted light under the sea, it might be even more alarming to clam-loving fauna.
The term of art is “spandrel”.
“Term of art” sounds right.
♫♪ You light up my life ♪♫
arrrrrgh
groooovy
Here is my long comment.
The normal way to test for distastefulness is to feed flesh or an entire test animal to a range of potential predators. This is easy to do but was not done by Ms. Dougherty. The main evidence she gives (in the links) for unpalatability is that clams seem excited and flash more when predator presence is simulated (i.e., the clam is disturbed) and that clams contain (gasp) sulfur. I am unaware of any chemically protected bivalves (other than from toxic wastes or ‘red tide’), but then again, the ocean is big.
I assume that the clam is attached to the substrate and, like others of their kind, ejects its gametes into the ocean to be fertilized. It difficult to imagine, with this degree of promiscuity, how mate attraction could be relevant. Dougherty’s answer also carries weight: clams can’t see good enough. As I learned it, clams use cilia-generated currents to bring in and filter suspended micro-organisms. In a sloshy ocean, light lures are not likely to provide much additional clam food. As I suggest for unpalatability, experiments, in this case on clam feeding rates in natural light and in the dark, are called for. If I’m not mistaken, these clams do not consume zooplankton.
One may be led astray by the bright red color of clam, for red is commonly a sign of aposematic (warning) coloration, but in the normal habitat of ‘Ctenoides’, 10-20 m underwater, most light is blue and a red clam will appear gray. The long mantel tentacles are probably sensory and may permit the clam to touch and detect approaching predators and snap shut (and maybe shoot away) before danger gets too close.
An additional hypothesis (to be tested) for flashing is that the iridescent lines disrupt the clams outline (gestalt) and makes it more difficult for a predator to recognize a clam as prey. The flashing could also act as a “pursuit inhibition” display that helps local (why do I say “local”?) predators that have unsuccessfully attempted to bite a clam to learn to not waste time on prey that are faster than they are. (The clam may benefit by not having to interrupt its feeding so often). It is also possible that under reduced lighting, the flashing movement draws predator attention away from the tentacles and thus becomes more likely to bump into one before it can sneak up on the clam.
Let’s see, that’s three additional hypotheses missed by the marine biologists. Finally, it may be that the silica inclusions that are the source of the flashing have evolved for some other function; the flashing may be just a non-adaptive by product of their physical arrangement. After all, despite the apparent design, what is the function of a rainbow?
On this last point, although the video is impressive, I suspect there has been cherry picking in presenting the most impressive scenes. After all, the light is reflected/refracted in specific directions, and we are not dealing with multi-directional bioluminescence. Would, I ask, the phenomenon be so striking if the ‘unsuccessful’ takes were put in the video? Dougherty in the National Geographic article is quoted to say, “Most animals don’t do something that’s energetically costly unless there’s [a payoff].” Although this is both true and a good guide for recognizing evolved adaptations, the article presents no evidence that ‘Ctenoides’ flashing is metabolically ‘costly’ to fitness. Although I am a hyper-adaptationist, and the present case is intriguing, more convincing evidence for adaptation, and for the nature of the adaptation, is sorely needed.
That was a most worth-reading long comment. Thanks.
I’m not sure what you mean – that the clams aren’t really that showy? They’re common in the pet trade, and that’s exactly what the flashing looks like. They do it pretty much constantly, and it’s dramatic and visible from all directions. Incidentally, it’s long been widely known that the flashing wasn’t bioluminescent, since they’ve been in the pet trade for decades.
Points well taken. I was unaware that the clams were part of the aquarium trade. The fact of their availability makes it all the more surprising that so little is known and so few experiments conducted on adaptive significance of flashing, if such exists. I stand corrected on my doubts that flashing would be conspicuously visible from all sides. It would be interesting to see the visual effect using natural illumination and different angles.
I think silica spheres are quite good retroreflectors, like cats eyes. If you’re viewing a clam with the light behind you, you’d see the full-strength flash.
The interesting thing (well, the other interesting thing) is, why they don’t ‘shine’ all the time, that is, why do they turn ‘off’ in between ‘flashes’?
sub
Watching that just sparked some ideas for some really funky labiaplasty treatments
Silica spheres? Exactly like those reflectorised road signs that blind you at night…
LOL, “not to attract John Travolta!”
I’ve seen disco clams while diving in Southeast Asia. I always have an urge to search for the electrical cord–it just doesn’t look like something a biological thing should be able to do.