Saturday, 16 December 2006

Killer bivalves

Popular fiction has it that a giant clam (Tridacna gigas) can close its shells so quickly that it can trap a diver. Bad news indeed for the unfortunate with a foot in the clam. The bivalve won't let go while a big wedge of rubber fin is irritating it. And sooner or later the air will run out. Or a tiger shark will spot the easy meal.

It's a dramatic image—but not an accurate one. Tridacna clams, like most other bivalves, feed by filtering suspended material from the water. They supplement their diet with nutrients supplied directly by symbiotic micro-organisms (zooxanthellae) in the tissue of the mantle. (It's a similar relationship to that between reef-building corals and zooxanthellae.)

But not all bivalves are filter-feeders. Some vacuum up detritus from the mud or sand. And a few prey on other animals.

Most filter-feeding infaunal bivalves (those that live in sediments) extend muscular siphons up into the water so they can draw in fresh water and suspended food and eject waste material. Water entering through the inhalant siphon passes over the gills, which do double duty as respiratory organs and as particle sorters. Any food particles that are too big are chucked out, as are those that are too small. But particles that are just right are sent to the mouth.

Carnivorous bivalves feed by either sucking in prey or scooping them up with the inhalant siphon. Parilimyidae and Cuspidariidae have long, tubular, flexible siphons that can be moved around to hoover up passing crustaceans. More sinister are the Poromyidae and Verticordiidae, in which the siphon is expanded into a 'cowl' that encloses prey.

Once inside the siphon, muscular structures called labial palps direct the hapless victim into the mouth. Although carnivorous bivalves lack teeth, the oesophagus and stomach are heavily muscled and can easily crush small crustaceans. Digestive enzymes do the rest.

None of these carnivorous bivalves is large. Most species don't reach 3 cm. But wouldn't it be fantastic if they were the size of Tridacna? Then we might have something to worry about.

13 comments:

Ex-student said...

You just get all this stuff from books. What do you actually know?

Darky said...

From that Poromya reference: Poromya also possesses red amoebocytes in the blood stream, which seem to carry a hemoglobin pigment. This and the several modifications described above point to a degree of evolutionary development that few other bivalves have achieved in adapting to new niches.

Cue Twilight Zone theme. They appear to be gathering forces for the appropriate muscle fibre development, ay.
Don't take your eye off them. A person could look away for, say, about another 3572 years and they could find their toe getting hoovered.

Seriously a good essay, teach.
I've just finished reading the latest dose of Dawkins, R and can't get my mind off questions of scale and perception. It is a wrestle that I love.
I hear that he and Lalla Ward have recorded The God Delusion as a kind of tag-team. It's on my Christmas list for one of the fraudband-enabled issue to hoover up from usenet for me if the publishers don't look smart about releasing it here.

Snail said...

I never used to pay much attention to bivalves, you know, but I'm developing a fondness for them in my dotage. Now that I'm no longer working in a group of terrestrial ecologists (not that there's anything wrong with that) and am surrounded by microbiologists and molecular geneticists, I'm starting to look at some of these previously ignored animals in a different light. (Ignored by me, that is. Others have been much less dismissive!) We're working up our enthusiasm to look at the seagrass-dwelling Solemya, which laughs in the face of deadly sulphur compounds. Perfect stuff for the micro and gel jocks.

I must blog about the carnivorous chiton from the western coast of North America. Not that I can remember the name at the moment. No doubt it'll come to me in the middle of the night. Anyway, I saw a bunch of them on display at the Monterey Bay Aquarium years ago. For some reason, I'd pictured them as being rather big ... It is all about scale and perception, isn't it?

Ooh, I like the idea of Richard D and Lalla Ward reading The God Delusion. Ideal stuff for the festive season.

Snail said...

What do you actually know?

Well, there are known knowns and ...

jj said...

... and then you could visit Kangaroo Island and do a study tour there and then we could all be ex-studenta.
:)

jj said...

Kangaroo Islands for chitons, I was meaning.
:)

Snail said...

KI must be crawling with chitons. None carnivorous as far as I know. All grazers.

Strange things, chitons. They have magnetite-impregnated teeth, no eyes and light detectors in the shell.

Duncan said...

Gee Snail, my old Ion Idriess books have suddenly lost some of their appeal! ;-)

Snail said...

You'll have to stick to his inland books!

AYDIN ÖRSTAN said...

I once had an encounter with a Tridacna:
http://snailstales.blogspot.com/2005/10/biggest-of-them-all-tridacna-gigas.html

Snail said...

That's a helluva big shell!

I think AIMS (Australian Institute of Marine Science) and James Cook University in Townsville had a breeding program for Tridacna. Something else for me to look up.

Darky said...

aörstan,

I get depressed thinking about all the stuff that museums don't have at the front of house for us proles.

Tridacna shells were a popular water dish in the backyards of the southern suburbs of Sydney when I stayed there about 35 years ago. I wonder whether their value has changed enough for people to be so casual about that kind of thing these days.

I liked the writer of the UMM article

Despite their classic movie depictions as "killer clams," there are no authentic cases of people being trapped and drowned by giant clams. Tridacnids are actually quite lethargic and slow about closing. Tridacnid-associated injuries are quite common however. They typically involve hernias, back injuries, and smashed toes induced when people lift adult clams out of the water unaware of their formidable weight in air.
my italics.

Tirac said...

Awwwww :( Giant clams don't actually eat people? My childhood has been ruined.