Clams Adrift on Ancient Currents

Here's a shot from the Shedd Aquarium in Chicago:

Giant Clam (Tridacna gigas), Shedd Aquarium, Cook Co, IL 1/20/2013

This is a Giant Clam (Tridacna gigas). They're normally found in the Indo-Pacific region, but are decreasing due to over-harvest for food and shells.

Since they're sessile as adults, it isn't surprising that they practice external fertilization. Their offspring typically float for approximately 2 weeks, if they can then find a suitable bottom to settle onto and start growing. As they grow, they capture algae known as zooxanthellae, which provide food through photosynthesis, and thereby increase the clam's growth rate considerably. (1)

Given the sort of broadcast spawning that this species and it's relatives practice, we should expect populations to show fairly little geographic variation in genetic structure, and indeed this is the case, with the entire 600 mile length of the Great Barrier Reef showing no such variation. (2) On the other hand, long-distance dispersal is probably fairly rare, so it's probably not surprising that widely separated populations show more genetic differentiation, at least in the related T. derasa. (3)

What was surprising is that in both this species (4) and the closely related T. maxima, (5) genetic dispersal events appear not to have followed modern surface currents! Clam larvae are definitely planktonic, so they're not swimming against currents in any meaningful sense, so unless they're somehow using deep-water currents, we're seeing the results of rather old events. Of course, Giant Clams can live over 100 years, and successful recruitment of such a long-lived species is typically quite low, so I guess it shouldn't be surprising that such echoes of the past are still visible.

Biologists are trained to consider the history of the organisms they're working on. Geologists are trained to consider the history of the Earth that those organisms live on. Mixing the two can't help but give us insights into this world we're all drifting through space on.

Jameson, S. C. (1976). Early life history of the giant clams Tridacna crocea Lamarck, Tridacna maxima (Roding) and Hippopus hippopus (Linnaeus). Pacific Science, 30(3), 219-233. 

Benzie, J. A. H., & Williams, S. T. (1992). No genetic differentiation of giant clam (Tridacna gigas) populations in the Great Barrier Reef, Australia. Marine Biology, 113(3), 373-377.

Macaranas, J. M., Ablan, C. A., Pante, M. J., Benzie, J. A. H., & Williams, S. T. (1992). Genetic structure of giant clam (Tridacna derasa) populations from reefs in the Indo-Pacific. Marine Biology, 113(2), 231-238.

Benzie, J. A. H., & Williams, S. T. (1995). Gene flow among giant clam (Tridacna gigas) populations in Pacific does not parallel ocean circulation.Marine Biology, 123(4), 781-787.

Benzie, J. A., & Williams, S. T. (1997). Genetic structure of giant clam (Tridacna maxima) populations in the West Pacific is not consistent with dispersal by present-day ocean currents. Evolution, 768-783.