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May 06, 2007

Reef fish show homing instinct


Clownfish on Blue, originally uploaded by Lindsay Beyerstein.

Research has shown that many species of reef fish begin life as floating larvae but return to their home reefs when they hatch.

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This is pretty cool and here’s how it works:

Where you and I have three semicircular canals in our middle ears to detect pitch, yaw, roll, and acceleration using the inertia of liquid, fish have little chambers with solid calcium carbonate chips called otoliths. As fish grow, their otoliths keep pace, adding layers like tree rings. Fish never quite stop growing, and neither do their otoliths. As otoliths’ growth rate, like the rest of the fish depends on food availability, temperature, reproductive effort, etc., which vary temporally (usually seasonally) otoliths can be used to age the fish. On a very fine scale, the center of the otolith may actually reveal daily growth. It’s not as clear-cut as the annual growth rings on a temperate zone tree, but a practiced ichthyologist can often age an otolith with considerable accuracy. (Other “aging structures” include scales, fin rays, and bones such as opercula and cleithra.)

That was as far as it went until relatively recently when it became possible to vaporize minute points on sectioned otoliths, ionize the ensuing gas, and analyze it with fancy-ass techniques like wavelength-dispersive electron microprobes, energy-dispersive electron microprobes, proton-induced X-ray emission, and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS).

This study used LA-ICPMS. - You remove an otolith from a fish’s head, grind it down till you’ve reached the center, wash it very, very thoroughly and burn microscopic dots along the resulting surface, analyzing the vapor produced along the way. The mass spectrogram part of the LA-ICPMS separates isotopes (check wikipedia if you’ve forgotten high school chemistry) of various elements that have been laid down in the matrix of the otolith. Since the inside of an otolith is metabolically inert, whatever elements (in whatever isotopic form and proportions) were sequestered there as it grew, remain unaltered. If a shad, for instance, is hatched and reared the first months of it’s life in a river with a particular ratio of selenium isotopes, its otoliths would record that fact, and years later that fish could be distinguished as an adult returning to spawn from another shad found alongside it that had strayed from another river with a different selenium isotopic ratio.

And here’s the cool part of this study. – They’ve developed a transgenerational tag. Juvenile fish can be tagged in a variety of ways, some quite ingenious and sophisticated (Check out the tour at PTAGIS for an example of fish tagging on a truly epic scale, using the same kind of tag the vet put in your dog), but none until now that permitted the fish to be tagged before they were even laid as eggs. These guys have tagged fish by injecting their mothers with a non-toxic, non-radioactive, isotope of barium. The barium is laid down in the eggs and remains in the center of the otoliths of the fish that hatch from those eggs for the rest of their lives. When the adults are caught their otoliths can be identified with the barium isotope injected into their mothers. The technique will permit tracking fish in all sorts of situations that were impossible before.

The paper is published in Science, and unavailable without a subscription or fee. A short paper describing some of the preliminary work is available here though. (Warning: pdf)

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