In Search of the Optimal Scumsucking Bottomfeeder
The Worm Turns
This long series of computer (and Lego) experiments, extending over almost 35 years, leads to reasonable conjectures about how primitive burrowing invertebrates could have created the intricate patterns seen in ichnofossils. But the question of why the worms evolved these peculiar habits is harder to resolve. The assumption has always been that they were optimizing their feeding strategy, maximizing the input of food while minimizing the effort to acquire it. This is still the leading hypothesis, but perhaps there is room for doubt.
One reason for skepticism is that many other species live by grazing or foraging, but none (as far as I know) organize their feeding with such compulsive geometric precision. Sheep and cattle do not crop a pasture in neat boustrophedonic rows. Of course there are many differences between a cow and a worm, most notably the worm's greater difficulty in seeing and moving. Still, similar obstacles confront other organisms, such as bark beetles, which must chew their way through wood. The beetles tend to avoid crossing their own path, but their galleries are usually stellate rather than spiralling or meandering.
Another question is why so many ichnofossil burrows are planar. Other burrowing animals range through a three-dimensional medium. Perhaps the worms were chasing prey that lived at some specific horizon within the sediment.
A related mystery is how the worm got to the middle of a spiral. Most simulations do not address this issue: The program merely puts the worm at an arbitrary starting point. A real worm cannot simply materialize in this magical way. If it is strictly confined to two-dimensional life, then it must cross its own path as it spirals outward. An alternative is to tunnel above or below the spiral-to-be to reach the center—but if the worm can pop into the third dimension whenever it wants to, the carefully crafted planar trails are not needed at all.
Finally, even if self-avoiding paths are advantageous to a forager, why do they have to be compact paths? After all, straight lines are also self-avoiding. Staying in one place and cleaning your plate seems tidy and well-mannered, but does such fussiness actually improve foraging efficiency?
Authors differ in their answers to this question. Some cite a patchy food distribution, suggesting that it's better to consume everything in one patch before going in search of another. Other workers argue that the deep sea is the most uniform habitat on earth, with little patchiness in the distribution of nutrients. To account for the compact fossil trails they cite the effect of competition from other worms. Although a straight trail will never cross itself, in a densely populated region many trails will cross one another. In other words, the worm's aim is not just to avoid itself but to avoid everyone else as well.
Whether the motivating factor is patchiness or competition, it remains to be shown that outward spirals and meanders are the best way to achieve the objective. For an animal foraging within a patch of seafloor—just as for a homeowner mowing a patch of lawn—spiralling in from the perimeter seems like an attractive alternative. An inward spiral might also be a useful tactic against competitors, since the initial outermost loop would effectively fence off a territory for private exploitation. I have tried to explore these possibilities in a few simulations of my own, but the results are inconclusive. The efficiency of spiralling into a patch depends critically on the shape of the patch; nonconvex shapes are troublesome. In the absence of patches, the algorithm for inward spirals has a hard time getting started: How does the worm decide how large a spiral to draw?
My personal guess is that the answers to all these questions will come not from computer simulations but from further examination of the fossil material itself. In the meantime it seems worth considering the possibility that foraging efficiency is simply not what these structures are all about. There is precedent for such a change of view. A living worm, Paraonis fulgens, makes spiral burrows that look much like some trace fossils. But Paraonis is not a forager; its burrows work like a spider's web in the sand to catch small organisms. And Seilacher has proposed that some deep-sea fossil traces are structures built not by foragers but by farmers: The tunnels were used to grow fungi, much as some ants do today.
When fossils such as Nereites and Helminthoida were first discovered, most of them were classified as plants. The sinuous paths were thought to be stems or fronds. Indeed, there was a whole phantom taxonomic group, the Fucoids, made up of plant species that are now understood to be animal trails. Is it possible that ichnology will undergo another such interpretive upheaval?
© Brian Hayes