"Every seashell has a story to tell if you're listening,
But underneath every shell there's a story as well if you've heard enough of the sea.
Then everything on the top will just suddenly stop seeming interesting,
So listen now to the sound of the things that are found under the ground."
(Metal Detector, They Might Be Giants)
It's easy to overlook activities that take place beneath the surface. Out of sight, out of mind, as the saying goes. Sub-surface processes become even easier to miss when they are also hidden beneath the sea. Yet oceans cover much of the planet, have done so throughout the history of life on Earth, and dominate the environments preserved in the rock record. Investigating the processes that take place in marine sediments is critical, therefore, to our understanding of how the world works, and how it worked in the past.
Bioturbation is one of the most important elements of this system, as burrowing organisms take surface nutrients down into the sediment, bring buried nutrients to the surface, and generate new infaunal habitats. But though all marine bioturbators are important, some are more important than others. Those species that produce, maintain and ventilate complex burrow systems in high densities, and which occur over large areas for long periods of time, can be the driving force behind entire ecosystems. These taxa are known as allogenic ecosystem engineers, and using laboratory aquaria, we studied the burrowing behaviour of one such organism, the ragworm (or clam worm) Alitta virens:
Herringshaw, L.G., Sherwood, O.A., McIlroy, D., 2010. Ecosystem engineering by bioturbating polychaetes in event bed microcosms. Palaios 25, 46-58. (http://palaios.geoscienceworld.org/cgi/content/full/25/1/46)
Alitta virens is common in shallow marine environments throughout the northern hemisphere, occurring in dense populations that can persist for many generations, over hundreds or possibly thousands of years. Depending on availability of food sources, they can be deposit feeders, scavengers, filter feeders, or carnivores, and this feeding flexibility can be reflected in the types of burrows they produce. During the course of our study we found evidence that, where organic detritus was present, Alitta would take up gardening, taking this material down into its burrow for cultivation. If no organic detritus was available, however, Alitta utilized a self-made mucus net, with which it trapped particles from the overlying seawater, and, in one case, even turned to cannibalism.
From our study, we were also able to show that a single, behaviourally diverse species can produce a variety of different burrow morphologies, the bioturbation type varying in settings with different sedimentary conditions, or changing over time in the same location. In the rock record, diversity of burrow morphology is often used as a proxy for ecological diversity, but our findings highlight the potential pitfalls of such interpretations.
As early migrants into new environments, tolerant of fluctuating conditions, opportunistic polychaetes such as Alitta modify their surroundings to improve their own chances of survival. In doing so, however, they create niches and nutrient fluxes that other organisms can exploit, leading to the development of new ecosystems. They have probably been responsible for such ecosystem engineering for many millions of years.
Perhaps most exciting of all, though, our worms made it onto the Palaios front cover!