by Joseph DeSisto
If I told you that worms can be gardeners, your first thought would surely be of the earthworms. It’s true, earthworms of the order Megadrilacea are extremely important to life on land, and in many cases are responsible for maintaining the soil structure necessary to support plants. A gardener would be wise to appreciate them.
Earthworms, however, are just one lineage in the highly diverse phylum Annelida, the segmented worms. There are more than 17,000 known species of annelids, with many more yet to be discovered, and most of these are marine. Among them are several species of worms that are not merely found in gardens — they are gardeners of their own food, namely algae.
The worms I am talking about belong to the large and diverse order Phyllodocida, and are commonly known as ragworms or clam worms. Ragworms look more like long centipedes than earthworms, but their “legs” are actually feathery gills with which they extract oxygen from seawater. They
Many ragworms construct burrows or tubes in which to live, and these burrows can be important in providing an environmentally stable shelter. This is crucial for those worms which live in the intertidal zone, where changes in temperature, moisture, and exposure can be violent. Many worms decorate their tubes with pebbles, fragments of shells, and other debris; the reason for this house-warming behavior is unclear (Berke and Woodin 2008).
Tubes, which are like burrows but have a hard shell, also provide a “home base” from which the ragworms can gather food. Many ragworms are predators; those that make burrows ambush their prey by darting out at small invertebrates that come too close. Other worms are scavengers, and grab bits of detritus that float by. Still others are at least partially herbivorous, and go one step further: they grab floating pieces of algae, attach them to the entrance of the tube, and construct a garden. These worms, however, are not mere gardeners, they are also ecosystem engineers — in some areas, gardening by ragworms can have major effects on their habitats (Shain 2009).
How does this happen? Woodin (1977) studied algal gardening in ragworms and found that the worms attached floating algae, especially sea lettuce (genus Ulva), to their tubes. This was already known, but Woodin also took detailed measurements of the micro-habitat created by the algae-covered burrows. She found that the presence of algae kept the burrows cool and wet during low tide, and so helped prevent the worms from overheating or drying out. So the worms were tending the algae not just for food, but to regulate the surrounding environment.
The algae benefit from this arrangement. Although the worms ate portions of the algae, they left enough for the plants to grow and even reproduce. Woodin also found that among algae that were attached to a substrate in the tidal flat, more than 80% had been attached by gardening worms (in particular, the species Nereis vexillosa and Platynereis bicanaliculata). Since attachment to a hard substrate, such as a worm’s tube, allows the algae to reproduce, it can be said that the ragworms are not merely gatherers but true gardeners that cultivate and harvest their crop. Without encountering a worm, a floating piece of algae is likely to wind up stranded on the shore.
Having plenty of attached algae provides benefits to other animals as well. Because the algae grows outside of the burrow, algal gardens provide a consistent source of food for herbivores that do not burrow. The algae also add sediment to the tidal flat, providing food and habitat for detritivores and scavengers. This in turn provides both food and cover for predators, including other species of ragworms. When the tide goes out, herbivores and carnivores alike benefit from the cover of algae to prevent them from overheating or drying out in the sun.
Algal gardening is one of the few known examples of true gardening in the natural world. When Woodin conducted his studies, only two species were known to participate in this behavior, but we now know that members of at least three families from several continents can be gardeners (Cosentino et al. 2014). Although the ragworms and their relatives within the Phyllodocida are fairly well-studied with respect to their ecology, algal gardening remains a relatively understudied phenomenon. Hopefully this will change as more people recognize and appreciate the complexity and diversity of these beautiful animals.
Berke, S.K. and S.A. Woodin. 2008. Tube decoration may not be cryptic for Diopatra cuprea (Polychaeta: Onuphidae). Biological Bulletin 214(1): 50-56.
Cosentino, A., G. Cantone, and S. Giacobbe. 2014. Neanthes rubicunda (Polychaeta: Nereididae) in the current-swept slope of the Strait of Messina (Mediterranean Sea). Marine Biodiversity Records 7: published online.
Díaz-Castañeda, V. and D.J. Reish. 2009. Polychaetes in Environmental Studies. In D. Shain (Ed.), Annelids in Modern Biology (pp. 205-227). Hoboken, New Jersey: Wiley-Blackwell.
Woodin, S.A. 1977. Algal “gardening” behavior by nereid polychaetes: Effects on soft-bottom community structure. Marine Biology 44: 39-42.