Swimming, swarming, spawning polychaete worms

Head of tiny polychaete worm

Late in the evening around 10:00 PM on March 9, 2002, excitement had broken out in the sea urchin tank in Augsburg’s general biology lab. In the darkness of the night, a multitude of tiny creatures were swarming in the tank. At first it was difficult to tell what these small swimming creatures were. However, an overhead projector was nearby, and strong side illumination of the tank using the projector gave a clear view, revealing their identity. The small creatures were in fact small worms a centimeter or so in length, swimming about and presumably spawning.

These worms normally stay hidden in the sand and rocks, though they are occasionally seen crawling about on the walls of the aquarium, especially at night. This particular evening though the worms were all over the place, zipping about, and attracted to the light like insects to a porch light on a summer night. The next several evenings not a single swimming worm could be found, so it appears the night of March 9 was indeed somewhat special.

Polychaeta Swimming

Though members of the same phylum (Annelida, the segmented worms) as the earthworms in your backyard, these tiny marine worms are quite unlike anything you are likely to find in your yard. While earthworms and most of the other worms on land and in freshwater are oligochaete worms, most species of marine worms (including the little swarmers in our aquarium) are polychaete worms (Note: the primarily freshwater leeches belong to yet a third common class of annelid worms).”Oligo” means few, and oligochaete means “few setae” (few bristles). “Poly” means many, so polychaete means “many setae”. Indeed, as can be seen in the photo below, the little swarming worms are equipped with abundant bristles, much more so than the earthworms in your garden. Many polychaete worms also have much more distinct heads (often with well-developed jaws) than oligochaete worms, and many have conspicuous leg-like appendages called parapodia (also visible below…the setae are attached to the parapodia).

Closeup of one of the swimming worms. We don’t know the identity of this particular species, though it appears to have arrived in our aquaria along with a shipment of sea urchins from Florida. Several similar worms were present among the detritus in the urchin shipping water.

Most marine invertebrates (animals without backbones) and marine fish have planktonic larval stages that typically look very little like the adults. These planktonic larvae drift in the water currents for days, weeks, or months (depending on the species), feeding on planktonic algae or other planktonic organisms until they eventually undergo a metamorphosis to the adult body form. They then settle out of the plankton into suitable habitats (the few lucky survivors that did not get eaten or carried to unsuitable locations do anyway), where they take up a lifestyle more similar to the adults. To give their offspring a good start in life and help them to be better swept away in the water currents, many marine invertebrates and fish either climb to the highest spot in the area or swim to the water surface (as is the case with these small swimming worms in our aquarium) before releasing eggs or larvae.

Quite a few species of polychaetes engage in this sort of spawning behavior, and at certain times of the year swarms of spawning polychaete worms can be quite conspicuous at night in coastal areas. As is the case with many marine creatures, spawning is often coordinated with lunar cycles. In the Florida Keys and the Caribbean some polychaete worms are even bioluminescent! These worms, known as bioluminescent threadworms, can be quite conspicuous in nearshore waters in the Spring. Just after sunset, females of these worms swim to the surface where they flash a bright green light that attracts the males.

A diagram of an epitoke of a polychaete worm from the Biodidac collection at http://biodidac.bio.uottawa.ca/.
(BIODIDAC is a bank of digital resources for teaching biology)

As might be imagined, swimming about for the purpose of spawning subjects polychaete worms to increased danger from predators, but some species minimize this risk by producing a tail region that breaks loose and swims, carrying the gametes to the surface. These dismembered swimming worm tails are called epitokes, and they allow the worm itself to remain safely hidden while its sperm or eggs are carried up into the water column for fertilization and dispersal on the water currents to new sites .

Polychaete worms of many shapes, sizes, and lifestyles are extremely abundant and diverse in marine (saltwater) habitats. Some are free-living like the ones discussed here, while others such as the feather duster worms (which live in tubes that they construct) live much more sedentary lifestyles. Many sorts can be found in reef aquaria such as Augsburg’s coral reef aquaria, and many more types can be found in the wild.