The Marine Worms

Who are the polychaetes?  You are probably quite familiar with the earthworm.  It is a member of the phylum Annelida; collectively, the species in this group are called the "segmented worms."  The segmented worms occur worldwide, from polar regions to the Tropics.   In size, they range from less than a millimeter to greater than a meter in length.  The group includes many close relatives to our garden variety earthworm, and even some more notorious worms -- the leeches.  Many species of segmented worms are found in terrestrial and freshwater ecosystems.  Mesenchytraeus provides an example from a more extreme environment; they live in glacial ice and get into trouble if exposed to temperatures much higher than 0° C.  However, the vast majority of the diversity in the phylum Annelida, about 8,000 species, are ocean-dwelling (marine) forms called polychaetes.

The king rag worm, Alitta virens.

Polychaete worms occur throughout the world ocean.  Some species can be found in the water column, but most of the worms are benthic, meaning they are associated with the sea floor. Their life history often includes a larval form that exists in the water column for a time and then settles to the bottom.

I selected the polychaetes for mention here because the average person probably associates the word "worm" with a bland, slimy creature that creeps its elongate body quietly through the Earth beneath our feet.  Therefore, they are out of sight and out of mind, and that's all the better for us.

The reality is that the "worm" is a catchall term that applies to a number of wildly different and highly successful groups of animals.  Additionally, many of the species exhibit beautiful forms (in the eye of the beholder) and unique features.

Closeup of the king rag worm.

They vary considerably in their feeding ecology.   Most polychaete worms are free living, but some are parasitic, even invading other worms.  You will be happy to know that none of them infect humans; you need not worry about being colonized the next time you hit the surf.

Scanning electron microscope image of a scale worm's mouth.

Many polychaetes, like the earthworm, are deposit feeders.  As they move through sediments, the sediments move through them, and the worms digest any organic material among the grains.  The king rag worm is one example, working its way through mudflats and achieving lengths greater than a meter.

Whale-bone worms, burrowed into the bone of a dead whale.

Many other species are predatory and bear jaws.  In some species the jaws are eversible, and snap at everything in their path. Check out this bobbit worm clip, linked here and inserted at the bottom of this post.  The BBS series The Blue Planet also includes footage of a polychaete crawling through a sponge in search of a meal, snapping its jaws at the small crustaceans within.  To the naked eye, their feeding structures do not seem terribly daunting, but examination of these worms with a light microscope or scanning electron microscope reveal a more horrific, alien world.

If you want an even stranger example, some polychaete worms specialize in dining on the bones of dead marine mammals.  The worms burrow into the bones of whales and other vertebrates that have fallen to the sea floor, where they feast and play an important role in the process of decomposition.

A sabellid worm growing among a coral colony.

Suspension feeding is a strategy for some species.  This is practiced by the sabellid fan worms, for instance, which live in a tube of their own construction.  (This in itself is accomplished in an interesting way.) Then, the worms extend their elaborate feeding structure into the water column to catch the unlucky passersby.

A Christmas tree worm, the body of which is within a burrow in the coral.

Some polychaetes make there homes in the structures produced by other organisms.  Christmas tree worms, for example, live in coral colonies and add small touches of color to the reefs.  These worms, as well as the fan worms mentioned above, are capable of detecting differences in light and can quickly retract their feeding structures.  They're clearly the inspiration behind some of the "plant" life in James Cameron's movie Avatar.

Of course, there is more to an animal than the manner by which it obtains food resources.  The path to reproduction in polychaetes can be a strange one too.

Large aggregation of Christmas tree worms, differing in color.

Unlike flatworms, most polychaete worms have separate sexes, and, in most cases, fertilization occurs externally.  Some species are asexual and reproduce by budding.  Myrianida, for example, is a worm that at times appears to have a tail, only the tail actually consists of new worms that are produced asexually by the parent.  There is a bit more to this story, of course, so I encourage you to look into it!  The Palolo worm is another interesting example, where the rear end of the animal consists of segments packed with sperm and egg (called epitokes) that rise to the sea surface where the gametes are released.  These nutrient-rich segments are actually collected for food in some regions, yum!

Myrianida pachycera and its tail of worms.

Tomopteris, a deep-sea swimming, bioluminescent species.

As is the case with other sexually reproducing species, following fertilization of the egg by a sperm cell, the egg soon divides to produce new individuals of many hundreds of cells.  Many marine invertebrates exhibit a life cycle that includes a free-living, swimming larval form.  Many species of polychaetes produce larvae called trochophores, some of which are non-feeding larvae that continue to develop from the nutrients remaining from the egg's yolk.  I could carry on with images and descriptions of reproductive modes among the polychaetes and the diversity of bizarre larval forms.

Plenty of other species would have been great candidates for this page as well (e.g., the parchment worm Chaetopterus).  While you can investigate this on your own, taking a peek at what is known about the thousands of species have been described, keep in mind that many species await discovery.  On a regular basis, we make new finds everywhere from the muddy floor of a local coastal bay to dark, deep-sea waters that remain largely unexplored.  We might see a doubling of the known species diversity in this group in the coming decades.  This is not too surprising.

Our technology is better, giving us a better reach into new environments.  Still, more goes into the discovery of new species than simply being in the right place.  You must have someone with knowledge of the group and who can recognize the form as unique.  On top of that, the person must have some level of excitement about the find and the willingness to spend time doing the work of formally describing it to the scientific community.  But how in the world does someone develop an interest in worms in the first place, especially when they are just slimy garden creatures that hide in the soil?