Animal Form and Function 3: Annelids and Nematodes

After the tour of seashore beauties in the previous chapter, the annelids and nematodes may seem dull.  The very limited array of annelids that most people encounter–mostly just the one–certainly don’t help that impression, and nematodes all look more-or-less alike.  Like any other animal phylum, though, the annelids have a few surprises for us.

For starters, not all earthworms are good old Lumbricus terrestris.  There are also Microchaetus rappi from Africa and Megascolides australis from Australia, visited below by David Attenborough.

The African worm is about six times that size but features in entirely too few videos.  Australia is also home to Terriswalkeris terraereginae, which grows to about one meter in size, glows in the dark, and is bright blue.

And in New Zealand, a short ways away, another giant earthworm glows so brightly that one can, the web sites say, read by wormlight.
A very different sort of spectacle comes from the earthworms’ aquatic kin.  Earthworms are part of a clade, Clitellata, that also includes leeches that my students examine in detail in class and a variety of freshwater worms with obvious chitinous setae.  The best known of these are the tubifex worms, a species whose endurance often makes it the sole inhabitant of highly polluted, oxygen-poor, mostly still water bodies, alongside midge larvae with similar survivability.  Their ability to encyst themselves when things get so bad that even they can’t tolerate them means that they can be stored refrigerated as fish food and pretty much ignored until needed.
One environment that has all of those problems in spades is a sewer, and that’s where things get interesting.  Tubifex worms live in burrows that they maintain long-term and aggregate in colonies.  In a sewer, those colonies can form huge clots that eventually block the movement of sewer-stuff, as revealed by this robot.

Notice that the worms all execute the escape reflex at once, giving their colony the appearance of a single pulsating sewer-heart.  And there’s probably one in your town, too: tubifex worms are more-or-less omnipresent thanks to human movements.

Leeches also show up just about everywhere.  Best known for their freshwater blood-sucking representatives, leeches are actually a quite diverse assemblage.  A handful of leeches are marine, for example, and another group is terrestrial, living amidst moist soil and foliage.  Many of these terrestrial leeches, rather than drawing blood from large animals, are voracious predators of smaller ones, as this Bornean monster shows.

The giant Bornean leech here is scarfing down a giant earthworm, using the same pumping motions that other leeches use to draw blood.

Naturally, the more photogenic annelids tend to be marine, dispersed among the polyphyletic assemblage known as the Polychaeta.  Most polychaetes are characterized by parapodia on their segments, which enable them to walk and swim via rhythmic paddling deceptively similar to a centipede’s walk.  The fireworm is a quintessential example, and provides my class with both a taste of their movement modes and a look at their colors.

In this video, they can watch a fireworm from much closer, and see the two lobes with separate sets of setae on each parapodium as well as the worm’s impressive jaws.  Unfortunately for it, it’s not very bright, so it manages to get outwitted and outrun by a snail.

Incidentally, they’re called fireworms because one does NOT want to touch them.

Other polychaetes are more benign, and feature prominently in many reef aquaria.  These feather duster worms are polychaetes whose assortment of head tentacles and cirri are replaced with a set of feathery tentacles called radioles, which serve as a combination respiratory and filter-feeding organ.  They secrete tubes in which they hide when disturbed or threatened.

One particularly striking feather duster is aptly known as the Christmas tree worm, so named for its spiral radioles.

And no mention of filter-feeding annelids would be complete without a mention of the Pogonophora and Vestimentifera.  Once regarded as distinct phyla, these two groups have been recently recognized as nesting deep within the polychaetes, their unusual anatomy related to their truly remarkable lifestyle.  Riftia and its kin are the most visible biota of “black smokers,” sites at the seafloor where sulfurous gases vent into the sea.  Chemoautotrophic bacteria feed on these sulfur compounds much like plants feed on sunlight, forming the basis of an ecology that, unlike almost all others on planet Earth, does not depend on the sun.

These vent worms maintain colonies of those sulfur-eating bacteria in their tissues, relying on them for sustenance the same way that many cnidarians rely on their internal algae.  And like those cnidarians, they can use this symbiosis to reach impressive, multi-meter sizes.

Which reminds me…

A few years ago, the Newquay aquarium in Cornwall was having problems.  Fish and coral in one of their displays kept disappearing or showing mysterious injuries, and a stakeout of the tank yielded no results.  Eventually, they dismantled the display bit by bit, and they discovered the culprit: a giant bristleworm about 1.3 meters long.

Unfortunately, I haven’t been able to find a source for this story other than the Daily Fail, so make of this tale what you will.  At least one other aquarium in England has a similar story, unless the Telegraph just caught the same rumor at a different point in its slide toward urban legend status.

And while we’re on this pleasantly nightmarish note…

Nematodes aren’t at all related to annelids, fitting almost at an opposite pole of the animal family tree.  The course treats them in the same session because the same dissection techniques are in play and to set up a comparative discussion of vermiform body plans.  Unfortunately, videos of nematodes tend to be quite ordinary (insofar as the worms themselves aren’t pretty odd) or trigger-warning-worthy medical exposes filled with human tragedy, so I’m not going to put any here.  I will, however, introduce their sister phylum, the Nematomorpha.  Nematomorphs, or horsehair worms, are mostly parasites of large insects.  Insects ingest their eggs when they drink water from contaminated ponds, and the worms develop inside their bodies.  When the worms metamorphose into their non-parasitic adult stage, they forcefully exit their hosts’ bodies.  Also, the worms can be MUCH longer than the insects they inhabit, to the point that the whole phylum is commonly known as “horsehair worms.”

One species that infects grasshoppers can alter its host’s brain chemistry, forcing the grasshopper to drown itself in order to bring the worms to their aquatic adult home.

Sleep tight.

Animal Form and Function 3: Annelids and Nematodes