Last week we briefly discussed the two types of annelids. Most terrestrial species belong to the group known as Oligochaetes and include the common earthworm. In the marine environment annelids tend to belong to the other group, known as Polychaetes. Polychaetes are divided into two groups based on whether or not they are able to move.
Mobile polychaetes are able to swim, crawl or burrow into the ocean bottom. These free-living worms are known as errant polychaetes and we’ve already seen two examples in this series in Eunice the bristle worm and Aphrodite the sea mouse. There is also an entire family of marine annelids that are incapable of movement, and among these, in time for the holidays, are the beautiful christmas tree worms. Worms that are unable to move are called sedentary polychaetes.
Even though many worms are armed with nasty defensive bristles, it doesn’t change the fact that all of them are soft-bodied. Errant polychaetes can make up for this by burrowing or hiding in crags and crevices. But sedentary worms are sitting ducks and need to have additional protections in place to protect them from predators. Most of these species are able to construct their own tubular homes made out of calcium carbonate, mud, shells or cemented sand where they live out their entire lives.
While some sedentary worms construct the tube beneath the sand, most attach the tube to a solid surface such as a rock, shell or coral head. Animals that live in tubes are called tubicolous, naturally.
This sessile existence requires a change in diet and feeding behavior. While errants can actively hunt or scavenge their meals, sedentary species have adapted to filter feeding passing plankton from the surrounding water. The feeding mechanism is an elaborately branched series of tentacles that protrude from the opening of the tube. Plankton is trapped in the tentacles and moved by tiny hair-like cilia lining the feeding apparatus towards the opening of the tube and into the mouth.
While discussing Eunice last week we saw the two types of paired appendages found on each of a polychaete’s hundred or so body segments. These were the bristle-like setae and the pseudo-feet parapodia. Not having any use for movement, the parapodia are reduced to minute stubs in sedentary species. The setae tend to be small and hooked, securing the animal within its tube and helping to prevent predators from pulling the entire animal out.
Like most marine animals, polychaetes breathe using gills. Normally the gills are simple extensions of the parapodia where gas is exchanged. Of course, this won’t work for sedentary species since little water is transferred between the inside and the outside of the tube. In this case oxygen and carbon dioxide are actually exchanged through the surface membranes of the feeding tentacles.
Looking at the structure of a sedentary worm, we can imagine a couple of other issues that would come up for a soft-bodied animal living inside a sealed and immovable tube. And yes, the opening of the tube can be sealed shut to enhance protection as well as prevent the worm from drying out in intertidal zones. A little trap-door structure attached to the feeding tentacles, called an operculum, can be closed tight when the tentacles are withdrawn into the body.
Since the rear end of the tube is sealed shut, wastes need to be ejected out of the anterior opening, and the digestive system of sedentary worms, unlike the straight line piping of other worms, has evolved so that the digestive tract doubles back under itself into sort of a U-shape.
As mentioned, sedentary polychaetes construct their tubes out of various materials. Christmas Tree worms, among others, make their homes out of calcium carbonate, the same material that makes up the bulk of clam and snail shells. Worms that use this compound are called Serpulids. So these animals are serpulid sedentary polychaete annelids.
Other diaries in this series can be found here.