Yes, the Christmas season is past (although hopefully all the good feelings it brings have lasted well into the new year)... or yet to come if you think I do my shopping this early (like my Mom and sisters)! However, Christmas... or more accurately Christmas tree worms... will be the focus of this week's column. "Yuck, icky" is the usual response when worms are mentioned (unless you speak a language other than English). However these worms really can be quite beautiful, even in the eyes of human "beholders."
These worms are related to common earthworms in that they have bodies with a series of segments. The segmented worms are grouped in the phylum Annelida. Typically each segment is similar, although some may have specialized structures for reproduction, feeding or other functions. Like most living things, "worms" originated in the sea and gradually invaded land as moist environments and vegetation made it habitable for them. Although terrestrial worms are often responsible for forming soils, marine worms generally aren't. Worms and arthropods like the lobster, shrimp and crab both possess segmented bodies and have been closely linked. In fact early biologists placed them together. However, I'd much rather eat an arthropod than a worm... and wouldn't even think of appearing on the "Fear Factor" TV show for dinner! Another major difference is that worms are soft bodied and lack the hard exoskeleton of their arthropod relatives.
Because worms like the Christmas tree worm are soft bodied, they make "easy to swallow" prey for the many predators lurking in "the mutual eating society" of kelp forests and other habitats. Some worms burrow into the sand or mud for protection. Others, like our subject, create hard tubes, often of calcium carbonate, for protection. These white tubes are usually covered with other growth so their full extent is not visible. One usually only sees the plumes of the feeding and breathing apparatus if it is extended. It may seem strange to have a single structure serve to gather food (mainly plankton and organic matter) as well as oxygen, but remember, our own mouths open into both the stomach and the lungs. The tubes of worms growing on the underside of rocks are often fully visible since there is no algal growth.
Our Christmas tree worms in southern California are relatively small. Their tubes may be up to about 4" long and the exposed feeding structures usually 1/2" or less. However, when I dive warmer waters, the tropical species are much larger in size. In fact, they appear to have been using steroids! Of course like our Governator, their steroid use is probably legal and there is no shame attached to such use unlike we have seen in baseball and other sports "heroes." In my case, the only steroids that have been effective are the ones focusing on my stomach!
The term "Christmas tree" worm may be going out of favor with some scientists due to its loose affiliation with Christian religions. I couldn't even find that phrase in the best marine invertebrate field book I own. However, it does not look anything like a Hanukkah bush, a Muslim helal (crescent moon) or a Hindu omkar, so I think it is an appropriate name. The "Christmas tree" is the branchial plume, or gill-like structure that extends from the head region. It has a spiral shape and tapers towards the end, suggesting the shape of a Christmas tree. This is easier to see in the larger tropical species than in our smaller ones.
The Christmas tree worm does not just come in green (that color seems to be rare). In fact there is a virtual rainbow of colors, suggesting the person who named them may have envisioned flocked trees in various hues. I've seen red, red-and-white, orange, blue, white, yellow and other colors. These worms are very sensitive to movement or shadows. As a diver passes over a rocky reef, they might see the individual worms quickly withdraw their colorful branchial plumes as the diver's shadow passes over them. Because of this sensitivity, it is often a real challenge to get good close-up images of these shy, retiring creatures. When they withdraw, these worms close up the end of their shell with an operculum like that used by snails.
The tube itself is constructed by organs in certain segments of the worm. Glands in one segment create the calcium that is used to build the shell. A second set of structures located towards the rear of the body store this calcium and are known to release it quickly if the animal is removed from its tube. There are also organic chemicals used in the shell and these are secreted by another structure known as the ventral shield.
Withdrawing into their tubes and closing the operculum is usually sufficient "defense" against most worm predators. However, these worms have developed a "fail safe" mechanism in addition. The end of their tube often has a very sharp, pointed projection. This spine will pierce the skin of a predator like a snail and "discourage" it from trying to feed on the occupant inside. In fact, these pointed tips have ruined many a good pair of dive gloves for me (not to mention a wetsuit or two) by tearing into them as I "crawl" along the reef. Now you know what present to put under MY Christmas tree next December!
© 2005 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.
(Top) Images of our local southern California
Christmas tree worms; (Bottom) sharp spine
on the worm's tube for defense, Christmas tree worms on steroids from Belize.
This document maintained by
Dr. Bill Bushing.
Material and images © 2005 Star Thrower Educational Multimedia