How would you answer the question, "What is a worm?" Um, well, it's a lot of things. Let's see, they're animals, generally roundish in cross section (or would you say radially symmetrical?), they have a head at one end... Oh yeah, and they're slimy. Anything else? That's about as good as I could do as well.
"Worm" is a very generic term. There are several "worm" groups (Phyla) and many other "wormy" animals in non-worm phyla that get called "worms" on the basis of their external appearance. Am I alone here or doesn't this remind you of all the "Eel" and "Shark" common labeling of fishes? What a bucket of worms! Sorry.
Many of the "real" worm groups are far from closely related. They include, but are definitely not limited to Flatworms (Tapes, Flukes, "free-living" Turbellarians), Segmented Worms (Earthworms, Bristle, Fire, Christmas Tree, Tubeworm, Tubifex), Roundworms (Vinegar, White, Ascaris, Heartworms, Guinea), Peanut Worms, Ribbon Worms, Tongue Worms, Acorn Worms, Beard Worms; and maybe the strangest of all, the Vestimentaferans. You might remember the last group from seeing them on teevee. They're the 'new' (they've been around for over 400 million years) ones found one and a half mile down below the Pacific Ocean at hot water vents. And that's not all the worms; should we mention rotifers, gastrotrichs, kinorhynchs...? Oh boy.
Who owns the largest worm collection in the world? You do, if you're a U.S. citizen; 12.8 million of them are the National Collection of Worms, in Room 218C of the Smithsonian Institution.
Though worms don't get much respect, due to their inconspicuousness and human bias, I want to go on record as stating that they are probably the most numerous macro-life in all marine habitats in terms of diversity and abundance. In accordance with this profusion, worms play many important roles in natural environments.
The importance of worms in marine environments deserves to be emphasized. They are ubiquitous (everywhere) and often in great numbers. There are measures of their populations in some sediments at millions per cubic yard. Allen (1994) cites a study of a single coral head at Heron Island on the GBR (Great Barrier Reef) that was found to harbor some 1400 worms of 103 different species! Inconspicuous indeed.
Worms are a major source of reef destruction, gnawing, chemically melting, eating all manner of life by way of all feeding modes; carnivorous, filter, detritivore... In turn they are being eaten by most everything; from crabs to blue whales. Their food-processing and burrowing actions recycle and return a tremendous amount of nutrient and mineral to the seas.
As an interesting anecdote, and to impress upon you both the breadth of worminess in the oceans, and how important it is for you to study up, I'd like to relate a personal experience to you. If you're squeamish about what you put in your mouth, I'd suggest skipping down to the next Heading.
As a boy growing up in the Philippines I had many novel adventures; one whose remembrance struck me right between the eyes as I sat in a Marine Invertebrate Zoology class one day. The local people in the smallish village where I lived for two years were pretty nonchalant where time-keeping was concerned; We were poor, had few watches, and pretty much ate when we we're hungry, slept when tired... except for certain celestial/tidal events. When the tides and moon were propitious everyone would get whipped into a high state of frenzy, and collect "palolo" at night. These 'things' could be scooped up in the reef shallows with our baskets by the bundle, and were they good to eat!
Skip ahead a few years and I'm at a lecture hearing about the same previously mysterious and forgotten organisms. They are "epitokes", sections of certain polychaete worms that on cue develop sets of eyes, break off, and swim to the surface to mass-reproduce. Yuck, I mean yum, I mean yuck, I mean yum. Oh blissful ignorance!
For Marine Aquarists:
There are many advantages of "having worms"; they help clean up wastes and uneaten food, in turn becoming food for your livestock. Through their tunneling through the substrate, worms prevent clogging and channeling, helping keep you water clean and clear. As you'll find in pursuing "wormy" matters, worms can also be of service to the marine aquarist as bio-indicators, ornament and more.
Besides, there isn't much you can do about getting them; Dear Reader, you've got worms! Or soon will have. No sense copping a denial plea with me. Many worms are microscopic as young, adults; some nearly transparent. look closely at any system that's been set up for a while, they are there.
All sorts of worms make their way into our tanks. Obvious sources are natural water if you use it, live rock, other invertebrates, algae, fishes, and fresh food.
Unfortunately, way too often an aquarist will overreact on discovery of "worm-sign" (shades of the book and movie Dune) Not to worry; as with the bacteria, indeed all living things, the vast majority of worms are harmless, or beneficial. The "real bad ones" in almost all cases, fall into one of three categories:
1) External/internal commensal to "space" to true parasites that can, could, should be removed/reduced in importance through proper collection, transport, dip/quarantine, and maintenance,
2) Those that are proliferating because you've overfed the system, the filtration or something else aquarist-wise is out of whack, and
3) Worms that get blamed for wiping out a tank, when the true cause was the system's owner over-reacting, and poisoning the whole thing trying to eliminate them.
Yes, I'm familiar with "bristle" and other noxious polychaete worm infestations that munch on expensive invertebrate livestock. There are better ways to eliminate them than chemicals. Traps of different sorts are so useful that they are marketed in pet-fish magazines; and there are numerous fishes that would love to help you curtail their populations. Check these out.
Polychaetous Annelids aka Bristleworms, Featherdusters... to aquarists are sub-divided into two sub-classes:
Worms are present in all environments, terrestrial, freshwater and marine. There are ones found in and on all our livestock. Almost all are "free-living", not harming anyone, though people seem more or only aware of the symbiotic and parasitic forms.
Hopefully now you share my concerns re the importance of worms and the general public's current ignorance and disdain for them. Worm groups, species, populations are vital to the planet and possibly to your marine system.
If you "have worms" or develop them in your set-up, don't be automatically alarmed. Probably they are there, and are doing you more good than you know; they will give you useful insights into successful marine aquarium keeping, and life.
Marine Hitchhiker/Critter ID (Maughmer, Toonen, Tompkins)
Allen, Gerald R. & Roger Steene. 1994. Indo-Pacific Coral Reef Field Guide. Tropical Reef Research, Singapore. p. 125.
Fenner, Bob. A diversity of aquatic life: Feather Duster Worms. FAMA 12/94.
Harris, Linda K. 1994. At the Division of Worms there are even worms to spare; Smithsonian has millions of them. Knight-Ridder Newspapers 11/30/94.
Kaufman, Les. 1973. Worms. Marine Aquarist 4(4):73.
Kent, M.L. & A.C. Olson, Jr. 1986. Interrelationships of a parasitic turbellarian (Paravortex sp.) (Graffillidae, Rhadocoela) and it's marine fish hosts. Fish Pathol. 21: 65-72
Lee, Tracy. 1988. The invertebrate corner; survey of the invertebrates, pt. II (worms). Marine Fish Monthly 3(3):88.
Wall, Jerry G. 1992. Little white worms. TFH 7/92.