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Phylum Annelida -- worms and leeches Rob Toonen Aquarium.Net May 1997

Rob Toonen covers the phylum Annelids, worms and leeches, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Reefkeeper's Guide to Invertebrate Zoology:

Part 7

Phylum Annelida -- worms and leeches

By Rob Toonen

I wanted to start this article with a simple justification for why reefkeepers should care about invertebrate zoology. A friend of mine had been browsing the net and happened to find this site. He didn't know I was writing for the Ezine, and checked out some of the articles I had written. The next time I saw him, he asked whether I got a lot of feedback on the articles. I told him that I do get a fair number of Email questions and comments each month, so many in fact, that it is getting difficult (bordering on impossible -- I apologize to those of you still waiting for a reply) to keep up with answering them all. He was obviously surprised. I asked him why. He told me that he was just surprised that many aquarists cared about the technical details of invertebrate zoology; he thought people simply wanted to know: "How do I keep animal X?" or "What does animal Y eat?" I guess that is probably true of many people, but I explained to him that I was really impressed at the general level of knowledge among reefkeepers (some of our best Invertebrate Zoology students here have been hobbyists), and that people seemed sophisticated enough to realize that an answer to "How do I keep X?" wasn't going to help them become a better aquarist in the long run. I don't know whether the readers share my philosophy or not, but I wanted to express it here, even if just for my personal satisfaction.

I have encountered VERY few animals (whether they were an invertebrate, mammal, bird or herptile) that I could not care for when I really set my mind to it (I most certainly cannot say the same of plants, however). That may seem like bragging, but I have keep a LOT of pets in my life (much to first my parent's and now my wife's chagrin!), and it is true. I think this stems not from any exceptional ability on my part, but rather a good basic training in traditional zoology (and a lack of training in botany). I just happen to know a lot about animals from my schooling, and that knowledge really helps when you try to decide what an animal needs to be happy and healthy. Simply knowing whether a particular species harbors photosynthetic symbionts or not tells you a LOT about under what sort of conditions that animal is likely going to thrive. I am writing this series in the hopes of passing along a better working knowledge of what animals are, and what makes them tick. I am of the opinion that this core of knowledge is at the basis of my success, and that any determined person who researches a topic well and plans to succeed (rather than jumping in blindly) is going to be successful in the end. To that end, I hope that people who are following along in this series are, in fact, learning something worthwhile, and that my efforts will lead to a better educated and more successful group of aquarists.

Anyhow, enough of that -- on to worms! Despite my intention last month that I would spend most of this article discussing polychaetes, I decided I first wanted to spend a bit more time discussing the other two classes of annelids: the oligochaetes ("earthworms" and many other forms, such as the freshwater worm Tubifex ), and the hirudinids (leeches) before moving onto the polychaetes . When I did that, I realized the article had become far too long, so I have cut it more-or-less in half, and will begin my discussion of the polychaetes in detail next month. It will take at least 2 articles to cover the polychaetes, so it will be a while before we move on to the next group (Phylum Mollusca).

If you're in doubt as to which class that nondescript worm you're looking at belongs, you can identify whether the worm is an oligochaete, a polychaete or a hirudinean by the general characteristics listed in this table (although some characters, such as seta and parapodia, will obviously be more useful than others, such as gonads):

  Oligochaeta Polychaeta Hirudinida
Setae Few - reduced in size if present Many - varying in length and size Absent
Parapodia Absent, although arrangement of setae roughly correspond to parapodial location Highly developed and obvious Absent
Head Reduced and lacking appendages Well developed and complex, often with many appendages Reduced and lacking appendages
Suckers Absent Absent Present
Sex Hermaphroditic Separate Hermaphroditic
Gonads Essentially 2 pairs of testes and 1 pair of ovaries Peritoneal lining seasonally proliferates sex cells Several paired testes and a single ovary
Fertilization Internal External Internal
Larval type Absent Trochophore Absent
Jaws Absent Very well developed Present in some
Metamerization Numerous Variable, but many Number of segments relatively constant, although number of annuli vary

(modified from Beck & Braithwaite, 1968)

This table is not entirely accurate, it does not include the odd groups (branchiobdellids, pogonophorans & vestimentiferans) which aquarists are unlikely to ever encounter (although some freshwater aquarists could encounter branchiobdellids, which parasitize or live commensally on crayfish), but it does indicate the major differences likely useful to an aquarist at home. The "odd groups" which I mentioned (branchiobdellids, pogonophorans & vestimentiferans) above have alternatively been classified as their own phylum, or as a member of oligochaetes or hirudinids, depending on the source. The exact relationship between these groups remains unclear, but it is obvious that all three of these groups are indeed annelids, rather than some unique phylum unto themselves. It will take a lot more work on these groups to determine to which others they are most closely related, but unfortunately with the miserable level of funding for invertebrate zoology these days, we will not know the answer for a long time yet. Because they will not been seen in a reef environment, I will not give them further consideration here.

Oligochaetes have earned the common and familiar name "earthworms," but it is really a misnomer. Many oligochaetes are, in fact, terrestrial, but there are nearly as many freshwater species (about 2900 species together), and there are about 200 marine and even some arboreal (tree-dwelling) species.

Mating Earthworms Lumbricus terrestrialis

Tropical arboreal species live in the accumulated water at the axils of certain epiphytic plants (such as orchids and bromeliads). Even more amazing (at least to me), is that there are many oligochaetes which are inhabitants of glaciers and ice fields and caves.

In general, oligochaetes lack obvious cephalic (head) sensory structures or elaborations. Like the leeches (discussed below), they possess a clitellum (a secretory region that functions in reproduction -- the light colored region directly behind the dark head in the photo above) and are hermaphroditic (possess both functional male and female reproductive organs, thus the "pair" of earthworms above are actually reciprocally fertilizing one another). The group is split into 2 or 3 orders (depending on the source): 1) the Lumbriculida , which are fairly large freshwater worms, most of which are found only in Lake Baikal, Russia; 2) the Moniligastrida (which many authorities consider a suborder of the Haplotaxida, below), which are primarily Asian earthworms, several of which exceed 1 m in length, and some can exceed 3 m (10 ft!); and 3) the Haplotaxida , which covers the vast majority of the oligochaete species in all habitats and from all parts of the world -- this group includes the tubificid (tubifex or sludge worms) and lumbricid (common earthworms everyone has seen in their backyards or in baitshops) worms.

Although few people think of anything other than tubificid or lumbricid worms when an oligochaete is mentioned, there are a wide variety of body forms and life-history modes among the oligochaetes, though not so diverse as the polychaetes (which I will discuss at length below) that bear mentioning. For example, species from the family Naididae , are found in marine, brackish and freshwater habitats worldwide. Some oligochaetes possess gills that could fool a casual observer into mistaking them for a polychaete (e.g., Branchiura ), some build protective tubes of sedimentary particles (e.g., Tubifex and Ripistes ), and others are parasitic. Although oligochaetes are far less spectacularly diverse in terms of species and body forms than the polychaetes (I'll get to them soon), they are still a more diverse group than most we have dealt with previously.

Hirudinids are the true leeches, of which there are about 500 species. These species are mostly in the subclass Hirudinea but there is also a family of oligochaete-like leeches placed in the subclass Acanthobdellida. Most authors also recognize the Branchiobdellida as members of this class, but the classification is not universally agreed upon, and some still argue that the branchiobdellids are oligochaetes rather than leeches. Just to make things more confusing, these classifications may change depending on the source. For example, I have used the classification of Brusca and Brusca (1990) to recognize the class Hirudinida and the subclass Hirudinea. Ruppert and Barnes (1994), however, designate the class Hirudinea and the subclass Euhirudinea. Fortunately, the under either classification the term hirudinea covers virtually all the leech diversity, because all but a few species are found in the same subclass.

People generally have a strong negative reaction to the mention of leeches, and identify them only as blood suckers. While there are certainly many blood feeding species (e.g., Hirudo medicinalis , pictured here, and Macrobdella decora ),

primarily on vertebrates, leeches actually range in their habits from parasitic to predatory to scavengers. About three quarters of the members of this class are ectoparasitic (parasites that feed on the blood/plasma/flesh of their host from outside the body, as shown in the photo).

Hirudo medicinalialis Medicinal leech at work

It is also not generally known that there are many terrestrial leeches. Land leeches in tropical SE Asia, especially in India I am told, cluster in such numbers on the legs of animals that travel on foot is nearly impossible. Some tropical species climb tall vegetation and await movement below them to drop from their perch and attach to the passer-by.

Leeches have a fixed number of segments in their bodies. There may be variable numbers of superficial annuli (external rings on the skin resembling the segments in the other classes), but these annuli mask the true body segments which generally number 32 to 34. The number of segments is remarkably constant for a group with a great deal of variation in size. Polychaetes vary in size by adding more segments, but regardless of whether you examine the tiniest leeches ( 1 cm) or the giant Amazonian leech, Haementeria ghiliani (which reaches just over a foot in length), the number of segments varies but little.

Although leeches are found in a wide variety of habitats, they are found primarily in shallow, vegetated sluggish streams or standing lakes and ponds. In favorable habitats, typically in areas of high organic pollution, populations may reach incredible densities; more than 10,000 individuals per square meter have been reported from some lakes in Illinois (Ruppert & Barnes 1994). As with most annelids, there are some members which can survive amazing physical hardship, and some species estivate ("hibernate") during periods of drought by burrowing into the benthos (mud at the bottom of a stream or pond) and can survive a loss of as much as 90% of their body weight (Ruppert & Barnes 1994). This is really an amazing feat -- were I a leech, I could hibernate and survive my body weight dropping to 20 lbs!

The group in which most people are interested, I am sure, are the polychaetes. Polychaetes are, by far, the most diverse class of the annelids, containing 8 to 10,000 described species (depending on who you ask), and more are being described/discovered every year. These animals range in length from a few millimeters (some members of the family Syllidae ) to over a meter (some members of the family Nereidae ); they may be less than 1 mm in diameter or as much as a couple of centimeters.

The diversity of body forms and lifestyles is reflected in the fact that there are about 86 families of described polychaetes (Ruppert & Barnes, 1994 -- although other sources give different numbers). Orders of polychaetes changes often and are basically meaningless to anyone but a specialist in this field -- the artificial groups sedenteria (the tube-building and other sedentary worms) and errantia (free-living worms) are as useful as are any real taxonomic distinctions to the classification of the polychaetes.

The Christmas Tree Worm

Spirobranchus giganteus

The errant polychaetes include some species that are strictly pelagic (live in the water column), some that crawl about beneath rocks and shells, some that are active burrowers in sand and mud, but also some that occupy stationary tubes (Ruppert & Barnes, 1994). On the other hand, many sedentary species construct and live in semi-permanent burrows or tubes of various complexity, but some species can leave the burrow or tube and start a new home. Only the obligate tubiculous (tube-building) species (such as the "Christmas-tree worm" Spirobranchus giganteus , pictured here) are typically confined to their tube and can only extend their heads from the tube opening.

Next month I will spend most of the article discussing why I think "bristleworms" have gotten a bad name, and what are some of the most common misconceptions about these amazing animals. I will talk about the "dreaded fireworms" and what to do if you encounter one (or more) in your tank, and I will answer some of the questions I see most commonly on the newsgroups in regard to bristleworms.

Literature Cited:

Beck, D.E., & L.F. Braithwaite, 1968. Invertebrate Zoology: Laboratory Workbook. 3 rd edition Burgess Publishing Co.. Minneapolis, Minn. 263 pp.

Brusca, R.C., & G.J. Brusca, 1990. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass. 922 pp.

Ruppert, E.E. & R.D. Barnes, 1994. Invertebrate Zoology, 6 th Edition . Saunders College Publishing, Harcourt Brace College Publishers, Orlando, FL. 1056 pp.

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Last modified 2006-11-18 20:02