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Aquarium.Net Jan 97

This month Rob continues his series on invertebrate zoology with Ribbon worms (Platyhelminthes and Nemertea)Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Reefkeeper's Guide to Invertebrate Zoology

Part 4:

Flat & Ribbon worms (Platyhelminthes and Nemertea)

By Rob Toonen

In the last article I tried to give a quick overview of some of the interesting and I think largely unknown details of Cnidarian biology.

Notoplana sp. A flatworm about 3 cm long. There are eyespots visible near the anterior end. Photo by Ron Shimek

This overview was far from comprehensive, but I tried to touch on some of the interesting research I have come across recently that I thought would be of general interest to hobbyists. Even though the article was longer than I intended, I still covered so little of the available information on cnidarians.

However, I will leave further details on the group to future articles and am instead going to move on to flatworms (Platyhelminthes) and ribbon worms (Nemerteans) in this article. Most people probably do not know much about either group, and I would hazard to guess that the majority of even reef-keepers have not noticed these animals in their aquariums. Observant hobbyists will eventually notice one or the other of these groups in their aquaria, because they are very common, so I thought I would give some information about both groups for the curious and/or particularly observant.

The Phylum Platyhelminthes includes about 20,000 species of parasitic and free-living flatworms (in fact the name Platyhelminthes comes from the Greek roots platy , meaning "flat" and helminth , meaning "worm"). They display a wide variety of body forms and are successful inhabitants of an even greater range of environments. Most flatworms are parasitic members of the classes Monogenea and Trematoda (there are roughly 9,000 species of flukes) and the Cestoda (there are about 5,000 species of tapeworms), but the class Turbellaria (about 4,500 species) is composed of primarily free-living forms from freshwater and marine habitats. Other turbellarians are terrestrial and some are even symbiotic in or on other invertebrates. I will only briefly mention the former three classes in this article, because these groups are generally only of concern to hobbyists as pathogens, and their biology and classification is probably not of general interest. As their name suggests, these animals are generally flattened and are typically elongate and/or oval. Some terrestrial forms are very elongate, and appear more ribbon-like than oval. Some species have tentacles on the anterior end, and are occasionally mistaken for nudibranchs by the untrained observer. The free-living forms generally range from less than 1 mm to over 30 cm in length, but the largest flatworms are tapeworms, some of which can attain lengths of several meters.

Unlike the previous groups we have discussed, the flatworms have three tissue layers, similar to the rest of the groups we will discuss, right through vertebrates. They are cephalized (have an identifiable head region), with a central nervous system. The central nervous system is typically composed of a pair of longitudinal nerve cords running the length of the body and connected by transverse commissures (forming a ladder-like nervous system) and culminating at the anterior cerebral ganglion (we cannot rightly call it a brain yet, but it is a large ball of cells in or near the head). They are also the first animals in this series to have an osmoregulatory/excretory structure.

These animals control their osmotic water balance with specialized structures called protonephridia , which function in much the same way as our own kidneys. Although many parasitic forms lack a gut (e.g., the tapeworms do not require a gut because they live within the digestive material of the host and directly absorb nutrients across the tegument -- the specialized 'skin' which allows these animals to survive in the digestive juices of the host gut), most free-living forms have a complex but incomplete gut (they, like the Cnidarians, lack an anus). The structure of the gut is used to classify members of the class Turbellaria, but only members of the orders Tricladida and Polycladida are likely to be encountered by the average aquarist. As the names suggest, Triclads have a 3 pronged gut (one anterior and two posterior), such as the common freshwater planarian Dugesia which everyone probably "examined" (we cut them up and watched the powers of regeneration) during their high school biology classes. Polyclads have a highly branched and irregular gut, and most of the large colorful reef species are polyclad turbellarians (e.g., Pseudobiceros , Pseudoceros , and Thysanozoon ).

In general, there is far more concern with the parasitic and/or predatory flatworms than there is interest about the care and maintenance of the free-living forms. Many flukes are parasitic on fishes and invertebrates, and many can infect mammals. There are a number of flatworms which commonly infect human hosts (e.g., Opisthorchis , the human liver fluke, Paragonimus , the human lung fluke, Schistosoma , the human blood fluke, and Diphyllobothrium , the broad fish tapeworm), but fortunately most of these species are freshwater and additionally require consumption of the intermediate host (the infectious stage of these animals develops in one or more species of host -- referred to as intermediate hosts -- prior to being able to reach maturity in the primary, or definitive host ) to infect us, and therefore do not tend to be a problem for reef-keepers (however, freshwater hobbyists ought to be aware that Schistosoma is carried by tropical snails, and is among the most widespread of tropical diseases; in this case, the infective stage -- cercariae -- do not require ingestion, and can penetrate even unbroken skin). The term intermediate host refers to one of the animals infected by these parasites en route to the host in which they reach sexual maturity. Although these seem like relatively simple animals, most parasitic flatworms have surprisingly complex life cycles. They are generally simultaneously hermaphroditic (possess both functional male and female sexual organs -- such animals typically are capable of mutual cross-fertilization and avoid self-fertilization, but self-fertilization does occur in rare cases among the flukes, and commonly among the tapeworms) and each class has an archetypal mode of reproduction. The typical pattern of reproduction among trematode flukes, for example, is that the eggs are passed from the definitive host (the host in which the parasite reaches sexual maturity) in the feces, and they hatch in water. A free-swimming stage (the miricidium ) locates and burrows into an intermediate host, where it undergoes one or more "generations" of asexual reproduction (during which thousands to hundreds of thousands of infectious cercariae are produced -- these may find a second or more intermediate hosts or locate the definitive host directly, depending on the species of parasite in question). The intermediate host (these commonly include molluscs, crustaceans and fishes) is then typically ingested by the definitive host, and the parasite matures to begin sexual reproduction and produce more eggs. If the parasite is ingested by some species other than the definitive host, that parasite does not generally survive or develop to reproductive maturity (e.g., black and grizzly bears are "dead-end hosts" for the broad fish tapeworm -- it can only reach sexual maturity in a human).

The free-living turbellarians typically have a less complicated reproductive cycle. They are still generally hermaphroditic and copulate to engage in mutual cross-fertilization, but most of these forms simply produce a few eggs which are either brooded or laid in protective egg capsules and develop directly into tiny turbellarians which look for all the world like the adults. There are however a number of exceptions, which produce a swimming larva. These larvae swim around for a few days prior to settling and metamorphosing into miniature adults, and because they do not feed, are relatively simple to raise in captivity. Most of the large polyclad flat worms of potential interest to reef-keepers will fall into this latter group.

A common flatworm in the shallow subtidal of the N. E. Pacific Photo by Ron Shimek

Unlike their cousins I mentioned above, the turbellarians are generally not parasitic. Some species are initially herbivorous, but become carnivorous as they mature. Others are herbivorous or detritivorous scavengers all their lives, living off microalgae and detritus found on the ocean floor. Most, however, are carnivorous predators or scavengers, feeding on nearly any available animal matter possible to consume. Their prey typically includes almost any invertebrates small enough to be captured and consumed, including protozoans, small crustaceans such as copepod and amphipod crustaceans, small polychaete (bristle) worms, tiny molluscs, etc. Many species also graze on the bodies of larger, but immotile invertebrates, such as sponges, tunicates, some cnidarians and even some barnacles. Obviously, some of these animals are not particularly desirable in a reef aquarium (the red "planarian" mentioned in Delbeek and Sprung, 1994, for example), but some make attractive and harmless additions to reef tanks. These animals are generally secretive and cryptic; they are hard to see and will remain out of sight for nearly their entire lives. In fact, most live rock shipments probably contain numerous small flatworms when introduced into the aquarium, but these animals seldom survive for long in new aquaria, and even if they do, they are so good at hiding that all but the most observant reef keepers will fail to notice them unless they become extremely abundant in the aquarium. A number of the particularly attractive and large polyclad worms are generally benign tank inhabitants worth considering, however. These worms ( Pseudobiceros and Pseudoceros in particular) can reach about 2-3 inches in length and the colors of some species rival those of the infamous nudibranchs. These free-living tropical reef species typically are generally frilled, and they can use these frilled edges of the body to swim by muscular contraction in an undulating flight that has to be seen to be truly appreciated. They occasionally appear for sale in petshops (often mistakenly identified for sale as nudibranchs), and should survive in a well established and stocked reef aquarium. Most are predatory scavengers, making a living by capturing the small polychaete worms and tiny crustaceans found in detrital accumulations. Although still a shy group of animals by nature, these larger species can occasionally be seen cruising around the aquarium during the day, or even swimming around the aquarium from time to time.

[an error occurred while processing this directive] The other group I wanted to discuss this month are the ribbon worms: Phylum Nemertea. There are only about 900 species of described nemerteans, which range in length from only about 1 cm to several meters when contracted. These animals have an amazing ability to stretch, however, and Brusca & Brusca (1990) report an individual of Lineus longissimus that was observed to stretch to nearly 60 m! Nemerteans generally live on the ocean floor, but some are pelagic, and others are commensal. These animals are long, flattened worms that superficially resemble some of the flatworms, however, they are different in a number of ways. First, these worms have a complete one-way gut in which food enters the mouth and undigested material is passed out the anus. Second, all members of this phylum are characterized by a highly eversible proboscis . The proboscis is an elongate, eversible (capable of being fired out from within the body) tube which is either fired from the mouth or through a special proboscis pore. The proboscis may be simply a long tube which entangles prey, or it may be equipped with stylets (tiny carpentry nail-shaped structures which are composed of crystalline calcium and phosphorous) to pierce and hold the prey while it is ingested. The proboscis is muscular and is generally wrapped around the prey to both physically "pin it down" and often, with the aid of toxic secretions, subdue or kill the prey item directly. Once the prey is subdued, the proboscis, which is often nearly the length of the entire worm when retracted and may extend up to three times the length of the animal when fully extended (only a portion of the proboscis is generally extended during eversion, however), is used to move the prey item to the mouth for ingestion.

Although nemerteans generally use their proboscis to capture prey, some are scavengers and others are commensal (live within the mantle cavity of bivalve molluscs and scoop collected particles from the feeding groove of their host) or feed on plant material. Most species are active predators on various small invertebrates, however, and the specificity of their prey preferences and how "picky" they are about their diet depends on the species in question.

The pinkish structure within this small shrimp (Heptacarpus kincaidi ) is a female of the parasitic turbellarian flatworm, Kronborgia pugettensis . Photo Ron Shimek

Some species are actually capable of sensing and tracking prey over long distances, whereas others seem to wander aimlessly until they physically contact a prey item. The species that actually hunt and track prey can recognize (most likely by some chemical cue) the trail left by their prey, and often fire their proboscis along the trail in front of them to capture the prey items. Some nemerteans fire their proboscis down the burrows of potential prey items to capture them and drag them out to be consumed.

As with the flatworms above, polychaete worms and small crustaceans seem to be the favorite prey items for many predatory nemerteans. Even active predators, however, also seem to spend a fair bit of time scavenging. Scavenging behavior is generally different from active predation because the proboscis is only rarely used. Instead, the worm simply ingests the food items directly with the mouth when scavenging.

As with the flat worms above, most nemerteans show remarkable powers of regeneration, and nearly all species can regenerate at least the posterior portions of the body. Those with the most amazing powers of regeneration are certain species of Lineus which regularly undergo transverse fission (body splits into several pieces), producing pieces which are often so small this process is sometimes referred to as simply fragmentation. If the pieces are extremely small a protective mucous cyst is often formed within which the new worm regenerates. Larger pieces regenerate to form a complete worm without the protection of a cyst. For such a small group (only 900 species) there is a remarkable diversity of reproductive modes among the species. These animals typically have separate sexes, although sequential and even simultaneous hermaphrodites are known, and they typically spawn either freely into the water column, or in gelatinous masses of mucous produced by the mating worms. There are also some species which copulate and have internal fertilization, some species have only the male spawn in the mucous mass and the sperm find their way to the eggs within the female's body. Some species have the young develop, nourished by yolk, in egg masses, some develop in the water column, where they must feed to gain sufficient energy to metamorphose into the adult body form, others also develop in the water column, but are provisioned with yolk so that no feeding is required to complete development, and finally others develop directly within the female's body. Obviously the ease with which these animals may be raised in captivity depends on the reproductive mode of the species in question.

Again, as in the flat worms above, some of these animals can be brilliantly colored and relatively benign additions to reef aquaria. Of course, there are also some species that are voracious predators which may cause damage to other species in a reef tank, but many tropical species appear happy to simply scavenge the detritus piles for left-overs. Unfortunately in the case of both groups I have discussed in this article, there is relatively little know about the biology of the group in general, but their abundance and ability to hide in and among tiny pieces of live rock make it likely that the observant hobbyist will encounter one or both these animals at some point in the life of their aquarium. I can offer no better advice than to watch the animal closely (separating unknown and potentially harmful animals into the sump or a separate tank is generally a good idea until it can be observed) and determine whether or not it presents a threat before sentencing an innocent and potentially interesting tank addition to the royal flush (which, for some reason, seems to be the favorite mode of execution for unwanted aquarium guests).

In most invertebrate zoology classes, a number of other small, wormlike phyla (such as gnathostomulids , gastrotrichs , kinorhynchs , priapulans , loriciferans , and acanthocephalans ) would be covered following flat and ribbon worms. Although these animals are common in live rock and especially in live sand, they are almost universally small, and are generally unobserved by even the most keen reef keepers. If you are curious about what these groups are, or what they look like, you can consult a general invertebrate zoology text book (such as Brusca & Brusca, 1990; Rupert & Barnes, 1996), or an in-depth aquarium text such as Moe (1993). Instead of filling in space with groups that are probably of little interest to the average aquarist, I will spend some time next month on the phyla Rotifera (rotifers) and the phylum Sipuncula (peanut worms).

Literature Cited:

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

Delbeek, J.C. & J. Sprung. 1994. The Reef Aquarium: a comprehensive guide to the identification and care of tropical marine invertebrates. , Ricordea Publishing: Coconut Grove, FL.

Moe, M.A., Jr., 1993. The Marine Aquarium Reference: Systems and Invertebrates. Green Turtle Publications, Plantation Florida, 512 pp.

Ruppert, E. E. & R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia, 1056 pp.

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Last modified 2006-11-18 18:31