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Nature Notes
by Bob Thomas

Why is it that so many invasive species are icky?

One somewhat rarely encountered example is a flatworm with a spade-shaped head reminiscent of a hammerhead shark that exudes copious amounts of slime. At last count, according to terrestrial planarian specialist Dr. Peter Ducey of the State University of New York at Cortland, there are five species in the United States, none of which is native.
• Bipalium kewense occurs across the southern states, gets 10 inches long, and has five thin longitudinal stripes.
• Bipalium adventitium has a more northern distribution, gets four inches long, and has one longitudinal dark stripe.
• Bipalium pennsylvanicum, only known from a couple of campuses in Philadelphia, has three longitudinal stripes (the middle much wider than in B. kewense).
• Bipalium vagum is in the southeast states, has three somewhat wider longitudinal stripes, a dark neck collar, and two dark spots on the head.
• Diversibipalium (Bipalium?) rauchi also occurs in Florida. It is ornately patterned with broad black and white bands and a red-orange lunate-shaped head. It is not known if this species is established, or is simply frequently introduced.

They are members of the Phylum Platyhelminthes, Class Turbellaria, Order Tricladida, and Family Bipaliidae. They lack a respiratory system, circulatory system, and a separate anus. The anus is combined with the mouth/pharynx: food goes in, feces goes out.

As a group, they are called broadhead planarians. The species that occurs in coastal Louisiana is Bipalium kewense, the so-called Kew greenhouse planarian, a species that was originally discovered in the Kew Gardens near London.

The provenance of all the naturalized species in the U.S. is not known, but most are from the tropic and temperate zones of southeastern Asia. Interestingly, as in the Kew greenhouse flatworm, most species were discovered in their non-native range.

Broadhead planarians are presumed to use potted plants, the most common vehicle used by tropical burrowing animals, in their movements around the world. Local examples of other species that arrived in tropical plants include brown anole lizards, Brahminy worm snakes, greenhouse frogs, and more.

Terrestrial planarians withstand desiccation, one of the perils of long distance travel, by coiling tightly and secreting loads of mucus to protect themselves.

Obviously, these flatworms require a moist environment and are negatively phototacic (they instinctively avoid light). Their preferred habitat includes piles of humus, protected cracks and crevices, flowerpot bases, and the like. As mentioned, they can withstand brief periods of drought by encapsulating themselves with their mucus. They are normally active at night and rainy periods, or near hose bibs and ponds. I find them most often during the cool, moist winter months.

When coiled in their hiding places, they tend to contract their bodies and be relatively short and wide. As they move from their hibernaculum, they may become very long and thin. The local species may reach 10 inches in length, but be only an eighth of an inch wide.

Planarians move by gliding along on a mucus cushion exuded by mucous glands that open into a ciliated strip called a creeping sole located along the median of their venter. Progression occurs as the cilia wave in the mucus, thus pushing the planarian forward. Their passage is marked by the presence of a visible mucus trail.

These flatworms do not cause plant damage, and are not harmful to humans or pets. Our species feeds exclusively on earthworms (as do two other species of naturalized planarians in the United States), which they track by following the earthworms’ gooey trails. Bipalium vagum specializes in slugs and snails, and I do not know what Diversibipalium rauchi eats.

Hunting behaviors involve the flatworm cruising about holding its spade-shaped head above the ground. It rocks its head back and forth, side to side, touching the ground only with tiny, finger-like chemoreceptor protrusions from the front rim of the hammerhead. Chemoreceptors may also be present on the lower surface of the head or in a ciliated groove on the venter.

Early studies suggested that the searching behavior is random, and contact must be made with its prey item to elicit the actual feeding response. The mouth of a planarian is on its belly near mid-body. When feeding, the flatworm crawls onto an earthworm, everts its muscular pharynx and exudes a digestive fluid over the surface of its meal. As the prey’s tissue is dissolved, the flatworm inserts its pharynx and swallows the juices. Small earthworms are completely consumed; large ones may not be totally eaten, but they always die.

Flatworms are hermaphroditic (containing both sexes) and the eggs are in somewhat hardened capsules (some refer to them as cocoons) that are attached to objects in damp places. Broadheads most commonly reproduce asexually via architomy, simply separating into an anterior and posterior piece by fission, with each part regenerating its missing components. These motile fragments are formed once or twice a month and a new head begins to form in about one week.

The hammerhead has tiny eyespots around its margin, and some species have the eyespots concentrated around the neck and running down the worm’s sides toward its posterior.

Though I earlier stated that broadhead planarians are not deemed dangerous, their diet purely of earthworms, coupled with a voracious appetite, may be a threat to the local earthworm population. Artioposthia triangulata, a New Zealand native terrestrial planarian, was accidentally introduced into Ireland in the 1960s. Studies have suggested that these animals are such effective earthworm predators that they have reduced populations below detection, and scientists fear their extinction.

Dr. Ducey is studying that aspect of the ecology of introduced terrestrial broadhead planarians in the United States.



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