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Natural History Observations on Bipalium cf. vagum Jones and Sterrer (Platyhelminthes: Tricladida), a Terrestrial Broadhead Planarian New to North America
Peter K. Ducey, Matthew McCormick, and Elizabeth Davidson

Southeastern Naturalist, Volume 6, Number 3 (2007): 449–460

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2007 SOUTHEASTERN NATURALIST 6(3):449–460 Natural History Observations on Bipalium cf. vagum Jones and Sterrer (Platyhelminthes: Tricladida), a Terrestrial Broadhead Planarian New to North America Peter K. Ducey1,*, Matthew McCormick1, and Elizabeth Davidson1 Abstract - An increasing number of exotic terrestrial planarian species have established populations worldwide. In North America, the most prominent invasive flatworms are three members of the broadhead planarian genus Bipalium. Herein we report observations on the morphology, predatory behavior, and reproduction of Bipalium cf. vagum, new to this continent and report its occurrence in Florida and Texas. Individuals of this species have a distinctive combination of head shape and pattern of dark dorsal pigmentation (large head spots, complete collar, and prominent median stripe) that distinguishes them from other members of the genus. Although the other North American species of Bipalium feed on earthworms, B. cf. vagum feeds exclusively on terrestrial mollusks. Their predatory behavior includes following mucus trails and subduing the prey by capping the prey’s head with the flatworm’s anterior end and wrapping the prey’s foot in the body of the planarian. Members of this species reproduce via egg capsules that contain small numbers of offspring. Because this is the first land planarian reported in North America that is a predator of mollusks, native land snails and slugs are unlikely to have effective defenses against it. Therefore, we should continue to monitor its geographic spread and potential ecological impact. Introduction Invasive species are increasingly being recognized as major threats to ecosystems worldwide (Cox 2004, Sakai et al. 2001). Although much research is now being directed to the ecological and evolutionary processes involved in invasion, relatively little attention has been focused on soil fauna despite their potential impact on native and agricultural ecosystems (Lee 1985, Wardle et al. 2004). Many species of terrestrial planarians have been described as invasive, exotic species in soils of the Northern Hemisphere; predominant among these are Arthurdendyus triangulatus (Dendy), invading the British Isles and Faroe Isles from a native range in New Zealand (e.g., Blackshaw and Stewart 1992, Christensen and Mather 1995); Bipalium kewense Moseley, now found in tropical and warm temperate regions worldwide from hypothesized origins in southeastern Asia (Winsor 1983); and B. adventitium Hyman, currently known only from North America but presumably from a homeland in Asia (Ducey and Noce 1998; Hyman 1943, 1954; Ogren and Kawakatsu 1998). In North America, although about a dozen species of exotic terrestrial planarians have been found (Ogren and Kawakatsu 1998), two species are 1Department of Biological Sciences, State University of New York at Cortland, Cortland, NY 13045. *Corresponding author - duceyp@cortland.edu. 450 Southeastern Naturalist Vol. 6, No. 3 most conspicuous: B. adventitium, occurring across the northern portion of the United States (Ducey and Noce 1998, Hyman 1954, Ogren 1984, Zaborski 2002), and B. kewense, found across the southern portion (Ducey et al. 2006, Ogren 1984, Ogren and Kawakatsu 1998). Both of these species are predators of earthworms and are most common in disturbed habitats associated with humans (Ducey and Noce 1998, Ogren 1984). They differ internally in morphology of their reproductive structures and externally in general body size, shape, and coloration (Ball and Sluys 1990, Ogren 1984, Ogren and Kawakatsu 1998). The two species, although both potentially hermaphroditic, also differ in reproductive ecology; B. adventitium reproduces sexually to produce egg capsules (Ducey et al. 2005, Ogren 1984), whereas B. kewense reproduces primarily via fragmentation, with only rare sexual production of egg cases (Connella and Stern 1969, Ducey et al. 2006, Winsor 1983). A third Bipalium species in North America, B. pennsylvanicum, known from only two sites in Pennsylvania, shares some morphological, reproductive, and behavioral features with B. adventitium, but differs in appearance (Ogren and Sheldon 1991). Because of their rapid dispersal, broad distribution, and effective predation on earthworms, the potential impact of these species on agricultural, horticultural, and natural ecosystems is of concern. To our knowledge, no other species of Bipalium have been reported living outdoors in North America, despite some 100+ members of the genus living in other parts of the world (Ogren and Kawakatsu 1998, Winsor 1983). Herein, we report on the occurrence of a member of the genus Bipalium new to North America, and describe aspects of its morphology, behavior, and reproduction. The external morphology of this species most closely resembles that of Diversibipalium sp. 10 of Wu et al. (2005) from Taiwan and B. vagum Jones and Sterrer, recently proposed for specimens from Bermuda (Jones and Sterrer 2005). Because the latter two taxa have not been formally synonymized and molecular comparisons between these flatworms and the North American species have not been completed, we will use the designation Bipalium cf. vagum to refer to the North American taxon throughout this document. We distinguish this species from the other members of the genus currently known from this continent, as well as from congeneric species reported from other parts of the world. Methods Animals Three individuals of B. cf. vagum were collected by Dan Hodgson in Cypress, Harris County, TX in May, 2005 in a suburban yard along with many B. kewense and a few Geoplana arkalabamensis Ogden and Darlington. They were transported alive to the State University of New York at Cortland, where we held them separately in plastic containers filled with moist paper towels at 19–23 °C while they were tested for predatory behaviors. In November, 2005, Matt Cormons sent photographs of what appears to 2007 P.K. Ducey, M. McCormick, and E. Davidson 451 be this species from Lehigh Acres, Lee County, FL, and in January, 2006, sent a live specimen collected by Grace Donaldson Cormons. The adult flatworms were maintained on a diet of Arion spp. and Deroceras sp. (slugs) and Discus spp., Helicodiscus spp., and Succinea spp. (snails) (all found locally). We fed mashed snails to the newly hatched offspring. The specimens, tissue samples, and photographs are held in Bowers Science Museum of SUNY at Cortland (Accession: P.K. Ducey; catalog numbers: 549, 549A, 550, 550A1, 550A1A, 550A2, 550A3, 550B1, 550B1A, 551, 551post, 574). We compared B. cf. vagum with hundreds of newly collected and laboratory raised B. adventitium and B. kewense. The B. adventitium were from California, Maryland, Michigan, New Hampshire, New York, Ohio, and West Virginia, and the B. kewense were from Arkansas, California, Georgia, Mississippi, North Carolina, and Texas. We maintained members of these species individually under the same physical conditions as B. cf. vagum, but fed them earthworms. All morphological comparisons were based on living specimens. Predatory behavior To examine the predatory choices and behavior of B. cf. vagum, we subjected each adult individual (n = 3, Texas) to a series of trials with different potential prey items. We ran the trials in plastic containers (25 cm x 14 cm) lined with moist paper towels and partly covered to reduce light. Test subjects were left undisturbed in the chambers for at least 5 min. for acclimation before testing. We presented prey items individually < 1.5 cm in front of the head of the flatworm. All potential prey items (including: Aporrectodea turgida, Amynthas sp., Eisenia fetida, Lumbricus rubellus, and L. terrestris [earthworms]; Arion spp. and Deroceras sp. [slugs]; Discus sp., Helicodiscus sp., and Succinea sp. [snails]; Armadillidium sp. and Porcellio sp. [isopods]; Oxidus sp. and Ptyoiulus sp. [millipedes]; and Exomala sp., Limonius sp., Phyllophaga sp., and Popilla sp. [beetle larvae]) were collected in Cortland County, NY, and each prey species was offered in at least six separate trials. We compared the results of these trials with other observations we have collected over the last ten years on the prey choice and predatory behaviors of B. adventitium and B. kewense (Ducey et al. 1999; Ducey and Noce 1998; Fiore et al. 2004; P.K. Ducey, unpubl. observ.) and with observations reported by others (Dindal 1970, Ogren and Sheldon 1991, Zaborski 2002). Results and Discussion External morphology of B. cf. vagum Like all members of the genus, members of B. cf. vagum have a broad, spatulate head that is wider than the body (Fig. 1). In life, the auricles may show very slight recurvature. A creeping sole runs medially the length of the ventral surface. Live adults were 3.7—4.9 cm long (mean = 4.0 cm, n = 4) and 0.3—0.4 cm wide (mean = 0.33 cm). All individuals possessed a thick 452 Southeastern Naturalist Vol. 6, No. 3 black transverse band on the neck (= collar) that is complete dorsally and extends well onto the ventral side, but does not include the creeping sole (Fig. 2b). In 3 of 7 individuals, a thin median stripe extended from the collar onto the head. All individuals had a dorsal base color of yellow-tan with three longitudinal dark stripes extending from the dark collar posteriorly the full length of the body. The median stripe is the darkest stripe (same color as the collar) and is about as broad as the diffusely pigmented lateral stripes. The head had two dark patches, separated from each other and the neck Figure 1. Bipalium cf. vagum from Cypress, TX—a new species for North America. 2007 P.K. Ducey, M. McCormick, and E. Davidson 453 collar by narrow lighter areas, giving the head the appearance of having two large “eyes.” Morphological comparison to other North American species of Bipalium General body size, shape, and base color of B. cf. vagum are similar to those of B. adventitium, although B. adventitium is slightly narrower in both the head and body. Throughout its range, B. adventitium has a single, narrow dorsal stripe, lacks the dark collar, and has only diffuse dark pigment concentrated distally on the head, making it easily distinguishable from B. cf. vagum. (Fig. 2). Although considerable intraspecific diversity of appearance has been attributed to B. kewense, at least some of this is due to historical misidentifications (Winsor 1983). Our collections of B. kewense from the United States agree with the descriptions, photographs, and illustrations for this species in the literature (e.g., Connella and Stern 1969, Ogren 1984, Winsor 1983). Similarities between B. kewense and B. cf. vagum include a median dark stripe, a pair of more diffuse lateral stripes, and a dark collar. However, in contrast to the appearance of B. cf. vagum, B. kewense has a median stripe that is very narrow, a collar that is incomplete dorsally and barely reaches the edge of the ventral surface, auricles that have distinct recurvature, an additional dark stripe along each lateral margin (= 5 total dorsal stripes), and a head with diffuse gray pigment concentrated medially (Fig. 2). Adults of B. kewense attain much greater length (regularly > 15 cm) than adults of B. cf. vagum; B. kewense of equivalent length with B. cf. vagum are considerably thinner. The original account of B. manubriatrium (now a synonym of B. kewense) in North America by Sharp (1892) described the median stripe as the broadest of the stripes, and Fletcher (1887) also stated that the median stripe could be as broad as the lateral stripes in some specimens of B. Figure 2. Diagrams of the typical head pigmentation for the three species of Bipalium widely distributed in North America. A) B. adventitium, B) B. cf. vagum, C) B. kewense. Photographs of the geographically restricted B. pennsylvanicum are available in Ogren (1987). 454 Southeastern Naturalist Vol. 6, No. 3 kewense. Winsor (1983) likewise reported that the degree of dark pigment on the head varies among individuals in B. kewense. However, none of the specimens of B. kewense from North America, from our studies and previous works (e.g., Chandler 1974, Connella and Stern 1969, Ogren 1984), nor specimens from around the world (Winsor 1983), closely match the appearance of B. cf. vagum. Ogren (1987) described B. pennsylvanicum as possessing three dorsal stripes, with the median stripe being prominent. His descriptions and illustrations suggest that B. pennsylvanicum differs from B. cf. vagum in lacking a dark collar, lacking dark head pigmentation, and having the median dorsal stripe that extends onto the head. Thus, B. cf. vagum can clearly be distinguished by external appearance from the three congeneric species previously known to occur on this continent. Interestingly, two public websites depicting pest animals in Florida have included photographs of what appear to be B. cf. vagum (Florida Nature 2006, University of Florida Institute for Food and Agricultural Services 2006). The flatworms depicted in the photographs appear identical to our live specimens from Florida and Texas and were originally labeled as B. kewense on the websites. One of those specimens was collected in Tallahassee (Florida Nature 2006), and the other in Gainesville (P.M. Choate, University of Florida, Gainesville, FL, pers. comm.). These sites are over 1000 km from the Cypress, TX locality and hundreds of kilometers from the Lee County, Fl site, indicating that B. cf. vagum may already be distributed across the Gulf Coast of the United States. Comparison to species of Bipalium outside North America Because the description of B. cf.vagum from Bermuda provided by Jones and Sterrer (2005) closely matches the appearance of the planarians we studied from Texas and Florida, and matches Diversibipalium sp. 10 from Taiwan (Wu et al. 2005), it is likely that all three studies involve the same species. However, such synonymy should await the outcome of molecular studies. To find possible previous descriptions of this species from elsewhere in the world, we also examined the literature that referred to three-lined species of Bipalium, including works by Graff (1899), Whitehouse (1919), Kaburaki (1922), Beauchamp (1939), Winsor (1983), Ogren (1987), and Sasaki (2001). We found, as did Jones and Sterrer (2005), that no previously described species of Bipalium had the combination of a dark complete collar, broad median stripe not reaching the head, and large head spots that we found in B. cf. vagum. Contrast in prey choice The individuals of B. cf. vagum that we tested preyed only on gastropods (Table 1), in contrast to the other North American Bipalium that feed exclusively on, or with strong preference for, earthworms (Dindal 1970, Ducey et al. 1999, Neck 1987, Ogren and Sheldon 1991, Zaborski 2002). In our laboratory trials, B. cf. vagum showed no predatory behavior toward any 2007 P.K. Ducey, M. McCormick, and E. Davidson 455 arthropods (isopods, millipedes, insects) or earthworms that were offered in 36 trials. Adults of B. cf. vagum did attack and eat most of the snails and slugs that were offered (Arion spp. and Deroceras sp. eaten in 8 of 9 trials; Discus sp. eaten in 5 of 6 trials; Helicodiscus sp. eaten in 4 of 6 trials; Succinea sp. eaten in 4 of 7 trials). Similarly, Jones and Sterrer (2005) reported that B. vagum in Bermuda fed on snails. In our laboratory, B. adventitium and B. kewense have refused to attack and eat live members of these same mollusk species on dozens of occasions over ten years (Ducey et al., 1999; P.K. Ducey, unpubl. observ.). Similarly, Okochi et al. (2004) reported that B. kewense on the Ogasawara Islands refused to eat snails. However, B. cf. vagum is not the only terrestrial planarian known to eat gastropods. Ogren and Sheldon (1991) found that B. pennsylvanicum would not prey upon live slugs, but would eat mashed slugs. Other terrestrial planarians that feed on mollusks include Australopacifica sp. (Okochi et al. 2004), Endeavouria septemlineata Hyman (Mead 1963), Geoplana burmeisteri Schultz and Muller (Ogren 1995), G. ventrolineata Dendy (Barker 1989), Platydemus manokwari de Beauchamp (Ogren 1995, Okochi et al. 2004), and two undescribed species of Bipalium from Japan and islands in the Pacific (Ogren 1995, Okochi et al. 2004). None of these molluscivorous terrestrial planarians is distributed within the continental United States. Predatory behavior of B. cf. vagum Adults of B. cf. vagum began searching and predatory behavior when they came in close proximity to a terrestrial mollusk or its mucus trail. Other terrestrial planarians are also known to follow trails of their prey (Fiore et al. 2005, Mead 1963). During a predatory encounter, B. cf. vagum moved its head into contact with the mollusk, then crawled upon the prey using body Table 1. Results of feeding trials with B. cf. vagum as predator. Eaten Rejected Mollusks – slugs Annelids – earthworms Arion spp. Amynthas sp. Deroceras sp. Aporrectodea turgida Eisenia fetida Mollusks – snails Lumbricus rubellus Discus sp. L. terrestris Helicodiscus sp. Succinea sp. Arthropods – isopods Armadillidium sp. Porcellio sp. Arthropods – millipedes Oxidus sp. Ptyoiulus sp. Arthropods – beetle larvae Exomala sp. Limonius sp. Phyllophaga sp. Popilla sp. 456 Southeastern Naturalist Vol. 6, No. 3 and head to enwrap it. Whether the prey was a snail or a slug, the flatworm usually attempted to use its own head to cover the head of the gastropod, with the effect of greatly reducing escape. A similar capping behavior is used by B. adventitium to subdue earthworms (Ducey et al. 1999, Fiore et al. 2005) and by Geoplana spp. eating slugs (Froehlich 1955). Wrapping the prey with the flatworm’s body to some extent is also reported for Geoplana spp. feeding on mollusks (Froehlich 1955) and for the three other species of Bipalium in North America eating earthworms (Dindal 1970, Ducey et al. 1999, Fiore et al. 2005, Neck 1987, Ogren and Sheldon 1991, Zaborski 2002). When attacking slugs, B. cf. vagum further reduced prey escape by sometimes lifting the prey from the substrate during the capping behavior. In trials with small snail prey (Discus sp. and Helicodiscus sp., 0.02—0.07 g), the flatworm would completely encircle the shell and position its pharynx over the aperture. In trials with mid-sized snails (Succinea sp., 0.2—0.9 g), the flatworm capped the snail’s tentacles and head and wrapped its body around the muscular foot and shell edge. The time for completion of the predation attempts varied depending on the size and species of mollusks involved. Most attacks on slugs appeared to be slowed, but usually not thwarted, by the mucus secreted by the prey. Some individuals of the genus Deroceras produced yellow mucus that inhibited expansion of the pharynx by the flatworms for nearly 30 min. This occurred despite the flatworm having full control over the slug’s movements. Ultimately, however, the prey was consumed. The flatworms fed easily and quickly on the small snails (Discus sp. and Helicodiscus sp.), sometimes completing attack and consumption in < 3 min. Mid-sized snails (genus Succinea) escaped in 3 of 7 trials, and in those trials that eventually led to consumption, the snails escaped and were recaptured several times before being subdued. The successful attacks occurred when a flatworm was able to encircle the underside of the shell with its body and cap the snail’s tentacles with its head and neck. Because we conducted trials only under laboratory conditions, it remains unclear how often prey would survive these attacks in nature. Reproduction Terrestrial planarians reproduce asexually via fragmentation (= fission), sexually through the production of egg capsules containing multiple embryos, or some combination of these methods (Ball and Sluys 1990, Ogren 1984, Winsor 1983). However, the specific reproductive strategy is known for only a few species (Ducey et al. 2005, Froehlich 1955). In North America, B. kewense primarily uses fragmentation (Hyman 1943, 1954; Ogren 1984; Winsor 1983), but rare instances of sexual reproduction in outdoor populations are known (Connella and Stern 1969, Ducey et al. 2006), whereas B. adventitium reproduces solely via egg capsules (Ducey et al. 2005, Ogren 1984). One of our B. cf. vagum produced an egg capsule (approximately 4 mm in diameter) from which 3 offspring hatched (3.0, 5.3, and 6.3 mg). These offspring had the same pigmentation pattern as the 2007 P.K. Ducey, M. McCormick, and E. Davidson 457 adults, although it took more than a week for the dark markings to fully appear in the larger individuals. Because of the internal yolk reserves, the base color of the offspring was lighter than that of the adults. The offspring eagerly ate mashed slugs. Thus, the reproductive strategy of B. cf. vagum is similar to that of B. adventitium, which produces similar egg capsules with a mean of 3.4 offspring per capsule and a mean offspring mass of 5.2 mg (Ducey et al. 2005). Bipalium in North America The individuals of B. cf. vagum described here represent the fourth species of the genus to be found in North America. Bipalium adventitium and B. kewense are already widespread and locally abundant throughout the temperate and subtropical parts of the continent, respectively (e.g., Ducey and Noce 1998, Neck 1987, Ogren and Kawakatsu 1998). Because these two species, and the geographically restricted B. pennsylvanicum, feed on earthworms, which are important ecosystem engineers, the flatworms could potentially affect physical and biotic features of agricultural, horticultural, and natural ecosystems. There is evidence that a terrestrial planarian that has invaded the British Isles and Faroe Isles— Arthurdendyus triangulatus (Dendy)—may be having significant impact on earthworm populations in some areas (Blackshaw and Stewart 1992, Christensen and Mather 1995). Although there are other members of the genus that eat mollusks elsewhere (Ogren 1995, Okochi et al. 2004), B. cf. vagum is the first mollusk-eating terrestrial planarian with confirmed populations in the continental United States. With a range that already includes at least parts of Texas and Florida, this species could potentially have a negative impact by preying upon rare native snails, as has been reported for other planarians in the Ogasawara Islands (Okochi et al. 2004). Because there are no other predators of terrestrial mollusks in North America that have a predatory archetype (sensu Cox and Lima 2006) similar to Bipalium, the native land mollusks could be quite vulnerable to this new predatory species, making ecological impacts more likely (Cox and Lima 2006). Alternatively, the impact of these planarians would be more complex if they prey preferentially on invasive snails and slugs. Hopefully, this paper will alert conservation biologists and land managers to the presence of this potentially damaging species. Continued monitoring of its geographic spread and ecological impacts is recommended. 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