Thursday, February 2, 2012

Leucochloridium paradoxum: Tasty Escargot


Leucochloridium paradoxum is one interesting parasite: not only does it cause a drastic deformity in its intermediate host, the Succinea snail, but it also provokes behavioral changes that seem to increase suicidal tendencies in the snail. L. paradoxum is a parasitic flatworm that is an endoparasite of both Succinea snails and of various birds, including crows, sparrows, and finches. The physical deformities caused by the parasite on the snails are attractive to birds, in which the ingested parasite then feeds off digested matter by the bird’s rectum [1]. The global distribution of the parasite depends largely on the range of its obligatory intermediate host – temperate forests of North America and Europe [2]. In regard to the ecological impact, L. paradoxum has no effect on humans, and its pathogenic effects on bird hosts are negligible since the parasite inhabits the rectum where it essentially feeds on waste that is about to be excreted [1].

Symbiont Description:

Leucochloridium paradoxum, also known as the green-banded broodsac, is a parasitic flatworm from the phylum Platyhelminthes and the class Trematoda [2]. This parasite is heteroxenous and includes a mollusk as an intermediate host. It is an endoparasite, with the egg as its only developmental stage that exists outside its hosts. It must remain moist to survive though, so egg mortality is relatively high since the parasites often do not reach appropriate hosts [1].

L. paradoxum has different developmental stages in its hosts and different means of acquiring nutrition. As a sporocyst in the snail, the worm absorbs nutrients through its tegument. As a fully developed, mature trematode in birds, adult L. paradoxum are long, dorsally flattened worms that have suckers for attachment to the rectum and feed on passing digested matter [2]. These adult parasitic worms are hermaphroditic and self-fertilize, but they can cross-fertilize when close to another worm [1].

Host Description:

The development of the parasite continues when the intermediate host Succinea snail, also known as the ambersnail, ingests the egg. The definitive host, where L. paradoxum reaches sexual maturity, is the bird. The parasite has little specificity for its definite host; it parasitizes many different bird species including Emberizidae and Passeridae (sparrows), Corvidae (crows and jays), and Fringillidae (finches) [2].

Life Cycle:

Because the parasite has two hosts, it has a heteroxenous life cycle. L. paradoxum begins its lifecycle as an egg in a bird dropping that is consumed by a Succinea snail. Within the snail the eggs hatch into miracidia, which then travel to the hepatopancreas of the snail to develop into sporocysts [1]. The sporocyst grows into long tubes that are divided into three parts: (1) a central body located in the snail's hepatopancreas where embryos are produced, (2) a swollen broodsac entering the snail’s tentacles atop its head, and (3) a tube connecting the broodsac to the central body. Embryos pass from the central body to the broodsac in the tentacles, where they mature into cercariae [3].

Sporocyst tubes outside of snails' tentacles.
This causes a drastic mutation of the tentacle into a pulsating, elongated, colorful eyestalk that mimics the appearance of a caterpillar [2]. The pulsating colors yellow, green, and red attract birds and allow for efficient transfer to the definitive host. Another mechanism to enhance transfer is behavioral changes in the snails. The pulsating is in response to light, and the mutated tentacles of the eyes inhibit the snail’s perception of light intensity. Therefore, normal snails inhabit dark areas under leaves, but infected snails expose themselves to light and birds [2].

The definitive host of the life cycle is a bird, which ingests the caterpillar-looking tentacles of snails containing hundreds of infective stages of L. paradoxum [4]. The cercariae develop into adult distomes in the bird’s digestive system. The adult parasitic worm attaches to the rectum and feeds on passing digested matter [2]. They are hermaphroditic and self-fertilize, but they can cross-fertilize when close to another worm. Eggs are excreted by the bird in its feces, which are then consumed by a snail to complete the life cycle of L. paradoxum [1].


L. paradoxum has no effect on humans. The types of birds that are definitive hosts (sparrows, crows, jays, and finches) are not consumed by humans, so transmission to humans is not likely. Also, its pathogenic effects on bird hosts are negligible since the parasite inhabits the rectum where it essentially feeds on waste that is about to be excreted [1]. Furthermore, after an infected snail’s tentacles have been picked off by a bird, the snail may still survive. However, this does not mean that the parasite is gone; the snail will continue to grow sporocysts and infect birds until it dies [2]. Because the parasite does not actively kill or prevent reproduction of the host species, the economic/ecological consequences of L. paradoxum are not grand.

An Example of an Extended Phenotype:

In our text, Claude Combes defines an extended phenotype as “a phenotype envisioned to occur beyond the physical limits of the organism to which the genes belong” [4]. In other words, the genome of the parasite can act on the phenotype of the host both morphologically or behaviorally. This is clearly the case in this parasite-host association. L. paradoxum causes a drastic deformity in the tentacles of Succinea snails by becoming pulsating, elongated, colorful eyestalks that mimic the appearance of a caterpillar. The mutated tentacles also inhibit the snail’s perception of light intensity, and therefore, the infected snails expose themselves to light and predators like birds [2].


[1] DeLaCruz, D. 2003. "Leucochloridium paradoxum" (On-line), Animal Diversity Web. Accessed February 01, 2012

[2] "Leucochloridium paradoxum" (On-line), Ecology of Life. Accessed February 01, 2012

[3] Schmidt, G.D., Roberts, L.S., Foundations of Parasitology, 6th ed. McGraw-Hill Comp., 2000

[4] Combes, Claude. Parasitism: The Ecology and Evolution of Intimate Interaction; Translated by Isaure De Buron and Vincent A. Connors; with a new Foreword by Daniel Simberloff. Chicago: University of Chicago, 2001.

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