About Leucochloridium paradoxum Carus, 1835
Identification: The simplest way to distinguish between species of Leucochloridium is by the appearance of the broodsacs located in their host snail’s tentacle. Leucochloridium paradoxum has broodsacs marked with green bands, dark brown and black spots, and a dark-brown or reddish-brown tip. As broodsacs mature, their colors become brighter and their tips become browner. Today, this visual identification can be confirmed using ribosomal DNA sequencing. A single snail can be infected by multiple Leucochloridium species at the same time.
Adult L. paradoxum are small parasites that live in the cloaca of birds, and they are not well-studied, making species-level identification more difficult. Useful distinguishing features for L. paradoxum include a round body, an oral sucker that is slightly larger than the ventral sucker, and gonads positioned close to the ventral sucker. Even with these traits, L. paradoxum cannot be reliably told apart morphologically from L. perturbatum.
Habitat: Leucochloridium paradoxum occurs in moist environments such as marshes, where its common intermediate host, Succinea snails, live.
Distribution: Leucochloridium paradoxum was first described from the sporocyst stage, collected from an island in the Elbe river at Pillnitz, near Dresden, Germany. Confirmed locations include Poland, the Czech Republic, Belarus, Belgium, the Saint Petersburg area of Russia, Denmark, Sweden, Norway, the Netherlands, and the United Kingdom. It has also been recorded in Japan, though this Japanese population may be a separate species. L. paradoxum is thought to be the species that infects an endemic semi-slug species on Robinson Crusoe Island in the Pacific; this is the only known record of the parasite from the Southern Hemisphere.
Life cycle: All Leucochloridium species share a similar life cycle, parasitizing both snails and birds. Unlike most typical trematodes, their life cycle is truncated: the same snail acts as both the first and second intermediate host.
After eggs are ingested by a snail, they hatch into miracidia in the snail’s stomach or midgut. These miracidia are 23 μm long, elliptical in shape, and contain only around 10 cells. Cilia, which partially form cirri, allow this stage to move, and a stylet at the front end is thought to help the miracidium penetrate the intestinal wall. Once a miracidium reaches the snail’s hepatopancreas, it disintegrates to release a germinal cell. This cell develops into the next life stage, a sporocyst, inside the hepatopancreas.
A mature sporocyst is made of multiple branches that spread through the snail’s haemocoel, and can account for one fifth or more of the snail’s total weight. Some branches develop into long tubes that end in a swollen broodsac. Broodsacs develop at staggered rates, so normally only two or three are mature at the same time. The basal part of the sporocyst asexually produces many tail-less cercariae, which develop directly into metacercariae inside the sporocyst, and deposit a thick mucoid coat around themselves. Mature metacercariae are oval, measuring 1.2 × 0.8 mm, and typically 100 to 250 metacercariae accumulate inside one broodsac.
A mature broodsac enters one or both of the snail’s tentacles, altering the tentacle’s appearance. The tentacle swells, and the pulsating, colorful, banded broodsac visible inside mimics the look of an insect larva such as a caterpillar. This mimicry encourages insectivorous birds, the parasite’s next host, to eat the tentacle or broodsac. Captive observations show that birds tear the broodsac out of the snail before eating it, so the snail may survive the encounter. Birds can also become infected by eating broodsacs that have spontaneously burst from the snail’s tentacle; these broodsacs survive for an hour and continue to pulsate after bursting.
If a broodsac is eaten by a bird, the metacercariae travel through the bird’s alimentary tract and attach in the gut. They have been found in the large intestine, cloaca, and bursa of Fabricius. After losing their mucoid coat, they develop into adult distomes around 1.5 mm long. Adult L. paradoxum have two suckers on their ventral side that anchor them to the gut wall, and a smooth dorsal surface. Adults are hermaphroditic, and release eggs into the bird’s faeces. In experimental settings, the first eggs appear in faeces 13 days after the bird is infected. Some eggs are then eaten by a snail, completing the life cycle.
In a Russian study, snails became infected in spring and summer. The resulting sporocysts began producing infective metacercariae the following spring, then died in late summer. The adult stage of the parasite is thought to live for weeks to months inside its bird host.
