The mature Aspidogaster conchicola is approximately 2.5 to 3.0 mm in length and 1.0 mm wide, and somewhat resembles a miniature conch. A single large ventral sucker, known as an opisthaptor, takes up most of the surface area of its underbelly. The opisthaptor is divided into adhesive depressions (termed loculi) formed by muscular septa, which are useful in classification. Aspidogaster conchicola has 64-66 loculi, arranged in four longitudinal rows. An exterior longitudinal septum, which is a horizontal flap of muscle, divides the body anteriorly. The function of the septum is unknown.
A wide buccal funnel (mouth) has its opening at the anterior end of a flexible neck-like process. Pharyngeal muscles move food entering the buccal funnel into the intestine. Unlike the related Digenean worms, aspidobothreans have a simple, unbranched digestive tract that ends in the cecum, a digestive sac surrounded by muscle. Wastes are removed by flame cell protonephridia, which channel material through excretory ducts into an excretory bladder, eventually exiting the body through a posteriorly located excretory pore.
An interesting feature of A. conchicola is its complex nervous system, a feature more characteristic of free-living rather than parasitic worms. Aspidogaster conchicola possesses an anterior cerebral commissure, an intricate organization of nerves. This presumably serves to coordinate the peripheral nerves, which are arranged in a ladder-type system. Numerous types of sensory receptors have been observed, particularly concentrated around the buccal funnel and opisthaptor, indicating that there is a good degree of neural coordination of locomotive and digestive action. (Bailey and Tompkin, 1971; Halton and Lyness, 1971; Hathaway, 1971; Roberts and Janovy Jr., 2000)
Aspidogaster conchicola is a parasite of freshwater bivalves, with a wide distribution in North America, Africa, and Europe. This small fluke has a low degree of host specificity and has been found in a wide variety of other hosts, including snails, fish, and turtles. Inside the typical molluscan host, the parasite will usually take residence within the pericardial cavity. (Huehner and Etges, 1977; Rohde, 1972)
Not being well adapted to any single host species, A. conchicola will readily develop in a wide variety of freshwater mollusk, snail, fish, or turtle families. In laboratory settings A. conchicola was able to survive in water for two weeks without a host, and in a saline solution for up to five weeks. This suggests that Aspidogaster is an archaic species, not too far removed from a free-living ancestor. (Rohde, 1972; Rohde, 1998)
Aspidogaster conchicola usually develops within mollusks, but is a facultative parasite of vertebrates, should their mollusk host be eaten. Their life cycle is still direct, requiring only one host for maturation. Infection occurs through intake of the egg, containing a fully developed larva, by the mollusk's siphon. Upon entering the host, the cotylocidium (larvae) will hatch and immediately begin maturation without further migration. At hatching, the larvae are 13 to 17 micrometers long. Since the cotylocidium does not seek the host as in other trematodes, it is unciliated. Instead, it has a simple posterior sucker lacking loculi. There have been no documented cases of Aspidogaster directly infecting a vertebrate host as a free egg or larva, while they have been found in the intestines of fish, so it is assumed that vertebrates are infected by ingestion of parasitized mollusks.
The large ventral opisthaptor appears to play a major role in Aspidogaster conchicola feeding behavior. Muscular action mechanically disrupts host tissues while marginal organs within the sucker release secretions for extracorporeal digestion. The predigested material is then sucked in through the buccal funnel.
Aspidogaster infections appear to be most prevalent and intense during the colder months, from November through April. (Bailey and Tompkin, 1971; Halton and Lyness, 1971; Huehner, 1987; Huehner, et al., 1989)
The primary hosts for Aspidogaster conchicola are freshwater mollusks, usually mussels. However, as a facultative parasite of vertebrates, it is not restricted to mussels, and will also enter into and develop within freshwater snails, fishes, and turtles while feeding on epithelial tissue. (Rohde, 1972)
The aspidobothreans do not hold any economic or medical significance for humans. However, they have several archaic characteristics which suggest that they are an ancient group, and so continue to be studied because they seem to represent a link between parasitic and free-living organisms. Since Aspidogaster has not developed any close associations with particular host species, it has been suggested that it is an archaic species of trematode, similar to the hypothetical ancestor from which the related digenean trematodes have evolved their complex life cycles exploiting multiple hosts. (Rohde, 1972)
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