Sound the ram's horn: Planorbella/Helisoma (Planorbidae)

The next snail species of interest once belonged to the genus Helisoma, but has recently been moved to the genus Planorbella. These are air-breathing freshwater snails. Two species in this genus are of interest as the first intermediate hosts of the carnivore trematode Alaria marcianae. These are Planorbella trivolvis and Planorbella campanulatum, commonly known as the marsh rams-horn and bell-mouth rams-horn respectively. However, the status of P. campanulatum as a valid species is under dispute.

Helisoma trivolvis
By Smithsonian Environmental
Research Center, CC BY 2.0
via Wikimedia Commons

Planorbella trivolvis looks a little different from the snails with spire-like shells that we have looked at before. Being a planorbid snail, it has a 15 - 18 mm flat coiling shell (called planispiral) with 3-4 whorls. And unlike the other snails, the shell is sinistral, i.e left-turning. It is light brown, dark brown, yellowish or chestnut colored. There are fine striae between the whorls. The mouth/aperture is large, with the lip being thickened internally, akin to a ram's horn/shofar. Both sexual and asexual reproduction is seen in these species and eggs are laid in small (~3mm) egg sacks.

Planorbella trivolvis is distributed throughout North America and has been introduced in other parts of the world. It prefers calm standing fresh waters (ecologically called "lentic") to running water (ecologically called "lotic"). The single foot allows locomotion under the surface of water.

In a 2007 survey of Planorbella snails from New Jersey, Klockars et al. recovered 8 types of cercariae. Two types of cercariae were identified as Echinostoma trivolis and Zygocotyle lunata while others were classified as armatae, brevifurcate apharyngeate, longifurcate pharyngate and brevifurcate pharyngate cercariae. Planorbella snails are first intermediate hosts of the following trematodes.

Helisoma trivolvis
By Say, 1817 -
Naturalis Biodiversity Center, CC0,
via Wikimedia Commons

1. Alaria:  As with other snails, Alaria miracidium infect the snail, develop thorough the sporocyst stage to become the cercarial stage, which are shed by the snail. The cercariae infect suitable amphibians such as tadpoles (frogs) and become the infective mesocercaria. Several other paratenic hosts can also be involved in this life cycle.

2. Zygocotyle lunata is a paramphistomid trematode found in the caecum of wild waterfowl (which are the definitive hosts) and may also incidentally affect ruminants. In North and South America, Z. lunata uses Planorbella as its first intermediate host. When Planorbella is not available, four species of Biomphalaria may serve as alternative first intermediate hosts. Z. lunata causes severe pathological changes in P. trivolvis and causes an apparent castration due to rupture and destruction of the snail's gonads.

3. Echinostoma trivolvis (and other related species) cause a zoonotic, intestinal food borne trematodiasis called echinostomiasis in humans and other vertebrate hosts. E. trivolvis uses amphibians and fish as second intermediate hosts. As with other food borne trematodiasis, humans become infected by consuming raw/undercooked second intermediate hosts. Symptoms point to a primarily gastrointestinal pathology in humans.

4. Bolbophorus confusus is a strigeoid trematode of American white pelicans (Pelicanus erythrorhynchous) that uses P. trivolvis as first intermediate host and about 28 species of fish as second intermediate hosts.

5. Riberoia ondatrae, a trematode that causes limb malformations in amphibians also uses P. trivolvis as intermediate host


References:
Fried, Bernard, et al. "The biology of the caecal trematode Zygocotyle lunata." Advances in Parasitology 69 (2009): 1-40.

Klockars, Jennifer, Jane Huffman, and Bernard Fried. "Survey of seasonal trematode infections in Helisoma trivolvis (Gastropoda) from lentic ecosystems in New Jersey, USA." Comparative Parasitology 74.1 (2007): 75-80.

Fox, Alfred Carter. The life cycle of Bolbophorus confusus (Krause, 1914) Dubois, 1935 (Tremotoda: Strigeoidea) and the effects of the metacercariae on fish hosts. Diss. Montana State University-Bozeman, College of Agriculture, 1965.

Johnson, Pieter TJ, et al. "Parasite (Ribeiroia ondatrae) infection linked to amphibian malformations in the western United States." Ecological Monographs 72.2 (2002): 151-168.