Theodoxus fluviatilis is considered holarctic, occurring in both the Nearctic and Palearctic biogeographic regions. However, they are more commonly seen in the Palearctic region, spreading far west to England, and east into southwestern Asia. Many patches within Europe are uninhabited, mainly the mountainous regions. This snail's ability to survive in freshwater allowed for migration throughout Eastern Europe, and later re-entry into the brackish waters of the Baltic Sea. (Bunje, 2005; Symanowski and Hildebrandt, 1984; Zettler, et al., 2004)
Commonly known as the river nerite, Theodoxus fluviatilis was first discovered in saltwater, originating in the Black Sea. Currently this species is mainly found in freshwater streams and rivers. The river nerite is also found along coastal regions of the Black and Baltic Seas, and also lives in estuaries, regions where freshwater rivers meet saltwater seas. Within this habitat, it lives on hard benthic substrates, typically rocks and submerged wood, and in calcium-rich waters. Theodoxus fluviatilis also lives as deep as 60 meters below the surface, and is sometimes found no higher than a centimeter above the water surface. (Bunje, 2005; Kirkegaard, 2006; Schneckli, 2009; Symanowski and Hildebrandt, 1984; Zettler, et al., 2004)
Although difficult to distinguish between different species of Theodoxus, Theodoxus fluviatilis has a few main morphological features used for identification. These include size, color patterns of the periostracum, the operculum and the radula. The common river nerite has a soft-tissued body encased within a round, calcareous shell. This species is known to absorb the inner walls of the shell, thus allowing for a more spacious environment inside the shell. This absorption may allow more room for their food intake. The operculum, a hard, proteinaceous plate protecting the body when fully retracted within the shell, is light red with broad ribs. The margin of the operculum is red.
The species has an average mass of 50 mg, an average shell length of 9 mm, and an average shell width of 6 mm. The different specimens, marine and freshwater, show some morphological variances. Freshwater specimens tend to be larger and thicker than the marine forms. Freshwater specimens also have a yellowish-green color, while marine forms are mostly black. Since they are dioecious, the male and female sexes are separate. Both have reproductive organs. The male's penis is located on the right side of their body, near the base. The female reproductive organs are located inside the mantle cavity. Within the mantle cavity, females have two openings, one for fertilization and the other for discharging their eggs. (Hickman Jr., et al., 2009; Kirkegaard, 2006; Orstan, 2007; Schneckli, 2009; Welter Schultes, 2009; Zettler, et al., 2004)
Theodoxus fluviatilis develops in a similar manner as most other gastropods, however, the snail develops within the egg, so there is no larval form. The shell forms as the egg is developing. Initially, the mouth is anterior and the anus is posterior. However, after a process known as torsion, the positions of the body parts change.
Torsion involves two steps, each being a 90 degree rotation. The first rotation is caused by a contraction of the foot retractor muscle. It rotates the shell and visceral mass 90-degrees counterclockwise, leaving the anus on the right side of the body. The second rotation is caused by further development of differentiating tissues. This allows for the mantle cavity to develop near the anus and an additional 90-degree rotation, placing the anus directly above the head and mouth.
The positioning of the anus above the head would normally result in sanitary problems, with wastes washing directly over the gills. However, this is resolved by another process known as coiling. Coiling is not the same as torsion, however it can occur at the same time in development as torsion. Coiling of the shell and visceral mass allow for the loss of the gill, auricle, and kidney, all on the right side of the mantle cavity, and thus allow the snails to avoid sanitation problems. Since the water-flow through the mantle cavity is unidirectional, it flows into the left side and out of the right side, carrying with it wastes from the anus, which is near the right side. The single gill on the left side is then exposed only to clean water, avoiding any such sanitation problems. Once eggs hatch, they are considered miniature adults and take 18 months to fully mature into adult snails. (Hickman Jr., et al., 2009; Orton and Sibley, 1990)
Theodoxus fluviatilis reproduces between April and October. These snails have separate sexes and reproduce sexually. The male reproductive organ is located on the right side of its body. The female reproductive organs are within the mantle cavity. In the mantle cavity, there are two openings, one for fertilization and another for discharging the eggs. Fertilization is internal. The nerites are oviparous, females laying calcified eggs, normally on benthic surfaces or on their shell near the opening. (Schneckli, 2009)
Theodoxus fluviatilis are dioecious and oviparous. They are semelparous, producing offspring all at once, after which the parent usually dies. During their mating season, anywhere from 50 to 200 eggs are laid. These eggs are collectively known as a capsule, and only one of them contains an embryo. Each egg is made of two spherical halves, which split when newborns hatch. The rest are used as a food source for the newborns. Embryos usually take about 100 days to fully develop. Since they are involved in a biennial life cycle, their eggs hatch some time in the fall as miniature adults and fully mature 18 months later. (Kirkegaard, 2006; Orstan, 2007; Orton and Sibley, 1990; Schneckli, 2009)
Not much is known on the parental investment of T. fluviatilis, but some were found with their eggs attached to their shell near the opening. This could be some form of protection of the eggs from possible predators or even by providing nutrients to their young. The young are precocial when they hatch. (Orstan, 2007)
Theodoxus fluviatilis grazes on hard benthic surfaces, feeding on diatoms, algae, and detritus. These nerites usually reside on stones, but some are found on submerged wood. Due to their delicacy and likelihood of being swept away by strong currents, the snails tend to stay away from softer plants. This species does not hibernate in the winter, and thus is active all year round. Theodoxus fluviatilits is more active at night than in the day, but is not considered to be truly nocturnal. As they are known to be grazers, their body only protrudes slightly from their shell, and their antennae are the only visible parts of their body. The antennae house well-developed eyes at their ends, which give the snails a visual sense of where they move. Theodoxus fluviatilis does not to migrate between freshwater and brackish water, but supspecies are adapted to either freshwater or saltwater. (Bunje, 2005; Kirkegaard, 2006; Schneckli, 2009; Symanowski and Hildebrandt, 1984)
Since they are grazing creatures that move very slowly while they feed, their home range is relatively small. However, no specific measurements of the home range are known.
Like many other gastropods, Theodoxus fluviatilis has a well-developed nervous system. Sense organs include eyes, statocysts, tactile organs, and chemoreceptors. Statocysts are fluid-filled cellular cysts used to sense direction of gravity, and thus sense equilibrium. Chemoreceptors are used to detect chemical stimuli. The snails also have a pair of eyes, each located at the end of their antennae that give them a visual sense perception of their habitat. Thus, they see their environment, sense movement, and also sense their state of balance. (Hickman Jr., et al., 2009; Schneckli, 2009)
As Theodoxus fluviatilis is an herbivore and a detritivore, it mainly feeds on phytoplankton (mainly diatoms), but also feeds on detritus and a variety of algae. Its radula is specialized to grind even the hardest phytoplankton, making it easier to digest. Detritus is the organic material that comes from dead or decomposing plants or animals. While developing, the larva will feed within the egg capsule. (Kirkegaard, 2006; Orton and Sibley, 1990; Schneckli, 2009; Zettler, et al., 2004)
Because their hard calcareous shell completely covers their body, most predators avoid Theodoxus fluviatilis as a potential food source. Their eggshells are also calcified and hardened, thus protecting them when predators attack. However, a few known predators include the European perch, Perca fluviatilis, as well as two crayfishes, Orconectes limosus and Astacus astacus. These animals have adaptations that allow them to break through the snails' calcareous shell. Orconectes limosus, endemic to North America, is an invasive species in Europe. (Schneckli, 2009)
Theodoxus fluviatilis plays an important role for the health and maintenance of large structuring macrophytes. They aid the growth of the perennial brown alga, Fucus vesiculosus. While other grazers inhibit the ability of Fucus to uptake nitrogen, T. fluviatilis does not, and thus aids in the growth of the algae. Not much is known about their symbiotic relationships with other animals. (Kautsky and Raberg, 2008)
There are no known positive effects of Theodoxus fluviatilis on humans.
There are no known adverse effects of Theodoxus fluviatilis on humans.
Theodoxus fluviatilis are considered extinct in the Czech Republic. They are also endangered in Switzerland and Latvia, and considered in jeopardy in other European countries including the Netherlands and Slovenia. They are also protected under law in Latvia. (Schneckli, 2009)
There are two subspecies, one freshwater and one saltwater. This freshwater form is called Theodoxus fluviatilis fluviatilis, while the salt-water species is known as Theodoxus fluviatilis littoralis. The freshwater species can survive in up to 1.5% saltwater concentrations, while the saltwater species can survive in up to 18% saltwater. Their ability to survive in both fresh and salt waters allowed for its adaption to different salt water concentrations. (Bunje, 2005; Kirkegaard, 2006; Symanowski and Hildebrandt, 1984; Zettler, et al., 2004)
Mahmoud Abdallah (author), University of Michigan-Ann Arbor, Phil Myers (editor), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
areas with salty water, usually in coastal marshes and estuaries.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
an animal that mainly eats decomposed plants and/or animals
particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.
union of egg and spermatozoan
a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.
mainly lives in water that is not salty.
An animal that eats mainly plants or parts of plants.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.
Found in northern North America and northern Europe or Asia.
fertilization takes place within the female's body
referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.
Having one mate at a time.
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)
mainly lives in oceans, seas, or other bodies of salt water.
offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
uses sight to communicate
young are relatively well-developed when born
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