Theodoxus fluviatilis

Geographic Range

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.

• Biogeographic Regions
• nearctic nearctic
  • introduced introduced
• palearctic palearctic
  • native native

• Other Geographic Terms
• holarctic holarctic

Habitat

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.

• Habitat Regions
• temperate temperate
• saltwater or marine saltwater or marine
• freshwater freshwater

• Aquatic Biomes
• benthic benthic
• lakes and ponds
• rivers and streams
• coastal coastal
• brackish water brackish water

• Other Habitat Features
• estuarine estuarine

Physical Description

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.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

Development

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.

Reproduction

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.

• Mating System
• monogamous monogamous

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.

• Key Reproductive Features
• semelparous semelparous
• sexual sexual
• fertilization fertilization
  • internal internal
• oviparous oviparous

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.

• Parental Investment
• precocial precocial
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female

Lifespan/Longevity

Theodoxus fluviatilis lives in nature for approximately two to three years. Longevity in an aquarium kept at room temperature has been recorded as two years.

Behavior

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.

• Key Behaviors
• motile motile

Home Range

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.

Communication and Perception

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.

• Communication Channels
• visual visual
• tactile tactile
• chemical chemical

• Perception Channels
• visual visual
• tactile tactile
• chemical chemical

Food Habits

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.

• Primary Diet
• herbivore herbivore
  • algivore
• detritivore detritivore

• Plant Foods
• algae
• phytoplankton phytoplankton

• Other Foods
• detritus detritus

• Foraging Behavior
• filter-feeding filter-feeding

Predation

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 potential predators include 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.

Ecosystem Roles

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.

Economic Importance for Humans: Positive

There are no known positive effects of Theodoxus fluviatilis on humans.

Economic Importance for Humans: Negative

There are no known adverse effects of Theodoxus fluviatilis on humans.

Conservation Status

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.

Other Comments

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.

Additional Links

Encyclopedia of Life

Contributors

Mahmoud Abdallah (author), University of Michigan-Ann Arbor, Phil Myers (editor), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects.

References

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