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💧Limnology

Molluscs are diverse invertebrates found in marine and freshwater environments. Gastropods and bivalves, two major classes, differ in body structure and lifestyle. Gastropods have a single shell and muscular foot, while bivalves have two hinged valves.

Molluscs play vital roles in aquatic ecosystems, influencing nutrient cycling and water clarity. They exhibit various adaptations, including specialized shell morphology, feeding strategies, and reproductive methods. Some species serve as bioindicators, while others are invasive and impact native ecosystems.

Mollusc classes

  • Molluscs are a diverse phylum of invertebrates found in both marine and freshwater environments
  • Two major classes of molluscs are gastropods and bivalves, which differ in their body structure and lifestyle

Gastropods

Top images from around the web for Gastropods
Top images from around the web for Gastropods
  • Gastropods are characterized by a single shell and a muscular foot used for locomotion
  • Include a wide variety of organisms such as snails, slugs, limpets, and nudibranchs
  • Gastropods have a well-developed head with eyes and tentacles for sensing their environment

Bivalves

  • Bivalves have a shell composed of two hinged valves that enclose their soft body
  • Lack a distinct head and instead have a muscular foot used for burrowing or anchoring
  • Examples of bivalves include clams, mussels, oysters, and scallops

Gastropod diversity

  • Gastropods exhibit a wide range of morphological and ecological diversity
  • Can be found in various aquatic habitats, from shallow streams to deep ocean trenches

Prosobranch snails

  • Prosobranch snails have gills located in front of their heart and a shell that can be closed by an operculum
  • Many prosobranch snails are grazers, feeding on algae and detritus (Neritina, Viviparus)
  • Some species have adapted to live in extreme environments, such as hydrothermal vents (Alviniconcha)

Pulmonate snails

  • Pulmonate snails have a lung-like organ for breathing air and lack an operculum
  • Includes many common freshwater snails (Lymnaea, Physa) and land snails (Helix)
  • Some pulmonate snails serve as intermediate hosts for parasites that infect humans and wildlife

Limpets

  • Limpets have a conical shell and a strong muscular foot that allows them to cling tightly to rocks
  • Primarily marine, limpets are important grazers in intertidal and subtidal zones
  • Keyhole limpets (Fissurellidae) have a hole at the apex of their shell for waste elimination

Bivalve diversity

  • Bivalves are filter feeders that play important roles in aquatic ecosystems
  • Diversity of bivalves includes both marine and freshwater species

Freshwater mussels

  • Freshwater mussels (Unionidae) are long-lived, sedentary bivalves found in rivers and lakes
  • Larvae (glochidia) are parasitic on fish, which aids in dispersal
  • Sensitive to water quality and habitat alterations, making them important bioindicators

Fingernail clams

  • Fingernail clams (Sphaeriidae) are small, ubiquitous bivalves in freshwater habitats
  • Have a short lifespan and high reproductive rate, allowing them to quickly colonize new areas
  • Play a significant role in nutrient cycling and serve as prey for many aquatic organisms

Zebra mussels

  • Zebra mussels (Dreissena polymorpha) are invasive bivalves native to the Black and Caspian Seas
  • Spread rapidly through human activities, such as boat traffic and ballast water discharge
  • Cause significant ecological and economic impacts by outcompeting native species and clogging water intake pipes

Mollusc adaptations

  • Molluscs have evolved various adaptations to survive in different aquatic environments
  • These adaptations include specialized shell morphology, feeding strategies, and reproductive methods

Shell morphology

  • Mollusc shells provide protection from predators and environmental stressors
  • Shell shape and thickness vary depending on the species' habitat and lifestyle (e.g., streamlined shells for burrowing, thick shells for protection)
  • Some molluscs, such as nudibranchs, have reduced or lost their shells entirely

Feeding strategies

  • Molluscs exhibit diverse feeding strategies, including grazing, filter-feeding, and predation
  • Many gastropods have a radula, a rasping tongue-like structure used for scraping algae or cutting prey
  • Bivalves use their gills to filter particles from the water column, capturing food such as phytoplankton and organic detritus

Reproduction

  • Molluscs display a variety of reproductive strategies, from external fertilization to brooding young within the shell
  • Some species, such as freshwater mussels, have a complex life cycle involving a parasitic larval stage on fish hosts
  • Certain gastropods, like apple snails (Pomacea), lay egg masses above the waterline to protect them from aquatic predators

Ecological roles

  • Molluscs play vital roles in aquatic ecosystems, influencing nutrient cycling, water clarity, and community structure
  • Their ecological functions include herbivory, filtration, and biofouling

Herbivory

  • Many gastropods, such as periwinkles (Littorina) and pond snails (Lymnaea), graze on algae and aquatic plants
  • Herbivorous molluscs can control algal growth and maintain the balance of primary producers in aquatic systems
  • In some cases, overgrazing by molluscs can lead to shifts in plant community composition

Filtration

  • Bivalves, such as mussels and clams, filter large volumes of water, removing suspended particles and improving water clarity
  • Filtration by bivalves can help mitigate the effects of eutrophication by reducing phytoplankton blooms
  • The presence of healthy bivalve populations can indicate good water quality in aquatic ecosystems

Biofouling

  • Some molluscs, particularly bivalves like zebra mussels (Dreissena polymorpha), can attach to and grow on submerged surfaces
  • Biofouling by molluscs can cause economic damage to infrastructure, such as water intake pipes and boat hulls
  • In natural systems, biofouling molluscs can provide habitat complexity and shelter for other organisms

Molluscs as bioindicators

  • Molluscs are often used as bioindicators to assess the health of aquatic ecosystems
  • Their sensitivity to pollution and ability to accumulate toxins make them valuable tools for monitoring water quality

Sensitivity to pollution

  • Many mollusc species are sensitive to pollutants, such as heavy metals, pesticides, and excess nutrients
  • Changes in mollusc populations or community composition can indicate the presence of environmental stressors
  • Freshwater mussels, in particular, are considered excellent bioindicators due to their long lifespans and sedentary nature

Accumulation of toxins

  • Molluscs can accumulate pollutants in their tissues over time, providing a record of environmental contamination
  • Bivalves, such as oysters and mussels, are commonly used in biomonitoring programs to detect and track pollutants
  • Analyzing contaminant levels in mollusc tissues can help identify sources of pollution and assess potential risks to human health

Invasive mollusc species

  • Invasive molluscs can have significant impacts on aquatic ecosystems and human activities
  • These species often spread through human-mediated transport, such as boat traffic and aquaculture

Impacts on ecosystems

  • Invasive molluscs can outcompete native species for resources, leading to declines in biodiversity
  • Some invasive species, like the New Zealand mud snail (Potamopyrgus antipodarum), can reach high densities and alter nutrient cycling
  • Invasive molluscs can also serve as vectors for the introduction of parasites and diseases

Control measures

  • Managing invasive mollusc populations often involves a combination of physical, chemical, and biological control methods
  • Physical control measures include manual removal, barriers, and water level manipulation
  • Chemical control, such as the use of molluscicides, can be effective but may have unintended consequences for non-target species
  • Biological control, using natural predators or parasites, is an emerging approach to managing invasive molluscs

Conservation of molluscs

  • Many mollusc species are threatened by habitat loss, pollution, and invasive species
  • Conservation efforts focus on protecting threatened species and their habitats

Threatened species

  • Numerous mollusc species are listed as threatened or endangered, particularly freshwater mussels and land snails
  • Factors contributing to mollusc declines include dam construction, channelization, and agricultural runoff
  • Conservation strategies for threatened molluscs include captive breeding programs, reintroductions, and habitat restoration

Habitat protection

  • Preserving and restoring mollusc habitats is crucial for their conservation
  • Protecting riparian zones and maintaining natural flow regimes can help support freshwater mollusc populations
  • Establishing marine protected areas can safeguard critical habitats for marine molluscs, such as oyster reefs and seagrass beds
  • Engaging local communities and stakeholders in conservation efforts is essential for the long-term protection of mollusc species and their habitats


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.