You are here:   Home » Fish & Wildlife » Species of Interest » Aquatic Invasive Species » Mollusc



Zebra Mussel

Montana ANS Priority Class 1 and Federal Injurious Wildlife Species


Zebra Mussel. Zebra Mussel
  • Dreissena polymorpha
  • Has caused profound ecological changes in freshwater ecosystems where it is established, including losses of phytoplankton and microzooplankton
  • Causes mortality of native clams and mussels
  • Cost municipal and industrial water facilities millions of dollars to control
  • If introduced into Montana will likely survive, become established and result in significant environmental and industrial damage
  • Transported overland in bilges, livewells, motors and attached to boats and trailers

General Characteristics

  • larvae not possible to see with naked eye
  • newly settled larvae feel like sand paper on smooth surfaces
  • juveniles are pepper corn size
  • mussel with brown to black stripes
  • adults can grow up to 2" long, typically are ¾" long


US Geological Survey (USGS)—Zebra Mussel

New Zealand Mudsnail

Montana ANS Priority Class 2


New Zealand Mudsnail Map. New Zealand Mudsnail Distribution Map (USGS)

New Zealand Mudsnail. Key shows the size of New Zealand Mudsnails.
  • Potamopyrgus antipodarum
  • Spread via stocking operations, by recreational anglers, or possibly aquarium release
  • Reproduces asexually and thus it only takes one to start a new population
  • Found in lakes and rivers
  • Can close operculum to avoid desiccation
  • Out competes native snails and other invertebrate
  • May impact fisheries by reducing food availability

General Characteristics

  • range in size from a grain of sand to 1/8" long
  • color brownish to bluish-green
  • operculum covers opening of shell, which is on left
  • densities reported over 500,000 per square meter!


US Geological Survey (USGS)—New Zealand Mudsnail