ABOUT MSEP

We are a not-for profit organisation established in 2000, whose work is centred around the understanding, conservation and promotion of the estuary’s natural and historical environments.

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Medway Swale Estuary Partnership | Freshwater
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Below are brief descriptions of several invasive species to look out the estuary’s freshwater tributaries, lakes and ditches. Click on the image to enlarge. Further information on all these species and others can be found here.

 

 

PLANTS

 

 

Floating Pennywort (Hydrocotyle ranunculoides)

 

NNSS_Image_2488

© GBNNSS

 

  • Fleshy stems and roundish blunt toothed leaves, held horizontal and usually dissected to the middle
  • Erect greenish flowers without petals
  • Forms thick mats which impede water flow and cause deoxygenation
  • Can grow up to 20cm per day

 

 

New Zealand Pygmyweed (Crassula helmsi)

 

© GBNNSS

© GBNNSS

 

  • Small round fleshy leaves up to 2cm long, arranged along the stem in pairs
  • Very small flowers, with small white petals
  • Can be sumerged, emergent and terrestrial
  • Forms dense inpenetrable mats that displaces other aqautic plant species

 

 

Parrots Feather (Myriophyllum aquaticum)

 

© GBNNSS

© GBNNSS

 

  • Aquatic perennial both emergent and sumergent forms
  • Blue-green feather like leaves, inwhols of 4-6
  • Can block ditches and dominate ponds
  • Emergent form dies back in winter, but submerged form present all year

 

 

Water Fern (Azolla filiculoides)

 

© GBNNSS

© GBNNSS

 

  • Very small free floating water plant
  • Leaves range from green to brown and have a fern like rough granular appearance
  • Black brown roots hang below the plant, which are easily broken
  • Forms dense mats on the surface of still waters

 

 

Water Primrose (Ludwigia grandiflora)

 

© Trevor Renals

© Trevor Renals

 

  • Creeping perennial water plant, with long oval willow like leaves
  • Large bright yellow flower, similar to that of a primrose
  • Can produce huge amounts of seed per year
  • Currently known at just a few site in the UK

 

Species Alert! Report any sightings as soon as possible to: alert_nonnative@ceh.ac.uk

 

 

 

Invertebrates

 

 

 

Asian Clam (Corbicula fluminea)

 

NNSS_Image_2692

© GBNNSS

 

  • Mollusc usually less than 25mm in size, but can grow up to  50 to 65mm
  • Yellow-green to brown round triangular shell, with evenly spaced surface ridges
  • Can reach high densities and outcompete native species for food and space
  • Threatens fish spawning grounds and native freshwater mollusc species

 

 

Chinese Mitten Crab (Eriocheir sinensis)

 

© FERA

© FERA

 

  • Only freshwater crab found in the UK
  • Migrates downstream in autumn to spawn in estuaries
  • Green, brown or grey in colour, with white tipped fron pincers covered in a dense matt of fine hairs
  • Long hairy legs, with a square body up to 86mm across
  • Undermines riverbanks through burrowing, leading to an increased risk of erosion

 

 

Killer Shrimp (Dikerogammarus villosus)

 

© Environment Agency

© Environment Agency

 

  • Up to 30mm in length (though usually around 10-20mm), with a curled and semi-transpararent body, two pairs of antennae and large powerful jaws
  • Predator of our native shrimp and other native fauna. Likely to disrupt ecosystems through direct predation and indirect effects across food chains
  • Carries parasites which could reduce fish stocks
  • Found in still or flowing fresh and brackish (estuarine) waters

 

Species Alert! Report any sightings as soon as possible to: alert_nonnative@ceh.ac.uk

 

 

Quagga Mussel – Dreissena rostriformis bugensis

 

© www.100thmeridian.org

© www.100thmeridian.org

 

  • First recorded in the UK in 2014
  • Small with stripy shell (similar to Zebra Mussel)
  • More rounded in cross section, unlike Zebra Mussel which is triangular
  •  Has an undulating seam between the valves (shells)
  • Rolls to the side when placed on its front, Zebra Mussel tends to lie flat
  • Has a small byssal groove, unlike Zebra Mussel which has a large groove towards its middle (which may not always be obvious)

 

Species Alert! Report any sightings as soon as possible to: alert_nonnative@ceh.ac.uk

 

 

Signal Crayfish (Pacifastacus leniusculus)

 

© GBNNSS

© GBNNSS

  • Much larger than the native White-clawed Crayfish, although junveniles of both species are very similar
  • Claws are bright red underneath, with a small turquoise/white spot on the surface
  • Spreads up and down stream and may cross land to colonise adjacent waterbodies, where it will dominate and replace our native crayfish
  • Carries Crayfish plague, which is deadly to our native crayfish

 

 

Zebra Mussel (Dreissena polymorpha)

 

© GBNNSS

© GBNNSS

 

  • Invasive mollusc up to 20mm in length
  • Brownish yellow in colour, with a characteristic dark and light coloured zigzag (zebra) banding
  • Can significantly reduce native biodiversity and alter whole freshwater systems by filteration
  • Has become a major economic pest, due to its ability to block water pipes