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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Below are brief descriptions of several key invasive species to look out for in the estuary. Click on the image to enlarge. Detailed factsheets can be viewed by following the links shown.




American tingle or oyster drill (Urosalpinx cinerea)


© D Fenwick


  • Tall conical yellowish or grey shell (sometimes with brown irregular marks), with a sharply pointed spire, up to 4cm high and 2cm broad
  • Up to 8 rounded whorls with pronounced ridges and ribs
  • Oval aperture, with an open canal at the base





Carpet Sea Squirt (Didemnum vexillum)





  • Uniform pale orange, cream or off-white in colour
  • The surface has a firm leathery texture, with a veined marbled appearance that has numerous open small pores, which close up out of water to produce pale spots
  • It can grow either as thin flexible sheets or hang down in long rope-like growths




Urgent action required if found. Send your image to alert_nonnative@ceh.ac.uk



Chinese Mitten Crab (Eriocheir sinensis)





  • 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)




  • Up to 30mm in length (though usually around 10-20mm)
  • Curled and semi-transpararent body,
  • two pairs of antennae and large powerful jaws
  • Predator of our native shrimp and other native fauna.
  • Found in still or flowing fresh and brackish water



Urgent action required if found. Send your image to alert_nonnative@ceh.ac.uk



Orange-tipped sea squirt (Corella eumyota)


© JDD Bishop

© JDD Bishop


  • Up to 8cm in length
  • Lies flat with its exhalant orange tinged siphon slighlty to the right.
  • Its tunic is translucent and smooth, whilst its gut forms a smooth curve around its hind end.





Red ripple bryozoan (Watersipora subtorquata)


© Andrew N Cohen – CRAB

© Andrew N Cohen – CRAB


Rigid encrusting colonies, up to several cm across, consisting of of 1 mm individuals arranged as a continuous sheet, often forming rounded lobes, sometimes with erect portions formed by back-to-back growth.
Colonies orange-red, especially at growing edges, sometimes dark sepia, blackish or deep purple.
Individuals elongate, each with rounded, darker spot (the operculum) at far end.





Veined rapa whelk or Mangrove oyster (Rapana venosa)



  • Dark veined knobbly shell, with an orange interior
  • Can grow up to 18cm in length
  • Large shell opening, with small teeth on the outer lip





Wakame or Japanese Kelp (Undaria pinnatifida)


© Kathryn Birch – CCW

© Kathryn Birch – CCW


  • Brown in colour
  • Stipe (stem) has very wavy edges or ruffles at the base
  • A broad blade, flattened with a distinctive midrib
  • Between 1-3 metres in length





Wireweed (Sargassum muticum)





  • Olive-brown in colour
  • Often over 1m in length
  • Tough wiry stipe with regularly alternating branches
  • Small, flattened oval blades
  • Spherical gas bladders
  • Lateral branches, which hang like a washing line out of water