The following resources have proven very useful to me during the development of www.seawater.no:

Taxonomy and general species identification sites:

Fish:

  • FishBase: A Global Information System on Fishes

Mollusks:

  • Sea Slug Forum: The worlds by far most extensive website about sea slugs, run by Australian Museum.
  • Scottish Nudibranchs: An extensive list and beautiful images of nudibranchs.

Sponges:

Cnidarians:

  • actiniaria.com: An extensive collection beautiful images of sea anemones from around the world.

Crustaceans:

  • crustacea.net: Provides illustrated, interactive keys to worlds crustaceans.