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Sea slug

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Sea slug is an informal name for a group of Gastropods which look similar, but are not monophyletic. Their similar shape and life styles is an example of convergent evolution. The group includes:

  • Nudibranchs
  • Cephalaspidea: a sub-order of Opisthobranch Gastropod molluscs. They usually possess a reduced internal shell.
  • Sacoglossa, a clade of Heterobranch sea slugs and snails which go in for kleptoplasty ('stealing' plastids). They eat algae and keep the chloroplasts for their own use.
  • Sea butterflies: a group of pelagic sea snails sometimes called pteropods. They belong to the clade Thecosomata.
  • Sea angels, or cliones, are Opisthobranch Gastropod molluscs of the clade Gymnosomata.
  • Sea hares are medium to large size marine Gastropod molluscs. They belong to two families of the Opisthobranchia.
  • Onchidiidae: a family of air-breathing sea slugs.

It would seem that their similar growth pattern, with the loss of torsion, bilateral symmetry, reduction of the Gastropod shell, and somewhat similar life styles, evolved a number of times in the Gastropods.