Sea slug

group of marine invertebrates with varying levels of resemblance to terrestrial slugs

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:

  • Heterobranchia
    • Nudibranchs: a clade with bilateral symmetry
    • 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.
    • Opisthobranchs: a long-used term now just an informal label. Polyphyletic.
      • 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.
      • Cephalaspidea: a sub-order of Opisthobranch gastropod molluscs. They usually possess a reduced internal shell.
  • Onchidiidae: a family of air-breathing sea slugs.
    • Sea butterflies: a group of pelagic sea snails sometimes called pteropods. They belong to the clade Thecosomata.
Sea slug
Nembrotha kubaryana.jpg
Nembrotha kubaryana
feeding on tunicates
Scientific classification
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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.