Midorigai australis from south-eastern Australia

October 20, 2001
From: Bill Rudman

PHOTOS: Animal on its food alga, Caulerpa simpliciuscula. West Head, Flinders, Westernport Bay, Victoria, Australia. March 1986. AM C149521. Shell length 4mm. PHOTOS: Bill Rudman

As I recently promised, here is some information on another of the bivalved sacoglossans, Midorigai australis, from south-eastern Australia. This is one of the very few species of bivalved sacoglossans which has evolved an elaborate colour pattern. The light green to cream patches in this species help to camouflage it on the its food plant. Many species of bivalved sacoglossans live on species of Caulerpa with large rounded bladders. In these associations the shape and size of the shell matches that of the plant's bladders and so it is well camouflaged. In the case of Midorigai the shell is much larger than the bladders of its food plant and so shape and size is of little camouflage value. The lighter patches on the mantle and body of the slug, match the highlighted parts of each algal bladder, making it, at least to my eyes, well camouflaged. Compare it with the other two common bivalved sacoglossans from south-eastern Australia: Edenticula typica which also has a colour pattern, and Tamanovalva babai, which does not.

• Burn, R.F. (1960) Australian bivalve gastropods. Nature, 187(4731): 44-46.
• Burn, R. (1989) Chapter 12 Opisthobranchs (Subclass Opisthobranchia). In: Shepherd, S.A. & Thomas, I.M. (Editors) Marine Invertebrates of Southern Australia Part 2, South Australian Government Printer, Adelaide.
• Wisely, B. (1962) An outline of the development of the bivalve gastropod Midorigai australis Burn, 1960. Journal of the malacological Society of Australia, 6: 37-

Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Oct 20) Midorigai australis from south-eastern Australia. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5519