South-eastern Australia from eastern South Australia, northern Tasmania to central Victoria.
Animal on its food alga, Caulerpa simpliciuscula. West Head, Flinders, Westernport Bay, Victoria, Australia. March 1986. AM C149521. Shell length 4mm. PHOTOS: Bill Rudman
" Shell small, up to 5 mm in length, broad, more or less quadrilateral with dorsal and ventral margins nearly parallel, protoconch depressed, colour yellowish. Body dark green, shell mantles with many radials of creamy-yellow patches, head and rhinophores with cream specks; neck and rhinophores stout. This species ranges from south-eastern South Australia and northern Tasmania to central Victoria from the lower intertidal to 2 m deep. Records from central N.S.W. need confirmation. It occurs on and eats only the green alga Caulerpa simpliciuscula, common in rock pools and channels on the open coastline. It is present throughout the year." ... description from Burn, 1989: 749.
This is another 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, 1(6): 37-39.
Rudman, W.B., 2001 (October 16) Midorigai australis Burn, 1960. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/midoaust