Four new species of dorid nudibranchs belonging to the genus Aldisa (Mollusca: Opisthobranchia), with a revision of the genus

Two new species belonging to the genus Aldisa were found in the cold-temperate N Pacific waters off southern British Columbia, Canada. These species are sympatric with Aldisa cooperi Robilliard & Baba, 1972. The cold-temperate Atlantic waters off the coast of S Africa revealed two new, sympatric species of Aldisa, the first recorded from the southern hemisphere. The morphological characteristics of these four new species are described in detail. As many known species of Aldisa are incompletely described, redescriptions of Aldisa binotata Pruvot-Fol, 1953; Aldisa cooperi Robilliard & Baba, 1972; Aldisa pikokai Bertsch & Johnson, 1982, and Aldisa zetlandica (Alder & Hancock, 1854) are provided. Specimens of Aldisa banyulensis Pruvot-Fol, 1951, and Aldisa berghi Vayssiere, 1901, were unavailable. A key to the 11 presently recognized species of Aldisa is included and the characteristics and relationships of this genus are discussed.