The systematics and phylogeny of the genus
Thordisa from the tropical Indo-Pacific and eastern Pacific are revised. Morphological and anatomical data from
Thordisa species were used to construct a preliminary phylogeny. The phylogenetic analysis demonstrates the monophyly of
Thordisa and its relationship to the outgroups
Asteronotus,
Halgerda, and
Hoplodoris. Five new species are described anatomically. Two of the species,
Thordisa luteola sp. nov., known only from tropical South Africa and
Thordisa nieseni sp. nov., from Costa Rica and Panama, are highly derived in several aspects of their morphology. Of the remaining three species,
Thordisa albomaculata [sic,
albomacula in main text] sp. nov. and
Thordisa tahala, both widespread in the Indo-Pacific, appear to be sister species to
Thordisa setosa Pease, 1860, known only from the Hawaiian Islands.
Thordisa oliva sp. nov., from the tropical Indian Ocean of South Africa and Japan, appears to be sister to
Thordisa diuda Marcus 1955, from Brazil. These two species share a unique apomorphy of having a pair of circular pits on the sides of the mouth.
