Diversification of filter-feeding nudibranchs: two remarkable new species of Melibe (Opisthobranchia: Tethyiidae) from the tropical western Pacific

Abstract

Two new species of the genus Melibe are described from the tropical western Pacific. Melibe colemani sp. nov. is described from northeastern Borneo, Malaysia. This species is transparent with a series of white interconnecting digestive gland ducts visible throughout the body. It is also characterized by having weakly denticulate jaws, highly ramified digestive gland tracks that extend throughout the body, and a reproductive system with an external nodular vaginal gland and a penis with a curved apex. Melibe coralophilia sp. nov., described from the Philippines and Malaysia, has a deep golden brown colour. The body is wide and densely covered with tubercles that form a mid-dorsal crest and cover the surface of the ‘cerata’. Internally, this species has a massive muscular buccal mass, jaws with a finely denticulate margin, tuberculate ganglia in the central nervous system and an expanded, rounded receptaculum seminis. Melibe coralophilia is associated with the scleractinean coral Porites sp. and the blue coral Heliopora coerulea although the nature of this association remains unknown. This species is also unusual in that the egg mass is carried on the right side of the body located posterior to the genital atrium and is found between the mantle and the foot. A morphological phylogeny suggests that M. colemani is most closely related to M. engeli Risbec, 1937, M. digitata Gosliner & Smith, 2003 and M. tuberculata Gosliner & Smith, 2003. Melibe coralophilia is a member of the Indo-Pacific clade of Melibe species, but its relationship to other taxa remains unresolved. Preliminary molecular data suggest a more complex relationship between Melibe species, but strongly support the notion that all of the species examined represent distinct lineages. The sister-group relationship of Tethys with Melibe is confirmed and the molecular phylogeny supports the monophyly of Melibe and Tethyiidae based on the combined molecular dataset.