Evolution & Systematics
Detorsion has allowed for the evolution and specialisation of the Opisthobranch body plan. As the chitinous shell was gradually lost, the animal evolved a larger body size, became more elaborate in its design and bright colouration. Through this process the positioning of the mantle cavity and organs have relocated from above the head downwards alongside the right side of the body towards the posterior end of the animal (Rudman 2000).
There has been considerable confusion surrounding the classification of Marianina rosea. Initially, this species of nudibranch was placed under the infraorder Dendronotina and further positioned into the family Tritoniidae (Pruvot-Fol,1930). It was placed in with the Tritoniids on the basis that M. rosea displayed a soft, elongate body with the edge of the mantle bearing a series of branched cerata that are lined in pairs, and contain secondary respiratory gills. Upon re-evaluation of the taxonomy of Tritoniids, Odhner (1936) relocated M. rosea into its own separate family, Marininidae or its synonym Aranucidae. Odhner (1936) renamed Marianina rosea as Aranucus bifidus on the basis of the separation of the right and left digestive glands.
Specimens of M.rosea are also often mistaken for Aeolids due to similarities in anatomical features. Upon further inspection of the species, the distinctive bright blood-red palmate rhinophoral clavi with their tubular sheaths are representative of the physiological features belonging to the family Tritoniidae (Marhsall &Willan, 1999)(Willan 1988). Willan (1988) suggests that M. rosea is the most primitive form of a Tritoniid nudibranch, that has undergone further 'Aeolidization' than its Tritoniid members.
Alice Pruvot-Fol classification of Marianina rosea (1930):
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Subclass: Heterobranchia
Infraclass: Opisthobranchia
Order: Nudibranchia
Suborder: Dexiarchia
Infraorder: Dendronotida
Superfamily: Tritonioidea
Family: Tritoniidae
Genus: Marianina
Species: Marianina rosea
Odhner classification of Marianinarosea (Aranucus bifidus) (1936):
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Subclass: Heterobranchia
Infraclass: Opisthobranchia
Order: Nudibranchia
Suborder: Dexiarchia
Infraorder: Dendronotida
Family: Aranucidae(Marianinidae)
Genus: Aranucus
Species: Aranucus bifidus
Recent taxonomy from Pola and Gosliner (2010) has now confirmed, through molecular phylogeny, that M. rosea does in fact belong to the family Tritoniidae. Through sequencing of two mitochondrial (cytochrome c oxidase subunit 1, and 16S rRNA) and one nuclear gene (Histone 3) Pola and Gosliner were able to confirm that suborders Bornellidae and Dendronotidae are now monophyletic. This resulted in M. rosea being transferred from the separate family Aranucidae back into Tritoniidae, which is now the widely accepted classification of Marianina rosea. The exact cladogram of Marianina rosea can be found in the publication by Pola and Gosliner (2010).
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