Hexabranchus Ehrenberg 1831
Creators: name: Valdés, Ángel
Publisher: name: Zenodo
Year: 2002
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Gastropoda
Nudibranchia
Hexabranchidae
Hexabranchus

description: GENUS HEXABRANCHUS EHRENBERG, 1831
Hexabranchus Ehrenberg, 1828 –31 [1831]: 30. Type species: Hexabranchus praetextus Ehrenberg, 1828, by subsequent designation of J. E. Gray (1847).
Heptabranchus A. Adams, 1848: 494–495. Type species: Heptabranchus burnettii A. Adams, 1848, by original designation.
Rhacodoris Mörch, 1863: 34. Type species: Doris lacera Cuvier, 1804, by original designation.
Aethedoris Abraham, 1877: 237. Type species: Aethedoris indica Abraham, 1877, by monotypy.
Albania Collingwood, 1881: 132–133. Type species: Albania formosa Collingwood, 1881, by monotypy.
Diagnosis
Dorsum smooth, lacking tubercles. Head with two large, flattened and lobate oral tentacles. Anterior border of the foot simple. Gill contractile, not retractile. Radula composed of simple, hamate teeth. Labial cuticle completely covered with rodlets and having several transverse grooves. Buccal mass with numerous and strong muscles attached. Reproductive system with a tubular, non differentiated prostate. Penis and vagina devoid of hooks. Vestibular or accessory glands absent.
Remarks
The genus Hexabranchus was originally introduced by Ehrenberg (1828 –31) based on three species: Hexabranchus praetextus Ehrenberg, 1831, Doris sanguinea Rüppell & Leuckart, 1830 and ‘ Doris laciniata Cuvier’ (error for Doris lacera Cuvier, 1804). Hexabranchus praetextus was subsequently selected by Gray (1847) as the type species. This species was detailed described and illustrated by Ehrenberg, (1828–31), and its features agree with the current usage of the name.
Adams (1848) described the genus Heptabranchus, type species by original designation Heptabranchus burnettii A. Adams, 1848, as being very close to Hexabranchus, but showing several differences in the number of gills and mantle widtH. In his opinion, these differences supported the separation of two different genera. Nowadays it is known that species of Hexabranchus can contract and spread out the mantle margin (Thompson, 1972), so the same animal is able to show a narrow mantle margin with the foot extending beyond it (as described by Adams, 1848) or a wide mantle completely covering the foot. In addition, the number of branchial leaves is variable among the same species. Therefore, there is no doubt that Heptabranchus is a junior synonym of Hexabranchus.
Mörch (1863) introduced the name Rhacodoris for Hexabranchus sensu Gray non Ehrenberg, with ‘ Doris laciniata Cuvier’ (error for Doris lacera Cuvier, 1804) as the type species by original designation. He also stated that Doris lacera was mistakenly reported as belonging to the genus Hexabranchus, from which it differs in having a special cavity for each branchial leaf and one common cavity for all the gill. The examination of the type material and original description of Doris lacera (Cuvier, 1804), show that this species clearly belongs to the genus Hexabranchus, and therefore Rhacodoris is a junior subjective synonym.
The genus Aethedoris and the species Aethedoris indica were erected by Abraham (1877) based on a picture of Alder & Hancock (1864; pl. 33, fig. 20) which represents a contracted, probably dead specimen. The two large and lobate oral tentacles shown in the picture clearly identified the specimen as belonging to the genus Hexabranchus, but they were considered by Abraham as the most striking feature of his new taxa. He interpreted them as a ‘bilobed head, each lobe being semicrescentic, with the apex curving backwards and the margin bearing 12–14 conical dentations’. The type material of Aethedoris indica could not be located in BMNH and is probably lost. However, it is very likely that the animal figured by Alder & Hancock was a dead specimen of Hexabranchus.
Collingwood (1881) introduced the new genus Albania with Albania formosa Collingwood, 1881 as the single included species (type by monotypy). The features of Albania are identical to those of the genus Hexabranchus. In this case the type material of Albania formosa is also lost but there are not doubts that this nominal species belong to the genus Hexabranchus.
description: Published as part of Valdés, Ángel, 2002, A phylogenetic analysis and systematic revision of the cryptobranch dorids (Mollusca, Nudibranchia, Anthobranchia), pp. 535-636 in Zoological Journal of the Linnean Society 136 (4) on pages 560-561, DOI: 10.1046/j.1096-3642.2002.00039.x, http://zenodo.org/record/4634200