Hexabranchus, EHRENBERG, 1831

Valdés, Ángel, 2002, A phylogenetic analysis and systematic revision of the cryptobranch dorids (Mollusca, Nudibranchia, Anthobranchia), Zoological Journal of the Linnean Society 136 (4), pp. 535-636 : 560-562

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 bor- der 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 .

HEXABRANCHUS SANGUINEUS ( CUVIER, 1804)

( FIGS 13 , 14 )

Doris lacera Cuvier, 1804: 452 , 453–465, 473, pl. 73, figs 1–3 (nomen oblitum).

Doris sanguinea Rüppell & Leuckart, 1830: 30–31 , pl. 1, fig. 1 (nomen protectum).

Hexabranchus praetextus Ehrenberg, 1828 –31 [1831]: 30–31, pl. 1, fig. 1A–C.

Heptabranchus burnettii A. Adams, 1848: 494 .

Aethedoris indica Abraham, 1877: 237 .

Albania formosa Collingwood, 1881: 133 , pl. 10, figs 1– 5.

Only the type species of synonyms of Hexabranchus are listed here; for a complete list of synonyms see Thompson (1972).

Type material

Doris lacera Cuvier , SYNTYPES: Indian Ocean (= Mer des Indes), date and exact locality unknown, two specimens, 30 and 76 mm preserved length, dissected ( MNHN). Hexabranchus praetextus Ehrenberg , SYN- TYPE: El Tûr (= Tor), Egypt, date unknown, one specimen, 125 mm preserved length ( MNHB 566). SYNTYPE: El Tûr (= Tor), Egypt, date unknown, one specimen, 110 mm preserved length, partially dissected ( MNHB 567). The holotypes of Heptabranchus burnettii (originally collected from Borneo), Aethedoris indica (originally collected from Madras, India) and Albania formosa (originally collected from Ke-lung, Formosa) could not be located at BMNH and are probably lost. The type material of other synonyms of Hexabranchus has not been traced.

Additional material

Reef near Hotel Coelacanth , North end of Moroni , Grand Comore Island, Mozambique Channel , 6 March 1975, one specimen, 104 mm preserved length, leg. S. Earle and A. Giddings ( CASIZ 068296 ). Tire Reef, 2 km north of Mora Mora Village, Madagascar, 9 April 1989, two specimens, 94–100 mm preserved length, leg. T . M. Gosliner ( CASIZ 071897 ) .

External morphology

The external morphology and behaviour of this species have been widely described. Thompson (1972) and Gohar & Soliman (1963) found wide chromatic variation.

The general colour of the living animals is very variable. It normally varies from pale orange to bright red. In some specimens there is a number of small white or yellowish dots on some areas or on the entire dorsum. Other specimens have large bright red or pinkish spots, or a pale concentric band. Sometimes the mantle margin is surrounded by a yellow line. The rhinophores are red to yellowish, with white spots in some specimens. The gill has normally the same colour as the dorsum, with the rachises of the branchial leaves white or yellowish. The dorsum is smooth. There are 7–9 tripinnate, non-retractile branchial leaves. The anal papilla is prominent, situated in the centre of the branchial circle of leaves. The rhinophores are elongate, having 45 lamellae in a 100-mm preserved length specimen.

Ventrally there are two large, flattened and lobate oral tentacles ( Fig. 13F ). The anterior border of the foot is simple.

Anatomy

The posterior end of the glandular portion of the oral tube has 18 strong retractor muscles ( Fig. 13E ) which attach to the body wall. The oval, muscular buccal bulb has several additional muscles attached together; two long and wide salivary glands connect with it at each side of the oesophageal junction. The buccal bulb is three times longer than the glandular portion of the oral tube. The labial cuticle is completely covered with simple rodlets ( Fig. 14D ). The radular formula is 36 ¥ 77.0. 77 in a 100-mm long specimen. Rachidian teeth are absent. The lateral teeth are narrow and elongate, having a single cusp and lacking denticles ( Fig. 14A ). The teeth from the middle portion of the half-row are larger than those closer to the medial portion of the radula ( Fig. 14B ). The outermost teeth are smaller and also lack denticles ( Fig. 14C ).

The ampulla is very long and convoluted. It branches into a short oviduct and the prostatic portion of the deferent duct ( Fig. 13C ). The oviduct enters the female gland mass near to its centre. There is no differentiated prostate, but a long, folded and tubular deferent duct ( Fig. 13B ). The prostatic region of the deferent duct expands into the huge ejaculatory portion, which opens into a short common atrium with the vagina. The vagina is long and wide. Near to its proximal end it joins the bursa copulatrix. From the bursa copulatrix leads another duct that connects to the uterine duct and the seminal receptacle. The bursa copulatrix is rounded in shape and several times larger than the elongate seminal receptacle ( Fig. 13B ).

In the central nervous system ( Fig. 13D ) the cerebral and pleural ganglia are fused and distinct from the pedal ganglia. The cerebral and pleural ganglia are entirely covered with large ganglionic tubercles. There are three cerebral nerves leading from each cerebral ganglion and two pleural nerves leading from each pleural ganglion. There is no separate abdominal ganglion on the right side of the visceral loop. The buccal ganglia are near to the rest of the central nervous system, joined to the cerebral ganglia by two relatively short nerves. Gastro-oesophageal, rhinophoral and optical ganglia are present. The pedal ganglia are clearly separated, having four nerves leading from each one. The pedal and parapedal commissures are enveloped together with the visceral loop.

The circulatory system ( Fig. 13A ) consists of a large heart and a single large blood gland situated beneath the central nervous system.

Remarks

First Eliot (1910) and then Thompson (1972) considered that most of the nominal species assigned to the genus Hexabranchus are synonyms. Only the Atlantic Hexabranchus mormosus Marcus & Marcus, 1962 was dubiously regarded as a different species for biogeographical reasons. The arguments of Eliot and Thompson are convincing, but despite the latter’s suggestion that Doris sanguinea Rüppell & Leuckart, 1830 has priority over other synonyms, the name Doris lacera Cuvier, 1804 is much older and must be the valid name for the Indo-Pacific species of Hexabranchus . A re-examination of the syntypes of Doris lacera confirmed they are conspecific with Hexabranchus sanguineus . Doris lacera has been ignored by all authors dealing with Hexabranchus . According to Article 23.9.1 ( ICZN, 1999), if a senior synonym has not been used as a valid name since 1899, and its junior synonym has been used for the same species in at least 25 papers, published by at least 10 authors in the immediately preceding 50 years and encompassing a span not less than 10 years, the usage of the junior synonym must be maintained. The name D. lacera has only been used as valid in its original description in 1804, whereas the name H. sanguineus is in constant usage in the literature. More than 30 papers, books and field guides using the name H. sanguineus as valid have been published during the last 20 years by more than 15 authors. Therefore, the name H. sanguineus is here conserved (nomen protectum) and H. lacer is regarded as invalid (nomen oblitum).

The type species of other synonymous generic names: Hexabranchus praetextus Ehrenberg, 1828 , Heptabranchus burnettii A. Adams, 1848 and Aethedoris indica Abraham, 1877 , are also regarded as synonyms of Hexabranchus sanguineus .

Figures

Figure 13. Hexabranchus sanguineus (CASIZ 071897). A, general view of the anatomy; scale bar = 2 mm. B, reproductive system; scale bar = 2 mm. C, detail of the ampula; scale bar = 2 mm. D, central nervous system; scale bar = 1 mm. E, lateral view of the buccal bulb; scale bar = 1 mm. F, ventral view of the mouth area; scale bar = 2 mm.

Figure 14. Hexabranchus sanguineus (CASIZ 071897), SEM images of the radula and jaws. A, inner lateral teeth; scale bar = 100 Mm. B, mid-lateral teeth; scale bar = 100 Mm. C, outer lateral teeth; scale bar = 150 Mm. D, jaw elements; scale bar = 10 Mm.