Goniobranchus rubrocornutus (Rudman, 1985)

Soong, Giun Yee, Bonomo, Lynn J., Reimer, James D. & Gosliner, Terrence M., 2022, Battle of the bands: systematics and phylogeny of the white Goniobranchus nudibranchs with marginal bands (Nudibranchia, Chromodorididae), ZooKeys 1083, pp. 169-210 : 169

Goniobranchus rubrocornutus (Rudman, 1985)

Figures 2g, h , 5e, f , 9a-f

Glossodoris marginata (Pease, 1860): Baba 1938: 11-12 (misidentification).

Chromodoris rubrocornuta Rudman, 1985: 83, 283-286, figs 12F, 20A, 25, 26A; Gosliner et al. 2008: 221, bottom photograph.

Goniobranchus rubrocornutus : Gosliner et al. 2015: 224, middle right photograph; Gosliner et al. 2018: 154, middle right photograph.

Goniobranchus cf. albonares (Rudman, 1985): Mehrotra et al. 2021: 104, fig. 9l (misidentification).

Type locality.

Hong Kong.

Type material.

AM C138518, one specimen, Flynn Point, 22.467°N, 114.333°E, Hoi Ha, Hong Kong, China, depth not available, 18 April 1983, collector not available. Not examined in this study due to the original description in Rudman (1985) being sufficient for comparisons.

Geographical distribution.

Widely distributed around the tropical and subtropical Indo-Pacific oceans ( Debelius and Kuiter 2007; Gosliner et al. 2008, 2015, 2018; Rudman 1985) with reports from Thailand ( Mehrotra et al. 2021), Malaysia, Philippines, Hong Kong, Palau, American Samoa, Marshall Islands ( Gosliner et al. 2008), Japan ( Nakano 2018; Ono and Katou 2020), Australia ( Rudman 1985), New Caledonia ( Hervé 2010), and the Marianas Islands ( Carlson and Hoff 2003).

Material examined.

CASIZ 203047 (morphotype A), one specimen (4 mm preserved), subsampled for molecular data and dissected, Verde Island Passage coast, 13.917°N, 120.617°E, Calatagan , Batangas Province, Luzon , Philippines, depth not available, 9 May 2014, T.M. Gosliner, 2014 Verde Island Passage Expedition. CASIZ 181235 (morphotype A), one specimen (4 mm preserved), dissected, Twin Rocks , 13.683°N, 120.883°E, Maricaban Strait , Mabini (Calumpan Peninsula), Batangas Province, Luzon , Philippines, depth not available, 22 May 2009, P. Paleracio, CAS Philippines Expedition May 2009. CASIZ 208563 (morphotype B), one specimen (3 mm preserved), subsampled for molecular data, School Beach , 13.516°N, 120.95°E, Batangas Channel , Puerto Galera , Oriental Mindoro Province, Mindoro, Philippines, 6-18 m depth, 13 April 2015, T.M. Gosliner, 2015 Verde Island Passage Expedition .

Description.

External morphology. Length of living animal 7-14mm. Body oval and elongated, with two marginal bands on the mantle edge. Six to nine unipinnate gill branches, 8-14 lamellae on rhinophores. The color patterns of this species can be divided into two distinct morphotypes. Morphotype A (Fig. 2g ) has a translucent creamy white body. The outermost portion of the mantle edge is surrounded by an orange band, followed by an irregular red band, followed by another irregular opaque white band. Gill branches and rhinophores are translucent, deep red with either red or white edges. Morphotype B (Fig. 2h ) has an opaque white body. The outermost portion of the mantle edge is surrounded by a red band, followed by a yellow submarginal band and both bands have similar widths. The gill and rhinophores are translucent deep red with bluish white tinged edges.

Buccal mass and radula. The muscular portion of the buccal mass approximately the same size as the oral tube length (Fig. 5e ). The chitinous labial cuticle is found at the anterior end of the muscular portion of the buccal mass and bears bifurcated and short jaw rodlets (Fig. 9a, b ). The radular formula of CASIZ 181235 is 39 × 27.1.27 (Fig. 9c ). The rachidian tooth is thin and linear. The inner and outer surface of the inner lateral teeth have two or three denticles on each side of the central cusp (Fig. 9d ). The central cusp on the inner lateral tooth is ~ 2 × the length of the adjacent denticles. The middle lateral teeth have a short central cusp with 5-7 denticles (Fig. 9e ). The outer lateral teeth have a rounded main cusp with 3-5 denticles (Fig. 9f ).

Reproductive system (Fig. 5f ). The thick, tubular ampulla narrows into a diverging short oviduct and long vas deferens. The proximal prostatic portion of the vas deferensis thin and convoluted and transitions into the muscular ejaculatory portion. The long, narrow, convoluted ejaculatory portion transitions into a wider, long penial bulb, which joins with the distal end of the vagina. The vagina is proximally narrow and elongated, transitions into a larger, spherical bursa copulatrix and large receptaculum seminis at its distal end. A moderately long uterine duct emerges from this junction of vagina, bursa copulatrix, and receptaculum seminis. The uterine duct connects the receptaculum seminis with the female gland mass. The female gland mass has smaller albumen and membrane glands and a larger mucous gland.

Remarks.

In this study, G. rubrocornutus morphotype A matched with Rudman’s (1985) G. rubrocornutus from Hong Kong: a creamy white translucent body with the outermost portion of the mantle edge surrounded by an orange band, followed by an irregular red band and an irregular opaque white band. The gill branches and rhinophores were translucent deep red with either red or white edges. Goniobranchus rubrocornutus morphotype B only has two marginal bands with the outermost red band followed by a yellow submarginal band, and this pattern does not match with Rudman’s description of G. rubrocornutus . In this case the inner white submarginal band may simply be masked by the opaque white body color of morphotype B rather than the cream body color of morphotype A. However, in our phylogenetic and species delimitation analyses, G. rubrocornutus morphotype B was clustered together with morphotype A and both morphotypes did not show any genetic differences (uncorrected pairwise distance = 0.0%). Thus, morphotype B very likely represents a different color variation of G. rubrocornutus . Recently, molecular work has revealed the presence of mimicry adaptation in chromodorid nudibranchs (e.g., Padula et al. 2016; Layton et al. 2018). Sympatric specimens of chromodorid nudibranchs with different color patterns were found to be the same species ( Layton et al. 2018), and this is also the case with our G. rubrocornutus morphotypes, where both morphotypes are sympatric. In this case, these variations are not likely different cases of mimicry, but simply color variants. Despite the variations observed here, few records of this species have been misidentified, with the exception of Mehrotra et al. (2021), where G. rubrocornutus was identified as G. cf. albonares . The specimen illustrated clearly has red rhinophores with white edging rather than white rhinophores and the orange, red, and opaque white marginal and submarginal bands that are characteristic of G. rubrocornutus .

Figures

Figure 2. a, b Goniobranchus albonares a CASIZ 191440, Papua New Guinea b CASIZ 228939, Philippines c-f Goniobranchus preciosus c CASIZ 208415, Morphotype A, Philippines d CASIZ 208574, Morphotype B, Philippines e CASIZ 176752, Morphotype C, Peninsular Malaysia f CASIZ 176761, Morphotype D, Peninsular Malaysia g, h Goniobranchus rubrocornutus g CASIZ 203047, Morphotype A, Philippines h CASIZ 208563, Morphotype B, Philippines. Photographs TMG. Scale bars: 1 cm.

Figure 5. a buccal mass of Goniobranchus albonares, CASIZ 228939 b reproductive system of Goniobranchus albonares, CASIZ 228939 c buccal mass of Goniobranchus preciosus, CASIZ 208574 d reproductive system of Goniobranchus preciosus, CASIZ 208574 e buccal mass of Goniobranchus rubrocornutus, CASIZ 203047 f reproductive system of Goniobranchus rubrocornutus, CASIZ 203047. Abbreviations: am, ampulla; bb, buccal bulb; bc, bursa copulatrix; ej, ejaculatory duct; es, esophagus; fgm, female gland mass; ot, oral tube; p, penis; pr, prostate; ra, radular sac; rs, receptaculum seminis; va, vagina; mu, mucous gland. Scale bars: 0.1 mm (a, b, e, f); 1 mm (c, d).

Figure 9. Scanning electron micrographs. Goniobranchus rubrocornutus, CASIZ 203047, Philippines. a jaw b jaw rodlets c radula d central teeth e mid-lateral teeth f outer lateral teeth.