Tritonia nigritigris Valdés & Lundsten & Wilson 2018, sp. nov.
Creators: name: Valdés, Ángel
name: Lundsten, Lonny
name: Wilson, Nerida G.
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Gastropoda
Nudibranchia
Tritoniidae
Tritonia
Tritonia nigritigris

description: Tritonia nigritigris sp. nov.
(Figs. 2A, 3A, 4–5)
Type material. Holotype: Guide Seamount, Pacific Ocean (37.036532, -123.330875), 1733.4 m depth, ROV Doc Ricketts (dive 882), 15 Aug 2016, 82 mm preserved length, dissected (LACM 3553), GenBank accession numbers: MH 756138 (COI), MH 756133 (16S).
Description. Body elongate, narrow (Fig. 2A). Notum outline with four invaginations, completely surrounded by numerous, densely packed glandular projections (Fig. 2A), giving the animal a ruffled appearance. Dorsum covered with numerous, densely packed elongate tubercles or projections, oriented in different directions, forming a zigzagging pattern in some areas. Velum wide, forming two triangular lateral projections, with several frontal irregular indentations. Rhinophores short and wide, with 9 simple lamellae, located on anterior end of notum. Rhinophoral sheaths elongate, with 12 branched papillae around distal edges. Foot smooth, rounded posteriorly, visible dorsally at posterior end and in notum invaginations. Genital and anal openings on right side of body. Background color of notum black. Dorsal tubercles primarily white to greyish-brown near posterior notum, giving the body a light appearance, becoming darker towards head, turning completely black on anterior notum; this gives a black and white appearance with a greyish transitional area mid-length, forming a wedge of black pigment. Notum edge primarily white, except for anterior-most end that becomes dark grey. Velum completely black. Rhinophoral sheaths black with grey edges. Rhinophores with black stalks and beige sheath papillae and clubs. Foot dark grey, surrounded by a thin white line.
Digestive system with large, muscular buccal bulb (Fig. 4A). Esophagus wide, connecting anteriorly into buccal bulb, where two large, flat salivary glands connect. Intestine emerging dorsally from left side of digestive gland, forming a loop towards the right side of body where it opens into the anus (Fig. 3A). Radular formula 44 × 50.1. 50 in holotype. Rachidian teeth narrow, with a large, sharp central cusp about twice as large as tooth bases and 3 irregular denticles on each side of central cusp (Fig. 5A). Lateral teeth hook-shaped, with sharp, elongate cusps lacking denticles (Fig. 5B). Outer lateral teeth very elongate with reduced bases (Fig. 5C). Jaws elongate, with no differentiate masticatory border (Fig. 5D).
Reproductive system with large, highly convoluted ampulla forming several loops (Fig. 4B), opening on female gland complex where the prostate connects. Prostate narrow, elongate, forming a single loop and expanding into a wide and muscular deferent duct, which connects with a simple, conical penis. Vagina elongate, narrow, expanding abruptly in its distal 1/4 th and opening into the spherical bursa copulatrix. The seminal receptacle is a small spherical evagination located close to mid-length in the vagina.
Biology. This species was collected at 1733.4 m depth on the Guide Seamount, about 55 nautical miles off Davenport, California (Fig. 1). The specimen was collected on a steep section of volcanoclastic rock. Dense populations of Keratoisis sp. (isidid octocoral) were observed in close proximity. Other organisms included comatulid and hyocrinid crinoids, primnoid octocorals, Evoloplosoma cf. claguei (sea star), additional unidentified sea stars, Acesta sp. (bivalve), Farrea sp. (hexactinellid sponge), and large and abundant Macrouridae (rattail fishes).
Phylogenetic position. Based on available sequence data and the phylogenetic analyses conducted, Tritonia nigritigris sp. nov. is sister to the Antarctic species Tritoniella belli Eliot, 1907 (Fig. 6).
Etymology. The species name is derived from the unique coloration of the holotype, in particular the broken, black stripes, which are reminiscent of those of a tiger.
Remarks. The most common species in relatively deep-waters of the eastern Pacific is Tritonia tetraquetra (Pallas, 1788), previously recognized as T. diomedea Bergh, 1894. Martynov (2006) considered that Pallas (1788) original description of Limax tetraquetra from the Kuril Islands corresponded to a specimen of the species commonly identified as T. diomedea (see Behrens & Hermosillo 2005) and therefore proposed to use the name T. tetraquetra for this species. Here we cautiously follow Martynov’s (2006) opinion, but clarification of the taxonomic status of this species requires comprehensive examination of specimens across its broad range (from the Kuril Islands to Panama), because as in other nominal species with similar ranges, multiple cryptic species may be involved.
Tritonia nigritigris sp. nov. is externally different from other species of Tritonia described to date. The most similar species morphologically is the deep-water species T. newfoundlandica Valdés et al., 2016, described from the North Atlantic (Valdés et al. 2016). Both T. nigritigris sp. nov. and T. newfoundlandica have narrow, elongate bodies with the anterior end wider, tapering posteriorly into a pointy, triangular end. However, whereas T. newfoundlandica is uniformly black, with the rhinophores, rhinophoral sheaths, velar appendages, and secondary gills brownish-red and the dorsum is smooth, lacking tubercles. On the contrary T. nigritigris sp. nov. has a more complex coloration of black, white and grey, and the dorsum is tuberculate.
The only two other deep-water species of Tritonia (found below 500 m depth) known to date are T. ingolfiana Bergh, 1899 described by Bergh (1899) from the mid-Atlantic ridge based on a single specimen collected at 887 m depth, and T. episcopalis Bouchet, 1977 described from 1035–2170 m depth also in the Atlantic. Bergh (1899) described his preserved specimen T. ingolfiana as grayish blue with yellow rhinophores. This is different from T. nigritigris sp. nov., which has a black to white body and the rhinophores, gills and velum are much lighter even in the preserved specimens. Other differences include the shape of the penis, which is very elongate and tightly packed in the penial sheath of T. ingolfiana (Bergh 1899: pl. 3 fig. 9), whereas the penis of T. nigritigris n. sp. is much shorter and straight. Tritonia episcopalis as well as T. newfoundlandica have a characteristic scoop in the penis, which is absent in T. nigritigris n. sp. The external coloration of T. episcopalis is bright violet with the gills of the same color and the rhinophores red, whereas in T. nigritigris n. sp. the body is black to white.
The sister species of Tritonia nigritigris n. sp., Tritoniella belli, is very distinct externally. Tritoniella belli ranges in color from uniform milky-white or translucent to yellow or orange in specimens (Wägele 1989b). As in T. nigritigris n. sp. the mantle edge is wavy and can be smooth or have finger-like processes. However, specimens of T. belli have a smooth dorsum typically with a longitudinal median ridge, whereas T. nigritigris n. sp. has a tuberculate dorsum and no ridge. In the phylogenetic analyses presented here, both T. nigritigris n. sp. and T. belli form a monophyletic group sister to Tritonia, supporting the separation of Tritoniella from Tritonia. Under this scenario, T. nigritigris n. sp. should be considered a member of Tritoniella. However, the phylogeny of Tritonia is poorly understood and several critical species have not yet been sequenced. Also, morphologically, T. nigritigris n. sp. appears more similar to some species of Tritonia than to T. belli making difficult to find morphological synapomorphies for Tritoniella. Until all these issues are resolved, we prefer to maintain T. nigritigris n. sp. in Tritonia.
description: Published as part of Valdés, Ángel, Lundsten, Lonny & Wilson, Nerida G., 2018, Five new deep-sea species of nudibranchs (Gastropoda: Heterobranchia: Cladobranchia) from the Northeast Pacific, pp. 401-433 in Zootaxa 4526 (4) on pages 404-408, DOI: 10.11646/zootaxa.4526.4.1, http://zenodo.org/record/2611755