Phyllidiella amphitrite, Yonow, 2020

Yonow, Nathalie, 2020, Red Sea Opisthobranchia 6: Phyllidiidae and their paradorid mimic: new species and new records (Heterobranchia, Nudibranchia, Doridina), ZooKeys 1006, pp. 1-34 : 1

Phyllidiella amphitrite sp. nov. Plate 1 ; Figures 1 , 2

Material.

Holotype. SMF 360585. Near Hurghada, Egypt, 09 August 2009, one specimen 28 × 15 mm, bent (pres., alcohol), leg. and photograph S. Kahlbrock.

Diagnosis.

Relatively smooth phyllidiid with pale yellow crests and cones (instead of rounded tubercles). White areas granulate, black areas smooth. Sub-margin with single series of yellow and white patches and small crests and pointed tubercles, margin smooth and pale. Rhinophores black, extending from pale yellow raised sheaths. Very distinctive oral tentacles, trilobed with a median ridge (not digitiform as those of other species of Phyllidiella ). Dorsal anus.

Description.

The photograph of the living specimen depicts an animal which vaguely resembles a smudged Phyllidiella ' Phyllidiella pustulosa ' with yellow pigment on the top of its crests, which are loosely arranged in groups with black lines around them (Plate 1 ). These tuberculate areas comprise a large white area and the tubercles arise very sharply as crests or cones, both pale yellow. These crests are low, and those around the margin are either low cones or rounded on top. There are six of these elongated crests forming a ridge along the midline, grouped into three polygonal areas with two single ones situated behind the anus. The rhinophores are located on the anterior sides of the first cluster, and issue from raised sheaths which are faintly yellow; the rhinophores are associated with rhino-tubercles. The visible part of the right rhinophore in the photograph is black and densely lamellated, rounded at the tip. The individual marginal tubercles are very small and nipple-like with a large creamy white base. They are present as a single series along the submargin and the margin is smooth and pale.

The preserved specimen is curved ventrally but everything is clearly visible (Fig. 1A ). The dorsal crests and cones are still present in the specimen and appear ‘dirty’ where they were coloured yellow. The anus is located on the posterior edge of the third tubercular cluster. The right rhinophore was removed and bears 17 lamellae, the lowest four of which are white, and a rounded distal tip. Ventrally, there is no black showing through the hyponotum, nor are there any black markings on the gill leaflets or oral tentacles in the preserved specimen. The foot sole has no black line. The oral tentacles, gill leaflets, and gonopore are all flesh-coloured (Fig. 1B, C ). The oral tentacles are trilobed with a median ridge: the right one (specimen viewed ventrally) is upstanding and the left one is folded over (Fig. 1C, D ). The foot is folded longitudinally (Fig. 1B, C, D ).

Anatomically, the dissection of the single specimen confirms placement in the genus Phyllidiella (see Fig. 2 ). The internal organs were covered by a dark visceral envelope and beneath this was a smaller envelope anteriorly covering the pharynx, pharyngeal bulb, oral glands, and nervous system. When this was removed, a large mass of large leaf-like oral glands covered the pharynx and pharyngeal bulb. The first is long and muscular, forming a large loop. Two strong muscles attach the elongated and bent pharyngeal bulb to the body wall. The bursa copulatrix is a solid sphere with a reddish patch on the ventral-most side.

Remarks.

The internal anatomy of this new species clearly places it in Phyllidiella : the visceral envelope is black, the pharyngeal bulb is elongate and folded, the pharynx is thick and muscular but becomes tubular, and there are leaf-like glands overlying the pharynx and the bulb. However, there are no other known species of Phyllidiella with yellow pigmentation or with such unusual oral tentacles. Despite these two differences, it is described as a new member of Phyllidiella due to similarities in the digestive system. Phyllidiella ' Phyllidiella pustulosa ' is one of the most common species in the Red Sea and Indo-West Pacific, but there are no records of it having yellow tubercles or crests on the tubercles.

Phyllidiella ' Phyllidiella pustulosa ' is always pink, green, or white underwater and in photographs, possibly depending on the lighting utilised; no species of Phyllidiella has any yellow pigmentation. There are, however, instances of very pale yellow markings in other genera, which may lead to misidentifications, e.g., Phyllidiella sp. in Gosliner et al. (2008: 295) which is in fact Phyllidia elegans : the pattern of black and tubercles is typical, and a yellow tinge is clearly visible on the rhinophores. For comparison, a photograph of a very pale Phyllidia varicosa is illustrated in Plate 2 , and another is available on Sea Slug Forum ( Adams 2003). There are specimens in the Red Sea of P. ' Phyllidia pustulosa ' with a more pointed appearance (Plate 3 ) which may eventually also be identified as a different species; but, it must be noted that these spikier variations also occur in other phyllidiid species, e.g., Phyllidia multifaria (Yonow, 1986: 1410, fig. 11i; Yonow 1988: 149, pl. 7).

While P. amphitrite looks like an aberrant form of P. ' Phyllidiella pustulosa ', the differences are enough to warrant specific separation for now, especially following the recent molecular work on the species complex in the western Pacific ( Stoffels et al. 2016; Bogdanov 2020; see Remarks for P. ' Phyllidiella pustulosa ' in Appendix 1). No other phyllidiid has trilobed oral tentacles, a character which needs further examination when more specimens are collected. Species of the Phyllidia pustulosa complex have triangular oral tentacles with a lateral groove and are tipped in black. With black rhinophores and a mass of leaf-like oral glands, P. amphitrite clearly does not belong to Phyllidia or Phyllidiopsis , and it is placed in Phyllidiella as the most parsimonious choice.

Etymology.

The name was chosen for the wife of the ruler of the sea, Poseidon, in Greek mythology. She was called Ἀμφιτρίτη, Amphitrite.

Figures

Plate 1. Phyllidiella amphitrite sp. nov., holotype SMF 360585, Hurghada, Egypt, 09 August 2009 (photograph S. Kahlbrock).

Figure 1. Phyllidiella amphitrite sp. nov., holotype SMF 360585 A dorsal view B ventral view C detail of oral tentacles and anterior foot margin D drawing of oral tentacles, ' lips', and anterior foot margin.

Figure 2. Phyllidiella amphitrite sp. nov., holotype SMF 360585, drawing of the digestive anatomy and the genitalia (the latter in heavier ink).

Plate 2. Phyllidia varicosa, not collected, very pale individual, Makadi Bay, Egypt, 2013 (photograph Hsini Lin).

Plate 3. Phyllidiella ' Phyllidiella pustulosa ', not collected, with pointed tubercles, Obhur Creek, Jeddah, Saudi Arabia, 1980 ' s (photograph W. Pridgen).