Zeusia herculea

Zeusia herculea (Bergh, 1894)

(Figs. 15 E–F, 20–21)

Aeolidia herculea Bergh, 1894: 128 –129, pl. 1, figs 8–12. Type locality: 33°08’N, 118°40’W (northeast of San Clemente Is., California), 757 m depth.

Aeolidia farallonensis Gosliner & Behrens, 1996: 351 –353, figs. 3–5. Type locality: 37°27.9’N, 123°02’W (off Farallones Is., California), 510 m depth.

Material examined. Blue whale skeleton (implanted on 5 Oct 2004), Monterey Bay, California (36.772, - 122.083), 1018 m depth, ROV Tiburon (dive 1117), 15 Aug 2007, 18 mm preserved length, dissected (SIO-BIC M12136), GenBank accession numbers: MH756131 (16S), MH 756142 (H3).

Description. Body elongate, wide, with numerous elongate, dorso-lateral cerata. Cerata arranged in densely packed rows, running from behind oral tentacles to posterior end of the body. Cerata increase in size in each row, lateral cerata typically shorter, dorsal cerata longer (Fig. 15 E–F). Oral tentacles tentacular, narrow, elongate (Figs. 15E). Foot corners absent (Figs. 15E). Rhinophores elongate, smooth (Figs. 15F). Color alive unknown, no live photographs available.

Digestive system with large, muscular buccal bulb (Fig. 20A). Esophagus narrow, short, connecting anteriorly into buccal bulb. Digestive gland with lateral branches entering cerata. Intestine emerging laterally from left side of digestive gland, forming a loop and opening into anus. Anus pleuroproctic, opening posterior to the genital opening, below the ceratal rows. Radular formula 19 × 0.1.0 in holotype. Radular teeth broad, arch-shaped (Fig. 21B), with 28–30 elongate acutely pointed denticles, lacking central cusp. Teeth progressively smaller towards posterior end of radula. Jaws elongate (Fig. 21A), no denticles on masticatory border.

Reproductive system with very elongate, convoluted ampulla forming numerous loops (Fig. 20B), opening into female gland complex next to prostate opening. Prostate tubular, very elongate, forming numerous loops, expanding abruptly into large, muscular deferent duct with several superficial folds. Penis simple, short, wide. Seminal receptacle oval, connecting to the female gland complex and the genital opening.

Biology. The single specimen was collected using an ROV suction sampler, while suctioning on and around vertebrae of an implanted Blue Whale carcass that was being surveyed as part of a time-series analysis of whalefall ecology (Lundsten et al. 2010b). The whale skeleton where the specimen was collected was in the late sulphophilic stage of degradation. Unidentified anemones, abundant polychate worms, Psathyrometra fragilis (Echinodea), Neptunea -Buccinum complex (gastropods), Chionoecetes tanneri (crabs), as well as Eptatretus sp., Lycenchelys sp., Lycodapus sp., and Sebastolobus sp. (fishes) were also observed at this time.

Phylogenetic position. Zeusia herculea is sister to the genus Aeolida (Fig. 14). Unfortunately, sequences of Zeusia hyperborea are not yet available for comparison.

Remarks. Kienberger et al. (2016) reviewed the systematics of Aeolidia papillosa and concluded it constitutes a complex of several species, two of which occur in the Northeast Pacific Ocean: A. papillosa (Linnaeus, 1761) and A. loui Kienberger, Carmona, Pola, Padula, Gosliner & Cervera, 2016. However, Kienberger et al. (2016) considered that A. herculea Bergh, 1894 probably constitutes a third, deep water species, distinguishable from A. loui by the presence of a pleuroproctic anus. Kienberger et al. (2016) also agreed with Martynov & Korshunova (2011) considering A. farallonensis Gosliner & Behrens, 1996 as a synonym of A. herculea.

More recently Korshunova et al. (2017), argued that species of Aeolidiidae with a pleuroproctic anus constitute a distinct genus, Zeusia Korshunova, Zimina & Martynov (2017), including two species, Z. hyperborea Korshunova, Zimina & Martynov, 2017 from the Barents Sea, and Z. herculea (Bergh, 1894) from the Northeast Pacific. Molecular evidence indicated that Zeusia is sister to Aeolidia supporting the separation of these two groups (Korshunova et al. 2017).

The specimen here examined has a pleuroproctic anus, as described by Korshunova et al. (2017) and is sister to other species of Aeolidia sequenced to date. Morphologically, our specimen is similar to the descriptions of A. herculea by Martynov & Korshunova (2011) and Gosliner & Behrens (1996) —as A. farallonensis —including details of the radular and reproductive anatomy. Because of the morphological similarities and the shared deepwater habitat between our specimen and previous descriptions of A. herculea, we consider it as a member of this species.