Voragonema tatsunoko, Lindsay, Dhugal & Pagès, Francesc, 2010

Lindsay, Dhugal & Pagès, Francesc, 2010, Voragonema tatsunoko (Trachymedusae: Rhopalonematidae), a new species of benthopelagic medusa, host to the hyperiid amphipod Mimonectes spandli (Physosomata: Mimonectidae), Zootaxa 2671, pp. 31-39 : 33-37

Voragonema tatsunoko sp. nov.

Voragonema with subumbrella rose-orange-coloured, peduncle medium, manubrium vermillion, beige-cream gonads pendant from just above the midpoint of the radial canals, circular canal with nine triangular-based filamentous centripetal expansions in each octant, about 1050 marginal tentacles of 2 types in 6–7 rows.

Species description. Umbrella slightly more conical than hemispherical, not as high (14 mm) as wide (19 mm) ( Fig. 1 A) [all measurements based on the living holotype which shrunk to approximately 70% of its original size after preservation; paratype umbrellar height was 16 mm]; mesoglea 2.8 mm thick in the apex and tapering towards the umbrella margin; exumbrella transparent with fine meridional ridges, subumbrella pale rose-orange ( Fig. 1 A, B, C), hemispherical in fresh specimens, with circular and crossed oblique-radial muscles.

Gastric peduncle medium, cylindrical, pale rose-orange, one quarter of subumbrella height, 1.8 mm in diameter and 4 mm in height.

Radial canals 8, whitish, translucent, straight, of uniform width but with perpendicularly striated tissue beneath the distal half causing them to appear broader.

Manubrium long, narrow, square and vermillion in fresh specimens ( Fig. 1 A, B, C) and brown when preserved ( Fig. 2 A, B, C), over twice the length of the peduncle; mouth with four flared simple lips; manubrial cavity not divided into gastric pouches; peduncle and manubrium height approximately equal to that of the subumbrella in fresh specimens.

Gonads beige-cream, laterally flattened, with distal margin of attachment at midpoint of radial canals, not extending onto upper one-third, distally pendant.

Ring canal rose-orange, wide, with nine triangular-based filamentous centripetal expansions between each radial canal.

Tentacles rose-orange and translucent, of two types, superimposed in 6–7 rows, following a regular pattern as follows: outermost row of 80 solid tentacles, with adnate bases, situated below each of the centripetal and radial canals, arranged in pairs below the centripetal canals but with the centralmost and those below the radial canals singular, in the pattern 2-2-1-2-2-1 per octant; second outermost row also with 80 solid tentacles, in pairs below each of the singular outermost tentacles; third outermost row with 48 large solid tentacles pointing downwards and situated below the spaces between the tentacles of the two outermost rows; up to 4 rows of thinner, hollow, annulate tentacles of a second type, approximately 108 tentacles per octant. Total tentacle number approximately 1075, with 208 of the larger, solid type and situated in the outer rows ( Fig. 3 A, B).

Statocysts free, numerous, approximately 20 per octant.

Velum translucent, pale rose-orange, 4.2 mm wide, with two marked muscle bands, the inner twice the width of the outer ( Fig. 2 A, B; Fig. 3 A).

Remarks. An immature male of the rare hyperiid amphipod Mimonectes spandli Stephensen and Pirlot, 1931 , measuring 6.6 mm from head to telson, was attached to the subumbrella next to a gonad in the paratype specimen ( Fig. 1 A, B, C). This is only the fourth record of this species in the literature and the first record outside of the Atlantic Ocean, being a significant range extension ( Vinogradov et al. 1996). Although this is the eleventh example of an association between a hyperiid amphipod of the Infraorder Physosomata and a gelatinous macroplanktonic host, it is only the second within the family Mimonectidae (Bovallius, 1885) , with Gasca et al. (2006) reporting a congener Mimonectes sphaericus Bovallius, 1885 (male) as an associate of the prayid siphonophore Nectadamas diomedeae (Bigelow, 1911) . Only one previous example of an association between a trachymedusa of the genus Voragonema and another organism exists in the literature. Larson et al. (1991) report an association between Crossota (= Voragonema ) pedunculata ( Bigelow, 1913) and an unidentified pycnogonid species between 830–910 m in the benthopelagic layer in the Bahamas.

The presence of centripetal expansions arising from the ring canal places this trachymedusan species in the genus Voragonema Naumov, 1971 . Voragonema tatsunoko Lindsay & Pagès, 2010 differs markedly from V. profundicola Naumov, 1971 by the markedly greater number of tentacles (1050 vs 500) and from other described species of Voragonema by the shape and number (9) of the centripetal canals (Table 1). Preservation in 5% formalin-seawater has lead to shrinkage and distortion in both of the specimens with one becoming more conical ( Fig. 2 C) while the other now exhibits a somewhat flattened exumbrella ( Fig. 2 A)—resembling that of V. profundicola in the diagram by Naumov (1971). The colour of the manubrium has also changed from vermillion to brown ( Fig. 1 vs. Fig. 2 A). The determination of whether V. profundicola does indeed have a flattened exumbrella or whether the original colour of the manubrium was in fact brown or some other colour will need to await collection of fresh specimens from the type locality of 6800–8700 m in the Kurile- Kamchatka Trench. As the other three species of Voragonema are benthopelagic in habitat, it can be inferred that V. profundicola may also be so. The loss of the world's deepest diving submersible platform, the ROV Kaiko, in May 2003 is therefore unfortunate and procuration of further specimens may only be possible following the development of a new submersible platform with the capability of reaching depths of over 7000 m.

In total, seven specimens of V. tatsunoko were observed during 64 minutes over the muddy bottom in the central axis of the Suruga Trough where the current speed was 0.1 knots, current direction 240˚. The specimen without the commensal amphipod pulsed at a rate of 0.7 Hz (11 times in 16.5 seconds), and that with the amphipod also pulsed at 0.7 Hz (7 times in 10 seconds), suggesting that the presence of the amphipod did not affect swimming ability. Other members of the benthopelagic fauna included a species of the lobate ctenophore Bathocyroe that had black pigmentation on the inner surfaces of the lobes and auricles as well as dark red-brown pigmentation on the gut, mysids, gammarid amphipods, Poralia rufescens Vanhöffen, 1902 , munnopsid isopods, oikopleurids and an orange-pigmented chaetognath.

In conclusion, the genus Voragonema presently comprises four species: V. profundicola Naumov, 1971 , V. pedunculata ( Bigelow, 1913) , V. l a c i n i a t a Bouillon et al, 2001, and the species described herein, V. tatsunoko Lindsay & Pagès, 2010 . The number and shape of the centripetal canals (Table 1) easily identifies this newlydescribed species.

Figures

FIGURE 1. Specimen of Voragonema tatsunoko with the hyperiid amphipod Mimonectes spandli Stephensen and Pirlot, 1931 attached [paratype: 2 K 1337 SS 6 h]. Specimen collected at 15: 24: 36 above muddy sediment in Suruga Bay at 1967 m depth on 7 April 2002 and photographed in an aquarium on board ship. Voragonema tatsunoko: lateral (A), apicolateral (B), and oral (C) views; scale: 10 mm.

FIGURE 2. Lateral views of the holotype of Voragonema tatsunoko [2 K 1337 SS 6 g], collected at 15: 18: 15, under darkfield illumination (A) and transmitted light (B), and the paratype [2 K 1337 SS 6 h] under transmitted light after removal of the parasitoid amphipod (C). Note the striations on the external surface of the exumbrella (A), reminiscent of those in Crossota rufobrunnea (Kramp, 1913); scale: 10 mm.

FIGURE 3. (A) Lateral view of the holotype of Voragonema tatsunoko [2 K 1337 SS 6 g], collected at 15: 18: 15; scale: 1 mm. (B) detail of the bell margin of the paratype [2 K 1337 SS 6 h], from the external surface showing the proximal part of the autotomized marginal tentacles superimposed in several rows together; scale: 1 mm.