Elisesione imajimai, Jimi & Eibye-Jacobsen & Salazar-Vallejo, 2018

Jimi, Naoto, Eibye-Jacobsen, Danny & Salazar-Vallejo, Sergio I., 2018, Description of Elisesione imajimai sp. nov. From Japan (Annelida: Hesionidae) and A Redescription of E. problematica (Wesenberg-Lund, 1950) and Its Confirmation Within Hesionini, Zoological Studies 57 (8), pp. 1-9 : 2-5

Elisesione imajimai sp. nov.

( Figs 1-2 ) urn:lsid:zoobank.org:act:A2AFFD68-4E59-45B0-A1EA-DB08BEFA7747

[Japanese name: Noumen-otohime-gokai]

Wesenbergia problematica: Imajima 2003: 138-139 , fig 81; Imajima 2007: 450, fig 143.

Elisesione problematica sensu Imajima 2003 : Salazar-Vallejo 2016: 5.

Description: Holotype 25 mm long, 3 mm wide (by chaetiger 7, without parapodia), with 16 chaetigers (right parapodium of chaetiger 5 removed, kept in 2.0 ml plastic tube with holotype; right parapodia of chaetigers 5-7 and 11 removed from paratypes by Dr. Minoru Imajima, kept in glass vial with type specimens).

Body cylindrical, tapered in posterior region ( Figs. 1A, B ), dorsally pinkish and ventrally whitish in life, whitish in ethanol, dorsal integument annulated, with 9-10 transverse wrinkles per segment ( Figs. 1A, B ). Reddish brown transverse bands and longitudinal marginal lines present along each segment in living specimen over a pale brown background ( Figs. 1A, B ). Transverse bands irregular, straight, homogeneous on chaetiger 1, across last segment and pygidium, other segments with transverse bands straight across their anterior margin, posterior margin of band projected posteriorly into irregular semicircular areas. Tentacular cirri, dorsal cirri and parapodial lobes whitish. Pigmentation of paratypes unknown (specimens in ethanol for over 50 years).

Prostomium wider than long, median line with a shallow depression, lateral margins rounded, slightly wider medially ( Fig. 1C ). Antennae digitate. Palps simple, blunt, shorter than antennae (1 / 2 - 4 / 5 times as long as antennae), positioned at same level, slightly external to antennae ( Fig. 1C ). Eyes present, two pairs, pinkish in living specimen, anterior eyes with pigmented areas transverse half-moon shaped, in posterior ones vertical ( Fig. 1C ).

Tentacular cirri long, thick, longest one reaches chaetiger 3 in paratype. Lateral cushions low, barely projected dorsally, slightly projected laterally, undivided ( Figs. 1A, 1B ).

Parapodia with chaetal lobes cylindrical, truncate, longer than wide ( Fig. 2A ); dorsal cirri thick, cirrophores cylindrical, smooth, 1.5-2 times longer than wide ( Fig. 2B ), cirrostyle basally cylindrical, smooth, annulated medially and distally, shorter than body width without parapodia. Ventral cirri basally smooth, rugose medially and distally, surpassing neurochaetal lobe, reaching up to half length of neurochaetal bundle ( Fig 2C ).

Acicula pale brownish, tapered; acicular lobe single, blunt, rounded ( Fig. 2D ). About 35 neurochaetae per bundle ( Fig. 2E ), shaft and blade pale brownish, blades 7-10 times longer than wide, with subdistal tooth 1 / 3 - 1 / 2 times as long as apical tooth, guard approaching apical tooth ( Fig. 2F ).

Posterior region barely thinner than median region. Cirri of prepygydial segment broken; pygidium smooth, depressed; anus dorso-terminal, open, with two pairs of lateral anal cirri ( Fig. 1B ) and about 8 anal papillae.

Pharynx dissected in paratype, smooth; dorsal papilla not seen, terminal papillae absent ( Fig. 1D ).

Paratype with oocytes in coelom, visible along chaetigers 6-14, each oocyte 300-500 μm in diameter, whitish in ethanol, about 150 oocytes per segment.

Type material: Holotype (NSMT-Pol H-665), sex unknown, Sagami Bay St. 2 (150-201 m depth, 35°07'N, 139°34'E to 35°06'N, 139°34'E) . Paratype (NMST-Pol R:604-1), Sagami St. 1 (150- 250 m depth, 35°07.0'N, 139°34.3'E to 35°07.3'N, 139°34.2'E), 30 mm long, 3 mm wide, mature female; paratype (NSMT-Pol R: 604-2), Sagami St. 1, 34 mm long, 4 mm wide, sex unknown.

Sequence: Partial sequence of the mitochondrial cytochrome c oxidase subunit I (COI) gene, 629 bp; 18S rRNA (18S) gene, 1713 bp; 28S rRNA (28S) gene, 982 bp, extracted from holotype specimen deposited in DDBJ (No. LC361352 - LC361354 ). 16S rRNA gene could not be determined.

Etymology: This species is named in honour of Dr. Minoru Imajima, in recognition of his great contributions to polychaete taxonomy in Japan.

Distribution: Only known from the type locality (Sagami Bay, Japan), 150-250 m depth.

Remarks: Elisesione imajimai sp. nov. resembles E. problematica ( Wesenberg-Lund, 1950, see below) because they have single acicular lobes and dorsal cirrophores that are about twice longer than wide. They differ in the length of the ventral cirri, size of palps, colour of aciculae, and the number of dorsal transverse wrinkles per segment.

In E. imajimai sp. nov. the ventral cirri surpass the neurochaetal lobe tip, the palps are 1 / 2 - 4 / 5 times as long as the antennae, the aciculae are pale brownish, and the dorsal integument has 9-10 transverse wrinkles per segment, whereas in E. problematica the ventral cirri hardly reach the tip of the neurochaetal lobe, the palps are as long as the antennae, the aciculae are black, and there are 13- 15 transverse wrinkles per segment ( Wesenberg-Lund 1950).

Furthermore, the neurochaetal blades of E. imajimai sp. nov. are 7-10 times longer than wide, whereas in E. problematica they are larger (8-12 times longer than wide). This feature was used in the discrimination of the two species in Salazar-Vallejo (2016), but the range difference cannot be regarded as diagnostic because of the small number of specimens currently known, and especially because of the few chaetae remaining in the holotype of E. problematica .

On the other hand, the finding of very large oocytes in the new species deserves two additional comments about some interesting features. First, egg size is to some degree related to dispersal potential. Strathmann (1977:373) simplified the mathematical models of Richard Vance, proposed a few years before, by indicating that “an increase in egg size is associated with… a decrease in the duration of the period between fertilization and metamorphosis.” Second, large eggs usually undergo lecitotrophic development. Wray and Raff (1991:48) and Giangrande et al. (1994:310) correlated egg size and developmental mode in echinoids, and indicated that species having eggs of about 300-500 µm have lecitotrophic larvae, whereas those having eggs about 600- 1000 µm have highly derived lecitotrophic larvae. Schroeder and Hermans (1975) also indicated that if polychaete eggs are larger than 180 µm in diameter, the young develop directly into juveniles without an intervening stage. These larvae/ juveniles usually spend a short period in the water column. These two features, combined with the low abundance of the species throughout their distribution range, could explain why the known species ranges are so restricted, confined in most instances to single localities.

Figures

Fig. 1. Elisesione imajimai sp. nov., A-C, holotype (NSMT-Pol H-665); D, paratype (NSMT-Pol R: 604-2). (A) anterior end, dorsal view; (B) posterior end, dorsal view (arrow points to a lateral cushion); (C) prostomium, dorsal view (white arrows point to tips of antennae, black arrow points to left palp); (D) pharynx, ventral view. Scale bars: A-B = 5 mm; C = 3 mm; D = 2 mm.

Fig. 2. Parapodial features of Elisesione imajimai sp. nov., A-F, paratype (NSMT-Pol R:604-2), chaetiger 5, right parapodium, posterior view. (A) parapodium; (B) dorsal cirrophore; (C) ventral cirrus; (D) acicular lobe (arrow points to tip of acicular lobe); (E) neurochaetal bundle; (F) blade of ventral neurochaeta (inset: tip of blade). Scale bars: A = 1 mm; B-E = 300 μm; F = 100 μm; F (inset) = 5 μm.