Ziminella abyssa Korshunova et al., 2017

Ziminella abyssa Korshunova et al., 2017 View in CoL

( Figs 3 D, E View FIG , 6 View FIG , 7)

Ziminella abyssa Korshunova et al., 2017: 20 View in CoL View Cited Treatment , 21, fig. 12.

Coryphella stimpsoni – Roginskaya, 1978: 169–177, fig. 1, 2, partim (not of Verrill, 1879).

Coryphella japonica – Martynov, 2013: 112-114 (partim), pl. 1.1–10, pl. 4, pl. 6.1–4, pl. 7.1–3 (not of Volodchenko, 1941).

Type material. Holotype ZSM Mol-20100647 deposited at Bavarian State Collection of Zoology, Munich. 153 paratypes ( ZMMU Op-248–Op-250, Op-252, Op-253, Op-255–Op-259, Op-264) deposited at Zoological Museum of Lomonosov Moscow State University.

Material studied. Sea of Japan, R / V “ Vityaz ”, st. 6658, 40°37’8”N 134°07’0”E, 3580 m depth, 16.06.1972, ZIN63735 View Materials (6 spm). All specimens were dissected GoogleMaps .

Type locality. Sea of Japan, 43°01’42.9”N 135°04’32.7”E, 2676 m depth GoogleMaps .

Description. External morphology (based on studied specimens): Body length up to 25 mm. Body wide. Foot wide, anterior corners short, rounded. Notal edge well-developed, continuous, forming short elevation above lateral body sides. Oral tentacles elongated, conical. Rhinophores slightly rugose, same length as oral tentacles, thinner than oral tentacles. Cerata in distinct rows, attached directly to well-defined notal edge. Anal opening on right side below notal edge, at beginning of posterior half of body. Reproductive opening on right side, on anterior part of body.

Coloration [after Korshunova et al., 2017]: Background body color translucent milky-white. Rhinophores light orange at base and pale on rest of length. Cerata dark violet.

Internal morphology (based on studied specimens): Jaws massive, oval-triangular with well-developed masticatory border bearing several rows of low blunt denticles or elevations. Radular formula: 22–31 × 1.1.1. Rachidian teeth large, triangular. Central cusp conical, wide and rather short, non-compressed by adjacent denticles. From 10 to 19 small slightly curved denticles on each side of cusp, sometimes bifurcated, commonly different in size. Lateral teeth narrow, slightly curved triangle blades, no denticulation traceable. Reproductive system diaulic. Ampulla long, narrow and convoluted. Vas deferens extremely long with slightly expanded prostatic part. Penis slightly folded, elongated when everted. Seminal receptacle absent.

Distribution. Currently this species is known only from the Sea of Japan [ Roginskaya, 1978; Martynov, 2013; Korshunova et al., 2017; this study], however further studies and sampling in adjacent regions are needed.

Remarks. This species was described based on historical collections (sampling by the R/V “ Vityaz ” in 1972–1976) and materials from the SoJaBio expedition in 2010 [ Korshunova et al., 2017]. The historical samples have been studied initially by Roginskaya [1978] and Martynov [2013], and were identified as either Coryphella stimpsoni or Coryphella japonica respectively. Furthermore, the collection of material suitable for the molecular studies allowed researchers to reconsider the deep-sea material of Z. japonica as a distinct species Z. abyssa [ Korshunova et al., 2017], although sequences of true Z. japonica were not available for comparison. Our results support the distinct status of Z. abyssa from Z. japonica ( Fig. 2B View FIG ), however our investigation of the radular morphology showed several differences with the original description of Z. abyssa ( Fig. 7). Roginskaya [1978], Martynov [2013], and Korshunova et al. [2017] have reported on the irregular denticulation or even smooth rachidian teeth. Roginskaya [1978] suggested that the high variability in denticulation of lateral and rachidian teeth was a result of radular wearing. Martynov [2013] identified asymmetrically effaced rachidian teeth and also suggested this was a result of feeding on burrowing sea anemones of the family Edwardsiidae . Korshunova et al. [2017] considered the irregular denticulation of the rachidian teeth to be a specific trait differentiating Z. abyssa from the remaining species of the genus. According to the latter authors, denticles of the rachidian teeth are commonly “fold-like or fork-like”, sometimes greatly irregular, and the larger denticles are typically intermingled with the smaller ones, which is an obvious trait of studied juvenile specimen [ Korshunova et al. 2017: 20, fig. 12I]. In our material the denticles on the rachidian teeth were not highly irregular ( Figs 7 F, H, J), but in several specimens their size was different and some of denticles were branched ( Fig. 7D). We did not detect any specific pattern of denticulation, like regular intermingling of larger denticles with smaller ones. Also, the anterior teeth rows had the same denticulation as the posterior teeth, with no sign of teeth wearing. Moreover, Korshunova et al. [2017] described small denticles on internal edge of the lateral teeth, however the referred figures display either only rachidian teeth [ Korshunova et al., 2017: 20, figs 12 F–H] or smooth lateral teeth [ Korshunova et al., 2017: fig. 12E]. In our material the lateral teeth were smooth with no evidence for any denticulation ( Figs 7 C, G, K).

Ziminella abyssa is clearly different from other described species considered distinct herein. The main differences are observed in radular characters, as this species has the highest number of denticles on the rachidian teeth among Ziminella , and these denticles commonly have different sizes.Also, in our material smooth lateral teeth were detected, which could be another distinctive feature of this species. It is possible that the coloration or some external features could be also different from those in other Ziminella species, but we do not have newly collected material to confirm this.

It is important to notice that both Martynov [2013] and Korshunova et al. [2017] identified a 2 mm specimen (ZMMU Op-264) to be a member of Ziminella abyssa . This specimen has very distinct rachidian teeth with narrow and long central cusps, highly protruding from the tooth surface [ Korshunova et al., 2017: fig. 12I]. This morphology is not typical for Ziminella , but is found in other Paracoryphellidae , such as Chlamylla intermedia (Bergh, 1899) , Paracoryphella parva (Hadfield, 1963) , Polaria polaris (Volodchenko, 1946) , see for example Korshunova et al. [2017], although no records of these species are known for the deep-sea waters of the Sea of Japan. We have studied a ‘juvenile’ specimen from the Sea of Japan collected from 3427–3431 m in depth (MIMB49484). The external appearance of this specimen ( Fig. 8A) is very similar to that of the ‘juvenile’ ZMMU Op-264 [ Martynov, 2013: pl. 2.9], in particular they have similar body width and arrangement of oral tentacles and rhinophores; the bases of the rhinophores are closely together ( Fig. 8A), which is not typical for Ziminella ( Fig. 3 View FIG ). The study of the radula of MIMB49484 shows that it has similar morphology to that of Polaria polaris : the rachidian teeth are triangular and bear long central cusps, and the lateral teeth are wide with large sharp denticles on the inner edge and long cusp ( Figs 8 B, C). Polaria polaris has a wide distribution in the Arctic waters [ Martynov, 2006 as Coryphella polaris ], and was also recorded from the Sea of Okhotsk and the southern Kuril Islands with a bathymetric limit of 325 m depth [ Martynov et al., 2015]. Our discovery extends its geographical range to the Sea of Japan, and its bathymetric range to 3431 m depth. Taking into account the external similar- ity between MIMB49484 and ZMMU Op-264, and also the different radular morphology of the latter from typical Ziminella , we consider ZMMU Op-264 does not belong to Ziminella abyssa . Nevertheless, we cannot precisely identify the species of ZMMU Op-264 as its rachidian teeth morphology does not fit the diagnosis of any described paracoryphellid species. This fact, along with the discovery of P. polaris in the abyssal depths of the Sea of Japan, suggests the species diversity of this area is much higher than previously thought and further dedicated studies with comprehensive sampling for molecular analysis are needed.