Investigating the genetic diversity of Sea Squirts in Rongcheng Bay, Shandong, China.

Ascidians, or Sea Squirts, are the most abundant class of the subphylum Tunicata and are distributed along shorelines worldwide. They are sessile marine invertebrates and are widely used as a model organism for developmental and evolutionary studies. Ascidians exhibit multiple morphological characteristics, from small colonial to colorful and large solitary forms. They are divided into three major well-accepted orders, namely, Phlebobranchia, Aplousobranchia, and Stolidobranchia, based on the branchial sac morphology of the adults. However, the class Ascidiacea is paraphyletic (i.e. not everything thought to be decended from the last common ancester of the group is considered to be am Ascidian) with the  Phlebobranchia and Aplousobranchia showing a close relationship with Thaliaceae (Pyrosomes, Salps, and Doliolids), a non-Ascidian Tunicate class, whereas the Stolidobranchia remains a distinct and monophyletic group. Over the course of several decades, the Ascidiacea have been shown to be an important class of ecological species because of their invasive potential along with their ability to adapt to new environments. Transportation of Ascidians attached to ship hulls as fouling material and within the ballast water of ships has enabled them to invade new territories. This phenomenon has major impacts on local marine biodiversity as well as aquaculture industries. Therefore, the Ascidiacea were recently considered as important model species for the study of nonindigenous species worldwide.

In a paper published in the journal Ecology and Evolution on 10 March 2020, Punit Bhattachan and Runyu Qiao of the Key Laboratory of Marine Genetics and Breeding at the Ocean University of China, and Bo Dong, also of the Key Laboratory of Marine Genetics and Breeding at the Ocean University of China, and of the Laboratory for Marine Biology and Biotechnology at the Qingdao National Laboratory for Marine Science and Technology, and the Institute of Evolution and Marine Biodiversity at the Ocean University of China, present the results of a comparative analysis of three Ascidian species from northeast of China, with samples from elsewhere in the world, using the cox1 gene sequence as a genetic marker to distinguish native from invasive ascidian populations.

Bhattachan et al. collected adults of three Ascidian species, Ciona robusta, Ciona savignyi, and Styela clava, from the Rongcheng Bay area of Shandong Province, which is a part of the Yellow Sea, in northeast China. These were maintained in the laboratory in seawater tanks with aeration and constant illumination, where species were identified morphologically, and internal tissues were collected for DNA extraction and sequencing.

Collection site of the Ascidian samples (black arrow). Bhattachan et al. (2020).

The cox1 sequences from three ascidian populations at different regions of the world were retrieved from the NCBI database to build multiple sequence alignments. Only the unique haplotype datasets were used for the multiple sequence alignments. Neighbor-Joining  and maximum parsimony  methods were employed to construct a phylogenetic tree with 1000 bootstrap estimations in the default setting using MEGA7.0. The barcode region of the cox1 sequence (accession no. HM151268.1) of the Sea Pinapple, Halocynthia roretzi, was used as an out-group.

Multiple sequence alignments of cox1 from the three Ascidian species were performed separately in ClustalW hosted by MEGA7.0 using default settings. Genetic diversity parameters, including haplotype number, haplotype diversity, nucleotide difference, mutation number per sequence, number of segregating sites, and nucleotide diversity, were estimated using DnaSP software.

Relationships among the three Ascidians cox1 haplotypes found globally, including those from China, were determined using a median-joining method in the network software. To infer the population structure and understand the connectivity between native and invasive ascidian populations, we performed molecular variance analysis using cox1 haplotypes from samples available in the database as well as those in Bhattachan et al.'s dataset study using the ARLEQUIN 3.11 software.

Morphological identification of the three Ascidian species. (b) Ciona robusta adult with oral siphon (os), atrial siphon (as), sperm duct (white arrow), oviduct (black arrow), and red colour at the tip of the sperm duct (arrowhead). (c) Ciona savignyi adult with oral siphon (os), atrial siphon (as), sperm duct (white arrow), and oviduct (black arrow). (d) Adult Styela clava with oral siphon (os) and atrial siphon (as). Scale bar represents 1 cm. Bhattachan et al. (2020).

Bhattachan et al. cloned the full length of the cox1 gene from genomic DNA of 50 individuals of the three Ascidian species. Each resulting sequence was subjected to BLASTN, with the results indicating that these sequences belonged to the three respective ascidian species. The open reading frames of the cox1 sequence from three species were variable. Bhattachan et al. identified a deletion polymorphism of cox1 in Ciona savignyi, but not in Ciona robusta and Styela clava. For instance, only a single 1560 and 1543 base pair-length of cox1 sequence was identified in Ciona robusta and Styela clava, respectively, whereas two different lengths of cox1 (1545 and 1548 base pairs) were identified in Ciona savignyi. All these sequences were deposited in the NCBI database.

Bhattachan et al. also retrieved the cox1 barcode sequences from the NCBI database Only the cox1 barcode regions of unique haplotypes were used for multiple sequence alignments and phylogenetic tree construction. The resulting phylogenetic trees allowed us to delineate different haplotypes among all of the samples. In the Ciona robusta tree, Bhattachan et al. found that the haplotypes (H_1 to H_9) from China did not form a single clade in either Neighbor-Joining or maximum parsimony trees, but rather clustered with some haplotypes from individuals originating from Korea and the USA. Similarly, the haplotypes (H_1 to H_16) of Ciona savignyi from China did not cluster in a single clade in either Neighbor-Joining or maximum parsimony trees. Instead, they grouped with other haplotypes from Korea and the USA. In addition, Neighbor-Joining and maximum parsimony trees did not resolve the haplotypes (H_1 to H_14) of Styela clava from China into a single clade either. Conversely, they formed a cluster with some haplotypes from New Zealand and the USA, which were invasive populations.

Bhattachan et al. used the cox1 gene for molecular diversity analysis. Nine haplotypes were identified among 14 Ciona robusta samples, 14 haplotypes among 19 Styela clava samples, and 16 haplotypes among 17 Ciona savignyi samples. The results of the comparative analysis using different genetic diversity parameters also revealed that Ciona savignyi was diverse compared with Ciona robusta and Styela clava. The haplotype diversity was comparatively higher in Ciona savignyi (0.993 + 0.038) than that in Ciona robusta (0.912 + 0.059), and Styela clava (0.947 + 0.038). Similarly, the detected average number of nucleotide difference in Ciona savignyi (20.618) was higher than that in Ciona robusta (8.143) and Styela clava (11.550). Nucleotide diversity and average number of mutations were also relatively higher in Ciona savignyi (0.02630, 0.05061) compared with Ciona robusta (0.01094, 0.01811) and Styela clava (0.01919, 0.03097), respectively.

A Tajima neutrality test produced negative values for all three species, but these values were significant only in the Ciona savignyi population, indicating that there was an excess of low-frequency polymorphisms, and the Ciona savignyi population was expanding. However, in the Ciona robusta/Styela clava populations, the values were not statistically significant, indicating that these two species populations did not deviate from the neutral expectations. Similarly, for Fu and Li's D* statistic, negative values were observed in all three species. The values from Ciona robusta and Ciona savignyi were statistically significant, whereas those from Styela clava were not. The results from these two analytical approaches indicate that the population of Ciona savignyi is undergoing positive selection and expansion.

Bhattachan et al. divided the three Ascidian species populations into native and invasive groups, with populations located within eastern Asian countries-like China, Japan, and Korea being considered as native groups. Since these species are believed to have originated from this region while the rest of the populations from other regions were grouped as invasive populations. Network analysis revealed that there were three haplogroups (1, 2, and 3) in Ciona robusta and Ciona savignyi, respectively. No haplogroups were found for Styela clava. In the Ciona robusta network, Bhattachan et al. found native populations in haplogroup 1, and haplogroup 3 consisted of invasive populations. On the other hand, haplogroup 2 was comprised mainly of native populations, including those from China, but few haplotypes were shared from invasive populations as well. Haplogroups 1 and 2 were connected with haplogroup 3. Similarly, in the Ciona savignyi network, Bhattachan et al. found native populations in haplogroup 1, but haplogroup 2 was entirely composed of only native populations, and haplogroup 3 consisted only of invasive populations. By contrast, there were no haplogroups present in the Styela clava network, and all haplotypes from both native and invasive populations, including those from China, were connected to each other.

Bhattachan et al. also performed a hierarchical analysis of molecular variance using cox1 haplotypes from both native and invasive populations of the three Ascidian species. There was no clear structure between native and invasive populations in Ciona robusta and Ciona savignyi, but these values were not statistically significant. In addition, we recorded a negative value for Styela clava, indicating that there was no population differentiation. By contrast, among populations of Ciona robusta, Ciona savignyi, and Styela clava, there were significant variations, with the highest level of variation appearing in Ciona savignyi. Surprisingly, within these variations, the highest value was recorded for Styela clava (77.37%),  followed by Ciona savignyi (22.77%) and Ciona robusta (21.07%).

Bhattachan et al. identified three Ascidian species from Northeast China using both morphological characteristics and genetic marker analysis. The tunic of Ciona spp. is soft and semi-transparent, whereas that of Styela clava is relatively rough and opaque. Since the tunic is mainly composed of a cellulose-like material resembling that of plants, we assume that tunic composition varies among different species. In addition, Ciona spp. absorb more water, as demonstrated by dry tunic weight, and potentially as a result, this organ became semi-transparent in nature. Furthermore, Ciona robusta is comparatively larger in size than Ciona savignyi. Recently, it was also revealed that the morpho-physiological properties play an essential role in the control of size between these two Ascidians. Hence, Bhattachan et al. use these characters to distinguish between them. It is also interesting to note that there is a red coloration at the tip of the sperm duct in Ciona robusta, which is absent in Ciona savignyi. The evolutionary and functional property of this pigmentation is not yet known. Strikingly, egg morphology also varies among these three species. For instance, long follicle cells are present on the outer covering of Ciona robusta eggs, comparatively shorter follicular cells overlay Ciona savignyi eggs, and no outer follicle cells are present on Styela clava eggs. Generally, the Ascidian egg consists of two layers of follicle cells, with a vitelline coat next to the egg membrane and several test cells between them. These outer follicle cells are vacuolated and elongated and are speculated to provide buoyancy to eggs in seawater. This may help Ascidian eggs disperse by the water current and thereby be transported to distant places. Follicle cells are also the first contact of sperm entry, and it is widely known that they function to prevent self-fertilization via a chemical reaction. Long follicle cells might have enabled a higher dispersal rate of Ciona robusta. This characteristic might also inhibit more self-fertilization in comparison to Ciona savignyi and Styela clava.

The genetic marker cox1 has been widely used for identification and characterization of genetic diversity. On the basis of barcode region of the cox1 gene from these three Ascidian species as well as other available sequences in the databases, Bhattachan et al. constructed the phylogenetic trees to infer their identification, which showed that the Ciona spp. from China was closely related to native populations, mostly from Korea to Japan. This result indicates that the Ciona spp. samples collected here from China are indeed native Ascidians, and these were not introduced from other geographical areas. However, Styela clava formed a clade with invasive populations. Bhattachan et al. also found that some haplotypes from invasive populations formed a cluster with native populations. This result indicates that there was incursion of native and invasive Ascidian populations to different parts of the world. A similar phylogenetic method was used for Ascidian identification in other geographical regions as well.

Ascidians are marine organisms with a relatively high level of genetic diversity, and there exist differences in levels of genetic diversity among the Ascidians themselves. How these differing levels of genetic diversity are maintained remains unknown. Bhattachan et al.'s current analyses confirmed that these Ascidians have a high level of genetic diversity, with Ciona savignyi exhibiting a comparatively high level of genetic diversity at the molecular level. One possible explanation might be that Ciona savignyi has a large effective population size, with differing life-history traits compared to Ciona robusta and Styela clava. Of note, a previous genome-wide study also revealed that Ciona savignyi exhibited the highest level of genetic diversity. Other comparative studies on Ascidians also confirmed that they have different evolutionary rates. This could be another reason causing the different levels of genetic diversity among these three species. In addition, the neutrality tests showed that Ciona robusta and Styela clava are undergoing neutral evolution, and Ciona savignyi is experiencing population expansion and positive selection. This also explains why Ciona savignyi exhibits a higher level of genetic diversity compared with Ciona robusta and Styela clava. Given the widespread distribution of Ascidians, it is possible to exhibit high genetic diversity across populations. This kind of observation is also seen in a wide range of other organisms.

Another important characteristic feature of Ascidians is their invasive potential. Some Ascidian species are dispersed to different geographical or ecological niches because of both anthropogenic and natural causes and are considered as invasive species. Bhattachan et al. compared the global cox1 haplotypes of these three Ascidians to understand their connectivity and population genetic structure. Global haplotypes were divided into native and invasive populations. The network analysis indicated that Ciona spp. formed haplogroups with separate native and invasive populations, although some haplotypes were shared. However, in the network of Styela clava, there was no such haplogroup formation as all of its haplotypes were interconnected, suggesting extensive incursion for this species in different geographical areas. A previous global study of Styela clava also suggested its extensive incursion, in which it was categorized as invasive species. In addition, a regional study of this species indicated the multiple sources of incursions. The results of the hierarchical analysis of molecular variance of the three species of Ascidian were also consistent with the network analysis. Bhattachan et al. found a weak population genetic structure in Ciona spp. and less genetic differentiation in Styela clava populations. An occasional gene flow between native and invasive populations of Ascidians might have occurred previously, most likely via ship transport. Bhattachan et al. clearly show that the Ciona robusta and Styela clava invasive potential is attributed to the neutral genetic diversity, whereas the invasive potential of Ciona savignyi might not be due to neutral evolution, but rather by population expansion and positive selection. Previous work indicated that a neutral force plays a role in the biological invasion and subsequent structuring of a population, but equally natural selection within biological invasion was also well characterised. It is worth noting that our analysis was based on the small sample size, because of the fewer collection sites. Increase of collection sites and sample sizes could be more accurate for the population genetic evaluation, but would not change the conclusion. Bhattachan et al.'s study reveals a global relationship between native and invasive populations and has implications in understanding the invasive potential of these three species. Thus, their work provides approaches useful for risk evaluation and management of invasive species.

See also...

Follow Sciency Thoughts on Facebook.

Asteroid (441987) 2010 NY65 passes the Earth.

Asteroid (441987) 2010 NY65 passed by the Earth at a distance of about 3 758 000 km (9.79 times the average distance between the Earth and the Moon, or 2.51% of the distance between the Earth and the Sun), slightly before 6.45 am GMT on Wednesday 24 June 2020. There was no danger of the asteroid hitting us, though were it to do so it would have presented a considerable threat. (441987) 2010 NY65 has an estimated equivalent diameter of 99-310 m (i.e. it is estimated that a spherical object with the same volume would be 99-310 m in diameter), and an object of this size would be predicted to be capable of passing through the Earth's atmosphere relatively intact, impacting the ground directly with an explosion that would be 200-7500 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater roughly 1-4.8 km in diameter and devastation on a global scale, as well as climatic effects that would last years or even decades.

Timelapse made from 411 images of (441987) 2010 NY65 made on 1 July 2016. Asteroid Tracker/Las Cumbres Observatory Network.

(441987) 2010 NY65 was discovered on 14 July 2014 by the  Wide-field Infrared Survey Explorer satellite.  The designation 2010 NY65 implies that the asteroid was the 1584th object (object Y65 - in numbering asteroids the letters A-Z, excluding I, are assigned numbers from 1 to 24, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., which means that Y65 = (24 x 65) + 24 = 1584) discovered in the first half of Ju;y 2010 (period 2010 N), while the designation 441987 implies that it was 441 987th asteroid ever discovered (asteroids are not given this longer designation immediately to avoid naming double or false sightings).

The calculated orbit of (441987) 2010 NY65. Asteroid Radar Research/JPL/NASA.

(441987) 2010 NY65 has a 366 day (1 year) orbital period, with an elliptical orbit tilted at an angle of 11.6° to the plain of the Solar System which takes in to 0.63 AU from the Sun (63% of the distance at which the Earth orbits the Sun, and slightly inside the orbit of the planet Venus) and out to 1.37 AU (137% of the distance at which the Earth orbits the Sun). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in June 2019 and the next predicted in June 2021. It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer). As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth,(441987) 2010 NY65 is also classified as a Potentially Hazardous Asteroid. (441987) 2010 NY65 also has occasional close encounters with the planets Venus, which it last came close to in May this year (2020) and is next predicted to pass in May 2053.

See also...

Follow Sciency Thoughts on Facebook.

Ophiambix kagutsuchi & Ophiambix macrodonta: Two new species of Brittle Stars from Japanese hot vents and cold seeps.

Brittle Stars, Ophiuroidea, contains the largest number of species within the Echinodermata and occurs in a great variety of marine habitats, such as muddy substrates, living infaunally in sediments, under rocks, in the interstices of Sponges and Hard Corals, and on surfaces of various Animals such as Soft Corals. They are globally one of the dominant deep-sea megafaunal groups, but diversity in chemosynthetic ecosystems such as hydrothermal vents and hydrocarbon seeps remains poorly understood. Since 1985, 10 species of Ophiuroids, including Ophiacantha longispina, Ophiactis tyleri, Ophienigma spinilimbatum, Ophiocten centobi, Ophioctenella acies, Ophiolamina epraewas, Ophiomitra spinea, Ophioplinthaca chelys, Ophiotreta valenciennesi rufescens, and Spinophiura jolliveti, have been collected from chemosynthetic ecosystems. As our knowledge grows, we have gained better understanding of Ophiuroids within these settings, and it appears Ophiuroid diversity has likely been underestimated. In Japanese waters, surveys of Ophiuroids have recorded 346 species, which represent approximately three-quarters of the North Pacific Ocean Ophiuroid fauna. However, no Ophiuroid species have been recorded from chemosynthetic habitats around Japan to date.

In a paper published in the journal Raffles Bulletin of Zoology on 17 April 2020, Masanori Okanishi of the Misaki Marine Biological Station of the University of Tokyo, Moe Kato of the School of Natural System at Kanazawa University, Hiromi Kayama Watanabe and Chong Chen of the Japan Agency for Marine-Earth Science and Technology, and Toshihiko Fujita of the National Museum of Nature and Science, describe two new species of Ophiuroids from chemosynthetic ecosystems in Japanese waters.

Both new species are placed in the enigmatic Ophiuroid genus Ophiambix, which was originally erected by Edward Lyman in 1880 for the monotypic Ophiambix aculeatus, and is known from the deep waters (146–5,315 m) of the Pacific and Atlantic Oceans. Out of the four species of Ophiambix currently considered to be valid, only Ophiambix aculeatus has been recorded from Japanese waters. Ophiambix has a characteristically flat body and their arms are well differentiated from the disc in a manner superficially similar to that of Asteroids.

The first new species is named Ophiambix kagutsuchi, in reference to Kagutsuchi, the god of fire in ancient Japanese mythology, referring to the hot-vent habit of the new species. The species was found at a series of hydrothermal vents in the Okinawa Trough, as well as on sunken wood found in the Ryukyu Trench, southwest of Japan, at a depth range of 276–1,979 m.

Ophiambix kagutsuchi, holotype (NSMT E-13071). (A) Aboral body; (B) aboral disc and proximal portion of arm; (C) oral disc and proximal portion of arms, arrow heads indicate oral papillae; (D) proximal portion of aboral surface of arm; (E) proximal portion of oral surface of arm; (F) distal portion of aboral surface of arm; (G) distal portion of oral surface of arm. Abbreviations: ASh, adoral shield; ASp, arm spine; D, dorsal arm plate; GS, genital slit; L, lateral arm plate; OP, oral plate; OS, oral shield; RS, radial shield; SD, supplementary dorsal plate; Te, tentacle scale; V, ventral arm plate. Scale bars 1 mm. Okanishi et al. (2020).

The disk of Ophiambix kagutsuchi is pentagonal, and about 4 mm in diameter. The aboral surface covered by small granules of almost uniform size, separated from each other, approximately 50–70 μm in diameter. Removal of granules shows underlying plates are scalar, circular in outline, and imbricating, each approximately 250–350 μm in diameter. Radial shields are triangular, distally wider, 350 μm in width, and 350 μm in length, sharpen towards the centre, and completely concealed by the granules. On the oral surface, the adoral shields are parallelogram-shaped, wider than long, approximately 250 μm in width, and 120 μm in length, one overlapping the other. The oral plates are trapezoidal, approximately 250 μm in width, and 180 μm in length at the radial edge, 250 μm in length at the abradial edge, and in contact with each other. The oral shields are pentagonal, slightly rounded, with a convex distal edge, approximately 360 μm in width, and 60–260 μm in length. The interradial oral disc area is narrow, covered by scales under thick skin, and approximately 200–300 μm in length. The genital slits are narrow, almost extending from the edge of the oral shield to two-thirds the height of the oral interradial disc, and 0.1 mm in width. The oral papillae and teeth are rudimentary, very thin, narrow and flat ossicles, approximately 10 μm in length, forming a continuous horizontal row on oral edge of dental and oral plates. The teeth and oral papillae quite similar in shape but for descriptive purposes, the ossicles on top of the dental plate are called teeth and ossicles on oral edge of oral plate are called oral papillae. With the exception of the oral-most row of papillae, there are 6 to 7 thin and fan-shaped teeth forming vertical row on the dental plate. There is a second tentacle pore inside the mouth slit.

Ophiambix kagutsuchi, paratype (NSMT E-13070), scaning electron microscope images of ossicles. (A)–(D) Vertebrae from proximal portion of arm, distal view (A), proximal view (B), oral view (C), aboral view, green illustration indicates 'T'-shaped groove (D); (E)–(G) lateral arm plates from proximal portion of arms, oral view (E), adoral view (F), inner view (G) an arrow head indicates a perforation; (H) ventral arm plates from proximal portion of arm, inner view; (I), (J) arm spines from distal (I) and proximal (J) portion of arm, arrow heads indicate secondary teeth. Orientations: ab, aboral side; ba, basal side; dis, distal side; ex, external side; in, inner side; o, oral side; pro, proximal side. Abbreviations: AF, aboral muscle fossae; DL, dorsal lobe; ES, elongated structure; LAC, lateral ambulacral canal; MO, muscle opening; NO, nerve opening; OF, oral muscle fossae; VL, ventral lobe. Okanishi et al. (2020).

On the proximally arm, theaboral surface is covered by small granules the same as those on the aboral disc, these decrease in number and disappearing on arm tip. After removal of the granules, the exposed dorsal arm plates are oblong, longer than wide, the proximal edge is slightly wider than the distal edge. There is a pair of fan-shaped supplementary dorsal arm plates on both lateral sides of each dorsal arm plate and irregularly shaped smaller supplementary plates on the distal side of the dorsal arm plates separating them from each other. On the middle portion of the arm, these supplementary plates disappear and the dorsal arm plates are in contact, gradually decreasing in size toward the arm tip. The lateral arm plates are thin, and widely separated from each other throughout the arm. The tentacle pore forms a large hole. On the proximal portion of the arm, the ventral arm plates are almost square with a concave distal edge, and toward the arm tip, become oblong, longer than wide, with a distal concave edge, contiguous throughout the arms. There are three flat and pointed arm spines on the proximal portion of each arm, the middle one is longest, the same length as the corresponding arm segment, the aboral-most half length of the middle spine, and the oral-most spine is shortest, approximately one-fifth length of the middle spine. The arm spines decrease in number to one toward the arm tip, transforming into a hook-shape, approximately the same length as the corresponding arm segment. There is one small, rudimentary tentacle scale at each tentacle pore, although small, tentacle scales present on the distal portion of each arm.

Ophiambix kagutsuchi, paratype (NSMT E-13050), scanning electron microscope images of ossicles. (A), (B) oral plates of abradial side (A) and adradial side (B), partly cracking, arrows indicate neural grooves; (C), (D) adradial genital plates, aboral view (C) and oral view, partly cracking (D); (E) inner view of a radial shield; (F) external view of a dental plate. Orientations: ab, aboral side; cen, centre of disc; o, oral side; per, periphery of disc. Abbreviations: AbMA, abradial muscle attachment area; AdMA, adradial muscle attachment area; AG, articulation for genital plate; DAT, depression for aboral tentacle; DOT, depression for oral tentacle; DW, presumable depression for water ring canal. Scale bars 100 μm. Okanishi et al. (2020).

The second new species is named Ophiambix macrodonta, which is a Latin adjective which means to have large teeth, referring to the flat, wide teeth of the species. The species is known only from a hydrocarbon seep site on Kuroshima Knoll, southeast of the Yaeyama Islands, part of the Ryukyu Island chain, southwest of Japan, at a depth range of 638–644 m.

Ophiambix macrodonta, holotype (NSMT E-13059). (A) Aboral body; (B) oral body; (C) aboral disc and proximal portion of arm; (D) aboral periphery of disc and proximal portion of arm; (E) oral disc and proximal portion of arms; (F) oral periphery of disc and proximal portion of arm; (G) a jaw; (H) top of a jaw, arrow heads indicate teeth and oral papillae. Abbreviations: ASh, adoral shield; DP, dental plate; GS, genital slit; L, lateral arm plate; OP, oral plate; OS, oral shield; RS, radial shield; Te, tentacle scale; V, ventral arm plate. Scale bars 1 mm. Okanishi et al. (2020).

The disc of Ophiambix macrodonta is pentagonal, and 6 mm in diameter. The aboral surface is covered by polygonal scales, approximately 250–350 μm in diameter, arranged in a mosaic pattern. Each scale is covered by small granules of uniform size, approximately 40–60 μm in diameter, in contact with each other and forming two or three circular rows on the periphery of each scale. The radial shields are oval, 850 μm in length and 500 μm in width, and almost completely concealed by granules except on the margins. The adoral shields are curved, bar-like, wider than long, approximately 500 μm in length and 150 μm in width, and separated from each other. The visible part of the jaws is trapezoid, approximately 500 μm in length and 250 μm in width, and contiguous. The oral shields are elliptical in shape, longer than wide, slightly acute on both adradial edges, and approximately 300 μm in length and 650 μm in width. The interradial oral disc area is narrow, covered by polygonal and mosaic scales, approximately 200–300 μm in length as those on aboral disc. The genital slits are long and wide, almost extending to the disc edge from distal edge of the oral shield, and 0.3 mm in width. The oral papillae and teeth are rudimentary, very thin, narrow and flat ossicles, approximately 10 μm in width, forming a horizontal row on the oral edge of the dental plate and oral plate. The second tentacle pore is inside the mouth slit.

Ophiambix macrodonta, holotype (NSMT E-13059) (A)–(E), (G), (H) and a paratype (NSMT E-13060) (F), scanning electron microscope images. (A) Aboral disc and proximal portion of arm; (B) aboral periphery of disc and proximal portion of arm; (C) granules (arrow heads) on aboral disc; (D) oral disc and proximal portion of arms; (E) jaws, an arrow head indicates teeth or oral papillae; (F) a jaw from lateral view, arrow heads indicate teeth and oral papillae; (G) proximal portion of aboral surface of arm, arrow heads indicate supplementary plates; (H) proximal portion of oral surface of arm. Abbreviations: ASh, adoral shield; ASp, arm spine; D, dorsal arm plate; GS, genital slit; RS, radial shield; V, ventral arm plate. Okanishi et al. (2020).

The hollowtype of Ophiambix macrodonta has two complete arms 25 mm and 26 mm, while the other three arms have lost their tips. The proximal portion of the arms is 1.5 mm wide and 1 mm high, and eliptical in cross section. The arms taper gradually towards the tip. The aboral surface is covered by dorsal arm plates and supplementary dorsal arm plates, with no granules. On the proximal portion of the arm, the dorsal arm plates are semicircular, and wider than long; the proximal edge is straight. There is one large, fan-shaped supplementary dorsal arm plate on both lateral sides of the dorsal arm plate. Three smaller, polygonal plates are at the distal edges of the dorsal arm plates and the two larger supplementary dorsal plates. Toward the arm tip, these supplementary plates decrease in size and gradually disappear with the dorsal arm plates becoming contiguous. The lateral arm plates are thin, and widely separated throughout the arm. On the proximal portion of the arm, the ventral arm plates are almost square, with slightly concave lateral edges and oblong. On the distal portion of the arm, the ventral arm plates are longer than wide, concave, and the lateral edges more pronounced. The ventral arm plates are contiguous throughout the arms. There are three flat arm spines on the proximal portion of arms, the aboral-most and middlevspines are flat and leaf-like, and the oral-most spine is cylindrical, narrow, and pointed. All three spines are equal in length to the corresponding arm segment. Toward the tip, the aboral-most and middle arm spines transform to a cylindrical and pointed shape, and the oral-most one transforms into a hook, approximately the same length as the corresponding arm segment. There is one small, triangular tentacle scale at each tentacle pore, although small, tentacle scales are present on distal portion of the arm.

Ophiambix macrodonta, holotype (NSMT E-13059). (A) Proximal portion of aboral surface of arm, a part enlarged in (B), arrow heads indicate supplementary plates; (C) distal portion of aboral surface of arm; (D) proximal portion of oral surface of arm; (E) distal portion of oral surface of arm; (F) distal portion of lateral surface of arm. Abbreviations: ASp, arm spine; D, dorsal arm plate; L, lateral arm plate; Te, tentacle scale; V, ventral arm plate. Scale bars 1 mm. Okanishi et al. (2020).

The difference in body size of the two new species Ophiambix macrodonta (3.3 to 7.0 mm in disc diameter) and Ophiambix kagutsuchi (0.8 to 3.2 mm in disc diameter) suggests that these could be interpreted as different sizes of the same species. However, the smallest specimen of Ophiambix macrodonta (3.3 mm in disc diameter), and the largest specimen of Ophiambix kagutsuchi (3.2 mm in disc diameter), were similar in size but exhibited the full set of respective diagnostic characters. All examined specimens of Ophiambix kagutsuchi, were collected from hydrothermal vents or sunken wood environments, whereas those of Ophiambix macrodonta, new species, were collected only from hydrocarbon seeps. Therefore, Okanishi et al. consider that these two taxa are indeed separate new species that can be distinguished from each other by morphological characters and by different environmental preferences.

See also...

Follow Sciency Thoughts on Facebook.