Antipathozoanthus tubus, Kise & Obuchi & Reimer, 2021

Kise, Hiroki, Obuchi, Masami & Reimer, James Davis, 2021, A new Antipathozoanthus species (Cnidaria, Hexacorallia, Zoantharia) from the northwest Pacific Ocean, ZooKeys 1040, pp. 49-64 : 49

Antipathozoanthus tubus sp. nov. Figures 1 , 2 , 3

Material examined.

Holotype. NSMT-Co 1742, collected from Iwa Beach , Sagami Bay, Kanagawa, Japan (35°09'36"N, 139°08'36"E) at a depth of 14 m by M. Obuchi, 26 July 2019, divided in two pieces, one portion fixed in 99.5% EtOH and the other in 5-10% saltwater formalin . Paratype. NSMT-Co 1743, collected from Kotogahama , Sagami Bay, Kanagawa, Japan (35°08'48"N, 139°09'05"E) at a depth of 14 m by M. Obuchi, 6 June 2020, divided in two pieces, one portion fixed in 99.5% EtOH and the other in 70% EtOH .

Material examined for comparison.

Antipathozoanthus obscurus NSMT-Co1602 (holotype), collected from Cape Bise , Motobu, Okinawa-jima Island, Japan, by J.D. Reimer, 14 August 2014 . Antipathozoanthus remengesaui NSMT-Co1603 (holotype), collected from Blue Hole , Palau, by J.D. Reimer, 15 September 2014 . Antipathozoanthus cavernus NSMT-Co1604 (holotype), collected from Sakurajima , Kagoshima, Japan, by J.D. Reimer, 20 September 2015 . Antipathozoanthus hickmani CMNH-ZG-05883 (paratype), collected from Roca Onan , Pinzon Island, Galapagos, Ecuador, by A. Chiriboga, 14 March 2007 .

Type locality.

Iwa Beach, Sagami Bay, Kanagawa, Japan

Description.

External morphology. Colonial zoantharian, with cylindrical polyps connected by well-developed dark red colored coenenchyme (Fig. 1a ). External branched tube of dead polychaete mostly covered by coenenchyme. Scapus of column dark red in situ, dark brown in preserved specimens. Capitulum of column orange in situ, dark violet in preserved specimens. Column and coenenchyme heavily encrusted with visible sand and silica particles in ectodermal tissue to outer mesoglea (Fig. 1c, d ). Preserved, contracted polyps 2.0-6.0 mm in height, 1.0-3.0 mm in diameter. In situ, opened polyps approximately <8.0 mm in height, <10 mm in diameter. Oral disk 5.0-8.0 mm in diameter, orange to light orange in coloration. Number of oral furrows the same as the number of tentacles, and cream white circular protrusion in central oral disk bears slit-like mouth aligned with directives. Tentacles arranged in two rows (15-17 inner endocoelic tentacles and 15-17 outer exocoelic tentacles), as long as the expanded oral disk diameter. Number of tentacles 30-34, transparent in coloration. 15-17 marginal teeth present under inner endocoelic tentacles (Fig. 1b ). Tips of tentacles usually cream in coloration. Capitular ridges indiscernible.

Internal morphology. Azooxanthellete. Mesentery number 30-34, complete 15-17, incomplete 15-17. Mesenteries in macrocnemic arrangement (Fig. 2c ). Mesoglea thickness 0.01-0.10 mm, and thicker than ectoderm. Developed siphonoglyph distinct and U-shaped. Mesenterial filaments present (Fig. 2a ). Endodermal marginal muscle, short comb-like mesogleal pleats supporting the entire length of the marginal muscle (cteniform endodermal marginal muscle: Fig. 2b ). Basal canals of mesenteries absent (Fig. 2d ). Additionally, possible gametes observed in several longitudinal sections.

Cnidae. Basitrichs and microbasic b -mastigophores, microbasic p -mastigophores, holotrichs, and spirocysts (Fig. 3 , Table 2 ).

Habitat and distribution.

Northwestern Pacific Ocean: Sagami Bay, Kanagawa, Japan at depths <14 m.

Associated host.

We could not identify host polychaete species as there were no polychaetes in the tubes. However, the tubes that Antipathozoanthus tubus sp. nov. was attached to may belong to species within the genus Eunice , as polychaete species that build parchment-like branched tubes have been reported from this genus (e.g., Díaz-Díaz et al. 2020).

Molecular phylogeny.

Both ML and BI phylogenetic analyses showed similar topologies as indicated in Fig. 4 . The genus Antipathozoanthus appeared as a monophyletic clade located within the family Parazoanthidae with strong nodal support (ML=100%, BI=1) and was close to a Parazoanthidae clade containing species associated with stalked hexactinellid sponges. Within Antipathozoanthus , two subclades were formed; one subclade consisted of the antipatharian-associated species A. macaronesicus , A. hickmani , A. remengesaui , and A. cavernus (ML = 100%, BI = 0.97), and the other subclade consisted of A. tubus sp. nov. and A. obscurus (ML = 82%, BI = 0.92). Genetic distances in COI, 16S-rDNA, and ITS-rDNA sequences between A. tubus sp. nov. and other Antipathozoanthus species were 0.000 to 0.009, 0.002 to 0.010, and 0.010 to 0.128, respectively. As well, A. tubus sp. nov. and other Antipathozoanthus species shared unique insertion/deletion patterns in 16S-rDNA sequences.

Remarks.

Antipathozoanthus tubus sp. nov. can be easily distinguished from A. remengesaui Kise et al., 2017, A. macaronesicus (Ocaña & Brito, 2003), and A. hickmani Reimer & Fujii, 2010 by the number of tentacles as well as different coloration; Antipathozoanthus remengesaui , A. macaronesicus , and A. hickmani have up to 42 tentacles ( Ocaña and Brito 2003; Reimer and Fujii 2010; Kise et al. 2017), while A. tubus sp. nov. has fewer tentacles (30-34). The dark red colored polyps and coenenchyme of A. tubus sp. nov. are not found in these other three Antipathozoanthus species. In addition, A. tubus sp. nov. differs from A. cavernus Kise et al., 2017 with regards to polyp coloration ( A. cavernus has orange or light orange polyps: Kise et al. 2017). Although A. tubus sp. nov. and A. obscurus Kise et al., 2017 are phylogenetically close, their COI, 16S-rDNA, and ITS-rDNA sequences are all unique (genetic distances in COI, 16S-rDNA and ITS-rDNA sequences between A. tubus sp. nov. and A. obscurus were 0.009, 0.03, and 0.12, respectively). As well, these two species can be separated by coloniality; polyps of A. obscurus are connected by a stolon forming a mesh network ( Kise et al. 2017), while polyps of A. tubus sp. nov. are connected by a well-developed coenenchyme. Furthermore, A. macaronesicus , A. remengesaui , A. cavernus , A. hickmani , and A. obscurus have holotrichs in their column ( Ocaña and Brito 2003; Reimer and Fujii 2010; Kise et al. 2017), while holotrichs were not observed in the column of A. tubus sp. nov.

Antipathozoanthus is a circumglobally distributed genus, as species have reported from the Indian, Pacific, and Atlantic Oceans ( Ocaña and Brito 2003; Sinniger et al. 2010; Reimer and Fujii 2010; Bo et al. 2012; Reimer et al. 2014; Kise et al. 2017), with members living from shallow waters ( A. obscurus at 3 m depth; Kise et al. 2017) to mesophotic depths (153-169 m for Antipathozoanthus sp. sensu Reimer et al. 2019). The most distinctive attributes of A. tubus sp. nov. are its substrate and habitat. Antipathozoanthus macaronesicus , A. hickmani , A. remengesaui , and A. cavernus are found on antipatharians within the families Antipathidae and Myriopathidae ( Ocaña and Brito 2003; Reimer and Fujii 2010; Sinniger et al. 2010; Bo et al. 2012; Kise et al. 2017), while A. obscurus is directly attached to coral reef carbonate ( Kise et al. 2017). On the other hand, A. tubus sp. nov. is the only species of the genus found to date on tubes of polychaetes. Four Antipathozoanthus species are known from low light environments; A. macaronesicus , A. remengesaui , and A. cavernus have been found in cave entrances, and A. obscurus is found in crevasses and/or coral reef caves ( Ocaña and Brito 2003; Kise et al. 2017). On the other hand, the habitat of A. tubus sp. nov. is not a low-light environment, but the specimens were instead found on a polychaete tube attached to exposed rock.

Within Parazoanthidae , until now, Isozoanthus altisulcatus Carlgren, 1939 is the only species described as living on the tubes of polychaetes. However, several morphological differences exist between A. tubus sp. nov. and I. altisulcatus . Capitular ridges are developed and conspicuous in I. altisulcatus , whereas they are indiscernible in A. tubus sp. nov. The marginal teeth on the capitulum found in A. tubus sp. nov. were not observed in I. altisulcatus . Although Carlgren (1939) did not describe the numbers of tentacles of I. altisulcatus , the numbers of mesenteries are 34-42 ( Carlgren 1939). As numbers of tentacles are known to be equal to the number of mesenteries ( Bourne 1900), the number of tentacles of I. altisulcatus is likely to be 34-42, which is greater than the number of tentacles of A. tubus sp. nov. (30-34).

Genetic distances of COI sequence between A. tubus sp. nov. and other Antipathozoanthus species can be considered as intra-generic differences based on previous comparisons of genetic distances ( Sinniger et al. 2010). As well, A. tubus sp. nov. shared unique insertion/deletion patterns in 16S-rDNA sequences with other Antipathozoanthus species. Thus, we consider that A. tubus sp. nov. should belong to the genus Antipathozoanthus and does not warrant the erection of a new parazoanthid genus.

Etymology.

Antipathozoanthus tubus sp. nov. is named from the Latin tuba, as this species is found on polychaete tubes. The Japanese name is ‘Iwa-tsuno-sunaginchaku’.

Figures

Figure 1. Images of external morphology of Antipathozoanthus tubus sp. nov. (holotype: NSMT-Co 1742) a colony on branched polychaete tubes in situ b close-up image of polyps in situ c colony on branched polychaete tubes in preserved condition d close-up image of closed polyp. Abbreviations: MT: marginal teeth, T: tube of polychaete. Scale bars: 10 mm (a, c), 5.0 mm (b), 0.5 mm (d).

Figure 2. Image of internal morphology of Antipathozoanthus tubus sp. nov. (holotype: NSMT-Co 1742) a longitudinal section of polyp b closed-up image of cteniform endodermal marginal muscle c, d cross-section of polyp. Abbreviations: A: actinopharynx, MF: mesenterial filament, CEMM: cteniform endodermal marginal muscle, DD: dorsal directives, VD: ventral directives, S: siphonoglyph, 5 th: 5 th mesentery from dorsal directives, M: mesoglea, CM: complete mesentery, IM: incomplete mesentery. Scale bars: 0.5 mm (a, c), 0.1 mm (b, d).

Figure 3. Cnidae in the tentacles, column, actinopharynx, and mesenterial filaments of holotype of Antipathozoanthus tubus sp. nov. Abbreviations: HL: holotrich large, HM: holotrich medium, B: basitrichs, BM: microbasic b - mastigophores, SBM: special microbasic b - mastigophores, PM: microbasic p - mastigophores, S: spriocysts.

Figure 4. Maximum likelihood tree based on combined dataset of COI, mt 12 S-rDNA, mt 16 S-rDNA, 18 S-rDNA, ITS-rDNA, and 28 S-rDNA sequences. Number at nodes represent ML bootstrap values (> 50 % are shown). White circles on nodes indicate high support of Bayesian posterior probabilities (> 0.95).