Sphaerodoridium songkhlaensis, Plathong & Plathong & Capa, 2020

Plathong, Jintana, Plathong, Sakanan & Capa, María, 2020, Two new species of Sphaerodoridae (Annelida) from the Gulf of Thailand, Zootaxa 4790 (1), pp. 57-75 : 64-73

Sphaerodoridium songkhlaensis sp. nov.

Figs 1B , 5 E–H, 6–10

Material examined. 48 specimens from Songkhla Sea , Gulf of Thailand ( Table 1). All specimens complete, from muddy mixed with sand and shells; coll. Marine Ecosearch Management Company (MEM). Holotype: PSUZC- POL-0010 (1 spec.), S09-6 (7°32’13” N, 100°42’21” E), 17 Feb 2015, 23.6 m . Paratypes: PSUZC-POL-0011 (1 spec. on SEM stub), S09-3 (7°32’01” N, 100°42’42” E), 18 Mar 2013, 24.0 m; PSUZC-POL-0012 (1 spec. on SEM stub), S08 (7°29’10” E, 100°47’06” E), 16 Oct 2013, 25.0 m; PSUZC-POL-0013 (1 spec. on SEM stub), S08 (7°29’10” E, 100°47’06” E), 28 Jan 2015, 25.0 m; PSUZC-POL-0014 (1 spec. on SEM stub), S08 (7°29’10” E, 100°47’06” E), 18 May 2016, 25.0 m; PSUZC-POL-0015 (1 spec. on SEM stub), S09-20 (7°31’44” N, 100°42’13” E), 23 Mar 2017, 24.0 m; PSUZC-POL-0016 (1 spec.), S09-8 (7°32’22” N, 100°42’30” E), 17 Mar 2013, 23.9 m; PSUZC-POL-0017 (1 spec.), S09-12 (7°31’56” N, 100°42’24” E), 17 Mar 2013, 23.8 m; PSUZC-POL-0018 (1 spec.), S09-5 (7°32’01” N, 100°42’30” E), 2 Mar 2016, 24.0 m; PSUZC-POL-0019 (1 spec. on SEM stub), S09-1 (7°32’13” N, 100°42’42” E), 17 Aug 2018, 24.0 m; PSUZC-POL-0063 (1 spec.), S09-16 (7°32’30” N, 100°42’59” E), 18 Feb 2015, 24.0 m; PSUZC-POL-0064 (3 specs, 2 on SEM stub), S09-5 (7°32’01” N, 100°42’30” E), 23 Aug 2019, 24.0 m; PSUZC-POL-0065 (1 spec.), S09-18 (7°31’44” N, 100°42’59” E), 1 Mar 2011, 24.0 m; PSUZC- POL-0066 (1 spec.), S09-7 (7°32’19” N, 100°42’24” E), 7 Mar 2011, 23.7 m; PSUZC-POL-0067 (1 spec.), S38 (7°37’29” N, 100°42’53” E), 27 Sep 2011, 25.0 m; PSUZC-POL-0068 (1 spec.), S21 (7°33’17” N, 100°46’43” E), 9 Feb 2012, 24.0 m; PSUZC-POL-0069 (2 specs), S08 (7°29’10” N, 100°47’06” E), 21 Feb 2013, 25.0 m; PSUZC- POL-0070 (1 spec.), S02 (7°31’44” N, 100°28’15” E), 5 Jun 2014, 10.0 m; PSUZC-POL-0071 (1 spec.), S09-1 (7°32’13” N, 100°42’42” E), 24 Mar 2017, 24.0 m; PSUZC-POL-0072 (1 spec.), S09-4 (7°31’60” N, 100°42’39” E), 24 Mar 2017, 24.0 m; AM W.52921 (1 spec.), S08 (7°29’10” N, 100°47’06” E), 23 Mar 2013 , 25.0 m; AM W.52922 (1 spec.), S09-22 (7°32’13” N, 100°42’30” E), 16 Aug 2018 , 24.0 m; MNCN 16.01 /18538 (1 spec.) S09-2 (7°32’04” N, 100°42’43” E), 24 Mar 2017 , 24.0 m; MNCN 16.01 /18539 (1 spec.), S09-3 (7°32’01” N, 100°42’42” E), 16 Aug 2018 , 24.0 m.

Additional material examined: Sphaerodoridium andamanense Bakken et al., 2002 (Holotype, PMBC 18540 ) .

Type locality. Songkhla Sea, Gulf of Thailand ( Fig. 1B ) in Block G 5/43 of petroleum concession area .

Diagnosis. Body short and ellipsoid, less than 2.0 mm long. Head appendages smooth and digitiform, without basal spurs. Dorsal macrotubercles with short stalk, and terminal papillae as inverted cones with a crenulated rim around distal almost circular surface; arranged in a single transverse row per segment with up to 10 macrotubercles. Dorsum without additional papillae. Ventrum with about eight large spherical papillae per segment, lateral larger, located near parapodial base, leaving a mid-ventral longitudinal gap; arranged in oblique rows. Parapodia with one or two spherical papillae on ventral and anterior surfaces. Acicular lobe from first segment. About 5–6 compound chaetae with medium blades (4–5 times as long as wide), showing intra-fascicle variation in serration (dorsal and ventral most chaetae with serration along complete edge; mid-chaetae with distal end without serration).

Description. Holotype with ellipsoid body, flat ventrum and convex dorsum; segmentation inconspicuous, 1.7 mm long, 0.47 mm wide with 18 chaetigers, female. ( Figs 6 A–B, 7A–C).

Prostomium with five appendages, including a pair of palps, a pair of lateral antennae and a median antenna.All appendages smooth, digitiform, about three or four times as long as wide; without basal spurs. Antenniform papillae absent. A pair of tentacular cirri shorter than lateral antennae. Four additional spherical papillae in head region ( Figs 8 A–B, 10A). A pair of rounded and sub-epidermal eyes present, black after preservation ( Fig. 6B ).

Chaetiger 1, without macrotubercles, with four small papillae ( Fig. 9A ). Chaetiger 2 with six dorsal macrotubercles arranged in a single transverse row (six or seven in paratypes) ( Fig. 9A ). Chaetigers from the third with nine dorsal macrotubercles ( Fig. 9 A–C), up to 10 in mid-body chaetigers and decreasing in number in posterior chaetigers ( Figs 5E , 7 A–B, 9C). Macrotubercles as inverted cones with a crenulated rim and a flattened or convex distal end; with a short stalk ( Fig. 9D ). All macrotubercles attached to anterior edge of segments; all similar in size except for chaetiger 1 and posterior segments where slightly smaller. Additional dorsal epithelial tubercles absent ( Figs 5E , 7 A–B, 9A–C). Ventrum with two longitudinal bands of sessile tubercles, as inverted cones, along the body, next to base of parapodia ( Figs 5F , 7C , 9 E–F), with four pairs of tubercles per segment forming oblique rows closing posteriorly ( Figs 5F , 9 E–F). Parapodia short and cylindrical, as long as wide, wrinkled. Acicular lobe from chaetiger 1, longer than parapodial papillae and projecting distally. Ventral cirri conical larger than acicular lobe but projecting as much in mid-body and posterior segments. First three chaetigers with parapodia provided with one or two spherical papillae: one on anterio-ventral surface, one on ventral surface ( Fig. 5G ); following chaetigers through with up to three papillae: one on anterior surface, two on ventral position, near the base of parapodia ( Fig. 5H ). Posterior one or two chaetigers lacking some papillae ( Figs 5H , 10A ). Each parapodium with 5–6 compound chaetae, arranged in a curved transverse row around acicular lobe ( Figs 5 G–H, 10A–C, E). Chaetae with blades 4–6 times longer than maximum width. Blades of each chaetiger with serration varying in length along cutting edge: one or two dorsal most chaetae with edge completely serrated; rest of chaetae with edge partially smooth ( Fig. 10 F–G). Pygidium terminal, with one mid-ventral digitiform anal cirrus projecting beyond last parapodia, flanked by two anal cirri similar in shape to dorsal macrotubercles ( Fig. 7C ).

Pharynx extending over two chaetigers. Females with “sexual structures”, as porous hemispherical papillae on base of parapodia of chaetiger 8 ( Fig. 10 C–D). Male copulatory organs present (?) on chaetiger 12 in paratype ( Fig. 10E ). Females with oblong eggs under translucent epithelium about 100–217 µm in length ( Fig. 6B ).

Variation. Unlike the holotype, which is a relaxed and turgent individual, most specimens have contracted heads and therefore it is difficult to assess arrangement and relative length of head appendages. All examined specimens lack macrotubercles in chaetiger 1, and four head papillae are visible only in some individuals (due to contraction of anterior end). Number of dorsal macrotubercles in chaetiger 2 ranges from six to seven. Chaetiger 3 with nine dorsal tubercles and following with 9–10. Dorsal macrotubercles are similar in all the material examined, regardless station or year collected. The stalk is short and even some tubercles seem sessile; distal tubercle has an inverted cone shape with a distal epithelial rim encircling an almost flat surface. These features are visible under optic and also electronic microscopy. Ventral tubercles are smaller in size but resemble in shape (cone shape), the rim is inconspicuous.

Etymology. The species name refers to the Songkhla Sea, the type locality.

Distribution: Only known from type locality - Songkhla Sea, Gulf of Thailand ( Fig. 1B ).

Habitat. Found in muddy mixed with sand and shells.

Remarks: The definition and circumscription of the genus Sphaerodoridium has changed recently, after phylogenetic analyses of DNA sequences ( Capa et al. 2019). Members of the previously considered Sphaerodoropsis Group 2 ( Borowski, 1994), characterised by bearing sessile dorsal macrotubercles arranged in six or more longitudinal rows and only one transverse row per segment, were found nested in a clade among those previously considered Sphaerodoridium , with more seven longitudinal rows of dorsal stalked macrotubercles ( Capa et al. 2019). The current diagnostic feature for Sphaerodoridium deals with the number of longitudinal rows of macrotubercles (over six) rather than their morphology (including both species with sessile and stalked macrotubercles). The number of accepted species in the genus is now 22, and none of them has been reported in the Gulf of Thailand before.

Sphaerodoridium songkhlaensis sp. nov. is characterized by the singular shape of the dorsal macrotubercles (as an inverted cone, with a conspicuous rim encircling the flattened distal surface) and the type of chaetae (with different morphologies on the same fascicle). There are two species of Sphaerodoridium and one of Clavodorum (sensu Capa et al. 2019) with similar dorsal macrotubercles (stalked and resembling an inverted cone or bell). These are Sphaerodoridium campanulata Borowski, 1994 from deep-sea sediments (1463 m) in Peru Basin, Sphaerodoridium andamanense Bakken, 2002 , from the Andaman sea, Thailand (29 m), and Clavodorum antarcticum Hartmann-Schröder & Rosenfeldt, 1990 from the Antarctic Peninsula (262 m). Sphaerodoridium campanulata is distinguished from S. songkhlaensis sp. nov. in the number, size and arrangement of the dorsal macrotubercles, having two kinds of tubercles (some stalked and spherical and other bell-shaped), arranged in two transverse rows per segment (6–8 + 3–4), while S. songkhlaensis sp. nov. has similar shaped macrotubercles crowned by a conspicuous epithelial rim, arranged in a single transverse row.

Sphaerodoridium andamanense was described as bearing up to 10 dorsal macrotubercles per segment, like S. songkhlaensis sp. nov. Even though the original description did not give details about the shape of macrotubercles, the drawings indicate an inverted cone shape (corroborated by direct examination of holotype. Differences between S. andamanense and S. songkhlaensis sp. nov. rely in the length of the stalk of dorsal macrotubercles, almost twice as long as the distal tubercle in S. andamanense ( Bakken, 2002) and much shorter than the tubercle in S. songkhlaensis sp. nov.; in the conspicuous epithelial rim, absent in S. andamanense ; and the number and arrangement of ventral tubercles: S. andamanense bears three pairs of long stalked tubercles per segment ( Bakken 2002) and the new species bears four pairs of sessile tubercles.

Sphaerodoridium songkhlaensis sp. nov. differs from Sphaerodoridium sp. C (as Sphaerodoropsis in Bakken, 2002 ) in the size of the dorsal macrotubercles, similar in the new species, and showing conspicuous variation in Sphaerodoropsis sp. C. Moreover, the new species has a pair of eyes, absent in Sphaerodoropsis sp. C, bears 5–6 chaetae per fascicle, with blades variable within fascicles (4–6 times longer than maximum width) while Sphaerodoropsis sp. C bears 6–8 chaetae per fascicle, all with short blades (three times as long as wide). It would, however, be interesting to find more material of both morphotypes to establish differences between intraspecific and interspecific variation.

Clavodorum antarcticum resembles S. songkhlaensis sp. nov. in the shape of the dorsal macrotubercles, as inverted cones, however due to the number of longitudinal rows of macrotubercles they are considered members of different genera ( Clavodorum with six and Sphaerodoridium more than six).

Figures

FIGURE 1. Sampling sites in the Songkhla Sea, Gulf of Thailand (dots). A. Stations where specimens of Geminofilum thailandica sp. nov. where collected (squares) B. Stations were Sphaerodoridium songkhlaensis sp. nov. were collected (triangles).

FIGURE 5. Stylised drawings. A–D. Geminofilum thailandica sp. nov. A. Dorsal tubercles of chaetigers 6–9; B. Ventral tubercles of chaetigers 6–9; C. Parapodial apendages and papillae in anterior chaetiger; D. Parapodial appendages and papillae in mid-body aposterior chaetiger; E–H. Sphaerodoridium songkhlaensis sp. nov. E. Dorsal tubercles of chaetigers 6–9; F. Ventral tubercles of chaetigers 6–9; G. Parapodial apendages and papillae in anterior chaetiger H. Parapodial apendages and papillae in mid-body and posterior chaetiger. The numbering system refers to chaetiger number, the concentric circles represent the volume of the parapodium (larger ones indicate basal and smaller distal areas) and the red axis are the imaginary lines dividing four sides of the parapodium: anterior ANT, dorsal DOR, posterior POS, ventral VEN.

FIGURE 6. Photographs of Sphaerodoridium songkhlaensis sp. nov. (A. PSUZC-POL-0013, B. PSUZC-POL-0019). A. Complete specimen, lateral view; B. Same, dorsal view.

FIGURE 7. Scanning electron micrographs of Sphaerodoridium songkhlaensis sp. nov. (A, B PSUZC-POL-0011, C PSUZC- POL-0012). A. Complete specimen, lateral view; B. Same, dorsal view; C. Complete specimen, ventral view.

FIGURE 8. Scanning electron micrographs of Sphaerodoridium songkhlaensis sp. nov. (PSUZC-POL-0064). A. Prostomium, frontal view; B. Same, from specimen with invaginated front end.Abreviations: lateral antenna la, median antenna ma, palp pa, tentacular cirri tc, ventral cirrus vc.

FIGURE 9. Scanning electron micrographs of Sphaerodoridium songkhlaensis sp. nov. (PSUZC-POL-0011–13). A, B. Anterior end, front view; C. Anterior end, dorsal view; D. Detail of macrotubercles, anterior chaetigers; E. Ventral tubercles, midbody chaetigers; F. Mid-body chaetigers, lateral view. Abreviations: number of chaetigers 1–6.

FIGURE 10. Scanning electron micrographs of Sphaerodoridium songkhlaensis sp. nov. (A. PSUZC-POL-0064, B–F. PSUZC- POL-0012).A.Anterior end, lateral view; B. Parapodium chaetiger 6; C. Parapodia chaetiger 5–9 with female copulatory organs located between chaetiger 8 and 9 (encircle), ventral view; D. Detail of female copulatory organs; E. Parapodia chaetiger 9–12 with male copulatory organs (encircle), ventral view; F–G. Compound chaetae. Abreviation: acicular lobe al, ventral cirrus vc.