Amphitritides skeletonensis, Bick & Zettler, 2024

Bick, Andreas & Zettler, Michael L., 2024, Description of three new species of Amphitritides Augener, 1922 (Terebellida, Annelida) from the coast of Namibia (South West Africa), Zootaxa 5446 (1), pp. 42-64 : 55-61

Amphitritides skeletonensis sp. nov.

( Figs 8–10 ; 11K–O, S )

Material examined. Holotype. Off Namibia: 18.000°S 11.650°E, depth 125 m, 30.08.2019, complete specimen ( ZSRO-P2689 ) . Paratypes. Off Namibia : 18.000°S 11.650°E, depth 125 m, 30.08.2019, 1 specimen ( ZSRO-P2690 ; 22.499°S 13.533°E, depth 156 m, 18.09.2011, 1 specimen ( ZSRO-P2691 ) ; 20.2225°S 12.6288°E, depth 154 m, 24.08.2011, 2 specimens ( SMF 32973 ) ; 20.175°S 12.710°E, depth 133 m, 24.08.2011, 4 specimens ( ZSRO-P2692 ) , 1 specimen ( SMF 32974 ) ; 20.079°S 12.872°E, depth 101 m, 06.09.2011, 1 specimen ( ZSRO-P2693 ) . Additional material. 25.0000°S 13.9168°E, depth 187 m, 07.09.2019, 3 specimens ( ZSRO-P2694 ) .

Diagnosis. Two pairs of stalked arborescent branchiae; first pair distinctly longer than second pair, often longer than body width. Without distinct lateral lobes on anterior segments, but midventral lobe on segment 1 present. Thorax with 28–31 segments. One pair of nephridial papillae on segment 3, inserted laterally and close to branchial stalk. Three pairs of genital papillae on segments 6–8, occurring at the anterior base of the corresponding notopodia and aligned with them. Notochaetae distally serrated. Neuropodia from segment 5, double rows of uncini on thoracic and first 6–7 abdominal segments, beginning on segment 11. Pygidium with crenulated margin.

Description. Large species, complete holotype 60 mm long, 6 mm wide, for 71 segments. Paratypes between 8 mm and 75 mm long, and 1.5 mm and 12 mm wide. Maximum 60–75 segments, but most individuals are incomplete. Anterior body usually distinctly swollen ( Fig. 8A, D ).

Body surface of dorsal and ventral sides very different from each other, especially on thorax. Ventral side with glandular structure, well stained with methylene blue; well-developed ventral shields, i.e., clearly separated from the lateral sides by furrows, from segments 5 to segment 14–15 usually; last shields may be narrower and shorter; mid-ventral groove usually begins immediately after last ventral shield ( Fig. 8C, D ). Dorsal side papillose; papillae arranged in more or less distinct rows, up to about segments 8–12 in 2 rows, thereafter 3 and later 4 rows of papillae per segment, segments appearing secondarily annulated as a result ( Figs 8A, B, E ; 9A, B ).

Prostomium at base of upper lip; basal part without eyespots; distal part forming shelf-like tentacular membrane from which numerous filiform and deeply grooved, ciliated buccal tentacles originate ( Figs 8B, C ; 9A ).

Peristomium well developed, with conspicuous hood-like upper lip, sometimes trilobate, with corrugated anterior margin, directed anteriorly; lower lip narrow and swollen; pharyngeal organ often visible ( Figs 8C ; 9A ).

Segment 1 dorsally narrow, conspicuously developed ventrally, forming mid-ventral lobe below lower lip; anterior margin concave and corrugated; lobe as long as lower lip or shorter (Figs (8A, C, D; 9A, B). Segments 2 and 3 without any lobes ( Figs 8A ; 9A, B ).

Two pairs of large arborescent branchiae on segments 2 and 3; first pair distinctly longer than second pair, usually about 2–4 times as long as second pair, often even longer than body width; first pair about 5–10 mm (exceptionally 14–16 mm) long; second pair always shorter than body width, about 2–4 mm long; main stems short and thick, irregularly branching dichotomously, with short branchial filaments terminally ( Figs 8A–D ; 9A, B ).

Notopodia starting from segment 4 and extending usually for 25–27 segments, i.e. up to segment 28–30 (n= 15 specimens), rarely also for 28 (up to segment 31; n= 1 specimen) segments; first pair of notopodia smaller than following pairs; notopodia short, rectangular to conical; all notopodia laterally aligned; notopodia and neuropodia clearly separated ( Figs 8D ; 9A–C ). Notochaetae indistinctly arranged in two or more rows; chaetae of both rows with symmetrical limbation and serrated tips, base of serrated tip distinctly wider; chaetae of first row shorter, serrated tip forms a distinct angle to the shaft; chaetae of second row longer, without angle to the shaft; bulbous wings in the middle of longer chaetae poorly developed; notochaetae of anterior and posterior thoracic segments all similar ( Figs 9E ; 10A–G ; 11K–M, S ).

Neuropodia beginning from segment 5; thoracic and abdominal neuropodia as long lateral ridges, distinctly raised from body surface, almost reaching ventral shields on thorax, reaching ventral groove on abdomen ( Figs 8A, C–E ; 9A, B ). Uncini arranged in double rows, in face-to-face arrangement, from segment 11 until abdominal segment 6 (n= 4 specimens) or 7 (n= 6 specimens) usually, sometimes up to abdominal segment 4 (n= 1 specimen) or 5 (n= 1 specimen); rows completely separated from each other ( Figs 9F , 11N ). Avicular unicini; thoracic uncini 50–58 µm wide and 38–47 µm high, abdominal uncini 45–50 µm wide and 34–40 µm high; triangular heel, distally pointed prow downwardly directed, pointed dorsal button inserted closer to prow than to base of main fang, convex base; thoracic uncini with 3–4 distinct rows of apical teeth above main fang, about 2–4 teeth on first row, 2–4 on second and 4–5 teeth on third and four row; dental formula MF: 2–4: 2–4: 4–5: 4–5; abdominal uncini with 2–3 rows above main fang, 2–3 teeth on first and 3–5 on second and third row; dental formula MF: 2–3: 3–5: 3–5 ( Figs 10H, I ; 11N–O ).

One pair of nephridial papillae on segment 3, inserted laterally and close to branchial stalk. Three pairs of tubular genital papillae on segments 6–8, originating at anteriorly base of the corresponding notopodia and aligned to them; all genital papillae of same size, clearly exceeding corresponding notopodia ( Figs 8F ; 9C, D ).

Pygidium terminal, margin crenulated ( Fig. 8E ).

The specimens pale after preservation.

No specimen with gametes observed.

Remarks. Amphitritides skeletonensis sp. nov. most closely resembles A. namibiensis sp. nov. described above (see Table 1 ). Both species have very long branchiae and 3 pairs of distinct genital papillae at the anterior base of notopodia of segments 6–8. However, they clearly differ in the number of thoracic segments ( A. skeletonensis sp. nov. with 28–31 and A. namibiensis sp. nov. with 24 segments), and the absence ( A. namibiensis sp. nov.) or occurrence ( A. skeletonensis sp. nov.) of double rows of uncini on the first abdominal segments. The uncini of A. skeletonensis sp. nov. are clearly larger than those in A. namibiensis sp. nov. and the thorax in A. skeletonensis sp. nov. is always clearly swollen, contrarily to A. namibiensis sp. nov.

Amphitritides skeletonensis sp. nov. differs from A. jirkovi sp. nov. mainly in that A. jirkovi sp. nov. has ten pairs of genital papillae, whereas A. skeletonensis sp. nov. has three pairs. In addition, A. jirkovi sp. nov. has neuropodia with double rows of uncini on all abdominal segments, whereas A. skeletonensis sp. nov. has only double rows on the anterior 4–7 abdominal segments (see Table 1 ).

Within the genera Amphitritides , Amphitrite (including Neoamphitrite ) and Paramphitrite no species has 2 pairs of branchiae, 3 pairs of genital papillae and 28–31 thoracic segments. Amphitritides bruneocomata and A. kuehlmanni , with a similar number of thoracic segments, also have 2 pairs of branchiae, but significantly more segments with genital papillae. Amphitritides bruneocomata has 15 and A. kuehlmanni 11–13 pairs of genital papillae. In addition, both species have double rows of uncini on almost all abdominal segments. All other species of these 3 genera with a similar number of segments always have 3 pairs of branchiae and significantly more genital papillae.

There are only minor differences between large (width 8–12 mm) and small (width 2–4 mm) individuals. Thus, the uncini in small specimens are smaller in general (24–28 µm high, 31–60 µm wide). As in A. namibiensis sp. nov. genital papillae are always visible, even in the smallest specimens which are presumably juveniles.

Etymology. The specific epithet skeletonensis is derived from the northern part of coast of Namibia, the Skeleton Coast (type locality).

Distribution. Although this species is not common, it had the widest distribution of all three species, described from Namibia, occurring between 18°– 25°S, in water depths between 101– 187m.

Habitat. Bottom water salinity between 34.9–35.5 psu, oxygen content between 4–112 µmol/l. Bottom water temperature was between 11–13 °C. The sediments were silty mud with grain sizes ranging from 20–150 µm. The organic content varied between 4–26 %. No information is available on the shape of the tubes.

Two more specimens of Amphitritides were found, which clearly differ in important diagnostic features from the species described above and those already known. These two individuals also differ from each other. However, their poor conservation status prevents us from formally describing these species until better preserved material becomes available.

Figures

FIGURE 8. Amphitritides skeletonensis sp. nov. methyl blue staining. A. Anterior end, lateral view. B. Anterior end, dorsal view. C. Anterior end, ventral view. D. Anterior end, ventrolateral view. E. Posterior end, lateral view. F. Segments 3–6, lateral view (note: nephridial and genital papillae). Scale bars. A 1 mm, B–E 2 mm, F 500 µm. Abbreviations: br1 branchia of first pair, br2 branchia of second pair, bt buccal tentacles gp genital papillae, np nephridial papilla, vs ventral shield.

FIGURE 9. Amphitritides skeletonensis sp. nov. SEM micrographs. A, B. Anterior end, lateral views. C. Segments 5–9, lateral view (note: genital papillae). D. Segments 6–10, dorsolateral view (note: genital papillae and rows of papillae on dorsal side). E. Thoracic notopodia. F. Thoracic uncini in double rows. Scale bars.A–E 500 µm, F 50 µm Abbreviations: br1 branchia of first pair, br2 branchia of second pair, gp genital papillae, ll lower lip, numbers refer to segments.

FIGURE 10. Amphitritides skeletonensis sp. nov. SEM micrographs. A. Thoracic notopodia (note: short and long chaetae). B. Shorter notochaetae of first row (note: distinct angle between shaft and serrated tip). C. Serrated tips of shorter notochaetae. D. Longer notochaetae of second row (note: angel between shaft and serrated tip absent). E. Shaft of notochaetae. F, G. Serrated tips of longer notochaetae. H. Three thoracic uncini from the region with uncini in double rows. I. Abdominal uncini. Scale bars. A 200 µm, B, D, I 20 µm, C, H 10 µm, E–G 5 µm.

FIGURE 11. Notopodial chaetae, thoracic and abdominal uncini of the new species of Amphitritides described herein. A–E. A. namibiensis sp. nov. A. Notopodial chaetae, segment 19. B–C. Notopodial chaetae, segment 10. D. Thoracic uncini, segment 10. E. Abdominal uncini, segment 31. F–J. A. jirkovi sp. nov. F. Notopodial chaetae, segment 10. G. Notopodial chaeta, segment 21. H. Thoracic uncini, segment 20. I. Abdominal uncini, segment 50. J. Thoracic uncinus, segment 20. K–O. A. skeletonensis sp. nov. K. Longer notopodial chaetae, segment 11. L–M. Shorter notopodial chaetae, segment 11. N. Abdominal uncini, segment 34. O. Abdominal uncini, segment 40. P. A. namibiensis sp. nov. thoracic chaetae with bulbous wings, segment 10 (arrows). R. A. jirkovi sp. nov. thoracic chaetae with bulbous wings, segment 8 (arrows), S. A. skeletonensis sp. nov. thoracic chaetae with indistinct bulbous wings, segment 15 (arrows). Scale bars. A, C–G, J–N, O, R, S 20 µm, B, H, I, O, P 50 µm.