Ctenophoricola tzetlini, Syomin & Kolbasova & Semenova & Neretina, 2025

Ctenophoricola tzetlini View in CoL sp. nov. Syomin and Neretina, 2024

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Figs 1–7 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7

Type material. Holotype ZMMU WS10257 View Materials , Indian Ocean , South Maldives, 0.79665°S, 72.48400°E; pelagic, 10 m depth; 12 December 2016. Originally complete adult female with 32 chaetigers, broke in two otherwise intact fragments during examination. GoogleMaps Body length 10.5 mm (12 mm with pygidial cirri). Maximum body width excluding parapodia 0.6 mm in anterior region and 0.5 in posterior region; maximum width with parapodia 3 mm (chaetiger 28); parapodia length up to 1.2 mm. Proboscis not everted. Body cavity filled with eggs, especially anterior region and parapodia of posterior region. Fixation: 96% ethanol. Parapodium from chaetiger 26 (right side) taken for DNA extraction. Parapodia of chaetigers 8, 14, 18, 19, 20 and 28 (right side) mounted on microscopic slides.

Diagnosis. Ctenophoricola with body relatively large and long. Prostomium conspicuous, non-retractable, nearly as wide as peristomium. Eyes with well-developed hemispherical external cornea directed forward. Two ovoid papillae arising from lip on either side of mouth. Anterior region lacks ciliation. Dorsal lobe of parapodium indistinctly conical. Chaetigerous lobe with aciculum and one long acicular chaeta. Posterior region with scattered bundles of cilia. Gut in posterior region with lateral caeca protruding into parapodia up to half the length of the parapodia. Pygidium large with pair of anal cirri longer than posterior parapodia.

Description of holotype. Body relatively long and slender, divided in two distinct regions ( Figs 1–2 View FIGURE 1 View FIGURE 2 ). Anterior region cylindrical, with reduced parapodia. Posterior region enlarged and flattened due to enormously expanded parapodia. Trunk cylindrical, its diameter smaller than in anterior region. Anterior region together with prostomium and peristomium with few irregularly distributed single cilia. Numerous bundles of short cilia scattered over posterior region with no apparent regularity. Gut straight in anterior region, with pair of lateral caeca per segment in posterior region ( Fig. 2 View FIGURE 2 ).

Prostomium non-retractable, bears a pair of eyes with well-developed hemispherical cornea directed forward ( Figs 3 View FIGURE 3 and 4 View FIGURE 4 ). Eyes with spherical lenses located directly under cornea. Internal eyes including retina elongated and darkly pigmented, reaching boundary of peristomium and first chaetiger in fixed specimen. Prostomium bears pair of small non-ciliated palps ( Figs 3B View FIGURE 3 ; 4A, B View FIGURE 4 ). They are possibly glandulous, with dense granular opaque white content. Peristomium similar in length and width to subsequent segments. Tentacular cirri not found ( Fig. 4C View FIGURE 4 ). Mouth surrounded by cushion-shaped lips ( Fig. 3B View FIGURE 3 ; 4A, B View FIGURE 4 ). Upper lip partly fused with bases of palps, opaque white. Inferior lip (or a pair of fused ventro-lateral lips) deep horseshoe-shaped, with dense dark pigmentation. On either side of mouth, one papilla emerges from lateral side of inferior lip. Each papilla consists of swollen base and thinner ovoid distal part, together exceeding corresponding palp two times ( Fig. 3B View FIGURE 3 ). Proboscis not everted in holotype.

Anterior body region with 18 chaetigers. Parapodia reduced, consisting of acute chaetigerous lobe and dorsal lobe. Chaetigerous lobe conical, with non-protruding aciculum and one long straight acicular chaeta located below aciculum. Acicular chaeta capable of being completely retracted inside parapodium ( Fig. 5A, C View FIGURE 5 ) or protruded far outside up to at least three lengths of lobe ( Fig. 5B View FIGURE 5 ). Dorsal lobe of indistinct conical shape with wide base ( Fig. 5 View FIGURE 5 ).

Posterior region with 14 chaetigers. Parapodia are an order of magnitude longer than those of anterior region and exceed corresponding body width up to 2.5 times. Dorsal and ventral lobes fused to chaetigerous lobe and overgrow it forming an expanded oval paddle-like structure with trilobed free end ( Fig. 6 View FIGURE 6 ). Longest and widest parapodia located in chaetiger 28 ( Fig. 6C View FIGURE 6 ). Parapodia decrease in length and become narrower in shape both forward and backward. Lateral caeca follow same pattern: they are short and just enter bases of anterior parapodia of posterior region ( Fig. 6B View FIGURE 6 ), protrude into parapodia to up to half their length in middle of region ( Fig. 6C View FIGURE 6 ), and become shorter again in posteriormost segments. Acicula long and slender, not protruding. Chaetae very fine, short capillaries; they only slightly protrude forming barely visible brush around tip of parapodium. In first and last parapodia of posterior body region, bases of chaetae attached within basal part of parapodium ( Fig. 6A–B View FIGURE 6 ). In biggest parapodia in middle of region, bases of chaetae moved towards end of parapodium, to around boundary of second and last thirds of its length; main part of internal space of parapodium is occupied by caecum ( Fig. 6C View FIGURE 6 ). Pygidium large, with tapering cirri longer than posterior parapodia ( Figs 1–2 View FIGURE 1 View FIGURE 2 ).

Coloration: fixed specimen transparent whitish with opaque mass of eggs filling body ( Fig. 1 View FIGURE 1 ). Palps and upper lip opaque white due to dense granular content. Inferior lip with papillae dark brown. Anterior region of body with little pigmentation: brown longitudinal spot dorsally on prostomium and first three segments ( Fig. 4C View FIGURE 4 ), and traces of brownish pigmentation at parapodial bases posteriorly. Posterior region of body with multiple reddish brown spots formed by branching chromatophores. Dorsally, scattered spots cover most of trunk; ventrally, spots form single transverse row in each segment running from one parapodial base to another. Parapodia with bold scattered bands along upper margin of dorsal parapodial lobes and indistinct bands on anterior and posterior surfaces.

Hosts absent or unknown.

Remarks. The original diagnosis of the genus ( San Martín et al. 2021) was amended to include the new species which differs in many points from previously described species, and stress the difference between Ctenophoricola and free-living genera of Alciopini .These amendments were as follows: 1) Body variably covered by cilia (“densely” in the original diagnosis; however, even the description of C. rousei from the original paper mentions “sparse” rather than “dense” ciliation; 2) Chaetae absent or represented by single acicular chaeta per parapodium in anterior body region (anterior region without chaetae in original diagnoses); 3) Posterior region enlarged due to larger segments and/or expanded parapodia: added to include the formally undescribed Ctenophoricola sp. from the Gulf of California (hereinafter referred to as Ctenophoricola sp. A ) and Ctenophoricola tzetlini sp. nov. in which segments of the posterior region are not enlarged; 4) Lips surrounding mouth: added because this character is common to all representatives of Ctenophoricola known so far and differs notably from typical morphology of mouth organs in Alciopini ; 5) Eyes extending through one to several anterior segments (extending through several anterior segments in original diagnosis).

Body of our specimen is considerably longer and comparatively more slender than in other Ctenophoricola species, which is illustrated by the following measurements. Its overall length (10.5 mm, 12 mm with pygidial cirri) exceeds 3.5 times the maximum lengths of valid species, and 2.1 times the maximum length of Ctenophoricola sp. A ( Table 3 View TABLE 3 ). Length to maximum width ratios in anterior and posterior body regions of C. tzetlini sp. nov. are 17.5 and 21, respectively. Meanwhile, the same ratios are around 3 and 4 times smaller, respectively, in both C. masanorii and C. rousei . Width excluding parapodia is not provided for Ctenophoricola sp. A , but measurements of its image from the original paper produce corresponding ratios ~ 9 and ~ 8, which is 2 and 2.5 times smaller, respectively. Body width to maximum parapodia length ratio in posterior region equals 0.3 in C. tzetlini sp. nov. This parameter could only be measured from figures in other species; it was only slightly bigger in Ctenophoricola sp. A (~ 0.5), but exceeded C. tzetlini sp. nov. 3–6 times in other species ( Table 3 View TABLE 3 ).

Ciliary cover varies considerably between different Ctenophoricola species. Whereas C. masanorii is “densely covered by short cilia”, body of C. rousei is “sparsely ciliate” ( San Martín et al. 2021). Information about Ctenophoricola sp. A is unavailable. In C. tzetlini sp. nov., ciliation differs between body regions. While anterior region has quite a few isolated cilia, posterior region bears numerous bundles of cilia.

Lower lips surrounding mouth are described in all Ctenophoricola species; however, no papillae or appendages on lips are mentioned by ( San Martín et al. 2021). Contrary to that, two papillae exceeding palps in size are found in our specimen.

Lateral caeca are absent in C. masanorii , “sometimes protrude into parapodia” by at least the width of the gut in C. rousei , and not described in Ctenophoricola sp. A ( San Martín et al. 2021). In our specimen, the lateral caeca consistently extend into the parapodia: they protrude into almost all parapodia of the posterior region, extending to half the length of the parapodia in the middle of the region.

All previously described Ctenophoricola species do not possess chaetae in parapodia of anterior region; only acicula are present ( San Martín et al. 2021). On the contrary, our specimen possesses long retractable acicular chaetae located below acicula. Capillary chaetae of the posterior region of our specimen are extremely short and fine; they just show up their tips at the end of a parapodial lobe. Contrary to that, in all three species described and depicted by San Martín et al. (2021), capillaries form a well-developed fan or fascicle. However, we are not quite confident with this character in our specimen, since this species is capable of retracting chaetae to a great extent (observed for acicular chaetae in its anterior body region).

Pygidium is “small with two rounded, short anal cirri” both in C. masanorii and C. rousei ( San Martín et al. 2021) . Our specimen has enlarged pygidium with a pair of long anal cirri, a feature which it shares with Ctenophoricola sp. A .

The most striking difference between C. tzetlini sp. nov. and two described Ctenophoricola species is that the eyes of the new species possess well-developed rigid cornea occupying the anterodorsal side of the prostomium, making the latter wide and presumably non-retractable. Contrary to that, prostomium is “small, semicircular, without external eyes” ( San Martín et al. 2021) in both C. masanorii and C. rousei . Internal eyes in these species reach posteriorly chaetigers 4 to 5 in fixed specimens (2 to 4 in vivo) ( San Martín et al. 2021). The supporting video shows that the eyes move forward and backward inside the worm’s body. Apparently, this is impossible for C. tzetlini sp. nov. in which internal portions of eyes are attached to rigid external cornea and extend only to the boundary between peristomium and the first parapodial segment. The description of Ctenophoricola sp. A lacks these details; but judging from the figure, its internal eyes reach the middle of the first chaetiger posteriorly and adjoin to some external elevations anteriorly. This makes it more similar to C. tzetlini sp. nov.

Etymology. The species is named after our teacher, colleague and friend, Professor Alexander Tzetlin, to honor his contribution to the study of annelid morphology and evolution, and for his incredible ability to share his wormloving attitude through years.

Phylogenetic analysis. The tree based on concatenated 18S rRNA and 28S rRNA sequences ( Fig. 7 View FIGURE 7 ) placed our specimen as a sister lineage to the only Ctenophoricola sequences available so far, Ctenophoricola masanorii San Martín et al., 2020 from the original study. Distance between Ctenophoricola lineages (0.012) is within the range of between-species distances in the analyzed dataset. Thus, distances between considered Vanadis species vary from 0.006 in close species group V. antarctica ( McIntosh, 1885) — V. longissima ( Levinsen, 1885) to 0.016 ( V. formosa Claparède, 1870 and V. minuta Treadwell, 1906 ); distance between two Rhynchonereella species is 0.012. The common node of Ctenophoricola lineages is well-supported ( Fig. 7 View FIGURE 7 ). However, all basal nodes within Alciopini including the one nesting Ctenophoricola within Alciopini lack support, thus leaving the position of the genus within Alciopini unresolved in this study.

Key to formally described Ctenophoricola species

Below, we present a short key including C. tzetlini sp. nov. and two other formally described species. We do not consider undescribed Ctenophoricola sp. A from the original paper by San Martin et al. (2021).

1 Prostomium without external eyes; parapodia of anterior region only with acicula; body relatively short and stout........ 2

- Prostomium with external eyes; parapodia of anterior region with acicular chaeta below aciculum; body relatively slender.............................................................................................. C. tzetlini View in CoL

2 Gut without lateral caeca; transverse pigmented bands on dorsum.................................... C. masanorii View in CoL

- Gut with lateral caeca in posterior region; scattered red spots on dorsum.................................. C. rousei View in CoL