Hediste sinesimplex, Costa & Rocha & Martins & Venâncio & Ureña & Almeida & Ferreira & Christoffersen & Antunes, 2025

Hediste sinesimplex View in CoL sp. nov.

Etymology: This species lacks fused “simple” chaetae, so in Latin form, sine means “without” and simplex for “simple” chaetae.

Type material / Material examined: Type material from Mediterranean Sea, Italy, Port of Livorno, Darsena Pisana area, Pisa, 43º41’57.89”N, 10º23’02.63”E, on muddy bottom intertidal sampled by hand, 30 cm depth, collected on April 1, 2021: Holotype: Complete specimen, 80 chaetigers, 53.5 mm long, 3.0 mm wide, GenBank accession number PV449855 ( NatMIP-APPh-0019), Paratypes: 9 complete specimens, 53.0 to 44.0 mm long, 2.9 to 2.5 mm wide ( NatMIP-APPh-0020 [ 3 specimens], NatMIP-APPh-0021 [ 3 specimens], NatMIP-APPh- 0022 [ 3 specimens]). Additional material: Spain, Platja de la Goja, Gola del Perellonet , Valencia, 39°18’29.63”N, 0°17’33.81”W, sampled by hand, collected on March 5, 2022: 17 specimens, 73.0 to 10.1 mm long, 4.5 to 1.0 mm wide ( NatMIP-APPh-0023), one of which was used for genetic analysis, GenBank accession number PV449856 GoogleMaps .

Apomorphic features: Posterior supra-acicular neurochaetae without fused “simple” chaetae; otherwise, by replacement, heterogomph falcigers are present.

Description: Anterior and median width body regions similar ( Fig. 2A View FIGURE 2 ), tapering towards posterior end, and slightly flattened; 51.0 to 40.0 mm long, 4.0 to 3.2 mm wide, with 60–80 chaetigers. Prostomium with two tiny unarticulated antennae; two pairs of eyes on lateral-posterior region like a trapezium ( Fig. 2A View FIGURE 2 ). Two palps emerging from lateral-middle of prostomium; robust palpophores, minute and ovate palpostyles (at antennal level). Proboscis subdivided between maxillary and oral ring, with conical paragnaths, and two brown stout maxillae with 5–8 teeth on anterior end; maxillary ring: area I with four paragnaths, II with 16, III with 45, IV with 20; oral ring: area V without paragnaths, area VI with 3–9, areas VII and VIII with 34. Four pairs of tentacular cirri (anterior modified cirri) near lateral-posterior region of prostomium, may reach chaetiger 8 ( Fig. 2A View FIGURE 2 ). Peristomium longer than subsequent chaetigerous segments. Parapodia biramous, more developed on median chaetigers ( Fig. 2D, G View FIGURE 2 ), decreasing toward anterior chaetigers ( Fig. 2B, C, F View FIGURE 2 ), as well as in posterior ones ( Fig. 2E, H View FIGURE 2 ); notopodia ( Fig. 3A–C View FIGURE 3 ) with three conical structures—notopodial ligule 1 (noLi1), pre-chaetal lobe 1 (prLi1, carrying chaetae alongside others, supported by a black aciculum), and notopodial ligule 2 (noL2); neuropodia carrying chaetae between pre-chaetal lobe 2 (prL2, supported by a black aciculum) and post-chaetal lobe (poL), and a neuropodial ligule (neLi) achaetiger ( Fig. 3B View FIGURE 3 ). Dorsal and ventral unarticulated cirri shorter than dorsal and ventral ligules, respectively. Chaetation process ( Figs 3C View FIGURE 3 ; 4 View FIGURE 4 ; 5 View FIGURE 5 ): anterior notochaetae homogomph spinigers ( Figs 3C View FIGURE 3 ; 4A, F View FIGURE 4 ); anterior neurochaetae, supra-acicular bundle: homogomph spinigers and heterogomph falcigers ( Figs 3C View FIGURE 3 ; 4D, I View FIGURE 4 ), with blades serrated ( Fig. 4I View FIGURE 4 ); infra-acicular bundle: heterogomph spinigers ( Figs 3C View FIGURE 3 ; 4C, H View FIGURE 4 ), and heterogomph falcigers with long and hook-type blades ( Fig. 4D, I View FIGURE 4 ). Median parapodia ( Fig. 2D, G View FIGURE 2 ): notochaetae homogomph spinigers; neuropodia with supra-acicular homogomph spinigers and heterogomph falcigers, and infra-acicular heterogomph spinigers and falcigers ( Fig. 4 View FIGURE 4 ). Posterior notopodia ( Fig. 5B, D View FIGURE 5 ) carrying homogomph spinigers; neuropodia: supra-acicular homogomph spinigers and heterogomph falcigers (to replace the simple chaetae, Fig. 5 View FIGURE 5 ); infra-acicular heterogomph spinigers and heterogomph and homogomph falcigers ( Figs 4 View FIGURE 4 ; 5B, D View FIGURE 5 ). Pygidium carrying two segmented anal cirri.

Additional description information: 3D models are presented for visual and graphic demonstration of the parapodia and chaetae (Videos S1,S3), and additional links are available on the Sketchfab website (parapodia: Models 1 (https://skfb.ly/owIEU), 3 (https://skfb.ly/owKoE); chaetae: Models 4 (https://skfb.ly/owKpS), 5 (https:// skfb.ly/owKpK), 6 (https://skfb.ly/owKpX), 7 (https://skfb.ly/owKpA).

Type locality: Mediterranean Sea , Italy: Port of Livorno , Darsena Pisana area, Pisa .

Distribution: Western Mediterranean Sea ( Spain to Italy, this study), Baltic and Eastern North seas ( Teixeira et al., 2022).

Ecological notes: In Italy, populations were found among Ficopomatus enigmaticus (Fauvel, 1923) tubes, in the vicinity of a port zone, ranging from muddy bottom to fine sand. In Spain, sediment samples were collected with a logarithmic mean grain size (Φ) of 2,116 and 0.39% of organic matter.

Remarks: The species Hediste sinesimplex sp. nov. mainly differs from other Hediste spp. in the posterior neuropodia (Model 2—https://skfb.ly/owKox), specifically by lacking fused “simple” chaetae in the supra-acicular neurochaetae ( Figs 4E, J View FIGURE 4 ; 5A, C View FIGURE 5 ; Model 8—https://skfb.ly/owKpP), which are replaced by heterogomph falcigers ( Figs 4D, I View FIGURE 4 ; 5B, D View FIGURE 5 ). This suggests the absence of this morphological feature, as observed in other groups of the family Nereididae , and many other polychaete taxa ( Cronk 2009), or character transformation, as a synapomorphic character process, from simple to falciger chaeta. Notwithstanding, the species Hediste sinesimplex sp. nov. shares all other synapomorphic features allowing for its integration in the genus Hediste , e.g. absence of proboscis paragnaths on position V, and posterior parapodia carrying bi-articulated supra-acicular homogomph falcigers. 3D models and videos (Models 1–8; Videos S1–S3) are provided that compare the posterior parapodia of Hediste sinesimplex sp. nov. to those of other Hediste spp. (represented by H. diversicolor ).

Phylogenetic analysis and species delimitation

We obtained a total of 657 COI gene base pairs (bp) for Hediste specimens. The best model of molecular evolution fitting dataset was: TIM2+F+I+I+R2. No base saturation was observed for the mitochondrial gene. All sequences obtained were deposited in the GenBank (accession numbers PV449855 and PV449856).

Our phylogenetic analysis recovered a topology in which Hediste sinesimplex sp. nov. (clade highlighted in yellow; Figure 6 View FIGURE 6 ) was recovered as a monophyletic clade that includes specific localities, including those of the present study. The genetic distance (based on the COI marker) of Hediste sinesimplex sp. nov. ranges from 5.6% to 15.2% when compared to other species of the genus Hediste ( Table 1).

The result of the ASAP model supports the split for the eight species and one lineage ( Figure S4 View FIGURE 4 ) and bPTP although it recovers the same scenario (nine groups), however, the a posteriori probability is lower than 0.95 ( Figure S5 View FIGURE 5 ). This result may be attributed to genetic similarity, small sample sizes, or suboptimal genetic markers.