Into the Depths of Patagonia: The First Troglobitic Species of Pleonaraius Attems, 1898 (Polydesmida, Dalodesmidae) from Argentina ^†

Abstract

A new troglobitic species of Dalodesmidae, Pleonaraius spelaeus n. sp., is described from Rolo Vergara Cave, Neuquén Province, Argentina. This species represents the fourth known troglobitic member of the family, the first troglobitic species of Dalodesmidae recorded in South America, and the first known troglobitic millipede from Argentina. Pleonaraius spelaeus n. sp. is distinguished from its congeners by the absence of cuticular pigmentation and a unique combination of gonopodal characters. Ecological notes, a key, and a distribution map of Pleonaraius species are also provided.

1. Introduction

The millipede family Dalodesmidae Cook, 1896 currently comprises approximately 55 genera and over 250 described species [1]. Its distribution is characteristically Gondwanan, encompassing regions such as New Caledonia, Australia, New Zealand, southern South America, southern Africa, and Madagascar [2].

Within South America, Chile stands out as the country with the highest diversity of Dalodesmidae, hosting 10 genera and 49 species [3,4]. In contrast, Argentina is known to harbor only a few representatives of the family, including Anaulacodesmus lacustris Schubart, 1954, recorded from Isla Victoria (Lago Nahuel Huapí) and Villa La Angostura (Parque Nacional Arrayanes); Kuschelodesmus atlanticus (Schubart, 1954) from Mar del Plata; Monenchodesmus monticola Silvestri, 1903 from Lago Lácar; M. inermis Silvestri, 1903 from Isla Victoria (Lago Nahuel Huapí); and Tsagonus valdiviae Chamberlin, 1957, also from Lago Nahuel Huapí.

Pleonaraius Attems, 1898 is a genus distinguished by a unique pore formula (5, 7, 9–19) [5] and a characteristic gonopodal structure: telopodite medially attached and fused on coxal region, medial branch robust and elongated, positioned posterior to lateral branch, which is simple and situated behind of solenomere, solenomere simple, though it may occasionally bear a spine-like basal process.

Currently, the genus comprises three species: P. pachyskeles Attems, 1898 (the type species), and P. omalonotus Silvestri, 1903, both distributed between Chile’s Maule and Los Lagos Regions [5,6,7,8], and P. spelaeus n. sp., from Rolo Vergara Cave, Neuquén Province, Argentina, herein described.

Millipedes that inhabit caves exclusively, known as troglobionts, often exhibit specialized morphological traits distinct from those of surface-dwelling species. These adaptations to life in permanent darkness (collectively referred to troglomorphisms) frequently include the reduction or complete loss of body pigmentation [9,10,11], thinner cuticle, and elongation of body appendages. Modifications may affect sensory structures, including the extension of antennae and tarsal claws, which enhance navigation and environmental perception in subterranean habitats [12,13].

This study describes Pleonaraius spelaeus n. sp., the first strictly cave-dwelling millipede species recorded in Argentina and the first representative of the genus Pleonaraius reported from the country. In addition to the species description, the work provides a key and a distribution map for all known species within the genus. Ecological notes on the newly described species are also included, offering insights into its subterranean habitat.

2. Materials and Methods

2.1. Study Area

Pleonaraius spelaeus n. sp. was discovered in Rolo Vergara Cave (coordinates: −37.2721, −69.8599), situated within the municipality of Chos Malal, in Neuquén Province, Argentina. Additional details on the cave’s characteristics and habitat will be presented in the “Ecological Remarks” section. The Chos Malal region lies within the northern extent of the Neuquén Basin, a prominent sedimentary basin in west-central Argentina. This area is geologically defined by the Chos Malal fold-and-thrust belt, a structural zone formed during the Andean orogeny. This tectonic activity produced a complex arrangement of folds and thrust faults that affect Mesozoic sedimentary strata, including deposits of marine, continental, and volcaniclastic origin [14]. Climatically, the region falls under the cold semi-arid (BSk) category of the Köppen–Geiger classification [15]. The local climate is marked by low annual precipitation, averaging around 347 mm per year, typically distributed across 56 days. Seasonal temperature fluctuations are pronounced, with mean annual temperatures ranging from 1 °C in winter to about 19 °C in summer.

2.2. Material Examined

Specimens were collected inside the cave using fine brushes moistened with 70% ethanol and were immediately transferred to vials containing 70% ethanol for preservation. The examined material is deposited in the following scientific collections: Museo de La Plata (MLP), Universidad Nacional de La Plata, Argentina (1♂, 2♀), and Coleção de Invertebrados Subterrâneos de Lavras (ISLA), housed at the Centro de Estudos em Biologia Subterrânea (CEBS), Universidade Federal de Lavras (UFLA), Brazil (2♂, 22♀ and 22 immatures).

2.3. Morphology and Analysis

Photographs of the habitus and gonopods were taken using a Zeiss Axio Zoom V16 stereomicroscope (Jena, Germany), with image capture facilitated by ZEN 2.1 software. Scanning electron microscopy (SEM) was conducted with a Hitachi TM4000 microscope (Minato-ku, Tokyo, Japan). Following SEM analysis, specimens were carefully removed from the stubs and returned to ethanol for preservation. Micrographs were assembled and edited using Adobe Photoshop CS6. Illustrations of the gonopods were produced by digitally tracing focus-stacked images in the Sketchbook Premium app. A distribution map was created in QGIS Desktop ver. 3.6.0. Color descriptions follow the standardized nomenclature of the 267 Color Centroids from the NBS/IBCC Color System [16].

The terminology used for description of gonopods, and somatic structures follows Golovatch, 2014 [5].

3. Results

Taxonomy

• Order Polydesmida Pocock, 1887
• Suborder Dalodesmidea Hoffman, 1980
• Family Dalodesmidae Cook, 1896
• Genus Pleonaraius Attems, 1898
• Pleonaraius Attems 1898: 268 (in genus key), 272 (relationships), 274 (first description, in German).
• Pleonaraius Attems 1914: 239 (in genus key; assigned to Sphaerotrichopidae).
• Pleonaraius Brölemann 1916: 556 (relationships).
• Pleonaraius Attems 1926: 145 (in genus key).
• Pleonaraius Verhoeff 1932: 1552 (relationships), 1579 (in genus key as Pleonarajus [sic]).
• Pleonaraius Verhoeff 1936: 6, 7 (in genus keys).
• Pleonaraius Attems 1940: 384 (in genus key), 425 (redescription in German).
• Pleonarius [sic] Chamberlin 1957: 19 (in genus key), 28 (introducing Pleonarius [sic] species).
• Pleonaraius Jeekel 1971: 347 (in genus catalogue).
• Pleonarius [sic] Demange & Silva 1976b: 38 (introducing illustrations of Pleonarius [sic] species).
• Pleonarius [sic] Hoffman 1980: 185 (in genus catalogue).
• Pleonarius [sic] Golovatch 2014: 259 (brief overview of genus).
• Pleonarius [sic] Parra-Gómez & Faúndez 2021: 145 (teratological case in Pleonaraius pachyskeles).
• Type species: Pleonaraius pachyskeles Attems, 1898, by monotypy.
• Diagnosis. A Dalodesmidae genus characterized by unusual pore formula (5, 7, 9–19) and the gonopod differs from other genera by telopodite contiguous medially, fused only in coxal region (cx). Medial branch (mb) long and strong, located posterior to lateral branch (lb). lb simple and located posterior to solenomere (sl). sl simple, although sometimes with a spine-shape additional process (pr) at their base.
• Pleonaraius spelaeus n. sp.

Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5

urn:lsid:zoobank.org:act:8ED5F9B2-612D-4C1C-9F4B-938DE88F57C7

Type material. HOLOTYPE. ARGENTINA—Neuquén • 1♂; Chos Malal, Rolo Vergara Cave; [−37.2721, −69.8599]; R.L. Ferreira leg.; 18/I/2023; MLP 21092. PARATYPES. ARGENTINA—Neuquén • 1♀; same data as holotype; MLP 21093; • 1♀; same data as holotype; MLP 21094; • 1♂; same data as holotype; ISLA 144037; • 1♂; same data as holotype; ISLA 144038; • 11♀, 9 immature; same data as holotype; ISLA 144039; • 1♀, 4 immatures; same data as holotype; ISLA 144040; • 1♀, 1 immature; same data as holotype; ISLA 144041; • 3♀, 1 immature; same data as holotype; ISLA 144042; • 1♀, 3 immatures; same data as holotype; ISLA 144043; • 1♀, 1 immature; same data as holotype; ISLA 144044; • 1♀; same data as holotype; ISLA 144045; • 3♀, 3 immature; same data as holotype; ISLA 144046.

Etymology. Spelaeus, adjective in the masculine gender. The epithet as a reference to the Latin word spelaeus, meaning “cave”.

Distribution. Only known from in Rolo Vergara Cave, municipality of Chos Malal, Neuquén Province, Argentina (Figure 5).

Diagnosis. Based on the gonopodal conformation, the new species differs from its congeners by the following combination of characters: mb simple (vs. bifurcated and complex in P. omalonotus) and directed ventrad at its apex (vs. directed laterad in P. pachyskeles and P. omalonotus). lb simple with apex acuminate (vs. apex suberect in P. pachyskeles). Presence of pr as in P. pachyskeles (vs. absence in P. omalonotus).

Description.

Measurements. Head + 20 rings (including telson), males 15–16 mm length and 1.5 mm wide (ring 10); females 17–18 mm length and 1.6 mm wide.

Color. In life and in ethanol between white 263 and pale yellow 89 with the exception of the antennae; light pink 4.

Head. Head about the same width as the collum (Figure 2A,B). Sulcus evident, clypeus and fronts slightly setose, vertex with 2 + 2 macrosetae (Figure 2B). Antennae long, reaching the fourth body ring when extended backward (Figure 1 and Figure 2A). Four apical sensory cones and one cluster of bacilliforms sensilla on laterodistal part of antennomere 6 and 7; antennomere 7 about 1/8 the size of antennomere 6 (Figure 2F). Relative lengths of antennomeres: 2 > 3 > 4 > 5 > 1 > 7.

Trunk. Collum convex and extended laterally (Figure 2A). Tergites smooth with two (1–4 rings) or three (5 to last) transverse rows of setae (Figure 2A,C,D). Paranota anteriorly rounded, posterior margins on body rings 1–4 rectangular, and on body rings 5–19 pointed posteriorly. Paranota directed posterolaterad, mostly thickened dorsoventrally on ozopore-bearing rings and progressively narrower (Figure 2A,C,D). Paranota of body rings 16–19 progressively decrease in lateral projection. Ozopore formula unusual (5, 7, 9–19) (e.g., see detail in Figure 2D), ozopores small and rounded, opening laterally on paranotal margins, somewhat thickened at the edges, without evidence of microsculpture or porosteles (Figure 2J). Sternites slightly setose, without modifications. Leg pair 2 of males with small, pointed coxal process bearing gonopores (Figure 2E). Spiracles small and rounded, without modifications (Figure 2H). Legs long; 1–2 pair shorts and simples, pair 3–7 gradually increasing in size and prefemora strongly swollen dorsally. Legs postgonopodal ca. of the same length as antenna. Relative podomere lengths: tarsus > femur > prefemur > tibia = postfemur. Claw ca. 1/8 the size of tarsus. Telson (Figure 2I) facing downwards. Paraprocts parallel to substrate and almost flat. Epiproct distally projected, flattened at its apex with four inconspicuous setae (spinnerets), each spinneret with a single low sheath, each setae inside a circular, deep, walled depression. Hypoproct rounded with two seta and apparently thickened at its apex.

Figure 1. Habitus of Pleonaraius spelaeus n. sp., male paratype (ISLA 144037). Scale bar: 1 mm.

Figure 1. Habitus of Pleonaraius spelaeus n. sp., male paratype (ISLA 144037). Scale bar: 1 mm.

Gonopods. Gonopod aperture oval, longitudinally elongated ca. 1.5× wide, lateral margins of aperture slightly extended ventrally on posterior margin (Figure 2G). Gonopods (Figure 3) contiguous medially, fused on coxal region (cx). Prefemoral region ca. 1/3 the length of acropodite, loosely setose. Solenomere (sl) extends into a simple, acuminate branch, directed ventrad and anteromesad, and then slightly bent posteriorly and anterolaterally at its apex. Additional process (pr) short and acuminate, located and attached to sl in basal region, extending ventral and anteromesally, and is mostly visible in lateral view. Lateral branch (lb), long and acuminate, ca. 2× of sl, begins laterally in prefemoral region, directed anteroposteriorly, and then slightly bent ventral and anteromesally. Medial branch (mb) stronger and longer, with a rounded apex; it is somewhat widened posteriorly at its base, directed anteriorly, and bent ventrally for approximately 2/3 of its length.

Female characters. Same general appearance and coloration as in males but first pairs of legs not enlarged.

Figure 2. Pleonaraius spelaeus n. sp., male paratype (ISLA 144038) (A). Anterior part of the body, dorsal view. (B). Head, anterior view. (C,D). Middle part of the body, dorsal view. (E). Anterior part of the body, ventral view. (F). Right antenna. (G). Aperture of gonopods, ventral view. (H). Spiracle in detail. (I). Telson, ventral view. (J). Right ozopore of ring 15. Dashed square in D shows the presence of ozopore in ring 11 and dashed circles in F show cluster of bacilliforms sensilla. Scale bars: (A–E,G,I) = 0.5 mm; (F) = 0.1 mm; (H,J) = 0.05 mm.

Figure 2. Pleonaraius spelaeus n. sp., male paratype (ISLA 144038) (A). Anterior part of the body, dorsal view. (B). Head, anterior view. (C,D). Middle part of the body, dorsal view. (E). Anterior part of the body, ventral view. (F). Right antenna. (G). Aperture of gonopods, ventral view. (H). Spiracle in detail. (I). Telson, ventral view. (J). Right ozopore of ring 15. Dashed square in D shows the presence of ozopore in ring 11 and dashed circles in F show cluster of bacilliforms sensilla. Scale bars: (A–E,G,I) = 0.5 mm; (F) = 0.1 mm; (H,J) = 0.05 mm.

Ecological remarks. Rolo Vergara Cave is a gypsum cave situated at an altitude of 1301 m a.s.l. It features a single entrance located at the base of a sinkhole approximately 20 m in diameter at its widest point (Figure 4A,B). From the entrance, a descending passage, partially obstructed by collapsed blocks, leads into the cave’s main conduit (Figure 4C). This conduit has an approximate length of 180 m from the mouth of the cavern to its end, not including the side galleries. An intermittent stream traverses this conduit, transporting organic material, primarily plant debris, which accumulates in various sections of the cave. Additionally, thin guano deposits from insectivorous bats are present on higher areas of the conduit, where they appear unaffected by water flow during the rainy season.

Figure 3. Gonopods of Pleonaraius spelaeus n. sp., male paratype (ISLA 144038). (A–D). Microphotographs. (B–E). Line drawings. (C–F). SEM photographs. (A–C). Ventral view. (D–F). Lateral view. Scale bar: 0.25 mm.

Figure 3. Gonopods of Pleonaraius spelaeus n. sp., male paratype (ISLA 144038). (A–D). Microphotographs. (B–E). Line drawings. (C–F). SEM photographs. (A–C). Ventral view. (D–F). Lateral view. Scale bar: 0.25 mm.

However, the bat colony’s presence appears to be seasonal, as no individuals were observed during the sampling event. During the sampling conducted on 18 January 2023, temperature within the cave ranged from 13.0 to 13.8 °C, while relative humidity was measured between 90% and 94%.

Specimens of Pleonaraius spelaeus n. sp. were observed along the main conduit, primarily associated with organic deposits (Figure 4D). While some individuals were found among plant debris, the highest densities (though still fewer than 10 specimens per guano pile) occurred on small accumulations of bat guano. Dozens of individuals, particularly immatures, were recorded during a single visit by one of the authors, suggesting the presence of a potentially large and well-established population. It is important to emphasize that the organic deposits observed within the cave do not characterize it as a mesotrophic or eutrophic system. Instead, the majority of the main conduit consists of clay-rich sediments, which appear to be seasonally reworked by water runoff. Hence, the cave can be considered oligotrophic.

The external environment surrounding the cave is subject to harsh conditions, characterized by extreme temperature fluctuations and significant variations in moisture throughout the year. Vegetation in the surrounding area is sparse, dominated by small shrubs, indicating high levels of solar exposure on the soil surface. Consequently, the existence of suitable microhabitats capable of supporting external populations of P. spelaeus n. sp. is highly unlikely, particularly given the troglomorphic features exhibited by the species, such as depigmentation and a thin cuticle.

The cave shows no evidence of anthropogenic disturbance and appears to be well preserved, likely due to its limited and sporadic visitation by speleologists. As biological surveys in the region have targeted only a limited number of caves, further sampling is recommended to assess whether P. spelaeus n. sp. is endemic to Rolo Vergara Cave or occurs more broadly across the area.

Figure 4. (A) Aereal view of the area where the cave in which Pleonaraius spelaeus n. sp. was found (yellow arrow indicates the cave entrance); (B) cave entrance at the bottom of the sinkhole; (C) cave conduct where the specimens were found and collected; (D) living male.

Figure 4. (A) Aereal view of the area where the cave in which Pleonaraius spelaeus n. sp. was found (yellow arrow indicates the cave entrance); (B) cave entrance at the bottom of the sinkhole; (C) cave conduct where the specimens were found and collected; (D) living male.

Figure 5. Distribution map of the genus Pleonaraius Attems, 1898.

Figure 5. Distribution map of the genus Pleonaraius Attems, 1898.

4. Discussion

Pleonaraius spelaeus n. sp., described from Rolo Vergara Cave in Neuquén Province, constitutes the first record of a millipede species strictly confined to cave habitats in Argentina. Moreover, it represents the first confirmed troglobitic species within the Dalodesmidae family in South America. Globally, only four dalodesmids species are currently known to be true cave dwellers: Atalopharetra clarkei Mesibov, 2005; A. eberhardi Mesibov, 2005; and Noteremus infimus Mesibov, 2009 from Australia; and now Pleonaraius spelaeus n. sp. from Argentina [17,18].

In addition to these, several dalodesmid species are considered accidental cave inhabitants, lacking clear troglomorphic adaptations. Examples include Dalodesmus speophilus Wesener, Akkari & Golovatch, 2025 from Madagascar, as well as several Australian species such as Tasmanodesmus hardyi Chamberlin, 1920; Asphalidesmus golovatchi Mesibov, 2009; Lissodesmus anas Mesibov, 2005; L. latus Mesibov, 2005; L. modestus Chamberlin, 1920; and L. perporosus Jeekel, 1984 [1,17,18,19].

Although a range of troglomorphic traits (both regressive and constructive) have been reported in millipedes (Diplopoda), such as elongation of legs and antenna, loss of pigmentation, reduction or complete absence of eyes, enlargement of Tömösváry organs, and modification of mouthparts for detritus filtration [10,20,21,22,23,24,25,26,27], differences in body size between troglobitic and epigean species may also reflect adaptation to cave environments [10].

In P. spelaeus n. sp., certain troglomorphic traits, such as depigmentation and a thin cuticle, are clearly present. Although precise measurements of antennae and legs in the congeners are lacking, the new species appears to exhibit a relative elongation of these appendages. This observation, based on direct comparison of available images of P. spelaeus n. sp., with those of the other two known Pleonaraius species, suggests that such traits may indeed be expressed among troglobitic dalodesmids, despite the limited availability of detailed morphometric data.

Regarding body size in males of the epigean species, P. pachyskeles measures ca. 18–20 mm in length and 2.0–2.3 mm in width at the midbody metazonae [18,28], while P. omalonotus measures ca. 18 mm in length and 2.3 mm in width [18]. These measurements indicate that epigean species of the genus are generally larger than the newly described troglobitic species, P. spelaeus n. sp., that measures ca. 16 mm in length and 1.5 mm in width at the midbody metazonae. A similar pattern of size reduction in troglobitic forms has been observed in the genus Pseudonannolene Silvestri, 1895 (Spirostreptida). However, the opposite trend occurs in some groups, such as Glomeridesmus Gervais, 1844 (Glomeridesmida) or Acanthophorella Antić & Makarov, 2016 (Chordeumatida), where troglobitic species tend to be larger than their surface-dwelling congeners [25,29,30,31].

In general, the study of subterranean fauna (and even soil-dwelling millipedes) in Argentina remains limited when compared to efforts in countries like Brazil or those in Europe. Given this gap, it is highly likely that additional, yet undescribed, species of Diplopoda adapted to subterranean environments exist, particularly within diverse South American lineages such as Chelodesminae and Spirostreptida.

• Key to species of Pleonaraius (based on male characters)

1.

mb simple; lb simple and without an obvious fold; sl directed anterolaterad at the base and apex in the middle of them, a mesad directed fold ……………………………………………………2

-

mb in ventral view with apex bifurcated and complex, directed laterad; lb in ventral view acuminate and only curved mesad by a smooth fold in distal part of process; without presence of pr………………………………………………………………………………P. omalonotus Silvestri, 1903

2.

mb in ventral view directed ventrad at its apex; lb in ventral view acuminate, only curved mesad; pr in lateral view strongly bent at the junction with sl, appearing with a roughly 90° angle……………………………………………….……………………………………….……P. spelaeus n. sp.

-

mb in ventral view directed laterad at its apex; lb in ventral view with sub-truncated apex; pr in lateral view forming a semicircle at the junction with sl…………………………………………………………………………………P. pachyskeles Attems, 1898