Polymerurus insularis, Magpali, Letícia, Machado, Danilo R. P., Araújo, Thiago Q. & Garraffoni, André R. S., 2021

Magpali, Letícia, Machado, Danilo R. P., Araújo, Thiago Q. & Garraffoni, André R. S., 2021, Long distance dispersal and pseudo-cryptic species in Gastrotricha: first description of a new species (Chaetonotida, Chaetonotidae, Polymerurus) from an oceanic island with volcanic rocks, European Journal of Taxonomy 746, pp. 62-93 : 68-82

Polymerurus insularis sp. nov.

urn:lsid:zoobank.org:pub:54D43422-15B7-4EC9-ACC9-E764D3A7BF6D Figs 2–8 ; Tables 2–3

Diagnosis

Elongated body with head separated from the trunk by slight neck constrictions. Total body length of 215–408 µm; furcal rami 93 µm long with 19–21 segments per side. Three-lobed, rounded head with a well-developed cephalion showing two lateral projections; one rugous pair of lateral pleurae and a ventral hypostomium present. Body mostly covered by pentagonal or hexagonal spined scales (Type 1 scales) of various sizes (3.5–11 µm) increasing in length progressively from anterior (3.5–8.5 µm) to posterior body (4.5–11 µm) ends and from ventral (3.5–7 µm) to dorsal surface (3.5–11 µm). Overall, the shape of the Type 1 scales is very distinct, with 5–6 corners and an anterior elevation that forms the spine base and opens towards a middle groove and a posterior cleavage.A triangular dorsal scale is found immediately anterior to the furca base, highly lifted and spineless (Type 2 scale). Dorsally on the furca base there is a spineless zone composed by a patch lacking cuticular ornamentation and flat, rounded or polygonal scales without spines measuring 2.5–5 µm (Type 4 scales). The posterior end of the ventral field is covered by a pair of keeled oval scales (Type 6 scales) and two pairs of small, elongated (Type 7 scales) or rounded scales (Type 8 scales), both bearing straight and long spines.

Etymology

From the Latin insularis meaning ‘belonging to an islandʼ, in reference to the type locality.

Material estudied

Holotype BRAZIL • adult (photographs, the specimen was destroyed); State of Pernambuco, Fernando de Noronha archipelago, Xaréu Reservoir ; ZUEC GCH 55 .

Specimen was examined while still alive under a compound microscope however, due to the fragility of the body, it was destroyed and is no longer available ( Garraffoni et al. 2019b). Photographs of the specimen are available at the Museum of Zoology of the University of Campinas under the access number ZUEC GCH 55 ( ICZN 2017: Article 73, Recommendation 73G, Statement 45). The holotype is shown in Figs 2A–C , 3A–I and 4B, D .

Paratypes BRAZIL • 3 adults specs (micrographs, the specimens were destroyed); same collection data as for holotype; ZUEC GCH 56 to GCH 58 • 2 specs (prepared for SEM); same collection datas as for holotype; ZUEC GCH 59 , GCH 60 .

Specimens were examined while still alive under a compound microscope, however, due to the fragility of their bodies, they were destroyed and are no longer available ( Garraffoni et al. 2019b). Photographs of the specimens are available at the Museum of Zoology of the University of Campinas, under the

access numbers ZUEC GCH 56 to 58 ( ICZN 2017: Article 73, Recommendation 73G, Statement 45). An example of paratypes are shown in Fig. 4A, C .

Other material

BRAZIL • 8 specs (examined while still alive under a compound microscope and, among those, three were prepared for DNA sequencing (no longer available)); same collection datas as for holotype.

Description

The description is based on both the holotype and 5 paratypes ( Figs 2–8 ; Table 2 ).

HABITUS. Specimens have a slender body with total length ranging from 215 µm to 408 µm, showing a slight neck constriction, represented by a small variation in width between the head and the neck. The cylindrical trunk is 31–55 µm wide at middle body length, showing little variation in width throughout its length, except for the set apart furcal rami, which are preceded by a pronounced constriction ( Fig. 2 ). Body widths at head, medium length and furca base are, respectively, 24–42 µm, 31–55 µm and 23– 25 µm.

HEAD. The three-lobed head is 24–42 µm wide and has three sets of distinct plates. The cephalion (U1–U6) is 19–25 µm long and 20–36 µm wide, with a free (detached from the cuticle) posterior portion and short paired lateral projections (lappets), each 10–18 µm long, posteriorly detached from the head and slightly lifted ( Figs 3A , 7A–C ). There is a pair of pleurae (U2–U6), each 12–15 µm long and presenting a rough texture with small parallel grooves, contrasting with the smooth surface of the cephalion ( Figs 3B , 7C ). The ventral hypostomion is a well-marked transversal bar, at U4, 13– 17 µm long and shaped like an arch (i.e., decreasing in thickness towards the lateral extremities) with a middle concavity ( Fig. 3C ). Two tufts of cilia (cephalic sensory bristles) are present on each side of the head. The first tuft is shorter, approximately 10.5 µm long and located below the cephalion projections (U1), while the second is situated between the cephalion and the pleurae (U2), bearing longer cilia with an average length of 14 µm ( Fig. 3B ). No dorsal sensory bristles were observed. Mouth is subterminal, with 8–13 µm of diameter and surrounded by a ring that is segmented with longitudinal ridges ( Fig. 3A–C ). The pharynx is relatively long, reaching up to one-quarter body length (adhesive tubes excluded) and situated between U2 and U20. It is 74 µm long, 25 µm wide at mid-length and divided in 2 regions, without any distinct constrictions. Pharynx becomes wider towards its inferior portion at the pharyngeal intestinal junction (from 19 µm to 27 µm) while at its superior end a distinct, although small, furrow is formed at the junction of the internal regions ( Fig. 3B ).

INTERNAL ANATOMY. Internally, a pair of protonephridia (105 µm long) are present, lateral to the intestine and posterior to the pharyngeointestinal junction (U29 – U75) ( Fig. 3E ).

FURCA. Is 109–117 µm long (¼ of the body) and furca base is 27 µm wide, showing a V-shaped gap with a large and distinct U-shaped middle furrow (“helmet like” shape, as described by Roszczak 1969). Presence of a single pair of very small ventral spines, one per side of the furrow ( Fig. 5C ). Furcal rami are 93 µm long (around 1/5 of body length) and appear to be composed of 19–21 segments ( Figs 4A , 6B , 7A , 8C ). Segments of the furcal rami show well marked ornamentations in both DIC and SEM images, with well defined edges and covered at both sides – sometimes almost entirely – with short, straight or slightly bent up spines (bristles) measuring around 4–8 µm ( Figs 4A–C , 8D–E ). These ornamentations decrease in thickness and length towards the posterior end of the furca, which seems to be partially due to the decrease in the number and length of the spines (bristles). In fact, thickness, number and length of the spines on the ornamentation are variable among the specimens ( Figs 2A , 4A–C , 8D–E ). The inner portion of the first 10 th of the furca length, which precedes segmentation, is smooth, while the outer portion is covered by the aforementioned bristles.

VENTRAL CILIATURE. Composed of two longitudinal bands of locomotory cilia with approximately 5–8.5 µm of length, starting at U1, immediately below the mouth line, and ending at U71, right before the location of the Type 6 scales ( Figs 3C, I , 6B , 7C ). Ventral ciliary bands are separated by the ventral interciliary field covered by small spined scales ( Figs 3I , 6B ). Although cilia are not well visible in the pictures and were not very well oriented for measurements, due to the position and preservation state of the individuals, they are depicted in the illustrated schematics ( Fig. 6B ).

DORSAL SCALES. Most of the cuticular armature of the body is composed of simple spined scales arranged in approximately 42 longitudinal columns, each column bearing 55 scales, and approximately 64 horizontal, alternate rows, with around 10 scales per row ( Fig. 2A ). Dorsolateral scales covering anterior, middle and most of the posterior body surface are roughly polygonal in shape (most presenting pentagonal or hexagonal form), with an elevated anterior portion, a distal incision and bearing a single spine (Type 1 scales – Figs 3D , 4D , 5A , 6A–C , 7D–E ). These polygonal scales are the most common type found on the specimens, covering the majority of the body, both dorsolaterally and ventrally. They are outlined by five to six corners, with two distinctive elements ( Figs 5A , 6A–C , 7D–E ): (a) an elevated anterior portion formed by the spine’s curvature – as it arises from the scale’s surface at middle range, from semi triangular keels – and (b) a longitudinal concavity delimited by two sloping edges that extend from the spine’s insertion to the posterior end of the scale, often ending in a gap, where the two most distal corners meet forming a wide angle. SEM images ( Figs 7A, D–E , 8A–B ) reveal that these scales are considerably overlapped (overlapping usually hides their anterior half). Dorsal scales arranged in the first five transversal rows are rounded, very small and rather close to each other, measuring 3.5–5.5 µm (scale type unidentified) ( Fig. 3A ). From the fifth transversal row, dorsal scales become predominantly polygonal (Type 1 scales), reaching 8.5 µm at the pharygeointestinal junction (U20 – Fig. 3A–B, D ). At the middle trunk (from the pharingeointestinal junction to the proximities of furca base, at U73– U75) they range from 5.5 to 9 µm, while at the lateral sides they become slightly larger, measuring 6.5–11 µm ( Figs 3D, I , 4D ). All dorsal scales are mostly parallel to the transverse axis of the body, with the exception of one distinct elevated spineless scale immediately anterior to the furca base (U71), which is 5–7 µm long and shaped as a triangle with well-marked edges (Type 2 scale – Figs 3G , 4A–C , 5B, D–E , 6A, C ). Laterally and below the Type 2 scale, at U72, there is a pair of small (4.5 µm) rounded scales, one at each side of the caudal field, bearing long spines (16–21 µm) which arise directly from the scale’s most posterior end, instead of from the middle, as it normally occurs (Type 3 scales – Figs 3G , 4B , 5B–D , 6A, C ). Immediately following this region (at U 62 in the paratype photographed with SEM and between U 71–75 in the holotype), there is a rather noticeable spineless field covered by: (a) a patch lacking cuticular ornamentation (situated medially from the surroundings of the furca base until the uppermost portion of the adhesive tubes), and (b) a complex of small, flat, rounded or polygonal and spineless scales (U73–U75), 2.5–5 µm long, covering the initial portion of each furcal rami (Type 4 scales) ( Figs 3G , 4A , 5D–E , 6A , 8A ). Below Type 2 scales there is a pair of spineless, wide and rounded scales, 7.5 µm long, presenting small indentations at their posterior ends (Type 5 scales – Figs 5C , 6A, C , 8B ).

DORSAL SPINES. Dorsolateral spines covering the head and neck (U1–U20) are rather short, ranging from 2–13 µm. The first two rows of spines, disposed immediately around and below the cephalic plates are much shorter (2–7 µm) and slightly curved, while the remaining rows along the neck contain longer and straighter spines. Through the upper and middle trunk (U21–U44), the dorsal spines increase progressively in length, with an average length of 11.5 µm and 15.5 µm, respectively, reaching their largest dimensions at the rear trunk (U45–U51), with an average range of 25.5µm. At rear trunk, towards the furca base (U75), dorsolateral spines become much longer (18–33 µm) and straighter, grouping into two pairs pair of small, tight clusters at the lateral edges of the body surrounding the anterior-most portion of the furca base ( Figs 2A , 3G , 4A–B ). Most spines arise at a medial point on a scale’s surface, under a small ridge-shaped elevation that is formed by the junction of: (a) the spine’s anterior end, as it rises from the scale, and (b) the superposition between the scale’s anterior portion and the posterior ends of the two scales in the former row ( Figs 6A–C , 7E ).

VENTRAL INTERCILIARY SCALES. Are small in the head and neck region (3.5–5 µm) and increase in size towards the middle trunk (4.5 µm) and rear trunk (4–7 µm), similarly to the dorsal and lateral scales. At the posterior end of the ventral surface, the scales are rounded or polygonal and flat (unidentified type). Among these scales is situated a particular and very distinct pair of oval scales at the middle of the posterior interciliary ventral field (U71); each scale bears a keel that emerges from the anterior portion of the scale and extends across its length (Type 6 scales – Figs 5F , 6B–C ). In the holotype, the largest scale of the pair is 10 µm long, with a keel 6 µm long, while the smaller scale is 9.5 µm long, showing a 7 µm long keel. However, such asymmetrical pattern might not be ubiquitous; it was only reported for the holotype because it was not possible to visualize and measure these structures in the paratypes. The most distal extremity of the ventral furca base (U75) is covered by two columns of small (3.5–7 µm long) scales (Types 7 and 8 scales) bearing spines that are very long and straight, measuring from 11 up to 20 µm. These scales are elongated antero-posteriorly and laterally narrowed, showing a middle furrow and lateral projections (Type 7 scales), or rounded and antero-posteriorly narrowed (Type 8 scales) ( Figs 5F , 6B–C ).

VENTRAL INTERCILIARY SPINES. Are shorter than the dorsal spines. They range in size from 5.5–15 µm, and become longer and thicker from anterior to posterior region and from ventral to ventrolateral sides.

EGG. A single egg was present in the holotype ( Fig. 3E ); it was 30 µm wide and situated between the cuticle and the intestine, extending from the upper to rear trunk (U21–U51). No sperm and reproductive organs were observed.

Taxonomic remarks

Specimens of the genus Polymerurus , when compared with the remaining members of Chaetonotida , are easily recognized due to their large size (some species are the longest known chaetonotidans), the presence of ring-like ornamentations – usually called segmentations – on the furcal rami, and a cephalion with prominent lateral expansions. Although all species of Polymerurus share these characteristics, it is possible to recognize among them very distinct morphotypes, generally based on their cuticular ornamentation and body outline. Regarding the latest, it is possible to distinguish completely straight outlines with absent body constrictions, as in P. serraticaudus ( Voigt, 1901) from more tenpin-like shapes as occurs in P. rhomboides . Furthermore, the caudal portion varies in length, thickness and segmentation type (partial or complete). In what accounts for cuticular coverage, species may present spined scales that lack a peduncle (column-like base) or pedunculated scales (stalked scales). Polymerurus insularis sp. nov. bears spined scales that lack a peduncle, and in terms of cuticular coverage resembles six other species: P. nodicaudus , P. serraticaudus , P. entzii (Daday, 1882) , P. nodifurca (Marcolongo, 1910) , P. paraelongatus ( Grosso & Drahg, 1986) and P. ringueleti (Grosso, 1975) . However, the new species has a specific set of characteristics that distinguishes it from these congeners: (1) a cuticular armature composed of simple spined scales with polygonal shape (Type 1 scales), (2) a single, spineless dorsal scale with a triangular shape located terminally next to the furca base (Type 2 scale), (3) a spineless zone composed by a patch lacking cuticular ornamentation and flat, rounded or polygonal scales without spines (Type 4 scales) (4) particular sets of terminal spined or keeled scales located both dorsally and ventrally around the furca base (Types 3, 5, 6, 7 and 8 scales). In addition to these characteristics, the new species can be differentiated from P. nodicaudus and P. paraelongatus by the presence of a U-shaped furca instead of a V-shaped furca, and from P. entzii by the absence of long spines in each of the ring-like ornamentations (segments) of the furca rami. Additionally, the new species contrasts with P. nodifurca by long and straight spines instead of short, curved spines, as found in the aforementioned species. Finally, spined-scales are present along the dorsolateral, lateral and ventrolateral regions in Polymerurus insularis sp. nov., which distinguishes it from P. ringueleti ; while the long, narrow and segmented furcal rami of the new species are very different from the short, thick and unornamented furca present in P. serraticaudus .

Phylogenetic relationships

The final alignments of 18S rDNA and 28S rDNA yielded 1761 and 4141 positions, respectively, while the concatenated alignment had 5902 positions. The phylogenetic reconstruction based on a multigene approach supported the currently recognized monophyly of the genus Polymerurus with a very high bayesian (1–0.98) and bootstrap (99–98) support for the phylogenetic signal on both internal and external nodes of the Maximum Likelihood ( Fig. 9 ) and Consensus trees (Supp. file 2). Accordingly, the two specimens of Polymerurus insularis sp. nov. were grouped together and nested within Polymerurus , with branch supports of 1 and 99. As expected, P. nodicaudus was recovered as sister clade to P. insularis sp. nov., reflecting the morphological similarities between the two species, such as spined scales with no peduncles, in contrast with the pedunculated scales of P. rhomboides .

Figures

Fig. 2. Light microscopy – DIC. Polymerurus insularis sp. nov., holotype (ZUEC GCH 55). Full body view. A. Dorsal view. B. Internal view. C. Ventral view. Scale bars = 30 µm.

Fig. 3. Light microscopy – DIC. Polymerurus insularis sp. nov., holotype (ZUEC GCH 55). A–C. Correspond to the most anterior third of the specimen. A. Anterior dorsal region. B. Anterior internal region. C. anterior ventral region. D–F. Correspond to the trunk. D. Dorsal trunk. E. Internal trunk. F. Ventral trunk. G–I. Correspond to the posterior third of the specimen. G. Dorsal posterior third. H. Internal posterior third. I. Ventral posterior third. Abbreviations: ce = cephalion; ct = cephalic bristles; eg = egg; hy = hypostomium; i = intestine; is = interciliary spines; lc = locomotory cilia; lce = lateral cephalic expansions; mo = mouth; ne = nephridia; pl = pleurae; ph = pharynx; PhIJ = pharyngealintestinal junction; sc-1 = Type 1 scales; sc-2 = Type 2 scale; sc-3 = Type 3 scales; sc-4 = Type 4 scales; vs = Type 1 ventral scale. Scale bars = 20 µm.

Fig. 4. Light microscopy – DIC. Polymerurus insularis sp. nov. Posterior region of the body. A. Paratype (ZUEC GCH 56). B, D. Holotype (ZUEC GCH 55). C. Paratype (ZUEC GCH 57). A–B. Posterior dorsal view. C. Posterior dorsolateral view. D. Posterior ventral view. Abbreviations: fr = furcal rami; sc-1 = Type 1 scales; sc-2 = Type 2 scale; sc-3 = Type 3 scales; sc-4 = Type 4 scales. Scale bars = 40 µm.

Fig. 5. Close-up of the different types of scales described forPolymerurusinsularis sp. nov.A–C. Paratype (ZUEC GCH 59). D, F. Holotype (ZUEC GCH 55). E. Paratype (ZUEC GCH 56). A–C. Scanning electron microscopy. D–F. Light microscopy – DIC. A. Detail of a section of the dorsal middle trunk, showing the most common type of scale, Type 1, with emphasis on its characteristic shape. B. Detail of a section of the dorsal posterior trunk, showing Type 2 and Type 3 scales. C–E. Detail of the transition between the dorsal posterior trunk and the dorsal furca base, showing the particular scale covering of this region. F. Detail of the transition between the ventral posterior trunk and the ventral furca base, showing the particular scale covering this region. Abbreviations: sc-1 = Type 1 scales; sc-2 = Type 2 scale; sc-3 = Type 3 scales; sc-4 = Type 4 scales; sc-5 = Type 5 scales; sc-6 = Type 6 scales; sc-7 = Type 7 scales; sc-8 = Type 8 scales; svs = small ventral pair of spines. Scale bars: A–C = 5 µm; D = 10 µm; E = 15 µm; F = 10 µm.

Fig. 6. Schematic illustration of dorsal and ventral posterior regions and type scales of Polymerurus insularis sp. nov. A–B. Paratype (ZUEC GCH 56). A. Dorsal view of the posterior end. Some Type 1 scales are faded for a better visualization of Types 2, 3 and 5 scales. B. Ventral view of the posterior end. C. Each type of scale, individually depicted (not to scale). Abbreviations: ff = furcal furrow; fr = furcal rami; lc = locomotory cilia; is = interciliary spines; sc-1 = Type 1 scales; sc-2 = Type 2 scale; sc-3 = Type 3 scales; sc-4 = Type 4 scales; sc-5 = Type 5 scales; sc-6 = Type 6 scales; sc-7 = Type 7 scales; sc-8 = Type 8 scales. Scale bars = 40 µm.

Fig. 7. Scanning electron microscopy. Polymerurus insularis sp. nov. A–B, D–E. Paratype (ZUEC GCH 59). C. Paratype (ZUEC GCH 60). A. Dorsal view of the body. B. Dorsolateral view of the head, showing the cephalion and its lateral projections. C. Lateral view of the head, highlighting the lateral pleurae. D–E. Dorsal trunk view, highlighting scale Type 1. Abbreviations: ce = cephalion; fr = furcal rami; lc = locomotory cilia; lce = lateral cephalic expansions; pl = pleurae; sc-1 = Type 1 scales. Scale bars: A = 40 µm; B–C = 5 µm; D = 20 µm; E = 10 µm.

Fig. 8. Scanning electron microscopy. Polymerurus insularis sp. nov. A–B, D. Paratype (ZUEC GCH 59). C, E. Paratype (ZUEC GCH 60). A–B. Posterior dorsal view. C. Detail of the furca base in posterior dorsolateral view. D–E. Details of the furcal rami. Abbreviations: fb = furca base; fr = furcal rami; sc-4 = Type 4 scales; sc-5 = Type 5 scales. Scale bars = 10 µm.

Fig. 9. Maximum Likelihood tree based on multigene approach with 18S and 28S sequences. Highlighted branches correspond to the Polymerurus Remane, 1927 species sequences. Values on the branches correspond, respectively, to: SH-aLRT support (%) / aBayes support / ultrafast bootstrap support (%).

Tables

Table 2 (continued on next page). Morphometric features of Polymerurus insularis sp. nov. All scale lengths for which the scale type is not specified are from Type 1 scales. Abbrevation: N = total number of measured adult specimens.

Features	Range (µm)	Mean (µm)	N
Total body length	215–408	322	5
Body length (furcal rami excluded)	139–308	222	5
Furcal rami length	91.5–94.5	93	2
Furcal rami + adhesive tubes length	109–117	113	2
Adhesive tubes length	16–23	19.5	2
Furca base width	26.5–27.5	27	1
Number of furcal rami segments per side	19–21	20	1
Length of ornamentation bristles on the furcal rami segments	4–8	6	3
Body width at head	24–42	32	4
Body width at medium length	31–55	40	4
Body width at furca base	23–25	24	4
Diameter of mouth ring	8–13	10	3
Number of cephalic lobes	3	3	1
Number of head cilia tufts	4	4	3
Length of first cilia tuft	9–12	10.5	1
Length of second cilia tuft	11–19.5	14	1
Cephalion length	19–25	23	3
Cephalion width	20–36	28	2
Lateral projections (lappets) length	10–18	14	3
Pleurae length	12–15	14	3
Hypostomium length	13–17	15	2
Pharynx length	74	74	1
Pharynx width at upper length	19	19	1
Pharynx width at medium length	25	25	1
Pharynx width at pharingeointestinal junction	27	27	1
Egg width	30	30	1
Protonephridia length	105	105	1
Length of locomotory cilia	5–8.5	7	1
Total number of longitudinal columns of scales	42	42	1
Number of scales in a single longitudinal column	55	55	1
Total number of horizontal rows of scales	64	64	1