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ZOOTAXA
ISSN 1175-5326 (print edition)
ISSN 1175-5334 (online edition)
Accepted by B. Neuhaus: 26 Apr. 2023; published: 15 May 2023 149
Zootaxa 5285 (1): 149–160
https://www.mapress.com/zt/
Copyright © 2023 Magnolia Press Article
https://doi.org/10.11646/zootaxa.5285.1.6
http://zoobank.org/urn:lsid:zoobank.org:pub:8C0B55F1-9478-454C-BF45-2560C1C1E17D
Mixtophyes toothlessi sp. n., a new Kinorhyncha species
(Anomoirhaga: Neocentrophyidae) from the Clarion-Clipperton Zone,
with a revision of terminology and taxonomic features of the family
ALBERTO GONZÁLEZ-CASARRUBIOS1,2 *, PEDRO MARTÍNEZ ARBIZU4 & NURIA SÁNCHEZ1,3
1Complutense University of Madrid (UCM), Faculty of Biology, Department of Biodiversity, Ecology and Evolution (BEE). Madrid,
Spain
2
�
albert23@ucm.es; https://orcid.org/0000-0003-1975-6900
3
�
nurisanc@ucm.es; https://orcid.org/0000-0003-4908-8755
4German Centre for Marine Biodiversity Research, Senckenberg am Meer, Wilhelmshaven, Germany.
�
pedro.martinez@senckenberg.de; https://orcid.org/0000-0002-0891-1154
*Corresponding author
Abstract
A new species of Mixtophyes Sánchez et al. 2014 is described from the Clarion-Clipperton Fracture Zone (north-eastern
Pacific Ocean). The new species is characterised by the presence of spinose processes on segments 1–11 in middorsal
position and on segments 1–10 in midlateral position, paired paradorsal setae on segments 2, 4–6 and 8–9, and unpaired
ones on segments 3 and 7, paired setae in paralateral and ventrolateral positions on segments 2–9. In addition, some
morphological characteristics of the family Neocentrophyidae and the genus Mixtophyes are discussed, and the diagnoses
of both taxa are updated. Lastly, the taxonomic nomenclature of Neocentrophyidae and the sexual characters of the genus
are accordingly emended.
Key words: mud dragon, meiofauna, polymetallic nodules, diversity, deep sea; taxonomy
Introduction
The latest research carried out at polymetallic nodule areas of the Clarion-Clipperton Fracture Zone (CCZ), in the
north-eastern Pacific Ocean, revealed a moderately diverse kinorhynch community (Sánchez et al. 2022). A total
of 16 kinorhynch species and 14 amplicon sequence variants (ASVs) were identified by morphological studies and
metabarcoding analyses respectively. Among the contract areas of the CCZ, the one studied by the BGR (German
Federal Institute for Geoscience and Natural Resources) is the most explored and diverse to date, with representatives
of 12 of the referred species.
The four missing species in the BGR area seem to be scarce across the CCZ, represented by only one or two
records. One of these species, namely Mixtophyes sp. 1, was collected from the UKSR I contract area (UK Seabed
Resources Ltd.) (Sánchez et al. 2019, 2022). Mixtophyes Sánchez et al., 2014 is a monospecific genus described
from the abyssal plain in the eastern Atlantic Ocean, which is easily distinguished from the other Neocentrophyidae
genus by a single, undifferentiated sternal plate on segment 1 and the presence of lateral terminal spines (Sánchez
et al. 2014). Since Mixtophyes abyssalis Sánchez et al., 2014 was described, only three specimens of the genus
have been found; two specimens of Mixtophyes sp. 1, which is described in the present contribution, and a single
additional specimen found at a depth of over 2,000 metres in the southern Gulf of Mexico (Landers et al. 2022),
which remains undescribed.
The aim of the present contribution is to describe a new species of Mixtophyes in order to increase our
knowledge in the diversity and morphological features of one of the genera with the least number of species. In
addition, some diagnostic characters of the family Neocentrophyidae and the genus Mixtophyes are discussed,
and the diagnoses of both taxa are emended. Also, the specific terminology for taxonomic and sexual dimorphic
GONZÁLEZ-CASARRUBIOS ET AL.
150 · Zootaxa 5285 (1) © 2023 Magnolia Press
characters of Neocentrophyidae is updated. Finally, the procedure for taking measurements of some traits in the
family is clarified and specified.
Material and methods
Abbysal sediment samples in which Mixtophyes specimens were present (samples 16 and 18) were collected from
the UKSR I contract area of the CCZ (UK Seabed Resources Ltd.) using a multicorer (each core of 9.4 cm of inner
diameter, 69 cm2 of total surface area) during the ABYSSLINE II cruise (TN319; R/V Thomas G. Thompson)
(Smith and Shipboard Scientific Party 2013) (Fig. 1). The upper 5 cm of sediment of each core were fixed on board
with 4 % buffered formalin for morphological examination. Then, meiofauna was extracted from the sediment
following the colloidal silica polymer Levasil protocol (Neuhaus & Blasche 2006). Subsequently, meiofauna was
sorted under a stereomicroscope and kinorhynchs were preserved in 70 % ethanol.
FIGURE 1. Map showing the collection points of the new species. A. General map of the Clarion Clippertone-Zone location. B.
Map of the CCZ, with the UKSR area marked in purple. C: Close-up of the sampling area; white dots mark the sampling points.
The colour gradient bar shows the water depth (m) under the medium sea level.
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Kinorhynchs for light microscopical (LM) observation were gradually transferred to glycerine, kept overnight
in 100 % glycerine and then mounted in Fluoromount-G® on glass slides. The specimen of the new species was
studied and photographed under an Olympus BX51 microscope with differential interference contrast (DIC) optics
and an Olympus DP70 camera. For scanning electron microscopy (SEM), specimen was transferred through a
graded series of ethanol until 100 % ethanol, chemically dried using hexamethyldisilazane (HMDS) through a
HMDS-ethanol series, mounted on a stub, coated with gold, and examined with a JEOL Ltd. JSM-6335F (CNME,
Complutense University of Madrid, Spain).
Identification to genus level was done following the Sørensen & Pardos (2020) keys. Taxonomic measurements
were taken following González-Casarrubios et al. (2023) (see “Segment measurements in Neocentrophyidae”
section in the discussion), using cellSens software. The spreadsheet with the most important measurements can be
found online in the Kinorhyncha Measurement Database (González-Casarrubios & Yamasaki 2022).
Line art illustration and image plate composition were performed with Adobe Photoshop and Illustrator CC-
2014 software. The bathymetric data and the outline of the contractor area were extracted from GEBCO (2014)
and International Seabed Authority (2020), respectively. The sampling points were generated with R (R Core Team
2019) using the package raster (Hijmans 2022). Type material of the new species is deposited in the collection
“Vermes” of the Museum für Naturkunde Berlin (MfN), Berlin, Germany.
Results
Phylum Kinorhyncha Reinhard, 1885.
Order Allomalorhagida Sørensen et al., 2015.
Class Anomoirhaga Herranz et al., 2022.
Family Neocentrophyidae Higgins, 1969.
Emended diagnosis
Neocentrophyidae, emended diagnosis.
Allomalorhagida with sternal plate on first trunk segment undivided or with partial divisions at the anteriormost
region of the plate; remaining trunk segments with one tergal and two sternal plates, except for the terminal segment,
consisting of a tergal and a single sternal plate. Articulated rigid lateral terminal spines on segment 11 present or
absent; non-articulated middorsal spinose processes on segments 1–9 in both sexes; middorsal spine on the last
two segments present or absent, commonly flexible and soft in males, otherwise with middorsal spinose processes;
non-articulated midlateral spinose processes on segments 1–9; processes may be absent or present on segments
10 and 11; long and flexible penile spines of males on segments 10 and 11 present or absent; pachycycli, peg and
socket joints, and apodemes absent or not well developed; seven placids: four dorsal and three ventral; fourteen
trichoscalids (7 dorsal and 7 ventral) present, but trichoscalid plates absent; four short, thin, non-articulated outer
oral styles and five longer articulated ones, composed of two units.
Genus Mixtophyes Sánchez et al., 2014
Mixtophyes, emended diagnosis.
Neocentrophyidae with a single sternal plate on the first trunk segment, without subdivision in its anterior
region; pairs of rigid lateral terminal spines on segment 11 in both sexes; middorsal spines, when present, flexible
and soft in appearance.
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Description of the new species
Mixtophyes toothlessi sp. n.
urn:lsid:zoobank.org:act:2555D525-A0A5-45B0-90FE-D58228184EE0
Synonymy. Mixtophyes sp. 1 (in Sánchez et al. 2019, 2022)
Material examined. Holotype, adult male, collected on 07/03/2015 at the Clarion-Clipperton Fracture Zone
(central-eastern Pacific Ocean), UKSR I contract area: 12°25.196′ N, 116°37.474′ W at 4137 m depth; mounted in
Fluoromount G®, deposited at MfN under accession number: ZMB ZMB 12822.
Paratype, adult of indeterminate sex, collected on 09/03/2015 at the CCZ, UKSR I contract area: 12°35.813′ N
116°29.610′ W at 4255 m depth; mounted for SEM deposited at the MfN under accession number: ZMB ZMB 12823.
Etymology. The species is named after Toothless, The Night Fury, the dragon of the novel series by Cressida
Cowell “How to train your dragon” and the film with the same title from DreamWorks Animation, 2010. Several
Kinorhyncha species have been described after famous dragons. The new species is reminiscent of Toothless for
its middorsal spinose processes, similar to the back protuberances of the dragon, and for having one of the lateral
terminal spines broken, like the tail in the animated character.
Diagnosis. Mixtophyes with spinose processes on segments 1–11 in middorsal position and on segments 1–10 in
midlateral position, slightly increasing in length towards the posterior segments; paired setae in paradorsal position
on segments 2, 4–6 and 8–9, and unpaired ones on segments 3 and 7. Paired setae in paralateral and ventrolateral
positions on segments 2–9. Cuticular ornamentation as a reticulate pattern of net-like ridges present at the anterior
margin of segment 1, both dorsally and ventrally.
Description. Head. With retractable mouth cone and introvert. The specimens have both structures retracted
inside the trunk, hence details on the morphology and distribution of oral styles and scalids cannot be provided.
Neck. Four dorsal and three ventral placids. Dorsal placids equal in size, rectangular, robust and concave (ca. 30–34
µm wide and 18–20 µm high); ventral ones with different sizes: midventral placid robust, broad (ca. 53 µm wide
and 14 µm high) lateroventral placids wider, weakly sclerotized (ca. 18–19 µm wide and 7–8 µm high) (Figs. 2A–B,
4C–D).
Trunk. With 11 segments. Segments 1 and 11 consisting of one tergal and one sternal plate; segments 2–9 with
one tergal and two sternal plates (Figs. 2A–B, 3A–H). Tergal plates slightly bulging middorsally, giving the trunk
a triangular shape in cross-section. Maximum width at segment 6, progressively tapering at the last trunk segments
but almost constant in width (Figs. 2A–D, 3A–H, 4A). Segments 1–9 with middorsal spinose processes slightly
increasing in length towards the posterior segments, surpassing the margin of the segment (Figs. 2A, 3A, C, E, G,
4B, 5A, C, E, H). Hairy midlateral spinose processes present on segments 1–10 (Figs. 2A–B, 3B, D, F, H, 4E–F,
5E–F). Spinose processes with marked notches at the free flap on both sides. No paradorsal intracuticular atria were
detected. Minute, dot-shaped glandular cell outlets (cuticular pores in Sánchez et al. 2014) as rounded perforations
in several positions throughout the cuticle on segments 1–11 (Figs. 2A–B, 5G). These glandular cells are usually
distributed on segments 2–10 as one or two dorsal pores close to the base of the middorsal spinose processes, one
or two pores in the middle region near the free flap, up to three pores around the dorsal muscular scars and up
to three pores close to the paralateral setae, located anterior and posterior to them. Ventrally, these structures are
distributed as two to four pores around the muscular scars and the middle region of the plates, plus two to three
pores surrounding the most lateral sensory spots and one to two pores located over the ventrolateral setae. Segment
1 with a high number of pores distributed over the entire surface. Segment 11 with only two subdorsal pairs of
pores and two pairs in the ventrolateral region, all of them near the free flap. Scale-like cuticular hairs as distributed
across the trunk on segments 1–11, absent at the muscular scar areas. Muscular scars oval to bolus-shaped, present
in laterodorsal and ventromedial positions on segments 1–10 (Fig. 2A–B). Except on segment 1, the ventromedial
muscular scars are close to the paraventral position. Longitudinal cuticular ridges in the anteriormost region of the
plates on segments 2–11, less conspicuous in segment 2, across all over the segment. Pachycycli and ball and socket
joints weakly developed. Primary pectinate fringe simple, with several short, pointed tips. Secondary pectinate
fringe and apodemes absent. Holotype male with sperm in gonads until segment 7.
Segment 1. Anterolateral margins of the tergal plate with horn-like extensions (Figs. 2A–B, 3B, 5A–B). Anterior
edge of both tergal and sternal plates denticulated, with a notorious reticulate pattern of cuticular wrinkles forming
a net-like ornamentation across the anterior margin of the plates (Figs. 2A–B, 3A–B, 5A–B). Spinose processes
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in middorsal and midlateral positions, surpassing the posterior margin of the segment (Figs. 2A–B, 3A–B, 5E).
Spinose processes of this and following segments consist of an elongate base ending in a flexible, acicular terminal
tip, and with a longitudinal band of minute cuticular hairs in the basal part of the structure. Type 1 sensory spots
in subdorsal (two pairs), laterodorsal, paralateral and ventromedial (two pairs) positions. Sensory spots of this and
following segments formed by numerous small papillae around a central pore or two pores (Figs. 4G, 5D). No cilia
were observed emerging from the pores.
TABLE 1. Measurements of male holotype (ZMB 12822) of Mixtophyes toothlessi sp. n. Abbreviations: CL, cumulative
length; LTS, lateral terminal spine; MDS, middorsal spine; MSW, maximum sternal width; S, segment; SW, standard
sternal width; TL, total trunk length. Numbers after abbreviations indicate the respective segment.
Character Holotype
TL (µm) 778
MSW-6 (µm) 170
MSW-6/TL (%) 21,81
SW-10 (µm) -
SW-10/TL (%) -
S1 (µm) 128
S2 (µm) 64
S3 (µm) 62
S4 (µm) 73
S5 (µm) 78
S6 (µm) 79
S7 (µm) 87
S8 (µm) 88
S9 (µm) 83
S10 (µm) 68
S11 (µm) 57
MDS10 (µm) 116
MDS11 (µm) -
LTS (µm) 483
LTS/TL (%) 62,10
CL (µm) 867
CL/TL (%) 111,52
Segment 2. Spinose processes in middorsal and midlateral positions. Paired setae in paradorsal, paralateral and
ventrolateral positions. Sensory spots in paradorsal, subdorsal (two pairs in the holotype, one pair in the paratype),
laterodorsal (two pairs) and ventromedial (two pairs) positions (Figs. 2A–B, 3A–B, 5E).
Segment 3. Spinose processes in middorsal and midlateral positions. Unpaired, paradorsal seta and paired
setae in paralateral and ventrolateral positions. Sensory spots in paradorsal, subdorsal (two pairs), laterodorsal and
ventromedial (two pairs) positions (Figs. 2A–B, 3C–D, 5E).
Segment 4. Spinose processes in middorsal and midlateral positions. Paired setae in paradorsal, paralateral and
ventrolateral positions. Sensory spots in paradorsal, subdorsal (two pairs), laterodorsal and ventromedial (two pairs)
positions (Figs. 2A–B, 3C–D, 4B).
Segment 5. Spinose processes in middorsal and midlateral positions. Paired setae in paradorsal, paralateral and
ventrolateral positions. Sensory spots in paradorsal, subdorsal (two pairs), laterodorsal and ventromedial (two pairs)
positions (Figs. 2A–B, 3C–D).
Segment 6. Spinose processes in middorsal and midlateral positions. Paired setae in paradorsal, paralateral and
ventrolateral positions. Sensory spots in subdorsal (two pairs), laterodorsal and ventromedial (two pairs) positions
(Figs. 2A–B, 3E–F). Paradorsal sensory spots could not be confirmed.
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FIGURE 2. Line art illustrations of adults of Mixtophyes thoothlessi sp. n. A. Male, ventral view. B. Male, dorsal view. C. Dorsal
overview showing the LTS length. Abbreviations: co, cuticular ornamentation; cr, cuticular ridge; gco, glandular cell outlet; mlp,
midlateral spinose process; ldss, laterodorsal sensory spot; lts, lateral terminal spine; mdp, middorsal process; mlp, midlateral
process; ms, muscular scar; msp, minute spine; mtp, midterminal process; pdse, paradorsal seta; pdss, paradorsal sensory spot;
plse, paralateral seta; sdss, subdorsal sensory spot; ss3, type 3 sensory spot; tsj, tergosternal junction; vlse, ventrolateral seta;
vmss, ventromedial sensory spot. Scalebar: 100 μm.
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FIGURE 3. Light micrographs of male holotype of Mixtophyes toothlessi sp. n. showing the main trunk characters. A. Dorsal
view of segments 1–2. B. Ventral view of segments 1–2. C. Dorsal view of segments 3–5. D. Ventral view of segments 3–5. E.
Dorsal view of segments 6–8. F. Ventral view of segments 6–8. G. Dorsal view of segments 9–11. H. Ventral view of segments
9–11. Abbreviations: lts, lateral terminal spine; mdp, middorsal spinose process; mlp, midlateral spinose process; msp, minute
spine; pdse, paradorsal seta; plse, paralateral seta; ss, sensory spot; ss3, type 3 sensory spot; vlse, ventrolateral seta. Numbers
after abbreviations indicate the corresponding segment. Sensory spots marked as dashed circles. Scalebars: A–H: 20 μm.
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Segment 7. Spinose processes in middorsal and midlateral positions. Unpaired, paradorsal seta and paired
setae in paralateral and ventrolateral positions. Sensory spots in paradorsal, subdorsal (two pairs), laterodorsal and
ventromedial (two pairs) positions (Figs. 2A–B, 3E–F).
Segment 8. Spinose processes in middorsal and midlateral positions. Paired setae in paradorsal, paralateral and
ventrolateral positions. Sensory spots in paradorsal, subdorsal (two pairs), laterodorsal and ventromedial (two pairs)
positions (Figs. 2A–B, 3E–F, 4E–F, 5C, H).
Segment 9. Spinose processes in middorsal and midlateral positions. Paired setae in paradorsal, paralateral and
ventrolateral positions. Sensory spots in paradorsal, subdorsal (two pairs), laterodorsal and ventromedial (two pairs)
position (Figs. 2A–B, 3G–H, 4E–F, 5C, H).
FIGURE 4. Light micrographs of male holotype of Mixtophyes toothlessi sp. n. showing trunk overviews and detailed trunk
characters. A. Dorsal overview. B. Detail of dorsal spinose process and setae of segment 4. C. Detail on dorsal placids. D.
Detail on ventral placis. E. Detail of lateral position of segment 8 showing the paralateral seta. F. Detail of lateral position of
segment 8 showing paralateral and ventrolateral setae. G. Close-up of a sensory spot and a glandular cell outlet of segment 3.
H. Detail of the middorsal spinose process of segment 10. I. Detailed dorsal view of segment 11. J. Detail on the midterminal
process of segment 11. Abbreviations: gco, glandular cell outlet; lts, lateral terminal spine; mdp, middorsal spinose process;
mds, middorsal spine; mlp, midlateral spinose process; mtp, midterminal process; pdse, paradorsal seta; plse, paralateral seta; ss,
sensory spot; ss3, type 3 sensory spot; vlse, ventrolateral seta. Numbers after abbreviations indicate the corresponding segment.
Sensory spots marked as dashed circles. Scalebars: A: 100 μm; C–D: 20 μm; B, E–J: 10 μm.
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FIGURE 5. Scanning electron micrographs of Scanning electron micrographs of adult paratype of paratype of Mixtophyes
toothlessi sp. n. A. Lateral overview. B. Detail on the anterior region of segment 1. C. Detail of the middorsal spinose process of
segment 9. D. Detail of a sensory spot. E. View of the right half of the tergal plates of segments 1–3. F. Dorsal view of segments
10–11. G. Glandular cell outlet. H. View of the right half of the tergal plates of segments 10–11. I. Putative bacterial epibionts. J.
Type 3 sensory spots of segment 11. Abbreviations: lts, lateral terminal spines; mdp, middorsal spinose process; mlp, midlateral
spinose process. Numbers after abbreviations indicate the corresponding segment. Sensory spots marked as dashed circles.
Scalebars: A: 100 μm; B–C, E–F, H: 10 μm; D, G, I–J: 1 μm.
Segment 10. Middorsal spinose process whose base ends in the middle region of the plate. This structure has
a pointed, thin free region, more flexible than the ones on the preceding segments Spinose processes in midlateral
position, surpassing the margin of the following segment. Two pairs of sensory spots in subdorsal and ventromedial
positions. Straight posterior margin of the segment, without notches in the middle region of the tergal plate (Figs.
2A–B, D, 3G–H, 4H, 5F, H).
Segment 11. Middorsal spinose process emerging from the posterior region of the segment. Midlateral spinose
processes absent. Small, midterminal process and minute, conical spines present in ventrolateral position, both
structures only confirmed in the LM specimen (Figs. 2A–B, 3H, 4J). Paired sensory spots in ventromedial position;
type 3 sensory spots present in subdorsal and ventromedial positions (Figs. 2A, 5J). Posterior margin of the tergal
plate with subdorsal and laterodorsal notches together with a pair of extensions over the lateral terminal spines.
Sternal plates with straight posterior margin and a pair of extensions over the lateral terminal spines (Figs. 2A–B,
3G–H, 4I, 5F, H). Long, slender lateral terminal spines (LTS:TL: 62 %) (Fig. 2C). The SEM specimen showed some
elongated, rounded structures on the sternal plates of segments 10 and 11 (Fig. 5I). These structures could be an
artefact of the fixative or a result of the critical point, being a cluster of crystals, but the option that they could be
small outcrops of epibiont bacilli or even some kind of fungus cannot be dismissed.
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TABLE 2. Summary of nature and arrangement of spinose processes, spines, setae and sensory spots in Mixtophyes
toothlessi sp. n. Abbreviations: cp, cuticular spinose process; LD, laterodorsal; lts, lateral terminal spine; LV, lateroventral;
MD, middorsal; ML, midlateral; msp, minute spine; mtp, midterminal process; PD, paradorsal; PL, paralateral; SD,
subdorsal; se, seta; sp, acicular spine; ss, sensory spot; VL, ventrolateral; VM, ventromedial. * Marks unpaired structures.
♂ male sexually dimorphic condition of a character.
Segment MD PD SD LD PL ML LV VL VM MV
1 cp* ss, ss ss ss cp ss, ss
2 cp* se, ss ss, ss (♂) ss, ss se cp se ss, ss
3 cp* se*, ss ss, ss ss se cp se ss, ss
4 cp* se, ss ss, ss ss se cp se ss, ss
5 cp* se ss, ss ss se cp se ss, ss
6 cp* se ss, ss ss se cp se ss, ss
7 cp* se*, ss ss, ss ss se cp se ss, ss
8 cp* se, ss ss, ss ss se cp se ss, ss
9 cp* se, ss ss, ss ss se cp se ss, ss
10 cp* ss, ss cp ss, ss
11 cp* ss3 lts msp ss, ss3 mtp
Discussion
Remarks on Mixtophyes toothlessi sp. n.
The new species described in the present contribution belongs to Mixtophyes. Mixtophyes toothlessi sp. n.
possesses a single, anteriorly undivided ventral plate on segment 1, segment 11 with a single, sternal plate and a
pair of lateral terminal spines. The absence of middorsal spines on segments 10 and 11 in the new species did not
fit with the original diagnosis of the genus and raises issues regarding secondary sexual characters within the group.
Nevertheless, the authors consider the plate configuration of segment 1 more relevant to phylogenetic history of
the family (Sánchez et al. 2014) than other traits such as the spine pattern; therefore, the genus diagnosis has been
emended (see section above).
Until now, Mixtophyes was a monospecific genus, with M. abyssalis as the single representative. However,
Mixtophyes toothlessi sp. n. is easily distinguished from M. abyssalis by its pattern of setae. The new species
has paradorsal setae on segments 2–9, whereas M. abyssalis presents paradorsal setae only on segments 7 and 9
(Sánchez et al. 2014). In the lateral series, M. abyssalis only possesses paralateral setae on segments 2 and 7–9 and
ventrolateral setae on segment 9. Mixtophyes toothlessi sp. n., on the other hand, bears paralateral and ventrolateral
setae on segments 2–9.
Secondary sexual characters in Mixtophyes and Neocentrophyidae
The lack of middorsal spines on the posterior segments of Mixtophyes toothlessi sp. n. male prevents a proper
discrimination of the sex in the specimens mounted for SEM. The two Mixtophyes investigated herein only differ
in the presence of an extra pair of subdorsal sensory spots in the LM specimen (holotype), which is clearly absent
in the SEM specimen (paratype). Sensory spots of its single congener show a sexually dimorphic distribution,
with two subdorsal pairs on segments 6 an 8 in females whereas males have a single pair (Sánchez et al. 2014).
Even though the differences observed between the two specimens of Mixtophyes toothlessi sp. n. can be due to
intraspecific variability, the possibility of a sexual dimorphism cannot be discarded. If so, the paratype would be a
female representative of the new species.
When the genus Mixtophyes was erected, the dimorphic midlateral features of segment 10 present only in males
of M. abyssalis were described as penile spines. However, if the paratype of M. toothlessi sp. n. is a female, a similar,
large and flexible structure is present on the same segment region. The mouldable apparency and the location of
the feature in males of M. abyssalis could have led to a misinterpretation, since the remaining neocentrophyids
have penile spines on the same region of segment 10. Therefore, the presence of such structure would not be an
intrinsically sexual character (like a penile spine) for the genus, but a sexually dimorphic feature of some species.
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Finally, the original Mixtophyes diagnosis referred that both sexes have middorsal spine on segment 11 whereas
sexual dimorphism exists on segment 10. Females bear non-articulated middorsal spinose process on this segment, but
males always have an articulated, flexible and soft middorsal spine. However, these traits are not observed in the new
species since the male holotype does not have any middorsal spine, neither in segment 10 nor in segment 11. Similarly,
the paratype specimen lacks a middorsal spine on the last segment (the pointed structure is not articulated), a condition
that should be present irrespective to the sex of the specimen. However, based on these results and the revision of
additional specimens of the family Neocentrophyidae (Neuhaus pers. comm.), it is possible that the description of
rigid, middorsal spines in Mixtophyes and in other genera of Neocentrophyidae may be due to misinterpretation.
Firstly, the appearance in LM of the middorsal structures on the most posterior segments highly depends on the
pressure exerted in the mounting of the specimens and on the developmental degree of the cuticular feature itself. In
the case of a middorsal process, the more a specimen is pressed dorsoventrally in the mounting, the more likely will
the process be pressed equally to both sides; and the stronger the basis of the process is developed anteriorly on a
segment as a keel, the more likely will the process fall down to one side of the segment during mounting. The basis
of the middorsal structure on segment 10 is less developed than on previous segments in many neocentrophyids,
including females of Mixtophyes abyssalis, and Paracentrophyes praedictus (Neuhaus pers. comm.), showing a broad
basis in dorsoventral LM mounts of specimens. The apparently articulated base could be an artifact originated from
the flattening of the process at the transition point between the keel fastening region and the free part, which becomes
posteriorly thinner than in the previous segments giving a spine-like appearance (see fig. 6E in Sánchez et al. 2014;
figs 6G, 7A, C, E, 10G, and 11E in in Sørensen et al. 2010; figs. 4H, 7C and 8A, F, H in Sørensen and Landers 2017).
Consequently, this structure in all females of the family would be a middorsal spinose process and not a spine.
Secondly, regarding segment 11, the spiny nature of the middorsal flexible and elongated whip-like structures
in most males of the family (except for M. toothlessi sp. n.) is relatively clear. Middorsal structure on segment 11 in
the female paratype RH37.88/USNM1207916 of Neocentrophyes satyai Higgins, 1969 seems to be articulated at its
base (see fig. 3N in Sánchez et al. 2014). Nevertheless, the revision of the other paratype RH37.27/USNM1207917
of the same species seems to indicate that this structure may not be articulated, being a spinose process rather than
a spine. The remaining neocenthrophyid females and M. toothlessi sp. n. bears a pointed, rigid structure on segment
11 but they are not basally articulated and hence they cannot be named as spines according to the definition of this
cuticular appendage.
Terminology of taxonomic characters in Neocentrophyidae
The nomenclature followed in the taxonomy of the family Neocentrophyidae is the one adopted by Higgins
(1969), when the family was erected. Since then, the term “perispinal seta” has been used in the species descriptions of
this family to name the setae located adjacent to (usually in the position immediately following) a spinous process.
This terminology is somewhat informative of the location with regard to other cuticular structure, a spine.
However, it is redundant and inaccurately at the same time. On one hand, Kinorhyncha descriptions follow
standardised positions; if a specific species is described with, for example, middorsal processes, it is not necessary
to report that the paradorsal seta is “perispinal”, because the terminology used for both structures already inform
about it. For this reason, we encourage to avoid the use of this terminology in future contributions of the family and
provide instead the position of the cuticular structure.
Segment measurements in Neocentrophyidae
Recently, González-Casarrubios et al. (2023) standardised the measurements in the phylum. According to this,
the segment length should be taken in the midlateral position, but the morphology of the family Neocentrophyidae
hampers to take appropriate measurements because of the presence of spinous processes in that position. Thus,
we propose that the measurements of the segment length should be taken in the paralateral position for the family
Neocentrophyidae, i.e. adjacent to the midlateral position within the tergal plate. In this way, the end of the pectinate
fringe could be undoubtedly identified as in the remaining kinorhynch families.
Acknowledgements
The authors want to acknowledge UK Seabed Resources for the invitation to the Abyssline program. The authors want
to thank the staff of the Senckenberg Research Institute, Deutsches Zentrum für Marine Biodiversitätsforschung,
GONZÁLEZ-CASARRUBIOS ET AL.
160 · Zootaxa 5285 (1) © 2023 Magnolia Press
Senckenberg am Meer for sorting the meiofauna samples, and all the participants and the staff involved in the
ABYSSLINE II cruise (TN319; R/V Thomas G. Thompson). We also thank Katja Uhlenkott, who helped to create
the map. Finally, the authors would like to thank the two anonymous reviewers and the editor Dr. Birger Neuhaus,
whose comments improved the first version of the manuscript.
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