New findings and two new species of the genus Colossendeis Jarzynsky, 1870 (Pycnogonida, Colossendeidae), with a morphometric analysis for species from the Southern Ocean

In this paper, specimens belonging to the pycnogonid genus Colossendeis Jarzynsky, 1870 collected during the Antarctic Polarstern XXIII/8 cruise (November 23, 2006, to January 30, 2007) were studied. In this collection, the following thirteen species have been identified, two of them being proposed as new species: Colossendeis frigida Hodgson, 1907, C. megalonyx Hoek, 1881, C. wilsoni Calman, 1915, C. engirmatica Turpaeva, 1974, C. arundorostris Fry & Hedgpeth, 1969, C. scotti Calman, 1915, C. lilliei Calman, 1915, C. drakei Calman, 1915, C. glacialis Hodgson, 1907, C. australis Hodgson, 1907, C. tortipalpis Gordon, 1932, C. pseudofrigida sp. nov., and C. bamberi sp. nov. All species are diagnosed and their taxonomic status discussed. C. frigida, C. engirmatica, and C. arundorostris extend their known depth range. C. arundorostris has been found for the first time outside the Ross Sea. The finding of C. engirmatica represents the first record after its original description. The new species are described, illustrated, and compared with their closest congeners. Finally, we carried out a cluster analysis (Bray-Curtis similarity index) based on a data-matrix of 49 Antarctic and Sub-Antarctic Colossendeis species and 26 morphological characters.


Introduction
The genus Colossendeis Jarynsky, 1870 has a relatively high number of species in Antarctic and Sub-Antarctic waters. Munilla and Soler-Membrives (2009) reported 37 species for this area, but the current number of Colossendeis species has to be regarded with caution. Most of these species have been described using a very small (if not a single) number of specimens and solely based on morphological characters. In this genus, there are several cases in which no clear species delimitations have been defined, mainly due to the lack of information about the variability of key morphological characters and the existence of similar morphological species. This situation has generated several controversial species and synonyms, such as C. robusta Hoek, 1881. This is the case of C. lilliei Calman, 1915, which has been considered a synonym of C. robusta by Child (1995), Munilla andBamber et al. (2022). However, this last synonym is contested by other authors (e.g. Cano-Sánchez and López-González 2007;Turpaeva and Rajsky 2013). Indirectly, Krabbe et al. (2010) corroborated the latter interpretation in their study on the phylogenetic position of C. megalonyx Hoek, 1881 within the genus Colossendeis. Another controversial taxonomic status is that of C. glacialis Hodgson, 1907 considered a junior synonym of C. robusta by some authors (Fry and Hedgpeth 1969;Child 1995) but considered a separate species by others (Pushkin 1993;Stiboy-Risch 1993;Cano-Sánchez and López-González 2007;Munilla and Soler-Membrives 2009;Turpaeva and Rajsky 2013;Bamber et al. 2022). Recently molecular or combined molecularmorphological studies have confirmed that C. robusta and C. glacialis must be considered distinct species (Dietz et al. 2015a;Dietz et al. 2019).
Currently, the most controversial taxonomic case within the genus Colossendeis is C. megalonyx, which is considered a complex species. Colossendeis megalonyx, C. frigida Hodgson, 1907, C. rugosa Hodgson, 1907, and C. scoresbii Gordon, 1932 are morphologically similar, and several authors expressed doubts about the separation of these species (Loman 1923;Gordon 1932;Fage 1956;Stock 1963). Fry and Hedgpeth (1969) considered these species as junior synonyms of C. megalonyx and proposed a set of four subspecies. Furthermore, subsequent authors have described new morphologically similar species to C. megalonyx (see Pushkin 1993;Child 1995;Turpaeva 2008;Turpaeva and Rajsky 2013) or considered that some of them presented enough morphological differences to be accepted as a single species, which is the case of C. scoresbii (Pushkin 1993;Child 1995), increasing the controversy about the status of C. megalonyx as a species complex.
In addition, molecular studies have revealed several cryptic lineages within the C. meglonyx complex, supporting the hypothesis that there are several unrecognized species within C. megalonyx (Krabbe et al. 2010;Dietz et al. 2013Dietz et al. , 2015bDietz et al. , 2019Dömel et al. 2020). Only C. scoresbii has been confirmed to be separated from C. megalonyx (Krabbe et al. 2010;Dietz et al. 2019). More recently, a combined molecular and morphological study has revealed a high morphological variation within the clades, hindering the possible differentiation of lineages in this C. megalonyx species complex (Dömel et al. 2020). Unfortunately, these clades have not been morphologically characterized and consequently have not been assigned to any Colossendeis binomen. The single exception is the clade that Krabbe et al. (2010) assigned to C. orcadense Hodgson, 1908 based on the absence of eyes. Krabbe et al. (2010) and Dietz et al. (2015bDietz et al. ( , 2019 noted that a taxonomic review of the C. megalonyx group is necessary. Another clade in these papers in need of taxonomic revision involved C. bouvetensis Dietz & Leese in Dietz et al. 2015a, C. lilliei, andC. wilsoni Calman, 1915, which are a set of species with many similar morphologic characters (see Dietz et al. 2019). This paper uses data from a number of Colossendeis species that were collected during the Polarstern cruise XXIII/8 (November 23, 2006-January 30, 2007. Each Colossendeis species is diagnosed, mainly using ratios between morphometric measurements of characters, and two new species are fully illustrated and described. The observed variability of some characters in comparison to previously published studies is briefly discussed. Finally, a qualitative cluster analysis using measurements and ratios between morphological characters of Colossendeis Antarctic and Sub-Antarctic species is presented.

Material and methods
A total of 338 adult specimens of Colossendeis were studied. They were collected at the South Shetlands Islands, Elephant Island, Bransfield Strait, Joinville Island, Dundee Island (Paulet Island) and Larsen B, during the Antarctic Polarstern cruise XXIII/8. Samples were collected using a bottom trawl and a small Agassiz trawl (Gutt 2008). Stations data are detailed in Table 1. The specimens were fixed in 10% buffered formalin and then transferred to 70% ethanol.
The taxonomic literature used for the identification of the present specimens included the most important works carried out in the last century on the fauna of Antarctic pycnogonids (e.g. Calman 1915;Gordon 1932Gordon , 1938Gordon , 1944Fry and Hedgpeth 1969;Pushkin 1993;Child 1995;among others). The list of synonyms and references for each identified Colessendeis species can be found in Fry and Hedgpeth (1969), Pushkin (1993), Child (1995), and Munilla and Soler-Membrives (2009). Additional information is referenced in the remarks section after the diagnosis section for each identified species.
Taking into consideration the absence of knowledge on phylogenetic weight of morphological features, a cluster analysis including all described Colossendeis species in the Southern Ocean (SO, Antarctic and SubAntarctic waters) and the Colossendeis specimens collected during the present cruise was carried out to detect trends in the behaviour of these variables, as well as to explore intra-and inter-specific variability, but is not necessarily looking for potential phylogenetic implications.
A set of traditional characters used in taxonomic studies of Colossendeis species was measured in each identified specimen, summarizing a total of 26 characters (SMTable 1). According to Cano-Sánchez and López-González (2016), the palp in Colossendeis species is 8-or 9-articled. The criteria used in the measurement of the different structures follow Fry and Hedgpeth (1969). When necessary, this kind of morphological information (as accurate as possible) was also obtained from original descriptions and illustrations of the type material of the different SO Colossendeis species.
PRIMER v6 software (Primer-E Ltd., Plymouth, UK) was used to perform a cluster analysis using a similarity matrix obtained by applying the Bray-Curtis similarity index to a nontransformed data-matrix and group average classification algorithm.
The examined specimens have been deposited at the Museu de Zoologia de Barcelona, Spain (MZB), and at the collection of the research group Biodiversidad y Ecología Acuática (BECA) at the University of Seville, Spain.

Results
Thirteen species were identified from the 338 adult specimens collected during the ANT XX111/8 cruise. Of these 13 species, two were new species and are described below.
The number of individuals per species in the newly collected specimens varies greatly (Fig. 1). The most abundant species was C. frigida with 188 specimens, followed by C. megalonyx with 32 specimens, and the minimum was only 1 specimen of C. wilsoni.
Three species extended their known depth range (C. frigida, C. engirmatica, and C. arundorostris), and C. arundorostris was found for the first time outside the Ross Sea (Table 2).

Diagnosis
Proboscis length at least 1.8 times longer than trunk. Second and third lateral processes separated by at least half their own diameter. Palp 9-articled, ratio 6 th /7 th palp article length greater than 6.8; ratio 7 th /8 th palp article less than 0.3; ratio length/ width of 7 th palp article subequal or less than 1. Oviger terminal article non-subchelate. Femur longest of the three longest leg articles; ratio femur/propodus length less than 3.5.

Remarks
Colossendeis arundorostris was described by Fry and Hedgpeth (1969) as a subspecies of C. megalonyx, and followed by some authors (Turpaeva 1974;Turpaeva and Rajsky 2013), while Child (1995) and Bamber et al. (2022) simply consider it as a junior synonym of C. megalonyx. However, other authors (Pushkin 1993;Munilla and Soler-Membrives 2009) consider C. arundorostris as a valid separate species. The C. arundorostris specimens (holotype plus our material) is within a group distinct from the grouping which includes C. megalonyx specimens (holotype plus our material) (Fig. 1). Colossendeis arundorostris is grouped with C. rugosa, C. arcanus Turpaeva, 2008 and C. bamberi sp. nov.  Fry and Hedgpeth (1969) proposed C. arundorostris as a new subspecies (C. megalonyx arundorostris) based on the presence of "trunk, proboscis, and appendages extremely slim and attenuated". Our study adds additional characters that can be used to identify C. arundorostris; the relative proportion between the length of the 6 th and 7 th palp article (ratio greater than 6.8) and that of the 7 th and 8 th palp article (ratio subequal or less than 0.4) (see also Table 3).

Diagnosis
Proboscis length longer than trunk, sloping downward, widely inflated from narrow base. Second and Fig. 1 Cluster analysis of Colossendeis species in the Southern Ocean (Representative Holotype specimens and Holotype specimens plus studied specimens) (N, number of examined specimens) third lateral processes separated by at least half their own diameter. Palps 9-articled, 7 th and 8 th articles subequal, and shorter than 9 th article length. Oviger terminal article subchelate. Legs with a femur/propodus length ratio greater than 2.5.

Remarks
The specimens of C. australis examined in this study agree with the original description (Hodgson 1907) and subsequent authors (Bouvier 1913;Calman 1915;Fry and Hedgpeth 1969;Pushkin 1993;Child 1995; -González 2007). This species is similar to C. speudochelata Pushkin, 1993. In fact, the holotypes of both species together with our C. australis material form a clearly defined grouping (see Fig. 1). However, characters such as >2.5 value in femur/propodus length relationship, and an inflated proboscis could be used to distinguish C. australis.  See sampling data of additional examined specimens of this species in Tables 1 and 2 and SMTable 3.

Etymology
This species is named in homage to the late Roger N. Bamber, for his important contribution to our current knowledge of the pycnogonid fauna.

Diagnosis
Proboscis length at least 1.7 times longer than trunk. Second and third lateral processes separated by at least half their own diameter. Palp 9-articled, ratio 6 th /7 th palp article length range between 4.2 and 6.5; ratio 9 th /8 th palp article less than 1.5; ratio length/width of the 7 th palp article less than 1.3. Oviger terminal article non-subchelate; terminal claw spatulate. Femur longest of the three longest leg articles; ratio propodus/claw subequal or less than 1.9.

Description of the holotype (male)
Size moderate for the genus, leg span 181.8 mm. Proboscis cylindrical, smooth, distal third slightly down curved, distal two-third dilated, distal third truncated distally, 1.9 times of trunk length.
Trunk without segmental suture-line traces, with lateral processes well separated, at least half of processes diameter; dorsal surface and processes smooth.
Abdomen erect, slightly upcurved, smooth. Ocular tubercle as a tall cone, four distinct eyes, anterior pair slightly larger than posterior.
Palps 9 articled, slender, longer than proboscis, with scarcely distributed tiny setules. Fourth article has a small pore on the external distal third. Sixth article at least four times the length of seventh. Seventh article as long as wide. Eighth article about 2.7 times of the length of seventh. Ninth article slightly longer than eighth.
Ovigers 10-articled and conventional, strigilis well developed, terminal article with 2 rows of elongated spines (inner row at least twice as long as previous one) and a field of shorter spines without clear pattern of rows; they are flat with pectinate borders (Fig. 2d). Terminal claw spatulate.
Walking legs slender and smooth. Second coxa longer than first or third coxa. Femur longest article and both tibiae progressively decrease in length. Tarsus about 1.4 propodus in length. Claw slightly curved, about 0.6 of propodal length. Sexual pores located ventrally on second coxa of all four legs.

Measurement of the holotype (mm)
Length of trunk ( Variability Table 3 shows the highest and lowest values of the ratios between characters of analysed specimens. Ocular tubercle shape is similar in all specimens, it is topped by a high pointed cone but some of them (MZB 2022(MZB -1446MZB 2022MZB -1454 are considerably shorter. In addition, the anterior area of the cephalic segment is variable from straight to slightly inclined following the ocular tubercle profile.

Distribution
At present, Colossendeis bamberi sp. nov. is known from its type locality (South Shetland Islands) and Elephant Island, Joinville Island and Larsen B (core-station South) ( Table 2) Remarks Colossendeis bamberi sp. nov. is comparable to C. arcanus, C. arundorostris, and C. rugosa. The new species is grouped with the above-listed species by sharing the following set of characters: (1) ratio 6 th /7 th palp article length longer than 4.2; (2) ratio 7 th /8 th palp article length subequal or shorter than 0.4; (3) second and third lateral processes separated by more than 0.5 their own diameter; (4) oviger terminal article not subchelate; and (5) eyes developed.
Colossendeis rugosa was included in the "C. megalonyx complex" by Fry and Hedgpeth (1969), and later only Pushkin (1993) considered it as a separate form from C. megalonyx. In our cluster analysis, C. rugosa is clearly separate from the grouping formed by C. arundorostris, C. arcanus and C. bamberi sp. nov. (see Fig. 1). Moreover, C. rugosa differs mainly from them by a higher ratio of 9 th /8 th palp article length and a lower ratio propodus/claw length (Table 3).
The length ratio of the 6 th /7 th palp article is considerably higher in C. arundorostris than in C. arcanus and in C. bamberi sp. nov., while conversely the ratio 7 th /8 th palp article length is lower in C. arundorostris (Table 3).
Colossendeis bamberi sp. nov. clearly differs from C. arcanus by the ratio proboscis/trunk length, the ratio length/width of the 7 th palp article, and the ratio propodus/ claw length, in all these comparisons the former species having higher values (Table 3).

Diagnosis
Proboscis length subequal or shorter than trunk. Second and third lateral processes separated by at least half their own diameter. Palp 9-articled, ratio 6 th /7 th palp article length subequal or less than 1.2. Ovigeral terminal article non-subchelate, with 4 rows of compound spines. Femur longest of the three longest leg articles, ratio propodus/claw length less than 1.3.

Remarks
The specimens of C. drakei examined for this study agree with the description provided by Calman (1915) and subsequent authors (Fry and Hedgpeth 1969;Pushkin 1993;Child 1995;Alexeeva 2021). These authors did not indicate any special condition concerning the ovigeral claw, but Dietz et al. (2015a) pointed out that the claw appears to be bifurcated. All our specimens, however, show a spatulate ovigeral claw.

Diagnosis
Proboscis length at least 1.5 times longer than trunk. Second and third lateral processes separated by at least half their own diameter. Palp 9-articled, 7 th palp article articulated anaxially with 8 th ; ratio 6 th /7 th palp article length subequal or greater than 6. Oviger terminal article subchelate. Second tibia shortest of the three longest leg articles.

Remarks
Turpaeva (1974) noted that this species closely resembles C. tortipalpis. Both species exhibit widely separated lateral processes, the 7 th palp article articulated anaxially with 8 th and the oviger terminal article subchelate. She pointed out the more separated lateral processes, the shape of ocular tubercle, the oviger structure, and the shorter claw of the legs of C. engirmatica were the differential characters.
The specimens here examined agree with the description provided by Turpaeva (1974), but they show a long claw in the legs, similar to propodus in length (a similar condition to that observed in C. tortipalpis), and the ocular tubercle ranges from rounded to pointed.
In our cluster analysis, C. tortipalpis and C. engirmatica are closely related (Fig. 1). They differ mainly in the relative proportion between the length of the 6 th and 7 th palp article, in C. engirmatiga group the ratio being longer than 6.
Only type of material was known before, and our specimens represent the first record of this species after its original description.

Diagnosis
Proboscis length at least 1.6 times longer than trunk. Second and third lateral processes separated by at least half their own diameter. Palp 9-articled, ratio 6 th /7 th palp article length less than 3.4; ratio 7 th /8 th palp article greater than 0.4; ratio length/width of the 7 th palp article subequal or greater than 1.3. Oviger terminal article not subchelate. Femur longest of the three longest leg articles; ratio femur/propodus subequal or length less than 3.6.

Remarks
Hodgson (1907) described Colossendeis frigida and p o i n t e d ou t th a t it w a s v e r y c l os e l y re l at e d t o C. megalonyx, and the arguments were followed by Calman (1915) and Pushkin (1993). However, Loman (1923) suggested that both species could not be separated (favour of C. megalonyx) and this was followed by subsequent authors (Gordon 1932(Gordon , 1938Fry and Hedgpeth 1969;Child 1995;Munilla and Soler-Membrives 2009;Turpaeva and Rajsky 2013;Bamber et al. 2022).
The specimens of C. frigida examined in this study agree with the description provided by Hodgson (1907) and Calman (1915). They can be diagnosed by the ratio of the 6 th /7 th palp article length and the ratio length/width of the 7 th palp article (see also Table 4). In our study, this species forms a clearly defined group, consisting of the holotype specimen and our 188 specimens (Fig. 1).

Diagnosis
Proboscis length subequal or shorter than trunk. Second and third lateral processes separated by at least their own diameter. Palp 9-articled, ratio 6 th /7 th palp article length greater than 1.2, three distal articles subequal in length, or 9 th slightly longer. Ovigeral terminal article nonsubchelate, with 4 rows of compound spines. Second tibia shortest of the three longest leg articles. Tarsus/propodus length ratio greater than 1.1.

Remarks
Colossendeis glacialis was a controversial species. Some authors considered this species to be a synonym of C. robusta (Fry and Hedgpeth 1969;Child 1995). Other authors (Stiboy-Risch 1993;Cano-Sánchez and López-González 2007;Dietz et al. 2015a) have provided morphological characters such as the relative proportion of the distal palp articles (see above), second tibia shorter than femur and first tibia, or the strigilis spination (see above) which could be used to separate this species from C. robusta. In addition, Dietz et al. (2015a) provided molecular evidence that supports C. glacialis as a separate species. Our specimens of C. glacialis agree with previous descriptions.

Diagnosis
Proboscis length subequal or longer than trunk, inclined downwards. Trunk with intersegmental suture-line traces.
Second and third lateral processes separated by less than 0.5 of their own diameter. Papls 9 articled, the 9 th palp article is shorter than 8 th and 7 th . Ovigeral terminal article nonsubchelate. Second tibia longest of the three longest leg articles. Tarsus subequal propodus length.

Diagnosis
Proboscis length less than two times the trunk. Second and third lateral processes separated by at least half their own diameter. Palp 9-articled, ratio 6 th /7 th palp article length range between 3.4 and 4.2; ratio 7 th /8 th palp article subequal or greater than 0.4; ratio length/width of the 7 th palp article subequal or greater than 1. Oviger terminal article nonsubchelate. Femur longest of the three longest leg articles; ratio femur/propodus length less than 3.5.

Remarks
Colossendeis megalonyx has an unresolved controversial taxonomic status. Fry and Hedgpeth (1969) proposed a set of subspecies to merge a number of previously proposed species into this species. This conception was followed by some authors (Turpaeva 1974;Turpaeva and Rajsky 2013). However, the characters used for separating at subspecies level could also be considered useful at species level. In fact, other authors (Pushkin 1993;Child 1995;Cano-Sánchez and López-González 2007;Munilla and Soler-Membrives 2009;Soler-Membrives et al. 2009;Weis et al. 2011; Melzer 2012) recognized valid species among part of Fry and Hedgpeth's C. megalonyx subspecies. Child (1995) and Cano-Sánchez and López-González (2007) pointed out the need to explore new sources of characters for better discrimination at the species level.
In the last decade, molecular analyses have been carried out on this C. megalonyx complex and a variable number of cryptic clades have been proposed (Krabbe et al. 2010;Dietz et al. 2015bDietz et al. , 2019Dömel et al. 2020). However no diagnostic morphological characters have been provided for these proposed clades, and they have not been assigned to any known species or subspecies name. In this study ( Fig. 1) a defined group is observed (including holotype plus our material), clearly differentiated from the species previously synonymized or included as a C. megalonyx complex. Colossendeis megalonyx can be differentiated using a set of characters, such as ratio proboscis/trunk length or ratio relative appendages articles length (see also  Table 4).

Material examined
Holotype

Etymology
This species is named after its close similarity to C. frigida Hodgson, 1907.

Diagnosis
Proboscis length at least 1.8 times longer than trunk. Second and third lateral processes separated by at least half their own diameter. Palp 9-articled, ratio 6 th /7 th palp article length less than 3.4; ratio 7 th /8 th palp article~0.5; ratio length/width of the 7 th palp article greater than 1.5. Oviger terminal article not subchelate; terminal claw spatulate. Femur longest of the three longest leg articles; ratio femur/ propodus length greater than 3.8; ratio femur/tarsus length greater than 3.

Description of the holotype (female)
Size moderate for the genus, leg span 213.7 mm. Proboscis cylindrical, smooth, nearly straight, distal two-third dilated, distal third truncated distally, 1.9 times trunk length.
Trunk without segmental suture-line traces, with lateral processes well separated, at least half of processes diameter; dorsal surface and processes smooth.
Abdomen slightly erect, slightly dilated distally, smooth. Ocular tubercle as a tall cone, four distinct eyes, anterior pair slightly larger than posterior.
Palps 9 articled, slender, longer than proboscis, with scarcely distributed tiny setules. Fourth article has a slight swelling with a pore on the external distal third. Sixth article about three times the length of seventh. Seventh article about 1.9 times longer than the width. Eighth article about 2 times the length of seventh. Ninth article longer than eighth.
Ovigers 10-articled and conventional, strigilis well developed, terminal article with 2 rows of elongated spines (inner row at least twice as long as previous one) and a field of shorter spines without clear pattern of rows; they are flat with pectinate borders (Fig. 3d). Terminal claw spatulate.
Walking legs slender and smooth. Second coxa longer than first or third coxa. Femur longest article, and both tibiae progressively decrease in length. Tarsus about 1.3 times propodus length. Claw slightly curved, about 0.7 times propodal length. Sexual pores located ventrally on second coxa of all four legs.

Distribution
At present, Colossendeis pseudofrigida sp. nov. is only known from its type locality at Elephant Island (Table 2).
The above-listed three species are included in the "C. megalonyx complex." These species can be differentiated by a set of characters shown in Table 4. The higher values of C. pseudofrigida sp. nov. in the ratios femur/propodus length and femur/tarsus length are revealed as the most differential characters between the new species and its three closest congeners.

Diagnosis
Proboscis length at least 1.5 times longer than trunk, widely inflated at mid length. Second and third lateral processes separated by less than 0.25 their own diameter. Palp 9-articled, 7 th article distinctly shorter than distal ones. Oviger terminal article subchelate. Femur longest of the three longest leg articles.

Remarks
The present specimens of C. scotti agree with the description provided by Calman (1915) and subsequent authors (Fry and Hedgpeth 1969;Pushkin 1993;Child 1995;Chimenz and Gravina 2001;Cano-Sánchez and López-González 2007;Alexeeva 2021). In fact, the measurements obtained from the holotype and our samples form a clearly defined group (Fig. 1). This species is easily differentiated by the subchelate oviger, a distinctly short 7 th palp article, and the lateral processes closely spaced. Gordon, 1932 Material examined See Tables 1 and 2 and SMTable 3.

Diagnosis
Proboscis length at least 1.5 times longer than trunk. Second and third lateral processes separated by at least half their own diameter. Palp 9-articled, 7 th palp article articulated anaxially with 8 th ; ratio 6 th /7 th palp article length less than 6.
Oviger terminal article subchelate. Second tibia shortest of the three longest leg articles.

Remarks
This species closely resembles C. engirmatica. See the Remarks section of C. engirmatica for distinction.

Diagnosis
Proboscis length less than 1.5 times that of trunk, inclined downwards. Second and third lateral processes separated by less than 0.25 their own diameter. Palps 8 articled. Ovigeral terminal article non-subchelate, with 3-4 rows of compound spines. Femur and first tibia subequal, second tibia longest. Tarsus equal to or a little longer than propodus.

Remarks
This species cannot be confused with any other Antarctic Colossendeis species because of its 8-articled palp.

Discussion
The genus Colossendeis Jarzynsky, 1870 has, on the one hand, a paraphyletic position in relation to the genera Decolopoda Eights, 1835and Dodecalopoda Calman & Gordon, 1933(Krabbe et al. 2010Dietz et al. 2015aDietz et al. , 2019Ballesteros et al. 2021), and, on the other hand, a possible junior synonym status with respect to the genus Decolopoda. However, in this paper, we have followed the criteria of Dietz et al. (2019), keeping the traditional classification in the use of the name Colossendeis and excluding the species with supernumerary segments.
Morphometric features, such as the length of articles and ratios of different structures (e.g. proboscis length/trunk length, among others) have been widely used in pycnogonids taxonomic papers to differentiate species (see Stock 1975;Bamber 2002;Cano-Sánchez and López-González 2018, among others), including those regarding the genus Colossendeis (see Bouvier 1913;Child 1995;Cano-Sánchez and López-González 2007, among others). However, the species of Colossendeis lack information on the interspecific morphometric variability and do not possess the structures commonly used in the taxonomy of other genera (e.g. cheliphores, auxiliary claws, dorsomedial tubercles). Hence, the use of those measurements and ratios is one of the main criteria in trying to differentiate Colossendeis species (e.g. Calman 1915;Gordon 1932;Fry and Hedgpeth 1969;Pushkin 1993;Child 1995;Cano-Sánchez and López-González 2007;Turpaeva and Rajsky 2013;Dietz et al. 2015a, among other).
The mode of reproduction remains unknown in Colossendeidae; nothing about mating behaviour or embryonic or early postembryonic development has ever been cited (Arnaud and Bamber 1987;Bamber 2007;Brenneis et al. 2017). Moreover, Child (1995) considers that "the sex [male/female or close-adult state/adult state] of Colossendeis species is relatively unimportant" for the taxonomy of the group, mainly because all of this information is largely unknown. However, corroborating at least the adult states should be the basis of the morphological descriptions and calculation of ratios discussed above in the description of Colossendeis species. In fact, the characteristics of sexual dimorphism in Colossendeis species have not been considered in traditional taxonomic studies. Dömel et al. (2020) did not find a clear separation between male and female measurements in their study of C. megalonyx complex species. Our cluster analysis does not discriminate between male and female specimens in any examined Colossendeis species (SMFig. 1). This is an important finding and is great to provide evidence towards confirming what has long been suspected.
Some authors have argued the need for new character sources to define Colossendeis species, especially for the controversial ones such as C. megalonyx and C. robusta (Child 1995;Cano-Sánchez and López-González 2007). Cano-Sánchez and López-González (2007) pointed to the molecular data as well as the detailed morphological characters such as the strigilis in the genus Colossendeis as the new sources. Subsequent integrative taxonomic studies on Colossendeis species have shown correspondence between the phylogeny of the studied species and the morphological data including a SEM detail of strigilis (Dietz et al. 2013(Dietz et al. , 2015a. In our study, this character has not been included in the similarity analysis because ovigers are related to cleaning behaviours, which cause their erosion in adult specimens (Munilla 1991;Cano-Sánchez and López-González 2007). The eroded ovigers may be a confusing character in some cases and may limit the taxonomic conclusions.
In the last decade, different genetic markers have been used to study the phylogeny of the genus Colossendeis, showing different results for the same complex species. Analyses based on mitochondrial cytochrome c oxidase subunit 1 (Cox1) sequences revealed six main clades in C. megalonyx complex, C. lilliei and C. scoresbii being well-supported linages (Krabbe et al. 2010), while the concatenated analyses combining Cox1 and the nuclear ribosomal gene region internal transcribed spacer (ITS) increased these to 15-20 clades, also observing some discordance when both markers are used separately in the C. megalonyx species complex (Dietz et al. 2015b).
In addition, several authors have also carried out integrative taxonomic studies, where molecular results have been consistent with the morphological characters analysed. Dietz et al. (2013) used Cox1 and nuclear histone 3 (H3) and showed the monophyly of C megalonyx complex excluding C. tenera Hilton, 1943 andC. angusta Sars, 1877, in concordance with their morphological analyses (morphometric comparison and SEM analyses of strigilis). In the same way, Dietz et al. (2015a) separated C. robusta from C. glacialis and C. bouvetensis using both Cox1 and ITS sequences, and from C. drakei using only Cox1 sequences; both results were consistent with the morphological characters including SEM analyses of strigilis.
Traditionally two groups of species are distinguished within the genus Colossendeis, these being "longitarsal" and "brevitarsal" according to the relation of the combined length of tarsus, propodus and claw compared to the length of the second tibia (Bouvier 1913). Most of the Antarctic and Sub-Antarctic Colosssendeis species described are found in the longitarsal group, and the monophyly of this group has been confirmed by molecular data (Dietz et al. 2015a(Dietz et al. , 2019. Although an integrative taxonomic study of this group has confirmed that the phylogeny is broadly consistent with morphological data for Antarctic and Sub-Antarctic Colossendeis species (see Dietz et al. 2019), the species-level taxonomy of some taxa has raised questions, such as the paraphyletic status of C. wilsoni with respect to C. bouvetensis (Dietz et al. 2019). Colossendeis wilsoni, except for having only eight palps, is from a morphologically very similar to C. lilliei, and C. boubetensis (Dietz et al. 2015a), the clade consisting of these three species is in need of taxonomic revision (Dietz et al. 2019). Our results from the cluster analysis are coincident with the close relationship among the three species, which form a clearly defined group (see Fig. 1).
Recently, Dömel et al. (2020) have studied the C. megalonyx species complex using an extensive set of genomic and morphometric data mainly to explore its speciation process. The authors conclude that differentiation of C. megalonyx complex lineages using morphometric data is possible in spite of the difficulties due to the high morphological variation within clades. However, these authors do not assign sets of morphological characters to clades and, what is more important from the taxonomic point of view, they do not assign putative species names to the observed clades in this "megalonyx complex." In the current state of knowledge, integrative taxonomic studies have a high scientific value but are not yet resolving the various existing taxonomic controversies, especially in the C. megalonyx complex. Although it is laborious, the character set used in our similarity cluster analysis seems to be useful to discriminate between species. The species of the traditional C. megalonyx complex are grouped into a clearly defined group (Fig. 1). Furthermore, we consider that future studies with an integrative molecular and morphological approach including the analysis of morphological variables (e.g. ratios between appendages articles) could contribute to partially resolve the taxonomic situation of the genus Colossendeis. the work on board and allowed us to collaborate in this Antarctic programme. Mr. Tony Krupa is thanked for reviewing the English version. Finally, the authors thank the two anonymous reviewers and the editorial office of Marine Biodiversity for all the constructive comments and suggestions that helped to improve the quality of an early version of the manuscript.
Funding Funding for open access publishing: Universidad de Sevilla/ CBUA. The participation of PJL-G in the Polarstern ANT XXIII/8 was possible thanks to the Spanish project CLIMANT (POL2006-06399/ CGL) of the Ministry of Education and Science.

Declarations
Conflict of interest The authors declare no competing interests.
Ethical approval All applicable international, national, and/or institutional guidelines for animal testing, animal care and use of animals were followed by the author.
Sampling and field studies All necessary permits for sampling and observational field studies have been obtained by the authors (or responsible researchers of the different research programs) from the competent authorities and are mentioned in the acknowledgements.
Data availability The data generated and analysed during this study are deposited in public repositories.
Author contribution EC-S and PJL-G conceived and designed the manuscript. PJL-G collected samples. EC-S conducted morphological description and numerical analysis. All authors interpreted data analysis. EC-S drafted a first manuscript, All authors critically reviewed and edited the final manuscript.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ .