Combining morphological and genomic evidence to resolve species diversity and study speciation processes of the Pallenopsis patagonica (Pycnogonida) species complex | Frontiers in Zoology


The sample set included specimens of Pallenopsis buphtalmus (corresponding to mitochondrial clade ANT_M in [26]), P. latefrontalis (ANT_F), P. notiosa (SUB_3) and P. yepayekae (Pye.1) as well as of further potential species within P. patagonica, i.e. ANT_C, ANT_D, ANT_K, ANT_L, SUB_1, SUB_2, SUB_4 and SUB_5. We refer to this set of putative species as the P. patagonica species complex (also P. patagonica sensu lato in [26]), since using the key in Child [12] would (erroneously) assign all those species to the morphospecies P. patagonica.

Genomic analyses

The obtained dataset consisted of 61 individuals of the Pallenopsis patagonica species complex. One individual of P. pilosa (Hoek, 1881) [42] genotyped by us and a previously published transcriptome assembly of Anoplodactylus insignis (Hoek, 1881) [42] [28] were added as outgroups in genetic analyses. When analyzing all Pallenopsis specimens on the nucleotide level, 821 out of 1607 targeted EOGs (Eukaryotic Orthologous Groups), which in our case are putative single-copy groups of orthologous genes, were recovered with a total alignment length of 474,954 bp. The data set used to infer a reliable root by including A. insignis was analysed on the amino acid level to reduce the branch length to the outgroup. This alignment included only EOGs for which a sequence of A. insignis was present and sites with a sequence coverage of at least 50%, which reduced the data set to 208 EOGs and 22,018 aa (corresponding to 66,054 bp). Furthermore, sequences that were outliers on the amino acid level were excluded. The models of evolution chosen by ModelFinder for the nucleotide data set were GTR + R2 for the first, TIM + R2 for the second and GTR + R4 for the third codon positions. For the amino acid alignment including A. insignis, JTT + F + R3 was chosen as the best fitting model.

Single-nucleotide polymorphism (SNP) calling for all Pallenopsis samples (i.e. including P. pilosa) resulted in 2527 SNPs from 168 EOGs. This data set was only used for construction of a phylogenetic tree.

Phylogenetic analyses of the amino acid data set revealed that the P. patagonica species complex represents a monophyletic group with P. pilosa and A. insignis representing a joint outgroup (Additional file 1). In particular P. pilosa was shown to be a sister group to the P. patagonica species complex, as assumed in previous studies [26, 81]. Further analyses were conducted with the nucleotide data set not including A. insignis. Separate phylogenetic analyses based on the EOG alignment (in the following referred to as the EOG data set) and the variant calling (in the following referred to as the SNP data set) including all Pallenopsis specimens resulted in phylograms with identical topologies (Fig. 1 and Additional file 2) but the EOG-based analysis had higher bootstrap support (bs) values and is discussed herein. Two major groups are discernible within the P. patagonica species complex, one including specimens assigned to all of the Antarctic clades (ANT) except ANT_N (from now on referred to as the “Antarctic supergroup”) and one including specimens from all Patagonian clades (SUB) plus ANT_N (from now on referred to as “Patagonian supergroup”). The “Antarctic supergroup” is comprised of two major lineages, ANT_C/D/M and ANT_F/K/L. More detailed divisions of those groups are in agreement with the clades delineated in Dömel et al. [26]. There is also a strong support for the geographical divide in ANT_D and P. latefrontalis (ANT_F) into specimens from the Antarctic shelf (both 100% bs) and sub-Antarctic islands (South Georgia with 99% bs, and Bouvet Island with 96% bs, respectively). Within the “Patagonian supergroup”, SUB_4 and SUB_5 together represent the basalmost group of the “Patagonian supergroup” with SUB_4 being paraphyletic with respect to SUB_5. Analogously, SUB_1 and SUB_2 appear not strictly monophyletic with respect to each other, since specimens from Burdwood Bank belonging to both clades group together. ANT_N is nested within the “Patagonian supergroup”, as are P. notiosa (SUB_3) and P. yepayekae.

Fig. 1
figure1

Phylogenetic tree of the Pallenopsis patagonica species complex. Maximum-Likelihood tree based on concatenated EOG sequences of all Pallenopsis samples. Asterisks (*) indicate samples that were also used in morphometric analyses. Bootstrap values are given next to the respective branches

For the principal component analyses (PCA) three SNP data sets were analysed. The first data set contained all specimens of the P. patagonica species complex and included 2543 SNPs from 175 EOGs. Furthermore, separate data sets for the “Patagonian supergroup” and the “Antarctic supergroup” yielded 2047 SNPs from 183 EOGs and 2487 SNPs from 216 EOGs, respectively. For the first SNP data set (P. patagonica species complex), 16 significant axes were found. There is a clear differentiation between five groups (Fig. 2a). All Antarctic clades cluster together, with the exception of ANT_N. The Patagonian clades are divided into four groups, SUB_1/2, P. notiosa (SUB_3), SUB_4/5 and P. yepayekae (Pye.1). Analyses of the data set divided into the two supergroups obtained no significant axes for the “Patagonian supergroup”. For the “Antarctic supergroup”, the first seven axes were significant and showed a differentiation into the clades previously proposed by Dömel et al. [26] (Fig. 2b).

Fig. 2
figure2

PCA from genomic data of the Pallenopsis patagonica species complex. PCA plots based on genomic data of a) all samples of the Pallenopsis patagonica species complex and b) samples of the “Antarctic supergroup”

For the clustering analyses, the cross entropy with the lowest median was chosen (Additional file 3). By this criterion, the best number of ancestral populations was seven (K = 7). The plot of the sparse nonnegative matrix factorization (sNMF) mostly supported the groupings obtained with the PCA. The differences were that ANT_K and ANT_L as well as SUB_1 and SUB_2 grouped together and showed similar proportions of the same ancestral populations (Fig. 3).

Fig. 3
figure3

sNMF analyses of the Pallenopsis patagonica species complex. Graphical illustration of ancestry proportion estimates for all samples with K = 7. Estimated proportions of ancestry populations are illustrated by different colors. Each horizontal bar represents one specimen

For selection tests, a sequence alignment including only positions that were present in at least 50% of the samples was used. This resulted in an alignment of 82,782 bp recovering 293 EOGs. Seventeen codons within 17 EOGs and 49 codons within 38 EOGs under selection using the Fast Unconstrained Bayesian AppRoximation (FUBAR) and the Mixed Effects Model of Evolution (MEME), respectively, were detected. Sixteen codons within 16 EOGs were shared between both methods. Furthermore, no branches under selection were detected, irrespective of the applied test (aBSREL or BUSTED; see Methods).

Morphology

Morphometrics

Morphometric measurements were taken for 37 individuals (a table including all measurements is provided in Additional file 4) but due to damage during trawling, transport, storage or preceding genetic analysis, distal articles of appendices and hence data for those were often missing. After averaging measurements for bilateral characters, the amount of missing data was reduced by about three quarters. For further analyses, filtered data sets including 38 and 39 characters for the absolute and relative values, respectively, were used. PCA plots using all specimens did not show separation into clades but a trend for a division of sub-Antarctic and Antarctic samples (Additional file 5).

To avoid the problem of overfitting, character sets optimal for species separation in discriminant analysis (LDA) were searched for using a heuristic approach. Therefore, only clades with a minimum of three individuals were included resulting in a data set of seven clades and 29 specimens. Absolute as well as relative values expressed as proportion of the trunk length were used.

For both data sets (absolute and relative values) multiple iterations of character selection were performed and it was recorded how often a character was added to an LDA model in individual optimizations and what its contribution was (see Table 1). The LDA plots of both data sets based on the character combinations with best performance clearly separated all clades from each other, except for clade ANT_D and ANT_F when looking at the absolute values (Fig. 4). Furthermore, analysis of cross-validation confusion matrices confirmed that these results were not dominated by overfitting artefacts, with the correctness rate being higher for the relative values (0.83) than for the absolute values (0.76) (Table 2). Here, ANT_F and SUB_5 had many misassignments (absolute data set). Analogously, PCAs for both data sets showed that the clades ANT_D and ANT_F could not be separated from each other for the data set including absolute values (see matrices of all PCs in the Additional file 6).

Table 1 Results of morphometric analyses of the Pallenopsis patagonica species complex for both data sets (absolute and relative values). Contributions to correctness rate (CR), characters combination for the best LDA performance and p-values for significant differences between geographic regions and sexes are listed
Fig. 4
figure4

LDA of the Pallenopsis patagonica species complex. Ordination of the filtered morphometric data set using different combination of characters for a) absolute values (ocular tubercle H, ocular tubercle W, eye H, forehead H, cheliphore 1), and b) relative values (trunk W12, ocular tubercle H, palp, cheliphore 2, WL4 coxa2)

Table 2 Cross-validation confusion matrices for morphometric data set of the Pallenopsis patagonica species complex using absolute and relative values

Significant differences of characters between clades were found for neither of the two data sets after Bonferroni correction. However, 33 and 14 significant differences between specimens from the different geographic regions (SUB and ANT) for absolute and relative value, respectively, were found (Table 1). In all cases, the characters of the Antarctic samples were larger than of the Patagonian ones. As for analyzed specimens, males were more frequent in sub-Antarctic (75%) and females preponderated in Antarctic clades (65%), characters were also tested for significant differences between sexes. There were five and eight significant differences for absolute and relative values, respectively, of which five characters for each data set also showed significant differences between geographic regions (see Table 1).

Morphological characters

Using the morphological key for Pallenopsis [82] from [13], all specimens analysed were assigned to P. patagonica. However, we observed consistent morphological features for several groups. Specimens that occur south of the Antarctic Polar Front are larger in body size and have longer legs in comparison to those from the Patagonian clades. Also, the distance between the lateral processes is longer for the Antarctic specimens. Furthermore, the rudimentary palp is larger for Antarctic individuals (Fig. 5).

Fig. 5
figure5

Boxplot showing size differences in morphological structures of the Pallenopsis patagonica species complex. All comparisons show that characters of samples from ANT (Antarctica) are significantly larger than from SUB (Patagonian) (log10 of absolute values used; p = 0.0000005, p = 0.00008, p = 0.00042 and p = 0.00012, respectively)

Specimens from Patagonian clades showed great variation and almost no suitable morphological characters for clade assignments. Only P. notiosa (SUB_3) can be distinguished from the others due to its rounded (rather than a pointed or slightly pointed) ocular tubercle and a very long second coxa, which exceeds the combined lengths of the first and third coxae (Fig. 6c,e).

Fig. 6
figure6

Prominent morphological characters of various lineages of the Pallenopsis patagonica complex. a, dorso-distally located crowning (see arrow) of lateral processes in PS82_143_2_2 (P. hiemalis; ANT_K). b, straight propodus of PS82_143_2_2 (P. hiemalis; ANT_K). c, rounded ocular tubercle of ZSM-A20111008 (P. notiosa; SUB_3). d, setae patches (see arrows) on dorsal-posterior margin of three trunk segments of JR262_1058 (P. aulaeturcarum; ANT_D). e, coxae of ZSM-A20111008 (P. notiosa; SUB_3). f, detailed view of second and third coxa with bifurcated setae on distal margins (see arrows) of PS77_211_6_1_4 (Pallenopsis sp. ANT_N). Scale bars = 1.5 mm

Specimens from Antarctica can morphologically be divided into two groups which can be distinguished by the setae patches on the dorso-posterior margin of the trunk segments (Fig. 6d), that vary in size for specimens of ANT_C/D/M but are absent in those of ANT_F/K/L and ANT_N. Two Antarctic clades were identified as already described species, namely P. buphtalmus (ANT_M) and P. latefrontalis (ANT_L). Pallenopsis buphtalmus (ANT_M) can be distinguished from the other Antarctic species due to relatively short accessory claws. For P. latefrontalis (ANT_L) the second coxa is characteristically shorter than the combined lengths of the first and third coxae. A straight rather than a curved propodus is distinctive of ANT_K (Fig. 6b). Also, the lateral processes in this clade display a dorso-distally located crowning that differs from the frequently occurring but much smaller thickenings (Fig. 6a). Those characters were also described for P. hiemalis by Hodgson [40] and Pushkin [68, 69] by their slim segmented body, cylindrical proboscis, rudimentary palps, ten-articled ovigera in males, and slender legs with one main and two auxiliary claws [82].

Pallenopsis aulaeturcarum sp. nov. Dömel & Melzer urn:lsid:zoobank.org:act:72E41F8B-0A6F-4A5B-815A-1C2CAB65AFA5 Figures 7 a-g, 9 a-e

Type material

Holotype: PS82_156_2_1 (ZSM-A20160629), female, Weddell Sea, − 75.507 (S), − 27.486 (W), January 2014, depth: 281.5 m.

Paratypes: PS82_121_1 (ZSM-A20160626), female, Weddell Sea, − 76.966 (S), − 32.945 (W), January 2014, depth: 265.2 m. First leg pair and ovigera loose in the jar, proboscis of this individual was used for further analyses with the scanning electron microscope (SEM); PS82_156_2_2 (ZSM-A20160630), female, Weddell Sea, − 75.507 (S), − 27.486 (W), January 2014, depth: 281.5 m; PS82_223_1 (ZSM-A20160730), male, Weddell Sea, − 75.522 (S), − 28.973 (W), February 2014, depth: 462 m, both ovigera damaged, cement gland tube used for sex determination; PS82_174_3 (ZSM-A20160637), male, Weddell Sea, − 74.491 (S), − 30.977 (W), February 2014, depth: 529.7 m, left oviger detached, no morphometric measurements available for this individual.

The type series is deposited in the Bavarian State Collection of Zoology, in the department Arthropoda varia.

Distribution

Weddell Sea, from eastern tip of the Antarctic Peninsula (− 63.686, − 56.859) to eastern Weddell Sea (− 70.940, − 10.489), and Bouvet Island (− 54.425, 3.524).

Diagnosis

Setae on posterior margin of trunk segments. More rows on ventral side (about three) than on dorsal side (one row). Abdomen oriented upwards.

Description (female)

Size moderate, leg span less than 65 mm. Trunk with distinct segment borders, ridges strongly expressed (Fig. 7a,b). Ridges on dorsal side smooth with few setae. Ventral surface covered with 2–3 rows of small clearly apparent spinules. Lateral processes separated by about the size of their diameter, U-shaped (Figs. 7a, 9a). Distal margins of all processes display fringe of small spinules. On dorsal side, these spinules are located on slight thickenings (Fig. 7b). Ocular tubercle situated on anterior end of cephalic segment. Top of ocular tubercle slightly bent backwards and pointed. Eyes prominent and pigmented, anterior eyes larger than posterior eyes. Proboscis sub-cylindrical, equally thick throughout and slightly directed downwards (Fig. 9a,b). It is about half the length of the trunk. Abdomen long, extending from the trunk oriented upwards and covered with few spinules (Figs. 7b, 9a). Cheliphores with two-articled scape, first article longer than second article (Fig. 7b). Ultimate cheliphore article (movable finger) equipped with setose pad. Moveable digit slightly longer than fixed digit, its tip curved. Inner margins straight and joined when closed. Setae pad has a triangular shape of which the whole length is attached to chela. Single-articled, laterally placed palp represents the rudimentary state typical for the genus (Fig. 7b). It takes the form of an elongated bulb that is twice as long as wide. Female oviger composed of ten articles (Fig. 7e). Proximal articles broaden slightly towards the distal part of each article. Second article equal in length to the third article. Fourth oviger article more swollen and the longest of all. From fourth article onwards, article length decreases. Oviger articles are setose, with all setae pointing distally. Legs with several short setae (Fig. 7f,g). First and third coxa sub-equal. Second coxa about twice the length of third coxa (Fig. 7a,f). Assemblage of short setae on ventral side of second and third coxa (Fig. 7h, 9d). Setae without bifurcation. Femur and first tibia about equal in size. Second tibia slightly longer than other leg articles. Tarsus is short and armed with one big spine on the ventral side near its distal part and a couple of smaller lateral spines. Propodus slightly curved, with three to four heel spines that differ insignificantly in length, but the distal spine is the largest (Fig. 7i, Fig. 9e). The remaining sole is covered with many shorter spines. Claw dorsally curved, its inner margin straight, its tip curved. Two auxiliary claws about half the length of main claw. Sexual pores on all second coxae on ventrodistal surface. In contrast to the male, the female lacks cement gland tubes (see below).

Fig. 7
figure7

Pallenopsis aulaeturcarum sp. nov. Dömel & Melzer (clade ANT_C). (a), dorsal view. (b), lateral view of male. (c), cheliphore. (d), male oviger. (e), female oviger. (f), walking leg with enlargement of cement gland tube (g), setae on third coxa (h), and propodus with claw and auxiliary claws (i)

Measurements (holotype in mm)

Length of trunk (anterior margin of first trunk segment to distal margin of fourth lateral processes), 9.80; trunk width (across first lateral processes), 3.98; proboscis length, 4.27; abdomen length, 3.91; third right leg, coxa 1, 1.45; coxa 2, 4.68; coxa 3, 2.78; femur, 13.74; tibia 1, 11.66; tibia 2, 15.08; tarsus, 0.32; propodus, 2.46; claw, 1.72; auxiliary claws, 0.63.

Different segments were measured in natural posture.

Male

The general habitus and size of the male are similar to those of the female. Differences are in the sexual characters: oviger ten-articled (as is typical for the genus), but longer than in the female (Fig. 7d). Second articles, nearly twice the length of third article. Fourth and fifth articles the longest and equal in size. Distal articles more setose than proximal articles, with setae pointing in various directions. Long cylindrical cement gland tube is located in the center of the ventral side of the femur in a small recess on top of a little swelling (Figs. 7g, 9c). It is about a third of the diameter of the femur and points away from the podomere’s surface in a nearly right angle. Sexual pores on ventral side of second coxae of third and fourth pair of legs.

Etymology

The specific name aulaeturcarum stands for “the yard (aula) of the Turks (turcae)” and is dedicated to the eponymous pub in Munich called “Türkenhof” that was frequently visited to discuss the complex and very variable morphology of Pallenopsis. The good atmosphere and drinks definitely improved the spirit and inspired the authors.

Remarks

This species belongs to the Pallenopsis patagonica s.l. species complex as defined in [26] and also analysed in [37]. In the previous studies, this species was defined as clade ANT_C or C, respectively.

There are no unique characters present for this new species which can be used to separate it from most other species of the genus, but the combination of its several diagnostic characters (shape of cheliphore pad, distances of lateral processes, presence of setae on ventral and dorsal side of trunk, as well as absence of long setae on legs and thickenings on lateral processes) makes it possible to distinguish it from the others.

Pallenopsis obstaculumsuperavit sp. nov. Dömel urn:lsid:zoobank.org:act:69F7ADB8-26BB-4183-A178-67EEBABAE8BE Figures 8 a-g, 9 f-j

Type material

Holotype: JR262_1058 (ZSM-A20160708), female, South Georgia, − 55.144, − 36.245, 195.21 m, November/December 2011, missing legs: 3rd and 4th right side, 4th left side; one loose leg in the jar.

Paratypes: JR262_48_5_2 (ZSM-A20160713), female, South Georgia, − 54.284, − 36.083, 124.08 m, November/December 2011; JR287_124_1 (ZSM-A20160691), male; South Georgia, − 53.764, − 36.681, 151 m, May 2013; JR287_152 (ZSM-A20160694), female, South Georgia, − 53.758, − 36.690, 145 m, May 2013, Proboscis of this individual was used for further analyses with the SEM; JR262_1597_2 (ZSM-A20160710), male, South Georgia, − 54.396, − 37.384, 174.98 m, November/December 2011; PS77_211_6_1_3 (ZSM-A20160696), female, Shag Rocks, − 53.402, − 42.668, 290.2 m, February 2011.

The type series is deposited in the Bavarian State Collection of Zoology, in the department Arthropoda varia.

Distribution

Southern Ocean, from sub-Antarctic islands (South Georgia and Shag Rocks; − 53.597, − 41.214) as well as the Antarctic continental shelf (west and east of the tip of the Antarctic Peninsula; − 63.389, − 60.120).

Diagnosis

Setae patches of half the width of lateral processes on first trunk segment and with size of width of whole lateral process for second and third trunk segment. Abdomen pointing downwards.

Description (female)

Size moderate, leg span less than 85 mm. Trunk with distinct segment borders, ridges strongly expressed (Fig. 8a,b). Ridges on dorsal side setae-rich with a setae patch of half the width of lateral processes on first segment and with size of width of whole lateral process for second and third trunk segment. Ventral surface covered with few setae. Lateral processes separated by about the size of their diameter, U-shaped (Figs. 8a, 9f). Distal margins of all processes display fringe of small spinules. On dorsal side, these spinules are located on slight thickenings (Fig. 8b). Ocular tubercle situated on anterior end of cephalic segment. Top of ocular tubercle slightly bent backwards and pointed. Eyes prominent and pigmented, anterior eyes larger than posterior eyes. Proboscis sub-cylindrical, equally thick throughout and slightly directed downwards (Fig. 9f,g). It is about half the size of the trunk. Abdomen long, extending ventrally from the thorax and covered with few spinules (Figs. 8a, 9f). Cheliphores with two-articled scape, first article longer than second article (Fig. 8c). Ultimate cheliphore article (movable finger) equipped with setose pad. Moveable digit slightly longer than fixed digit, its tip curved. Inner margins straight and joined when closed. Setae pad has a triangular shape of which half the length is attached to chela whereas other half protrudes. Single-articled, laterally placed palp represents the rudimentary state typical for the genus (Fig. 8b). It takes the form of an elongated bulb that is twice as long as wide. Female oviger composed of ten articles (Fig. 8e). Proximal articles broaden slightly towards the distal part of each article. Second article longer, nearly twice the size of third article. Fourth oviger article more swollen and the longest of all. From fourth article onwards, article length decreases. Oviger articles are setose, with all setae pointing distally. Legs with several short setae (Fig. 8f,g). First and third coxa sub-equal. Second coxa about twice the length of third coxa. Assemblage of conspicuous setae on ventral side of second and third coxa, brush-like (Figs. 8h, 9i). Setae without bifurcation. Femur and first tibia about equal in size. Second tibia longest leg article. Tarsus is short and armed with one big spine on the ventral side nearer its distal part and a couple of smaller lateral spines. Propodus slightly curved, with three to four heel spines that differ insignificantly in length, but the distal spine is the largest (Figs. 8i, 9j). The remaining sole is covered with many shorter spines. Claw dorsally curved, its inner margin straight, its tip curved. Two auxiliary claws about one-half the length of main claw. Sexual pores on all second coxae on ventrodistal surface. In contrast to the male, the female lacks cement gland tubes (see below).

Fig. 8
figure8

Pallenopsis obstaculumsuperavit sp. nov. Dömel (clade ANT_D). (a), dorsal view. (b), lateral view of male. (c), cheliphore. (d), male oviger. (e), female oviger. (f), walking leg with enlargement of cement gland tube (g), setae on third coxa (h), and propodus with claw and auxiliary claws (i)

Fig. 9
figure9

Images of Pallenopsis aulaeturcarum sp. nov. Dömel & Melzer (clade ANT_C) (ae) and Pallenopsis obstaculumsuperavit sp. nov. Dömel (clade ANT_D) (fj). a, f, micro-computed tomography (μCT) of specimens in lateral view; scale bar = 2.5 mm. b, g, ventral view of proboscis; scale bar = 1.5 mm. c, h, detail view of cement gland tube on femur (male); scale bar = 1.5 mm. d, i, detailed view of coxae with setae on posterior margin of the third coxa (see arrow); scale bar = 1.5 mm. e, j, propodus with claw and accessory claws; scale bar = 1.5 mm. a, PS82_121_1; b, d, e, PS82_156_2_1; c, PS82_185_1; f, JR287_152; gj, JR287_124_3

Measurements (holotype in mm)

Length of trunk (anterior margin of first trunk segment to distal margin of fourth lateral processes), 14.33; trunk width (across first lateral processes), 7.40; proboscis length, 6.35; abdomen length, 6.36; third right leg, coxa 1, 2.53; coxa 2, 7.59; coxa 3, 3.2; femur, 19.76; tibia 1, 15.91; tibia 2, 24.50; tarsus, 0.81; propodus, 4.43; claw, 2.46; auxiliary claws, 1.49.

Different segments were measured in natural posture.

Male

The general habitus and size of the male is similar to the female. Differences are in the sexual characters: oviger also ten-articled, typical for genus, but longer than female (Fig. 8d). Second articles longer, nearly twice the length of third article. Fourth and fifth articles the longest and equal in size. Distal articles more setose than proximal articles, with setae pointing in various directions. Small cylindrical cement gland tube is located in the center of the ventral side of the femur on top of a little swelling (Figs. 8g, 9h). It is as high as its diameter and points away from the podomere’s surface in a nearly right angle. Sexual pores on ventral side of second coxae of third and fourth pair of legs.

Etymology

The specific name obstaculumsuperavit stands for “the one that overcame (superare) the obstacle (obstaculum)”. Pallenopsis obstaculumsuperavit has been reported from the Antarctic continental shelf and South Georgia, which are separated by deep sea representing a barrier for the dispersal of many brooding invertebrates.

Remarks

This species belongs to the complex Pallenopsis patagonica s.l. defined in [26] and also analysed in [37]. In the previous studies, this species was defined as clade ANT_D or D, respectively.

Combining morphological and genetic data

There is a significant positive correlation of greater morphological distances with larger genetic distances for both genetic distances calculated based on COI (r = 0.36, p <  0.0001; Fig. 10a) and EOG sequences (r = 0.51, p <  0.0001; see figure provided in the Additional file 7). There is only a small difference between both correlations and SUB_2 has high intraspecific genetic distances between specimens from Burdwood Bank and the Falkland Islands or the Patagonian shelf (Additional file 7). When dividing the genetic COI distances, which are available for all morphologically analysed individuals, into ranges (< 2.5% = intraspecific; > 2.5% = interspecific), the morphological distances are always higher for specimens that occur in allopatry than for those in sympatry. However, there is no significant difference between the genetic COI distance ranges, except for genetic distances above 10% (Fig. 10b).

Fig. 10
figure10

Morphological against genetic distances. Morphological distances plotted against uncorrected genetic COI distances a) for each individual with regression line (r = 0.36, p <  0.0001) and b) for genetic ranges differentiated into sympatric (white) and allopatric (grey) samples of the Pallenopsis patagonica species complex. Dashed line separates intraspecific (left) and interspecific (right) genetic distances



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