Linyphiidae, Blackwall, 1859

Breitling, Rainer, 2021, A completely resolved phylogenetic tree of British spiders (Arachnida: Araneae), Ecologica Montenegrina 46, pp. 1-51 : 20-22

Linyphiidae

This family is particularly difficult to analyse, not just because it is the largest of the British spider families, but also because a large part of recent taxonomic work has been dedicated to splitting genera into poorly supported smaller units on the basis of typological arguments, instead of identifying convincing relationships between genera; together with the traditionally poor genus concepts in this group, this has created such a degree of confusion that even a considerable amount of detailed phylogenetic analyses (both molecular and morphological) have not been able to completely clarify the situation, and the phylogenetic relationships of many genera remain unresolved at all levels. Additionally, while the molecular and morphological analyses show some convergence in a few important areas of the tree, a large fraction of the published trees is still highly unstable, and the addition of new characters or species can lead to major rearrangements (see, e.g., the discussions in Miller & Hormiga 2004 and Paquin et al. 2008). The proposal advanced here can only be a very first attempt at providing a comprehensive phylogenetic hypothesis for this group (within strict geographical limits).

The framework for the proposed linyphiid phylogeny is provided by the molecular analyses of Wang et al. (2015) and Dimitrov et al. (2017). This is complemented by the increasingly comprehensive morphological analyses of the entire family or large subgroups in Duperré & Paquin (2007), Gavish et al. (2013), Hormiga (1993, 1994, 2000), Hormiga & Scharff (2005), Miller & Hormiga (2004), Paquin et al. (2008), and Sun et al. (2012). Most importantly, the relative placement of genera required a much larger degree of personal interpretation of the traditional taxonomic and morphological literature. The British Linyphiidae were comprehensively analysed in terms of pedipalp morphology by Merrett (1963) and Millidge (1977), and less comprehensively in terms of their female genitalia by Millidge (1984, 1993). This information was complemented by the phylogenetic assessments implicitly (and rarely explicitly) contained in the works of Wiehle (1956, 1960) and Roberts (1987), as well as a thorough assessment of the morphological data encoded in the interactive key of linyphiid species by Anna Stäubli ( Stäubli 2020, http://www.araneae.nmbe.ch). The barcode analyses presented in Breitling (2019b) provided additional information, but were mostly used for determining the relationships within genera.

The internal topology of Agyneta is based on a careful interpretation of barcode data, in conjunction with a morphological analysis. Agyneta is a good example of a genus where species identification is challenging and the resulting mis-identifications cause difficulties in interpreting barcode database information. In the British fauna, Meioneta and Agyneta seem to be mutually monophyletic and could be maintained as subgenera, but in the global context, they should remain unified following Dupérré (2013), probably together with a number of smaller genera, such as Tennesseellum , Mesasigone and Nippononeta , in Agyneta s. lat.

Following Breitling (2019b), Saaristoa is considered a junior synonym of Aphileta , and Centromerita a junior synonym of Centromerus . In both cases, the proposed phylogenetic hypotheses support this synonymy, as it is necessary to maintain the monophyly of all named genera.

Collinsia is treated as a junior synonym of Halorates , following Buckle et al. (2001), Millidge (1977), Roberts (1987), and Tanasevitch (2009). As the proposed tree shows, it would be impossible to maintain C. inerrans in the same genus as C. holmgreni / C. distinctus , if H. reprobus is excluded. Joining the two genera in Halorates s. lat. seems more conservative in the short run, than a splitting off of C. inerrans (in Milleriana ), in the absence of a comprehensive revision of this and several related genera. The barcode data indicate a general confusion in this group, where most genera are not recovered as monophyletic. This is not fully reflected in the proposed tree, which gives priority to the morphological similarities; e.g., in its COI barcode, Mecynargus paetulus seems to be closer to H. inerrans than to the type of its genus, and H. inerrans closer to M. paetulus than to H. holmgreni ; complementary information from a larger range of molecular markers would be required to justify such a major rearrangement.

Dicymbium is treated as a subgenus in a considerably expanded genus Savignia , resulting in a number of new combinations, as shown in the tree. This change in rank is consistent with earlier proposals by Millidge (1977) concerning the expansion of Savignia to include most of the members of his “ Savignya genus group”. It is also supported by both molecular and morphological evidence as discussed in Frick et al. (2010) and Breitling (2019d). Savignia (Dicymbium) brevisetosa is certainly not a subspecies of S. (D.) nigra in the current sense, as the two occur sympatrically. The genitalia are indistinguishable and the two forms are not clearly ecologically distinct, although syntopic occurrence apparently is rare; it is therefore quite likely that they are synonymous, brevisetosa merely being a geographically restricted variant of the male prosomal morphology, as suggested by Roberts (1987). However, the genetic barcode data show two clusters (BINs) among the Dicymbium specimens, which could indicate the presence of two closely related species, one of which might correspond to the brevisetosa form, occasional intermediate specimens being the result of sporadic hybridisation. The two forms are therefore here considered conservatively as semispecies.

Erigone maritima is considered a separate species, distinct from E. arctica s. str., based on the considerable barcode gap between Nearctic and Palaearctic specimens identified as “ Erigone arctica ” s. lat. Whether the palaearctic species can be meaningfully subdivided into subspecies is currently an open question; given the high mobility and vast range of Erigone species, which are among the most frequent aeronauts, a relevant subspecific differentiation seems rather unlikely. Many of the morphologically well-defined Erigone species show a surprisingly narrow barcode gap, indicating relatively recent differentiation and arguing further against the probability of the existence of morphologically all but cryptic subspecies.

Mermessus (sub Eperigone ) was considered as probably closely related and possibly the sister group of Erigone s. lat. by Millidge (1987), and the barcode data support this placement.

Erigone longipalpis meridionalis is a phantom species as defined by Breitling et al. (2015, 2016) and probably only represents intraspecific variation of E. longipalpis . It is thus considered a nomen dubium and not included in the tree.

Frontinellina is considered a junior synonym of Frontinella , because of the close genetic affinities between representatives of the two genera.

Hilaira is considered a senior synonym of Oreoneta . When separating Oreoneta from Hilaira, Saaristo & Marusik (2004) point out that H. nubigena and H. pervicax are also not conspecific with the type species of Hilaira , H. excisa . Instead of creating three poorly delimited genera, it is far more informative to consider the three groups as subgenera within a monophyletic genus Hilaira s. lat., sister to Sciastes . For the subgenus including H. nubigena and H. pervicax , the name Utopiellum (type species: H. herniosa ) would be available, and it is here used in the tree.

Maso and Pocadicnemis are strongly united in the barcode data; their position relative to other higher erigonines is less clear. They are placed in the same group by Merrett (1963; Group E) and Locket & Millidge (1953; all tibiae with 1 dorsal spine; with Tm IV), but these are rather large groups, and the morphology of the two genera does not indicate a particularly close relationship to each other or other genera.

Oryphantes is considered a senior synonym of Anguliphantes , Improphantes , Mansuphantes and Piniphantes , following Breitling (2019b), and Palliduphantes antroniensis is also transferred to Oryphantes s. lat., where it belongs on the basis of its genital morphology (Bosmans in Heimer & Nentwig 1991), as confirmed by barcode information. As explained in Breitling (2019d), the synonymy is also supported by the observation of Wang et al. (2015) that a combination of a large number of genetic markers, including mitochondrial (COI and 16S) as well nuclear sequences (18S, 28S, H3), recovers Anguliphantes and Oryphantes as mutually polyphyletic with strong bootstrap support.

Millidge (1977) and Merrett (1963) point out similarities between Ostearius and Donacochara / Tmeticus , and Wiehle (1960) places Ostearius in his Donacochareae. However, this traditional placementDzof Ostearius in a clade with Tmeticus and Donacochara has long been dubious, and it is not supported by any of the recent analyses. Even the sister group relationship between the latter two is not strongly supported by any of the newer data. Hormiga (2000) and subsequent morphological assessments place Tmeticus far from Ostearius . The barcode data also do not indicate a close relationship: there, Ostearius is sister to Eulaira , matching Millidge’s earlier morphology- based proposal ( Millidge 1984).

Pelecopsis susannae is transferred to Parapelecopsis , based on similarity of genitalia and absence of dorsal spines on its tibiae. As this indicates that the boundary between the two genera is not quite clear, they are here treated as subgenera of Pelecopsis s.lat., and in the global context Parapelecopsis should possibly be discarded altogether.

Poeciloneta is treated as a senior synonym of Agnyphantes and Obscuriphantes . While the necessity of this merger is not obvious in the context of the British fauna, where each of these genera is represented by a single species, the global analysis shows that this move is required to obtain a monophyletic genus Poeciloneta .

In the case of morphologically homogeneous genera, where even the species boundaries have long been ambiguous and species groups have been fluid at best, in the absence of genetic data the proposed phylogenetic relationships can be little more than a poorly educated guess. The genus Porrhomma is a good example of this situation. The preferred tree presented here is based on a rather subjective assessment of the morphological affinities of the included species.

The placement of Pseudomaro as sister of Mioxena is based on unpublished data on the morphology of the males (A. Grabolle https://wiki.arages.de/index.php?title= Pseudomaro_aenigmaticus ). These indicate that the two genera may even be synonymous, but a formal synonymisation shouldDzawait a formal publication of the description of male Pseudomaro specimens.

Savignia is here considered in the broadest sense, as discussed in Breitling (2019d). It includes the former genera Dicymbium, Minyriolus , Glyphesis , Araeoncus , Diplocephalus and Erigonella . Various earlier authors, including Bosmans (1996), Frick et al. (2010), Holm (in lit. in Millidge 1977), and Millidge (1977) had already found that this group is so homogeneous and the genera so poorly defined that they should probably be merged in a single genus. The barcode results confirm this assessment. The subgenus assignments try to identify monophyletic groups, at least within the context of the British fauna, but they are tentative only, given that no comprehensive global analysis of the genus group has been performed, and their practical value could be debated. Savignia connata jacksoni is considered an infrasubspecific variant of Savignia connata , following Roberts (1987), and is therefore not included separately in the tree.

Figure 2 shows a mapping of selected morphological characters used for traditional “pragmatic” classifications of British linyphiids onto the proposed phylogenetic tree of this family.

Liocranidae

The status and phylogeny of this family are controversial; morphological analyses by Bosselaers & Jocqué (2002) and Ramírez (2014) do recover the family as presently defined as strongly polyphyletic. The molecular results of Wheeler et al. (2017) agree. Only the morphological study of Bosselaers & Jocqué (2013), which analyses the densest sample of species, including all genera found in the British Isles , presents a monophyletic Liocranidae s.lat. The preferred tree presented here shows a compromise between the different analyses: while it proposes that the British representatives of Liocranidae are united in a monophyletic group, it modifies the arrangement of genera suggested by Bosselaers & Jocqué (2013) to match the observation by Ramírez (2014) that Liocranum and Apostenus are more closely related to each other than to Agroeca (which Ramírez wants to remove to Clubionidae ). Scotina was not included in the study by Ramírez (2014), but is morphologically closer to Agroeca , although historically, the species of this genus have been placed in Agroeca , Liocranum , and Apostenus ( S. palliardii in all three).

Relationships within Scotina are based on the phylogeny proposed by Bosselaers & Jocqué (2013). The arrangement within Agroeca follows Braun’s (1967) division of the genus into two species groups, ( A. lusatica , A. brunnea , A. dentigera ) vs. ( A. cuprea , A. proxima ). This contradicts the results of Bosselaers & Jocqué (2013), but is supported by barcode data. The placement of A. lusatica (sister to A. dentigera ) and of A. inopina (sister to A. cuprea ) is based on the stated morphological similarities in Grimm (1986).

Figures

Figure 2. Selected morphological character states mapped onto the phylogenetic tree of British linyphiid spiders. The framework provided by the phylogenetic information allows the clear identification of patterns of character evolution, including a considerable degree of homoplasy for all characters. The definition of characters and character states is based on the linyphiid identification key by Anna Stäubli, provided on the Spiders of Europe website. The order of columns (from left to right) follows the order of the legends (from top to bottom). The left block of columns refers to characters applying to the male, the right to characters of the female. The female of Centromerus minutissimus has not yet been described.