Phylogenomics, Classification, and Lifestyle Evolution in Raft- and Nursery web- Spiders (Araneae: Dolomedidae and Pisauridae)

Abstract

Pisauridae Simon, 1890 or "nursery web spiders" are a global and heterogenous assemblage of spider genera with diverse lifestyles, containing web builders and webless species, as well as terrestrial and semi-aquatic species, notably "fishing spiders", genus Dolomedes Latreille, 1804. Incomplete, unresolved, or conflicting phylogenies have so far hampered testing for Dolomedes and pisaurid monophyly and evolution. Here, we broadly address these questions within a phylogenomic and comparative framework. Our goals are i) reconstruction of a robust phylogeny to test the monophyly of Dolomedes and Pisauridae and to amend Dolomedes classification; ii) estimation of evolutionary shifts and trends in lifestyles and capture webs; and iii) evaluation of hypotheses of morphological trait association with a semi-aquatic lifestyle. To this end we generate subgenomic data (ultraconserved elements or UCE) for 53 Dolomedes species and 28 pisaurid genera. We analyze these data using maximum likelihood, Bayesian, and multi-species coalescence approaches, as well as using two different phylogenetic time calibration methods, RelTime and MCMCtree. Consistent across analytical approaches, our phylogenies reject the monophyly of both Pisauridae and Dolomedes. "Pisaurid" genera fall into three clades: 1) Focal Clade I groups the majority, including Pisaura Simon, 1886, hence representing true pisaurids; 2) Focal Clade II = Blandinia Tonini et al., 2016 is sister to Trechaleidae Simon, 1890 and Lycosidae Sundevall, 1833; 3) Focal Clade III with fishing and raft spiders groups Dolomedes, Megadolomedes Davies and Raven, 1980, and Ornodolomedes Raven and Hebron, 2018 and is sister to Focal Clade II, Trechaleidae, and Lycosidae. Our taxonomy, based on complementary taxa and morphological evidence, resurrects Dolomedidae Simon, 1876 to include Dolomedes and the Oceanic genera Bradystichus Simon, 1884, Megadolomedes, Caledomedes Raven and Hebron, 2018, Mangromedes Raven and Hebron, 2018, Ornodolomedes, and Tasmomedes Raven and Hebron, 2018. Both RelTime and MCMCtree analyses yield comparable divergence estimations: Pisauridae origin is estimated between 29 and 40 Ma; Blandinia between 21 and 34 Ma; Dolomedidae between 10 and 17 Ma; and Dolomedes between 9 and 16 Ma. In order to avoid misleading significant correlations and/or over-resolved ancestral states, we performed taxon sampling bias correction in all evolutionary analyses. Evolutionary analyses reconstruct semi-aquatic lifestyle as ancestral to a large clade containing pisaurids, lycosids, trechaleids, Blandinia, and dolomedids, with several reversals to terrestrial lifestyle. Capture webs evolved at least three times, with reversals. Counter to expectation, the evolution of lifestyles and capture webs are independent. Although leg and tarsus lengths do not indicate lifestyles, semi-aquatic taxa are significantly larger than terrestrial ones. We explain this pattern with a biomechanical threshold over which surface tension can be broken while spiders forage under water. Our time-calibrated analyses indicate that the evolution of terrestrial and web-building lifestyles from semi-aquatic ancestors in Pisauridae coincided with cooling and drying climates in the mid-Miocene. We therefore hypothesize that climatic changes have acted as strong selection pressures toward lifestyle diversification.