The stable environment of subterranean realms is characterized by constant darkness, temperature and humidity, and scarcity of resources. This led to similar adaptations in different lineages of animals, such as the reduction of eyes and pigmentation. It is common textbook knowledge that blindness in cave insects is compensated for by transformations of other sensorial structures, especially the antennae with their rich array of sensilla. We tested this hypothesis with 33 species of Leiodidae of the tribe Leptodirini (Coleoptera) with and without eyes and from hypogean and epigean environments. We documented and compared the number, types, arrangement and density of smooth and furrowed antennal sensilla on certain flagellomeres. Our statistical analysis that took effects of body size and phylogeny into consideration showed that (1) the number of these sensilla does not differ between hypogean or epigean beetles; (2) the same applies to length and diameter of the antennal sensilla; (3) there is a difference in density, but unexpectedly it is lower in hypogean species. Our finding thus contrasts with widely accepted earlier interpretations for those external antennal sensilla in the studied Leptodirini, showing that sensillar patterns are scarcely affected in these subterranean beetles if at all, and even less dense in blind and cave-living species. Our results thus add a new facet to the evolution of cave animals.

The biotas of old islands formed from continental terranes usually have a more complex biogeographical history than those of young oceanic islands, including taxa which have originated by vicariance and/or colonisation, over a variety of timescales. The Tyrrhenian Islands of Corsica, Sardinia and the Tuscan Archipelago in the Mediterranean have a complex geological history, shaped by interactions between the African and Eurasian plates since the Mesozoic. Our understanding of the historical biogeography of Tyrrhenian endemics remains limited for many groups, including freshwater invertebrates. Here we use a time-calibrated phylogeny, derived from mitochondrial and nuclear DNA sequence data, to explore the evolutionary history of Tyrrhenian endemic Hydraena water beetles, an ecologically important group in the islands’ streams. Whilst no endemic Hydraena appear to date from the initial separation of Corsica-Sardinia from the European mainland in the Oligocene, we show that Tyrrhenian species stem from five colonisation events, occurring at different intervals in the last ca. 15 Ma, at least two endemic lineages apparently arising through isolation at the end of the Messinian Salinity Crisis. Hydraena reflexa Rey, 1884, long considered a geographical form or subspecies of the widespread H. pygmaea Waterhouse, 1833, is reinstated as a valid species, endemic to Corsica and Sardinia. H. reflexa can be distinguished from H. pygmaea on external and aedeagal characters, documented here in detail. Specimens of ‘ H. reflexa ’ from southern continental Italy are shown to differ from those on Corsica and Sardinia, having identical male genitalia to H. pygmaea from elsewhere in its range. Genetically, this Calabrian form also clusters with H. pygmaea , and may have arisen through past introgression between a reflexa -like ancestor and H. pygmaea following the connection of the Calabrian block with the nascent Apennines in the Pliocene. The degree of genetic divergence seen between H. pygmaea and H. reflexa suggests that they diverged approximately 5.5 Ma in the Miocene, following the isolation of Corsico-Sardinian and mainland populations at the end of the Messinian.

Minute moss beetles (Hydraenidae) are one of the most speciose and widespread families of aquatic Coleoptera, with an estimated 4000 extant species, found in the majority of aquatic habitats from coastal rock pools to mountain streams and from the Arctic Circle to the Antarctic islands. Molecular phylogenetic works have improved our understanding of the evolutionary history of the megadiverse Hydraena , Limnebius and Ochthebius in recent years, but most genera in the family have not yet been included in any phylogenetic analyses, particularly most of those which are restricted to the Southern Hemisphere. Using a multimarker molecular matrix, sampling over 40% of described species richness and 75% of currently recognized genera, we infer a comprehensive molecular phylogeny of these predominantly Gondwanan Hydraenidae. Whilst the genera we focus on are morphologically diverse, and currently classified across all four hydraenid subfamilies, our phylogenetic analyses suggest that these Gondwanan genera may instead constitute a single clade. As a result of our findings, the African genus Oomtelecopon Perkins syn.n. is shown to nest within Coelometopon Janssens, the New Zealand Homalaena Ordish syn.n. and Podaena Ordish syn.n. are synonymised with Orchymontia Broun, and the South African Pterosthetops Perkins syn.n. is synonymised with Prosthetops Waterhouse, resulting in Pterosthetopini Perkins syn.n. being synonymised with Prosthetopini Perkins. Mesoceratops Bilton & Jäch gen.n. is erected to accommodate six former members of Mesoceration Janssens, which is shown to be polyphyletic. We propose the replacement name Orchymontia ordishi Jäch & Bilton nom.n. for Homalaena dilatata Ordish, 1984 (now a junior homonym); altogether 39 new combinations are proposed. Our Bayesian divergence times infer an origin for this ‘Gondwana group’ of genera in Africa plus Madagascar in the mid‐Cretaceous and suggest that both vicariant and dispersal processes, together with extinctions, have shaped the biogeographic history of these beetles in the Southern Hemisphere during the Cretaceous, resulting in geographically conserved extant lineages. Finally, we reconstruct ancestral habitat shifts across our phylogeny, revealing numerous changes in habitat occupancy in these genera, including multiple origins of fully terrestrial, humicolous taxa in different regions.

Abstract Adaptation to life in caves is often accompanied by dramatically convergent changes across distantly related taxa, epitomized by the loss or reduction of eyes and pigmentation. Nevertheless, the genomic underpinnings underlying the evolution of cave-related phenotypes are largely unexplored. We investigated genome-wide gene evolutionary dynamics in three distantly related beetle tribes with at least six instances of independent colonization of subterranean habitats, inhabiting both aquatic and terrestrial underground systems. Our results indicate that remarkable genomic changes mainly driven by gene family expansions occurred prior to underground colonization in the three tribes, suggesting that genomic exaptation may have facilitated a strict subterranean lifestyle parallelly across beetle lineages. The three tribes experienced both parallel and convergent changes in the evolutionary dynamics of their gene repertoires. These findings pave the way towards a deeper understanding of the genomic toolkit of hypogean fauna from a macroevolutionary perspective.

Morphologically cryptic lineages confound many estimates of global biodiversity and are often discovered in ecologically specialized taxa, subject to strong morphological constraint. Such a situation may apply in many extreme environments, including supralittoral rockpools, where dramatic fluctuations in water availability and salinity impose strong selection pressures on the inhabitants. Here we explore the genetic diversity and phylogeography of supralittoral rockpool Ochthebius beetles in the eastern Atlantic and western Mediterranean, using a combination of mitochondrial and nuclear markers and dense geographical sampling of the three recognized widespread species. Our results point to the existence of morphologically cryptic lineages within all currently named taxa and suggest that the distribution of these is linked to both historical and contemporary marine hydrogeography; a combination of ocean currents and winds apparently driving the spatial patterns observed. The main contemporary barrier to dispersal for Ochthebius is located around the Ibiza Channel, whilst the Messinian Salinity Crisis appears to have been the ultimate driver of lineage diversification in these insects. Our results show that oceanographic processes do not just shape the evolution of fully marine species, but also impact significantly on the terrestrially derived inhabitants of the coastal zone.

ABSTRACT Chemical cues in subterranean habitats differ highly from those on the surface due to the contrasting environmental conditions, such as absolute darkness, high humidity or food scarcity. Subterranean animals underwent changes to their sensory systems to facilitate the perception of essential stimuli for underground lifestyles. Despite representing unique systems to understand biological adaptation, the genomic basis of chemosensation across cave-dwelling species remains unexplored from a macroevolutionary perspective. Here, we explore the evolution of chemoreception in three beetle tribes that underwent at least six independent transitions to the underground through a phylogenomics spyglass. Our findings suggest that the chemosensory gene repertoire varies dramatically between species. Overall, no parallel changes in the net rate of evolution of chemosensory gene families were detected prior, during, or after the habitat shift among subterranean lineages. Contrarily, we found evidence of lineage-specific changes within surface and subterranean lineages. Our results reveal key duplications and losses shared between some of the lineages transitioning to the underground, including the loss of sugar receptors and gene duplications of the highly conserved ionotropic receptors IR25a and IR8a, involved in thermal and humidity sensing among other olfactory roles in insects. These duplications were detected both in independent subterranean lineages and their surface relatives, suggesting parallel evolution of these genes across lineages giving rise to cave-dwelling species. Overall, our results shed light on the genomic basis of chemoreception in subterranean beetles and contribute to deepen our understanding of the genomic underpinnings of adaptation to the subterranean lifestyle at a macroevolutionary scale.

We redefine the genus Troglocharinus Reitter, 1908 based on a phylogenetic analysis with a combination of mitochondrial and molecular data. We recovered the current Speonomites mengeli (Jeannel, 1910) and S. mercedesi (Zariquiey, 1922) as valid, separate species within the Troglocharinus clade, not directly related to Speonomites Jeannel, 1910, a finding corroborated by a detailed study of the male and female genitalia. In consequence, we reinstate Speonomus mercedesi Zariquiey, 1922 stat. nov. as a valid species, transfer both of them to the genus Troglocharinus, T. mengeli (Jeannel, 1910) comb. nov. and T. mercedesi (Zariquiey, 1922) comb. nov., and redescribe the genus. The study of new material from the distribution area of the former S. mengeli revealed the presence of two undescribed species, T. sendrai sp. nov. and T. fadriquei sp. nov., which we describe herein. We designate the lectotype of Speonomus vinyasi Escolà, 1971 to fix its identity, as among its syntypes there are two different species. In agreement with the results of the phylogenetic analyses we establish the synonymy between the genus Speonomites and Pallaresiella Fresneda, 1998 syn. nv.

The chemosensory system has experienced relevant changes in subterranean animals, facilitating the perception of specific chemical signals critical to survival in their particular environment. However, the genomic basis of chemoreception in cave-dwelling fauna has been largely unexplored. We generated de novo transcriptomes for antennae and body samples of the troglobitic beetle Speonomus longicornis (whose characters suggest an extreme adaptation to a deep subterranean environment) in order to investigate the evolutionary origin and diversification of the chemosensory gene repertoire across coleopterans through a phylogenomic approach. Our results suggested a diminished diversity of odourant and gustatory gene repertoires compared to polyphagous beetles that inhabit surface habitats. Moreover, S. longicornis showed a large diversity of odourant-binding proteins, suggesting an important role of these proteins in capturing airborne chemical cues. We identified a gene duplication of the ionotropic coreceptor IR25a, a highly conserved single-copy gene in protostomes involved in thermal and humidity sensing. In addition, no homologous genes to sugar receptors or the ionotropic receptor IR41a were detected. Our findings suggest that the chemosensory gene repertoire of this cave beetle may result from adaptation to the highly specific ecological niche it occupies, and that gene duplication and loss may have played an important role in the evolution of gene families involved in chemoreception. Altogether, our results shed light on the genomic basis of chemoreception in a cave-dwelling invertebrate and pave the road towards understanding the genomic underpinnings of adaptation to the subterranean lifestyle at a deeper level.

SummaryA new representative of the subtribe Trechodina, Iberotrechodes spinosus n. gen., n. sp., subterranean and known so far from a single cave system of the Cantabrian chain (NW Spain), is described. The external morphology of the new species is highly derived within Trechodina, in particular for the peculiar and unique shape of its elytra, and likely due in part to its subterranean habits. A phylogenetic analysis based on six nuclear and mitochondrial genes placed it unambiguously within the subtribe Trechodina of Trechinae, as sister to all extent species of Trechodes plus Sporades, distributed in Africa, Madagascar, Asia, New Caledonia and Australia. Using a Bayesian molecular clock approach we estimated the separation of Iberotrechodes n. gen. and its sister clade to have occurred in the early Paleocene, at ca. 61 Ma. The biogeographic implications of this discovery are discussed in the context of the unusual abundance of phylogenetically and geographically isolated species of subterranean Carabidae in the Iberian peninsula. Finally, new phylogenetic relationships are highlighted within Trechodina, such as the sister-group relationship between the genera Pachytrechodes (Tanzania) and Himalotrechodes (Nepal), and the African origin of the genus Amblystogenium (Crozet Islands).

The long-term geological stability of aquatic habitats has been demonstrated to be a determinant in the evolution of macroinvertebrate fauna, with species in running (lotic) waters having lower dispersal abilities, smaller ranges and higher gene flow between populations than species in standing (lentic) environments. Lotic species have been hypothesized to be more specialised, but the diversification dynamics of both habitat types have not been studied in detail. Using a speciose lineage of water beetles we test here whether diversification rates are related to the habitat preference of the species and its consequences on turnover, which we expect to be higher for lotic taxa. Moreover, we tested whether life in lotic environments is acting as an evolutionary dead-end as it is considered an ecological specialisation. We built a comprehensive molecular phylogeny with 473 terminals representing 421 of the 689 known species of the tribe Hydroporini (Coleoptera, Dytiscidae), using a combination of sequences from four mitochondrial and two nuclear genes plus 69 mitogenomes obtained with NGS. We found a general pattern of gradual acceleration of diversification rate with time, with 2-3 significant diversification shifts. However, habitat is not the main factor driving diversification in Hydroporini based on SecSSE analyses. The most recent common ancestor of Hydroporini was reconstructed as a lotic species, with multiple shifts to lentic environments. Most frequent transitions were estimated from lentic and lotic habitats to the category "both", followed by transitions from lotic to lentic and lentic to lotic respectively, although with very similar rates. Contrary to expectations, we found little evidence for differences in diversification dynamics between habitats, with lotic environments clearly not acting as evolutionary dead-ends in Hydroporini.

Physiological traits are key in determining the vulnerability of narrow range, highly specialized animals to climate change. It is generally predicted that species from more stable environments possess lower thermal tolerance breadths and thermal plasticity than those from more variable habitats – the so‐called ‘climatic variability hypothesis’. However, evolutionary trade‐offs between thermal breadth and its plasticity are also seen in some taxa, and the evolution of thermal physiology remains poorly understood. Subterranean environments are excellent systems for exploring these issues, being characterized by stable climatic conditions, with environmental variability increasing predictably from deep to shallow habitats. Acclimation capacity will be fundamental in determining the sensitivity of subterranean species to climate change, since they have poor dispersal capacity and limited possibility to exploit thermally different microhabitats in the uniform cave environment. We assessed critical thermal maximum (CT max ) and short‐term heat acclimation capacity in three related beetles (Leiodidae: Leptodirini) with differing degrees of specialization to the subterranean environment (deep, shallow and facultatively subterranean, respectively) and therefore exposed to contrasting thermal variability in nature. Only the facultative subterranean species showed any acclimatory capacity, also having the highest CT max across the taxa studied. However, this species might experience the highest thermal stress in its habitat under climate change. The studied subterranean specialists will be poorly able to cope physiologically with temperature increase, but in contrast exposed to lower magnitude and rate of warming. Our results fit the climatic variability hypothesis, suggesting that adaptation to cave conditions has selected against the retention of acclimation mechanisms. We show that the pathways that determine vulnerability of subterranean species to climate change depend on their degree of specialization to deep subterranean environments. This information, combined with evaluation of exposure to climatic changes at their present locations, is fundamental in identifying species or populations at greatest risk.

Understanding sources of variation in animal thermal limits is critical to forecasting ecological responses to climate change. Here, we estimated upper and lower thermal limits, and their capacity to respond to thermal acclimation, in several species and populations of diving beetles (Dytiscidae) from diverse geographic regions representative of variable climate within South Africa. We also considered ecoregions and latitudinal ranges as potential predictors of thermal limits and the plasticity thereof. For upper thermal limits, species showed significant variation and limited acclimation-related plasticity. Lower thermal limits responded to acclimation in some cases and showed marked variation among species that could be explained by taxonomic affiliation and ecoregion. Limited acclimation ability in the species included in this study suggest plasticity of thermal limits will not be a likely buffer for coping with climate change. From the present results for the Dytiscidae of the region, it appears the group may be particularly susceptible to heat and/or drought and may thus serve as useful indicator species of ecosystem change. Understanding how these climate-related impacts play out at different spatial and temporal scales will have profound implications for conservation management and functional responses, especially important in a region already showing a trend for warming and drying.

We describe a new subterranean species of the genus Exocelina Broun, 1886 (Coleoptera: Dytiscidae) from the Malay Peninsula. Almost all of the 196 species of that genus are epigean and distributed mainly in New Guinea, Australia, Oceania and New Caledonia. One epigean species is, however, known from China. The discovery of a species on the Malay Peninsula fills that distribution gap to some degree.

The evolution of the coleopteran suborder Adephaga is discussed based on a robust phylogenetic background. Analyses of morphological characters yield results nearly identical to recent molecular phylogenies, with the highly specialized Gyrinidae placed as sister to the remaining families, which form two large, reciprocally monophyletic subunits, the aquatic Haliplidae + Dytiscoidea (Meruidae, Noteridae, Aspidytidae, Amphizoidae, Hygrobiidae, Dytiscidae) on one hand, and the terrestrial Geadephaga (Trachypachidae + Carabidae) on the other. The ancestral habitat of Adephaga, either terrestrial or aquatic, remains ambiguous. The former option would imply two or three independent invasions of aquatic habitats, with very different structural adaptations in larvae of Gyrinidae, Haliplidae and Dytiscoidea.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

We describe four species of Ochthebius subgenus Cobalius (Coleoptera, Hydraenidae) from coastal rockpools of the western Palaearctic Region: O. anzar sp. nov. (south Morocco, plus a possible specimen from Lanzarote, Canary Islands), O. cortomaltese sp. nov. (Malta), O. evae sp. nov. (Atlantic coast of Morocco and Mediterranean coast of south Spain), and O. gorgadensis sp. nov. (Ilha de Santiago, Cabo Verde). A molecular phylogeny of all described species of Cobalius recovered three clades: (1) O. serratus Rosenhauer, 1864 as sister to the newly defined O. algicola group, including most of the Macaronesian species of Cobalius ( O. algicola Wollaston, 1871 from Madeira, O. freyi d’Orchymont, 1941 from Azores, O. lanthanus Ribera and Foster 2018 from Gran Canaria and Lanzarote, and O. balfourbrownei Jäch, 1989 and O. gorgadensis sp. nov. from Cabo Verde); (2) O. anzar sp. nov. as sister to the species of the newly defined O. biltoni group ( O. biltoni Jäch and Delgado, 2017 from Sicily, plus O. evae sp. nov. and O. cortomaltese sp. nov.); and (3) the newly defined O. lejolisii group, including O. lejolisii Mulsant and Rey, 1861 (Atlantic coast from south Morocco to the British Islands) plus O. subinteger Mulsant and Rey, 1861 (western Mediterranean), O. adriaticus Reitter, 1886 and its subspecies, O. celatus Jäch, 1989 and O. asper Sahlberg, 1900 stat. rev., previously considered a synonym of O. subinteger (all from the Central and Eastern Mediterranean). We note the strong intrapopulational morphological variation and the contrasting lack of consistent diagnostic characters between populations, despite the deep genetic divergence between lineages found within some species. We also discuss the widespread evolutionary convergence in beetles living in coastal habitats, sometimes in different genera or even families.

Three new species of the very rarely collected water beetle genus Adelphydraena Perkins, 1989 are described from Brazil, Guyana, and Suriname. Illustrations of the male genitalia of the two previously known species in the genus, A. orchymonti Perkins and A. spangleri Perkins from Venezuela, are presented for the first time. High resolution digital images of the habitus of all species are given, and geographical distributions mapped. New species described are Adelphydraena amazonica n. sp. (Brazil), A. spinosa n. sp. (Guyana), and A. surinamensis n. sp. (Suriname). Mitochondrial and nuclear DNA sequence data is given for three of the five species currently known (A. orchymonti, A. spangleri, and A. amazonica). The monophyly of the genus Adelphydraena was strongly supported, but relationships between the sequenced species remain uncertain.

Specialization to extreme environments is often considered an evolutionary dead-end, leading to irreversible adaptations and reduced evolvability. There is, however, mixed evidence for this macroevolutionary pattern, and limited data from speciose lineages. Here, we tested the effect of habitat specialization to hypersaline waters in the diversification rates of aquatic beetles of the genusOchthebius(Coleoptera, Hydraenidae), using a molecular phylogeny with more than 50% of the 546 recognized species, including representatives of all but one of the nine recognized subgenera and 17 species groups. Phylogenies were built combining mitochondrial and nuclear genes, with the addition of 42 mitochondrial genomes. Using Bayesian methods of character reconstruction, we show that hypersaline tolerance is an irreversible ecological specialization that arose multiple times. Two lineages ofOchthebiusexperienced a significant increase in diversification rates, one of them inhabiting hypersaline waters, but there was no overall correlation with habitat or any significant decrease in diversification rates despite the irreversibility of hypersaline tolerance. Our study tested for the first time the impact of hypersaline habitat specialization on diversification rates, finding no support for it to be an evolutionary dead-end. On the contrary, multiple and ancient lineages fully adapted to these extreme osmotic conditions have persisted and diversified over a long evolutionary timescale.

Beetles have colonized water many times during their history, with some of these events involving extensive evolutionary radiations and multiple transitions between land and water. With over 13,000 described species, they are one of the most diverse macroinvertebrate groups in most nonmarine aquatic habitats and occur on all continents except Antarctica. A combination of wide geographical and ecological range and relatively accessible taxonomy makes these insects an excellent model system for addressing a variety of questions in ecology and evolution. Work on water beetles has recently made important contributions to fields as diverse as DNA taxonomy, macroecology, historical biogeography, sexual selection, and conservation biology, as well as predicting organismal responses to global change. Aquatic beetles have some of the best resolved phylogenies of any comparably diverse insect group, and this, coupled with recent advances in taxonomic and ecological knowledge, is likely to drive an expansion of studies in the future.