Evolution of salinity tolerance in the diving beetle tribe Hygrotini (Coleoptera, Dytiscidae)

Underestimated diversity: Cryptic species and phylogenetic relationships in the subgenus Cobalius (Coleoptera: Hydraenidae) from marine rockpools

Marine rockpools are isolated patches of habitat in the supratidal environment (the so-called splash zone), at the transition between sea and land, found along the rocky shores worldwide and characterized by harsh conditions for life. Nonetheless, few specialized invertebrates successfully colonized this peculiar environment. Among them several members of the water beetles Ochthebius Leach, 1815, subgenus Cobalius Rey (1886), which are found almost exclusively in supratidal and upper-most intertidal marine rockpools from the eastern Atlantic Ocean (Cape Verde, Canary Islands and Morocco, Madeira, Azores) throughout the whole Mediterranean basin. The subgenus Cobalius before 2020 was considered to include ten valid species, based on morphological differences. In late 2020, four additional new species were described. However, recent molecular phylogenetic studies have uncovered further cryptic diversity suggesting the presence of multiple undetected species within this group, highlighting that the species boundaries remain unclear and systematics and taxonomy are in need of revision. In this study we provide a molecular phylogeny based on DNA sequence data from mitochondrial and nuclear genes obtained from ten described species belonging to the subgenus Cobalius, and extensive taxon sampling, in order to better understand the phylogenetic relationships within this genus and to infer the biogeographic processes behind its diversification. We also used a molecular clock to define a time window for diversification of distinct clades within the subgenus, and explore aspects of its evolutionary history. Finally, we used three species delimitation methods (PTP, GMYC and ABGD) to clarify taxonomy and validate species boundaries. Our phylogenetic and biogeographic results identified sixteen independent lineages grouped in four main clades and the possible origin of Cobalius was estimated to be in the Early Miocene (~22 Mya) in W Mediterranean area. Moreover, species delimitation methods suggest there are between 16 and 24 putative species, most of them diverged during the Late Miocene, Pliocene and Pleistocene (6.0-0.11 Mya).

Geographic variation in acclimation responses of thermal tolerance in South African diving beetles (Dytiscidae: Coleoptera)

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.

A subterranean species of Exocelina diving beetle from the Malay Peninsula filling a 4,000 km distribution gap between Melanesia and southern China

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.

Role of cuticle hydrocarbons composition in the salinity tolerance of aquatic beetles

Salinity tolerance has enabled the colonization of inland saline waters and promoted species diversification in some lineages of aquatic insects. However, the mechanisms behind this tolerance, particularly the role of cuticle hydrocarbons (CHCs), are not well-known. We characterized the CHC profile of eight species of two water beetle genera (Nebrioporus, Adephaga: Dytiscidae and Enochrus, Polyphaga: Hydrophilidae), which span the fresh-hypersaline gradient, to: i) determine the interspecific variation of CHC composition in relation to species' salinity tolerance; ii) explore plastic adjustments in CHC profiles in response to salinity changes at the intraspecific level in saline-tolerant species. CHC profiles were highly species-specific, more complex and diverse in composition, and characterized by longer-chain-length compounds in the species with higher salinity tolerance within each genus. Higher salinity tolerance in the Enochrus species was also associated with an increase in the relative abundance of branched alkanes, and with a lower proportion of n-alkanes and unsaturated compounds. These CHC characteristics are related with improved waterproofing capacity and suggest that reducing cuticle permeability was one of the key mechanisms to adapt to saline waters. Similar CHC composition patterns were found at the intraspecific level between populations from lower and higher salinity sites within saline-tolerant species of each genus. These saline species also displayed an extraordinary ability to adjust CHC profiles to changing salinity conditions in the laboratory in a relatively short time, which reflects great plasticity and a high potential to deal with daily and seasonal environmental fluctuations in the highly dynamic saline habitats.

Water Beetles as Models in Ecology and Evolution

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.