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The principal eyes of jumping spiders (Salticidae) integrate a dual-lens system, a tiered retinal matrix with multiple photoreceptor classes and muscular control of retinal movements to form high resolution images, extract color information, and dynamically evaluate visual scenes. While much work has been done to characterize these more complex principal anterior eyes, little work has investigated the three other pairs of simpler secondary eyes: the anterior lateral eye pair and two posterior (lateral and median) pairs of eyes. We investigated the opsin protein component of visual pigments in the eyes of three species of salticid using transcriptomics and immunohistochemistry. Based on characterization and localization of a set of three conserved opsins (Rh1 - green sensitive, Rh2 - blue sensitive, and Rh3 - ultraviolet sensitive) we have identified potential photoreceptors for blue light detection in the eyes of two out of three species: Menemerus bivittatus (Chrysillini) and Habrocestum africanum (Hasarinii). Additionally, the photoreceptor diversity of the secondary eyes exhibits more variation than previous estimates, particularly for the small, posterior median eyes previously considered vestigial in some species. In all three species investigated the lateral eyes were dominated by green-sensitive visual pigments (RH1 opsins), while the posterior median retinas were dominated by opsins forming short-wavelength sensitive visual pigments (e.g. RH2 and/or RH3/RH4). There was also variation among secondary eye types and among species in the distribution of opsins in retinal photoreceptors, particularly for the putatively blue-sensitive visual pigment formed from RH2. Our findings suggest secondary eyes have the potential for color vision, with observed differences between species likely associated with different ecologies and visual tasks. 

Roots are common features in basaltic lava tube caves on the island of Hawai‘i. For the past 50 years, new species of cave-adapted invertebrates, including cixiid planthoppers, crickets, thread-legged bugs, and spiders, have been discovered from root patches in lava tubes on different volcanoes and across variable climatic conditions. Assessing vegetation on the surface above lava tube passages, as well as genetic characterization of roots from within lava tubes, suggest that most roots belong to the native pioneer tree, ‘ōhi‘a lehua (Metrosideros polymorpha). Planthoppers are the primary consumers of sap at the base of the subsurface food web. However, root physicochemistry and rhizobiome microbial diversity and functional potential have received little attention. This study focuses on characterizing the ‘ōhi‘a rhizobiome, accessed from free-hanging roots inside lava tubes. Using these results, we can begin to evaluate the development and evolution of plant-microbe-invertebrate relationships. We explored lava tubes formed in flows of differing elevations and ages, from about 140 to 3000 years old, on Mauna Loa, Kīlauea, and Hualālai volcanoes on Hawai‘i Island. Invertebrate diversity was evaluated from root galleries and non-root galleries, in situ fluid physicochemistry was measured, and root and bare rock fluids (e.g., water, sap) were collected to determine major ion concentrations, as well as non-purgeable organic carbon (NPOC) and total nitrogen (TN) content. To verify root identity, DNA was extracted, and three sets of primers were used. After screening for onlyMetrosiderosspp., the V4 region of the 16S rRNA gene was sequenced and taxonomy was assigned. Root fluids were viscous and ranged in color from clear to yellow to reddish orange. Root fluids had 2X to 10X higher major ion concentrations compared to rock water. The average root NPOC and TN concentrations were 192 mg/L and 5.2 mg/L, respectively, compared to rock water that had concentrations of 6.8 mg/L and 1.8 mg/L, respectively. Fluids from almost 300 root samples had pH values that ranged from 2.2 to 5.6 (average pH 4.63) and were lower than rock water (average pH 6.39). Root fluid pH was comparable to soil pH from montane wet forests dominated by ‘ōhi‘a (Selmants et al. 2016), which can grow in infertile soil with pH values as low as 3.6. On Hawai‘i, rain water pH averages 5.2 at sea level and systematically decreases with elevation to pH 4.3 at 2500 m (Miller and Yoshinaga 2012), but root fluid pH did not correlate with elevation, temperature, relative humidity, inorganic and organic constituents, or age of flow. Root fluid acidity is likely due to concentrated organic compounds, sourced as root exudates, and this habitat is acidic for the associated invertebrates. From 62 root samples, over 66% were identified to the genusMetrosideros. A few other identifications of roots from lava tube systems where there had been extensive clear-cutting and ranching included monkey pod tree, coconut palm,Ficusspp., and silky oak. The 16S rRNA gene sequence surveys revealed that root bacterial communities were dominated by few groups, including Burkholderiaceae, as well as Acetobacteraceae, Sphingomonadaceae, Acidobacteriaceae, Gemmataceae, Xanthobacteraceae, and Chitinophagaceae. However, most of the reads could not be classified to a specific genus, which suggested that the rhizobiome harbor novel diversity. Diversity was higher from wetter climates. The root communities were distinct from those described previously from ‘ōhi‘a flowers and leaves (Junker and Keller 2015) and lava tube rocky surfaces (Hathaway et al. 2014) where microbial groups were specifically presumed capable of heterotrophy, methanotrophy, diazotrophy, and nitrification. Less can be inferred for the rhizobiome metabolism, although most taxa are likely aerobic heterotrophs. Within the Burkholderiaceae, there were high relative abundances of sequences affiliated with the genusParaburkholderia, which includes known plant symbionts, as well as the acidophilic generaAcidocellaandAcidisomafrom the Acetobacteraceae, which were retrieved predominately from caves in the oldest lava flows that also had the lowest root pH values. It is likely that the bacterial groups are capable of degrading exudates and providing nutritional substrates for invertebrate consumers that are not provided by root fluids (i.e., phloem) alone. As details about the biochemistry of ‘ōhi‘a have been missing, characterizing the rhizobiome from lava tubes will help to better understand potential plant-microbe-invertebrate interactions and ecological and evolutionary relationships through time. In particular, the microbial rhizobiome may produce compounds used by invertebrates nutritionally or that affect their behavior, and changes to the rhizobiome in response to environmental conditions may influence invertebrate interactions with the roots, which could be important to combat climate change effects or invasive species introductions. 

Abstract The Hawaiian Islands are known to harbour a rich and diverse fauna of troglobionts (obligate subterranean species). To date, 74 obligate cavernicolous arthropod species have been documented from across the main Hawaiian islands, the majority of which were from Hawaiʻi Island, and mostly from lava tubes of Kilauea volcano, the youngest volcano on the island. A recent bioinventory of the Kipuka Kanohina lava tube system on the south-western side of Mauna Loa volcano revealed the existence of previously unknown cave-adapted species. Among them is the first cave-adapted species of the planthopper genus Iolania, Iolania frankanstonei Hoch & Porter sp. nov. Morphological and molecular data suggest that the species is closely related to the epigean (i.e. surface-dwelling) species Iolania perkinsi, which occurs in surface environments on Hawaiʻi Island. Thus, parapatric speciation is assumed, further corroborating the assumption that adaptive shifts are the major evolutionary patterns underlying the evolution of troglobionts on young oceanic islands. 

Knowledge of crustacean vision is lacking compared to the more well-studied vertebrates and insects. While crustacean visual systems are typically conserved morphologically, the molecular components (i.e. opsins) remain understudied. This review aims to characterize opsin diversity across crustacean lineages for an integrated view of visual system evolution. Using publicly available data from 95 species, we identified opsin sequences and classified them by clade. Our analysis produced 485 putative visual opsins and 141 non-visual opsins. The visual opsins were separated into six clades: long wavelength sensitive (LWS), middle wavelength sensitive (MWS) 1 and 2, short wavelength or ultraviolet sensitive (SWS/UVS) and a clade of thecostracan opsins, with multiple LWS and MWS opsin copies observed. The SWS/UVS opsins were relatively conserved in most species. The crustacean classes Cephalocarida, Remipedia and Hexanauplia exhibited reduced visual opsin diversity compared to others, with the malacostracan decapods having the highest opsin diversity. Non-visual opsins were identified from all investigated classes except Cephalocarida. Additionally, a novel clade of non-visual crustacean-specific, R-type opsins (Rc) was discovered. This review aims to provide a framework for future research on crustacean vision, with an emphasis on the need for more work in spectral characterization and molecular analysis. This article is part of the theme issue ‘Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods’. 

Estimating stomatopod species diversity using morphology alone has long been difficult; though over 450 species have been described, new species are still being discovered regularly despite the cryptic behaviors of adults. However, the larvae of stomatopods are more easily obtained due to their pelagic habitat, and have been the focus of recent studies of diversity. Studies of morphological diversity describe both conserved and divergent traits in larval stomatopods, but generally cannot be linked to a particular species. Conversely, genetic studies of stomatopod larvae using DNA barcoding can be used to estimate species diversity, but are generally not linked to known species by analyses of morphological characters. Here we combine these two approaches, larval morphology and genetics, to estimate stomatopod species diversity in the Hawaiian Islands. Over 22 operational taxonomic units (OTUs) were identified genetically, corresponding to 20 characterized morphological types. Species from three major superfamilies of stomatopod were identified: Squilloidea (4 OTUs, 3 morphotypes), Gonodactyloidea (9, 8), and Lysiosquilloidea (6, 7). Among these, lysiosquilloids were more diverse based on larval morphotypes and OTUs as compared to previously documented Hawaiian species (3), while squilloids had a lower diversity of species represented by collected larvae as compared to the seven species previously documented. Two OTUs / morphotypes could not be identified to superfamily as their molecular and morphological features did not closely match any available information, suggesting they belong to poorly sampled superfamilies. The pseudosquillid, Pseudosquillana richeri, was discovered for the first time from Hawaiʻi. This study contributes an updated estimate for Hawaiian stomatopod diversity for a total of 24 documented species, provides references for identification of larval stomatopods across the three major superfamilies, and emphasizes the lack of knowledge of species diversity in more cryptic stomatopod superfamilies, such as Lysiosquilloidea.