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Subterranean organisms provide excellent opportunities to investigate morphological evolution, especially of sensory organs and structures and their processing areas in the central nervous system. We describe the gross morphology of the brain and some cephalic sensory organs (olfactory organ, eye, semicircular canals of the inner ear) and the swim bladder (a non‐sensory accessory structure) of subterranean species of pencil catfishes of the genus Ituglanis Costa and Bockmann, 1993 (Siluriformes, Trichomycteridae) and compare them with an epigean species of the genus, Ituglanis goya Datovo, Aquino and Langeani, 2016. We compared qualitatively the size of the different brain regions and sense organs of the subterranean species with those of the epigean one, searching for modifications possibly associated with living in the subterranean environment. Our findings suggest that species of Ituglanis exhibit sensory characteristics that are preadaptive for the subterranean life, as only slight modifications were observed in the brains and sense organs of the subterranean species of the genus when compared with the epigean one. Because most subterranean fish species belong to lineages putatively preadapted for subterranean life, our results, discussed in the context of available information on the brain and sense organs of other subterranean species, help identify general trends for the evolution of the brain and sensory organs of subterranean fishes in general. [ABSTRACT FROM AUTHOR]

Genetic divergence in the presence of gene flow has been well documented, but there is little information on the specific factors maintaining divergence. The present study investigates this in the Mexican tetra (Astyanax mexicanus), an excellent model for studying this question because surface and cave populations differ markedly in phenotype and genotype but are interfertile. Previous population studies documented significant gene flow among cave and surface populations, but they focused on analyses of neutral markers whose evolutionary dynamics likely differ from those of genes involved in cave adaptation. The present study advances our understanding of this question by focusing specifically on the genetics responsible for eye and pigmentation reduction, signature traits of cave populations. Direct observations of two cave populations over the course of 63 years verify that surface fish frequently move into the caves and even hybridize with the cave fish. Importantly, however, historical records show that surface alleles for pigmentation and eye size do not persist but are rapidly eliminated from the cave gene pool. It has been argued that regression of eyes and pigmentation was driven by drift, but the results of this study suggest that strong selection actively eliminates surface alleles from the cave populations. [ABSTRACT FROM AUTHOR]

Sinocyclocheilus longicornus sp. nov. is described from the Pearl River basin in Hongguo Town, Panzhou City, Guizhou Province, Southwest China. Based on the presence of the long horn-like structure on the back of the head, Sinocyclocheilus longicornus sp. nov. is assigned to the Sinocyclocheilus angularis species group. Sinocyclocheilus longicornus sp. nov. is distinguished from its congeners by a combination of morphological characters: (1) presence of a single, relatively long horn-like structure on the back of the head; (2) pigmentation absent; (3) reduced eyes; (4) dorsal-fin rays, ii, 7; (5) pectoral-fin rays, i, 13; (6) anal-fin rays, iii, 5; (7) pelvic-fin rays, i, 7; (8) lateral line pores 38-49; (9) gill rakers well developed, nine on first gill arch; and (10) tip of adpressed pelvic fin not reaching anus. [ABSTRACT FROM AUTHOR]

Modern accumulations of genetic data offer unprecedented opportunities for understanding the systematic classification and origins of specific groups of organisms. The genus Sinocyclocheilus is among the most cave abundant genera in Cyprinidae, with 76 recognized species, belonging to 4 species groups. Recent phylogenetic studies have shown that the classification of species groups within the genus Sinocyclocheilus remains controversial. In this study, we constructed a sequence supermatrix of 26 species from 4 species groups of the genus Sinocyclocheilus using the mitochondrial genome to reveal phylogenetic relationships, historical biogeography and patterns of species diversification in the genus Sinocyclocheilus. Phylogenetic analysis strongly supports the monophyletic groups of the 3 species groups (S. jii, S. cyphotergous, and S. tingi groups) except the S. angularis group. Phylogenetic analysis showed that S. anshuiensis and S. microphthalmus, which were recognized as numbers of S. angularis group, formed a strongly supported independent clade. Therefore, we propose a new species group, the S. microphthalmus group, which contains S. anshuiensis and S. microphthalmus. Biogeographic reconstruction suggests that the living Sinocyclocheilus may have originated in north‐central Guangxi at the late Eocene and dispersed outward after a vicariance at 32.31 Million years ago (Ma). Early diversification is focused on the late Oligocene (ca. 25 Ma), which is related to the second uplift of the Qinghai−Tibetan Plateau and the lateral extrusion of the Indochina at the Oligocene/Miocene boundary. Our results suggest that 2 uplifts of the Qinghai−Tibetan Plateau and climate change in the Miocene may have influenced the diversification of the Sinocyclocheilus lineage. [ABSTRACT FROM AUTHOR]

Limited data are available about the accumulation of heavy metals in cave fish. In current study, eight species of fish in six caves were selected and the heavy metal contents of Cr, As, Cd, Pb, Zn, Cu, Ni, and Hg. The results showed the average concentration of these metals in the order Zn > Cu > Cr > Ni > Cd > As > Pb > Hg. The bio-concentration factors of Cu, Zn, Cd, and Pb were greater than 5000. The outcome of the health risk assessment suggests that consumption of the viscera or gills of cave fish from the karst caves in Libo can pose considerable health risks.

Limited data are available about the accumulation of heavy metals in cave fish. In current study, eight species of fish in six caves were selected and the heavy metal contents of Cr, As, Cd, Pb, Zn, Cu, Ni, and Hg. The results showed the average concentration of these metals in the order Zn > Cu > Cr > Ni > Cd > As > Pb > Hg. The bio-concentration factors of Cu, Zn, Cd, and Pb were greater than 5000. The outcome of the health risk assessment suggests that consumption of the viscera or gills of cave fish from the karst caves in Libo can pose considerable health risks. [ABSTRACT FROM AUTHOR]

Sinocyclocheilus longibarbarus and Sinocyclocheilus punctatus were collected from a karst cave Libo County, southwest of China. The two Sinocyclocheilus species can be distinguished obviously by external morphological characteristics. In this study, the complete mitochondrial genome sequences of two species were assembled, and both sequences reflected gene organization typical for mitochondrial DNA of the genus Sinocyclocheilus, comprising of 13 protein-coding genes (PCGs), 22 transfer RNA genes (tRNAs), 2 ribosomal RNA genes (rRNAs), and a large non-coding control region. Phylogenetic analysis showed that S. punctatus was first clustered together with S. mutipunctatus, and S. longibarbarus was closely related to S. yishanensis. The complete mitogenome of two species may provide useful information for the further taxonomic and phylogenetic studies.

Animals often show high consistency in their social organisation despite facing changing environmental conditions. Especially in shoaling fish, fission–fusion dynamics that describe for which periods individuals are solitary or social have been found to remain unaltered even when density changed. This compensatory ability is assumed to be an adaptation towards constant predation pressure, but the mechanism through which individuals can actively compensate for density changes is yet unknown. The aim of the current study is to identify behavioural patterns that enable this active compensation. We compared the fission–fusion dynamics of two populations of the live-bearing Atlantic molly (Poecilia mexicana) that live in adjacent habitats with very different predator regimes: cave mollies that inhabit a low-predation environment inside a sulfidic cave with a low density of predatory water bugs (Belostoma sp.), and mollies that live directly outside the cave (henceforth called “surface” mollies) in a high-predation environment. We analysed their fission–fusion dynamics under two different fish densities of 12 and 6 fish per 0.36 m2. As expected, surface mollies spent more time being social than cave mollies, and this difference in social time was a result of surface mollies being less likely to discontinue social contact (once they had a social partner) and being more likely to resume social contact (once alone) than cave mollies. Interestingly, surface mollies were also less likely to switch among social partners than cave mollies. A random walk simulation predicted each population to show reduced social encounters in the low density treatment. While cave mollies largely followed this prediction, surface mollies maintained their interaction probabilities even at low density. Surface mollies achieved this by a reduction in the size of a convex polygon formed by the group as density decreased. This may allow them to largely maintain their fission–fusion dynamics while still being able to visit large parts of the available area as a group. A slight reduction (21%) in the area visited at low densities was also observed but insufficient to explain how the fish maintained their fission–fusion dynamics. Finally, we discuss potential movement rules that could account for the reduction of polygon size and test their performance.

Abstract Background The phenotypes of sperm are generally believed to be under the control of the diploid genotype of the male producing them rather than their own haploid genotypes, because developing spermatids share cytoplasm through intercellular bridges. This sharing is believed to homogenize their content of gene products. However, not all developing spermatids have identical gene products and estimates are that alleles at numerous gene loci are unequally expressed in sperm. This provides scope for the hypothesis that sperm phenotypes might be influenced by their unique haplotypes. Here we test a key prediction of this hypothesis. Results The haploid hypothesis predicts that phenotypically different sperm subpopulations should be genetically distinct. We tested this by genotyping different sperm subpopulations that were generated by exposing sperm to a chemical dye challenge (Hoechst 33342). Dye treatment caused the cells to swell and tend to clump together. The three subpopulations of sperm we distinguished in flow cytometry corresponded to single cells, and clumps of two or three. Cell clumping in the presence of the dye may reflect variation in cell adhesivity. We found that allelic contents differed among the three populations. Importantly, the subpopulations with clumped sperm cells were significantly enriched in allelic combinations that had previously been observed to have significantly lower transmission success. Conclusions We show that at least one sperm phenotype is correlated with its haploid genotype. This supports a broader hypothesis that the haploid genotypes of sperm cells may influence their fitness, with potentially significant implications for the transmission of deleterious alleles or combinations of alleles to their offspring.

Trichomycterus rosablanca is described as a new troglobitic catfish species from caves in southeastern Santander, Colombia. These caves are drained by the Carare River of the Magdalena River basin. The new species is characterized by the advanced condition in the typical troglomorphisms found in other congeneric cave-dwelling species, such as absence of eyes and pigmentation. Trichomycterus rosablanca is diagnosed by the following putative autapomorphies: 1) presence of a circular foramen in the main body of the interopercle, dorsal to the interopercular plate supporting the odontodes, and 2) presence of a single sensory pore in the posteriormost section of the infraorbital canal. Trichomycterus rosablanca can be distinguished from all known Trichomycterus species from Colombia by having the supraorbital canal interrupted in the nasal section, resulting in the pattern of s1, s2, s3, and s6 sensory pores, and the lachrimal/antorbital bone not enclosing the anteriormost section of the infraorbital canal. The genetic distinctiveness of Trichomycterus rosablanca is confirmed by GMYC and genetic distance method analyses of the cytochrome C oxidase subunit I gene sequence. The description of this species places Colombia as the second most diverse country in the continent in terms of number of cave fish species and calls the attention on the conservation efforts needed to guarantee the permanence of this remarkable diversity of hypogean fishes.

Trait loss represents an intriguing evolutionary problem, particularly when it occurs across independent lineages. Fishes in light-poor environments often evolve "troglomorphic" traits, including reduction or loss of both pigment and eyes. Here, we investigate the genomic basis of trait loss in a blind and depigmented African cichlid, Lamprologus lethops , and explore evolutionary forces (selection and drift) that may have contributed to these losses. This species, the only known blind cichlid, is endemic to the lower Congo River. Available evidence suggests that it inhabits deep, low-light habitats. Using genome sequencing, we show that genes related to eye formation and pigmentation, as well as other traits associated with troglomorphism, accumulated inactivating mutations rapidly after speciation. A number of the genes affected in L. lethops are also implicated in troglomorphic phenotypes in Mexican cavefish (Astyanax mexicanus) and other species. Analysis of heterozygosity patterns across the genome indicates that L. lethops underwent a significant population bottleneck roughly 1 Ma, after which effective population sizes remained low. Branch-length tests on a subset of genes with inactivating mutations show little evidence of directional selection; however, low overall heterozygosity may reduce statistical power to detect such signals. Overall, genome-wide patterns suggest that accelerated genetic drift from a severe bottleneck, perhaps aided by directional selection for the loss of physiologically expensive traits, caused inactivating mutations to fix rapidly in this species. [ABSTRACT FROM AUTHOR]

We compared the behavior, including spatial distribution, reaction to stimuli, activity phases, and agonistic interactions, as well as diet and reproduction, of the troglobitic Eigenmannia vicentespelaea and that of its epigean relative, E. trilineata, both from São Domingos karst area, central Brazil. We utilized ad libitum underwater observations, complemented by physicochemical water variables, habitat descriptions, and collections of specimens. Differences in behavioral aspects include the absence of cryptobiotic habits and an extended spatial and temporal activity in E. vicentespelaea that were not present in E. trilineata, and the foraging angle, which was approximately 30° in E. vicentespelaea and 90° in E. trilineata. The agonistic behaviors recorded for the troglobitic E. vicentespelaea could be related to the preservation of a character state that is present in its epigean relatives. Eigenmannia vicentespelaea and E. trilineata might be considered benthophagous invertivores, similar to other Eigenmannia electric fishes, with no evidence of seasonality in the volume and diversity of prey items in their stomachs, suggesting that there is a food spectrum common to the two species. Both epigean and troglobitic Eigenmannia fish from Goiás reproduce during the dry season, with no indication of reproductive peaks during this period.

Background: The phenotypes of sperm are generally believed to be under the control of the diploid genotype of the male producing them rather than their own haploid genotypes, because developing spermatids share cytoplasm through intercellular bridges. This sharing is believed to homogenize their content of gene products. However, not all developing spermatids have identical gene products and estimates are that alleles at numerous gene loci are unequally expressed in sperm. This provides scope for the hypothesis that sperm phenotypes might be influenced by their unique haplotypes. Here we test a key prediction of this hypothesis. Results: The haploid hypothesis predicts that phenotypically different sperm subpopulations should be genetically distinct. We tested this by genotyping different sperm subpopulations that were generated by exposing sperm to a chemical dye challenge (Hoechst 33342). Dye treatment caused the cells to swell and tend to clump together. The three subpopulations of sperm we distinguished in flow cytometry corresponded to single cells, and clumps of two or three. Cell clumping in the presence of the dye may reflect variation in cell adhesivity. We found that allelic contents differed among the three populations. Importantly, the subpopulations with clumped sperm cells were significantly enriched in allelic combinations that had previously been observed to have significantly lower transmission success. Conclusions: We show that at least one sperm phenotype is correlated with its haploid genotype. This supports a broader hypothesis that the haploid genotypes of sperm cells may influence their fitness, with potentially significant implications for the transmission of deleterious alleles or combinations of alleles to their offspring. [ABSTRACT FROM AUTHOR]

Trichomycterus rosablanca is described as a new troglobitic catfish species from caves in southeastern Santander, Colombia. These caves are drained by the Carare River of the Magdalena River basin. The new species is characterized by the advanced condition in the typical troglomorphisms found in other congeneric cave-dwelling species, such as absence of eyes and pigmentation. Trichomycterus rosablanca is diagnosed by the following putative autapomorphies: 1) presence of a circular foramen in the main body of the interopercle, dorsal to the interopercular plate supporting the odontodes, and 2) presence of a single sensory pore in the posteriormost section of the infraorbital canal. Trichomycterus rosablanca can be distinguished from all known Trichomycterus species from Colombia by having the supraorbital canal interrupted in the nasal section, resulting in the pattern of s1, s2, s3, and s6 sensory pores, and the lachrimal/antorbital bone not enclosing the anteriormost section of the infraorbital canal. The genetic distinctiveness of Trichomycterus rosablanca is confirmed by GMYC and genetic distance method analyses of the cytochrome C oxidase subunit I gene sequence. The description of this species places Colombia as the second most diverse country in the continent in terms of number of cave fish species and calls the attention on the conservation efforts needed to guarantee the permanence of this remarkable diversity of hypogean fishes. [ABSTRACT FROM AUTHOR]

A new cave-dwelling fish, Triplophysa anshuiensis, is described here based on specimens collected from a karst cave in Guangxi Zhuang Autonomous Region, China, interconnected with the Hongshui River system, a tributary of the Xijiang River in the Pearl River (Zhu Jiang) Drainage. The species can be distinguished from its congeners by a combination of morphological characters. A key to the cave-dwelling species of Triplophysa in the Xijiang River is provided. [ABSTRACT FROM AUTHOR]

A new cave-dwelling fish Triplophysa luochengensis is described based on specimens collected from a karst cave in Guangxi Zhuang Autonomous Region, China, that is interconnected to the Hongshui River drainage. The species can be distinguished from its congeners by a combination of characters: eyes degenerated, anal fin with six branched rays, caudal fin with 16-17 branched rays, pectoral-fin length 72·4-95·8% of the distance between pectoral-fin origin and pelvic-fin origin, lateral head length 26·2-28·2% of standard length ( LS), eye diameter 7·5-8·6 of LS, body covered by sparse scales, lateral line complete and 7-8 pre-operculo-mandibular pores. Dark pigments irregularly present on dorsum of head, dorsum and flank. [ABSTRACT FROM AUTHOR]

We compared the behavior, including spatial distribution, reaction to stimuli, activity phases, and agonistic interactions, as well as diet and reproduction, of the troglobiticEigenmannia vicentespelaeaand that of its epigean relative,E. trilineata, both from São Domingos karst area, central Brazil. We utilizedad libitumunderwater observations, complemented by physicochemical water variables, habitat descriptions, and collections of specimens. Differences in behavioral aspects include the absence of cryptobiotic habits and an extended spatial and temporal activity inE. vicentespelaeathat were not present inE. trilineata, and the foraging angle, which was approximately 30° inE. vicentespelaeaand 90° inE. trilineata. The agonistic behaviors recorded for the troglobiticE. vicentespelaeacould be related to the preservation of a character state that is present in its epigean relatives.Eigenmannia vicentespelaeaandE. trilineatamight be considered benthophagous invertivores, similar to otherEigenmanniaelectric fishes, with no evidence of seasonality in the volume and diversity of prey items in their stomachs, suggesting that there is a food spectrum common to the two species. Both epigean and troglobiticEigenmanniafish from Goiás reproduce during the dry season, with no indication of reproductive peaks during this period. [ABSTRACT FROM AUTHOR]

Samples from seven different locations of the genus Pimelodella were genetically examined, two caves (exclusively subterranean, upper Tocantins River and São Francisco River) and five epigean (from upper Paraná River basin). Cytogenetic analyses revealed the same diploid number (2n=46) for all species besides similarities in both number and location of nucleolar organizer regions and C bands. FISH with 5S rDNA probes and CMA3 staining indicated significant differences among the studied species. Application of PCR-RFLP in ATPase 6 and 8 mitochondrial genes allowed building a minimum evolution phenogram identifying the close evolutionary relationship among groups. Both chromosomal and molecular data were useful to infer the relationships among studied Pimelodella species.Amostras de sete diferentes localidades do gênero Pimelodella foram geneticamente analisadas, duas cavernícolas (exclusivamente subterrâneas, alto rio Tocantins e rio São Francisco) e cinco epígeas (provenientes da bacia do alto Paraná). Análises citogenéticas revelaram o mesmo número diploide (2n=46) para todas as espécies, além de similaridades no número e localização das regiões organizadoras de nucléolo e bandas C. FISH com sondas de rDNA 5S e marcação com CMA3 indicaram diferenças significativas entre as espécies estudadas. A aplicação da técnica de PCR-RFLP nos genes mitocondriais ATPase 6 e 8 permitiu a construção de um fenograma de evolução mínima identificando uma estreita relação evolutiva entre as espécies estudadas.

Two cave populations of Aspidoras albater catfish with reduced eyes and pigmentation were recently found near the city of Posse, São Domingos karst area, Goiás State, representing the first known case of a troglomorphic callichthyid catfish. Cave specimens are described and compared to epigean specimens of A. albater, but morphometric differences between the epigean and the cave populations are not adequate to delimit as distinct species.Duas populações de Aspidoras albater com olhos e pigmentação reduzidos foram descobertas recentemente em cavernas do município de Posse, área cárstica de São Domingos, Goiás, representando o primeiro caso conhecido de Callichthyidae troglomórfico. Os espécimes cavernícolas são descritos e comparados com espécimes epígeos de A. albater, mas as diferenças morfométricas entre eles não são suficientes para delimitá-los como espécies distintas.

The Cueva del Azufre in Tabasco, Mexico, is a nutrient-rich cave and its inhabitants need to cope with high levels of dissolved hydrogen sulfide and extreme hypoxia. One of the successful colonizers of this cave is the poeciliid fish Poecilia mexicana, which has received considerable attention as a model organism to examine evolutionary adaptations to extreme environmental conditions. Nonetheless, basic ecological data on the endemic cave molly population are still missing; here we aim to provide data on population densities, size class compositions and use of different microhabitats. We found high overall densities in the cave and highest densities at the middle part of the cave with more than 200 individuals per square meter. These sites have lower H2S concentrations compared to the inner parts where most large sulfide sources are located, but they are annually exposed to a religious harvesting ceremony of local Zoque people called La Pesca. We found a marked shift in size/age compositions towards an overabundance of smaller, juvenile fish at those sites. We discuss these findings in relation to several environmental gradients within the cave (i.e., differences in toxicity and lighting conditions), but we also tentatively argue that the annual fish harvest during a religious ceremony (La Pesca) locally diminishes competition (and possibly, cannibalism by large adults), which is followed by a phase of overcompensation of fish densities.

Sinocyclocheiluslongicornus sp. nov. is described from the Pearl River basin in Hongguo Town, Panzhou City, Guizhou Province, Southwest China. Based on the presence of the long horn-like structure on the back of the head, Sinocyclocheiluslongicornus sp. nov. is assigned to the Sinocyclocheilusangularis species group. Sinocyclocheiluslongicornus sp. nov. is distinguished from its congeners by a combination of morphological characters: (1) presence of a single, relatively long horn-like structure on the back of the head; (2) pigmentation absent; (3) reduced eyes; (4) dorsal-fin rays, ii, 7; (5) pectoral-fin rays, i, 13; (6) anal-fin rays, iii, 5; (7) pelvic-fin rays, i, 7; (8) lateral line pores 38-49; (9) gill rakers well developed, nine on first gill arch; and (10) tip of adpressed pelvic fin not reaching anus., (Cheng Xu, Tao Luo, Jia-Jun Zhou, Li Wu, Xin-Rui Zhao, Hong-Fu Yang, Ning Xiao, Jiang Zhou.)

The brain is a plastic organ, and so intraspecific studies that compare results obtained from wild individuals with those from common-garden experiments are crucial for studies aiming to understand brain evolution. We compared volumes of brain regions between reproductively isolated populations of a neotropical fish, P oecilia mexicana, that has locally adapted to perpetual darkness ( Cueva Luna Azufre), toxic hydrogen sulphide in a surface stream ( El Azufre) or a combination of both stressors ( Cueva del Azufre). Wild fish showed habitat-dependent differences: enlarged telencephalic lobes and reduced optic tecta were found in fish living in darkness and sulphidic waters, in darkness without hydrogen sulphide or exposed to light and sulphide; fish from the sulphidic cave additionally showed enlarged cerebella. Comparison with common-garden reared fish detected a general decrease in brain size throughout populations in the lab, and little of the brain size divergence between lab-reared ecotypes that was seen in wild-caught fish. The pronounced differences in brain region volumes between ecotypes in the wild might be interpreted within the framework of mosaic evolution; however, the outcomes of common-garden experiments indicate a high amount of phenotypic plasticity. Our study thus highlights the importance of combining the investigation of brain size in wild populations with common-garden experiments for answering questions of brain evolution. [ABSTRACT FROM AUTHOR]

Rhamdiopsis krugi is a highly specialized troglobitic (exclusively subterranean) catfish from phreatic water bodies of caves located within two separated metasedimentary basins in the region of Chapada Diamantina, Bahia state, Brazil. In order to test the hypothesis of isolation with differentiation of the groups from the Una- Utinga and Irecê metasedimentary basins, we compared five populations among themselves and with an epigean species of Rhamdiopsis. This was accomplished using geometric morphometrics, a powerful tool for detecting differences in body shape at population and species levels. All studied samples differed significantly from each other, the epigean sample being the most distinct and the Una Basin populations clustering together. Geological and hydrological barriers explain the differences among the subterranean populations. We discuss our results together with the autapomorphies found in R. krugi, which validate its monophyly. These results imply an old age for the R. krugi clade, more than 10 Myr; alternative hypotheses are also presented. We propose a two-step vertical colonization model of the subterranean habitat through the hyporheic zone by an epigean ancestral, with a progressive acquisition of the autapomorphies characterizing R. krugi. For conservation purposes, the two differentiated sets of populations should be considered and referred to as R. krugi 'Una morphotype' and R. krugi 'Irecê morphotype'. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 114, 136-151. [ABSTRACT FROM AUTHOR]

Sinocyclocheilus longibarbarus and Sinocyclocheilus punctatus were collected from a karst cave Libo County, southwest of China. The two Sinocyclocheilus species can be distinguished obviously by external morphological characteristics. In this study, the complete mitochondrial genome sequences of two species were assembled, and both sequences reflected gene organization typical for mitochondrial DNA of the genus Sinocyclocheilus, comprising of 13 protein-coding genes (PCGs), 22 transfer RNA genes (tRNAs), 2 ribosomal RNA genes (rRNAs), and a large non-coding control region. Phylogenetic analysis showed that S. punctatus was first clustered together with S. mutipunctatus, and S. longibarbarus was closely related to S. yishanensis. The complete mitogenome of two species may provide useful information for the further taxonomic and phylogenetic studies. [ABSTRACT FROM AUTHOR]

This article presents information on a study which described five specimens of the new blind loach Triplophysa lihuensis sp. nov. (Teleostei: Balitoridae) which were collected from a cave in Renguang Village in Lihu Town in Nandan County in Guangxi Province, China. The species has been distinguished from all other congeneric species by having a combination of characters which include the absence of eyes and scales, lips with furrows and dorsal fin origin closer to caudal-fin base than to snout tip.

Across a variety of taxa, large offspring have been demonstrated to have a fitness advantage over smaller offspring of the same species. However, producing large offspring often comes at the cost of having to produce fewer young, and the payoff (and thus, evolutionary outcome) of this trade-off is expected to vary between environments. Atlantic mollies ( Poecilia mexicana: Poeciliidae, Teleostei), inhabiting a sulfidic cave and various non-sulfidic surface habitats in Tabasco (Mexico), are reproductively isolated and evolved divergent female life-history traits: females of the cave ecotype produce considerably fewer, but larger offspring. Stressful (sulfidic) environments may favor the production of larger offspring, as they are better able to cope with chemical stressors. It remains to be determined though if increased offspring survival outweighs the fitness cost of producing fewer but larger offspring even under benign laboratory conditions. We tested 30-day newborn survival of offspring from wild-caught P. mexicana females from diverging populations in a low-density, no predation, no cannibalism, and ad-libitum-food, benign laboratory environment. Survival rates were highly skewed towards larger cave molly offspring; however, surface molly females still had a higher fitness than cave molly females in terms of higher total numbers of surviving offspring. Our study provides evidence for an innate fitness advantage of larger cave molly offspring. Furthermore, the observed differences in life-history strategies could promote further divergence and reproductive isolation among these ecotypes of P. mexicana, because cave molly females immigrating into the adjacent surface habitats would most likely be selected against. [ABSTRACT FROM AUTHOR]

Abstract: Our study was designed to evaluate if, and to what extent, restrictive environmental conditions affect otolith morphology. As a model, we chose two extremophile livebearing fishes: (i) Poecilia mexicana, a widespread species in various Mexican freshwater habitats, with locally adapted populations thriving in habitats characterized by the presence of one (or both) of the natural stressors hydrogen sulphide and darkness, and (ii) the closely related Poecilia sulphuraria living in a highly sulphidic habitat (Baños del Azufre). All three otolith types (lapilli, sagittae, and asterisci) of P. mexicana showed a decrease in size ranging from the non-sulphidic cave habitat (Cueva Luna Azufre), to non-sulphidic surface habitats, to the sulphidic cave (Cueva del Azufre), to sulphidic surface habitats (El Azufre), to P. sulphuraria. Although we found a distinct differentiation between ecotypes with respect to their otolith morphology, no clear-cut pattern of trait evolution along the two ecological gradients was discernible. Otoliths from extremophiles captured in the wild revealed only slight similarities to aberrant otoliths found in captive-bred fish. We therefore hypothesize that extremophile fishes have developed coping mechanisms enabling them to avoid aberrant otolith growth – an otherwise common phenomenon in fishes reared under stressful conditions. [Copyright &y& Elsevier]

Photoautotrophs are generally considered to be the base of food webs, and habitats that lack light, such as caves, frequently rely on surface-derived carbon. Here we show, based on analysis of gut contents and stable isotope ratios of tissues (13C:12C and 15N:14N), that sulfur-oxidizing bacteria are directly consumed and assimilated by the fish Poecilia mexicana in a sulfide-rich cave stream in Tabasco state, Mexico. Our results provide evidence of a vertebrate deriving most of its organic carbon and nitrogen from in situ chemoautotrophic production, and reveals the importance of alternative energy production sources supporting animals in extreme environments. [ABSTRACT FROM AUTHOR]

Blind Mexican cave fish ( Astyanax fasciatus) lack a functional visual system and have been shown to sense their environment using a technique called hydrodynamic imaging, whereby nearby objects are detected by sensing distortions in the flow field of water around the body using the mechanosensory lateral line. This species has also been noted to touch obstacles, mainly with the pectoral fins, apparently using this tactile information alongside hydrodynamic imaging to sense their surroundings. This study aimed to determine the relative contributions of hydrodynamic and tactile information during wall following behaviour in blind Mexican cave fish. A wall was custom built with a 'netted' region in its centre, which provided very similar tactile information to a solid tank wall, but was undetectable using hydrodynamic imaging. The fish swam significantly closer to and collided more frequently with the netted region of this wall than the solid regions, indicating that the fish did not perceive the netted region as a solid obstacle despite being able to feel it as such with their pectoral fins. We conclude that the touching of objects with the pectoral fins may be an artefact of the intrinsic link between pectoral fin extensions and tail beating whilst swimming, and does not function to gather information. During wall following, hydrodynamic information appears to be used strongly in preference to tactile information in this non-visual system. [ABSTRACT FROM AUTHOR]

Vision is precluded in the darkness of hypogean environment, which is the reason why cave fish have to direct their locomotor activity using information only acquired through alternative sensory channels. The aim of the present study was to evaluate whether the presence of an object or its morphological features can be used as landmark in orienting the swimming activity of the blind cave cyprinid Phreatichthys andruzzii Vinciguerra 1924. We could verify, by means of a food conditioning procedure, that Phreatichthys is capable of (a) recognising and memorising the presence/absence of an object, (b) discriminating between different shapes and (c) utilising such an information to orient its locomotion in space. The results of our tests show that morphological cues can act as landmarks for Phreatichthys, thus supporting the hypothesis that an orienting mechanism based on such type of information can operate in the permanent darkness of cave habitats. [ABSTRACT FROM AUTHOR]

Life-history traits are very sensitive to extreme environmental conditions, because resources that need to be invested in somatic maintenance cannot be invested in reproduction. Here we examined female life-history traits in the Mexican livebearing fish Poecilia mexicana from a variety of benign surface habitats, a creek with naturally occurring toxic hydrogen sulfide (H2S), a sulfidic cave, and a non-sulfidic cave. Previous studies revealed pronounced genetic and morphological divergence over very small geographic scales in this system despite the absence of physical barriers, suggesting that local adaptation to different combinations of two selection factors, toxicity (H2S) and darkness, is accompanied by very low rates of gene flow. Hence, we investigated life-history divergence between these populations in response to the selective pressures of darkness and/or toxicity. Our main results show that toxicity and darkness both select for (or impose constraints on) the same female trait dynamics: reduced fecundity and increased offspring size. Since reduced fecundity in the sulfur cave population was previously shown to be heritable, we discuss how divergent life-history evolution may promote further ecological divergence: for example, reduced fecundity and increased offspring autonomy are clearly beneficial in extreme environments, but fish with these traits are outcompeted in benign habitats. [ABSTRACT FROM AUTHOR]

Local adaptation to divergent environmental conditions can promote population genetic differentiation even in the absence of geographic barriers and hence lead to speciation. But what mechanisms contribute to reproductive isolation among diverging populations? We tested for natural and sexual selection against immigrants in a fish species inhabiting (and adapting to) nonsulphidic surface habitats, sulphidic surface habitats and a sulphidic cave. Gene flow is strong among sample sites situated within the same habitat type, but low among divergent habitat types. Our results indicate that females of both sulphidic populations discriminate against immigrant males during mate choice. Furthermore, using reciprocal translocation experiments, we document natural selection against migrants between nonsulphidic and sulphidic habitats, whereas migrants between sulphidic cave and surface habitats did not exhibit increased mortality within the same time period. Consequently, both natural and sexual selection may contribute to isolation among parapatric populations, and selection against immigrants may be a powerful mechanism facilitating speciation among locally adapted populations even over very small spatial distances. [ABSTRACT FROM AUTHOR]

Life history traits within species often vary among different habitats. We measured female fecundity in mollies ( Poecilia mexicana) from a H2S-rich cave and from a neighbouring surface habitat, as well as in laboratory-reared individuals of both populations raised in either light or continuous darkness. Compared to conspecifics from surface habitats, cave-dwelling P. mexicana had reduced fecundity (adjusted for size) in the field. In the laboratory, the fecundity of surface mollies was higher in light than in darkness, whereas fecundity in the cave mollies was almost unaffected by the ambient light conditions. Our results suggest a heritable component to the reduction in fecundity in female cave mollies. Moreover, the reduced plasticity in fecundity of cave mollies in response to light conditions might be an example of genetic assimilation or channelling of a life history trait in a population invading a new environment. [ABSTRACT FROM AUTHOR]

Extreme habitats are characterized by the presence of physio-chemical stressors, but also differ in aspects of the biotic environment, such as resource availability or the presence of competitors. The present study quantifies variation in trophic ecology of a small livebearing fish ( Poecilia mexicana, Poeciliidae) across four different habitats that included nonsulphidic and sulphidic surface waters, as well as a nonsulphidic and a sulphidic cave. Resource use in different habitat types was investigated using gut content analysis. Populations diverged in resource use from a diet dominated by algae and detritus in nonsulfidic surface habitats to a diet including invertebrate food items in the other habitats. Poecilia mexicana in cave habitats further exhibited a higher dietary niche width than conspecifics from surface habitats. The condition of P. mexicana was analysed using storage lipid extractions. Fish from sulphidic and cave habitats exhibited a very poor condition, suggesting resource limitation and/or high costs of coping with extreme conditions. Finally, divergence in resource use was correlated with variation in viscerocranial morphology. A common garden experiment indicated both a genetic and plastic basis to the morphological variation observed among field populations. It is suggested that the morphological diversification is an adaptation to the differential use of resources among populations. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95, 517–528. [ABSTRACT FROM AUTHOR]

Subterranean cave ecosystems are characterized by perpetual darkness, almost constant ambient temperature, limited source of food supply and relatively high humidity. The occurrence of circadian rhythms in organisms living in such an ecosystem always attracts chronobiologists to understand the phenomena of time-measuring mechanisms. Few attempts have been made to correlate such rhythmic patterns of the organism with the putative periodicities in weak zeitgebers. In the present study, the effects of periodic feeding schedules on the characteristics of circadian rhythm in the vertical swimming activity of the cave loach Nemacheilus evezardi were examined. Results reveal that periodic feeding at 18:00 has the ability to synchronize the vertical swimming activity rhythm. It seems that periodic restricted feeding could act as a powerful zeitgeber of circadian rhythms in subterranean organisms. [ABSTRACT FROM AUTHOR]

Shoaling behavior protects fishes from avian and piscine predation, but at the same time costs of group living arise due to several mechanisms including increased food competition. Most cave fishes live in an environment in which avian and piscine predators are lacking, and cave environments are often characterized by low food availability, leading to increased food competition. Altogether, this should favor the reduction of shoaling in cave fishes. We compared shoaling behavior (i.e. the tendency to associate with a stimulus shoal) among surface dwelling populations of the Atlantic molly, Poecilia mexicana, and two cave forms of that species. The first cave population of P. mexicana originated from the Cueva del Azufre and was previously recognized as the only cave form of a poeciliid fish. The second cave population examined came from a cave that was discovered only recently (Cueva Luna Azufre). In both cave forms shoaling behavior was reduced compared with surface dwelling mollies. [ABSTRACT FROM AUTHOR]

The cave molly, Poecilia mexicana, from the Cueva del Azufre, a sulfur cave in Tabasco, Mexico, ranks among the best-studied cave fishes worldwide, despite being known from a single population only. Here we describe a newly discovered second population of cave-dwelling P. mexicana from a nearby, but mostly non-sulfidic cave (Luna Azufre). Despite apparent similarities between the two populations (such as reduced eye diameter and reduced pigmentation), a geometric morphometric analysis revealed pronounced morphological differentiation between the two cave forms. [ABSTRACT FROM AUTHOR]

Cave fishes need to rely on non-visual senses, such as the sense of smell or the lateral line to communicate in darkness. In the present study, we investigated sex identification by females of a cave-dwelling livebearing fish, Poecilia mexicana (cave molly), as well as its surface-dwelling relatives. Unlike many other cave fishes, cave mollies still possess functional eyes. Three different modes of presentation of the stimulus fish (a male and an equally sized female) were used: ( i) the stimulus fish were presented behind wire-mesh in light, allowing the focal female to perceive multiple cues, ( ii) the experiment was carried out under infrared conditions, such that only non-visual cues could be perceived and ( iii) the stimulus fish were presented in light behind transparent Plexiglas, allowing for the use of visual cues only. Females of all populations examined preferred to associate with the stimulus female in at least one of the treatments, but only when visible light was provided, suggesting that far-range sex recognition is limited or even absent in the cave molly under naturally dark conditions. [ABSTRACT FROM AUTHOR]

The phototactic behaviour of surface- and cave-dwelling Atlantic mollies Poecilia mexicana was examined. Surface-dwelling Atlantic mollies, as well as light- and dark-reared cave fish showed photophilic behaviour under all light intensities used (620, 50 and 3 lx). [ABSTRACT FROM AUTHOR]

1. Caves are often assumed to be predator-free environments for cave fishes. This has been proposed to be a potential benefit of colonising these otherwise harsh environments. In order to test this hypothesis, the predator–prey interaction of a belostomatid (predator) and a cave fish (prey) occurring in the Cueva del Azufre (Tabasco, Mexico) was investigated with two separate experiments. 2. In one experiment, individual Belostoma were given a chance to prey on a cave fish, the cave form of the Atlantic molly ( Poecilia mexicana), to estimate feeding rates and size-specific prey preferences of the predator. In the other experiment, population density of Belostoma was estimated using a mark–recapture analysis in one of the cave chambers. 3. Belostomatids were found to heavily prey on cave mollies and to exhibit a prey preference for large fish. The mark–recapture analysis revealed a high population density of the heteropterans in the cave. 4. The absence of predators in caves is not a general habitat feature for cave fishes. None the less predation regimes differ strikingly between epigean and hypogean habitats. The prey preference of Belostoma indicates that cave-dwelling P. mexicana experience size-specific predation pressure comparable with surface populations, which may have implications for life-history evolution in this cave fish. [ABSTRACT FROM AUTHOR]

Previous studies revealed that females of a cave form of the livebearing fish Poecilia mexicana (cave molly) have maintained the ancestral visual preference for large males, but — as an adaptation to life in darkness — they have evolved the novel capability to assess male size non-visually. Here we examined the mechanisms by which non-visual mate choice for large body size occurs. Are sex- and species-specific chemical cues involved in this preference for large conspecifics? We gave focal females an opportunity to associate with a large and a small stimulus fish in simultaneous choice tests, whereby the females could perceive either multiple cues (visual plus non-visual) from the stimulus fish, solely non-visual cues in darkness, or solely visual cues. Stimulus fish were two conspecific males, conspecific females, or heterospecific females (Xiphophorus hellerii). Cave molly females showed a significant preference for large conspecific males and for large conspecific females in all treatments. When a large and a small swordtail female were presented, cave molly females showed a preference for the larger fish only when exclusively visual cues from the stimulus fish were available. The non-visual preference for large body size appears to be mediated by species- but not by sex-specific cues, suggesting that species-specific chemical cues play an important role during mate choice. [ABSTRACT FROM AUTHOR]

Abstract: Living in extreme habitats typically requires costly adaptations of any organism tolerating these conditions, but very little is known about potential benefits that trade off these costs. We suggest that extreme habitats may function as refuge from parasite infections, since parasites can become locally extinct either directly, through selection by an extreme environmental parameter on free-living parasite stages, or indirectly, through selection on other host species involved in its life cycle. We tested this hypothesis in a small freshwater fish, the Atlantic molly (Poecilia mexicana) that inhabits normal freshwaters as well as extreme habitats containing high concentrations of toxic hydrogen sulfide. Populations from such extreme habitats are significantly less parasitized by the trematode Uvulifer sp. than a population from a non-sulfidic habitat. We suggest that reduced parasite prevalence may be a benefit of living in sulfidic habitats. [Copyright &y& Elsevier]

We investigated genetic differentiation and migration patterns in a small livebearing fish, Poecilia mexicana, inhabiting a sulfidic Mexican limestone cave (Cueva del Azufre). We examined fish from three different cave chambers, the sulfidic surface creek draining the cave (El Azufre) and a nearby surface creek without the toxic hydrogen sulphide (Arroyo Cristal). Using microsatellite analysis of 10 unlinked loci, we found pronounced genetic differentiation among the three major habitats: Arroyo Cristal, El Azufre and the cave. Genetic differentiation was also found within the cave between different pools. An estimation of first-generation migrants suggests that (i) migration is unidirectional, out of the cave, and (ii) migration among different cave chambers occurs to some extent. We investigated if the pattern of genetic differentiation is also reflected in a morphological trait, eye size. Relatively large eyes were found in surface habitats, small eyes in the anterior cave chambers, and the smallest eyes were detected in the innermost cave chamber (XIII). This pattern shows some congruence with a previously proposed morphocline in eye size. However, our data do not support the proposed mechanism for this morphocline, namely that it would be maintained by migration from both directions into the middle cave chambers. This would have led to an increased variance in eye size in the middle cave chambers, which we did not find. Restricted gene flow between the cave and the surface can be explained by local adaptations to extreme environmental conditions, namely H2S and absence of light. Within the cave system, habitat properties are patchy, and genetic differentiation between cave chambers despite migration could indicate local adaptation at an even smaller scale. [ABSTRACT FROM AUTHOR]

Most eucaryotic organisms classified as living in an extreme habitat are invertebrates. Here we report of a fish living in a Mexican cave (Cueva del Azufre) that is rich in highly toxic H2S. We compared the water chemistry and fish communities of the cave and several nearby surface streams. Our study revealed high concentrations of H2S in the cave and its outflow (El Azufre). The concentrations of H2S reach more than 300 μM inside the cave, which are acutely toxic for most fishes. In both sulfidic habitats, the diversity of fishes was heavily reduced, and Poecilia mexicana was the dominant species indicating that the presence of H2S has an all-or-none effect, permitting only few species to survive in sulfidic habitats. Compared to habitats without H2S, P. mexicana from the cave and the outflow have a significantly lower body condition. Although there are microhabitats with varying concentrations of H2S within the cave, we could not find a higher fish density in areas with lower concentrations of H2S. We discuss that P. mexicana is one of the few extremophile vertebrates. Our study supports the idea that extreme habitats lead to an impoverished species diversity. [ABSTRACT FROM AUTHOR]

Cave animals are widely recognised as model organisms to study regressive evolutionary processes like the reduction of eyes. In this paper, we report on the regressive evolution of species discrimination in the cave molly, Poecilia mexicana, which, unlike other cave fishes, still has functional eyes. This allowed us to examine the response to both visual and non-visual cues involved in species discrimination. When surface-dwelling females were given a chance to associate with either a conspecific or a swordtail ( Xiphophorus hellerii) female, they strongly preferred the conspecific female both when multiple cues and when solely visual cues were available to the female. No association preference was observed when only non-visual cues were provided. In contrast, cave-dwelling females showed no preference under all testing conditions, suggesting that species recognition mechanisms have been reduced. We discuss the role of species discrimination in relation to habitat differences. [ABSTRACT FROM AUTHOR]

In many species, male mating behaviour is correlated with male body size, with large males often being preferred by females. Small surface-dwelling Poecilia mexicana males compensate for this disadvantage by being more sexually active and using sneaky copulations. In a cave-dwelling population, however, small males do not show this behaviour. Do small males alter their behaviour in the presence of a large rival? Here, we investigated the influence of male competition on male mating behaviour in the cave form. Two males of different sizes were mated with a female either alone or together with the other male. No aggressive interactions were observed between either fish. There was no statistically significant difference in the frequency of sexual behaviours between the two treatments. In both treatments, large males were more sexually active than small males. Thus, small cave molly males do not switch to an alternative mating behaviour in the presence of a larger rival. Possibly, the extreme environmental conditions in the cave (e.g. low oxygen content and high levels of hydrogen sulphide) favour saving energetic costs, resulting in the absence of alternative mating behaviour in small males. [ABSTRACT FROM AUTHOR]

The Mexican tetra Astyanax fasciatus has evolved a variety of more or less color- and eyeless cave populations. Here we examined the evolution of the female preference for large male body size within different populations of this species, either surface- or cave-dwelling. Given the choice between visual cues from a large and a small male, females from the surface form as well as females from an eyed cave form showed a strong preference for large males. When only non-visual cues were presented in darkness, the surface females did not prefer either males. Among the six cave populations studied, females of the eyed cave form and females of one of the five eyeless cave populations showed a preference for large males. Apparently, not all cave populations of Astyanax have evolved non-visual mating preferences. We discuss the role of selection by benefits of non-visual mate choice for the evolution of non-visual mating preferences. [ABSTRACT FROM AUTHOR]

Abstract: Prior to this study, it was believed that epigean and hypogean Astyanax differ markedly in their display of agonistic behavior. Research suggested that surface-dwelling individuals were extremely aggressive whereas their blind, cave-dwelling counterparts tended to show little or no aggressive behavior. Aggression in Astyanax was thought to be triggered by visual stimuli because surface fish in a dark environment or surface fish blinded late in life did not show aggression. Here, we demonstrate that surface fish blinded early on in their embryonic development are highly aggressive as adults. We also report the first case of a population of blind cave-dwelling Astyanax that is highly aggressive. We conclude that reduced aggression is not the only evolutionary pathway for troglobitic Astyanax and that there is some degree of developmental plasticity in the releaser of aggression and in the selection of its triggering stimuli. [Copyright &y& Elsevier]