Light-sensitive motile iridophores and visual pigments in the neon tetra, Paracheirodon innesi

Although motile iridophores in the longitudinal stripes of neon tetra skin are under control of the sympathetic nervous system, they also respond to light directly and show circadian color changes. Using neon tetra skin, we found that the photoresponse of iridophores depends on light intensity, and that light near 500 nm is most effective. RT-PCR demonstrated the expression of mRNAs encoding rhodopsin and two kinds of cone opsins (Pi-green1 and Pi-green2) in neon tetra skin where the light-sensitive iridophores exist. These mRNAs are also expressed in the lateral eyes. The cone opsin genes, Pi-green1 and Pi-green2, show high similarity with the g101 and g103 genes of unique green cone opsins (belonging to the MWS/LWS group) of the blind Mexican cavefish. These results show that Pi-green1, Pi-green2, and/or rhodopsin may play important roles in the photoresponse of neon tetra iridophores, which are most sensitive to light near 500 nm.     

Ionoregulatory specializations for exceptional tolerance of ion-poor, acidic waters in the neon tetra (Paracheirodon innesi)

To better understand how fish are able to inhabit dilute waters of low pH, we examined ionoregulation in exceptionally acid-tolerant neon tetras (Paracheirodon innesi), which are native to the ion-poor, acidic Rio Negro, Amazon. Overall ion balance was unaffected by 2-wk exposure to pH 4.0 and 3.5. Measurements of unidirectional Na+ fluxes during exposure to pH 3.5 showed that tetras experienced only a mild, ionic disturbance of short duration (相似文献   

Neural control of motile activity of light-sensitive iridophores in the neon tetra

Experiments with skin pieces revealed that the sympathetic nervous system controls the activity of the light-sensitive iridophores in the stripes of the neon tetra. The spectral peak reflected from the cells was shifted toward longer wavelengths as a result of a direct interaction between norepinephrine and alpha-adrenoceptors present on the cell membrane. Adenosine accelerated the recovery from the effects of the amine. Such regulation seems to operate when fish are in an excited state or under stress. Since alpha-melanophore-stimulating hormone (alpha-MSH), melanin-concentrating hormone (MCH) and melatonin caused the responses only at high concentrations, it is possible that these peptides and amine do not affect the properties of the light-reflecting cells in vivo.     

Microsatellite markers for the cardinal tetra Paracheirodon axelrodi,a commercially important fish from central Amazonia

The cardinal tetra Paracheirodon axelrodi is a very popular aquarium fish and the most important ornamental fishery resource for riverine communities inhabiting the Rio Negro floodplain in central Amazonia. Here we describe the isolation and characterization of 14 microsatellite DNA loci for cardinal tetras. Number of alleles and heterozygosity per locus in a sample of 30 fish ranged from 2 to 22 and from 0.49 to 0.92, respectively. These markers provide powerful tools for studies on conservation and management of cardinal tetra resources and for investigating evolutionary processes underlying population diversification in Amazonian flooded forest fishes.     

Extremely high variability in the S72 intron of the Amazonian cardinal tetra (Paracheirodon axelrodi)

Here, a combination of a DNA screening technique, direct sequencing and cloning was used to carry out a large-scale assessment of intron variability in cardinal tetras – the first to be conducted for an Amazonian forest fish. To the best of the authors' knowledge, the levels of DNA variability reported in this study are the highest ever documented for an intron marker.     

Comparative development in Moenkhausia pittieri and Paracheirodon innesi (Ostariophysi: Characiformes) with comments on heterochrony and miniaturization in the Characidae

Developmental heterochronies between the small‐sized Moenkhausia pittieri and the miniature Paracheirodon innesi were determined by describing and comparing external morphological development in both species. Despite being classified as miniature, P. innesi was larger and more developed in the flexion and post‐flexion larval stages than M. pittieri. Broader patterns within the Characidae suggest that body size reduction in P. innesi is tied to accelerated development and higher growth rate along with early maturation and precocious cessation of growth, which are characteristic of miniature species. Comments on adipose‐fin loss in Characidae as a consequence of developmental truncation are included.     

Phylogeography of a flooded forest specialist fish from central Amazonia based on intron DNA: the cardinal tetra Paracheirodon axelrodi

1. Historic and extant landscape structures and environmental conditions are known to influence phylogeographic patterns and population histories in organisms from Amazonia. Recent work suggests that events of the Tertiary Period have played a significant role in generating biodiversity in this exceptionally rich but understudied region. However, species distributions and population structures are also affected by recent environmental and physical changes, especially within highly dynamic ecosystems. 2. The cardinal tetra is a small luminous fish native to the dynamic flooded forest ecosystems of the Rio Negro and Orinoco basins of South America. This very popular aquarium fish represents the single most important species in the ornamental fishery of the Rio Negro, an activity of high socio‐economic value for local riverine communities. Here we use a fine‐scale sampling regime and sequence data from the second intron of the Ribosomal protein S7 (S7 2) to investigate population structure, colonisation history and genealogical relationships in cardinal tetras of the Rio Negro. 3. High levels of S7 2 polymorphisms revealed phylogeographic patterns across several temporal settings that appear associated with the complex dynamics of the region. Our results suggest a long history of isolation and persistence of cardinal tetra populations in the headwaters and upper regions of the Rio Negro and recent events of colonisation within the incipient Rio Negro floodplain. These colonisation events were followed by recent population expansions likely facilitated by the establishment of the extensive Rio Negro floodplain, an anabranching ecosystem of Holocene age. 4. Our reconstruction of the phylogeographic history of cardinal tetras based on S7 2 intron sequence data reflects the complex influence of both geomorphological and climatic events through time in the Rio Negro basin. Our findings also indicate that in order to maintain the cardinal tetra fishery as a sustainable activity for rural Amazonians, the fishery should be restricted to the middle Rio Negro, the region of the basin where population replenishment is more likely to occur.     

Leucophores and iridophores of Fundulus heteroclitus: Biophysical and ultrastructural properties

Classical light microscopic studies on pigmentation of Fundulus heteroclitus (killifish) indicated that there are three groups of light reflecting cells; one group on the surface of scales reflects white light, while two other deeper groups (the melaniridophores and the stratum argenteum) are iridescent. The results presented here show that: (1) The scale leucophores reflect white light by a Tyndall light-scattering mechanism, by virtue of the presence of randomly oriented organelles of “novel” morphology. (2) The iridophores of the melaniridophores contain stacks of irregularly-spaced, large reflecting platelets which function as an imperfect multiple thin layer interference system. (3) The stratum argenteum consists of a continuous layer(s) of iridophores with reflecting platelets which are so regularly packed as to approach an ideal multiple thin layer interference system. (4) In all three types of light reflecting cells, the dimensions and packing (orientation) of the reflecting organelles satisfactorily account for the chromogenic properties of the cells, including colors as viewed under transmitted, reflected, or polarized light. (5) The spacial relationships between these light reflecting cells and adjoining melanophores are different for each type of light reflecting cell. Furthermore, we propose to replace the term reflecting platelet with refractosome.     

Motile iridophores of a freshwater goby,Odontobutis obscura

Summary Reflecting chromatophores in the dermis of the skin of a freshwater goby, Odontobutis obscura, are of an iridophore type. These chromatophores contain numerous reflecting platelets, which are similar to those in iridophores of other fish and amphibian species. It was found that these iridophores are motile, i.e., these cells respond to certain stimuli with translocation of the platelets within the cells. K⁺ ions induced dispersion of the platelets in excised scale preparations, but not in excised scales from chemically denervated fish. Norepinephrine and melatonin also induced dispersion of the platelets. Alpha-MSH was effective in aggregating these organelles into the centrospheres of the cells. The conclusions reached are: (1) iridophores of O. obscura are motile; (2) the movement of the iridophores is under nervous and hormonal control.     

Ultrastructural observations of motile iridophores from the freshwater goby, Odontobutis obscura

The ultrastructure of "motile" iridophores of Odontobutis obscura and the changes in cell shape related to the motility were studied with electron microscopy. Various structural details were revealed by this method, and their importance is discussed. Of particular interest were the abundant microfilaments observed in the cortical cytoplasm. Cross-sectional profiles of iridophores showed that, in an iridophore in the dendritic state, the platelets were scattered randomly throughout the centrosphere and its processes, so that the centrosphere appeared to be rather flat. In the punctate state, the platelets were gathered, in groups or in stacks with regular arrangements, in the centrosphere, which appeared to be ovoid in shape. The most notable finding was that, at this time, the processes from which the platelets were lost remained there without retracting. The results indicates clearly that the motility of the goby iridophores involves the migration of platelets within the fixed contour of each cell and that no amoeboidal changes in the shapes of the cells occur.     

Why is the neon tetra so bright? Coloration for mirror-image projection to confuse predators? “Mirror-image decoy” hypothesis

The neon tetra (Paracheirodon innesi), which inhabits blackwater streams of the Amazon basin, has extremely bright coloration characterized by metallic blue-green stripes. To clarify the ecological function of this coloration, we examined the appearance of living neon tetra. They changed color in response to lighting and background conditions, and became less conspicuous under each condition to the human eye. Although they appeared bright in colorless clear water, their stripes appeared darker in blackwater. In addition, the visible area of their stripes was small and their brightness decreased, unless they were observed within a limited viewing angle (approximately 30° above the horizon). The results show that from the viewpoint of approaching submerged predators, a bright mirror image of the stripes is projected onto the underside of the water’s surface, providing a dramatic visual target while the real fish remains less conspicuous. Based on these results, we hypothesize that the neon tetra’s bright coloration is an effective predator evasion strategy that confuses predators using bright mirror images.     

Ion flux and cortisol responses of cardinal tetra,Paracheirodon axelrodi (Schultz, 1956), to additives (tetracycline,tetracycline + salt or Amquel®) used during transportation: contributions to Amazonian ornamental fish trade

The cardinal tetra, Paracheirodon axelrodi (Schultz), is the most important ornamental fish exported from Brazil and Colombia. During the transport of this species, Amazon fishermen usually add unmeasured amounts of tetracycline and/or table salt to the water in an effort to mitigate stress and mortality. The aim of the present study was to analyze the effect of salt, the antibiotic tetracycline and the ammonia chelating agent Amquel^® in the transport of this species. Cardinal tetras were submitted to a 24‐h boat transport in water containing either tetracycline hydrochloride (2.5 mg L^?1), Amquel^® (26.41 mg L^?1), tetracycline + table salt (2.5 mg L^?1 + 66.67 mg L^?1), or additive‐free water (control treatment). Whole‐body cortisol measurements showed that fish transported with any water additive had lower cortisol levels compared to control. However, based on ionoregulatory and water quality parameters, the addition of tetracycline, Amquel^® or the combination tetracycline + salt to the water of transport of cardinal tetras is not recommended when compared to control. Although Amquel^® helped stabilize the pH and diminish total ammonia levels in the transport water, this compound dramatically increased net Na⁺ and Cl^? uptake, and therefore compromised cardinal tetra ionoregulation.     

Regressive evolution in the Mexican cave tetra, Astyanax mexicanus

The evolutionary forces driving the reduction of eyes and pigmentation in cave-adapted animals are unknown; Darwin famously questioned the role of natural selection in eye loss in cave fishes: "As it is difficult to imagine that eyes, although useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse"[1]. We studied the genetics of eye and pigmentation regression in the Mexican cave tetra, Astyanax mexicanus, by mapping and quantitative trait loci (QTL) analysis. We also mapped QTL for the putatively constructive traits of jaw size, tooth number, and numbers of taste buds. The data suggest that eyes and pigmentation regressed through different mechanisms. Cave alleles at every eye or lens QTL we detected caused size reductions, consistent with evolution by natural selection but not with drift. QTL polarities for melanophore number were mixed, however, consistent with genetic drift. Arguments against a role for selection in the regression of cave-fish eyes cited the insignificant cost of their development [2, 3], but we argue that the energetic cost of their maintenance is sufficiently high for eyes to be detrimental in the cave environment. Regression can be caused either by selection or drift.     

Biochemical characterization of crystals from the dermal iridophores of a chameleon Anolis carolinensis

The biochemical characteristics of dermal iridophore crystals from Anolis carolinensis have been investigated. Iridophores isolated by collangenase-hyaluronidase treatment were sonicated and their contents fractionated through sucrose. Pure iridophore crystals so obtained were examined by chromatography and electron diffraction. They were found to be pure hydrated crystalline form. The suggestion is made that the subcrystalline structure of this guanine does not play a role in color production by the iridophore.     

Autotrophic energy sources for Paracheirodon axelrodi (Osteichthyes, Characidae) in the middle Negro River, Central Amazon, Brazil

The cardinal tetra (Paracheirodon axelrodi) is the most abundant species of the Brazilian ornamental fish trade, constituting more than 80% of the ornamental fish collected in the middle Negro River basin. Stable isotope analyses were used to identify the autotrophic carbon sources and trophic position for the cardinal in relation to the plant groups at the base of its foodchain. Filamentous algae, tree and plant leaves and cardinals were collected in stream habitats, flooded forest and interfluvial swamps (campos) during peak flood, falling water and low water periods. δ¹⁵N values of the cardinal in relation to the plants at the base of the food chain indicated a trophic position of omnivore. Values of δ¹³C for the plants ranged from −43.1 to −26.4‰, with averages of −37.6, −30.4, and −29.4‰ for filamentous algae, flooded forest leaves, and campo leaves, respectively. The δ¹³C values for the cardinal ranged from −35.0 to −27.9‰, with an average of −31.4‰. Relative contributions of plants to fish carbon were estimated in a two end-member mixing model which determined that the leaves (flooded forest and campo leaves combined) and filamentous algae had average relative contributions to cardinal carbon of 71% and 29%, respectively. However, seasonal variation in the relative contributions was encountered throughout the hydrological cycle. The cardinals least enriched in ¹³C were encountered in November during the falling water period, indicating that they had perhaps recently migrated down from interfluvial campos where filamentous algae production is significant. Considering that algal production has been reported to be less than 1% of total primary production in the Negro River, these results could suggest some evidence of selective herbivory in the cardinal’s food chain. Handling editor: J. M. Melack