Ultrastructure of spermatogenic cells and spermatozoa in Hoplias malabaricus (Teleostei, Characiformes, Erythrinidae)

The differentiation of spermatids in Hoplias malabaricus is characterized by chromatin compaction, flagellum development, nuclear rotation, nuclear fossa formation, and excess cytoplasm elimination. In the resulting spermatozoon, the head is round and the nucleus contains chromatin compacted in thick filaments, peripherically arranged, to a central electron-lucent area. The acrosome is absent. The nuclear fossa is eccentric but not pronounced. The proximal centriole penetrates it and is oblique to the flagellum. The long midpiece has several converging elongate vesicles, forming membranous hoops in the initial segment of the flagellum, but has no cytoplasmic channel. The mitochondria are elongate and branched or C-shaped and located around the initial segment of the axoneme. The lateral flagellum does not show lateral projections. The ultrastructural characteristics of H.malabaricus spermatozoa are similar to the Cypriniformes.     

Cardiac function of two ecologically distinct Neotropical freshwater fish: Curimbata, Prochilodus lineatus (Teleostei, Prochilodontidae), and trahira, Hoplias malabaricus (Teleostei, Erythrinidae)

An isometric muscle preparation was used to investigate the importance of the ventricular sarcoplasmic reticulum (SR) and extracellular Ca(2+) (2.5 up to 10.5 mM) to force generation at 25 degrees C (acclimation temperature) in two ecologically distinct Neotropical teleost fish: Curimbata (active species), and trahira (sedentary species). The post-rest force was studied with and without 10 muM ryanodine in the medium. The positive inotropism observed for both species in response to increases on extracellular Ca(2+) reflected a greater Ca(2+) influx through sarcolemma, as well as an increase in Ca(2+) liberation from the SR by the Ca(2+)-induced Ca(2+) release mechanism. The significant post-rest potentiation recorded for the curimbata and trahira control preparations (3.22+/-0.24 to 6.55+/-0.77 mN mm(-2) and 0.74+/-0.07 to 2.26+/-0.26 mN mm(-2), respectively), was completely inhibited by the addition of ryanodine to the bathing medium, suggesting a potential functionality of SR for both species. Considering the differences in these species habitats, modes of life and levels of activity and the fact of a probable SR Ca(2+) cycling in a physiological temperature, we suggest that the functionality of the SR in these species is probably related to their phylogeny.     

Molecular and karyotypic phylogeography in the Neotropical Hoplias malabaricus (Erythrinidae) fish in eastern Brazil

The sedentary, predatory characin Hoplias malabaricus has one of the widest distributions of freshwater fishes in South America and is characterized by seven karyomorphs (A–G) that occur in sympatric and allopatric populations. Karyotypical patterns of variation in wild populations have been interpreted as evidence of multiple lineages within this nominal species, a possibility that may limit the validity of experimental data for particular karyomorphs. This study used the phylogeographic and genealogical concordance between cytogenetic (N = 49) and molecular (mitochondrial DNA) (N = 73) data on 17 samples, collected in 12 basins from south‐eastern and north‐eastern Brazil, to assess the systematic value of cytogenetic data. Cytogenetic patterns show a sex chromosome system in the 2n = 40F karyomorph. Molecular and cytogenetic data indicate a long, independent evolutionary history of karyomorphs and a coastal origin of continental populations in south‐eastern Brazil. The lack of fit with molecular clock expectations of divergence between groups is likely to be due to strong demographic fluctuations during the evolution of this species complex. The results indicate that karyotypical identification provides a reliable baseline for placing experimental studies on Hoplias spp. in a phylogenetic context.     

Cytotaxonomy in distinct populations of Hoplias aff. malabaricus (Characiformes,Erythrinidae) from lower Paranapanema River basin

Cytogenetic and morphometric analyses were carried out in Hoplias aff. malabaricus specimens from six distinct populations from the lower Paranapanema River basin, located between the states of Paraná and São Paulo, Brazil. Measurements were also taken from a specimen collected in Surinam. In a population from a fish farm station at Universidade do Norte do Parana (EPUNOPAR), two sympatric cytotypes (2n = 40 and 2n = 42 chromosomes) are found. A population from a fish farm station at Universidade Estadual de Londrina (EPUEL) shows 2n = 42 meta–submetacentric chromosomes for males and females with a simple sex chromosome system of XX/XY type. Populations from the Vermelho and Rancho Alegre Rivers, Três Bocas Stream and Paranapanema River have 2n = 39 chromosomes in males and 2n = 40 chromosomes in females, showing a multiple sex chromosome system of X₁X₁X₂X₂/X₁X₂Y type. Twenty morphological variables were studied. These measurements were used for an analysis of the canonical variables and standard analysis of proportional measurements. The most variable measurements among the specimens are the maxilla length (MXL) and the pre‐dorsal distance (PDD). Analysis of canonical variables indicates three distinct groups in the first canonical axis formed by: (1) Três Bocas Stream, (2) Rancho Alegre + Vermelho River + EPUNOPAR and (3) EPUEL + Paranapanema River. This axis retained 79·4% of information from the original matrix. Analysis of morphometrics reveals differences among populations from the Paranapanema River basin and between these and the specimen from Surinam. The morphometric and cytogenetic differences among the studied populations suggest a species complex.     

Final maturation and ovulation provoked by supply of homologous hypophysis in Hoplias m. malabaricus (Pisces, Erythrinidae)

The final maturation and the ovulation were induced in recently collected specimenes of Hoplias m. malabaricus that show oocytes with complete vitellogenesis, by using homologous hypophysis. The hormonal supply was a gradual and positive results were obtained with 9 to 10 hypophysis/kg injected according to 3 different methods. The time of oocytes' maturation from the first dose ranges between 32 and 60 hours a 18 degrees C. The stages of maturation of the oocyte were controlled by intraovaric cannulations using a numerical index to determine the position of the germinal vesicle.     

A tandemly repetitive centromeric DNA sequence of the fish Hoplias malabaricus (Characiformes: Erythrinidae) is derived from 5S rDNA

A substantial fraction of the eukaryotic genome consists of repetitive DNA sequences that include satellites, minisatellites, microsatellites, and transposable elements. Although extensively studied for the past three decades, the molecular forces that generate, propagate and maintain repetitive DNAs in the genomes are still discussed. To further understand the dynamics and the mechanisms of evolution of repetitive DNAs in vertebrate genome, we searched for repetitive sequences in the genome of the fish species Hoplias malabaricus. A satellite sequence, named 5SHindIII-DNA, which has a conspicuous similarity with 5S rRNA genes and spacers was identified. FISH experiments showed that the 5S rRNA bona fide gene repeats were clustered in the interstitial position of two chromosome pairs of H. malabaricus, while the satellite 5SHindIII-DNA sequences were clustered in the centromeric position in nine chromosome pairs of the species. The presence of the 5SHindIII-DNA sequences in the centromeres of several chromosomes indicates that this satellite family probably escaped from the selective pressure that maintains the structure and organization of the 5S rDNA repeats and become disperse into the genome. Although it is not feasible to explain how this sequence has been maintained in the centromeric regions, it is possible to hypothesize that it may be involved in some structural or functional role of the centromere organization.     

Ontogenetic allometric coefficient changes: implications of diet shift and morphometric traits in Hoplias malabaricus (Bloch) (Characiforme, Erythrinidae)

This study evaluated the relationship between body size and digestive tract characteristics of the important predatory freshwater fish Hoplias malabaricus , which is widely distributed in South America. The allometric coefficients were calculated for the mass and standard length ( L _S) relationships for two different L _S groups: (1) between 20 and 100 mm (characterized as insectivores) and (2) >100 mm (characterized as piscivores). Differential growth measured from the allometric coefficient, b , between the insectivore ( b < 3) and the piscivore ( b > 3) groups was detected. Anterior intestine length and pyloric caeca zone length showed significant differences between groups. Two complementary hypotheses were developed to explain the differential growth: (1) H. malabaricus has a digestive tract adapted to a piscivorous diet, which is independent of its ontogenetic stage of development, and (2) the negative allometry observed in group 1 individuals agrees with a general behavioural strategy, allowing individuals to grow in L _S during a shorter period of time.     

Gill chloride cell proliferation and respiratory responses to hypoxia of the neotropical erythrinid fish Hoplias malabaricus

The effect of chloride cell proliferation on the respiratory function was evaluated by measuring oxygen consumption (VO2) and ventilatory parameters during normoxia and gradual hypoxia in the tropical fish Hoplias malabaricus. Chloride cell proliferation was induced by keeping fish in deionized water, and the effect on the respiratory function was measured on the 1st, 2nd, and 7th day in this water using a flow-through respirometry system. Plasma osmolarity and the partial pressure of arterial oxygen (PaO2) and carbon dioxide (PaCO2) were measured under conditions of normoxia and severe hypoxia. Chloride cell proliferation on the lamellae significantly increased the water-blood diffusion distance on the 2nd and 7th day in deionized water. VO2 was kept constant until the critical oxygen pressure (PcO2) of 21.6+/-0.9 mmHg in both the control and deionized water fish was reached. The ventilatory parameters were higher in deionized water fish in normoxia, and increased during hypoxia, matching decreases in the water's partial O2 pressure. Impairment of the respiratory function was evidenced by the decrease of PaO2 of deionized water fish in normoxic condition. However, despite the changes in the epithelial morphology of gills in fish kept in deionized water, H. malabaricus proved be a hypoxic-tolerant tropical species.     

Cytogenetic variation of repetitive DNA elements in Hoplias malabaricus (Characiformes - Erythrinidae) from white,black and clear water rivers of the Amazon basin

Hoplias malabaricus is a common fish species occurring in white, black and clear water rivers of the Amazon basin. Its large distribution across distinct aquatic environments can pose stressful conditions for dispersal and creates possibilities for the emergence of local adaptive profiles. We investigated the chromosomal localization of repetitive DNA markers (constitutive heterochromatin, rDNA and the transposable element REX-3) in populations from the Amazonas river (white water), the Negro river (black water) and the Tapajós river (clear water), in order to address the variation/association of cytogenomic features and environmental conditions. We found a conserved karyotypic macrostructure with a diploid number of 40 chromosomes (20 metacentrics + 20 submetacentrics) in all the samples. Heteromorphism in pair 14 was detected as evidence for the initial differentiation of an XX/XY system. Minor differences detected in the amount of repetitive DNA markers are interpreted as possible signatures of local adaptations to distinct aquatic environments.     

The effects of long-term food deprivation on respiration and haematology of the neotropical fish Hoplias malabaricus

Growth of adult traíras Hoplias malabaricus ceased and body mass ( M ) decreased during starvation periods of 30, 60, 90, 150, 180 and 240 days. Hepatic reserves were mobilized in fish starved for 30 days, but liver mass of fish starved for longer periods was not significantly different from those starved for 30 days. Perivisceral fat bodies were consumed gradually, being completely exhausted after 240 days of food deprivation. Length of starvation was associated with a significant decrease in the oxygen uptake ( V o₂). In spite of this reduction, the respiratory frequency ( f _R) was kept nearly constant during the starvation periods. The haematocrit and the number of red blood cells decreased after 150 and 240 days of starvation, respectively. These parameters did not recover after refeeding (after 90 and 240 days of starvation). This hypometabolic state in response to food deprivation contributed to energy conservation during these periods. Traíras can survive food deprivation for periods of up to 180 days without reductions in metabolism and when they do become hypometabolic, normal metabolic rates are rapidly restored upon refeeding.     

Responses of the teleost Hoplias malabaricus to hypoxia

Synopsis Oxygen uptake (VO₂) during graded hypoxia, rate of hypoxia acclimation, breathing frequency (f_R), breath volume (V_{S, R}) and gill ventilation (V_G) were measured in Hoplias malabaricus. Normoxia and hypoxia acclimated fish had similar and constant VO₂ and V_G in a range of water PO₂ from 150 to 25 mmHg. Hypoxia acclimated fish showed significantly higher VO₂ in severe hypoxia (PO₂ <15 mmHg). Normoxia acclimated fish showed symptoms similar to hypoxic coma after 1 h of exposure to water PO₂ of 10 mmHg whereas the same symptoms were observed only at PO₂ of 5 mmHg for fish acclimated to hypoxia. Fish required 14 days to achieve full acclimation to hypoxia (PO₂ ≥25 mmHg). Lowering of water PO₂ from 150 to 25 mmHg resulted in normoxic fish showing a 3–2 fold increase in V_G. The increase was the result of an elevation in V_{S, R} rather than f_R. Among normoxia acclimated specimens, small fish showed a higher V_G per unit weight than the large ones in both normoxia (PO₂ =150 mmHg) and hypoxia (PO₂ = 15 mmHg). A decrease in the ventilatory requirement (V_G/VO₂) with increased body weight was recorded in hypoxia (PO₂ = 15 mmHg).     

Comparative chromosome mapping of 5S rDNA and 5SHindIII repetitive sequences in Erythrinidae fishes (Characiformes) with emphasis on the Hoplias malabaricus 'species complex'

Chromosomal localization of 5S rDNA and 5SHindIII repetitive sequences was carried out in several representatives of the Erythrinidae family, namely in karyomorphs A, D, and F of Hoplias malabaricus, and in H. lacerdae, Hoplerythrinusunitaeniatus and Erythrinus erythrinus. The 5S rDNA mapped interstitially in two chromosome pairs in karyomorph A and in one chromosome pair in karyomorphs D and F and in H. lacerdae. The 5SHindIII repetitive DNA mapped to the centromeric region of several chromosomes (18 to 22 chromosomes) with variations related to the different karyomorphs of H. malabaricus. On the other hand, no signal was detected in the chromosomes of H. lacerdae, H. unitaeniatus and E. erythrinus, suggesting that the 5SHindIII-DNA sequences have originated or were lost after the divergence of H. malabaricus from the other erythrinid species. The chromosome distribution of 5S rDNA and 5SHindIII-DNA sequences contributes to a better understanding of the mechanisms of karyotype differentiation among the Erythrinidae members.     

Chloride cell responses to ion challenge in two tropical freshwater fish,the erythrinids Hoplias malabaricus and Hoplerythrinus unitaeniatus

Chloride cell (CC) responses to ion challenge and plasma ion concentration were evaluated in two ecologically distinct erythrinids, Hoplias malabaricus, an exclusively water-breathing species, and Hoplerythrinus unitaeniatus, a facultative air-breathing fish, at one, two, seven, and 15 days of exposure to deionized water and to ion-rich water. H. malabaricus displayed high CC proliferation on filament and lamellar epithelium during exposure to deionized water and significant CC proliferation in the filament epithelium on the first day of exposure to water rich in NaCl and Ca2+ and in the lamellar epithelium on the first, second, and seventh day of exposure to such water. CC proliferation in H. unitaeniatus occurred only in the lamellar epithelium of fish exposed to deionized water. CC proliferation on both species was not accompanied by significant increase of CC density in contact with the external medium. The increase in the CC fractional area (CCFA) resulted from the increase of individual CC apical surface area on the first and second days of exposure to deionized water in H. malabaricus and only on the first day in H. unitaeniatus. Plasma ions in both erythrinid species showed transitory changes and, on the fifteenth day of exposure to the two types of experimental water, the plasma ion concentration was similar to the control fish. The CC responses of these erythrinid fish showed that CC proliferation depends on previous CC density in the gill and is not related solely to exposure to ion-poor water. Furthermore, CC proliferation in gill epithelium did not always involve an increase of CC density in contact with the external medium.     

New and previously described species of Urocleidoides (Monogenoidea: Dactylogyridae) infecting the gills and nasal cavities of Hoplias malabaricus (Characiformes: Erythrinidae) from Brazil

During an investigation of the biodiversity and structure of parasite communities among native populations of the erythrinid fish Hoplias malabaricus (Characiformes) from 7 rivers in Brazil, the following monogenoidean (Dactylogyridae) species were found: Urocleidoides malabaricusi n. sp., Urocleidoides naris n. sp., Urocleidoides cuiabai n. sp., Urocleidoides brasiliensis n. sp., and Urocleidoides eremitus Kritsky, Thatcher, and Boeger, 1986. Specimens considered as a new genus of Dactylogyridae were also found. The new species are mainly distinguished from the other 14 species of Urocleidoides in the general morphology of their copulatory and anchors/bar complexes. These new species and the new dactylogyrid genus are described, as well as supplemental observations and new illustrations of U. eremitus are provided. The present findings expand the known geographical distribution of species of Urocleidoides to southeastern and midwestern Brazil. A high speciation of this genus in the tropics is hypothesized, and it is briefly discussed.     

Genetic differentiation among distinct karyomorphs of the wolf fish Hoplias malabaricus species complex (Characiformes,Erythrinidae) and report of unusual hybridization with natural triploidy

In this study, genetic differentiation between karyomorphs A (2n = 42) and D (2n = 39/40) of the wolf fish Hoplias malabaricus, which is comprised of several cryptic species that present a wide variety of diploid chromosome numbers and sex chromosome systems, resulting in the identification of seven distinct karyomorphs (A–G), was investigated using a combination of molecular and cytogenetic tools. Deep sequence divergences for both karyomorphs were observed and indicate a long period of reproductive isolation between karyomorphs A and D. Additionally, one individual with 61 chromosomes was identified, which, as far as is known, is the first case of natural triploidy resulting from the hybridization between these highly differentiated karyomorphs of H. malabaricus. Molecular and cytogenetic analyses revealed that this allotriploid specimen carries two sets of maternal chromosomes from karyomorph D (2n = 40) and one set of chromosomes from karyomorph A (n = 21). Moreover, ribosomal sites and active nucleolus organizer regions from both parental contributors were found in the triploid hybrid. Considering the significant genetic distances between karyomorphs A and D, one of the primary reasons for the lack of recurrent reports of hybridization in the H. malabaricus species complex may be due to post‐zygotic barriers, such as hybrid sterility or unviability.     

Shoals Receive more Attacks from the Wolf-Fish (Hoplias malabaricus Bloch, 1794)

We investigated whether the piscivorous wolf‐fish, Hoplias malabaricus (Bloch, 1794), showed preferences for attacking a ‘straggler’ or a shoal using guppies, Poecilia reticulata (Peters, 1860), as prey. Predators should show a preference to attack single prey items rather than grouped prey to overcome the confusion effect, which has been shown to negatively affect their capture success. However, they may attack groups more frequently because they are more likely to detect them because of their greater conspicuousness. In our study we looked at predator choice for grouped or single prey and whether this was affected by larger shoals being more conspicuous. We offered Hoplias binary choices of a single guppy (straggler) and shoals of 2, 6 and 10 fish. Hoplias preferred to attack the shoal over the straggler with regards to both frequency of attacks (initial and subsequent) and attack duration. The implications for predator–prey interactions of such a preference are discussed.