Gametogenesis and Development of Gymnocharacinus Bergi (Pisces, Characidae): Reproductive Mode Relative to Environmental Stability

Gymnocharacinus bergi (Pisces, Characidae) is classified as 'endangered' (IUCN 1996). It has been given particular attention among Patagonian fishes due to the reduction of some of its morphological structures, particularly scales, its extreme southern distribution among the Characiformes and its strong adaptation to a thermal environment in northeastern Patagonia. This paper describes the main aspects of gametogenesis, induced breeding, spawning and development of G. bergi, whose life history traits are related to the peculiar characteristics of its restricted environment. The temperature and flow stability of the Valcheta Stream make many of the environmental variables predictable. Other variables related to photoperiod undergo predictable fluctuation during the year. Gymnocharacinus bergi has synchronous gametogenesis and a rather short breeding season, which is earlier than for Paranensean characids. Its reproductive mode, courtship and spawning behaviour, and the absence of sexual dimorphism are common to other small characins from lotic environments. Gymnocharacinus bergi is a sedentary species with precocial characters, such as low fecundity and slightly adhesive, yolk-rich oocytes which are large in relation to the small size of the female. Larvae are robust, development is indirect and adulthood prolonged. These peculiar characteristics indicate a reproductive style very different from that of the main groups of Patagonian fish fauna.     

Aggressive Behaviour Between Gymnocharacinus Bergi (Pisces, Characidae) And Other Neotropical Fishes From A Thermal Stream In Patagonia

Gymnocharacinus bergi is a stenothermal endangered fish only found in the headwaters of a thermal stream within the temperate Somuncurá Plateau (Patagonia, Argentina). In spite of the presence of other fish species in the Valcheta Stream basin, none of these has been found at the headwaters. Temperature and salinity cannot account for the absence of extremely eurytopic Cnesterodon and Jenynsia. In addition, there is no evidence of diet overlapping or territory defence between these species and G. bergi. Therefore, we hypothesise that agonistic behaviour might be related to G. bergi's isolation. Experiments were performed in order to analyse interspecific interaction between G. bergi and individuals of the other two species. Cnesterodon decemmaculatus showed a submissive behaviour while Jenynsia multidentata was clearly aggressive. G. bergi decreased its aggressive behaviour towards C. decemmaculatus throughout the experiment. However, when placed with J. multidentata, its aggressive behaviour did not diminish. We suggest that the characteristics of the agonistic behaviour displayed by J. multidentata, opposite from those displayed by C. decemmaculatus, might account for the absence of J. multidentata in G. bergi's present habitat. With respect to C. decemmaculatus, other factors such as its low counter current swimming ability might play a major role in this phenomenon.     

Length–weight relationships for three small reef‐fishes from the Argentine coast: Helcogrammoides cunninghami (Smitt, 1898), Ribeiroclinus eigenmanni (Jordan, 1888), and Hypleurochilus fissicornis (Quoy and Gaimard, 1824)

In this study the first length–weight relationships are provided for Helcogrammoides cunninghami and Ribeiroclinus eigenmanni inhabiting subtidal rocky areas in northern Patagonia, and for Hypleurochilus fissicornis inhabiting intertidal rocky areas in Buenos Aires Province. Specimens were collected between 2009 and 2016, from seven different locations in northern Patagonia and Buenos Aires Province (Southwest Atlantic Coast; 37°–42°S), using a trawling dredge or a small hand net. New maximum lengths were recorded for R. eigenmanni and H. fissicornis.     

Steganoderma (Steganoderma) valchetensis n. sp. (Digenea: Zoogonidae) from the relict fish Gymnocharacinus bergi (Osteichthyes: Characidae) in Argentina

A new species of Steganoderma (Steganoderma) (Digenea: Zoogonidae) was found in the digestive tract of the southernmost, endemic and scale-less characid, Gymnocharacinus bergi, inhabiting thermal headwaters of the Valcheta Creek in northern Patagonia, Argentina. Steganoderma valchetensis n. sp. is included in the subgenus Steganoderma because of having a sessile ventral sucker and caeca reaching to the testes. This new species can be distinguished from other species of this genus on the basis of body-length, sucker-ratio, the length of the cirrus-sac, the morphology of the seminal vesicle and gonads, and the location of the vitelline follicles and genital pore. The taxonomic status of the species of Steganoderma, parasitising the freshwater fishes of Patagonia, in southern Argentina, and Chile, is also discussed.     

Lethal temperatures of a Neotropical fish relic in Patagonia, the scale-less characinid Gymnocharacinus bergi

Southern South America has a rather low fish species diversity. Gymnocharacinus bergi, the southernmost characid fish of the world, is the only member of Characoidei in the Argentine Patagonia. The isolation of this species in an endorheic stream has been linked to the thermal conditions of its habitat, the head-waters of the Valcheta Stream, which is the only site where this species occurs. We provide information on the distribution and thermal habitat of this species and other fishes in the Valcheta Stream. The responses of G. bergi to high and low temperatures were assessed in the laboratory under different temperatures and heating and cooling rates. Our results suggest that G. bergi is unable to extend its distribution to the colder waters nearby, as well as to waters with greater temperature fluctuations. We discuss the implications of our experimental data, the habitat of G. bergi, and the known responses of a few other paranensean fishes to temperature, within the framework of the thermal ecology of freshwater fishes.     

Interactions Between Habitat Use and Patterns of Abundance in Coral-dwelling Fishes of the Genus <Emphasis Type="Italic">Gobiodon</Emphasis>

Coral-dwelling fishes from the genus Gobiodon are some of the most habitat specialised fishes on coral reefs. Consequently, we might expect that their population dynamics will be closely associated with the abundance of host corals. I used a combination of log-linear modelling and resource selection ratios to examine patterns of habitat use among eight species of Gobiodon in Kimbe Bay, Papua New Guinea. I then used multiple regression analysis to investigate relationships between the abundance of each species of Gobiodon and the abundance of the corals they inhabited. Each species of Gobiodon used one or more species of coral more frequently than expected by chance. The pattern of habitat use exhibited by each species of Gobiodon did not vary among reef zones or among reefs with different exposures to prevailing winds, despite changes in the relative abundances of corals among reef zones. This consistency in habitat use might be expected if the coral species inhabited confer considerable fitness advantages and, therefore, are strongly preferred. For most species of Gobiodon, abundances among reef zones and exposure regimes were correlated with the abundance of the coral species usually inhabited. Therefore, it appears that habitat availability helps determine abundances of most species of Gobiodon in Kimbe Bay. In addition to correlations with habitat availability, the abundances of G. histrio, G. quinquestrigatus, G. rivulatus (dark form) and the group others were also associated with particular reef zones and exposure regimes. Therefore, in these species, reef type appears to influence patterns of abundance independently of coral availability. In contrast to other species of Gobiodon, the abundance of the most specialised species, Gobiodon sp.A, was not closely associated with the abundance of the only coral species it inhabited. This study demonstrates that even for habitat specialised species, the relationship between habitat availability and abundance varies widely and is multiscale.     

Contribution à l'etude des thecamoebiens de la Province de Buenos Aires

Résumé La faune des Thécamoebiens fut étudiée dans deux étangs (La Brava et De los Padres) situe dans la Province de Buenos Aires (Argentina). Le nombre d'espèces trouvé en tout a été de 14, quelques-unes desquelles représentées par de différentes formae. Une nouvelle forma Centropyxis compressa (Cushman), forma brava n.f. a été trouvée et décrite, et une espèce de Pontigulasia a été proposée en nomenclatura aperta.

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Summary The Thecamoebiens of two lakes (La Brava and de los Padres) located in the Province of Buenos Aires (Argentina) were studied. The total number of species found is 14, several of them are represented by various formae. A new forma was found and described, namely Centropyxis compressa (Cushman), forma brava n.f. and one species of Pontigulasia was put in nomenclatura aperta.

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Departamento de Ciencias Biologicas, Facultad de Ciencias Exactas y Naturales Universidad de Buenos Aires     

Fine‐scale habitat preferences influence within‐river population connectivity: a case‐study using two sympatric New Zealand Galaxias fish species

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Chemosensory mediation of food searching in the Buenos Aires tetra <Emphasis Type="Italic">Hyphessobrycon anisitsi</Emphasis> (Characidae)

Food searching behavior of the Buenos Aires tetra Hyphessobrycon anisitsi has been investigated before and after experimentally induced anosmia combined with visual deprivation. The threshold concentration of a water extract of familiar food (chironomid larvae) for fish with intact olfaction was 10^–6 g/L. Induced anosmia led to complete loss of the search reaction to food odor. The chemosensory behavioral response to the water extract of food used at high concentrations (10^-1-10^-2 g/L) was not restored in fish after 1.5, 5, or 13.5 months of anosmia. The obtained results show that the external sense of taste is absent or extremely poorly developed in the Buenos Aires tetra.     

Fright Reaction in Gymnocharacinus bergi (Pisces, Characidae), A Relic Fish from Patagonia

Fright reaction, mediated by pheromones, is an antipredatory mechanism in the scale-less mojarra Gymnocharacinus bergi (Pisces, Characidae). The aim of this study was to characterise and quantify the fright reaction of this species and to analyse the alarm response to different doses of alarm substance. The alarm response consisted in a general decrease in the swimming speed and a tendency to remain at the bottom of the test aquarium. A threshold effect was induced by a very low dose of alarm substance, and the intensity of the fright reaction was positively correlated to the dose concentration. The main characteristics of the alarm response appear to be consistent with G. bergi's environmental conditions such as: a higher predation risk in the upper stratum, the abundant pond-weeds of the stream and the great flow and fast speed of the clear waters.     

Microhabitat association and temporal stability in reef fish assemblages on massive <Emphasis Type="Italic">Porites</Emphasis> microatolls

A massive Porites microatoll generally has three types of microhabitat at the top, side, and base of the microatoll. The purpose of the present study was to analyze microhabitat associations of reef fish on microatolls to determine whether habitat characteristics play an important role in the structuring of reef fish assemblages in a patchy habitat. We also investigated temporal stability of reef fish assemblage structures over a period of 17 months to determine whether fish assemblage structures vary in a random manner. The results of correspondence analysis indicated species-specific habitat associations for pomacentrids (five species) and labrids (seven species). The degree of temporal stability of fish assemblage structures, calculated by Piankas index, was relatively high in a large-sized microatoll (0.503–0.831: 3.6m in diameter), in which microhabitat associations of fishes were clearly observed. The present study suggests that a microhabitat association is one of the important factors responsible for organization of reef fish assemblages in a microatoll.     

Food consumption by the cock fish,Callorhynchus callorhynchus (Holocephali: Callorhynchidae), from Patagonia (Argentina)

Synopsis Food consumption by the holocephalanCallorhynchus callorhynchus, the cock fish, was studied in the northern San Matías Gulf, Argentine Patagonia. Relative gut fullness was assessed objectively with reference to estimated maximum fullness. Analysis of stomach contents of samples from the commercial catch shows maximum gut fullness during the spring. No diel cycle in gut fullness could be detected in the course of an experimental survey. Rate of gastroenteric emptying was estimated on board a research vessel by sacrificing at varying time periods fish kept in circulating sea water. Daily consumption rate by an adult fish of average size was estimated to be 1.4% of its body weight, very close to figures estimated for elasmobranchs from cold temperate waters; this is the first estimation of consumption rate by a holocephalan. Approximately half of the cock fish diet is composed of scallops, including the commercially importantAequipecten tehuelchus. It was calculated that consumption of this species in the study area reaches 81 metric tons per month, which corresponds to 5–7% of its estimated natural mortality.     

Swimming abilities of juvenile estuarine fishes: implications for passage at water control structures

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Biomass structure and dry matter dynamics of subtropical alluvial and exotic Ligustrum forests at the Río de la Plata,Argentina

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Species diversity and abundance of freshwater fishes in irrigation ditches around rice fields

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Sponge-dwelling Fishes of Northeastern Brazil

In this first attempt to survey the Brazilian sponge-dwelling fishes we present a list of collected fishes, with notes on their distribution, abundance and habitat preferences. Risor ruber, an obligate sponge-dwelling goby, and Scorpaenodes tredecimspinosus, never before collected in association with sponges, are recorded for the first time in the western South Atlantic based on collections made in localities on the northeastern Brazilian coast. Previous work on Risor ruber indicated that it preferred massive sponges, but in our study the majority of the specimens were found in the lumen of tubular sponges, Aplysina lacunosa, together with other fishes and invertebrates, mostly crustaceans. Elacatinus figaro, originally described as a cleaner goby, is also a sponge-dweller that occurs in waters much deeper than previously thought. The sponge community off northeastern Brazil represents the only shelter for several species of fishes in a desert of rubble and flat rocky bottoms, perhaps functioning as habitat oases. We also found four Brazilian endemic species of fishes associated with sponges in depths greater than 50m, which contradicts a previous hypothesis suggesting that endemic fishes in Brazilian coastal waters are restricted to depths less than 50m.     

Carbon dioxide exchange characteristics of C4 Hawaiian Euphorbia species native to diverse habitats

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Life history adaptations of phyllopods in response to predators,vegetation, and habitat duration in north-eastern Natal

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Cloning and characterization of class I Mhc genes of the zebrafish,Brachydanio rerio

The zebrafish (Brachydanio rerio) offers many advantages for immunological and immunogenetic research and has the potential for becoming one of the most important nonmammalian vertebrate research models. With this in mind, we initiated a systematic study of the zebrafish major histocompatibility complex (Mhc) genes. In this report, we describe the cloning and characteristics of the zebrafish class I A genes coding for the chains of the heterodimer and thus complete the identification of all four classes and subclasses of the Mhc in this species. We describe the full class I cDNA sequence as well as the exon-intron organization of the class I A genes, including intron sequences. We identify three families of class I A genes which we designate Bree-UAA,-UBA, and -UCA. The three families originated about the time of the divergence of cyprinid and salmonid fishes. All three families are members of an ancient lineage that diverged from another, older lineage also represented in cyprinid fishes before the radiation of teleost orders. The fish class I A genes therefore evolve differently from mammalian class I A genes, in which the establishment of lineages and families mostly postdates the divergence of orders.The nucleotide sequence data reported in this Papershave been submitted to the EMBL/GenBank nucleotide sequence databases and have been assigned the accession numbers Z46776–Z46779     

BOOK REVIEWS

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