Electric organ discharges of the gymnotiform fishes: III. Brachyhypopomus

We measured and mapped the electric fields produced by three species of neotropical electric fish of the genus Brachyhypopomus (Gymnotiformes, Rham phichthyoidea, Hypopomidae), formerly Hypopomus. These species produce biphasic pulsed discharges from myogenic electric organs. Spatio-temporal false-color maps of the electric organ discharges measured on the skin show that the electric field is not a simple dipole in Brachyhypopomus. Instead, the dipole center moves rostro-caudally during the 1st phase (P1) of the electric organ discharge, and is stationary during the 2nd phase (P2). Except at the head and tip of tail, electric field lines rotate in the lateral and dorso-ventral planes. Rostro-caudal differences in field amplitude, field lines, and spatial stability suggest that different parts of the electric organ have undergone selection for different functions; the rostral portions seem specialized for electrosensory processing, whereas the caudal portions show adaptations for d.c. signal balancing and mate attraction as well. Computer animations of the electric field images described in this paper are available on web sites http://www.bbb.caltech.edu/ElectricFish or http://www.fiu.edu/∼stoddard/electricfish.html. Accepted: 22 September 1998     

The electric organ discharges of the gymnotiform fishes: I. Apteronotus leptorhynchus

We present high temporal and spatial resolution maps in 3-dimensions of the electric field vector generated by the weakly electric fish, Apteronotus leptorhynchus. The waveforms and harmonic composition of the electric organ discharge (EOD) are variable around the fish but highly stable over long times at any position. We examine the role of harmonics on the temporal and spatial characteristics of the EOD, such as the slew rate and rostral-to-caudal propagation. We also explore the radial symmetry of the fish's field. There are major differences in the direction of the electric field vector at the head and caudal body. In the caudal part of the fish, the electric field vector rotates during the EOD cycle. However, rostral of the pectoral fin, the field magnitude and sign oscillate while maintaining relatively constant orientation. We discuss possible functional ramifications of these electric field patterns to electrolocation, communication, and electrogenesis.Abbreviations EOD electric organ discharge - EO electric organ - RMS root mean square - ADC analog-to-digital converter     

Optimal foraging and feeding mode shifts in fishes

Synopsis Most optimal foraging models for fishes are based on particulate feeding behavior. But many obligate planktivores also filter zooplankton. I suggest that feeding mode shifts (e.g. from particulate feeding to filtering) may be predictable from the costs and benefits of foraging in various modes. Quantitative examples of feeding mode shifts in three species of fishes (northern anchovy, pacific mackerel and alewife) from 3 different families support this hypothesis. Feeding mode shifts seem to depend on relative profitability of each mode, but improvements in model predictions will need to include the effects of spatial and temporal patchiness on encounter rates of prey of various sizes.     

Regulation and modulation of electric waveforms in gymnotiform electric fish

Weakly electric gymnotiform fish specialize in the regulation and modulation of the action potentials that make up their multi-purpose electric signals. To produce communication signals, gymnotiform fish modulate the waveforms of their electric organ discharges (EODs) over timescales spanning ten orders of magnitude within the animal’s life cycle: developmental, reproductive, circadian, and behavioral. Rapid changes lasting milliseconds to seconds are the result of direct neural control of action potential firing in the electric organ. Intermediate-term changes taking minutes to hours result from the action of melanocortin peptides, the pituitary hormones that induce skin darkening and cortisol release in many vertebrates. Long-term changes in the EOD waveform taking days to weeks result from the action of sex steroids on the electrocytes in the electric organ as well as changes in the neural control structures in the brain. These long-term changes in the electric organ seem to be associated with changes in the expression of voltage-gated ion channels in two gene families. Electric organs express multiple voltage-gated sodium channel genes, at least one of which seems to be regulated by androgens. Electric organs also express multiple subunits of the shaker (Kv1) family of voltage-gated potassium channels. Expression of the Kv1 subtype has been found to vary with the duration of the waveform in the electric signal. Our increasing understanding of the mechanisms underlying precise control of electric communication signals may yield significant insights into the diversity of natural mechanisms available for modifying the performance of ion channels in excitable membranes. These mechanisms may lead to better understanding of normal function in a wide range of physiological systems and future application in treatment of disease states involving pathology of excitable membranes.     

Parasitoids deter foraging by Argentine ants (Linepithema humile) in their native habitat in Brazil

The Argentine ant, Linepithema humile, has invaded sites across Africa, Australia, Europe, and North America. In its introduced ranges it eliminates native ants and tends agricultural pests. Few studies have examined the ecology of Argentine ants in their native habitat. This study examined the effects of parasitoid flies, genus Pseudacteon, on the foraging behavior of Argentine ants in part of their native range in southern Brazil. Pseudacteon parasitoids commonly attacked Argentine ants, but not other ant species, in daylight at temperatures above 18°C. Argentine ants abandoned food resources and returned underground in the presence of parasitoids. Parasitoid attack rates diminished as Argentine ants retreated underground. Where parasitoids were present, Argentine ants were abundant at food resources only during times of day when parasitoids were inactive. Where parasitoids were absent, Argentine ants were abundant at food resources throughout the day. Overall, the presence of parasitoids explained observed variation in Argentine ant foraging far better than temperature, although temperature had some effect. The results suggest that Pseudacteon parasitoids inhibit the ability of Argentine ants to gather food resources in their native habitat in Brazil. Received: 11 December 1997 / Accepted: 12 June 1998     

Effects of prey size and foraging mode on the ontogenetic change in feeding niche ofColostethus stepheni (Anura: Dendrobatidae)

The feeding niche ofColostethus stepheni changes during ontogeny. Small individuals eat small arthropods, principally mites and collembolans, and larger frogs eat bigger prey of other types. The shift in prey types is not a passive effect of selection for bigger prey. There is a strong relationship between electivity for prey types and frog size, independent of electivity for prey size. Four indices of general activity during foraging (number of movements, velocity, total area utilized and time spent moving), which are associated with electivity for prey types in adult frogs and lizards, did not predict the ontogenetic change in the diet ofC. stepheni. Apparently, the behavioral changes that cause the ontogenetic change inC. stepheni are more subtle than shifts in general activity during foraging. Studies of niche partitioning in communities of anurans that do not take into consideration ontogenetic changes in diet and seasonal changes in the size structures of populations present a partial and possibly erroneous picture of the potential interactions among species.     

Carnivore taphonomy in South America: a review of actualistic studies and their implications in the southern Neotropics

Abstract

Actualistic studies on mammalian carnivore taphonomy in southern South America are reviewed here, including pumas, small cats, foxes, and other, smaller carnivores. Patterns for different carnivore taxa and their variation are elicited. Also temporal and spatial variability is analysed, and comparisons are made to other carnivores and regions. While generally these Neotropical carnivores produce low damage intensity, stronger modifications and their implications are described as well. These patterns and their variation are put in context by taking into account the particular physical and biotic conditions in the southern Neotropics.     

The biogeography of otophysan fishes (Ostariophysi: Otophysi): a new appraisal

Aim To present a new hypothetical history of the otophysan fishes. Location World‐wide. Methods Utilization of recent information about otophysan phylogeny, palaeontology, biogeography and external relations. Results Anatomical, mitochondrial DNA and karological research has indicated that the Cypriniformes is the most primitive of the four orders that comprise the Otophysi. The Siluriformes is next in the phylogenetic progression followed by the Characiformes and the Gymnotiformes. A molecular clock estimate indicated that the divergence time between the cypriniforms and the remaining otophysans took place about 250 Ma. But, otophysans have apparently never occurred on Madagascar‐India and that land mass did not separate from Africa until 158–160 Ma. As we know that some otophysans existed in Africa–South America prior to 115 Ma, they must have originated between 115 and 160 Ma. Main conclusions Comparison of phylogeny and biogeography appears to indicate that the Otophysi originated in South America, the location of the apomorphic Gymnotiformes. It seems that the gymnotiforms did not radiate in time to get across to Africa and the characiforms did not expand in time to catch the connection to Asia. Apparently, it was only the ancestors to the cypriniform and siluriform lines that expanded early enough to reach Asia in the late Jurassic. There remains a gap of about 80 Myr between the oldest known otophysan fossil and the approximate time of dispersal to Asia. Further progress awaits more phylogenetic or fossil discoveries.     

Ecomorphological diversification and convergence in fluvial cichlid fishes

Synopsis We compared ecological and morphological patterns among cichlid faunas from three different biotic regions: the Río Tortuguero/Rio Sarapiquf in Costa Rica, the Río Apure drainage in Venezuela, and the Upper Zambezi drainage in Zambia. Cichlids comprise 19 percent of the fish fauna in the Tortuguero and Upper Zambezi drainages and 6.5 percent in the Apure drainage. Cichlid faunas exhibited similar patterns of habitat and food resource utilization, although vegetation-dwelling is more common and detritivory and herbivory are rarer in the Apure fauna. We hypothesize that South American ostariophysan fishes were more preadapted than cichlids to exploit detritivore and herbivore niches. The Zambezi cichlid fauna shows less ecomorphological diversification than the other two faunas, even though the degree of dietary diversification is similar among faunas. Chaetobranchus flavescens from the Venezuelan fauna is the only species that specializes on zooplankton as an adult, and algae grazing (Neetroplus nematopus) and specialized fruit feeding (Cichlasoma tuba) were represented only in the Costa Rican fauna. Based on the most recent hypothesized phylogeny for the family Cichlidae, we identified numerous interfaunal ecomorphological and feeding niche convergences. Patterns of ecomorphological divergence in relation to cladogenesis indicate a faster rate of evolutionary niche diversification in Central American cichlids compared with the two other faunas.     

Sonic and electric fish: at the crossroads of neuroethology and behavioral neuroendocrinology

Field and laboratory studies of weakly electric and sound-producing teleost fishes demonstrate how steroidal and non-steroidal hormones mediate the translation of neural events into behavior. The development of this research program has depended upon an interdisciplinary neuroethological approach that has characterized the neurophysiological properties of the motor and sensory pathways that lead to the production and detection of easily quantified highly stereotyped behaviors, namely, electric organ discharges (EODs) and vocalizations. Neuroethological studies of these teleosts have now integrated a behavioral neuroendocrinology approach that has provided several examples of how hormone-sensitive neurobiological traits contribute to adaptive behavioral plasticity in natural habitats. As such, these studies provide guideposts for comparable studies in other groups of teleosts and vertebrates in general.     

Body size,energetic and foraging mode of raptors in central Chile

Summary An inferential analysis of the foraging mode (opportunist or mediate by prey selection) of a taxonomic assemblage of raptors in central Chile was conducted. The analysis of energetic aspects such as daily requirements of predators and energy supplied by the preys, with estimations of prey relative abundances and their incidence in the diet of raptors, led us to conclude that contrarly to the opportunistic hunting mode suggested by others authors, these predators apparently present prey selection (on a biomass basis). This phenomenon is particularly evident in raptors of small body size.     

The effects of exposure in sandy beach surf zones on larval fishes

The influence of wind and wave exposure on larval fish assemblages within a large bay system was investigated. Larval fishes were sampled from two areas with vastly different exposure to waves and wind, namely the windward and leeward sectors of Algoa Bay. In total, 5702 larval fishes were collected using a modified larval seine. Of these, 4391 were collected in the leeward and 1311 in the windward sector of the bay, representing a total of 23 families and 57 species. Dominant fish families included Clinidae, Engraulidae, Kyphosidae, Mugilidae, Soleidae and Sparidae, similar to the situation elsewhere, highlighting continuity in the composition of larval fish assemblages and the utilization of surf zones by a specific group of larval fishes. Nineteen estuary‐associated marine species occurred within the surf zones of Algoa Bay and dominated catches (86·7%) in terms of abundance. Postflexion larvae comprised > 80% of the catch, indicating the importance of the seemingly inhospitable surf zone environment for the early life stages of many fish species. The greatest species diversity was observed within the windward sector of the bay. Distance‐based linear modelling identified wave period as the environmental variable explaining the largest proportion of the significant variation in the larval fish assemblage. The physical disturbance generated by breaking waves could create a suitable environment for fish larvae, sheltered from predators and with an abundance of food resources.     

Effects of zooplankton availability and foraging mode on cannibalism in three dragonfly larvae

Summary Cannibalism is likely to operate as a form of population control in dragonfly larvae. I performed aquarium experiments to investigate the effect of foraging activity and zooplankton availability on cannibalism in three dragonfly larvae. Large Cordulia aenea larvae showed low activity, and large Leucorrhinia dubia larvae showed high activity irrespective of zooplankton availability. In contrast, large Coenagrion hastulatum larvae changed from high activity in the absence to low activity in the presence of zooplankton. Small Cordulia aenea larvae were active in the absence of large conspecifics irrespective of zooplankton availability. In the presence of large conspecifics they showed a reduced activity when zooplankton were present. Small L. dubia larvae showed high activity and small Coenagrion hastulatum larvae low activity irrespective of pressence or absence of zooplankton and large conspecifics. In all three species cannibalism was highest in the absence of zooplankton. In the absence of zooplankton cannibalism was low in Coenagrion hastulatum compared to the other two species. On the contrary, in the presence of zooplankton, cannibalism did not differ between the three species.     

Detection of multiple stimulus features forces a trade-off in the pyramidal cell network of a gymnotiform electric fish's electrosensory lateral line lobe

Modification of an existing neural structure to support a second function will produce a trade-off between the two functions if they are in some way incompatible. The trade-off between two such sensory functions is modeled here in pyramidal neurons of the gymnotiform electric fish's medullar electrosensory lateral line lobe (ELL). These neurons detect two electric stimulus features produced when a nearby object interferes with the fish's autogenous electric field: (1) amplitude modulation across a cell's entire receptive field and (2) amplitude variation within a cell's receptive field produced by an object's edge. A model of sensory integration shows that detection of amplitude modulation and enhancement of spatial contrast involve an inherent mechanistic trade-off and that the severity of the trade-off depends on the particular algorithm of sensory integration. Electrophysiology data indicate that of the two algorithms for sensory integration modeled here for the gymnotiform fish Brachyhypopomus pinnicaudatus, the algorithm with the better trade-off function is used. Further, the intrinsic trade-off within single cells has been surmounted by the replication of ELL into multiple electrosensory map segments, each specialized to emphasize different sensory features. Accepted: 14 June 1997     

Sex and individual differences in foraging behavior of Japanese cormorants in years of different prey availability

Japanese cormorants, Phalacrocorax capillatus, are sexually dimorphic seabirds with males that are heavier and that dive deeper than females. Sex differences in prey composition and foraging behavior of those rearing chicks at Teuri Island, Hokkaido, were examined by collecting food-loads from parents in 1992–1998 and by radio-tracking ten birds each in 1997 and 1998 when prey availability was different. Males fed more on benthic and epibenthic fishes (82% mass) than did females (34%) while females fed more on epipelagic and coastal fishes (53%) than did males (18%). Males made longer dives (53 s) at feeding sites closer to the island (7 km) than females (39 s, 13 km) in 1997. In 1998 when the availability of epipelagic fish seemed to be higher, there were no sex differences in dive duration and distance to the feeding sites (35 s and 9 km for males, 36 s and 10 km for females). This sex difference in foraging behavior with a poor availability of epipelagic fish suggests that high diving ability possibly enables males to feed on demersal fish. Birds specializing in coastal shallow waters around the island made long dives; hence they were probably foraging in bottom layers. Those foraging in deeper shelf waters made short dives and they were thought to forage in surface layers. Electronic Publication     

Insect herbivores foraging on seedlings in an unlogged rain forest in Guyana: spatial and temporal considerations

About 9,000 individuals of sap-sucking and leaf-chewing insects, representing 345 species, were collected from 10,000 seedlings belonging to 5 rainforest tree species in an unlogged forest in Guyana. For the 40 most common species, it was possible to estimate their host specificity, diurnal activity, seasonal distribution, spatial aggregation, abundance and body weight. Most species were generalists but more specialised species tended to show a higher spatial aggregation, a more restricted diurnal activity and a higher seasonality. Although insect abundance was highest at the onset of the long wet season in May, seasonal amplitude was not pronounced. The combined effects of host and rainfall explained 14.5% of the total variance in insect seasonality, which was poorly explained by the leaf production of seedlings. The spatial distribution of insects was often aggregated and, overall, explained by host effect, production of young foliage, number of conspecific trees within a radius of 50m, and number of dead seedlings at each collecting station. However, these variables explained only 7% of the variance in spatial distribution. The lack of notable influence of leaf production and other important variables recorded at the collecting stations suggests that the seedlings represent a marginal food resource for most of the insect species collected.     

Body Temperature and Thermoregulatory Behaviour of the Lizard Ameiva ameiva in Central Amazonian Forests

Body temperatures and thermoregulatory behaviour of the teiid lizard Ameiva ameiva inhabiting the edge and the understory were studied in Central Amazonian forests. Despite of differences in the thermal profile of the habitats, the mean body temperature was the same for active lizards observed at the edge or inside the forest, where only slight peculiarities in thermoregulatory behaviour were observed. A. ameiva is capable of maintaining body temperature significantly above microhabitat temperature.     

The Role of Mammals in Creating and Modifying Seedshadows in Tropical Forests and Some Possible Consequences of Their Elimination

Mammal populations are increasingly hunted, yet the consequences of their disappearance from tropical forests have only recently been explored. Here, we summarize current research on the role of mammals in seed dispersal and postdispersal processes, such as seed predation and secondary dispersal, in different tropical regions. We evaluate how mammal features influence seedshadows and ultimately forest regeneration. Finally, we discuss the potential effect of changes in seedshadows caused by the elimination of many medium- and large-sized mammals. The complex role that mammals play in creating and modifying seedshadows in tropical forests cannot be easily quantified, and in this review we emphasize the variation that exists both within and among mammal taxa and across continents. To bridge this gap in information, we suggest that more studies should evaluate the relative importance of the disappearance of both seed dispersers and seed predators for particular plant species so that we may begin to understand the balance between these two influences. We also suggest that future studies identify ecological redundancy in nonhunted vertebrates within any particular community to evaluate compensatory behavior that may help ameliorate some of the negative effects of hunting of large and medium mammals.     

Three new species of Curimatopsis (Characiformes: Curimatidae) from the Amazon basin

Three new species of Curimatopsis are described from major tributaries of the Amazon basin. Curimatopsis guaporensis n. sp., from the Rio Madeira, belongs to the Curimatopsis evelynae clade and can be distinguished by the distinctive shape of the dark blotch on the caudal peduncle and by the position and shape of the nostrils. Curimatopsis pallida n. sp., from the Rio Negro, also related to C. evelynae, is distinguished from all congeners by the complete absence of pigmentation on the lateral surface of the caudal peduncle. Curimatopsis jaci n. sp., apparently endemic to the upper Rio Tapajós, belongs to the Curimatopsis macrolepis clade and differs from all congeners in details of body pigmentation. Meristic and morphometric features supplement diagnoses for the three new species. These species are hypothesized to belong to the two main clades of Curimatopsis on the basis of previous studies of osteology and external morphology and supplement a recent genetic study that revealed several cryptic and yet undescribed species within the genus. An updated identification key to the species of Curimatopsis is also provided.