Ontogeny and evolution of electric organs in gymnotiform fish

In order to further our understanding of the evolution of electric organs in the Neotropical gymnotiform fish, we studied the ontogeny of the electric organs in eight species. In Eigenmannia virescens, Sternopygus macrurus, and Apteronotus leptorhynchus the earliest electrocytes are located between muscle fibres of the hypaxial muscle (Type A electrocytes). We present arguments that these Type A electrocytes represent the plesiomorphic condition. In S. macrurus, in addition to the electrocytes in the hypaxial muscle, additional electrocytes were found in the epaxial muscle. In A. leptorhynchus a neurogenic organ develops later during ontogeny in the medial part of the hypaxial muscle in addition to the early myogenic organ. In E. virescens the early electrocytes in hypaxial muscle will degenerate later during ontogeny, and this organ will be replaced functionally by electrocytes located in the caudal appendage and below the hypaxial muscle. In Electrophorus electricus, two Gymnotus species, Rhamphichthys sp., and Brachyhypopomus pinnicaudatus the first electrocytes were found below the hypaxial muscle (Type B electrocytes); they are assumed to be the more derived stage. In R. sp., and B. pinnicaudatus the electrocytes of Type B developed directly into the adult organ. In the two Gymnotus ssp. electrocytes were also found in the medial part of the organ in-between muscle fibres of the hypaxial muscle. In E. electricus a germinative zone was observed to separate from the ventral myotome. This zone is generating electrocytes continuously so that, as a consequence, the relative proportion of electric organ to muscle increases greatly. In 45 mm long E. electricus a separation of low voltage orientation pulses and high voltage trains of pulses (shocks) was observed. A first appearance of Hunter’s organ was found in 140 mm specimens of E. electricus. The first discharges of all species studied were head- positive, with the exception of R. sp., which produced a triphasic discharge, its main component, however, being head-positive. The arguments presented indicate that the Type A electrocytes found in E. virescens, S. macrurus, and A. leptorhynchus would represent the plesiomorphic condition. On the basis of the evidence regarding the formation, cytological appearance, and anatomical location, as well as the early electrical recordings, we would hypothesise that during the evolution of gymnotiforms wave type species evolved first, and in a second step pulse type species followed. This view, however, is corroborated by only some phylogenetic hypotheses.     

Independent evolution of the specialized pharyngeal jaw apparatus in cichlid and labrid fishes

Background  

Fishes in the families Cichlidae and Labridae provide good probable examples of vertebrate adaptive radiations. Their spectacular trophic radiations have been widely assumed to be due to structural key innovation in pharyngeal jaw apparatus (PJA), but this idea has never been tested based on a reliable phylogeny. For the first step of evaluating the hypothesis, we investigated the phylogenetic positions of the components of the suborder Labroidei (including Pomacentridae and Embiotocidae in addition to Cichlidae and Labridae) within the Percomorpha, the most diversified (> 15,000 spp) crown clade of teleosts. We examined those based on 78 whole mitochondrial genome sequences (including 12 newly determined sequences) through partitioned Bayesian analyses with concatenated sequences (13,933 bp).     

A temporal analysis of testosterone-induced changes in electric organs and electric organ discharges of mormyrid fishes

The electric organ discharge (EOD) of several species of mormyrid fishes within the genus Brienomyrus is sexually dimorphic during the breeding season: the duration of the male's EOD is much longer than the duration of the female's (for a review see Hopkins, 1986). The mormyrid used here, Brienomyrus sp., exhibits similar alterations in the duration of the triphasic EOD after treatment with testosterone, as do other members of this genus (for reviews see Bass, 1986a,b). In this experiment, animals were intraperitoneally implanted with pellets of either 11-ketotestosterone or 17 a-methyltestosterone, and the time course of the changes in the duration of each of the three phases of the EOD were quantified. Additionally, the time course of changes in the morphology of the electric organ, after testosterone treatment, was also quantified using electron microscopic techniques. The results suggest that the change in the duration of the first phase of the EOD is due exclusively to the change in the thickness of the electrocyte body: this is consistent with a model proposed by Bennett and Grundfest (1961) for the electrogenesis of a triphasic EOD. Changes in the duration of the second and third phases of the EOD are highly correlated with the changes in the surface area of the posterior and anterior faces of the electrocyte, respectively. The results support the hypothesis that gonadal steroid hormone-induced changes in the EOD are due to structural changes in the electrocyte's membranes, and that all of the observed changes in the discharge of this system can be explained by the action of steroid hormones on the peripheral target cells (electrocytes).     

The anatomy, physiology, functional significance and evolution of specialized hearing organs of gerbilline rodents

Middle and inner ear anatomy correlates with neurophysiological responses to a wide range of sound frequencies for species of the Gerbillinae representing generalized, intermediate, and specialized anatomical conditions. Neurophysiological data were recorded from 81 specimens of 13 species representing six genera. Anatomical parameters involved in the process of hearing were correlated with the neurophysiological data to assess the effects of different degrees of anatomical specialization on hearing. The 13 species tested in this manner have graphic curves of auditory sensitivity of remarkably similar disposition over the frequencies tested and to those published for Kangaroo Rats. Ears with anatomical specializations show greater auditory sensitivity. The natural history of the Gerbillinae, particularly the kinds of predators, degree of predation, and habitat is reviewed and utilized to interpret the significance of the degree of auditory specialization in the forms studied and to evaluate the prevailing hypothesis that these specializations enhance the ability of these rodents to survive in open desert situations by detecting and evading predators. The middle ear anatomy of five additional genera and species was also studied. Thus, data on the entire spectrum of gerbilline middle ear morphology provide an evolutionary sequence. Certain anatomical parameters of the organ of Corti show a degree of specialization parallel to that of features of the middle ear. The morphological changes and possible functional roles of these features are considered. A very high correlation exists for degree of specialization and aridity of habitat, thus specialization increases with increasing aridity. This increased specialization may result from more effective predation in open xeric environments. Auditory acuity for a wide range of low frequency sounds augmented by auditory specialization is hence more advantageous here. There does not appear to be selection for hearing at particular frequencies in this range. The peaks of greatest auditory sensitivity appear to correspond to the resonant frequencies of the different components of the middle ear transformer and cavity.     

Convergent evolution of weakly electric fishes from floodplain habitats in Africa and South America

An assemblage of seven gymnotiform fishes in Venezuela was compared with an assemblage of six mormyriform fishes in Zambia to test the assumption of convergent evolution in the two groups of very distantly related, weakly electric, noctournal fishes. Both assemblages occur in strongly seasonal floodplain habitats, but the upper Zambezi floodplain in Zambia covers a much larger area. The two assemblages had broad diet overlap but relatively narrow overlap of morphological attributes associated with feeding. The gymnotiform assemblage had greater morphological variation, but mormyriforms had more dietary variation. There was ample evidence of evolutionary convergence based on both morphology and diet, and this was despite the fact that species pairwise morphological similarity and dietary similarity were uncorrelated in this dataset. For the most part, the two groups have diversified in a convergent fashion within the confines of their broader niche as nocturnal invertebrate feeders. Both assemblages contain midwater planktivores, microphagous vegetation-dwellers, macrophagous benthic foragers, and long-snouted benthic probers. The gymnotiform assemblage has one piscivore, a niche not represented in the upper Zambezi mormyriform assemblage, but present in the form of Mormyrops deliciousus in the lower Zambezi and many other regions of Africa.     

Sexual signals in electric fishes

Electroreceptive bony fishes of Africa (the Mormyriformes) and South America (the Gymnotiformes) detect and communicate with conspecifics by their continuously discharging electric organs. Laboratory studies of members of each group are beginning to reveal the mechanisms of communicating with and finding mates, offering much scope for future studies of the behavioral ecology of electric fishes.     

Molecular evolution of haemoglobins of polar fishes

The Arctic and the Antarctic differ by age and isolation of the respective marine faunas. Antarctic fish are highly stenothermal, in response to stable water temperatures, whereas the Arctic ones are exposed to seasonal and latitudinal temperature variations. The knowledge of the mechanisms of phenotypic response to cold exposure in species of both polar habitats offers fundamental insights into the nature of environmental adaptation. In the process of cold adaptation, the evolutionary trend of Antarctic fish has led to unique specialisations, including modification of haematological characteristics, e.g. decreased amounts and multiplicity of haemoglobins.Unlike Antarctic Notothenioidei, Arctic teleosts have high haemoglobin multiplicity. Although the presence of functionally and structurally distinct haemoglobins is a plesiomorphic condition for many perciform-like fishes, it seems that the oxygen-transport system of teleost fish in the Arctic region has been adjusted to temperature differences and fluctuations of Arctic waters, much larger than in the Antarctic. The amino-acid sequences used to gain insight into the evolution history of α and β globins of polar fish have clearly shown that Antarctic and Arctic globins have different phylogenies, leading to the hypothesis that the selective pressure of environment stability allows the phylogenetic signal to be maintained in the Antarctic sequences, whereas environmental variability would tend to disrupt this signal in Arctic sequences.     

Oxygen consumption in weakly electric Neotropical fishes

Weakly electric gymnotiform fishes with wave-type electric organ discharge (EOD) are less hypoxia-tolerant and are less likely to be found in hypoxic habitats than weakly electric gymnotiforms with pulse-type EOD, suggesting that differences in metabolism resulting from EOD type affects habitat choice. Although gymnotiform fishes are common in most Neotropical freshwaters and represent the dominant vertebrates in some habitats, the metabolic rates of these unique fishes have never been determined. In this study, O₂ consumption rates during EOD generation are reported for 34 gymnotiforms representing 23 species, all five families and 17 (59%) of the 28 genera. Over the size range sampled (0.4 g to 125 g), O₂ consumption of gymnotiform fishes was dependent on body mass, as expected, fitting a power function with a scaling exponent of 0.74, but the O₂ consumption rate was generally about 50% of that expected by extrapolation of temperate teleost metabolic rates to a similar ambient temperature (26°C). O₂ consumption rate was not dependent on EOD type, but maintenance of scan swimming (continuous forwards and backwards swimming), which is characteristic only of gymnotiforms with wave-type EODs, increased O₂ consumption 2.83±0.49-fold (mean±SD). This suggests that the increased metabolic cost of scan swimming could restrict gymnotiforms with wave-type EODs from hypoxic habitats.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

The evolution of sex-change mechanisms in fishes

Synopsis Five distinct sex-change mechanisms are identified among sequentially hermaphroditic fishes based on socio-ecological characteristics. The primary determinants of the sex-change mechanisms appear to be social organization and mating system, which in turn depend on resource distribution in space and time. The ability of a single individual to control all mating in the social unit, which is related to the size of the social unit, differentiates three suppression mechanisms from two induction mechanisms. Sex-change suppression, which is characteristic of species with small group size and rigid dominance hierarchies, refers to inevitable sex change in the absence of group dominance. Ability to migrate between resource patches differentiates protogynous suppression (e.g. inLabroides dimidiatus) from protandrous suppression (e.g. inAmphiprion spp.). Early sex change appears to have evolved from protogynous suppression under special conditions involving the loss of mating control by a single dominant individual in certain species (e.g.Centropyge spp. ). Sex-change induction, which is characteristic of species with large social groups lacking rigid dominance hierarchies, refers to the requirement that sex change must be induced by specific characteristics of (or changes in) the social group, regardless of dominance status. Ability to distinguish sex, or its importance, differentiates sex-ratio induction (e.g.Anthias squamipinnis) from size-ratio induction (e.g.Thalassoma spp.). Alternative models account for the possibility that all cases of sex change require stimulation from smaller conspecifics (universal induction-inhibition model) or that all fish have the genetic capacity to switch mechanisms, depending on changing ecological conditions and resulting changes in mating system (behavioral-scaling model). Neurophysiological models suggest that induction mechanisms, which require at least two categories of environmental stimuli, may have evolved from the simpler suppression mechanisms, which require only one kind of input from the environment.     

Structure of tendon organs in fishes of the genus Polymixia

Summary Modified branchiostegal rays 1 through 3 support the proximal end of the paired hyoid barbels in the beardfish (Beryciformes: Polymixiidae). The polymixiid barbel is unusual in that it has an unique intrinsic muscular system. Using silver impregnation and electron microscopic techniques, unencapsulated, free nerve endings were located within the tendon of the third modified branchiostegal ray. Branchiostegal rays 1 and 2 do not have any free nerve endings associated with their tendons, however. It is suggested that the free nerve endings are proprioceptors acting as stretch-sensitive mechanoreceptors, and that branchiostegal ray 3 acts as part of a sensory apparatus for monitoring the positional state of the barbel. Branchiostegal rays 1 and 2 merely provide support for the barbel.Abbreviations used in Figures BA barbel - br r 3 branchiostegal ray 3 - IM intermandibularis - IOP interoperculum - LIM interoperculomandibular ligament - MD mandible - MX maxilla - OP operculum - PM premaxilla - POP preoperculum - SOP suboperculum     

Characterisation of the muscular organs of paramphistomes from Asian fishes

The histological characteristics of the pharynx, terminal genitalium and acetabulum are described for four species of paramphistomes from fishes from Malaysia and Thailand. New pharyngeal and terminal genitalium types are designated for Osteochilotrema malayae, Helostomatis indica, Protocladorchis pangasii and P. chinabutae and new acetabular types for H. indica and P. pangasii.     

Functional aspects of the integument in polypterid fishes

The integument and the axial musculoskeletal system of fishes belonging to the family Polypteridae (Osteichthyes, Actinopterygii) are re-examined with the purpose of clarifying the mechanical functions of the squamation. The major integumentary axes of bone and collagen are laid down in such a pattern as to resist that deformation of the skin resulting from axial body torsion. This resistance will have the effect of reducing or preventing such torsion and maintaining the strictly transverse plane of locomotory undulations.     

The evolution of microsatellite loci in salmonid fishes

In ten species of salmonid fishes, sequences of five microsatellite loci were determined. Considerable differences in the structure of the same microsatellites in different species were found. It was demonstrated that the evolution of microsatellites was a complex process, including changes in the copy number, point mutations, and extended deletions and insertions, leading either to the formation of microsatellites or to their loss.