A comparison of adrenergic stress responses in three tropical teleosts exposed to acute hypoxia

Experiments were performed to assess the afferent and efferent limbs of the hypoxia-mediated humoral adrenergic stress response in selected hypoxia-tolerant tropical fishes that routinely experience environmental O(2) depletion. Plasma catecholamine (Cat) levels and blood respiratory status were measured during acute aquatic hypoxia [water Po(2) (Pw(O(2))) = 10-60 mmHg] in three teleost species, the obligate water breathers Hoplias malabaricus (traira) and Piaractus mesopotamicus (pacu) and the facultative air breather Hoplerythrinus unitaeniatus (jeju). Traira displayed a significant increase in plasma Cat levels (from 1.3 +/- 0.4 to 23.3 +/- 15.1 nmol/l) at Pw(O(2)) levels below 20 mmHg, whereas circulating Cat levels were unaltered in pacu at all levels of hypoxia. In jeju denied access to air, plasma Cat levels were increased markedly to a maximum mean value of 53.6 +/- 19.1 nmol/l as Pw(O(2)) was lowered below 40 mmHg. In traira and jeju, Cat release into the circulation occurred at abrupt thresholds corresponding to arterial Po(2) (Pa(O(2))) values of approximately 8.5-12.5 mmHg. A comparison of in vivo blood O(2) equilibration curves revealed low and similar P(50) values (i.e., Pa(O(2)) at 50% Hb-O(2) saturation) among the three species (7.7-11.3 mmHg). Thus Cat release in traira and jeju occurred as blood O(2) concentration was reduced to approximately 50-60% of the normoxic value. Intravascular injections of nicotine (600 nmol/kg) elicited pronounced increases in plasma Cat levels in traira and jeju but not in pacu. Thus the lack of Cat release during hypoxia in pacu may reflect an inoperative or absent humoral adrenergic stress response in this species. When allowed access to air, jeju did not release Cats into the circulation at any level of aquatic hypoxia. The likeliest explanation for the absence of Cat release in these fish was that air breathing, initiated by aquatic hypoxia, prevented Pa(O(2)) values from falling to the critical threshold required for Cat secretion. The ventilatory responses to hypoxia in each species were similar, consisting generally of increases in both frequency and amplitude. These responses were not synchronized with or influenced by plasma Cat levels. Thus the acute humoral adrenergic stress response does not appear to stimulate ventilation during acute hypoxia in these tropical species.     

Foveate vision in deep-sea teleosts: a comparison of primary visual and olfactory inputs

The relative importance of vision in a foveate group of alepocephalid teleosts is examined in the context of a deep-sea habitat beyond the penetration limits of sunlight. The large eyes of Conocara spp. possess deep convexiclivate foveae lined with Müller cells comprising radial shafts of intermediate filaments and horizontal processes. Photoreceptor cell (171.8 x 10(3) rods mm(-2)) and retinal ganglion cell (11.9 x 10(3) cells mm(-2)) densities peak within the foveal clivus and the perifloveal slopes, respectively, with a centro-peripheral gradient between 3:1 (photoreceptors) and over 20:1 (ganglion cells). The marked increase in retinal sampling localized in temporal retina, coupled with a high summation ratio (13:1), suggest that foveal vision optimizes both spatial resolving power and sensitivity in the binocular frontal visual field. The elongated optic nerve head is comprised of over 500 optic papillae, which join at the embryonic fissure to form a thin nervous sheet behind the eye. The optic nerve is divided into two axonal bundles; one receiving input from the fovea (only unmyelinated axons) and the other from non-specialized retinal regions (25% of axons are myelinated), both of which appear to be separated as they reach the visual centres of the central nervous system. Comparison of the number of primary (first-order) axonal pathways for the visual (a total of 63.4 x 10(6) rod photoreceptors) and olfactory (a total of 15.24 x 10(3) olfactory nerve axons) inputs shows a marked visual bias (ratio of 41:1). Coupled with the relative size of the optic tecta (44.0 mm3) and olfactory bulbs (0.9 mm3), vision appears to play a major role in the survival of these deep-sea teleosts and emphasizes that ecological and behavioural strategies account for significant variation in sensory brain structure.     

A new species of coelacanth. Genetic and morphologic proof

To test the assumption of the existence of a possible new population of coelacanth in Indonesia, we sequenced the mitochondrial DNA from cytochrome b and 12S rDNA genes, and described the morphological features of the specimen discovered on July 1998. Significant differences in base sequences revealed that the Comorean and the Indonesian coelacanth belong to distinct populations. Estimation of divergence time, high transition-transversion ratios, preponderance of third position silent substitution, strong morphological differentiation and geographical isolation led us to consider that the Indonesian coelacanth is a new species, Latimeria menadoensis n. sp., closely related to L. chalumnae.     

A histologic survey of diencephalic circumventricular organs in teleosts with special reference to osteoglossomorphs

Diencephalic circumventricular organs of various teleosts were studied histologically. Special attention was paid to osteoglossomorphs. The neurohypophysis of osteoglossomorphs (Arapaima, Notopterus, Xenomystus, andGymnarchus) is well differentiated into the median eminence and the neural lobe. The pituitary organization of these species is an intermediate between that of holosteans and of more advanced teleosts. The saccus vasculosus is absent inPantodon andGymnarchus, but it is well developed inNotopterus andXenomystus. The light microscopically discernible pineal is absent inGymnarchus: this may be the only species that lacks the pineal among teleosts. The paraphysis is found in various species including most osteoglossomorphs and some perciforms. In advanced teleosts such as gobiids and tetraodontids, the saccus dorsalis and velum transversum are absent, but the diencephalic choroid plexus is well developed instead. Some evolutionary trends are apparent in the occurrence and organization of these circumventricular organs among teleosts.     

The fibrous structure of coelacanth scales: A twisted ‘Plywood’

Isopedin is a network of collagen bundles present in the scales of most fishes. The scales of coelacanths show a remarkable three-dimensional arrangement of this network which is similar to a regularly twisted plywood. The successive fibrous layers cross at an angle which differs slightly from a right angle. It results that the whole system is twisted. The progressive rotation of the fibril direction is right-handed. Certain preferential orientations of fibrils have been observed, namely parallel to the growth rings. Such arrangements also exist in the embryonic cornea of birds and in the cuticle of certain insects, but do not present such an extensive and regular development.     

Microglia of teleosts: facing a challenge in neurobiology

This review is concerned with recent literature on teleost fish CNS microglia. It covers not only various aspects of these cells, notably comparing them with mammalian microglia, but also points out the several potentialities neural tissue of teleosts exhibits in neurobiological research. The relationships between neurons and glial cells are considered in fish, aiming at an integrated picture of the complex ways neurons and glia communicate and collaborate in normal and injured neural tissues. In addition, attention has been paid to different teleost models according to their availability, easy maintenance in experimental conditions, possibilities of embryos manipulation and sequenced genome. The recent setting up of successful protocols for fish glia and mixed neuron-glia cultures, together with the molecular facilities offered from genome knowledge, should provide a new boost to studies about microglia and neuron-microglia relationships.     

Toll-like receptor signaling in teleosts

Toll-like receptors(TLRs) sit at the top of the immune system pyramid. They form a paramount family of immune sentinels capable of sensing diverse microbe-associated molecular patterns(MAPMs), danger/damage-associated molecular patterns(DAMPs), and other signals. These perceptions trigger immediate innate immunity and instruct subsequent adaptive immunity. TLRs are highly glycosylated type I transmembrane glycoproteins that share a conserved tripartite domain architecture(LRR, TM and TIR domains), classified into six subfamilies(TLR1, TLR3, TLR4, TLR5, TLR7, TLR11) in vertebrates. Upon ligand engagement, TLRs form homodimers or heterodimers to activate immune responses via SMOCs, orchestrated by intrinsic and pathogen-directed negative regulators, glycosylation modification, etc. TLR signaling culminates in the production of inflammatory cytokines, interferons, inflammasomes, immune cell activation, apoptosis, etc. Teleosts, as the largest and most diverse group among the extant vertebrates, manifest important economic value and are crucial for understanding the evolution of vertebrate immunity. To date, teleosts contain 20 TLRs(TLR1–5, TLR7–9, TLR13, TLR14, TLR18–23, TLR25– 28) with expansions and losses in different species, and most of them possess more or less variants. Almost all teleostean TLRs localize in organelles, such as endosomes and lysosomes, sensing not only pathogens and DAMPs but also trophic factors and environmental stresses(hypoxia, temperature, microplastics, etc.). Most ligands for TLRs remain undetermined in teleosts. The adaptors consist of My D88, TIRAP, TRIF, SARM1, BCAP and SCIMP, but without TRAM; however, half of the corresponding relationships between TLRs and adaptors remain unknown in teleosts. Neofunctionalization often emerges during evolution in teleostean TLRs. Here, a systematic review of TLR signaling in teleosts, from the perspective of comparative immunology, presents the current understanding of the functions and mechanisms of teleosts. Additionally, it provides strong evidence of a divergent TLR signaling repertoire with the species-specific variation among teleosts. These are expected to benefit novel adjuvants, aquaculture, fish immunology, and comparative immunology.     

Fish fingers: digit homologues in sarcopterygian fish fins

A defining feature of tetrapod evolutionary origins is the transition from fish fins to tetrapod limbs. A major change during this transition is the appearance of the autopod (hands, feet), which comprises two distinct regions, the wrist/ankle and the digits. When the autopod first appeared in Late Devonian fossil tetrapods, it was incomplete: digits evolved before the full complement of wrist/ankle bones. Early tetrapod wrists/ankles, including those with a full complement of bones, also show a sharp pattern discontinuity between proximal elements and distal elements. This suggests the presence of a discontinuity in the proximal-distal sequence of development. Such a discontinuity occurs in living urodeles, where digits form before completion of the wrist/ankle, implying developmental independence of the digits from wrist/ankle elements. We have observed comparable independent development of pectoral fin radials in the lungfish Neoceratodus (Osteichthyes: Sarcopterygii), relative to homologues of the tetrapod limb and proximal wrist elements in the main fin axis. Moreover, in the Neoceratodus fin, expression of Hoxd13 closely matches late expression patterns observed in the tetrapod autopod. This evidence suggests that Neoceratodus fin radials and tetrapod digits may be patterned by shared mechanisms distinct from those patterning the proximal fin/limb elements, and in that sense are homologous. The presence of independently developing radials in the distal part of the pectoral (and pelvic) fin may be a general feature of the Sarcopterygii.     

Chondrichthyan cytogenetics: A comparison with teleosteans

Summary Cytogenetic studies on cartilaginous fish conducted in recent years have shown that these vertebrates have peculiarities associated both with the karyotypes and the size and composition of their DNAs. Although the data for this group, which includes about 1000 extant species, are still fragmentary, there appear to be more differences than similarities with teleosts; e.g., chromosome sets are characterized by a high diploid number (2n=up to 106) and are often rich in acrocentric elements and in microchromosomes. From the quantitative standpoint, chondrichthyan genomes are relatively large (2C=up to 34 pg DNA/n), exhibiting sometimes wide interspecific variability (Squalidae).The few studies on genome composition for these species have revealed marked difference between chondrichthyans and teleosteans in the ratio of the amount of GC-rich DNA to the total increase in genome. Moreover, thermal denaturation of the genomes of six selachians revealed derived curves that are characteristic of heterogeneity in nucleotide distribution, which has not been evidenced in most of the teleosteans investigated thus far.Finally, for the first time in selachians, an investigation was conducted using restriction enzymes, the results of which showed a pattern of chromosome labeling that was in some cases (Alu I) similar to and in others (Hae III, Hind III) different from that of teleosteans.     

A pressure chamber for determination of depth tolerance in teleosts

The construction and operation of a pressure tank suitable for experimental work on teleosts is described. The tank can be used to simulate depths down to 60 m. The design permits water to flow through the tank at all pressures in order to provide oxygen and remove metabolites but there is no increase in dissolved gases with increased pressure.     

Study of glomerular vasculature in teleosts with the resin-replica method

Glomerular vasculature was investigated in the carpCyprinus carpio, the scorpionfishSebastiscus marmoratus, and the marine catfishPlotosus lineatus with the resin-replica method. An afferent arteriole was connected with a glomerulus in every fish. It was slender in the carp, whereas the scorpionfish and marine catfish possessed thick afferent arterioles. The glomerular capillaries were sinusoidal. The divergences, convergences, and windings of these capillaries were not well developed in any of the fish. The glomerular capillaries converged into an efferent arteriole in the carp and scorpionfish. In the marine catfish, on the other hand, most of the glomeruli had two efferent arterioles.     

Glucose dehydrogenase in teleosts: tissue distribution and proposed function

Tissue extracts of skeletal muscle, heart, eye, brain, liver, kidney, gill and stomach were electrophoretically examined for glucose dehydrogenase (EC 1.1.1.47) activity in 21 species of marine teleost fishes. Glucose dehydrogenase expression was detected only in liver extracts. Considerable interordinal variation was found in levels of enzymatic activity. Available data support the hypothesis that glucose dehydrogenase provides NADPH for the mixed-function oxidase system in teleosts.     

Oocyte growth and development in teleosts

Oocyte growth and development is an important issue in fish and fisheries biology. This paper reviews the information available on oocyte growth patterns and the rates and dynamics of oocyte growth in teleosts. In synchronous spawners, the weight of the gonad may represent as much as 40% of the overall body weight of the fish. In asynchronous spawners, the weight of the mature ovary is considerably less than in synchronous ovulators, but the ovary shows a more regular periodicity and may grow repeatedly many times during the breeding season. There is a huge variability in egg size in teleosts, with the largest known measuring up to 8 cm in diameter. Within the limits of variance set by genetic constraints, egg size may vary between populations of the same species. Oocytes in all teleosts undergo the same basic pattern of growth: oogenesis, primary oocyte growth, cortical alveolus stage, vitellogenesis, maturation and ovulation. The mechanisms that control oocyte growth are addressed in this review, albeit that the available information, as in all other vertebrates, is very limited. The main hormones that have been shown to affect ovarian growth are gonadotrophin, thyroid hormones, growth hormone, insulin and insulin-like growth factors. An overview of the determinants of fecundity, with particular reference to oocyte recruitment and atresia, is the focus of the second part of the paper. Genetics and nutrition have major effects on fecundity, and studies so far suggest that the determinants of fecundity usually operate during the early part of gametogenesis. The role of atresia in determining fecundity is less clear. The final part of this review highlights some areas of study that are priorities for research on ovarian development in fish.