Respiratory vasculatures of the intertidal air-breathing eel goby, <Emphasis Type="Italic">Odontamblyopus lacepedii</Emphasis> (Gobiidae: Amblyopinae)

Lacking a propensity to emerge over the mud surface, the eel goby, Odontamblyopus lacepedii, survives low tide periods by continuously breathing air in burrows filled with hypoxic water. As with most marine air-breathing fishes, O. lacepedii does not possess an accessory air-breathing organ, but holds air in the buccal–opercular cavity. The present study aimed to clarify how the respiratory vasculature has been modified in this facultative air-breathing fish. Results showed that the gills apparently lacked structural modifications for air breathing, whereas the inner epithelia of the opercula were richly vascularized. Comparison with two sympatric gobies revealed that the density of blood capillaries within 10μm from the inner opercular epithelial surface in O. lacepedii (14.5 ± 3.0 capillaries mm⁻¹; mean ± s.d., n = 3) was significantly higher than in the aquatic non-air-breathing Acanthogobius hasta (0.0 ± 0.0) but significantly lower than in the amphibious air-breathing mudskipper, Periophthalmus modestus (59.1 ± 8.5). The opercular capillary bed was supplied predominantly by the 1st efferent branchial arteries (EBA1) and drained by the opercular veins, which open into the anterior cardinal vein. Deep invaginations at the distal end of the EBA1 and the junction with EBA2 are suggestive of blood flow regulatory sites during breath-holding and apnoeic periods. It remains to be investigated how blood flow through the gills is maintained during breath holding when the buccal–opercular cavity is filled with air.     

Respiratory,cardiovascular, and hemolymph acid‐base changes in the amphibious crab,Cardisoma guanhumi,during immersion and emersion

The cardiorespiratory and hemolymph acid base status of bimodal breathing crabs, Cardisoma guanhumi, was monitored during the transition from breathing air to breathing water. Upon immersion, oxygen uptake (MO₂) decreased by half. Ventilatory frequency (fsc) increased more than 5 fold, causing a decrease in hemolymph carbon dioxide partial pressure (PCO₂). This was nearly fully compensated for by a gradual decrease in hemolymph bicarbonate concentration ([HCO₃ ^‐]) over 96 hours post‐immersion. After one to two weeks of immersion, when crabs were removed from the water, oxygen uptake initially increased, but eventually returned to the initial immersed value. Heart rate was unchanged but fsc slowed dramatically. The decreased ventilation resulted in a buildup of hemolymph PCO₂, causing a respiratory acidosis that was slowly compensated for by increased hemolymph [HCO₃ ^‐]. C. guanhumi appears to be a truly amphibious crab with respiratory and acid‐base adaptations found in both fully aquatic and fully terrestrial species.     

Metabolism,gas exchange,and acid‐base balance of giant salamanders

The giant salamanders are aquatic and paedomorphic urodeles including the genera Andrias and Cryptobranchus (Cryptobranchidae), Amphiuma (Amphiumidae), Siren (Sirenidae), and Necturus (Proteidae, of which only N. maculosus is considered ‘a giant'). Species in the genera Cryptobranchus and Necturus are considered aquatic salamanders well adapted for breathing water, poorly adapted for breathing air, and with limited abilities to compensate acid‐base disturbances. As such, they are water‐breathing animals with a somewhat fish‐like respiratory and acid‐base physiology, whose habitat selection is limited to waters that do not typically become hypoxic or hypercarbic (although this assertion has been questioned for N. maculosus). Siren and Amphiuma species, by contrast, are dependent upon air‐breathing, have excellent lungs, inefficient (Siren) or no (Amphiuma) gills, and are obligate air‐breathers with an acid‐base status more similar to that of terrestrial tetrapods. As such, they can be considered to be air‐breathing animals that live in water. Their response to the aquatic hypercarbia that they often encounter is to maintain intracellular pH (pH_i) and abandon extracellular pH regulation, a process that has been referred to as preferential pH_i regulation. The acid‐base status of some present‐day tropical air‐breathing fishes, and of Siren and Amphiuma, suggests that the acid‐base transition from a low PCO₂‐low [] system typical of water‐breathing fishes to the high PCO₂‐high [] systems of terrestrial tetrapods may have been completed before emergence onto land, and likely occurred in habitats that were typically both hypoxic and hypercarbic.     

Physiological consequences of prolonged aerial exposure in the American eel,Anguilla rostrata: blood respiratory and acid-base status

Summary The physiological consequences of prolonged air-exposure on blood respiratory and acid-base properties were examined in the American eel (Anguilla rostrata). Eels displayed a low capacity for aerial gas transfer as indicated by pronounced increases and decreases in arterial CO₂ and O₂ tensions, respectively. The increase in arterial CO₂ tension contributed to severe extracellular acidosis. The decrease in arterial O₂ tension, combined with a marked reduction in red blood cell pH and concomitant Bohr and Root effects, caused arterial O₂ content to decline to levels that were insufficient to support metabolic requirements, aerobically. Consequently, the rate of anaerobic glycolysis increased during air-exposure as suggested by a gradual elevation of blood lactate levels after 12 h. Increased anaerobic glycolysis and associated ATP hydrolysis and/or degradation of internal ATP stores further depressed blood pH as metabolic acid, produced by these processes, entered the circulation. Unlike other fishes previously examined, red blood cell pH was not regulated preferentially during the extracellular acidosis but simply conformed to the in vitro relationship between red blood cell and whole blood pH. Although capable of surviving prolonged air-exposure, the results demonstrate nevertheless and perhaps not surprisingly that eels, unlike true amphibious fishes that utilize gills or buccal epithelia for gas transfer, are not particularly well-adapted for gas exchange in air but do display an unusual tolerance to hypoxemia.Symbols and abbreviations B buffer value - Hct hematocrit - RBC red blood cell     

Gross structure of the respiratory organs and dimensions of the gill in the mud–skipper, Periophthalmodon schlosseri (Bleeker)

In Periopkrlialnwdon scldosseri the respiratory organs consist of the gills, the suprabranchial and opercular chambers. The gills are more suited for aerial than aquatic respiration as is shown by the presence of the vascular papillae, blood sinusesand dilated blood vessels in their lamellae. The gill lamellae possess a surface coat of sulphated mucopolysaccharides that prevents water loss during exposure to the air. The filaments of the outer hemibranchs in the first gill arch are reduced to nearly one quarter of those of its posterior hemibranch. The gill area in relation to body weight shows a high slope value ( b =0·93).     

A study on the epidermal structure of <Emphasis Type="Italic">Periophthalmodon</Emphasis> and <Emphasis Type="Italic">Periophthalmus</Emphasis> mudskippers with reference to their terrestrial adaptation

Epidermal structures of three species of Periophthalmus (Ps.) and two species of Periophthalmodon (Pn.) were investigated in relation to their lifestyle. All species of both genera lack a dermal bulge, which species of other two oxudercine genera, Boleophthalmus and Scartelaos, have in their epidermis. In Periophthalmus and Periophthalmodon species, which are highly terrestrial, the middle cells are well developed in the epidermis and the capillaries are distributed in the surface of the epidermis on the head and dorsal body. In Periophthalmus species and Pn. septemradiatus, the capillaries and blood vessels are also distributed in the epidermis of the abdomen, superficially in Ps. modestus and deeply in other species. In Ps. modestus, the capillaries are also densely distributed on the surface of the epidermis in the caudal area, whereas in other species, the epidermal capillaries and blood vessels of this area are located deep with a very low density. In Pn. schlosseri, the epidermal capillaries are not found in either the abdominal area or caudal area. A comparison of the distribution of epidermal capillaries among Boleophthalmus, Periophthalmodon, Periophthalmus, and Scartelaos species revealed that the skin makes a larger contribution to respiration in the species having a more terrestrial lifestyle. Goblet mucous cells are completely lacking in Periophthalmus species, whereas slimelike materials were often found on the skin surface of Periophthalmus species. This finding suggests that Periophthalmus species have some unknown mechanism for producing mucus. In Pn. schlosseri, exposure of the dense capillary net on the surface of the head is likely to increase cutaneous respiration, but it also makes the fish an attractive target of bloodsucking insects.Supplementary material to this paper is available in electronic format at http://dx.doi.org/10.1007/s10228-003-00173-7     

Amphibious adaptations in a newly recognized amphibious fish: Terrestrial locomotion and the influences of body size and temperature

Amphibious animals are adapted for both aquatic and terrestrial habitats. The conflicting requirements for dual habitats are perhaps most pronounced in the air‐breathing fishes, which represent an intermediate stage between the totally aquatic habitat and terrestrial colonization. A key requirement for amphibious fishes is terrestrial locomotion. The different densities and compositions of air and water impose constraints for efficient terrestrial locomotion that differ from those required for aquatic locomotion. I investigated terrestrial locomotion in a small South African fish, Galaxias ‘nebula’, by exposing 60 individual fish to air in specially designed raceways and quantifying movement type and occurrence as a function of availability of water, fish size and environmental temperature. Nebula showed a sustained undulating form of terrestrial locomotion characteristic of amphibious fishes and also a transient ballistic locomotion (jumps) typical of fully aquatic species. Terrestrial movement was influenced by fish size, with medium‐sized fish undertaking more jumps towards water, and fewer jumps away from water, than their smaller or larger conspecifics. In contrast, axial undulation was mainly influenced by temperature. However, there was no consistent pattern in temperature effects presumably because temperature is just one of a suit of environmental factors that may affect terrestrial locomotion. Nebula's amphibious adaptations allow it to cope with the unpredictability inherent in its natural environment.     

CO2‐metabolism in early tetrapods revisited: inferences from osteological correlates of gills,skin and lung ventilation in the fossil record

One of the most important physiological changes during the conquest of land by vertebrates was the increasing reliance on lung breathing, with the concomitant decrease in importance of gill breathing. The main problem involved here was to cope with the excessive accumulation of CO₂ in the body and to avoid respiratory acidosis. In the past, several often mutually contradicting hypotheses of CO₂‐elimination via skin, lungs and gills in early tetrapods have been proposed, based on theoretical physiological considerations and comparison with extant air‐breathing fishes and amphibians. This study proposes a revised scenario of CO₂‐elimination in early tetrapods based on fossil evidence, that is recently identified osteological correlates of gills, skin structure and mode of lung ventilation. In stem tetrapods of the Devonian and Carboniferous, O₂‐uptake via the lungs by buccal pumping was decoupled from CO₂‐release via internal gills, and the rather gas‐impermeable skin played a minor role in gaseous exchange. The two main lineages of crown‐group tetrapods, the amphibian and amniote lineage, used different strategies of CO₂‐elimination. As in stem tetrapods, O₂‐uptake and CO₂‐release remained always largely decoupled in temnospondyls, which ventilated their lungs via buccal pumping and relied mainly on their internal gills for CO₂‐release. Temnospondyls were not able to reduce their internal gills before their skin became more gas permeable and their body size was reduced, to shift from internal gills to the skin as the major site of CO₂‐elimination, a pattern that is retained in most lissamphibians. In contrast, internal gills were lost very early in stem amniote evolution. This was associated with the evolution of the more effective aspiration pump that allowed the elimination of the bulk of CO₂ via the lungs, leading to a coupled O₂‐uptake and CO₂‐loss in stem amniotes and later in amniotes.     

Bimodal oxygen uptake in juveniles and adults amphibious fish,Channa (= Ophiocephalus) marulius

Oxygen uptake of Channa marulius was studied under water with and without access to air. There was a significant increase in the oxygen uptake through the gills when access to air was prevented. However, this value (0.863 ± 0.058 mlO₂/indiv./h) was quite low in comparison to the total bimodal oxygen uptake (2.04 ± 0.14 mlO₂/indiv./h) in juveniles. In adult fish the oxygen uptake per unit time increased appreciably (4.673 ± 0.404 mlO₂/indiv./h). In juveniles as well as in adults the air breathing dominated over aquatic breathing. This fish showed a definite circadian rhythm in the bimodal oxygen uptake during different hours of the day.This work was performed in the Ichthyology Laboratory, P. G. Dept. of Zoology, Bhagalpur University, and was supported by a research grant from Bhagalpur University     

Aerial release of CO2 and respiratory exchange ratio in intertidal fishes out of water

Synopsis Many fishes in the marine intertidal zone have the ability to exchange respiratory gases in air during brief periods out of water. Previous studies on marine amphibious fishes such as mudskippers and rockskippers have shown that they release CO₂ in air at a rate consistent with its release in aquatic respiration. Respiratory Exchange Ratios (RER, CO₂ released per O₂ consumed) are between 0.7 and 1.0, similar to Respiratory Quotients (CO₂ produced metabolically per O₂ consumed). However, previous studies of temperate intertidal fishes emerged only at low tides have been less consistent, with two species reported similar in ability to the amphibious skipper fishes, and two others reported with much lower RERs in air. This study examines 12 species of fishes, from six families (Batrachoididae, Cottidae, Gobiesocidae, Kyphosidae, Pholididae, and Stichaeidae), present in the intertidal zone of California. All 12 species exchanged O₂ and CO₂ in air. Like the amphibious skipper fishes of marine tropical waters, many intertidal fishes of the temperate zone release CO₂ during low tide emergence in quantities sufficient to match its metabolic production. These results indicate that CO₂ release is not hampered by the change in respiratory medium from water to air.     

A comparative study on the responses of the gills of two mudskippers to hypoxia and anoxia

A comparison of branchial enzyme profiles indicates that the gills of Periophthalmodon schlosseri would have a greater capacity for energy metabolism through glycolysis than those of Boleophthalmus boddaerti. Indeed, after exposure to hypoxia, or anoxia, there were significant increases in the lactate content in the gills of P. schlosseri. In addition, exposure to hypoxia or anoxia significantly lowered the glycogen level in the gills of this mudskipper. It can be deduced from these results that the glycolytic flux was increased to compensate for the decrease in ATP production through anaerobic glycolysis. Different from P. schlosseri, although there was an increase in lactate production in the gills of B. boddaerti exposed to hypoxia, there was no significant change in the branchial glycogen content, indicating that a reversed Pasteur effect might have occurred under such conditions. In contrast, anoxia induced an accumulation of lactate and a decrease in glycogen in the gills of B. boddaerti. Although lactate production in the gills of these mudskippers during hypoxia was inhibited by iodoacetate, the decreases in branchial glycogen contents could not account for the amounts of lactate formed. The branchial fructose-2,6-bisphosphate contents of these mudskippers exposed to hypoxia or anoxia decreased significantly, leaving phosphofructokinase and glycolytic rate responsive to cellular energy requirements under such conditions. The differences in response in the gills of B. boddaerti and P. schlosseri to hypoxia were possibly related to the distribution of phosphofructokinase between the free and bound states.Abbreviations ADP adenosine diphosphate - ALD aldolase - ALT alanine transaminase - AST aspartate transaminase - ATP adenosine triphosphate - CS citrate synthase - EDTA ethylenediaminetetra-acetic acid - EGTA ethylene glycol tetra-acetic acid - F6P fructose-6-phosphate - F-1,6-P₂ fructose-1,6-bisphosphate - F-2,6-P₂ fructose-2,6-bisphosphate - FBPase fructose-1,6-bisphosphatese - GAPDH glyceraldehyde-3-phosphate dehydrogenase - GDH glutamate dehydrogenase - -GDH -glycerophosphate dehydrogenase - GPase glycogen phosphorylase - HK hexokinase - HOAD 3-hydroxyacyl-CoA dehydrogenase - IDH isocitrate dehydrogenase - IOA iodoacetic acid - LDH lactate dehydrogenase - LO lactate oxidizing activity - MDH malate dehydrogenase - 3-PG 3-phosphoglyceric acid - PEP phosphoenolpyruvate - PEPCK phosphoenolpyruvate carboxykinase - PGI phosphoglucose isomerase - PGK phosphoglycerate kinase - PFK 6-phosphofructo-1-kinase - PIPES piperazine-N, N-bis-(2-ethanesulphonic acid) - PK pyruvate kinase - PMSF phenylmethylsulphonyl fluoride - PR pyrurate reducing activity - SE standard error - SW seawater - TPI triosephosphate isomerase     

Regulation of ventilation in the caiman (Caiman latirostris): effects of inspired CO2 on pulmonary and upper airway chemoreceptors

In order to study the relative roles of receptors in the upper airways, lungs and systemic circulation in modulating the ventilatory response of caiman (Caiman latirostris) to inhaled CO₂, gas mixtures of varying concentrations of CO₂ were administered to animals breathing through an intact respiratory system, via a tracheal cannula by-passing the upper airways (before and after vagotomy), or via a cannula delivering gas to the upper airways alone. While increasing levels of hypercarbia led to a progressive increase in tidal volume in animals with intact respiratory systems (Series I), breathing frequency did not change until the CO₂ level reached 7%, at which time it decreased. Despite this, at the higher levels of hypercarbia, the net effect was a large and progressive increase in total ventilation. There were no associated changes in heart rate or arterial blood pressure. On return to air, there was an immediate change in breathing pattern; breathing frequency increased above air-breathing values, roughly to the same maximum level regardless of the level of CO₂ the animal had been previously breathing, and tidal volume returned rapidly toward resting (baseline) values. Total ventilation slowly returned to air breathing values. Administration of CO₂ via different routes indicated that inhaled CO₂ acted at both upper airway and pulmonary CO₂-sensitive receptors to modify breathing pattern without inhibiting breathing overall. Our data suggest that in caiman, high levels of inspired CO₂ promote slow, deep breathing. This will decrease dead-space ventilation and may reduce stratification in the saccular portions of the lung.     

Development of the neo-morphic air breathing organs in three genera of estuarine gobies

The rudiment of the neo-morphic organ for O2 uptake arises in the form of a gill mass formed by the gill material of the embryonic 5th gill arch. Ectodermal induction to the gill mass takes place in the post-embryonic stage of development to produce a respiratory epithelium of the neo-morphic air breathing organ. The respiratory epithelium of the opercular chamber and the buccal cavity is formed by the cells of the gill mass alone. The respiratory epithelium of the suprabranchial chamber is formed by the cells of the gill mass as well as the gill lamellae derived from the dorsal aspects of the functional gill arches (1 to 4). Extension of the suprabranchial chamber into the buccal region anteriorly is a device to increase the respiratory surface area available for O2 uptake by air. The epithelial position of the blood capillaries in the suprabranchial chamber of Periophthalmodon schlosseri signifies terrestrial nature of the fish.     

Thermal specialization across large geographical scales predicts the resilience of mangrove crab populations to global warming

The broad prediction that ectotherms will be more vulnerable to climate change in the tropics than in temperate regions includes assumptions about centre/edge population effects that can only be tested by within‐species comparisons across wide latitudinal gradients. Here, we investigated the thermal vulnerability of two mangrove crab species, comparing populations at the centre (Kenya) and edge (South Africa) of their distributions. At the same time, we investigated the role of respiratory mode (water‐ versus air‐breathing) in determining the thermal tolerance in amphibious organisms. To do this, we compared the vulnerability to acute temperature fluctuations of two sympatric species with two different lifestyle adaptations: the free living Perisesarma guttatum and the burrowing Uca urvillei, both pivotal to the ecosystem functioning of mangroves. The results revealed the air‐breathing U. urvillei to be a thermal generalist with much higher thermal tolerances than P. guttatum. Importantly, however, we found that, while U. urvillei showed little difference between edge and centre populations, P. guttatum showed adaptation to local conditions. Equatorial populations had elevated tolerances to acute heat stress and mechanisms of partial thermoregulation, which make them less vulnerable to global warming than temperate conspecifics. The results reveal both the importance of respiratory mode to thermal tolerance and the unexpected potential for low latitude populations/species to endure a warming climate. The results also contribute to a conceptual model on the latitudinal thermal tolerance of these key species. This highlights the need for an integrated population‐level approach to predict the consequences of climate change.     

Morphological, Behavioral, and Physiological Characterization of Bimodal Breathing Crustaceans

SYNOPSIS Bimodal breathing crustaceans, while representing astage in the transition from the aquatic to terrestrial habitat,also constitute a distinct group that can be characterized bymorphological, behavioral, and physiological traits. Morphologically,this group displays reduced gill surface area and enlarged branchialchambers. The lining of the branchial chambers, the branchiostegites,also has increased surface area and is highly vascularized.The branchiostegites can be smooth, cutaneous epithelia, orthey can have more complex evaginations or invaginations tofurther increase surface area. In addition, the thickness ofthe branchiostegal epithelium is greatly reduced, compared tothat in the gills, thus minimizing the diffusion distance betweenair and hemolymph. These animals maintain a store of water inthe branchial chamber that covers the gills and allows for simultaneousgas exchange with two media (air and water). There is also apartitioning of gas exchange, with oxygen uptake occurring preferentiallyfrom air across the branchiostegites, and carbon dioxide excretionoccurring across the gills into the branchial water. One criticalfactor that appears to separate bimodal breathing crustaceansfrom fully terrestrial, exclusively air-breathers is the abilityof the latter group to excrete CO₂directly into air across thegills and branchiostegites. It is suggested that the incorporationof the enzyme carbonic anhydrase into the membrane fractionof the branchiostegites may have been one of the key molecularevents which allowed for pulmonary CO₂ excretion into air.     

Voluntary emergence and water detection in a newly recognized amphibious fish

Galaxias ‘nebula’, a small fish which has adaptations for air‐breathing but is not known to be amphibious, voluntarily emerged from water and, in an unfamiliar environment, moved preferentially towards an alternative water source. Nebula may thus be considered one of the few truly amphibious fishes, and their ability to detect water provides a selective advantage which aids their survival in unpredictable natural environments.     

Hypoxia-induced developmental plasticity of larval growth,gill and labyrinth organ morphometrics in two anabantoid fish: The facultative air-breather Siamese fighting fish (Betta splendens) and the obligate air-breather the blue gourami (Trichopodus trichopterus)

Larval and juvenile air breathing fish may experience nocturnal and/or seasonal aquatic hypoxia. Yet, whether hypoxia induces respiratory developmental plasticity in larval air breathing fish is uncertain. This study predicted that larvae of two closely related anabantid fish—the facultative air breather the Siamese fighting fish (Betta splendens) and the obligate air breathing blue gourami (Trichopodus trichopterus)—show distinct differences in developmental changes in body, gill, and labyrinth morphology because of their differences in levels of dependency upon air breathing and habitat. Larval populations of both species were reared in normoxia or chronic nocturnal hypoxia from hatching through 35–38 days postfertilization. Gill and labyrinth variables were measured at the onset of air breathing. Betta splendens reared in normoxia possessed larger, more developed gills (~3× greater area) than T. trichopterus at comparable stages. Surface area of the emerging labyrinth, the air breathing organ, was ~ 85% larger in normoxic B. splendens compared to T. trichopterus. Rearing in mild hypoxia stimulated body growth in B. splendens, but neither mild nor severe hypoxia affected growth in T. trichopterus. Condition factor, K (~ 1.3 in B. splendens, 0.7 in T. trichopterus) was unaffected by mild hypoxia in either species, but was reduced by severe hypoxia to <0.9 only in B. splendens. Severe, but not mild, hypoxia decreased branchial surface area in B. splendens by ~40%, but neither hypoxia level affected Trichopodus branchial surface. Mild, but not severe, hypoxia increased labyrinth surface area by 30% in B. splendens. However, as for branchial surface area, labyrinth surface area was not affected in Trichopodus. These differential larval responses to hypoxic rearing suggest that different larval habitats and activity levels are greater factors influencing developmental plasticity than genetic closeness of the two species.     

O2 uptake through water during early life of Heteropneustes fossilis (Bloch)

The O₂ uptake through water has been measured in case of Heteropneustes fossilis during development and growth and its relationship to body size established. A higher rate of O₂ uptake during the early phase of ontogenesis is related to intense growth of the respiratory surface area and increasing metabolic demand of the fish.The logarithmic plot of data for O₂ uptake in relation to body size shows a statistically significant two-component curve; one related to the fish when it is a fully aquatic breather and the other when it changes to bimodal gas exchange. The onset of the air breathing habit brings about a 40% drop in O₂ uptake through water, which is made good through the newly developed air breathing organ.     

Gill morphology and morphometry of the facultative air-breathing armoured catfish,Corydoras paleatus,in relation on aquatic respiration

The Neotropical armoured catfish Corydoras paleatus is a facultative air-breathing teleost commonly exported as ornamental fish. In this species, air breathing enables it to survive and inhabit freshwater environments with low oxygen levels. Therefore, it is important to analyse the gills from a morphological aspect and its dimensions in relation to body mass with reference to aquatic respiration. For that, the gills were analysed using a stereoscopic microscope for morphometric studies, and structural and ultrastructural studies were carried out to compare the four branchial arches. Furthermore, two immunohistochemical techniques were used to locate and identify the presence of a Na⁺/K⁺ pump. The characterization of the potential for cell proliferation of this organ was assessed using an anti-PCNA antibody. The results show that gills of C. paleatus present some characteristics related to its diet and lifestyle, such as the limited development of gill rakers and the abundance of taste buds. In addition, other special features associated with the environment and bimodal breathing were observed: scarce and absent mucous cells (MCs) in the gill filaments and branchial lamellae, respectively, and the localization of mitochondria-rich cells (MRCs) covering the basal third of the branchial lamellae, which reduces the gill respiratory area. A peculiar finding in the gill epithelium of this armoured catfish was the presence of mononuclear cells with sarcomeres similar to myoid cells, whose functional importance should be determined in future studies. Finally, in C. paleatus, the interlamellar space of gill filaments is an important site for cell turnover and ionoregulation; the latter function is also performed by the branchial lamellae.