Increased ventilation by fish leads to a higher risk of parasitism
increasingly smaller and the number per unit of lung volume increases. The evolution of the . fish or 4) by diffusion through the lungs, respiratory organs present in relationship with the arterioarterial vasculature, i.e., the pillar cells. .. the high pressures to which they are subjected during the immersion. Air ventilation volumes, O2 and CO2 partial pressures of exhaled gas, O2 elimination (gas exchange ratio = for intestine). O2 extraction several species of intestinal breathing fishes (Jeuken, ; W. W. Burggren & B. R. . during each breathing sequence were diverted away from the air in the reservoir and. However, the slow diffusion rate of oxygen relative to carbon dioxide limits the size when they are dissolved in water or an aqueous solution, thus respiratory Arthropods, annelids, and fish use gills; terrestrial vertebrates utilize internal lungs. The lungs and alveoli and their relationship to the diaphragm and capillaries.
The sequence of air flow in the breathing cycle of lungfish and amphibians such as bullfrogs is similar. However, unlike air-breathing fish, which must open their mouth to aspirate ambient air into their buccal cavity at the onset of the breathing cycle, frogs aspirate air via their nostrils. Lung ventilation usually occurs episodically in bullfrogs. The breathing cycle has recently been described by Gargaglioni and Milsom Typically, after a bout of lung breathing, a series of elevations and depressions of the floor of the buccal cavity follows, called buccal oscillations.
It has been suggested that they may be remnants of the mechanisms of gill ventilation used by the premetamorphic tadpole stages, and homologous to gill ventilations in fish, and that their rhythm may reflect vestiges of the central rhythm generator for gill ventilation Buccal oscillations and lung ventilations are produced by the same muscles. The primary difference between these two events is the force of the contraction and the positions of the glottis and nares Table 1.
In resting animals, buccal oscillations occur more or less continuously and are interrupted by periodic lung ventilations, which normally occur at a time when another buccal oscillation would have been initiated.
However, the fact that lung ventilation always occurs at a time when a buccal oscillation would otherwise have occurred suggests that if there are separate rhythm generators, they are entrained to a large degree. There are some circumstantial evidence for the existence of two central respiratory rhythm generators in the bullfrog 4. Hypercapnia had no effect on the frequency of lung inflations but reduced both the occurrence of buccal oscillations and their instantaneous frequency when they did occur.
This might suggest that there are separate rhythms for lung inflation and buccal oscillation, which can be uncoupled. A number of investigators have used in vitro preparations of the larval or adult anuran brainstem to examine the mechanisms of respiratory rhythmogenesis Recordings of fictive breathing in isolated brainstem preparations revealed spontaneous neural output from the roots of cranial nerves V, VII, X, and XII.
However, these bursts were synchronous, implying that the spatiotemporal relationships between bursts of activity in these nerves in the intact animal rely on feedback from peripheral receptors.
Microinjections of glutamate into rostral areas of the bullfrog brainstem, near the VIIth motor nucleus, caused a brief excitation of fictive breathing. Extracellular recording from in vitro brainstem, spinal cord preparations of Rana catesbeiana tadpoles, and adults revealed that it is possible to manipulate the two types of neural activity associated with buccal or lung breathing independently, using pharmacological agents 4.
Superfusion of an in vitro brainstem-spinal cord preparation from the bullfrog tadpole with chloride-free saline eliminated the rhythmic bursts associated with gill ventilation while augmenting lung bursts, indicating that the former arise from a GABAergic, network-type rhythm generator whereas lung ventilatory rhythms arise from pacemaker cells However, there is some evidence for maturation of respiratory rhythmogenesis from a pacemaker-driven process in the tadpole to a network-driven process in the adult frog and for relocation of the site of the RRG during development.
This apparent discrimination is of interest in comparison to the situation described in fish, where gill ventilation may depend on pacemaker cells located in the RF, and in adult mammals, where lung ventilation may be dependent on activity in neural networks 4, This may have evolved from the generator for the feeding rhythm, which can be recruited by the respiratory RRG during forced ventilation in fish, or when air-breathing fish gulp air at the water surface, as described above.
Although similar data are not available for anuran amphibians, which may have lost this function, these data raise important considerations regarding the evolution of the control of ventilation in amphibians, which imply that descending fibres from the brainstem, innervating spinal motoneurons, can have important roles in some species, anticipating their roles in the supposedly more advanced tetrapods.
Amphibians often breathe intermittently, with bouts of ventilation interrupted by quiescent periods or, in aquatic species or stages, submersion. Intermittent breathing patterns are common in lower vertebrates, such as amphibians and reptiles, and contrast with the continuous breathing patterns of non-diving birds and mammals in their apparent lack of constancy and intrinsic rhythm Figure 2.
In this model, lung ventilations are induced when a certain PaO2 or PaCO2 threshold is reached and breathing ceases when the blood gas values have been brought back within a certain range The observation that breathing is completely suppressed when convective requirements are met by unidirectional ventilation 17 indicates that lung ventilation is conditional upon a minimal stimulatory input.
However, unidirectionally ventilated toads can still display episodic breathing or fictive ventilation 19although this experimental procedure has been assumed to maintain blood gases constant and, in paralysed animals, lung distension constant, and thus produce only tonic chemoreceptor and mechanoreceptor input. These data imply that the mechanisms underlying episodic breathing may be an intrinsic property of the central respiratory control system, a view which seems to be confirmed by the observation that the motor output from a brainstem-spinal cord preparation of the bullfrog was episodic, in the absence of any possible feedback from the periphery The central generation of these episodic breathing patterns has been localised to the nucleus isthmi in the brainstem of the bullfrog This mesencephalic structure is the neuroanatomical equivalent of the pons in mammals, which contributes to the control of breathing pattern.
Reptiles Reptiles are typically committed air-breathers, having dry scaly skin and well-developed lungs. They are an ancient and highly polyphyletic class of vertebrates. Extant members show highly diverse respiratory and cardiovascular mechanisms, including some they share with the amphibians, such as an incompletely divided circulatory system and periodic ventilation, often combined with periods of submersion.
Lizards, in common with all other reptiles except some crocodilians lack a diaphragm. However, unlike modern amphibians they do have ribs, and lung ventilation has long been considered to be generated by intercostal muscles acting on the rib cage. As lizards run in a serpentine manner, employing segmental muscles from the body wall, it was asserted by some investigators that they are unable to breathe while running Figure 3.
However, some lizards have been shown to utilise an alternative mode of ventilation involving a gular pump, which alternates with the costal pump see Table 1.
Following a short passive expiration, a bout of buccal pumping caused a progressive increase in lung volume, followed by breath-hold Figure 4.
X-ray imaging of the varanid lizard, Varanus exanthematicus, revealed that it used an accessory gular pump when walking, thus overcoming the supposed mechanical constraint on active lung ventilation during exercise reviewed by Ref. As well as the intercostal muscles, crocodilians have a unique mechanism for lung ventilation in the form of the diaphragmatic muscle that moves the liver and viscera back and forth to aspirate the lung see Table 1. This muscle is inserted on the pelvic girdle, which is also rotated to change abdominal volume.
Chelonians have their ribs fused to the carapace and lung ventilation is achieved by movements of the forelimb and shoulder girdle, together with the glottis Table 1.
The existence of anatomically and functionally separate thoracic and gular respiratory pumps in lizards would seem to require separate sites of central respiratory rhythm generation. However, this interesting possibility remains unexplored. Putative sites of respiratory pattern generation, having similarities in neural organisation and activation to those extensively documented for mammals, have been described for turtles.
Recent evidence from study on the isolated brainstem indicates that the RRG in turtles requires calcium-activated cation channels and resembles the group-pacemaker model described for mammals, though during synaptic inhibition blockade the rhythm generator appeared to be transformed into a pacemaker-driven network However, the direct contribution of these populations of neurons and their potential integration of sensory information, in determining the generation of respiratory movements, remains unclear As in amphibians, it has been suggested that the initiation of bouts of discontinuous breathing in reptiles may relate to thresholds for stimulation of central and peripheral chemoreceptors rather than to patterns dictated by a central rhythm generator However, unidirectionally ventilated alligators display episodic breathing so that centrally generated rhythmicity may have a role in its initiation 1.
In turtles, the basic output of the RRG is episodic, even under experimental conditions when all sensory feedback appears to be tonic Experiments performed on reptiles demonstrated that mild anaesthesia and brainstem section at the level of the rostral rhombencephalon metencephalon abolish these breathing episodes, i. Vagotomy also affects the breathing pattern by reducing the number of breaths per episode in crocodilians.
It is interesting to note, however, that vagotomy had no effect on the breathing pattern when it was performed after episodic breathing had been abolished by a caudal midbrain transection reviewed in Ref. Afferent innervation of the respiratory and circulatory systems Fish Chemoreceptors. Fish typically respond to ambient hypoxia with a reflex bradycardia and increased ventilatory effort.
These changes were interpreted as adaptive responses that improved respiratory gas exchange but the bradycardia is now considered to be a protective response for the highly aerobic cardiac muscle 1. The chemoreceptors inducing these responses vary among the studied species of fish in their location, distribution, innervation and also the reflex triggered by each receptor population.
Many sites have been suggested as the reflexogenic origin of the O2 chemosensory responses. The gill arches, innervated by cranial nerves IX and X, are the ubiquitous site of O2 receptors in all fishes studied Other sites can contribute to that response. Among them are the walls of the orobranchial cavity, innervated by cranial nerves V and VII 24 and the spiracle or pseudobranch, innervated by nerves VII and IX 23 and maybe the brain Activity recorded peripherally from branchial respiratory branches of the trout showed an exponential increase in afferent activity with a progressive decrease in O2 supply.
These responses resemble the recorded responses of the mammalian carotid body, similarly innervated by the IXth cranial nerve.
They were shown to have similar embryonic origins 26innervation and even chemoreceptive mechanisms, suggesting that they may be the evolutionary antecedents of the mammalian carotid body The O2 receptors in fish occur in distinct populations, which characteristically generate very specific reflex responses.
The populations of receptors associated with hypoxic bradycardia are restricted to the first gill arch in some species like Salmo gairdneri, Gadus morrhua and Hoplias malabaricus, but can be found throughout the other gill arches in other species, like Ictalurus punctatus, the pacu, Piaractus mesopotamicus and the tambaqui, Colossoma macropomum.
The hypoxic ventilatory response, on the other hand, only arises from receptors confined to the gill arches in a few species like Ictalurus punctatus. In many others, such as Hemipterus americanus, Hoplias malabaricus, Colossoma macropomum, and Piaractus mesopotamicus, total gill denervation fails to eliminate the hypoxic ventilatory response and the remaining receptors appear to occur at extrabranchial sites that include the orobranchial cavity These populations of O2 chemoreceptors can also be directed to monitor either the internal or the external environment.
The chemoreceptors that engender a hypoxic bradycardia appear to sense the O2 levels in the water passing over the gills. Changes in ventilation are most often triggered in response to changes in O2 tension in the blood passing through the gills 29, Many teleosts have receptors that sense changes in both water and blood 28,29, Immunofluorescence techniques against serotonin have made it possible to directly identify putative O2 receptors on the gills by visualising the presence of 5-HT in dense-cored vesicles in neuroepithelial cells NEC.
Furthermore, these NECs are innervated by the same nerves and located at the gill sites that would provide internal and external O2 monitoring as inferred by the reflexogenic experiments reported above. That was also observed by Coolidge et al. The authors described innervated NEC at the filament tips in all species in a prime location to sense PwO2, in agreement with physiological data. The presence of these internal chemoreceptors varied between the species and correlated with their ability to respond to internal arterial hypoxia.
Furthermore, the authors also described a group of non-innervated NEC in the lamella that might have a paracrine role, acting directly on the pillar cells to enhance respiratory surface area when exposed to aquatic hypoxia.
Control of respiration in fish, amphibians and reptiles
To date, we are not able to link species-specific patterns of O2 receptor distribution to particular phylogenetic or life history traits, and it may prove rewarding to continue to try to map locations and distribution as a function of phylogeny, habitat and life history. Short-term plasticity, due to a previous history of exposure to hypoxia, may also lead to changes in responsiveness and even functionality of receptors The respiratory muscles in fish contain length and tension receptors, in common with other vertebrate muscles, and the gill arches bear a number of mechanoreceptors with various functional characteristics.
These mechanoreceptors will also be stimulated by the ventilatory movements of the gill arches and filaments and may be important in stabilising the respiratory rhythm. When gill arches of a lightly anaesthetised fish were artificially moved the respiratory rhythm was regularly reset by the imposed movements in a manner related to the phase of the respiratory cycle at which the movement was imposed, with 1: These experiments suggest that phasic mechanoreceptor activity serves to stabilise the generation of the respiratory rhythm, preventing the central generating circuits from being disrupted by other inputs.
Central stimulation towards the brain of nerves innervating respiratory muscles in the carp with short trains of electrical stimuli also entrained the respiratory rhythm to the imposed stimuli The branchial branches of the IXth and Xth cranial nerves innervate a range of tonically and phasically active mechanoreceptors as well as chemoreceptors on the gill arches of fish and project directly to a dorsal sensory nucleus lying above the equivalent motor nuclei in the medulla The sensory area in turn projects centrally to the respiratory motor nuclei.
However, sensory fibres from branchial receptors may terminate in different locations within the brainstem and consequently have different effects on integration. Some vagal afferent fibres seem to project to vagal motoneurons innervating branchial muscles by short loops, either directly or via the RF and may be involved in the reflex contraction of adductor muscles on the gill filaments in response to mechanical stimulation of the gill filaments or gill rakers.
Stronger stimulation may induce the coughing reflex with simultaneous contraction of respiratory pump muscles that receive inputs from the RF. Vagal afferent fibres also connect with the trigeminal complex that receives inputs from proprioceptors in the respiratory pump muscles innervated by the trigeminal Vth cranial nerve. As proprioceptive reflexes are involved in entrainment and stabilisation of the respiratory rhythm then their inputs must be connected fairly directly with the rhythm-generating neurons 1.
Chemoreceptor stimulation transmitted in the vagus nerve, that affects ventilation, may be relayed via the medulla.
However, microinjection of glutamic acid into identified areas of the vagal sensory projection in sculpin, identified by injection of a fluorescent tracer, elicited specific, highly localised responses, including changes in ventilation frequency and amplitude Glutamate has been identified as the neurotransmitter for afferents into the nucleus tractus solitarii NTS in mammals.
Evidence for the involvement of baroreceptors in vasomotor control in fish was long contentious and it has been proposed that the evolution of barostatic control of the heart may be associated with the evolution of air-breathing because the gills of fish are supported by their neutral buoyancy in water. Ventilation of the gills generates hydrostatic pressures, which fluctuate around, but predominantly above ambient levels.
Arterial blood pressures in the branchial circulation of fish and the pressure difference across the gill epithelia are relatively low, despite the fact that the highest systolic pressures are generated in the ventral aorta, which leaves the heart to supply the afferent branchial arteries.
Consequently, the need for functional baroreceptors in fish is not clear. However, in teleosts, injection of adrenaline, which raised arterial pressure, caused a bradycardia, abolished by atropine, while low frequency oscillations in blood pressure, similar to the Mayer waves in mammals, were abolished by injection of the a-adrenoreceptor antagonist yohimbine.
These data imply active regulation of vasomotor tone and the balance of evidence indicates that functional arterial baroreceptors may exist in the branchial circulation of teleost fishes 1. Aquatic surface respiration and air-breathing in fish It has generally been considered that hypoxia, consequent upon stagnation of tropical freshwater habitats, was the environmental spur for the evolution, in the Devonian era, of air-breathing in many bony fishes, which use a variety of different ABO, from modified swimbladders to diverticula of the branchial chambers There are also a large number of highly derived marine teleosts that occupy the intertidal zone and which evolved air-breathing abilities and an amphibious lifestyle independently of the freshwater air-breathers The selection pressures may have been an ability to tolerate emersion during low tide and to escape extremes of salinity and hypoxia in tidepools.
These species typically use the skin, gills, and branchial chambers as ABO. However, it has been argued on the basis of fossil evidence and the ionic composition of blood plasma that terrestrial vertebrates have evolved from freshwater ancestors, rather than from amphibious marine ancestors Chemoreceptor control of aquatic surface respiration.
It has been suggested that true air-breathing evolved from a behaviour known as aquatic surface respiration ASR As the name implies, ASR involves rising to the surface and ventilating the layer of water in contact with the atmosphere, which is richer in dissolved oxygen than the underlying bulk water.
Many teleost species have evolved this behavioural response in both temperate and tropical environments, freshwater and marine. Many species also hold an air bubble or bubbles in their mouth when they perform ASR, which may have a dual role of increasing oxygen levels in the bucco-opercular cavity, and maintaining the fish buoyant at the water surface 38and may have been the behavioural antecedent to true air-breathing. Indeed, although ASR is a behavioural response to hypoxia, it is in fact a reflex that is driven by oxygen-sensitive chemoreceptors 24, These chemoreceptor sensory modalities and innervations would appear to be homologous, therefore, to those that drive reflex gill hyperventilation in all fish groups studied to date 1, Thus, ASR may use the pre-existing sensory arm of such hypoxic ventilatory reflexes, integrating a new motor output that involves rising to the water surface to ventilate the surface layer.
Presumably, cessation of this behaviour is also driven by information from the same chemoreceptors Clearly, ASR is a much more complex chemoreflex than changes in gill ventilation, with a very large behavioural component, which must involve significant inputs from higher brain centres Teleost fish exhibit behavioural modulation of gill ventilation patterns, and such higher-order inputs to the respiratory medulla must, presumably, have been a prerequisite for the evolution of the complex motor responses of ASR and true air-breathing in fishes.
One major ecological cost to reflexes such as ASR and air-breathing is that they place fish at significantly greater risk from aerial predation by birds.
Exposure of the grey mullet to a model avian predator delayed the onset of ASR in hypoxia or in response to direct chemoreceptor stimulation with cyanide. Furthermore, the fish surfaced preferentially under a sheltered area in their experimental chamber or close to the walls Figure 5A. In turbid water, the fish could not see the model predator and it had no effect on the onset of ASR but, in turbidity, all the mullet preferentially surfaced around the walls of their chamber Figure 5B.
In oxygen-poor water, the octopus may increase its ventilation fold, indicating a more active control of respiration than appears to be present in other classes of mollusks. Many crustaceans crabs, shrimps, crayfish are very dependent on their gills. As a rule, the gill area is greater in fast-moving crabs Portunids than in sluggish bottom dwellers; decreases progressively from wholly aquatic, to intertidal, to land species; and is greater in young crabs than in older crabs.
Often the gills are enclosed in protective chambers, and ventilation is provided by specialized appendages that create the respiratory current.
As in cephalopod mollusks, oxygen utilization is relatively high—up to 70 percent of the oxygen is extracted from the water passing over the gills in the European crayfish Astacus. A decrease in the partial pressure of oxygen in the water elicits a marked increase in ventilation the volume of water passing over the gills ; at the same time, the rate of oxygen utilization declines somewhat.
Although more oxygen is extracted per unit of time, the increased ventilation increases the oxygen cost of breathing. The increased oxygen cost, together with the decrease in extraction per unit of volume, probably limits aquatic forms of crustaceans to levels of oxidative metabolism lower than those found in many air-breathing forms.
This is largely due to the lower relative content of oxygen in water and the higher oxidative cost of ventilating a dense and viscous medium compared with air. Not all crustaceans meet a reduction in oxygen with increased ventilation and metabolism. The square-backed crabs Sesarma become less active, reducing their oxidative metabolism until more favourable conditions prevail.
Respiratory organs of vertebrates In most vertebrates the organs of external respiration are thin-walled structures well supplied with blood vessels. Such structures bring blood into close association with the external medium so that the exchange of gases takes place across relatively small distances. There are three major types of respiratory structures in the vertebrates: The gills are totally external in a few forms as in Necturus, a neotenic salamanderbut in most they are composed of filamentous leaflets protected by bony plates as in fish.
Some fishes and numerous amphibians also use the body integument, or skin, as a gas-exchange structure. Both gills and lungs are formed from outpouchings of the gut wall during embryogenesis. Such structures have the advantage of a protected internal location, but this requires some sort of pumping mechanism to move the external gas-containing medium in and out.
The quantity of air or water passing through the lungs or gills each minute is known as the ventilation volume. The rate or depth of respiration may be altered to bring about adjustments in ventilation volume.
The ventilation volume of humans at rest is approximately six litres per minute. This may increase to more than litres per minute with increases in the rate of respiration and the quantity of air breathed in during each respiratory cycle tidal volume.
Certain portions of the airways trachea, bronchi, bronchioles do not participate in respiratory exchange, and the gas that fills these structures occupies an anatomical dead space of about millilitres in volume.
Of a tidal volume of millilitres, only millilitres ventilate the gas-exchange sites. The maximum capacity of human lungs is about six litres. During normal quiet respiration, a tidal volume of about millilitres is inspired and expired during every respiratory cycle.
The lungs are not collapsed at the close of expiration; a certain volume of gas remains within them. At the close of the expiratory act, a normal subject may, by additional effort, expel another 1, millilitres of gas.
Even after the most forceful expiratory effort, however, there remains a residual volume of approximately 1, millilitres.
By the same token, at the end of a normal inspiration, further effort may succeed in drawing into the lungs an additional 3, millilitres. The gills The gills of fishes are supported by a series of gill arches encased within a chamber formed by bony plates the operculum. A pair of gill filaments projects from each arch; between the dorsal upper and ventral lower surfaces of the filaments, there is a series of secondary folds, the lamellaewhere the gas exchange takes place.
The blood vessels passing through the gill arches branch into the filaments and then into still smaller vessels capillaries in the lamellae. Deoxygenated blood from the heart flows in the lamellae in a direction counter to that of the water flow across the exchange surfaces. In a number of fishes the water-to-blood distance across which gases must diffuse is 0.
The countercurrent flow of blood through the lamellae in relation to external water flow has much to do with the efficiency of gas exchange.
Laboratory experiments in which the direction of water flow across fish gills was reversed showed that about 80 percent of the oxygen was extracted in the normal situation, while only 10 percent was extracted when water flow was reversed. The uptake of oxygen from water to blood is thus facilitated by countercurrent flow; in this way, greater efficiency of oxygen uptake is achieved by an anatomical arrangement that is free of energy expenditure by the organism. Countercurrent flow is a feature of elasmobranchs sharks, skates and cyclostomes hagfisheslampreys as well as bony fishes.
A number of vertebrates use externalized gill structures. Some larval fishes have external gills that are lost with the appearance of the adult structures. A curious example of external gills is found in the male lungfish Lepidosiren.
At the time the male begins to care for the nest, a mass of vascular filaments a system of blood vessels develops as an outgrowth of the pelvic fins. The fish meets its own needs by refilling its lungs with air during periodic excursions to the water surface. When it returns to the nest, its pelvic-gill filaments are perfused with well-oxygenated blood, providing an oxygen supply for the eggs, which are more or less enveloped by the gill filaments.
It is theoretically possible for a skin that is well supplied with blood vessels to serve as a major or even the only respiratory surface. In terrestrial animals a moist integument also provides a major avenue of water loss. A number of fishes and amphibians rely on the skin for much of their respiratory exchange; hibernating frogs utilize the skin for practically all their gas exchanges. The lung The lungs of vertebrates range from simple saclike structures found in the Dipnoi lungfishes to the complexly subdivided organs of mammals and birds.
An increasing subdivision of the airways and the development of greater surface area at the exchange surfaces appear to be the general evolutionary trend among the higher vertebrates.
In the embryo, lungs develop as an outgrowth of the forward portion of the gut. The lung proper is connected to the outside through a series of tubes; the main tube, known as the trachea windpipeexits in the throat through a controllable orifice, the glottis.
At the other end the trachea subdivides into secondary tubes bronchiin varying degree among different vertebrate groups. The trachea of amphibians is not divided into secondary tubes but ends abruptly at the lungs. The relatively simple lungs of frogs are subdivided by incomplete walls septaand between the larger septa are secondary septa that surround the air spaces where gas exchange occurs. The diameter of these air spaces alveoli in lower vertebrates is larger than in mammals: The alveolus in the frog is about 10 times the diameter of the human alveolus.
The smaller alveoli in mammals are associated with a greater surface for gas exchange: An important characteristic of lungs is their elasticity. An elastic material is one that tends to return to its initial state after the removal of a deforming force. Elastic tissues behave like springs. As the lungs are inflated, there is an accompanying increase in the energy stored within the elastic tissues of the lungs, just as energy is stored in a stretched rubber band.
Long-term effects of environmental stressors, including oxygen deficiency, on parasitism are well documented [ 1516 ]. Increased parasite burden is primarily related to impaired immune responses that characteristically develop within several days or over longer periods [ 17 - 19 ].
Possible short-term effects e. A link between abrupt oxygen depletion and parasite acquisition would be expected if an increase in ventilation flow were to bring small passively floating trematode cercariae to the vicinity of fish gills. The present study used rainbow trout, Oncorhynchus mykiss, to test whether a short-term reduction in oxygen concentration resulted in increased ventilation rate and thus increased parasite load when in the presence of cercariae of Diplostomum pseudospathaceum.
We monitored operculum beat frequency and amplitude together with the number of acquired parasites at three levels of oxygen concentration. We also checked whether parasite activity was influenced by oxygen level.
Our hypothesis was that ventilation by fish is an important variable linking parasitism and stress due to low oxygen levels. Methods Fish, parasites and infection procedure D. Rainbow trout, which is a commonly farmed fish in Finland, is known to serve as a suitable host for D.
In the pilot experiment in Augustinfluence of oxygen depletion on infection with Diplostomum was observed. The main experiment designed to test whether ventilation by fish influences cercarial acquisition was carried out in August All cercariae were pooled into one suspension and the cercariae density was estimated by taking ten 1-ml samples.
Ventilation activity Ventilation activity of fish was estimated while fish were maintained at three different concentrations of oxygen: Two of the tanks were tightly covered and kept in the dark, while two additional tanks were illuminated and aerated.
Following exposure to parasites, fish were taken out of the boxes and the final oxygen concentration was measured. Fish were killed by an overdose of MS, measured, weighed, and inspected for the number of parasites in the eye lenses. A total of 36 fish were tested, 12 fish at each of the three levels of oxygen saturation. To assess the ventilation activity of fish exposed to D. Two minute video recordings and 15—20 photo frames were obtained for each fish.
The number of opercula beats per minute was estimated from video recordings opercula beat rate, OBR min The means of these measurements were used for further analysis. Volume of a pump stroke ml was assessed for each fish using data on mean ventilation amplitude and approximating the volume of the water pumping chamber ml to a sphere with the diameter equal to ventilation amplitude.
The volume of a pump stroke was calculated by subtracting the spherical volume of the chamber with closed opercula from the volume of the chamber when the opercula were open.
We assumed the distribution of cercariae within the water column was close to homogeneous.