THE STRUCTURE AND FUNCTIONS OF THE THELEOSTEAN SWEMBLADDER

A sensation for inflation: initial swim bladder inflation in larval zebrafish is mediated by the mechanosensory lateral line

Larval zebrafish achieve neutral buoyancy by swimming up to the surface and taking in air through their mouths to inflate their swim bladders. We define this behavior as 'surfacing'. Little is known about the sensory basis for this underappreciated behavior of larval fish. A strong candidate is the mechanosensory lateral line, a hair cell-based sensory system that detects hydrodynamic information from sources like water currents, predators, prey, and surface waves. However, a role for the lateral line in mediating initial inflation of the swim bladder has not been reported. To explore the connection between the lateral line and surfacing, we utilized a genetic mutant (lhfpl5b-/-) that renders the zebrafish lateral line insensitive to mechanical stimuli. We observe that approximately half of these lateral line mutants over-inflate their swim bladders during initial inflation and become positively buoyant. Thus, we hypothesize that larval zebrafish use their lateral line to moderate interactions with the air-water interface during surfacing to regulate swim bladder inflation. To test the hypothesis that lateral line defects are responsible for swim bladder over-inflation, we show exogenous air is required for the hyperinflation phenotype and transgenic rescue of hair cell function restores normal inflation. We also find that chemical ablation of anterior lateral line hair cells in wild type larvae causes hyperinflation. Furthermore, we show that manipulation of lateral line sensory information results in abnormal inflation. Finally, we report spatial and temporal differences in the surfacing behavior between wild type and lateral line mutant larvae. In summary, we propose a novel sensory basis for achieving neutral buoyancy where larval zebrafish use their lateral line to sense the air-water interface and regulate initial swim bladder inflation.

Buoyancy and hydrostatic balance in a West Indian Ocean coelacanth Latimeria chalumnae

Background

Buoyancy and balance are important parameters for slow-moving, low-metabolic, aquatic organisms. The extant coelacanths have among the lowest metabolic rates of any living vertebrate and can afford little energy to keep station. Previous observations on living coelacanths support the hypothesis that the coelacanth is neutrally buoyant and in close-to-perfect hydrostatic balance. However, precise measurements of buoyancy and balance at different depths have never been made. 

Results

Here we show, using non-invasive imaging, that buoyancy of the coelacanth closely matches its depth distribution. We found that the lipid-filled fatty organ is well suited to support neutral buoyancy, and due to a close-to-perfect hydrostatic balance, simple maneuvers of fins can cause a considerable shift in torque around the pitch axis allowing the coelacanth to assume different body orientations with little physical effort.

Conclusions

Our results demonstrate a close match between tissue composition, depth range and behavior, and our collection-based approach could be used to predict depth range of less well-studied coelacanth life stages as well as of deep sea fishes in general.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01354-8.

Preself-Feeding Medaka Fry Provides a Suitable Screening System for in Vivo Assessment of Thyroid Hormone-Disrupting Potential

Endocrine-disrupting chemicals are assessed based on their physiological potential and their potential associated adverse effects. However, suitable end points for detection of chemicals that interfere with the thyroid hormone (TH) system have not been established in nonmammals, with the exception of amphibian metamorphosis. The aims of the current study were to develop an in vivo screening system using preself-feeding medaka fry (Oryzias latipes) for the detection of TH-disrupting chemicals and elucidate the underlying molecular mechanism. 17α-Ethinylestradiol (EE2: <100 ng/L) did not induce mRNA expression of estrogen-responsive genes, vitellogenins (vtgs) mRNA. Meanwhile, coexposure with thyroxin (T4) induced an increase of vtg expression. TH-disrupting chemicals (thiourea (TU), perfluorooctanoic acid (PFOA), and tetrabromobisphenol A (TBBPA)) significantly suppressed EE2 (1,000 ng/L)-induced vtg1 expression, while T4 rescued their expression as well as that of thyroid hormone receptor α (tRα) and estrogen receptors (esrs). These results were supported by in silico analysis of the 5'-transcriptional regulatory region of these genes. Furthermore, the esr1 null mutant revealed that EE2-induced vtg1 expression requires mainly esr2a and esr2b in a TH-dependent manner in preself-feeding fry. Application of preself-feeding medaka fry as a screening system might help decipher the in vivo mechanisms of action of TH-disrupting molecules, while providing an alternative to the traditional animal model.

Coordinated gas release among the physostomous fish sprat (Sprattus sprattus)

Previous experimental studies suggest that the production of sound associated with expelling gas from an open swimbladder may play a role in communication. This would suggest non-random gas release. We used deployed echosounders to study patterns of gas release among a fjord population of sprat (Sprattus sprattus). The echosounder records concurrently revealed individual fish and their release of gas. The gas release primarily occurred at night, partly following recurrent temporal patterns, but also varying between nights. In testing for non-randomness, we formulated a data-driven simulation approach. Non-random gas release scaled with the length of the analyzed time intervals from 1 min to 6 h, and above 30 min the release events in more than 50% of the intervals were significantly connected.

Is Barotrauma an Important Factor in the Discard Mortality of Yellow Perch?

In physoclistous fishes, barotrauma caused by rapid decompression during capture may be an important source of fishing mortality that is unquantified for some fisheries. We developed a predictive logistic model for barotrauma incidence in Yellow Perch Perca flavescens and applied this model to Ohio's recreational and commercial fisheries in Lake Erie where fisheries managers implicitly consider discard mortality to be negligible in current stock assessment. As expected, capture depth explained most of the variation in incidence, with comparatively small effects of season, sex, and size categories. Measurements of whole body and gonad density provided limited explanation for the categorical effects. Both fisheries spanned a range of depths (7.6 to 16.8 m) that corresponded to a broad range of barotrauma incidence (13 to 74%). Using a recent example, we estimated that additional fishing mortality due to barotrauma in discards was approximately six-fold higher in the commercial than recreational fishery. Overall, this additional mortality was &lt;1% of lake-wide population size estimates. Thus, the assumption that all discarded Yellow Perch survive is unlikely to result in a detectable bias in population estimates. One caveat is that we still do not understand how strong year-classes might influence discard mortality via increased discard rate and barotrauma incidence for small fish.

Intra- and Intersexual swim bladder dimorphisms in the plainfin midshipman fish (Porichthys notatus): Implications of swim bladder proximity to the inner ear for sound pressure detection

The plainfin midshipman fish, Porichthys notatus, is a nocturnal marine teleost that uses social acoustic signals for communication during the breeding season. Nesting type I males produce multiharmonic advertisement calls by contracting their swim bladder sonic muscles to attract females for courtship and spawning while subsequently attracting cuckholding type II males. Here, we report intra- and intersexual dimorphisms of the swim bladder in a vocal teleost fish and detail the swim bladder dimorphisms in the three sexual phenotypes (females, type I and II males) of plainfin midshipman fish. Micro-computerized tomography revealed that females and type II males have prominent, horn-like rostral swim bladder extensions that project toward the inner ear end organs (saccule, lagena, and utricle). The rostral swim bladder extensions were longer, and the distance between these swim bladder extensions and each inner-ear end organ type was significantly shorter in both females and type II males compared to that in type I males. Our results revealed that the normalized swim bladder length of females and type II males was longer than that in type I males while there was no difference in normalized swim bladder width among the three sexual phenotypes. We predict that these intrasexual and intersexual differences in swim bladder morphology among midshipman sexual phenotypes will afford greater sound pressure sensitivity and higher frequency detection in females and type II males and facilitate the detection and localization of conspecifics in shallow water environments, like those in which midshipman breed and nest.

Physostomous channel catfish, Ictalurus punctatus, modify swimming mode and buoyancy based on flow conditions

The employment of gliding in aquatic animals as a means of conserving energy has been theoretically predicted and discussed for decades. Several studies have shown that some species glide, whereas others do not. Freshwater fish species that widely inhabit both lentic and lotic environments are thought to be able to adapt to fluctuating flow conditions in terms of locomotion. In adapting to the different functional demands of lentic and lotic environments on fish energetics, physostomous (open swim bladder) fish may optimise their locomotion and activity by controlling their net buoyancy; however, few buoyancy studies have been conducted on physostomous fish in the wild. We deployed accelerometers on free-ranging channel catfish, Ictalurus punctatus, in both lentic and lotic environments to quantify their swimming activity, and to determine their buoyancy condition preferences and whether gliding conserves energy. Individual comparisons of swimming efforts between ascent and descent phases revealed that all fish in the lentic environment had negative buoyancy. However, all individuals showed many descents without gliding phases, which was contrary to the behaviour predicted to minimise the cost of transport. The fact that significantly fewer gliding phases were observed in the lotic environment, together with the existence of neutrally buoyant fish, indicated that channel catfish seem to optimise their locomotion through buoyancy control based on flow conditions. The buoyancy optimisation of channel catfish relative to the flow conditions that they inhabit not only reflects differences in swimming behaviour but also provides new insights into the adaptation of physostome fish species to various freshwater environments.

Does the hearing sensitivity in thorny catfishes depend on swim bladder morphology?

Background

Thorny catfishes exhibit large variations in swim bladder morphology. These organs are of different sizes, forms and may have simple or branched diverticula. The swim bladder plays an important role in otophysans because it enhances their hearing sensitivity by transmitting sound pressure fluctuations via ossicles to the inner ear.

Methodology/Principal Findings

To investigate if a form-function relationship exists, the swim bladder morphology and hearing ability were analyzed in six species. The morphology was quantified by measuring the length, width and height and calculating a standardized swim bladder length (sSBL), which was then used to calculate the relative swim bladder length (rSBL). Hearing was measured using the auditory evoked potential (AEP) recording technique. Two species had simple apple-shaped and four species heart-shaped (cordiform) bladders. One of the latter species had short unbranched diverticula on the terminal margin, two had a secondary bladder and two had many long, branched diverticula. The rSBL differed significantly between most of the species. All species were able to detect frequencies between 70 Hz and 6 kHz, with lowest thresholds found between 0.5 and 1 kHz (60 dB re 1 µPa). Hearing curves were U-shaped except in Hemidoras morrisi in which it was ramp-like. Mean hearing thresholds of species possessing smaller rSBLs were slightly lower (maximum 8.5 dB) than those of species having larger rSBLs.

Conclusions/Significance

The current findings reveal a relationship between swim bladder form and its function among thorny catfishes. Relatively smaller swim bladders resulted in relatively better hearing. This is in contrast to a prior inter-familial study on catfishes in which species with large unpaired bladders possessed higher sensitivity at higher frequencies than species having tiny paired and encapsulated bladders.

Effects of multiple scattering, attenuation and dispersion in waveguide sensing of fish

An ocean acoustic waveguide remote sensing system can instantaneously image and continuously monitor fish populations distributed over continental shelf-scale regions. Here it is shown theoretically that the areal population density of fish groups can be estimated from their incoherently averaged broadband matched filtered scattered intensities measured using a waveguide remote sensing system with less than 10% error. A numerical Monte-Carlo model is developed to determine the statistical moments of the scattered returns from a fish group. It uses the parabolic equation to simulate acoustic field propagation in a random range-dependent ocean waveguide. The effects of (1) multiple scattering, (2) attenuation due to scattering, and (3) modal dispersion on fish population density imaging are examined. The model is applied to investigate population density imaging of shoaling Atlantic herring during the 2006 Gulf of Maine Experiment. Multiple scattering, attenuation and dispersion are found to be negligible at the imaging frequencies employed and for the herring densities observed. Coherent multiple scattering effects, such as resonance shifts, which can be significant for small highly dense fish groups on the order of the acoustic wavelength, are found to be negligible for the much larger groups typically imaged with a waveguide remote sensing system.

The ecological conditions that favor tool use and innovation in wild bottlenose dolphins (Tursiops sp.)

Dolphins are well known for their exquisite echolocation abilities, which enable them to detect and discriminate prey species and even locate buried prey. While these skills are widely used during foraging, some dolphins use tools to locate and extract prey. In the only known case of tool use in free-ranging cetaceans, a subset of bottlenose dolphins (Tursiops sp.) in Shark Bay, Western Australia habitually employs marine basket sponge tools to locate and ferret prey from the seafloor. While it is clear that sponges protect dolphins' rostra while searching for prey, it is still not known why dolphins probe the substrate at all instead of merely echolocating for buried prey as documented at other sites. By ‘sponge foraging’ ourselves, we show that these dolphins target prey that both lack swimbladders and burrow in a rubble-littered substrate. Delphinid echolocation and vision are critical for hunting but less effective on such prey. Consequently, if dolphins are to access this burrowing, swimbladderless prey, they must probe the seafloor and in turn benefit from using protective sponges. We suggest that these tools have allowed sponge foraging dolphins to exploit an empty niche inaccessible to their non-tool-using counterparts. Our study identifies the underlying ecological basis of dolphin tool use and strengthens our understanding of the conditions that favor tool use and innovation in the wild.

Adrenergic control of swimbladder deflation in the zebrafish (Danio rerio)

Many teleosts actively regulate buoyancy by adjusting gas volume in the swimbladder. In physostomous fishes such as the zebrafish, a connection is maintained between the swimbladder and the oesophagus via the pneumatic duct for the inflation and deflation of this organ. Here we investigated the role of adrenergic stimulation of swimbladder wall musculature in deflation of the swimbladder. Noradrenaline (NA), the sympathetic neurotransmitter (dosage 10(-6) to 10(-5) mol l(-1)), doubled the force of smooth muscle contraction in isolated tissue rings from the anterior chamber, caused a doubling of pressure in this chamber in situ, and evoked gas expulsion through the pneumatic duct, deflating the swimbladder to approximately 85% of the pre-NA volume. These effects were mediated by beta-adrenergic receptors, representing a novel role for these receptors in vertebrates. No effects of adrenergic stimulation were detected in the posterior chamber. In a detailed examination of the musculature and innervation of the swimbladder to determine the anatomical substrate for these functional results, we found that the anterior chamber contained an extensive ventral band of smooth muscle with fibres organized into putative motor units, richly innervated by tyrosine hydroxylase-positive axons. Additionally, a novel arrangement of folds in the lumenal connective tissue in the wall of the anterior chamber was described that may permit small changes in muscle length to cause large changes in effective wall distensibility and hence chamber volume. Taken together, these data strongly suggest that deflation of the zebrafish swimbladder occurs primarily by beta-adrenergically mediated contraction of smooth muscle in the anterior chamber and is under the control of the sympathetic limb of the autonomic nervous system.

Autonomic control of the swimbladder

The swimbladder of teleost fishes is the primary organ for controlling whole-body density, and thus buoyancy. The volume of gas in the swimbladder is adjusted to bring the organism to near neutral buoyancy at a particular depth. Swimbladder morphology varies widely among teleosts, but all species are capable of inflating and deflating this organ under reflex control by the autonomic nervous system, to achieve neutral buoyancy. Here we review the control of effectors within the swimbladder, including acid-secreting cells, vasculature and musculature, that are involved in determining gas volume. This control system is complex. It incorporates the "classical" efferent elements of the autonomic nervous system, the spinal autonomic and cranial autonomic limbs and their neurotransmitters (typically noradrenaline (NA)/adrenaline (ADR), and acetylcholine, respectively), but also non-adrenergic, non-cholinergic neurotransmitters such as peptides, purines and nitric oxide. The detailed patterns of autonomic innervation of swimbladder effectors are not well understood, nor are the relationships of terminals releasing non-adrenergic, non-cholinergic neurotransmitters onto these effectors. Furthermore, in most cases the complement of postjunctional receptor subtypes activated by adrenergic, cholinergic and other neurotransmitters, and the biological effects of these neurochemicals, have not been completely established. In order to clarify some of these issues and to provide insight into basic principles underlying autonomic control of swimbladder function, we propose the zebrafish as a potentially useful model teleost.

From inflation to flotation: contribution of the swimbladder to whole-body density and swimming depth during development of the zebrafish (Danio rerio)

Teleost fishes have body tissues that are denser than water, causing them to sink. Many teleosts therefore possess a gas-filled swimbladder that provides lift, allowing fish to attain neutral buoyancy. The importance of the swimbladder as a buoyancy aid during changing body sizes over ontogeny and its role in determining the swimming depth of fish remain unclear. In this study, we have used the zebrafish (Danio rerio) to investigate changes in the size and shape of the swimbladder during development and examine whether these changes affect the hydrostatic contribution of the swimbladder during swimming. Our results showed that swim-up behavior is critical for larvae to first inflate their swimbladder, decrease body density, and attain neutral buoyancy. Following inflation, we found a strong linear correlation between fish volume and swimbladder volume over ontogeny. This trend was supported by measures of the density of zebrafish, which was conserved within a narrow range between 1.00 +/- 0.001 and 0.996 +/- 0.001 g/cm(3) despite an increase in the swimming depth of zebrafish, which occurred upon transition to a double-chambered organ. Finally, we demonstrated that the contribution of the swimbladder keeps the fish within 1.7% of neutral buoyancy throughout larval development.

Fractional rate of change of swim-bladder volume is reliably related to absolute depth during vertical displacements in teleost fish

Fish must orient in three dimensions as they navigate through space, but it is unknown whether they are assisted by a sense of depth. In principle, depth can be estimated directly from hydrostatic pressure, but although teleost fish are exquisitely sensitive to changes in pressure, they appear unable to measure absolute pressure. Teleosts sense changes in pressure via changes in the volume of their gas-filled swim-bladder, but because the amount of gas it contains is varied to regulate buoyancy, this cannot act as a long-term steady reference for inferring absolute pressure. In consequence, it is generally thought that teleosts are unable to sense depth using hydrostatic pressure. Here, we overturn this received wisdom by showing from a theoretical physical perspective that absolute depth could be estimated during fast, steady vertical displacements by combining a measurement of vertical speed with a measurement of the fractional rate of change of swim-bladder volume. This mechanism works even if the amount of gas in the swim-bladder varies, provided that this variation occurs over much longer time scales than changes in volume during displacements. There is therefore no a priori physical justification for assuming that teleost fish cannot sense absolute depth by using hydrostatic pressure cues.

Low-frequency target strength and abundance of shoaling Atlantic herring (Clupea harengus) in the Gulf of Maine during the Ocean Acoustic Waveguide Remote Sensing 2006 Experiment

The low-frequency target strength of shoaling Atlantic herring (Clupea harengus) in the Gulf of Maine during Autumn 2006 spawning season is estimated from experimental data acquired simultaneously at multiple frequencies in the 300-1200 Hz range using (1) a low-frequency ocean acoustic waveguide remote sensing (OAWRS) system, (2) areal population density calibration with several conventional fish finding sonar (CFFS) systems, and (3) low-frequency transmission loss measurements. The OAWRS system's instantaneous imaging diameter of 100 km and regular updating enabled unaliased monitoring of fish populations over ecosystem scales including shoals of Atlantic herring containing hundreds of millions of individuals, as confirmed by concurrent trawl and CFFS sampling. High spatial-temporal coregistration was found between herring shoals imaged by OAWRS and concurrent CFFS line-transects, which also provided fish depth distributions. The mean scattering cross-section of an individual shoaling herring is found to consistently exhibit a strong, roughly 20 dB/octave roll-off with decreasing frequency in the range of the OAWRS survey over all days of the roughly 2-week experiment, consistent with the steep roll-offs expected for sub-resonance scattering from fish with air-filled swimbladders.

Nervous control of fish swimbladders

The swimbladder of teleost fish receives a rich and complex innervation by nerve fibres of the autonomic nervous system. While an understanding of the form and function of a non-adrenergic, non-cholinergic innervation is slowly emerging, the pattern of control by the "classical" cholinergic and adrenergic innervation is becoming relatively well understood. This short review describes the autonomic innervation patterns, and attempts to summarise the role of cholinergic and adrenergic pathways in the control of gas secretion and resorption in the teleost swimbladder.

Historical reconstructions of evolving physiological complexity:O2 secretion in the eye and swimbladder of fishes

The ability of some fishes to inflate their compressible swimbladder with almost pure oxygen to maintain neutral buoyancy, even against the high hydrostatic pressure several thousand metres below the water surface, has fascinated physiologists for more than 200 years. This review shows how evolutionary reconstruction of the components of such a complex physiological system on a phylogenetic tree can generate new and important insights into the origin of complex phenotypes that are difficult to obtain with a purely mechanistic approach alone. Thus, it is shown that oxygen secretion first evolved in the eyes of fishes, presumably for improved oxygen supply to an avascular, metabolically active retina. Evolution of this system was facilitated by prior changes in the pH dependence of oxygen-binding characteristics of haemoglobin (the Root effect) and in the specific buffer value of haemoglobin. These changes predisposed teleost fishes for the later evolution of swimbladder oxygen secretion, which occurred at least four times independently and can be associated with increased auditory sensitivity and invasion of the deep sea in some groups. It is proposed that the increasing availability of molecular phylogenetic trees for evolutionary reconstructions may be as important for understanding physiological diversity in the postgenomic era as the increase of genomic sequence information in single model species.

Sound production mechanism in carapid fish: first example with a slow sonic muscle

Fish sonic swimbladder muscles are the fastest muscles in vertebrates and have fibers with numerous biochemical and structural adaptations for speed. Carapid fishes produce sounds with a complex swimbladder mechanism, including skeletal components and extrinsic sonic muscle fibers with an exceptional helical myofibrillar structure. To study this system we stimulated the sonic muscles, described their insertion and action and generated sounds by slowly pulling the sonic muscles. We find the sonic muscles contract slowly, pulling the anterior bladder and thereby stretching a thin fenestra. Sound is generated when the tension trips a release system that causes the fenestra to snap back to its resting position. The sound frequency does not correspond to the calculated resonant frequency of the bladder, and we hypothesize that it is determined by the snapping fenestra interacting with an overlying bony swimbladder plate. To our knowledge this tension release mechanism is unique in animal sound generation.

Sound-producing mechanisms and recordings in Carapini species (Teleostei, Pisces)

Carapus boraborensis, C. homei and Encheliophis gracilis are three species of Carapidae that display the ability to penetrate and reside in the holothurian Bohadschia argus. This study describes both the particular morphology of the sound-producing structures and, for the first time, the sounds produced by each species. The study of the structures composing the sound-producing system seems to indicate that the action made by the primary sonic muscles (i.e. the pulling and releasing of the front of the swim bladder) might be responsible for the sound emissions of these three species by provoking a vibration of a thinner zone in front of the swim bladder (swimbladder fenestra). The sounds were only emitted and recorded when several individuals of the same species were inside the same sea cucumber. They were composed of serially repeated knocks and were heard as drum beats or drum rolls. Their specific differences were mainly defined as variations in the timing or grouping of the knocking sounds. The recordings of these sound productions demonstrate a vocal ability for the three species, linked with the presence of particular organs associated with sound production. Moreover, the ecological significance of the sounds and of the sound apparatus system is discussed.

Functional morphology of the gas-gland cells of the air-bladder of Oreochromis alcalicus grahami (teleostei: cichlidae): an ultrastructural study on a fish adapted to a severe, highly alkaline environment

Oreochromis alcalicus grahami is a small cichlid fish that lives in shallow peripheral lagoons of Lake Magadi, Kenya. The internal surface of the air-bladder is highly vascularized, illustrating possible utilization as an accessory respiratory organ. The wall of the bladder consists of five distinct tissue layers. From the outer to the inner surfaces are: a squamous, undifferentiated epithelial cell; a collagen-elastic tissue space; a smooth muscle tissue band; an inner connective tissue space; and columnar gas-gland cells projecting into the lumen. The cell membrane over the perikarya of the gas-gland cells was sporadically broken. The disruptions were ascribed to possible physical damage by discharging gas-bubbles. Juxtaluminally, the gas-gland cells attached across tight junctions. The cells have highly euchromatic nuclei and conspicuously large intracytoplasmic secretory bodies. On the blood capillary facing (basal) aspect, the cell membrane of the gas-gland cells is highly amplified. The cells insert onto a smooth muscle tissue band. The morphological features and the topographical arrangement of the gas-gland cells in O. a. grahami are indicative of an operative exchange of materials between them and the underlying tissue components especially the blood capillaries. For a fish that subsists in hot, highly saline and alkaline water heavily invested by avian predators and where the partial pressure of oxygen diurnally shifts from virtual anoxia to hyperoxia, development of a versatile air-bladder for efficient buoyancy control conforms to the functional demands placed on it by a unique environment. Illustratively, instead of the gas-gland morphology in O. a. grahami resembling that in the freshwater fishes, the group from which the fish has evolved, it compares more closely to that of the marine fish. This similarity suggests amazing parallel evolutionary adaptation to biophysically corresponding aquatic milieus.

Cod courtship song: a song at the expense of dance?

Sexually selected characters may reveal information about individual quality during mate choice. Fin display and sound emitted with the aid of specific drumming muscles are characters described as being of importance in the reproductive behaviour of cod (Gadus morhua L.). We examined whether the mass of drumming muscles or fin size was sexually dimorphic, and whether these characters could provide information about male cod that was potentially of benefit to mate-seeking females. The mass of drumming muscles, but not fin size, was sexually dimorphic, with males having larger muscles than females. Neither the mass of drumming muscles nor fin size apparently revealed information about traits that may be associated with parasite resistance in males (i.e., parasite intensities and leukocyte densities). However, variation in fertilization potential (i.e., spermatocrit level) among males was related to both mass of drumming muscles and fin size. Thus, by evaluating sound and fin size, mate-seeking females may obtain information about fertilization ability among males. This may be of particular importance for females in a species whose eggs commonly remain unfertilized. Furthermore, males with large drumming muscles and small fins had low spermatocrit levels. This may reflect reductions in sperm density resulting from frequent ejaculations by attractive males. A costly allocation of resources for the development of drumming muscles at the expense of fin muscles used for propulsion is presented as a tentative explanation as to why females should pay attention to these particular traits during courtship. Increased investment in "song" may thus appear at the expense of "dance."

Responses of the swimbladder of the carp to sound stimulation

The oscillations of the swimbladder anterior chamber of the carp (Cyprinus carpio) following stimulation with tones of 300-1500 Hz were studied by the method of holographic interferometry. The oscillation amplitude appeared to be maximal at frequencies close to the resonance frequency of an air bladder of equivalent volume as well as at frequencies corresponding approximately to the second and third harmonics of the resonance frequency. A change in the frequency of the sound signal or in the instantaneous pressure amplitude could result in spatial displacement of the oscillation centers on the swimbladder wall. The interference picture which resulted from recording the swimbladder oscillations over the tested frequency range was not observed on the holograms recorded within 20-24 h after the fish had been killed.

Physiological significance of Root effect hemoglobins in trout

Root effect hemoglobins are found in trout and salmon. These functionally unique hemoglobins are believed to be intricately involved in oxygen secretion to the swimbladders of many fishes. This has also been proposed as their primary physiological role in trout; however, such an oxygen secretory function is unlikely in these fish. Trout swimbladders characteristically contain very high concentrations of nitrogen and the anatomical structures associated with swimbladder gas secretion are absent from trout. Also, trout appear to fill their swimbladders by physical deposition of gas, with the following of surface air, rather than by chemical secretion, thus obviating a role of Root effect hemoglobins at the swimbladder. A chemical secretion of gas is likely involved in oxygen secretion to the eye. The eyes of trout, as those of many fishes, contain very high concentrations of oxygen which exceed those found in the blood or ambient water. Data are consistent with a physiological role of trout Root effect hemoglobins in oxygen secretion to the eye; they are not consistent with a role in any gaseous secretion to the swimbladder.

Layered membranes: a diffusion barrier to gases in teleostean swimbladders

Flattened cells are dispersed in an overlapping fashion within the submucosa of closed (physoclistous) swimbladders. Within the cells are located round platelets best seen by phase or interference type microscopy. At the electron microscope level the platelets are revealed as repeated, numerous layers of membranous discs. Each disc is a bilayer with an enlarged rim. The bilayer is strongly and preferentially reactive to osmium tetroxide. Swimbladder walls are known to be highly resistant to passage of gases. The randomly disposed platelets with the highly ordered layering of membranes may provide that physical resistance to gas movement.

Overlapping Platelets: A Diffusion Barrier in a Teleost Swimbladder

Overlapping platelets are layered within the connective tissue of the wall of a closed (physoclistous) swimbladder. The close, staggered arrangement of the platelets is viewed as a physical barrier that can interfere with the diffusion pathway of gas molecules. The result is a more efficient retention of gas pressures within the swimbladder.

Prevalence and morphology of Eimeria gadi (Fiebiger, 1913) in the haddock

The coccidian parasite Eimeria gadi was found in the haddock, Melanogrammus aeglefinus, taken from the Nova Scotian fishing banks. The haddock infection rates ranged from a high of 58% on Emerald Bank to a low of 4% on Georges Bank, the average being 32%. There was no relationship between sex and degree or prevalence of infection. Although the probability of an occurrence of infection increased with size, small fish with heavy infections were observed. The degree of infection had no apparent effect on the condition factor (length/weight) of the fish. The infection rate reached a maximum in the fall of the year while the heaviest infections were observed in the spring. It is evident from the data that the infection is fatal. The parasite mass, appearing as a creamy viscous to a yellow semisolid material in the swimbladder, consisting of various parasite stages, fibrous and cellular debris, and lipid material. Some aspects of the sporocyst stage are described. No other gadoids from the Nova Scotian banks were found to be infected; however, a single specimen of the fourbeard rockling, Enchelyopus cimbrius, from St. John's, Newfoundland, was found to be heavily infected with E. gadi.

Autonomic nerve control of the swimbladder of the goldsinny wrasse, Ctenolabrus rupestris

The autonomic nerve control of the swimbladder of the goldsinny wrasse has been studied by experiments with isolated strips from the muscularis mucosae, drug effects on gas secretion in vivo, fluorescent histochemistry and quantitative analysis of catecholamine content. Vagotomy, atropine and mecamylamine significantly inhibited induced gas secretion, suggesting a cholinergic vagal secretory innervation of the gas gland. The anterior part of the mucosa (secretory part) is contracted by acetylcholine and alpha-adrenoceptor agonists. These responses are competitively antagonized by atropine and phentolamine or yohimbine respectively. The posterior (resorbent) part of the mucosa is contracted by acetylcholine, but beta-adrenoceptor agonists relax the acetylcholine-precontracted preparations. These responses are competitively blocked by atropine and propranolol respectively. The pA2-values from all experiments with antagonists are well comparable with values obtained for the same drugs in other teleost or mammalian tissues. Tyramine, 5-hydroxytryptamine or phenylephrine had mixed effects on the posterior part. This effect is at least in part due to release of nervously stored catecholamines. Fluorescent histochemistry revealed a strong innervation by both smooth and varicose adrenergic fibres in all parts of the swimbladder, noradrenaline being the dominant catecholamine. Yellow fluorescent cells, which may contain 5-hydroxytryptamine, were also seen in the swimbladder mucosa. No conclusive evidence for a cholinergic innervation of the muscularis mucosae was obtained.