Does muscle fatigue limit advertisement calling in the oyster toadfish Opsanus tau?

The role of acoustic signals in fish reproductiona)

This paper outlines my research path over three decades while providing a review on the role of fish sounds in mate choice and reproduction. It also intends to provide advice to young scientists and point toward future avenues in this field of research. An overview of studies on different fish model species shows that male mating acoustic signals can inform females and male competitors about their size (dominant frequency, amplitude, and sound pulse rate modulation), body condition (calling activity and sound pulse rate), and readiness to mate (calling rate, number of pulses in a sound). At least in species with parental care, such as toadfishes, gobies, and pomacentrids, calling activity seems to be the main driver of reproductive success. Playback experiments ran on a restricted number of species consistently revealed that females prefer vocal to silent males and select for higher calling rates. This personal synthesis concludes with the suggestion to increase knowledge on fish mating signals, especially considering the emerging use of fish sounds to monitor aquatic environments due to increasing threats, like noise pollution.

Alarm calls of the same individual vary during a response to the same predator in Gunnison’s prairie dogs (Cynomys gunnisoni)

Many animals emit vocalizations in a repetitive series, but are all the calls within a series structurally the same? To answer this question, we recorded the barks of adult female Gunnison’s prairie dogs (Cynomys gunnisoni (Baird, 1855)) during 5 min experimental presentations of several terrestrial stimuli. We measured eight variables (primarily pitch and duration measures) of the first, middle, and last barks in each bout of barking produced by each of 24 females, as well as the duration of inter-bout intervals, the number of barks per bout, and the rate of barking per bout. We found that first barks were significantly longer and higher pitched than middle or last barks. Some of these differences were affected by the number of barks in a bout. Regardless of bark position, barks became longer and lower pitched in later bouts, and inter-bout intervals, number of barks per bout, and the rate of barking per bout all declined in later bouts. Our results show that bark structure can vary even within a single context and within a short period of time. Thus, variation due to call position within and across bouts of calling is a potentially important confound for studies examining other sources of acoustic variation.

Origin and evolution of sound production in Serrasalmidae

Among piranhas, sound production is known in carnivorous species, whereas herbivorous species were thought to be mute. Given that these carnivorous sonic species have a complex sonic apparatus, we hypothesize that intermediate forms could be found in other serrasalmid species. The results highlight the evolutionary transition from a simple sound-producing mechanism without specialized sonic structures to a sonic mechanism involving large, fast-contracting sonic muscles. Hypaxial muscles in basal herbivores primarily serve locomotion, but some fibres cause sound production during swimming accelerations, meaning that these muscles have gained a dual function. Sound production therefore seems to have been acquired through exaptation, i.e. the development of a new function (sound production) in existing structures initially shaped for a different purpose (locomotion). In more derived species (Catoprion and Pygopristis), some fibres are distinguishable from typical hypaxial muscles and insert directly on the swimbladder. At this stage, the primary function (locomotion) is lost in favour of the secondary function (sound production). In the last stage, the muscles and insertion sites are larger and the innervation involves more spinal nerves, improving calling abilities. In serrasalmids, the evolution of acoustic communication is characterized initially by exaptation followed by adaptive evolution.

Agonistic sounds signal male quality in the Lusitanian toadfish

Acoustic communication during agonistic behaviour is widespread in fishes. Yet, compared to other taxa, little is known on the information content of fish agonistic calls and their effect on territorial defence. Lusitanian toadfish males (Halobatrachus didactylus) are highly territorial during the breeding season and use sounds (boatwhistles, BW) to defend nests from intruders. BW present most energy in either the fundamental frequency, set by the contraction rate of the sonic muscles attached to the swimbladder, or in the harmonics, which are multiples of the fundamental frequency. Here we investigated if temporal and spectral features of BW produced during territorial defence reflect aspects of male quality that may be important in resolving disputes. We found that higher mean pulse period (i.e. lower fundamental frequency) reflected higher levels of 11-ketotestosterone (11KT), the main teleost androgen which, in turn, was significantly related with male condition (relative body mass and glycogen content). BW dominant harmonic mean and variability decreased with sonic muscle lipid content. We found no association between BW duration and male quality. Taken together, these results suggest that the spectral content of fish agonistic sounds may signal male features that are key in fight outcome.

Variation in swim bladder drumming sounds from three doradid catfish species with similar sonic morphologies

A variety of teleost fishes produce sounds for communication by vibrating the swim bladder with fast contracting muscles. Doradid catfishes have an elastic spring apparatus (ESA) for sound production. Contractions of the ESA protractor muscle pull the anterior transverse process of the 4th vertebra or Müllerian ramus (MR) to expand the swim bladder and elasticity of the MR returns the swim bladder to the resting state. In this study, we examined the sound characteristics and associated fine structure of the protractor drumming muscles of three doradid species: Acanthodoras cataphractus, Platydoras hancockii, and Agamyxis pectinifrons. Despite important variations in sizes, sounds from all three species had similar mean dominant rates ranging from 91-131 Hz and showed frequencies related to muscle contraction speed rather than fish size. Sounds differed among species in terms of waveform shape and their rate of amplitude modulation. In addition, multiple distinguishable sound types were observed from each species: three sound types from A. cataphractus and P. hancockii, and two sound types from A. pectinifrons. Though sounds differed among species, no differences in muscle fiber fine structure were observed at the species level. Drumming muscles from each species bear features associated with fast contractions, including sarcoplasmic cores, thin radial myofibrils, abundant mitochondria, and an elaborated sarcoplasmic reticulum. These results indicate that sound differences between doradids are not due to swimbladder size, muscle anatomy, muscle length, or Müllerian ramus shape, but instead result from differences in neural activation of sonic muscles.

Spatiotemporal variability and sound characterization in silver croaker Plagioscion squamosissimus (Sciaenidae) in the Central Amazon

Background

The fish family Sciaenidae has numerous species that produce sounds with superfast muscles that vibrate the swimbladder. These muscles form post embryonically and undergo seasonal hypertrophy-atrophy cycles. The family has been the focus of numerous passive acoustic studies to localize spatial and temporal occurrence of spawning aggregations. Fishes produce disturbance calls when hand-held, and males form aggregations in late afternoon and produce advertisement calls to attract females for mating. Previous studies on five continents have been confined to temperate species. Here we examine the calls of the silver croaker Plagioscion squamosissimus, a freshwater equatorial species, which experiences constant photoperiod, minimal temperature variation but seasonal changes in water depth and color, pH and conductivity.

Methods and Principal Findings

Dissections indicate that sonic muscles are present exclusively in males and that muscles are thicker and redder during the mating season. Disturbance calls were recorded in hand-held fish during the low-water mating season and high-water period outside of the mating season. Advertisement calls were recorded from wild fish that formed aggregations in both periods but only during the mating season from fish in large cages. Disturbance calls consist of a series of short individual pulses in mature males. Advertisement calls start with single and paired pulses followed by greater amplitude multi-pulse bursts with higher peak frequencies than in disturbance calls. Advertisement-like calls also occur in aggregations during the off season, but bursts are shorter with fewer pulses.

Conclusions and Significance

Silver croaker produce complex advertisement calls that vary in amplitude, number of cycles per burst and burst duration of their calls. Unlike temperate sciaenids, which only call during the spawning season, silver croaker produce advertisement calls in both seasons. Sonic muscles are thinner, and bursts are shorter than at the spawning peak, but males still produce complex calls outside of the mating season.

A superfast muscle in the complex sonic apparatus of Ophidion rochei (Ophidiiformes): histological and physiological approaches

In teleosts, superfast muscles are generally associated with the swimbladder wall whose vibrations result in sound production. In Ophidion rochei, three pairs of muscles were named 'sonic' because their contractions affect swimbladder position: the dorsal sonic muscle (DSM), the intermediate sonic muscle (ISM), and the ventral sonic muscle (VSM). These muscles were investigated thanks to electron microscopy and electromyography in order to determine their function in sound production. Fibers of the VSM and DSM were much thinner than the fibers of the ISM and epaxial musculature. However, only VSM fibers had the typical ultrastructure of superfast muscles: low proportion of myofibrils, and high proportions of sarcoplasmic reticulum and mitochondria. In females, each sound onset was preceded by the onset of electrical activity in the VSM and the DSM (ISM was not tested). The electromyograms of the VSM were very similar to the waveforms of the sounds: means for the pulse period were 3.6±0.5 ms and 3.6±0.7 ms, respectively. This shows that the fast VSM (ca. 280 Hz) is responsible for the pulse period and fundamental frequency of female sounds. DSM electromyograms were generally characterized by one or two main peaks followed by periods of lower electrical activity which suggests a sustained contraction over the course of the sound. The fiber morphology of the ISM and its antagonistic position relative to the DSM are not indicative of a muscle capable of superfast contractions. Overall, this study experimentally shows the complexity of the sound production mechanism in the nocturnal fish O. rochei.

Sound production by the West Australian dhufish (Glaucosoma hebraicum)

Biological examinations of Glaucosomatid fish species have suggested that they could produce sound via swimbladder vibration, using "sonic" muscles. However, there have been few reported instances of it in the family. West Australian dhufish (Glaucosoma hebraicum) is an iconic teleost, endemic to Western Australia. Dissection of G. hebraicum in this study identified the presence of "sonic" muscle pairs in immature and sexually mature individuals. The muscle tissue originates in the otic region of the skull with its insertion at the anterior of the swimbladder. Recordings of sounds were acquired from two male G. hebraicum, at a range of 1 m, during capture. Calls comprised 1 to 14 swimbladder pulses with spectral peak frequency of 154 ± 45 Hz (n = 67 calls) and 3 dB bandwidth of 110 ± 50 Hz. The mean of all call maximum source levels was 126 dB re 1 μPa at 1 m with the highest level at 137 dB re 1 μPa at 1 m. The confirmation of sound production by G. hebraicum and the acoustic characteristics of those sounds could be used to gain a better understanding of its ecology and, particularly, whether the production of sound is associated with specific behaviors, such as reproduction.

Parvalbumin characteristics in the sonic muscle of a freshwater ornamental grunting toadfish (Allenbatrachus grunniens)

The grunting toadfish, Allenbatrachus grunniens, is an ornamental fish in freshwater aquariums, and it has the ability to produce sounds. The sonic muscle of the toadfish is the fastest vertebrate muscle ever measured, and the rates of Ca(2+) transport and cross-bridge dissociation are also the fastest. Parvalbumins (PAs) are Ca(2+)-binding proteins that help in muscle relaxation in vertebrates. Several PA isoforms have been identified in variable ratios in different muscle types. Both male and female grunting toadfish have intrinsic sonic muscles attached to their swim bladders, but no significant difference in morphology between male and female sonic muscles has been observed. In this study, we used SDS-PAGE and western blotting to characterize the total PA expression and to identify the PAs from the sonic muscle and the white body muscle of A. grunniens. Although the total PA concentrations were similar in sonic and white muscles, there were differences in the isoform percentages. Two and four PA isoforms were identified from sonic muscle and white muscle, respectively. The estimated sizes of PA1, PA2, and PA3 in the sonic muscle of the grunting toadfish were 10, 10.5, and 10.5 kDa, respectively, and the isoelectric points of PA1, PA2, and PA3 in the grunting toadfish were 4.77, 4.58, and 4.42, respectively. In the sonic muscle, the primary PA isoform was PA1, which comprised more than 94 % of total PA, whereas PA2 comprised only 5 % of the total PA content. In contrast, in white muscle, the primary isoform was PA2, which comprised 58 % of the total PA. Both PA1 (with PA1a) and PA3 represented approximately 20 % of the total PA in white muscle. These results indicate that there is no positive correlation between a high PA content and the speed of muscle relaxation; however, PA1 might have the greatest effect on the relaxation of the grunting toadfish's sonic muscle.

Brown meagre vocalization rate increases during repetitive boat noise exposures: a possible case of vocal compensation

This study investigated whether or not boat noise causes variations in brown meagre (Sciaena umbra) vocalizations recorded in a nearshore Mediterranean marine reserve. Six nocturnal experimental sessions were carried out from June to September 2009. In each of them, a recreational boat passed over vocalizing fish 6 times with 1 boat passage every 10 min. For this purpose three different boats were used in random order: an 8.5-m cabin-cruiser (CC), a 5-m fiberglass boat (FB), and a 7-m inflatable boat (INF). In situ continuous acoustic recordings were collected using a self-standing sonobuoy. Because boat noise levels largely exceeded both background noise and S. umbra vocalizations in the species' hearing frequency range, masking of acoustic communication was assumed. Although no immediate effect was observed during a single boat passage, the S. umbra mean pulse rate increased over multiple boat passages in the experimental condition but not in the control condition, excluding that the observed effect was due to a natural rise in fish vocalizations. The observed vocal enhancement may result either from an increased density of callers or from an increased number of pulses/sounds produced by already acoustically active individuals, as a form of vocal compensation. These two explanations are discussed.

In situ source levels of mulloway (Argyrosomus japonicus) calls

Mulloway (Argyrosomus japonicus) in Mosman Bay, Western Australia produce three call categories associated with spawning behavior. The determination of call source levels and their contribution to overall recorded sound pressure levels is a significant step towards estimating numbers of calling fish within the detection range of a hydrophone. The source levels and ambient noise also provide significant information on the impacts anthropogenic activity may have on the detection of A. japonicus calls. An array of four hydrophones was deployed to record and locate individual fish from call arrival-time differences. Successive A. japonicus calls produced samples at various ranges between 1 and 100 m from one of the array hydrophones. The three-dimensional localization of calls, together with removal of ambient noise, allowed the determination of source levels for each call category using observed trends in propagation losses and interference. Mean source levels (at 1 m from the hydrophone) of the three call categories were calculated as 163 ± 16 dB re 1 μPa for Category 1 calls (short call of 2-5 pulses); 172 ± 4 dB re 1 μPa for Category 2 calls (long calls of 11-32 pulses); and 157 ± 5 dB re 1 μPa for Category 3 calls (series of successive calls of 1-4 pulses, increasing in call rate).

Seasonal changes in atrophy-associated proteins of the sonic muscle in the big-snout croaker, Johnius macrorhynus (Pisces, Sciaenidae), identified by using a proteomic approach

In most sciaenids, males possess sonic muscles and produce sound through the contraction of these muscles and amplification of the swim bladder. The sonic muscles in some fishes exhibit seasonal changes in size. For example, they are hypertrophic in the spawning season, and atrophic in the non-spawning months. The protein profiles of the sonic muscle, red muscle, and white muscle in the Johnius macrorhynus were shown by two-dimensional electrophoresis (2-DE) and were compared to reveal differential protein expressions. About 80 up-regulated protein spots in the sonic muscle, and 30 spots related to six contractile proteins (fast muscle myosin heavy chain, skeletal alpha actin, alpha actin cardiac, tropomyosin, myosin light chain 2, and myosin light chain 3), four energy metabolic enzymes (enolase, acyl-CoA synthetase, creatine kinase, and cytochrome P450 monooxygenase), and two miscellaneous proteins (DEAD box protein and cyclin H) were identified. Seasonal hypertrophy and atrophy of the sonic muscles related to the reproductive cycle were verified in male big-snout croaker. The contents of some proteins were significantly different in the muscles under these conditions. The levels of cytochrome P450 monooxygenase, fast muscle myosin heavy chain, DEAD box proteins, isocitrate dehydrogenase, and creatine kinase were up-regulated in the hypertrophic muscle, but the levels of alpha actin cardiac, myosin light 2, and myosin light 3 were lower than in the atrophic muscle. Potential reasons for these differences in protein expression related to physiological adaptation are discussed.

An Intermediate in the evolution of superfast sonic muscles

BACKGROUND

Intermediate forms in the evolution of new adaptations such as transitions from water to land and the evolution of flight are often poorly understood. Similarly, the evolution of superfast sonic muscles in fishes, often considered the fastest muscles in vertebrates, has been a mystery because slow bladder movement does not generate sound. Slow muscles that stretch the swimbladder and then produce sound during recoil have recently been discovered in ophidiiform fishes. Here we describe the disturbance call (produced when fish are held) and sonic mechanism in an unrelated perciform pearl perch (Glaucosomatidae) that represents an intermediate condition in the evolution of super-fast sonic muscles.

RESULTS

The pearl perch disturbance call is a two-part sound produced by a fast sonic muscle that rapidly stretches the bladder and an antagonistic tendon-smooth muscle combination (part 1) causing the tendon and bladder to snap back (part 2) generating a higher-frequency and greater-amplitude pulse. The smooth muscle is confirmed by electron microscopy and protein analysis. To our knowledge smooth muscle attachment to a tendon is unknown in animals.

CONCLUSION

The pearl perch, an advanced perciform teleost unrelated to ophidiiform fishes, uses a slow type mechanism to produce the major portion of the sound pulse during recoil, but the swimbladder is stretched by a fast muscle. Similarities between the two unrelated lineages, suggest independent and convergent evolution of sonic muscles and indicate intermediate forms in the evolution of superfast muscles.

Sound production in the longnose butterflyfishes (genus Forcipiger): cranial kinematics, muscle activity and honest signals

Many teleost fishes produce sounds for social communication with mechanisms that do not involve swim bladder musculature. Such sounds may reflect physical attributes of the sound-production mechanism, be constrained by body size and therefore control signal reliability during agonistic behaviors. We examined kinematics of the cranium, median fins and caudal peduncle during sound production in two territorial chaetodontid butterflyfish sister species: forcepsfish (Forcipiger flavissimus) and longnose butterflyfish (F. longirostris). During intraspecific agonistic encounters, both species emit a single pulse sound that precedes rapid cranial rotation at velocities and accelerations that exceed those of prey strikes by many ram-and suction-feeding fishes. Electromyography showed that onsets of activity for anterior epaxialis, sternohyoideus, A1 and A2 adductor mandibulae muscles and sound emission are coincident but precede cranial elevation. Observations indicate that sound production is driven by epaxial muscle contraction whereas a ventral linkage between the head and pectoral girdle is maintained by simultaneous activity from the adductor mandibulae and sternohyoideus. Thus, the girdle, ribs and rostral swim bladder are pulled anteriorly before the head is released and rotated dorsally. Predictions of the hypothesis that acoustic signals are indicators of body size and kinematic performance were confirmed. Variation in forcepsfish sound duration and sound pressure level is explained partly by cranial elevation velocity and epaxial electromyogram duration. Body size, however, explains most variation in duration and sound pressure level. These observed associations indicate that forcepsfish sounds may be accurate indicators of size and condition that are related to resource holding potential during social encounters.

Localization and source level estimates of black drum (Pogonias cromis) calls

A four hydrophone linear array was used to localize calling black drum and estimate source levels and signal propagation. A total of 1025 source level estimates averaged 165 dB(RMS) relative (re:) 1 μPa (standard deviation (SD)=1.0). The authors suggest that the diverticulated morphology of the black drum swimbladder increase the bladder's surface area, thus contributing to sound amplitude. Call energy was greatest in the fundamental frequency (94 Hz) followed by the second (188 Hz) and third harmonics (282 Hz). A square root model best described propagation of the entire call, and separately the fundamental frequency and second harmonic. A logarithmic model best described propagation of the third harmonic which was the only component to satisfy the cut-off frequency equation. Peak auditory sensitivity was 300 Hz at a 94 dB re: 1 μPa threshold based on auditory evoked potential measurements of a single black drum. Based on mean RMS source level, signal propagation, background levels, and hearing sensitivity, the communication range of black drum was estimated at 33-108 m and was limited by background levels not auditory sensitivity. This estimate assumed the source and receiver were at approximately 0.5 m above the bottom. Consecutive calls of an individual fish localized over 59 min demonstrated a mean calling period of 3.6 s (SD=0.48), mean swimming speed of 0.5 body lengths/s, and a total distance swam of 1035 m.

Comparative study on sound production in different Holocentridae species

BACKGROUND

Holocentrids (squirrelfish and soldierfish) are vocal reef fishes whose calls and sound-producing mechanisms have been studied in some species only. The present study aims to compare sound-producing mechanisms in different Holocentridae genera (Holocentrus, Myripristis, Neoniphon, Sargocentron) from separate regions and, in some cases, at different developmental stages. An accurate comparison was made by recording six species while being hand-held, by observing TEM) the sonic muscles and by dissections of the sound-producing mechanism.

RESULTS

In all these species, calls presented harmonics, their dominant frequency was between 80 and 130 Hz and they were composed of trains of 4 to 11 pulses with gradual increasing periods towards the end of the call. In each case, the calls did not provide reliable information on fish size. The sounds were produced by homologous fast-contracting sonic muscles that insert on articulated ribs whose proximal heads are integrated into the swimbladder: each pulse is the result of the back and forth movements of the ribs. Small differences in the shape of the oscillograms of the different species could be related to the number of ribs that are involved in the sound-producing mechanism. These fish species are able to make sounds as soon as they settle on the reef, when they are 40 days old. Comparison between Neoniphon from Madagascar and from Rangiroa in French Polynesia showed a new, unexpected kind of dialect involving differences at the level of pulse distribution. Neoniphon calls were characterised by a single pulse that was isolated at the beginning of the remaining train in Madagascar whereas they did not show any isolated single pulses at the beginning of the call in Rangiroa.

CONCLUSION

This family cannot use the acoustic fundamental frequencies (or pulse periods) of grunts to infer the size of partners. Pulse duration and number of pulses are statistically related to fish size. However, these characteristics are poorly informative because the correlation slope values are weak. It remains other features (sound amplitude, resistance to muscle fatigue, calling frequency) could be used to assess the body size. Characteristics of the sound producing mechanisms are conservative. All species possess fast-contracting muscles and have the same kind of sound producing mechanism. They do show some change between clades but these differences are not important enough to deeply modify the waveforms of the calls. In this case, our description of the grunt could be considered as the signature for the holocentrid family and can be used in passive acoustic monitoring.

The energetic basis of acoustic communication

Animals produce a tremendous diversity of sounds for communication to perform life's basic functions, from courtship and parental care to defence and foraging. Explaining this diversity in sound production is important for understanding the ecology, evolution and behaviour of species. Here, we present a theory of acoustic communication that shows that much of the heterogeneity in animal vocal signals can be explained based on the energetic constraints of sound production. The models presented here yield quantitative predictions on key features of acoustic signals, including the frequency, power and duration of signals. Predictions are supported with data from nearly 500 diverse species (e.g. insects, fishes, reptiles, amphibians, birds and mammals). These results indicate that, for all species, acoustic communication is primarily controlled by individual metabolism such that call features vary predictably with body size and temperature. These results also provide insights regarding the common energetic and neuromuscular constraints on sound production, and the ecological and evolutionary consequences of producing these sounds.

Variability in the sonic muscles of the Lusitanian toadfish (Halobatrachus didactylus): acoustic signals may reflect individual quality

Animal vocalizations are good examples of signals that have been shaped by sexual selection and often contribute to resolve contests or the choice of mates. We relate the mass of the sound-producing muscles of a highly vocal fish species, the Lusitanian toadfish ( Halobatrachus didactylus (Bloch and Schneider, 1801)), with the sender’s physical features, such as body size, and reproductive and body condition. In this species, both sexes are known to emit sounds during agonistic interactions and males rely on their mate attraction vocalizations to reproduce. Sonic muscles were highly variable among males (CV = 40%) and females (CV = 33%) and showed sexual dimorphism. Regression analysis showed that variability in the sonic muscles was best explained by total length and fish condition in males and females. Liver mass in both genders, and the mass of the testes accessory glands, also explained sonic muscle variability. These variables explained 96% and 91% of the sonic muscle mass variability in males and females, respectively. As in teleost fishes sonic muscle mass correlates to particular sound acoustic features, we propose that in the Lusitanian toadfish sounds can inform the receiver about the sender’s quality, such as body size and condition, which are critical information in contests and mate choice.

Individual, temporal, and population-level variations in circulating 11-ketotestosterone and 17beta-estradiol concentrations in the oyster toadfish Opsanus tau

Sex steroid hormones are important for reproduction in all vertebrates, but few studies examine inter-individual, temporal, and population-level variations, as well as environmental influences on circulating steroid levels within the same species. In this study we analyzed plasma 11-ketotoestosterone (11-KT) and 17beta-estradiol (E(2)) levels in the oyster toadfish to test for 1) individual and temporal variations by serially sampling the same individuals during the reproductive and post-reproductive period, 2) variations in steroid levels among toadfish obtained from different sources or maintained under different holding conditions, and 3) correlations with environmental parameters. Results from serial sampling showed marked inter-individual variations in male 11-KT levels in two separate groups of toadfish, but no temporal differences from June to September. Females also showed inter-individual variations in E(2) concentrations, but most had elevated levels late in the reproductive season coincident with oocyte growth prior to winter quiescence. E(2) concentration, but not 11-KT, was positively correlated with water temperature, and negatively correlated with daylength and lunar phase. Maricultured toadfish held under constant conditions had elevated levels of E(2) and 11-KT that should be considered when using these fish for experimentation. This study provides important comparative information on the relationship between individual variations in steroid levels, and how they relate to physiological and environmental correlates in a model marine teleost.