Saccular potentials of the vocal plainfin midshipman fish, <Emphasis Type="Italic">Porichthys notatus</Emphasis>

The plainfin midshipman fish, Porichthys notatus, is a vocal species of teleost fish that generates acoustic signals for intraspecific communication during social and reproductive behaviors. All adult morphs (females and males) produce single short duration grunts important for agonistic encounters, but only nesting males produce trains of grunts and growls in agonistic contexts and long duration multiharmonic advertisement calls to attract gravid females for spawning. The midshipman fish uses the saccule as the main acoustic endorgan for hearing to detect and locate vocalizing conspecifics. Here, I examined the response properties of evoked potentials from the midshipman saccule to determine the frequency response and auditory threshold sensitivity of saccular hair cells to behaviorally-relevant single tone stimuli. Saccular potentials were recorded from the rostral, medial and caudal regions of the saccule while sound was presented by an underwater speaker. Saccular potentials of the midshipman, like other teleosts, were evoked greatest at a frequency that was twice the stimulus frequency. Results indicate that midshipman saccular hair cells of non-reproductive adults had a peak frequency sensitivity that ranged from 75 (lowest frequency tested) to 145 Hz and were best suited to detect the low frequency components (≤105 Hz) of midshipman vocalizations.     

Differential effects of 11-ketotestosterone on dimorphic traits in a teleost with alternative male reproductive morphs

Species with dimorphic males typically have one morph (type I) associated with territoriality and courtship, and another morph (type II) associated with cuckoldry. In teleost fishes, type I males generally have higher levels of the androgen 11-ketotestosterone (KT) than type II males. Is KT causal to phenotypic differences between morphs? We investigated this question in the midshipman fish (Porichthys notatus) in which type I males have detectable levels of KT during the breeding season, whereas levels in type II males are usually undetectable. Type I midshipman will either cuckold or hold territories and court females, whereas type II males are only known to cuckold. Acoustic courtship by type I's is supported by the sonic motor nucleus, which innervates a sound-producing sonic muscle. Type I males have larger sonic motor nuclei and larger sonic muscles than type II males, consistent with the more dynamic vocal repertoire of type I's. Here, we tested whether intraperitoneal KT implants in adult type II males would induce type I male-like traits in brain, sonic muscle, and behavior. Type II's treated with KT did not differ from blank-implanted type II's in sonic motor nucleus volume. Sonic muscle mass increased in KT-implanted type II's, but did not reach the relative mass naturally observed in type I's. While neither territoriality nor courtship were induced, cuckoldry behavior intensified in KT-implanted type II's. Thus, for some but not all characters, KT exaggerated the expression of already existing type II male traits rather than inducing the type I male-like traits of territoriality, courtship, and an expansive vocal motor system.     

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.     

Androgen effects on vocal muscle structure in a teleost fish with inter- and intra-sexual dimorphism

The plainfin midshipman fish Porichthys notatus has both interand intra-sexual dimorphism in the sound-producing (vocal or sonic) muscles attached to the swimbladder wall. The “Type I” and “Type II” male morphs differ in that dramatic structural changes related to sexual maturity occur in the mass, the area of mitochondria-filled sarcoplasm, and the myofiber number of the sonic muscles of Type I males, but not in those of Type II males (nor of females). Androgen implantation for 9 weeks markedly increased the relative sonic muscle size in juvenile males, juvenile females, and Type II males, whereas estradiol or cholesterol treatment did not. The principal androgen effect on myofiber structure was an increase in the area of mitochondria-filled sarcoplasm. The ratio of sarcoplasm area to myofibril area (Sr/Mf) increased by 1.4- to 2-fold in myofibers of all androgen-treated groups, with the greatest structural change occurring in juvenile males. When androgen implants were removed from juvenile males, the muscle mass and Sr/Mf ratio reverted toward the unimplanted juvenile phenotype. Total fiber number in sonic muscle increased significantly in juvenile males following androgen implantation but did not detectably change in juvenile females or Type II males. These results suggest: (1) sonic muscle in Porichthys notatus is an androgen target tissue, (2) fiber structure and fiber number are androgen-sensitive features, and (3) there exist sex- and morph-specific patterns of sonic muscle responsiveness to androgen implants. © 1993 Wiley-Liss, Inc.     

Nonsequential developmental trajectories lead to dimorphic vocal circuitry for males with alternative reproductive tactics

Midshipman fish, Porichthys notatus, have two male reproductive morphs: type I males generate long duration advertisement calls (“hums”) to attract females to a nest; type II males sneak-spawn and, like females, do not produce mate calls but generate short duration agonistic calls. A vocal pacemaker circuit includes: motoneurons in the caudal brain stem and rostral spinal cord that innervate vocal/sonic muscles; pacemaker neurons that are located ventrolateral to motoneurons and establish their fundamental discharge frequency; and a ventral medullary nucleus that couples the motoneuron-pacemaker circuit bilaterally. Transneuronal biocytin transport identified morph-specific developmental trajectories for the vocal circuit. Among nonreproductive, juvenile type I males, motoneuron soma size and motor nucleus volume increase most during a stage prior to sexual maturation. An additional increase in motoneuron size and nucleus volume is coupled to the greatest increase in pacemaker soma size at a stage coincident with the onset of sexual maturity; ventral medullary neurons show similar growth increments during both stages. Type II males (and females) mature with no or little change in cell size or motor nucleus volume. The results indicate that alternative mating tactics are paralleled by alternative developmental trajectories for the neurons that determine tactic-specific behaviors, in this case vocalizations. Together with aging data based on otolith growth, the results support the hypothesis that alternative male morphs in midshipman fish adopt nonsequential, mutually exclusive life history tactics. © 1996 John Wiley & Sons, Inc.     

Diet and cannibalism in plainfin midshipman Porichthys notatus

The macroscopic and microscopic diversity of potential food items available in the nests of plainfin midshipman Porichthys notatus were quantified and compared with items that were found in the stomach and intestine (digestive tract) of the guarding males. In this species, males occur as one of two possible reproductive morphs: guarder males that care for young and sneaker males that parasitize the courtship and care of guarder males. Although it was predicted that guarder males would have fewer feeding opportunities due to their confinement to the nest, they in fact had more food items in their digestive tracts than did sneaker males and females. Date in the breeding season (a proxy of care duration) and body condition were not correlated with the amount of food consumed by guarder males. The main type of food consumed was P. notatus embryos; 69% of all guarder males sampled had cannibalized offspring. By comparing the diet of both sexes and tactics, this study sheds light on some of the strategies designed to cope with the costs of providing parental care.     

Aromatase activity in hindbrain vocal control region of a teleost fish: divergence amoung males with alternative reproductive tactics

Type I male midshipman fish acoustically court females, whereas type II males do not but instead sneak or satellite spawn to compete with type I males for fertilizations. ''Singing'' type I males diverge from type II males and females in the organization of an expansive hindbrain pacemaker–motoneuron circuit that establishes the physical attributes of vocalizations. Here, levels of aromatase activity were determined in homogenates of brain by measuring the conversion of ³H-androstenedione (AE) to ³H-oestrone (E₁) and ³H-oestradiol (E₂). Levels were highest in the telencephalon–preoptic area and similar for all morphs. Lower levels were in a region including the diencephalon, midbrain and cerebellum, although levels were significantly higher in females compared with type I males. In the vocal hindbrain region, aromatase levels were three- to five-fold higher in type II males and females than in type I males, and in castrated type II males than in castrated type I males. Conversion of testosterone to oestrogen in type II males and females may effectively prevent testosterone-induced maturation of the vocal system that characterizes type I males. Aromatase may thus be a key enzyme regulating the expression of individual-specific brain circuitry and behaviours among members of one sex.     

Coding of concurrent vocal signals by the auditory midbrain: effects of duration

Neural selectivity to signal duration within the auditory midbrain has been observed in several species and is thought to play a role in signal recognition. Here we examine the effects of signal duration on the coding of individual and concurrent vocal signals in a teleost fish with exceptionally long duration vocalizations, the plainfin midshipman, Porichthys notatus. Nesting males produce long-duration, multi-harmonic signals known as hums to attract females to their nests; overlapping hums produce acoustic beats at the difference frequency of their spectral components. Our data show that all midbrain neurons have sustained responses to long-duration hum-like tones and beats. Overall spike counts increase linearly with signal duration, although spike rates decrease dramatically. Neurons show varying degrees of spike rate decline and hence, differential changes in spike rate across the neuron population may code signal duration. Spike synchronization to beat difference frequency progressively increases throughout long-duration beats such that significant difference frequency coding is maintained in most neurons. The significance level of difference frequency synchronization coding increases by an order of magnitude when integrated over the entirety of long-duration signals. Thus, spike synchronization remains a reliable difference frequency code and improves with integration over longer time spans.     

Otolith morphology varies between populations,sexes and male alternative reproductive tactics in a vocal toadfish Porichthys notatus

In this study, the morphology of sagittal otoliths of the plainfin midshipman fish Porichthys notatus was compared between populations, sexes and male alternative reproductive phenotypes (known as ‘type I males or guarders’ and ‘type II males or sneakers’). Sagitta size increased with P. notatus size and changes in shape were also detected with increasing body size. Porichthys notatus sagittae begin as simple rounded structures, but then elongate as they grow and take on a more triangular and complex shape with several prominent notches and indentations along the dorsal and caudal edges. Moreover, the sagittae of the two geographically and genetically distinct populations of P. notatus (northern and southern) differed in shape. Porichthys notatus from the north possessed taller sagittae with deeper caudal indentations compared to P. notatus from the south. Sagitta shape also differed between females and males of the conventional guarder tactic. Furthermore, guarder males had smaller sagittae for their body size than did sneaker males or females. These differences in sagittal otolith morphology are discussed in relation to ecological and life history differences between the sexes and male tactics of this species. This is the first study to investigate teleost otolith morphology from the perspective of alternative reproductive tactics.     

Does proximity to aquatic pollution affect reproductive traits in a wild‐caught intertidal fish?

How chronic exposure to aquatic pollution affects reproductive traits was assessed in nesting wild-caught plainfin midshipman Porichthys notatus in areas with low and high contaminant exposure on Vancouver Island, British Columbia. Males in high-exposure areas had a greater degree of testicular asymmetry, sperm with shorter heads and fewer live eggs in their nests. The results of this study provide important insights into the potential consequences of contaminant exposure on the reproductive physiology of wild-caught fishes.     

Exposure to Advertisement Calls of Reproductive Competitors Activates Vocal-Acoustic and Catecholaminergic Neurons in the Plainfin Midshipman Fish,Porichthys notatus

While the neural circuitry and physiology of the auditory system is well studied among vertebrates, far less is known about how the auditory system interacts with other neural substrates to mediate behavioral responses to social acoustic signals. One species that has been the subject of intensive neuroethological investigation with regard to the production and perception of social acoustic signals is the plainfin midshipman fish, Porichthys notatus, in part because acoustic communication is essential to their reproductive behavior. Nesting male midshipman vocally court females by producing a long duration advertisement call. Females localize males by their advertisement call, spawn and deposit all their eggs in their mate’s nest. As multiple courting males establish nests in close proximity to one another, the perception of another male’s call may modulate individual calling behavior in competition for females. We tested the hypothesis that nesting males exposed to advertisement calls of other males would show elevated neural activity in auditory and vocal-acoustic brain centers as well as differential activation of catecholaminergic neurons compared to males exposed only to ambient noise. Experimental brains were then double labeled by immunofluorescence (-ir) for tyrosine hydroxylase (TH), an enzyme necessary for catecholamine synthesis, and cFos, an immediate-early gene product used as a marker for neural activation. Males exposed to other advertisement calls showed a significantly greater percentage of TH-ir cells colocalized with cFos-ir in the noradrenergic locus coeruleus and the dopaminergic periventricular posterior tuberculum, as well as increased numbers of cFos-ir neurons in several levels of the auditory and vocal-acoustic pathway. Increased activation of catecholaminergic neurons may serve to coordinate appropriate behavioral responses to male competitors. Additionally, these results implicate a role for specific catecholaminergic neuronal groups in auditory-driven social behavior in fishes, consistent with a conserved function in social acoustic behavior across vertebrates.     

Early development of the motor and premotor circuitry of a sexually dimorphic vocal pathway in a teleost fish

The plainfin midshipman fish (Porichthys notatus) has a caudal hindbrain vocal motor circuit that has been proposed to share a common embryonic origin with the hindbrain vocal networks of other vertebrates. In midshipman, this vocal circuit includes three groups of neurons: sonic motor, pacemaker, and ventral medullary. Here, transneuronal transport of biocytin or neurobiotin was used to delineate the early ontogeny of the three hindbrain vocal nuclei and their pattern of connectivity. The organization of the vocal nuclei was studied in animals beginning soon after hatching until the nuclei have the adult phenotype at the time fish become free-swimming. There is a clear sequence of events whereby motoneurons establish their connections with the sonic muscle prior to establishing connections with premotor neurons; developmental milestones of the vocal pathway parallel those of the sonic muscle. The results also indicate that sexual differentiation of the vocal motor system in midshipman begins early in development, well before any evidence of sexual maturation. Embryonic males and females differ in the relationship between soma size and body length for the three hindbrain nuclei. Males are also more variable than females in body mass, volume of the sonic motor nucleus, and motoneuron cell size.     

Acoustic communication in the Bocon toadfish (<Emphasis Type="Italic">Amphichthys cryptocentrus</Emphasis>)

The soundscapes of many coastal habitats include vocalizations produced by species of the family Batrachoididae (toadfish and midshipman). We describe the calling and grunting behavior of male Amphichthys cryptocentrus, a tropical toadfish, and predict how these vocalizations are influenced by conspecifics. We recorded individual males, which produced broadband grunts and multi-note, harmonic “boatwhistle” calls. Grunts were either in combination with calls or stand-alone. We used a null model to test if these latter grunts were produced at random or in response to calls from conspecifics. The model supports the hypothesis that grunts were in response to calls from neighboring males, suggesting acoustic competition. Using the most conservative estimate of hearing abilities we predict that males responded to the second harmonic of neighbor’s calls (230 Hz) at amplitudes of approximately 100–125 dB re 1μPa²/Hz. We also observed that call and grunt rates increased when males were exposed to higher rates of acoustic activity from neighboring fish. Fish used grunts to respond to background calls that occurred at different amplitudes, suggesting they responded to the calls of multiple neighboring fish and not just the highest amplitude neighbor. This communication with multiple fish within hearing range suggests a communication network in which the spatial distribution of individual toadfish relative to one another will impact their vocal behavior. Thus, the density and distribution, and not just abundance, of these toadfish at a given site will influence the characteristics of the chorus and the role of this species in the local soundscape.     

Steroid hormones and paternal care in the plainfin midshipman fish (Porichthys notatus)

The present study investigated the relationship between plasma steroid hormone levels and the expression ofpaternal behavior in the plainfin midshipman fish (Porichthys notatus), where males may simultaneously care for multiple clutches in different stages of development. Blood samples were collected from free-living parental males during that part of the breeding season when males may be found in various stages of parental care. Plasma 11-ketotestosterone levels were significantly higher in males with empty nests and nests containing only eggs than in males with nests containing embryos. All males with nests containing embryos had undetectable testosterone levels, whereas testosterone levels were detectable in many males with empty nests or nests containing only eggs. Estradiol levels were detectable in only a few males from nests with no eggs or nests containing only eggs. Cortisol levels were not correlated with stage of paternal care or with handling time. These results follow the frequently reported vertebrate pattern of declining androgen levels over the course of the breeding season or during the period of parental care. However, many male midshipman guarding nests containing only eggs had androgen levels similar to those of males whose nests contained no offspring. Thus the pattern of androgen levels exhibited by reproductively active parental male midshipman may reflect a compromise between investment in paternal care versus courtship and/or territoriality.     

Steroid regulation of brain aromatase expression in glia: female preoptic and vocal motor nuclei

Expression of the enzyme aromatase, which converts androgens to estrogens, is known to be regulated by gonadal steroids in brain areas linked to reproduction and related behaviors in several groups of vertebrates. Previously, we demonstrated in a vocal fish, the plainfin midshipman, that both males and females undergo seasonal changes in brain aromatase mRNA expression in the preoptic area (POA) and the dimorphic sonic/vocal motor nucleus (SMN) that parallel seasonal variation in circulating steroid levels and reproductive behavior. We tested the hypothesis that steroids are directly responsible for seasonal modulation of aromatase in females because they show the most dramatic fluctuations of testosterone (T) and 17beta-estradiol (E2) throughout the year. Adult female midshipmen were ovariectomized and administered T, E2, or blank (control) implants. We then quantified aromatase mRNA expression within the POA and SMN by in situ hybridization. Both T- and E2-treated females had elevated mRNA expression levels in both brain areas compared to controls. T affected aromatase expression in a level-dependent manner, whereas E2 showed a decreased effect at higher circulating levels. This study demonstrates that seasonal differences in brain aromatase expression in female midshipman fish may be explained, in part, by changes in levels of circulating steroids.     

Coping with aquatic hypoxia: how the plainfin midshipman (Porichthys notatus) tolerates the intertidal zone

Although plainfin midshipman (Porichthys notatus) are primarily known for their alternative reproductive tactics, and the dimorphic male subtypes, in which Type-I males demonstrate parental investment and mate attraction, and Type-II males ‘sneak’ fertilization and show no investment after fertilization, little is known about the physiology and tolerance to low aquatic oxygen while nesting in the intertidal zone. In May 2007, females and Type-I and Type-II males were collected, and in June 2009, only Type-I males were collected from nest sites on the coast of Vancouver Island, British Columbia. In the 2007 season, an initial assessment of hypoxia tolerance and nest parameters was recorded for the three subtypes of midshipman. Historical evidence indicates that Type-I males remain on the nest for prolonged periods, and our results suggest they can cope with repeated bouts of aquatic hypoxia by elevating their hematocrit and tolerating high lactate levels. The 2009 season was directed at examining the aquatic hypoxia tolerance of only the Type-I male. Hypoxic (~15 % air saturated water) Type-I males had oxygen consumption rates at ~12 % of the normoxic control (~100 % air saturated water) and a P_crit, the critical oxygen tension, when a fish switches from oxyregulator to oxyconformer, could not be determined; an indication that these fish are solely oxyconformers. With prolonged exposure to aquatic hypoxia, Type-I males displayed significant elevations in plasma and tissue lactate (heart), tissue glucose (liver), and a depression in gill Na⁺/K⁺ATPase and catalase activities. Results suggest that male Type-I midshipman survival in the intertidal zone is enhanced by metabolic depression and tolerance to anaerobic byproducts.     

Seasonally Variable Eusocially Selected Traits in the Paper Wasp, Mischocyttarus mexicanus

The expression of alternative traits that benefit eusocial individuals but are not directly involved in reproductive differences among those individuals, which I call ‘eusocially selected traits’, may vary in response to environmental changes if this increases an individual's inclusive fitness. In this study, I describe traits that separate individuals within the reproductive division of labor of Mischocyttarus mexicanus, a eusocial paper wasp, and determine whether observed eusocially selected traits vary across seasons. I examined M. mexicanus because females initiate new nests throughout most of the year where they experience different conditions depending on the season. Findings from this study suggest two main conclusions: (1) phenotypic differences among M. mexicanus females are mixed, showing specialized, generalized, and context‐dependent eusocially selected traits and (2) a female's position within the reproductive division of labor may be influenced by its state. The presence of context‐dependent traits, e.g. large females initiated solitary nests in the spring and grouped nests during the summer, suggests that the payoff for pursuing different positions within the reproductive division of labor changes across seasons. The expression of context‐dependent eusocially selected traits also suggests that, roles, instead of castes, may better reflect the reproductive division of labor among individuals of eusocial species like M. mexicanus.     

Social network predicts loss of fertilizations in nesting males of a fish with alternative reproductive tactics

Alternative reproductive tactics (ARTs) evolve when there is strong intra-sexual competition between conspecifics for access to mates. Typically, larger “bourgeois” males reproduce by securing the access to reproductive resources while smaller “parasitic” males reproduce by stealing fertilizations from larger males. A number of factors can influence the reproductive success of each tactic, including intrinsic (e.g. size) and extrinsic (e.g. tactic relative frequency) variables. An example where plastic ARTs occur is the peacock blenny Salaria pavo, with large males reproducing by defending nests and attracting females (bourgeois tactic) and small males reproducing by achieving sneaked fertilizations (parasitic tactic). In this study, we conducted field observations on individually tagged animals to determine their social network and collected eggs from 11 nests to determine the fertilization success of each male tactic. Paternity estimates for 550 offspring indicated an average fertilization success for nest-holder males of 95%. Nest-holder male morphological traits and social network parameters were tested as predictors of fertilization success, but only the number of sneakers present in the nest-holder’s social networks was found to be a predictor of paternity loss. Although male morphological traits had been previously found to be strongly correlated with reproductive success of nest-holder males, as measured by the number of eggs collected in the male’s nest, no correlation was found between any of the measured morphological traits and fertilization success for these males. The results suggest a stronger influence of the social environment than of morphological variables in the proportion of lost fertilizations by nest-holder males of this species.     

Morphometric changes associated with the reproductive cycle and behaviour of the intertidal-nesting, male plainfin midshipman Porichthys notatus

Morphometric changes in body condition, liver, sonic muscle and gonadal development associated with the annual reproductive cycle and behaviour of the intertidal-nesting male plainfin midshipman Porichthys notatus were investigated. Body condition of type I males rapidly increased during the pre-nesting (PN) period, peaked at the beginning of the summer nesting cycle and then gradually declined to lowest levels during the non-reproductive (NR) period. The gonado-somatic index of type I males peaked during PN and then declined during the summer nesting cycle to lowest levels at the end of the nest cycle and during NR. Indices of sonic muscle and liver of type I males were lowest during NR, gradually increased during PN and then peaked during the summer nesting cycle. Results indicate that body condition and fecundity of type I males were positively correlated with body mass at the end of the nest cycle. These findings as they relate to the annual reproductive cycle and behaviour of the type I male P. notatus are discussed.