Seasonal variation of steroid hormone levels in an intertidal-nesting fish, the vocal plainfin midshipman

Abstract

This study characterized the seasonal variation of the steroid hormones testosterone (T), 11-ketotestosterone (11-KT), 17β-estradiol (E₂), and cortisol (F) as they relate to the gonadal development and reproductive behavior of the plainfin midshipman fish, Porichthys notatus. The plainfin midshipman is a deep-water teleost that seasonally migrates into the shallow intertidal zone where type I, or “singing,” males build nests, acoustically court and spawn with females. The gonadosomatic index and plasma steroid levels were measured from adult type I males and females collected over four time periods (non-reproductive, pre-nesting, nesting, and post-nesting) that corresponded to seasonal fluctuations in midshipman reproductive biology and behavior. Among type I males, plasma levels of T and 11-KT were low during the winter non-reproductive period, gradually increased during seasonal recrudescence of the testes in the spring pre-nesting period, and then peaked at the beginning of the summer nesting period. In the latter half of the nesting period and during the fall post-nesting period, plasma levels of T and 11-KT were low or non-detectable. Low, detectable levels of E₂ were also found in the plasma of 50% or more type I males during every seasonal period except the winter non-reproductive period. Among females, plasma levels of T and E₂ were low throughout the year but briefly peaked in April during the spring pre-nesting period when ovaries underwent seasonal recrudescence. Plasma F levels were correlated with collection depth and were lower in males than females when fish were collected deeper than 120 m. The sex-specific peaks of steroid hormone levels in male and female midshipman may serve differential functions related to the physiology, reproductive behavior, and vocal communication of this species.

Introduction

Gonadal steroid hormones can have profound influences on the central nervous system and behavior of vertebrates either through organizational effects during early development or through activational effects in adults (Nelson, 2000). The activational effects of gonadal hormones during reproduction are often associated with seasonal cycles of steroid hormone production and gametogenesis. Such seasonal changes can ultimately influence reproductive behaviors and are necessary for successful reproduction in all vertebrates.

Cyclical changes in the reproductive hormones of teleost fishes are widely known to occur in association with reproductive cycles and have been investigated mainly to understand the mechanisms of reproductive behavior, gametogenesis, and gonadal steroidogenesis (Fostier et al., 1983; Goetz, 1983). Seasonal changes in circulating levels of gonadal steroid hormones during the reproductive cycle are described for a variety of freshwater and marine teleost species (reviews: Fostier et al., 1983; Pankhurst and Carragher, 1991). In marine teleosts, reproductive periodicity of gonadal steroid hormone levels has been documented for wild caught populations of flatfish (Campbell et al., 1976; Harmin et al., 1995; Methven et al., 1992; Wingfield and Grimm, 1977), mullet (Dindo and MacGregor, 1981), salmon (Ueda et al., 1984), cod (Pankhurst and Conroy, 1987), orange roughy (Pankhurst and Conroy, 1988), rockfish (Nagahama et al., 1991; Takano et al., 1991), sardines (Matsuyama et al., 1994; Murayama et al., 1994), grouper (Johnson et al., 1998), kingfish (Poortenaar et al., 2001), tuna (Stequert et al., 2001), eelpout (Larsson et al., 2002), damselfish (Barnett and Pankhurst, 1994; Pankhurst et al., 1999; Sikkel, 1993), goby (Bonnin, 1979; Pierantoni et al., 1990), and toadfish (Modesto and Canario, 2003a). The focus here is on the plainfin midshipman (Porichthys notatus), another member of the same family of teleosts as toadfish (Batrachoididae), that has been the subject of intensive neuroethological investigation.

A series of multidisciplinary investigations has now characterized the vocal-acoustic behaviors and neurobiology of midshipman fish showing, in part, how midshipman serve as a model for identifying the mechanisms of auditory reception, neural encoding, and vocal production shared by all vertebrates (review: Bass and McKibben, 2003). An important component of these studies has been to establish sex differences in vocal-acoustic traits and showing how steroidal and peptidergic hormones and neurohormones can influence the development, maintenance and seasonal changes in the expression of those traits. What has been lacking from these studies, and is now documented in this report, is a portrayal of naturally changing levels of circulating gonadal steroid hormones.

The plainfin midshipman seasonally migrates from deep ocean sites to breed in the intertidal zone where the production and reception of vocal signals are essential to its reproductive success. Midshipman have three adult reproductive morphs, females and two male morphs (types I and II), that can be distinguished by a suite of behavioral, somatic, endocrinological and neurobiological characters (reviews: Bass, 1996, Bass, 1998). Type I, “singing” males build nests, acoustically court females, and provide parental care for fertilized eggs, whereas type II males neither build nests nor sing to females, but instead satellite or sneak spawn to steal fertilizations from type Is (Brantley and Bass, 1994). After spawning, females leave the seasonal breeding grounds and return to deep offshore sites (Brantley and Bass, 1994).

Seasonal periodicity of reproduction in the plainfin midshipman is concurrent with changes in vocal behavior and the physiological performance of the peripheral auditory system. Midshipman generate highly stereotyped advertisement and agonistic vocalizations. In particular, during the breeding season (May–August), type I males alone produce a long duration (>1 min), multiharmonic advertisement call or “hum” with a fundamental frequency near 90–100 Hz and several prominent harmonics (Bass et al., 1999). Observations of reproductive behavior together with underwater playback experiments demonstrate the role of the hum in attracting gravid females to a type I male’s nest (Brantley and Bass, 1994; Ibara et al., 1983; McKibben and Bass, 1998, McKibben and Bass, 2001). We recently discovered that seasonal changes in type I male vocal behavior (humming during the summer) are accompanied by changes in the frequency sensitivity of the peripheral auditory system such that summer, reproductive females were better suited than winter, non-reproductive females to encode the higher harmonic components of male advertisement calls (Sisneros and Bass, 2003a). This finding suggests that the seasonal enhancement of higher frequency encoding in females may represent an adaptive response to improve the detection of multiharmonic hums. A possible mechanism for the plasticity of frequency sensitivity in midshipman is one that is dependent on seasonal changes in circulating levels of gonadal steroids, like those reported here.

This study reports the seasonal cycle of the gonadal steroids testosterone (T), 11-ketotestosterone (11-KT), and 17β-estradiol (E₂) in the plasma of wild-caught populations of plainfin midshipman fish. We relate the plasma levels of gonadal steroids with the gonadal development and reproductive behavior of the midshipman. In addition, we also report plasma cortisol (F) levels, in part, because of the general influence of glucocorticoids on behavioral plasticity across vertebrates (review: Romero, 2002) and on the vocal motor system of batrachoidids (Remage-Healey and Bass, 2002, Remage-Healey and Bass, 2003). This study also complements earlier reports of circulating levels of gonadal and adrenal steroids that only focused on midshipman during the reproductive summer period (Brantley et al., 1993b; Knapp et al., 1999, Knapp et al., 2001). The results of the present study have previously been reported in abstract form (Forlano et al., 2003).