Seasonal Variation in Androgen Levels in the Oyster Toadfish

Male Oyster Toadfish Opsanus tau produce an advertisement call, the boatwhistle, using sexually dimorphic sonic muscles attached to the swimbladder. The fundamental frequency and duration of the boatwhistle change seasonally suggesting hormonal modulation of the output of pattern generators in the brain. The toadfish has an unusual protracted reproductive cycle in which testes contain mature sperm throughout the year, and females develop large eggs during late summer and fall for spawning the following spring although some may mate in the fall. This study quantified gonad development and plasma androgens in males and females throughout a seasonal cycle to relate them to the prolonged reproductive cycle and to quantitative changes in boatwhistle parameters. Median levels of testosterone (T) and 11- ketotestosterone (11KT) in males peak in May during the early part of the spawning season (461 pg/mL for T and 3746 for 11KT) and decline to 153 and 43 pg/mL, respectively, in June although spawning continues into July. A minor increase in gonosomatic index (GSI) and levels of both androgens (180 and 94 pg/mL, respectively) occurs in October. Median levels of T (328 pg/mL) and GSI in females also peak in May. In June, T levels drop in spawned females but remain elevated in those still gravid. Ovaries start to develop in late summer, and T levels increase above levels of individuals spawned in June. A spring peak in T in unspawned females and increasing levels in the fall correlate with estradiol (E) levels. Androgen levels do not correlate with the seasonal cycle in boatwhistle parameters suggesting that some other factor is responsible.

Fig. 1.

Monthly values of gonosomatic index (A), plasma testosterone (B), and 11-ketotestosterone (C) for individual male Oyster Toadfish. Horizontal bars indicate the median of each sampling period.

Fig. 2.

Monthly ratios of plasma 11-ketotestosterone to testosterone for male Oyster Toadfish (A) and of estradiol to testosterone for females (B). Horizontal bars indicate the median of each sampling period.

Fig. 3.

Relationship of plasma 11-ketotestosterone to T in male toadfish in May 1994 and May 1995. The linear regression (11KT = −461.7 + 4.597 T, r² = 0.68, P < 0.0001) excludes the five fish with highest 11-ketotestosterone levels that were presumably spermiating (Fostier et al., 1983).

Fig. 4.

Monthly values of gonosomatic index (A), plasma testosterone (B), and estradiol (C) for individual female Oyster Toadfish. Horizontal bars indicate the median of each sampling period

Fig. 5.

Relationship of testosterone (A) (Spearman Rho r = 0.81, P < 0.0001) and estradiol (B) (r = 0.68, P < 0.0002) to gonosomatic index for females in June 1994. The cluster of points near the origin came from females that had spawned

Fig. 6.

Comparison of the seasonal cycle of boatwhistle parameters (median fundamental frequency and duration) (A) and androgen levels (B). Boatwhistle parameters are redrawn from Fine (1978) and androgen levels are from this study. There are different time scales and years in (A) and (B), and letters in (B) stand for months. Note drop off in androgen levels between May and June at times when fundamental frequency is increasing and duration is relatively constant.

Contributor Notes

(MLF) Department of Biology, Virginia Commonwealth University, Richmond, Virginia 23284-2012; (MSJ) UNOS, 700 North 4th Street, Richmond, Virginia 23218; and (DWM) Department of Obstetrics and Gynecology, Medical College of Virginia Commonwealth University, Richmond, Virginia 23298. (MLF) mlfine@vcu.edu Send reprint requests to MLF.

Accepted: 24 Oct 2003