Sol et al.: Gonadal development and changes in plasma reproductive steroids in Pleuronectes vetulus 
867 
B 
Figure 6 
Mean ±SE of (A) gonadosomatic index (GSI) and (B) plasma steroid 
concentrations in male English sole at each stage of test icular develop- 
ment. “a” denotes significantly higher levels than levels observed in 
regressed fish; “b” denotes significantly higher levels than levels ob- 
served in early recrudescence fish; and “c” denotes significantly higher 
levels than levels observed in spawned out fish I AN OVA, Fisher’s PLSD, 
95% Cl). Numbers in parentheses represent animals at each testicular 
developmental stage. 
in plasma steroid levels may have occurred 
in male English sole and would account 
for the significant difference in plasma 
steroid levels between spent and regressed 
males. 
A few male English sole sampled in July 
expressed milt even though reproductive 
steroid levels were low. Release of milt was 
also observed in winter flounder (Harmin 
et al., 1995) and Atlantic halibut (Methven 
et al., 1991) a few months earlier than vi- 
tellogenesis in females. Moreover, in win- 
ter flounder (Harmin et al., 1995), milt 
remained expressible a few months after 
the spawning season even though steroid 
levels were low during this time. Histo- 
logical analyses of English sole testis 
showed that initiation of spermiogenesis 
does begin in July in a substantial propor- 
tion of sole, in spite of their low plasma 
steroid concentrations. This phenomenon 
has also been observed in other male fish, 
but the mechanism accounting for it is not 
entirely clear. Borg (1994) suggests that 
concentrations ofT and 11KT in the testis 
itself may be quite high in early spermato- 
genesis, sufficient to stimulate spermio- 
genesis even when plasma concentrations 
of T and 11KT are low. The early produc- 
tion of mature sperm may be typical of 
most northern latitude, temperate zone 
teleosts in which fully developed sperma- 
tozoa are formed before winter and stored 
for discharge in spring (Lofts, 1987). 
It is unclear which steroid induces final 
oocyte maturation in English sole. In all 
teleosts which have been studied, final 
maturation is triggered by a surge in 
plasma concentrations of a maturational 
gonadotropin (GTH-II). GTH-II stimulates 
gonadal production of progesterones and 
related compounds, the C21 steroids, 
which induce final oocyte maturation and 
production of sperm (Swanson, 1991). In 
most teleosts, 17a, 20p-P is believed to be 
a potent MIS (Scott and Canario, 1987, 
1992; Inbaraj et al., 1995). However, in 
flatfish species that have been studied, low or 
nondetectable levels of 17a, 20j3-P were found ( Howell 
and Scott, 1989; Truscott et al., 1992b; Mugnier et 
al., 1995). Similarly, 17a, 20(3-P was not detected in 
spawning English sole, and in some flatfish species, 
other steroids have been suggested as the MIS. For 
example, in winter flounder 17P-hydroxy-5p-andro- 
stan-3-one and T were correlated with oocyte stages 
characterized by germinal vesicle migration (Truscott 
et al., 1992b), whereas in turbot (Scophthalmus maxi- 
mus L.) 17a, 21p,21-trihydroxy-4-pregnene-3-one-20 
has been shown to induce final maturation (Mugnier 
et al., 1995). Recent studies also suggest that in pla- 
ice (Pleuronectes platessa) and Dover sole (Solea 
solea) 17a, 20P-P actually does induce final matura- 
tion but is metabolized before it can be detected in 
