Body color change and serum steroid hormone levels throughout the process of sex change in the adult wrasse, <Emphasis Type="Italic">Pseudolabrus sieboldi</Emphasis>

Gonadal sex steroid hormones are the principal factors that directly control the gonadal and morphological alterations during sex change in hermaphrodite fish; however, the physiological mechanism of action by which these hormones govern body coloration is poorly understood. The protogynous wrasse Pseudolabrus sieboldi is a good model for understanding the physiological mechanisms of gonadal and body color change during sex change in hermaphrodite fish. To obtain information on the relationship between sex steroids and body color change during the process of gonadal sex change, we analyzed body color, gonadal histology, and serum levels of sex steroids. Body color was analyzed using a quantitative analytical method based on the hue value. Compared to other body parts of the fish, the anal fin changed color the most, becoming increasingly redder in association with gonadal changes that converted ovaries to testes. Levels of serum 11-ketotestosterone (11KT) increased as the gonadal sex change proceeded, whereas no significant change was observed in estradiol-17β (E2) levels. Moreover, we found a significant correlation between the hue value of the anal fin and serum 11KT levels, but not E2 levels. These results suggest that androgen, but not estrogen, plays a principle role in the changes in both gonadal morphology and body color in the transformation from female to male in this species. To our knowledge, this is the first quantitative demonstration of the relationship between body color and serum steroid levels during sex change in fish.     

Comparison of feeding habits of myctophid fishes and juvenile small epipelagic fishes in the western North Pacific

To examine the potential trophic competition between myctophids and small epipelagic fishes in the nursery grounds in spring, we compared the stomach contents of dominant myctophids (Symbolophorus californiensis, Ceratoscopelus warmingii and Myctophum asperum; n = 179) and juvenile epipelagic fishes (Japanese sardine, Sardinops melanostictus, Japanese anchovy, Engraulis japonicus, chub mackerel, Scomber japonicus, and spotted mackerel, S. australasicus; n = 78) that were simultaneously collected at nighttime with a midwater trawl net around the Kuroshio-Oyashio transition zone in the western North Pacific. It was clear that the neritic copepod Paracalanus parvus s.l. was the most abundant species in NORPAC samples (0.335 mm mesh size) taken at the same stations. Diets of dominant myctophid fishes differed from those of the juvenile epipelagic fishes; Japanese sardine and anchovy mostly preyed upon P. parvus s.l. (23.6% of stomach contents in volume) and Corycaeus affinis (16.1%), respectively. Both chub and spotted mackerels mainly preyed upon the seasonal vertical migrant copepod, Neocalanus cristatus (15.9 and 14.7%, respectively). On the contrary, myctophid fishes probably do not specifically select the abundant neritic copepods. Namely, S. californiensis mostly preyed upon a diel vertical migrating copepod, Pleuromamma piseki (22.7 and 30.6% in stomach of juvenile and adult, respectively), while C. warmingii and M. asperum preyed on Doliolida (43.0% in stomach of juvenile C. warmingii), appendicularians (11.0% in stomach of juvenile M. asperum), and Ostracoda (6.3% in stomach of adult C. warmingii). Feeding habits of myctophid fishes seem adapted to their prey animals; low rate of digested material (less than 30% in volume) in stomachs of S. californiensis may be linked to the movement of P. piseki, hence S. californiensis can easily consume this copepod at night since they are more concentrated at night than daytime. High rate of digested material (over 40%) of M. asperum and adult C. warmingii suggest that they feed not only at night but also during the daytime in the midwater layer. Thus, myctophid fishes actually fed in the surface layer but less actively than the small epipelagic fishes. These results suggest that the potential for direct food competition between myctophids and small epipelagic fishes is low in the nursery ground, but there remains a possibility of indirect effects through their prey items, since the above gelatinous animals feed on common prey items as juveniles of Japanese sardine and anchovy.