It is well known that elevated testicular temperature can lead to low sperm count and other reproductive problems in male mammals. Just ask any father who switched from briefs to boxers when trying to have a baby. It turns out that some fish, such as pejerrey and Florida largemouth bass, also experience heat-induced gonadal damage, except that both males and females are affected. And as fish are unable to regulate their body temperature, increasing global temperatures could have a damaging effect on reproduction and worldwide fish populations.

While the mechanism is poorly understood in fish, in mammals, gonadal degeneration generally occurs through a process called apoptosis. Apoptosis is essentially when a cell commits suicide through a number of characteristic steps. Lauro Satoru Ito from the Tokyo University of Marine Science and Technology and colleagues from Brazil and Japan sought to determine whether apoptosis occurs during warm water-induced gonadal degeneration in male and female pejerrey, whose natural habitat in the shallow lagoons of South America can reach 31–32°C.

To characterize gonadal degeneration and germ cell loss and examine the occurrence of apoptosis, the authors exposed fish to a prolonged heat stress(16 weeks at 29°C) or a short heat stress (36 h at 31°C and then returned to 24°C), or held them at a comfortable 24°C. Then the team sampled the fishes' gonads at various time points (hours, days, weeks) to look for histochemical and biochemical signs of gonadal degeneration, germ cell loss and apoptosis.

The team found clear evidence of apoptosis in the gonadal cells of both male and female pejerrey. Remarkably, all of the fish exposed to a prolonged heat stress survived and only 11% of those exposed to a short heat stress died. Furthermore, all the fish fed normally except for fish during the short 31°C exposure. Thus, this experiment demonstrates that warm temperatures,which may have no effect on survival or even feeding, can nonetheless have negative effects on reproduction.

Not surprisingly, the severity of the gonadal damage was proportional to the magnitude of the heat stress. In addition, males were more susceptible to heat stress than females. Interestingly, Ito and his team observed substantial individual variability in heat sensitivity. For example, some male pejerrey exposed to prolonged heat stress were completely devoid of sperm-producing germ cells at the end of the study while other fish from the same treatment group appeared completely normal. However, the basis of this variability is unknown.

This study provides valuable information in the field of thermal reproductive biology, which is imperative in the face of global warming and its potentially catastrophic consequences for fish populations. The work performed by Ito and colleagues shows that warm temperatures can harm the gonads of pejerrey in a similar manner to mammals. In addition, pejerrey require a lower range of temperatures to successfully reproduce than they do to simply survive. This means that if these fish want to have babies, they need to keep cool. If only fish could wear boxer briefs!

Ito, L. S., Takahashi, C., Yamashita, M. and Strussmann, C. A. (
). Warm water induces apoptosis, gonadal degeneration, and germ cell loss in subadult Odontesthes bonaiensis(Pisces, Atheriniformes).
Physiol. Biochem. Zool.