Macroscopic gonadal deviations and intersex in developing whitefish Coregonus lavaretus

Abstract

In Lake Thun, Switzerland, a significant number of whitefish Coregonus lavaretus were found to exhibit abnormal gonadal morphology. To determine the time at which the morphological deviations from a normal gonadal arrangement develop, we conducted experiments designed to: (1) examine whether the morphologic gonadal variations develop during or after gonadal differenti- ation, and (2) characterise the entire ontogenetic gonadal development in whitefish from hatching until maturity. For these experiments, whitefish were reared under controlled conditions with distinct water sources and temperature regimes (Lake Thun water: 4 to 22°C; spring water: 8 to 9°C). Gonadal development in the spring water group was clearly delayed with respect to age, but similar to the Lake Thun water group with respect to degree-days in Celsius (°D). Undifferentiated gonads were first seen at 65 d post-hatch and 491°D. Ovarian differentiation (starting from 1734 to 1820°D) preceded testicular differentiation (starting from 1989 to 3673°D). The first fish with mature germ cells were recognised at 8163 to 8356°D. Morphological gonadal deviations became evident during gonadal differentiation. Constrictions and asymmetries developed for the first time in the first (0+) or second (1+) year-of-life, at a body length of 13 to 18 cm. Aplasia and compartmentation were recorded in the third (2+) year-of-life at a body length of 16 to 18 cm. Intersex fish occurred at a fre- quency of 8 to 9% during gonadal sex differentiation. Because no evidence for the presence of exo- genic endocrine active substances was revealed, we suppose that there is a naturally increased mosaic intersex condition in whitefish during the ontogenetic gonadal differentiation process.