Intersexuality in Gammarus Fossarum Koch, 1835 (Amphipoda)

We present the first description of intersexes in Gammarus fossarum Koch, 1835 (Amphipoda). Intersexes were found in monthly samples collected in two streams in different regions of Germany. The frequency of intersexuality differed considerably between the populations from the two streams. The Lockwitzbach, located in Sachsen, showed a frequency of 8.8% intersexes, which is much higher compared to the K?rsch (located in Baden-W?rttemberg) with 0.6%. In contrast to the K?rsch, there were also differences between the two sampling sites in the Lockwitzbach. No seasonal pattern in intersex frequency was observed. Intersexes occurred in all length classes of the adolescent and adult gammarids examined. They were significantly larger than females, but showed no difference in body length compared to males. External sex characteristics of intersex individuals have been described in detail, while in histological analyses of internal sex characteristics ovarian tissue was found exclusively. Microsporidians, parasites often associated with intersexuality, were not detected. The intersexes seemed to be functional females, regarding their ability to breed.