Hypothesis: Inbreeding depression is manifest in lower sperm production.
Organism: Freshwater crustaceans (clam shrimp - Eulimnadia texana), from the southwestern U.S., which have high levels of inbreeding.
Methods: Comparisons of semithin sections of the male gonad among selfed and outcrossed siblings from four families.
Results: Twofold reduction in sperm production in inbred relative to outcrossed males. Inbreeding depression in males higher than previous estimates from hermaphrodites.
Conclusions: Inbreeding markedly reduces sperm production. The observed low levels of sperm production can explain both the low average outcrossing rates as well as the variation in these rates reported in previous studies of these crustaceans.