The adaptive significance of polymorphism in the rotifer Asplanchna. Humps in males and females

Abstract

Experiments show that humps or body wall outgrowths protect males and cruciform females of Asplanchna sieboldi from predation by conspecific, cannibal females. Humped prey were eaten by large cannibals to a significantly lesser extent than similarly-sized or even larger, non-humped prey. Since males and male-producing females, which are typically cruciform, co-occur with the largest, most cannibalistic morphotypes, their protection by humps may greatly increase the efficiency of sexual reproduction. An analysis of female diets and the occurrence of humped males and females in the different species of Asplanchna shows that these humped forms are found only in those species in which the females can eat comparatively large prey. This relationship provides strong indirect evidence for the theory that humps in both males and polymorphic females specifically evolved as a defense mechanism against predation by conspecific, cannibal females.