Defenses of Males Against Cannibalism in the Rotifer Asplanchna: Size, Shape, and Failure to Elicit Tactile Feeding Responese

In Asplanchna, ♂ ♂ are considerably smaller than the ♀ ♀. In A. sieboldi this sexual dimorphism may be further accentuated because the ♀ ♀ are trimorphic in size and shape. The intermediate—sized, cruciform, and especially the largest, campanulate morphs are cannibalistic. However, the ♂ ♂ are protected from these cannibalistic ♀ ♀ by 2 different types of mechanisms. They can fail to elicit contact feeding responses and they can have body sizes and body wall outgrowths sufficiently large to prevent attacking ♀ ♀ from capturing them. The degree to which each of these mechanisms is developed can vary greatly between clones or taxa, may be inversely related, and may be a function of the voracity and size of cannibalistic ♀ ♀ to which the ♂ ♂ are generally exposed. Campanulate ♀ ♀ from clone 12C1, readily attacked both ♀ ♀ from the smallest, saccate morph of their own clone and also ♀ ♀ from clone 10C6, but they rarely attacked ♂ ♂ of their own clone. Campanulate ♀ ♀ from clone 10C6 regularly attacked ♂ clonemates, but cruciform ♀ ♀ from this clone responded more frequently to ♂ ♂ from clone 12C1 than to ♂ ♂ from their own clone. On a given diet, ♂ ♂ from clone 10C6 were about 1.5 x larger and had more pronounced body wall outgrowths than those from clone 12C1. In both clones, the sizes of ♂ ♂ and mictic (♂—producing) ♀ ♀ were influenced by the diet, being larger in populations fed on the rotifer Brachionus calyciflorus than those fed on the ciliate Paramecium aurelia. It may be adaptive for ♂ ♂ to be as small and simply constructed as possible; they have only a rudimentary digestive system, and so energy for their production must come entirely from their mictic—♀ parents. Therefore, ♂ size and body wall outgrowth development may represent a balance between production costs and ability to withstand cannibalism. Evolution of a mechanism preventing attack by cannibalistic ♀ ♀ may decrease the requirements for defensive structures and increase the efficiency of ♂ production and hence subsequent resting egg production.