A hypothesis about the origin of sperm storage in the Eubrachyura, the effects of seminal receptacle structure on mating strategies and the evolution of crab diversity: how did a race to be first become a race to be last?

MC LAY, COLIN; LÓPEZ GRECO, LAURA SUSANA

ZOOLOGISCHER ANZEIGER

ELSEVIER GMBH

Año: 2011 vol. 250 p. 378 - 406

0044-5231

The origins and evolution of sperm storage in Brachyura are enigmatic: sperm is either stored in seminal receptacles, accessable via the vulvae on the sixth thoracic sternite, or in spermathecae at the border between the seventh and eighth sternites. Crabs with spermathecae are collectively refered to as “podotremes” while crabs with seminal receptacles belong to the Eubrachyura. The position of gonopores is the primary basis for subdividing the Eurachyura into the Heterotremata (female vulvae + males with coxal gonopores) and Thoracotremata (female vulvae + males with sternal gonopores). We present a hypothesis about the evolution of seminal receptacles in eubrachyuran female crabs and argue that the sternal gonopore has been internalized into chitin-lined seminal receptacles and the vulva is in fact a secondary aperture. The loss of some or all of the ancestral chitinous seminal receptacle lining was linked to ventral migration of the oviduct connection. Male and female strategies are to maximize gamete fertilization. The most important variable for females is sperm supply, enhanced by long-term storage made possible by the seminal receptacle. To maximize their fertilization rates males must adapt to the structure of the seminal receptacle to ensure that their sperm are close to the oviduct entrance. The major evolutionary impetus for female mating strategies was derived from the consequences of better sperm conservation and the structure of the seminal receptacle. The advantages were all to the females because their promiscuity and sperm storage allowed them to produce more genetically variable offspring, thereby enhancing variation upon which natural selection could act. We extend our arguments to Brachyura as a whole and offer a unifying explanation of the evolution of seminal receptacles, comparing them with the spermathecae found in “Podotremata”: they were independent solutions to the same problem: maintaining spermsperm supply during evolutionary carcinization.