An organism’s method of reproduction is the result of a tug of war battle between mother and offspring over a limited energy supply. It is the mother’s goal to allocate an equal amount of nutrients to all her offspring (past, current, and future) while the current offspring tries to increase the maternal investment they receive (Crespi and Semeniuk, 2004). The two major solutions to this problem are lecithotrophy and matrotrophy. The trophic relationship between a lecithotrophic mother and its offspring is limited to the energy the mother invests in the yolk before fertilization (Trexler and DeAngelis, 2003). While lecithotrophic mothers make a single energy investment, matrotrophic mothers invest energy in their offspring throughout gestation (Trexler and DeAngelis, 2003). When the embryo’s yolk reserve runs out, the matrotrophic mother uses structural adaptations to establish a direct nutritive relationship with her offspring (Lombardi, 1996) (Wourms, 1993).
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Images from: www.monsterfishkeepers.com and www.cosmomagazine.com
The elasmobranchs are the oldest extant jawed vertebrates, consisting of sharks, skates, and rays (Wourms, 1993). This clade can be viewed as the guinea pig clade that experimented with multiple forms of reproduction, including 5 different types of viviparity (Crespi and Semeniuk, 2004) (Dulvy and Reynolds, 1997). Elasmobranchs were the first organisms to use forms of vivparity and placental nourishment used by more derived vertebrates (Lombardi, 1996).
40% of sharks, skates, and rays have maintained the ancestral form of reproduction and lay eggs that develop external to the mother’s body (oviparity) (Crespi and Semeniuk, 2004). The other 60% of Elasmobranchs have transitioned to various forms of viviparity or live birth. Out of the 12-15 transitions from oviparity to vivparity seen within this clade, 10-11 of these transitions were to lecithotrophy and 4-5 of these transitions were to matrotrophy (Dulvy and Reynolds, 1997). Elasmobranchs are a perfect model species for understanding the evolution of vertebrate reproduction as their diversity allows us to evaluate the advanatages and disadvantages of each method.