Ion and Osmoregulation in Prenatal Elasmobranchs: Evolutionary Implications |
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Authors: | KORMANIK GREGG A. |
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Affiliation: | Department of Biology, University of North Carolina at Asheville Asheville, North Carolina 28804 The Mount Desert Island Biological Laboratory Salsbury Cove, Maine 04672 |
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Abstract: | The elasmobranchs represent a fascinating series of experimentsin the evolution of maternal support for developing embryos.In oviparous species, eggs are enclosed in a tough, fibrouscapsule. The capsule is very permeable and the embryonic tissuesare bathed in a solution ionically similar to sea water withinhours of oviposition. In the primitively viviparous speciesSqualus acanthias, early embryos in egg capsules are retainedin utero and are bathed in a solution similar to maternal plasma.Several months into the 22 month gestation period the embryosare capable of independent iono- and osmoregulation in a uterinesolution that resembles sea water. Embryos of more advancedviviparous species develop in a solution that is similar tomaternal plasma. Iono- and osmoregulation by these embryos wouldappear to beminimal. It is clear that in the oviparous elasmobranchs,the ability of the egg/embryo to maintain salts and urea atappropriate levels is present at the earliest stage of development.The ability of prenatal elasmobranch embryos to iono- and osmoregulatewould allow the evolution ofa diverse array of reproductivestrategies in the elasmobranchs. |
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