Xenotoca eiseni, a Viviparous Teleost, Possesses a Trophotaenial Placenta for Maternal Nutrient Intake

Abstract

Over 500 species of teleost fish have been identified as live-bearers. These fish breed by internal fertilization, and, in some species, the embryo weight increases during pregnancy. This indicates that these fish likely possess the specific machinery required to absorb maternally derived nutrients. Approximately 170 viviparous species are included in the order Cyprinodontiformes. This chapter focuses on a viviparous teleost species, Xenotoca eiseni, which belongs to the family Goodeidae. Members of the family Goodeidae have a unique structure called the “trophotaenial placenta,” which is a branching, ribbonlike structure that extends from the perianal region of the goodeid embryo. The trophotaenial placenta is a hindgut-derived pseudoplacenta that allows the absorption of maternal nutrients during the prenatal stage. The trophotaeniae preliminarily regress when the embryo is born. Because the offspring can ingest food orally soon after birth, the trophotaeniae become unnecessary. Immunohistochemistry indicates that caspase-3-activated cells with fragmented nuclei are present in the regressed processes of the fry immediately after birth. This finding suggests that the trophotaeniae are rapidly resorbed by apoptosis during the last phase of the pregnancy. Such prenatal regression of pseudoplacentae has not been reported in other viviparous vertebrates. Therefore, the small teleost might be a convenient model for understanding the diversity of viviparity in vertebrates.