Fish, Amphibian, Bird and Reptile Placentation
The fishes are non-amniotes, which have evolved a wide variety of modifications for successful viviparity (Amoroso 1960; Hogarth 1976; Wourms 1981; Mossman 1987). The most widely used system is an increased secretion of nutrients into the ovarian or uteri
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Fish, Amphibian, Bird and Reptile Placentation
3.1
Fish
3.1.1
Introduction
The fishes are non-amniotes, which have evolved a wide variety of modifications for successful viviparity (Amoroso 1960; Hogarth 1976; Wourms 1981; Mossman 1987). The most widely used system is an increased secretion of nutrients into the ovarian or uterine cavity and absorption via the fetal gut or skin surface, but a number of genera have produced specialized structures, both embryonic and extraembryonic, some of which closely parallel amniote placentas.
3.1.2
Classification
Fishes: Class Chondrichthyes (cartilaginous fish) Subclass Elasmobranchii, selachians About half the 100 families have viviparous members; only two of these families (Carcharinidae and Syphonidae, including sharks, skates and rays) have either a yolk sac placenta (Fig. 3.1) or trophonemata. The latter are long extensions of the uterine epithelium producing abundant secretion (Wourms 1981). Class Osteichthyes (bony fish) Subclass Actinopterygii This includes more than 99% of all living bony fish. Fourteen out of the 425 families are viviparous, and five of these have structures ranging from fin elaborations, gut extensions (trophotaeniae) and expansions to extra-embryonic pericardial sac elaborations analogous to the vertebrate amnion and chorion (Fig. 3.2).
P. Wooding, G. Burton, Comparative Placentation, © Springer-Verlag Berlin Heidelberg 2008
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3 Fish, Amphibian, Bird and Reptile Placentation
Table 3.1 Dry weight increases during gestation in fishes % Change in dry Reproductive Species weight of conceptus pattern Chondrichthyes; Cartilaginous fish Oviparous Scyliorhinus caniculus-20 Viviparous Squalus acanthias Mustelus mustelus Dasyatis violacea Mustelus canis Gymnura micrura Odontaspis Taurus
−40 370 1,600 1,050 4,900 1,200,000
Scoliodon laticaudus
5,800,000
Uterine secretion
Oviparous
None
Yolky egg Yolky egg Trophonemata Yolk sac placenta Trophonemata Intrauterine cannibalism Yolk sac placenta and appendiculae
Serous Mucous Lipid Mucous Lipid
Osteichthyes; Bony fish Oviparous Salmo vivideus Viviparous Poeciliopsis monacha Poecilia (lebistes) sp.
−40
Oviparous
−40 0
Heterandria formosa
4,000
Anableps dowei
800,000
Ameca splendens
15,000
Embiotoca lateralis
20,400
Jennynsia sp.
24,000
Yolky egg Yolk, small pericardial sac Large smooth pericardial sac, simple Follicular placenta, large specialized pericardial sac Villous follicular placenta and Trophotaeniae from anus Fin extensions, gut expansion and trophonemata from ovarian wall
Wourms (1993)
Data from Wourms (1981).
It is clear that fish viviparity has evolved many times in many different genera (Amoroso 1960; Wourms 1981) and even three times in a single genus (Reznick et al. 2002). A decrease in the amount of yolk after retention in the body of the mother necessitated an efficient system of nutrient transfer. In fishes, the placenta is only one, and not necessarily the most efficient, solution to the problem. In fish physiology a rough measure of the efficiency of v
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