Different type of Embryonic development 1813310 1 Embryonic

Different type of Embryonic development 1813310 徐楠 1

Embryonic development types • Species can be assigned to categories based on where embryonic development occurs and whether developing embryos are dependent on maternal versus yolk provisioning. • According to this, the embryonic development can be divided into three types: oviparity , viviparity, ovoviviparity. 2

Oviparity 卵生 3

Oviparity Characters: • The mother releases eggs • Rely on yolk for nutrition • Doesn’t have a high survival rate. Chimaeras, some sharks and skates, and most bony fishes. 4

Ovoviviparity 卵胎生 Scorpaenidae: benthic, oviparous or ovoviviparous, carnivorous 5

Ovoviviparity Characters: • Develop inside the mother • Depend on the yolk that was laid down during oogenesis Some sharks, scorpaenids, zoarcid eelpouts, and arguably, guppies. 6

Ovoviviparity • Live-bearing characterizes about half of the 1200 species of Chondrichthyes and about 500 (2%) of the 25, 000+ species of bony fishes (most of those with internal fertilization). • However, many intermediate conditions exist involving both yolk and maternal secretions, blurring the distinctions and making ovoviviparity difficult to define. • Although live birth usually follows internal fertilization, the two are not synonymous. Many elasmobranchs have internal fertilization but lay eggs which develop for many months outside the female. 7

Ovoviviparity *To be or not to be, that is the question! • Some live-bearing sharks have evolved an additional form of nourishment. • After about 3 months, yolk reserves are exhausted and the young then feed directly on eggs ovulated by the female (oophagy). • Oophagous sharks include Threshers, Whites, Makos, and Sand Tigers. . Sand Tigers have carried this one step further. The first embryo to consume its yolk then turns on its siblings and eats them (embryophagy) before assuming an oophagous existence. At birth, Sand Tiger litters are composed of two large (1 m long) young, one in each uterus. 8

Viviparity 胎生 9

Viviparity Characters: • Develop inside the mother • Have an exchange of nutrients with the mother • Placental exist The Carcharhinidae, as well as the closely related hammerheads (Sphyrnidae) 10

NO! 11

Viviparity *A special structure: • Yolk is consumed and then the spent yolk sac attaches to the uterine wall to form a yolk-sac placenta. • The stalk of the yolk sac, which is attached to the embryo between the pectoral fins, forms an umbilical cord that transports nutrients and oxygen to the embryo and carries metabolic wastes to the mother. • In a construction strongly analogous to the mammalian condition (although involving different embryonic tissues). 12

Viviparity • In some sharks, such as the Sharpnose and hammerheads, the umbilical cord diversifies further and develops appendicula or outgrowths that serve as additional sites for exchange of materials, including uptake of nutrients in histotroph or “uterine milk” secreted by special cells in the wall of the uterus. • • In the myliobatoid stingrays and manta rays, nourishment is solely via ingestion of uterine milk without a placental connection (uterine viviparity). umbilical cord outgrowths Uterine milk may be absorbed through the skin and mouth and also via modified gill filaments that exit from the spiracle and gill slits of the developing embryo. placenta 13

Let’s be organized… A summary of embryonic development and nutrition in chondrichthyans 14

“ “Virtually every known form of vertebrate possible maternal–fetal relationship may be found in fishes”. 15