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Matrotrophic mothers invest energy in their offspring from fertilization to birth (Trexler and DeAngelis, 2004). Pregnant matrotrophs do not rely on previously collected energy stores, but increase their feeding rate in response to their day to day energy needs (Jonsson). Any energy in excess to the amount of energy the mother requires to satisfy her maintenance costs is used for the nourishment of her embryos (Trexler and DeAngelis, 2004) (Jonsson). Although some offspring of matrotrophic species have an initial dependence on yolk, all matrotrophs depend on maternal nutrients during some stage of gestation (Lombardi, 1996) (Crespi and Semenuik, 2004).
There are 3 main forms of matrotrophy: oophagy, histophagy, and placental. All 3 forms include an initial lecithotrophic phase that is followed by maternal nutrient provision.
- Oophagy: In oophageous reproduction the female’s largest ova are fertilized while groups of the smaller, unfertilized ova are packaged in egg cases as trophic eggs (Joung and Hsu, 2002) (Hamlett, 1998). Once oophageous embryos have depleted their initial yolk reserve they depend on these trophic eggs for food. (Joung and Hsu, 2002) (Hamlett, 1998). The embryos store the nutritive eggs in the cardiac portion of their stomach, resulting in a bloated belly that is easily mistaken for a yolk sac (Wourms,1993).
A unique form of oophagy is adelphophagy or intrauterine cannibalism. In this method of oophageous reproduction embryos supplement yolk reserves by eating trophic eggs and smaller brood mates (Trexler and DeAngelis, 2004) (Hamlett, 1993). The consumption of one embryo by another may be the result of unequal feeding due to the embryos physical position in relationship to the mother’s cloaca (as well as early development of teeth) (Joung and Hsu, 2002) (Wourms, 1993). Embryos positioned further from the cloaca do not get as many trophic eggs as embryos positioned directly anterior to it (Joung and Hsu, 2002). When the mother stops producing trophic eggs in the third stage of pregnancy, the largest embryo eats the smaller embryos (Joung and Hsu, 2002) (Wourms, 1993).
Example: Oophagy and Adelphophagy is the least specialized form of matrotrophic viviparity and is most prevelent in Lamniform sharks such as the Shortfin Mako and the Sand Tiger Shark (Wourms, 1993) (Gilmore,1993).
- Histophagy: Histophageous mothers transfer nutrients to their offspring through placental analogues formed by modifications to the maternal uterine epithelium (Crespi and Semenuik, 2004) (Wourms, 1993). One such modification is vascularized villa called trophonemata that project off the maternal uterine wall and into the mouth and spiracles of the embryo (Hamlett, 1983) (Wourms, 1993). Thin, watery mucus from the uterus hypertrophies into the trophonemata where it becomes a thick, protein-lipid rich histotroph (or uterine milk) (Hamlett, 1983) (Wourms, 1993). The trophonemata secrete histotroph directly into the gut of the embryo (Wourms, 1993).
Example: Histophagy is the most common form of elasmobranch viviparity found in Bat Ray, Butterfly Ray, sawfish (Dulvy and Reynolds, 1997)
- Placental: Placental Elasmobranchs depend on a yolk sac placenta to form a direct maternal-fetal exchange organ that supplements the embryo's limited yolk reserve (Crespi and Semenuik, 2004). Halfway through gestation the yolk sac becomes a placenta that attaches to maternal uterus. The functional change of the yolk sac corresponds to functional changes in the germ layers of the yolk sac (Hamlett, 1983) (Wourms, 1993). Yolk sac endoderm is used for absorption and transfer of nutrients, but placental endoderm simly acts as a boundary layer. The placental ectoderm maintains the respiratory functions it performed when it was part of a yolk sac but picks up nutritional functions as a placental germ layer (Wourms, 1993). The yolk sac placenta is not the same as the chorio-allantoic placenta found in mammals and retiles (Hamlett, 1983). The yolk sac placenta is a chorio-vitelline placenta in which the chorion fuses with delicate connective tissue called serosa rather than to the allantois (Hamlett, 1983).
Example: Smooth dogfish, Blue shark, Hammerhead shark
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