A team of biologists from the University of Copenhagen, Denmark, have discovered a rare fish that suckles its young while they are embryos inside the mother’s body . Mammals are known to suckle their young after birth and pass nutrients to them in the milk supplied to the mammary glands. But nothing similar has ever been seen in fish.
The species in question is thezoarcido (Zoarces viviparus) and, as the researchers explain inJournal of Experimental Marine Biology and Ecology, uses ovarian follicles to breastfeed its offspring. This could explain why this species can givegive birth between 30 and 400 pups between 3 and 5 centimeters long, a relatively large size, which are usually born in winter, when the waters are extremely cold.
In mammals, ovarian follicles produce eggs that then mature as they pass into the uterus where fertilization occurs. Once in the uterus, the embryo obtains nutrients and oxygen in the placenta through the umbilical cord, and after birth it continues to nourish itself with breastfeeding.
In the zoarcid, the ovum is also produced and matured in the ovary with the follicles, as in mammals, but fertilization and the entire gestational period take place right there, because these fish lack a uterus. After the reserves of the gestational sac have been depleted,zoarids attach their mouths to an ovarian follicle, which has a channel at its tip through which fluids and nutrients can flow. This follicular fluid is a rich source ofproteins, fatty acids and glucose. And there is alsooxygen saturated, which helps ventilate the gills of the developing fish.
The zoarcid lives near the coastal waters of large parts of Europe, from the English Channel to the Baltic Sea and the White Sea in the Arctic Ocean. Among fish, it is the one that undergoes one of the longest pregnancies, which can last up to six months.
