By Matt Walker
Editor, Earth News
An artist's impression of the live-bearing European eelpout
A fish that suckles its young has been discovered by biologists.
More surprising, the young suckle while still within their mother's body.
Mammals are well known to suckle their offspring, passing nutrients via milk supplied by mammary glands; a process that occurs after the young are born.
But the European eelpout is the first fish known to do so, using ovarian follicles to suckle its offspring, which explains why it can give birth to such large, live baby fish.
Details of the discovery are published in the Journal of Experimental Marine Biology and Ecology.
The European or viviparous eelpout (Zoarces viviparus), also known as the viviparous blenny, is found in near coastal waters throughout large parts of Europe, from the southern parts of the English Channel, around the British Isles into the Baltic Sea and north to the White Sea.
Among fish it has one of the longest known pregnancies, lasting approximately six months.
It gives birth to live young, and unusually it does so during winter when water temperatures are extremely cold.
Other fish, such as guppies and mollies, also give birth to live young.
But they tend to have short gestation times lasting a few weeks or less, and their embryos feed on yolk from egg sacs within their mother's body.
So it has been a mystery how the European eelpout feeds its young, as egg yolk would not sustain embryos that develop over six months.
"The exact mechanisms behind the development of the young has been a topic of fairly intensive research for the past half century," says Professor Peter Skov of the University of Copenhagen.
Now a team of researchers from Denmark, led by Prof Skov, has found out how the eelpout does it.
They have discovered that eelpout embryos suckle from their mothers.
"This has never previously been documented in the eelpout," says Prof Skov.
The embryos actually suckle from ovarian follicles, ingesting nutrients and gases from these internal structures.
In mammals, follicles in the ovaries produce eggs, which then mature as they are passed towards the uterus, where fertilisation takes place.
The egg develops into an embryo in the uterus gaining nutrients and oxygen via the placenta by way of the umbilical cord, before continuing to gain nourishment after birth by suckling.
"In the eelpout, the egg is also produced and matured in the ovary, by the follicles as with mammals," Prof Skov told the BBC.
"But fertilisation takes place here as does the entire gestational period, since the eelpout has no uterus."
After depleting the egg's yolk reserves, the eelpouts attach their mouths to an ovarian follicle, which has a canal in its tip via which fluid and nutrients can flow.
This follicle fluid is rich in proteins, fatty acids and glucose.
It is also saturated in oxygen, which helps ventilate the gills of the developing fish.
Each embryo latches onto a single follicle.
"This ensures an equal distribution of nutrients," says Prof Skov.
The researchers made the discovery serendipitously, as they were researching the fish's physiology.
Prof Skov was dissecting a recently deceased pregnant female, when he came across the embryos already attached by their mouths to the ovarian follicles.
"We were extremely excited to have made this discovery," he says.
"Our work changes the way we think about reproduction in the eelpout, and this may extend to other fish."