Bizarre propagation of bone eaters clarified

Worm females living in whale bones hold miniature males as harem

Adult zombie worm - female - isolated from the bone. © Miyamoto et al. / The Science of Nature
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It has been known for some time that the bone-eating Osedax worms are strange creatures. Now it turns out that their reproduction is abundantly unusual: males of this "zombie worm" get no chance for an independent life, instead they arise only when there are larvae in excess anyway. Then some of them settle on the fallopian tube of the female and stay there all their lives. This particular strategy allows the worms life on their unusual site - sunken whale bones on the seabed, according to the researchers in the journal "Science - The Science of Nature".

The Zombie worms are enigmatic sea creatures: Since their discovery in 2004, researchers have been trying to get behind the habits of animals known as bone-eater worms. So far, not much is known about them, except that their only food source - at the same time the place of residence of the animals - are sunken whale bones on the seabed. Noteworthy: The animals belonging to the genus of annelid worms have neither a mouth nor its own digestive system. Instead, they live in a close dependency relationship with marine bacteria. These provide them directly with the nutrients from the bones. The microbes live in branched foothills of the worm body, with which this anchored in the whale bone and at the same time provides the bacteria access to the bone.

Little men live in the fallopian tube

But not only the strange eating habits of the marine worms gave the researchers long puzzles, their sexual behavior is anything but usual: So it has been suspected for some time that the decision whether the larvae males or females arise, depending on the environmental conditions. In addition, males and females hardly resemble each other in appearance. The male representatives are microscopically small relative to the few centimeters females. But that's not all: The little men are usually found directly in the fallopian tubes of females and thus have no independent life.

Based on these observations, the researchers of the "Japan Agency for Marine-Earth Science and Technology" now particularly the larval development to get to the bottom. In laboratory experiments, they examined to a few on the seabed caught specimens of the zombie worm in more detail. They took embryos from the larval mucus of some females and offered the resulting larvae a bone as a nursery. Once the young worms had settled on the bone, the researchers cut the bones to gain an insight into the hitherto secret life of the worms.

An Osedax japonicus colony on a whale bone. Miyamoto et al. / The Science of Nature

Males grow only when there are many larvae

The results confirmed that the larvae are free-floating during the first ten days - until they settle on a bone, if present, and become sedentary within one day. The young seaworms are helped by secreted mucus, which allows them to form their tubes into the bones. A short time later, limbs and female genitalia develop. Because only the females nest in the bones, males do not settle down, but live as a harem in the trunk system of a female. display

In the researchers' experiments it was also shown that male zombie worms only occur if a large number of free-swimming larvae were given to already sexually mature females. Only then did some of the larvae become male. These larvae did not nostril themselves into the bones, but immediately sat down on the fallopian tubes of mature females. This is where their metamorphosis to dwarf males begins, some of which even migrate into the fallopian tubes, while others persevere before the opening. Six weeks after having received the bone, the females lay the first fertilized eggs using the male harem that lives in and around their fallopian tubes.

The researchers are convinced that the special combination of early free-swimming larvae and the sexual development of the animals allows zombie survivors to survive on food sources that are rich in food but highly isolated. According to the scientists, the speed of maturity development in particular is a key to the effective and rapid development of the food source bone. However, what conditions in nature are necessary to induce the development of male zombie warmers remains to be studied. (Natural Sciences, 2013; doi: 10.1007 / s00114-013-1024-7)

(Natural Sciences, 13.03.2013 - KBE)