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July 02, 2015

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To reproduce, bizarre flatworm may have sex with own head

June 2, 2015
Courtesy of the University of Basel
and World Science staff

Fail­ing to find a mat­ing part­ner is a dent to an­y­one’s re­pro­duc­tive prospects, but in the flat­worm spe­cies Macros­to­mum hys­trix it might in­volve a real head­ache. 

Zo­ol­o­gists say they have dis­cov­ered the ex­tra­or­di­nary lengths to which this critter will in or­der to re­pro­duce, in­clud­ing ap­par­ently in­ject­ing sperm in­to its own head.

A microscopic image of M. hystrix shows the anterior eyes (1) in the head, followed by the paired testes (2), paired ovaries (3), developing eggs (4), the female genitalia containing three mature eggs (5), and the male genital region in the tail of the worm (6). (Credit: Lukas Schärer)


“As far as we know, this is the first de­scribed ex­am­ple of hy­po­der­mic self-injection of sperm in­to the head,” said Ste­ven Ramm of the Uni­vers­ity of Ba­sel in Switz­er­land, a co-author of the re­port on the find­ings. 

“To us this sounds trau­mat­ic, but to these flat­worms it may be their best bet if they can­not find a mate but still want to re­pro­duce.”

The find­ings are pub­lished in the jour­nal Pro­ceed­ings of the Roy­al So­ci­e­ty B.

The ab­sence of a mate usu­ally spells dis­as­ter for sex­u­ally re­pro­duc­ing an­i­mals. But some “sim­ul­ta­ne­ous hermaphrodites”—an­i­mals who have both male and female sex or­gans at the same time—have de­vel­oped an al­ter­na­tive: self-fer­til­iz­a­tion. It’s an im­per­fect so­lu­tion, as any off­spring pro­duced by so-called “self­ing” are in­bred, but still, in ev­o­lu­tion­ary terms, it’s bet­ter than not re­pro­duc­ing at all.

Pre­vi­ous stud­ies had found that M. hys­trix can switch to “self­ing” when iso­lat­ed from mat­ing part­ners, a be­hav­ior found in many but not all sim­ul­ta­ne­ous hermaphrodites.

The flat­worms are trans­par­ent, so their in­sides are vis­i­ble un­der mi­cro­scopes. Ex­am­in­ing them, the zo­ol­o­gists found that un­der self­ing con­di­tions, when her­maph­ro­ditic in­di­vid­u­als had to use their own sperm to fer­ti­lize their own eggs, the worms had very few sperm in their tail re­gions. This is in stark con­trast to worms kept in a group, which con­tained most sperm in their tails, close to where fer­til­iz­a­tion ac­tu­ally oc­curs. In­stead, iso­lat­ed worms had more sperm in their head re­gion.

This im­plies a rath­er strange in­semina­t­ion route, the in­ves­ti­ga­tors said: by us­ing its needle-like male sex­u­al or­gan, an iso­lat­ed worm can inject sperm in­to the front part of its body, from where the sperm then moves through the body to­wards the eggs. Such a con­vo­lut­ed route is likely needed be­cause, al­though hermaphrodites, there are no in­ter­nal con­nec­tions be­tween the wor­m’s male and female re­pro­duc­tive sys­tems.


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Failing to find a mating partner is a dent to anyone’s reproductive prospects, but in the flatworm species Macrostomum hystrix it might involve a real headache. Zoologists say they have discovered the extraordinary lengths to which this animal is willing to go in order to reproduce, including apparently injecting sperm into its own head. “As far as we know, this is the first described example of hypodermic self-injection of sperm into the head,” said Steven Ramm of the University of Basel in Switzerland, a co-author of the report on the findings. “To us this sounds traumatic, but to these flatworms it may be their best bet if they cannot find a mate but still want to reproduce.” The findings are published in the journal Proceedings of the Royal Society B. The absence of a mate usually spells disaster for sexually reproducing animals. But some simultaneous hermaphrodites—animals who have both male and female sex organs at the same time—have developed an alternative: self-fertilization. It’s an imperfect solution, as any offspring produced by so-called “selfing” are inbred, but still, in evolutionary terms, it’s better than not reproducing at all. Previous studies had found that M. hystrix can switch to “selfing” when isolated from mating partners, a behavior found in many but not all simultaneous hermaphrodites. The flatworms are transparent, so their insides are visible under microscopes. Examining them, the zoologists found that under selfing conditions, when hermaphroditic individuals had to use their own sperm to fertilize their own eggs, the worms had very few sperm in their tail regions. This is in stark contrast to worms kept in a group, which contained most sperm in their tails, close to where fertilization actually occurs. Instead, isolated worms had more sperm in their head region. This implies a rather strange insemination route, the investigators said: by using its needle-like male sexual organ, an isolated worm can self-inject sperm into its own anterior body, from where the sperm then moves through the body towards the eggs. Such a convoluted route is likely needed because, although hermaphrodites, there are no internal connections between the worm’s male and female reproductive systems.