The bizarre creature was captured in a trawler net by mistake and was already dead when it was pulled aboard.
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After taking a few pictures of it, the fishermen apparently threw it back into the sea because they thought they may get in trouble with fishing authorities.
However, experts are now studying the photos with huge interest because it appears to be the first ever case of conjoined twin porpoises ever discovered.
Twinning isn't unheard of in marine life but it's rare that such a specimen ever finds its way into human hands. According to experts at the University of Rotterdam, this porpoise probably died shortly after it was born.
There are hairs on the upper lip, which should have fallen out after it was born - indicating it died very young.
Twins in general are very rare in porpoises, because of the size of the mother's body.
The porpoise was caught in a trawler net
"Normal twins are extremely rare in cetaceans," Erwin Kompanje, of the Erasmus MC University Medical Center in Rotterdam, told New Scientist.
"There is simply not enough room in the body of the female to give room to more than one fetus."