Officials at Uozu Aquarium in Toyama Prefecture are scratching their heads after finding twin baby banded houndsharks in a tank that only held three female adults.

The female twins, named Mana and Kana after well-known twin actresses, were found in the aquarium’s tank on May 8, the officials said. They said it might be a case of parthenogenesis, a type of asexual reproduction in which offspring develop from unfertilized eggs. Although parthenogenesis has been observed in other shark species, the case in the city of Uozu, if confirmed, would be the first ever for banded houndsharks, they said.

The aquarium noted that a baby shark appeared under similar circumstances in April 2013, but they decided not to look into it because it died immediately, the officials said.

According to Kenji Nohara, a lecturer at Tokai University who is well-versed in the shark propagation, females of some shark species have the ability to store sperm in their bodies after mating. But Nohara said such a scenario is unlikely for the sharks at the aquarium, because the last of the three adult banded houndsharks was brought to the aquarium in January 2012, and it would be difficult to assume they mated in the wild and used sperm that was stored four years ago.

Mana and Kana, which have grown to about 30 cm long, are expected to be put on display until the end of this month. The aquarium will decide whether to ask Nohara to conduct a DNA test on them.

Banded houndsharks inhabit waters south of Hokkaido. It is an ovoviviparous fish that produces eggs that develop within the maternal body. A banded houndshark usually propagates in springtime and gives birth to 10 to 20 fry at a time.