In 2011, Tashima took part in the 10th generation auditions for the idol group Morning Musume. She passed 3 rounds and advanced to the final, but was not chosen to join the group. In 2012, the idol group HKT48 held its 2nd generation audition.
Tashima participated and passed.
She was added to the group as trainee member along with 33 other winners of the audition. She became soon a hot topic throughout Japan, after it was reported that AKB48 and HKT48 producer Yasushi Akimoto commented on her as being "the new Jurina Matsui", a member who joined AKB48"s sister group SKE48 at the age of 11 and was immediately appointed one of the front positions in the next AKB48 single and featured on its cover art
Another surprise came when Tashima, in spite of officially being a trainee member, was appointed the central position in her band"s first original song "Hatsukoi Butterfly", to be released as a B-side on the 29th single by AKB48. In February 2013 it was announced that HKT48 will release its debut single, titled "Suki! Suki! Skip!", and that Tashima, again, was going to hold the central position in the title song.
In HKT48"s second single "Melon Juice", Meru Tashima was again featured in a central position.
This time there were two "centers", her and Mio Tomonaga.