The tourists were killed on December 24 and their murder was followed by a shooting attack against the Israeli embassy in Nouakchott in February.
Mohamed Abadllahi Ould Tiyib, chief prosecutor, said Sidna was detained along with another suspect, Khadim Ould Semane.
"The two most important suspects have been arrested. I have seen them. They are in detention at the gendarmerie," he said.
A security source said three others were also detained.
"Several searches are under way and those detained have been transferred to a high-security location," said the source that asked not to be named.
The spate of attacks, claimed by al-Qaeda's North African branch, prompted the cancellation of the Dakar rally for the first time in its history, triggering concerns that al-Qaeda was extending its operations into Mauritania from neighbouring Algeria and Morocco.