They are facing charges of hijacking, kidnapping and armed robbery after they allegedly seized the yacht and its crew, Jean-Yves Delanne and his wife Bernadette, both aged 60, off the coast of Somalia in 2008.
They face life in prison if convicted.
The six, aged between 21 and 35, were captured and flown to France after French special forces stormed the yacht, the Carre d'As IV, and rescued the couple. A seventh suspect was killed in the raid.
One of the suspects was a minor at the time of the crime but the court granted the defence's request to hold the trial in public and not behind closed doors.
The suspects had reportedly demanded a ransom of $2 million (€1.5 million) for the couple's release.
But in the French courtroom on Tuesday only one of them admitted to taking part as an "underling" in the hostage-taking.
"I was in such a financial situation, I have six children, it was then that I crossed paths with someone who recruited me," said Ahmed Hamoud Mahmoud, a fisherman who is accused of being one of the leaders of the operation.
Another Somali suspect claimed he himself was "kidnapped" by pirates who commandeered his boat to carry out the operation.
One suspect spoke of being grabbed by pirates when he got into trouble in the Gulf of Aden en route to Yemen to look for work, while another said all he did was cook the food.
Their trial, which resumes Wednesday and is expected to last to November 30th, marks the first time France has brought alleged Somali pirates to court.
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Somali suspects in three other cases are currently awaiting trial.
Dozens of ships, mainly merchant vessels, have been seized by gangs off Somalia's 3,700-kilometre coastline in recent years.
The pirates travel in high-powered speedboats and are armed with automatic weapons and rocket-propelled grenades. They sometimes hold ships for weeks until they are released for large ransoms paid by governments or owners.