Here the ventral grooves are expanded to allow the enormous amounds of water to enter the mouth
The blue whale eats mostly krill, which are extremely small, shrimp-like animals. The blue whale starts to do this about 6 months old. When the blue whale is at it's peak of feeding during the season, it can consume more than 4-6 tons of krill in one day. To consume this much krill, the ventral grooves expand to a point to let the blue whale take in 50 tons of water in one gulp. Then once the krill are inside the mouth, the water is forced out by a perfect fit of the upper ridge of the mouth and the tongue and the baleen plates keep the krill inside to be swallowed by the blue whale.