What does MLDB stand for?

What does MLDB stand for?
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Last’ week winner was Allen McCloud who correctly stated that dolphins are positively buoyant at the surface but negatively buoyant at depth. Now, for this week’s quiz: Concerning a specific part of a dolphin’s anatomy, what does the acronym MLDB stand for? Think you know the answer? Surf on over to thedolphnipod.com and click on Dolphin Quiz – leave your answer in the comments section. Winners will randomly be chosen from the correct answers, and will be announced on next week’s show.