If you’re a shellfisherman and it’s low tide, you’d better be out on the flats. ‘‘Once you pull on your oil gear, it’s pretty much business as usual when you go out,’’ says Pat Woodbury, who with his wife, Barbara, works a 5-acre shellfish grant on Indian Neck in South Wellfleet, where they plant hundreds of thousands of tiny seed clams each year. He admits that when the two of them start the season in March, they’re ‘‘wimpy.’’ Barring problems like red tide, the Woodburys supply about 30 restaurants in the Boston area with their littleneck clams. Out on the flats, each late spring day brings something new as the land warms up: spawning mud snails, scuttling crabs, the return of the laughing gulls. And Woodbury says there are other perks to those days when low tide falls especially early. ‘‘You can take a left turn on Route 6 and there’s almost no traffic out.’’