Yes, MASSASAUGAS is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, S 1, S 1, A 1, S 1, A 1, U 1, G 2, A 1, S 1.

Is it a valid word?

MASSASAUGAS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

small pygmy rattlesnake
pygmy rattlesnake found in moist areas from the Great Lakes to Mexico; feeds on mice and small amphibians

Also: ground rattler, sistrurus miliaris, massasauga rattler, sistrurus catenatus.