In Polynesian tattoo designs, shells are very common in many designs, especially turtle shells. Turtle shell is a symbolization of turtle, which is a very important sea creature in all the cultures of Polynesian triangle. It has more meanings than other symbols. Some complex turtle designs could have quite a lot of meanings, according to its elements embedded in. Basically, turtle shells symbolize longevity, wellness, fertility and peace.
Other shell symbols are mainly sea shells. They often have many variations and styles. Seashells are a main source of works of art. Sea shells often symbolize shield, protection and intimacy. Bivalve shells symbolize couples and marriage. Compared to other Polynesian symbols, sea shells are not commonly seen in many tattoo designs.
Below are several design and photo samples of turtle shells and sea shells.
(From Tattootribes)
http://www.apolynesiantattoo.com/polynesian-tattoo-meanings/polynesian-tattoo-symbols-meanings-shells.html