Shellfish Island

Shellfish Island is a non-canon island in Paradise and the location of the treasure Amanda's father sought. It is actually a giant oyster that became covered in sand and vegetation after lying dormant for years.

Geography
When its shell is closed, Shellfish Island appears to be a small, tropical island covered in beaches and thick vegetation. It had one large, short hill that covered the whole island. In reality, it is an oyster with a thick shell that became covered in sand and plants that grew on it. When it opens, the shell is a pale purple color and has deep ridges. Inside its shell, it holds a giant pearl that is at least three times the size of a normal person.

Shellfish Island is located between Mamanato Island and Samua Island, but neither are ever shown.

Inhabitants
Amanda's father lived on Shellfish Island for some time while trying to locate the island's treasure. Amanda and her siblings Milia and Holy were taken to the island by Maccus and Bonney and stayed there temporarily while hiding out from the Bayan Pirates and searching as well.

Past
Over many years, an oyster became covered in sand and vegetation, taking the appearance of an island.

Amanda's father heard of a great treasure and searched for Shellfish Island with Bayan. After he was betrayed and injured by Bayan, he gave his children an Eternal Pose to an island close to Shellfish Island, a map reading tool, and a map drawn on his eldest daughter's back, hoping that they could find the treasure he searched for.

The children were captured by the Bayan Pirates, and after they escaped with the help of Maccus and Bonney, they traveled to Shellfish Island and began looking for the island's treasure.

Open Upon the Great Sea! A Father's Huge, HUGE Dream!
The Straw Hat Pirates came to Shellfish Island and meet the bunch of escapees, quickly realizing that they are posing as a family and are searching for a hidden treasure. After Amanda and Luffy were kidnapped by Zap, the crew and others left the island to rescue them.

When they returned, they worked together to piece together Amanda's father's tools and clues and discovered how to access the treasure. Luffy, Zoro, and Sanji descended one of the island's caves and bisected three fleshy stalks. These were actually muscles holding the two halves of the shell together, and once they opened, the island's true form and treasure was revealed; it was a giant oyster with a massive pearl inside. Upon seeing the pearl, Amanda realized her father's dream and appreciated his sacrifice.

Trivia

 * Shellfish Island's name was not mentioned at all in the episode special or the credits, but was printed on the front cover of the special's DVD release.