You Do Need power plug adapter while traveling from Saint Lucia to Tuvalu.

Here is Why You Need An Adapter

The outlet and plug type used in Tuvalu is I and in Saint Lucia is G. This is how they look like.

Saint Lucia

In Saint Lucia, outlets of type G are in use.

Type G

Tuvalu

In Tuvalu, outlets of type I are in use.

Type I