The trident is the weapon of Poseidon, or Neptune, the God of the Sea in classical mythology. The trident may occasionally be held by other marine divinities such as Tritons in classical art, and trident may be depicted in medieval heraldry as well, sometimes held by a merman-Triton.

The trident symbol shows the cross pierced by a U. Thus it takes on the symbol of the cross.  The U shape represents the chalice or cup into which the Water  - spiritual input - is poured.