Aquarium

Each answer has a well-known mother:

• Finding Nemo — DEBBIE REYNOLDS is the mother of CARRIE FISHER.
• Tuna — REBECCA is the mother of ESAU.
• Seel — DAENERYS TARGARYEN is the mother of DRAGONS.
• Pisces — PENELOPE is the mother of TELEMACHUS.
• Mahi-Mahi — ELASTIGIRL is the mother of JACKJACK.
• Sailor Swifties — GOLDE is the mother of TZEITEL.
• Sharks — NECESSITY is the mother of INVENTION.
• Dolphin — KIM KARDASHIAN is the mother of NORTH WEST.
• Shellfish — THE STATUE OF LIBERTY is the mother of EXILES.

Fitting the mothers into the boxes at the bottom yields the following completed grid:

KIMKARDASHIAN
PENELOPE
DEBBIEREYNOLDS
DAENERYSTARGARYEN
ELASTIGIRL
NECESSITY
REBECCA
GOLDE
THESTATUEOFLIBERTY

The highlighted column spells SPONGEBOB, whose mother is MARGARET SQUAREPANTS.