This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Mermaids The Body Found
The poor captive mermaid. The thing's last year or two on Earth is trapped inside a tiny cylinder, unable to see its friends and family, and captive of strange beings who treat it like a petri dish.
The mer-person's Heroic Sacrifice of themself to distract the Megalodon from the rest of its pod. It really doesn't help that he gazes wistfully at who might very well be his mate and child before he slashes his chest.
The confiscation of research materials by South African police, implied under orders from the US Navy.