Short answers, pulled from the story.
There are 40 extant species of dolphins, spread across five families: Delphinidae (oceanic dolphins), Platanistidae (Indian river dolphins), Iniidae (New World river dolphins), Pontoporiidae (brackish dolphins), and the probably extinct Lipotidae (Chinese river dolphin). All belong to the parvorder Odontoceti, the toothed whales.
Dolphins are descended from land-dwelling artiodactyls and split from the lineage that includes the Indohyus approximately 48 million years ago. The earliest cetaceans first took to the sea around 49 million years ago and became fully aquatic 5-10 million years later. Their closest living relatives are hippopotamuses, which share a common semi-aquatic ancestor that diverged from other artiodactyls roughly 60 million years ago.
Bottlenose dolphins develop unique signature whistles during their first year of life that remain consistent throughout their lifetime, functioning as individual names. Dolphins address one another by mimicking a specific individual's whistle. They can recognise the signature whistle of an individual they have not encountered for over twenty years.
Tail-walking in the Port River population of Indo-Pacific bottlenose dolphins near Adelaide, South Australia, originated with a female named Billie in 1988. Billie learned the trick during two weeks of rehabilitation at a marine park and brought it back to the wild. Another dolphin named Wave copied her and passed the skill to her daughters Ripple and Tallula. By 2011, up to 12 dolphins in the group were observed tail-walking, with only females appearing to learn it.
Chile, Costa Rica, Hungary, and India have all declared dolphins to be non-human persons and banned the capture and import of live dolphins for entertainment. India also designated the dolphin as its national aquatic animal specifically to protect the endangered Ganges river dolphin.
Queensland's shark culling program, in operation since 1962, has killed roughly 50,000 sharks and thousands of dolphins as bycatch. Since 2014, at least 32 dolphins have been killed in Queensland's shark nets and drum lines. A shark culling program in KwaZulu-Natal has killed at least 2,310 dolphins.