It was a question many Herald readers wanted answered - what the heck is a pseudorca?

The black marine mammals hit the headlines in a dramatic way yesterday after a boatload of tourists saw a killer whale flick a pseudorca high into the air, break its back and eat it and its calf.

The attack occurred at the Black Rocks, about 7km from Paihia, after a pod of eight killer whales - which are orca without the "pseud" - chased more than a dozen pseudorca.

The Herald yesterday asked scientists for more information about the creatures and discovered the pseudorca - sometimes known as black fish or false killer whales - are members of the Delphinade (ocean dolphin) family, with killer whales and other dolphins.

Whale and dolphin researcher Dr Ingrid Visser said they got their name because their skulls were similar to orca.

"It [the pseudorca] is similar to a pilot whale but the skull closely resembles an orca.

"So when they first found the skulls they thought it was an orca.

"When they found out it wasn't, they called it a false orca or pseudorca.

"They look more like a pilot whale ... with a more pointed face."

Pseudorca live in warm waters in the tropics and subtropics.

Dr Visser said they were seen every year in Northland.

Dolphin researcher Dr Karen Stockin said comparatively little was known about the sociable creatures, which sometimes formed mixed groups with pods of bottlenose dolphins.

The squid- and fish-eaters were listed in New Zealand as "not threatened" but there were no estimates of their population, she said.

Adults were between 4.5-6m long and newborns 1.5-1.8m long.

In rare cases pseudorca have been known to breed with bottlenose dolphins to make wholphins (so called even though both species are dolphins).

But Dr Stockin said it was not known whether that happened in the wild or only among animals in captivity.

In 2001, Department of Conservation workers in Gisborne took a skin sample from a beached pseudorca, which was used to show that their meat was being sold in Japanese markets.