NEW YORK — A Discovery network special that speculated about whether a giant prehistoric shark could still exist has drawn a passionate response from viewers and starkly raised the question about the worth of big ratings.

The program, “Megalodon: The Monster Shark Lives,” opened Discovery’s annual “Shark Week” during the weekend. With an estimated 4.8 million viewers, it had the largest audience of any show in the 26 years that Discovery has made “Shark Week” a part of its summer programming, the Nielsen company said.

Yet it drew a heated response online from viewers who said airing a “mockumentary” compromises the network’s reputation.

“It’s the ultimate ‘Shark Week’ fantasy,” said Michael Sorensen, Discovery’s senior director of programming. “The stories have been out there for years, and with 95 percent of the ocean unexplored, who really knows?”

Discovery’s “Shark Week” Web page and Facebook and Twitter sites filled after the show with complaints from fans who objected to the program, saying they were surprised a science-based network seriously discussed the existence of a fearsome creature when there is no evidence it exists today. The “Megalodon” special was reminiscent of two shows on sister network Animal Planet about mermaids, which also got big audiences.

In a blog post, actor Wil Wheaton said he is a regular viewer of “Shark Week” but he was disgusted by the show. He said Discovery owes an apology to viewers who have grown to trust the network for its presentation of science.

“Discovery Channel betrayed that trust during its biggest viewing week of the year,” Wheaton wrote. “Discovery Channel isn’t run by stupid people, and this was not some kind of a mistake. Someone made a deliberate choice to present a work of fiction that is more suited for the SyFy channel as a truthful and factual documentary. That is disgusting.”

Discover magazine’s Christie Wilcox wrote that the show’s “evidence was faked, the stories fabricated, and the scientists portrayed on it were actors. The idea that Megalodon could still be roaming the ocean is a complete and total myth.”

Wilcox wrote that “You used to expose the beautiful, magical, wonderful sides of the world around us. Now, you just make (stuff) up for profit. It’s depressing. It’s disgusting. It’s wrong.”

At the end of the special, Discovery aired three disclaimers. Discovery said that none of the institutions or agencies that appear in the film is affiliated with it in any way. The network also said that “though certain events and characters in this film have been dramatized, sightings of ‘submarine’ continue to this day.” Discovery would not say what events that referred to.