Blue Planet II's bird-eating fish horrifies viewers

If you need an idea for a Halloween costume, why not dress up as a giant trevally? Last night the enormous fish provided one of the most frightening moments of the TV year, when it swallowed a bird mid-flight in the first episode of Blue Planet II.

Viewers have described themselves as "traumatised" by the incident, in which the 40kg fish devoured a sooty tern in a remote stretch of water in the Seychelles. 

Meanwhile, a Twitter user called "Spookymath" took the long view, writing: "Just wait a few million years until these motherf------ evolve and start snatching planes out of the air."

One of the show's photographers, Miles Barton, recalled capturing the fish's "extraordinary behaviour" in an interview with Radio Times. According to Barton, the attack took place too quickly for him to see it clearly at the time. "The fish manages to project itself a metre out of the water and then expands its mouth to the size of a small football to totally encompass the bird," he said. "It happens so fast you don’t really see it until you play it back."

Giant wrasse usually hunt alone, but in last night's episode they descended on the area en masse, due to the "abundance of prey", in the words of the programme's presenter David Attenborough.

The predatory fish (caranx ignobilis) seemed to be ignobilis by name and ignoble by nature, but at least one viewer acknowledged that both the wrasse and the bird were simply fulfilling their place in the food-chain.

The episode, which was praised by The Telegraph's Gerard O'Donovan as "mesmerising" in a five-star review, also featured another remarkable wrasse: a female Asian Sheepshead Wrasse was shown changing into a male. Scientists believe that the fish, also called the kodubai, switches its gender in order to pass on its genes more easily.