Jaws is a 1975 Steven Spielberg movie about a small town being terrorized by a huge shark. Finding Nemo is a 2003 Pixar movie about a clownfish searching for his missing son. Apart from sharing watery settings and shark antagonists, the two films bear little similarity, though Finding Nemo does feature a callback to Jaws when the aforementioned fishy predator takes a torpedo to the mouth, mimicking the way that Spielberg's beast was slain.
This reference point got Redditor u/CinnaSol thinking, and from there they extrapolated an entire shared universe theory that posits that the shark in Jaws and the shark in Finding Nemo are father and son. Here's what they had to say about it in the r/FanTheories subreddit:
"On the set of Jaws they had multiple shark models for filming. All of them were named "Bruce" after Speilberg's lawyer. This could be more than just an homage to the film Jaws -- Bruce [in Finding Nemo] could be named after his father. Bruce in Finding Nemo never knew his father. It's possible he could've been closer to his mother. In fact, it's apparently fairly common for great whites to leave their infant children behind since they can swim at birth.
"Bruce in Finding Nemo has a support group of sharks where their entire mantra goes against being mindless killing machines. Maybe because his mother told him about how monstrous his father was? Even by great white standards. We don't know why Brce formed a support group like this. It could be that he's avoiding trying to be like his dad."
The TL;DR version is that Jaws' Bruce abandoned his newborn son and breeding partner to pursue a career of hunting skinny-dipping teens in New England. Mama shark attempted to steer Bruce Jr. down a better path in life before dying or being outgrown by him.
For those who didn't already know, the name of the shark in Jaws was indeed Bruce, so that key aspect of this wacky idea does check out. (Speilberg's other pet name for the animatronic creature was "the great white turd" because of how much real-life strife the thing caused him during the difficult filming process.) Since Finding Nemo wears its reference points on its sleeve -- including nods to Deep Blue Sea and -- it would be more surprising if Bruce wasn't named after its forebear from Jaws.
The other reason for the name choice is that calling an Australian shark "Bruce" is intended as an Aussie in-joke. In Australia -- the nearest land mass to where Finding Nemo takes place -- Bruce is a popular name often associated with stereotypical manliness. There's even a Monty Python sketch on the subject.
One Reddit commenter has an alternative naming explanation. "Maybe Bruce (or his parents) named him that in honor of the famous man-eating shark. After all, Bruce's whole thing is "fish are friends, not food." He doesn't say anything about humans. Maybe he's inspired by this shark who proved that rather than feeding on those you share a community with, you should feed on the land-dwellers who regularly invade your community." That would make Finding Nemo's Bruce the Ocean Master of the Pixar universe, right?
But, as another commenter points out, location could be the killer here, as Jaws takes place off the coast of Northeast America, oceans away from Finding Nemo's Australian setting. But, the theory's author has a pretty solid comeback to plug this potential hole. "The whole horror behind Jaws is that [the shark isn't] supposed to be there. Massachusetts isn't normal migration for a great white, I don't think. But I'm pretty sure we're led to believe that the shark isn't a normal shark to begin with. [...] My theory just states that before he got all the way out there, he mated first."
So, there you have it: Finding Nemo's Bruce is either the son of an infamous mass murderer trying to distance himself from his family's legacy, or he carries the mantle of a radical Fish Justice Warrior in the hopes of living up to his namesake. You'll probably never watch the Pixar classic the same away again.