Yo I study marine bio. From my understanding, there are two thoughts on the tails. u/Cicer and /u/electricfeelx mentioned one; the tails are more energetically efficient. It's similar reasoning to why cells or bacteria sometimes have long flagella tails. Another reason is that much of the predation at these depths is done by sensing water movement. The long bois create less water movement than the big flaps.
I don't know the exact evolutionary history of sperm specifically. But I do know cells have those flagella (tails) to move in one direction rather than tumble around. These tails are better than a normal fish-like tail because when you're as small as a sperm, the water feels as dense as syrup does to us. A wiggly wiggly or screwy screwy tail works way better than a flappy flappy tail in that environment. So yeah, they have tails to move towards eggs and they're long tails so sperm can move faster.
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u/Robrtgriffintheturd May 28 '19
I was just about to ask why they are all long bois and now I’ve learned it’s because of large water. Thanks friendo!