Student or Learner
Unlike land animals, fish mostly use their muscles for quick bursts of activity, such as fleeing from an enemy.
Since, they don't need to store oxygen in their muscle cells, they have little myoglobin. That's why their meat looks white. But some fish, such as tuna and mackerel, are more active than others. Therefore, those fish have red meat.
Is this comma a mistake or does it contain a meaning? I have never seen such structure.
The comma should not be there. "Since" here means "Because".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.