The aquatic larvae of a midge, they're called 'bloodworms' because of the similarity in appearance to human blood - this is n accident, they contain a hemoglobin molecule similar to our own blood.
Some people are allergic to this and can still use them as fish food only if they don't come in contact with them.
This is the only fish food that contains selenium, an essential trace mineral. The lack of sufficient selenium can cause reproductive failures (eggs will fungus) and note that many fish are from areas of high selenium.
Bloodworms can only contain decent levels of selenium if harvested from areas where it is abundant. Distribution of the mineral is spotty. Only Germany and Finland have decent amounts of it everywhere; it occurs naturally in Germany, Finland has been adding it to all farmlands for some time now.
This important food is commonly fed from frozen as a staple, but can be collected, usually in the spring.