Why Do Bloodworms Occasionally Have a White Skin?
Another frequently asked question: I occasionally notice some bloodworms to have a white skin, or a bloodworm to have a white cap on its head. Is it safe for my fish to eat these white-skinned or white-capped worms?
As explained here, bloodworms are the aquatic, larval life stage of the midge species Chironomus plumosus. Chironomids belong to the superorder of the Endopterygota, so those insects which undergo a complete meta-morphosis between their larval and adult life phases. In the case of Chironomids, 4 visually clearly different life stages can be distinguished:
Of these 4 stages the larval stage is the most important as adult Chironomids are known to hardly forage anymore during their adult phase. The only function the adults have is to find a mate and reproduce. As they hardly take in any energy the adult phase is kept short[1]. In the larval stage the larvae actually pass through 4 different larval stages called instar 1 to instar 4; a batch of mostly instar 4 larvae is pictured below.
Each time a larva molts to the next instar phase it becomes a lot bigger and as such the existence of these four instar phases is one of the reasons why the size of bloodworms varies throughout the year as explained here.
Before starting to pupate the instar 4 larvae secrete a silky material covering their body; these are the ‘white-skinned’ bloodworms you might occasionally notice to be present. Although maybe unsightly, these white-skinned bloodworms can and will be eaten by your fish, aquatic amphibians, invertebrates and reptiles without a problem.
But as most people don’t like to see these white-skinned bloodworms, we utilize a mechanical separation step as one of the cleaning steps during the processing of the harvested bloodworms before they are to be packaged and frozen. However, in case too many harvested bloodworms are found to be preparing for entering the pupa life stage (as for example pictured below), the batch is not processed further but sold off as live food to fish, shrimp and crab breeders.
The white cap an occasional bloodworm seems to be wearing is the head capsule. Sometimes molting to the next larval phase occurs incompletely, whereby the head capsule might stay connected to the larval body. Again, this does not cause the larva to become inedible for its predators.
Manufacturers of fly-fishing hooks actually try their best camouflaging their hooks by letting these resemble a chironomid larva or pupa. Just before the developed larva (well, it is more a soon to emerge adult now) will emerge from the pupa, the pupa floats to the water surface and it is at this point that chironomids are at their most vulnerable life phase because the pupa is completely defenseless and at the mercy of patrolling fish at this point.
But indeed, these white skinned or white-capped bloodworms are quite unsightly and we therefore do our best to remove such larvae if present (depends on the time & water temperature) during processing.
