Transparent larvae of the phantom midge (Corethra).
Corethra (Corethra plumicornis) (Chaoborus crystallinus) is a transparent mosquito larva reaching a length of 16 mm. Corethra is translucent, with a spindle-shaped body, and has a clearly defined head with well-developed jaws. It reaches a length of 10–12 mm. It inhabits shallow areas of large deep lakes, small marshy and peat bogs, or bodies of water rich in humus and silt, as well as puddles. It usually lives in the water column and rarely descends to the bottom.
Corethra stays in the water column thanks to its swim bladders. It has two pairs of them: one lies directly behind the head, and the other is on the third body segment from the eye. The bladders are curved in the shape of arcs and contain gases secreted by the organism. It moves with lateral strokes of its body, additionally assisted by a caudal fin consisting of a whole bunch of downward-hanging hairs. The color and shade of Corethra’s transparent body depend on the food base of the reservoir and can vary from a greenish and yellow tint to a dirty red.
Corethra begins to pupate with the onset of warmth, turning into a pupa that has two ear-like appendages on its head, and its abdomen ends with a fin-like locomotion organ. This pupa stays near the water surface and sinks deeper only when disturbed. The adult midge emerges from it after 12 days and has a size of 6 mm. It feeds on plankton and small crustaceans, which it catches with its ladle-shaped mouthparts.
Corethra is a predator, feeding on small crustaceans (Daphnia, Cyclops) and young larvae of other mosquito species, not shunning fish fry either. Therefore, it is used to feed already grown fish. This should be taken into account, as live Corethra in an aquarium poses a danger to fish roe and newly hatched fry. Adult midges do not feed on blood.
Corethra is caught with a net throughout the year in shallow areas of clean bodies of water, in small forest puddles, forest lakes, and shallow bogs surrounded by birch forests. The Corethra fishing season begins with the onset of cold weather. It is better to fish in the cold season, even under the ice; in the warm season, the larvae turn into pupae and then into midges.
These are wonderful, unpolluted food organisms found only in clean water. It is not a vector of dangerous disease pathogens. Corethra lives in the aquarium for a very long time, does not burrow into the substrate, and fish have the opportunity to hunt it. However, it is less nutritious than bloodworms. For adult fish, it is an easily digestible food. Corethra can be used to feed most fish species, periodically replacing it with bloodworms or other food. Feeding exclusively on Corethra is not recommended, as it is less nutritious than bloodworms and fish quickly get “tired” of it. Analysis has shown that Corethra contains 9.8% dry matter, of which protein accounts for 41.2%, fats 5.4%, and ash 4.5%. Corethra contains unsaturated fatty acids (liquid fats), which enhance the readiness and ability of fish to reproduce.
It is used by many aquarists for comprehensive feeding of most fish species in combination with other types of food. The frequency of feeding fish with Corethra is 2-3 times a week. Corethra can be used to feed fish in both live and frozen forms, although the former is not very preferred due to the fear of introducing all kinds of infections into the aquarium. It is considered one of the cleanest foods; the risk of infecting fish with infectious or parasitic diseases is much lower than when using many other live food organisms.
Live Corethra is insensitive to dissolved oxygen in water and can live in the aquarium for a certain time until eaten. It does not burrow into the substrate like live bloodworms, does not spoil the water, and is in no way inferior to them in caloric content. Live Corethra has a certain buoyancy that allows it to swim both in the water column and near the water surface – this is made possible by clearly visible air bubbles located in the front and rear parts of the body. Due to its predatory lifestyle, the larva is dangerous to fry. Feeding with both live and frozen Corethra should be dosed to avoid overfeeding.
Frozen Corethra can be considered with a sufficient degree of objectivity and safety as a staple food for many fish species, provided certain restrictions on single dosage are met. However, its disadvantage is that uneaten frozen Corethra floating on the surface spoils very quickly and must be removed using a fine net; otherwise, if left uneaten, Corethra quickly spoils and becomes covered with Saprolegnia fungus, which will inevitably cause gastrointestinal tract disorders or other diseases.
Indicators of good quality frozen Corethra are light, transparent larvae without a specific odor. Dark or “milky” colored larvae indicate that they were dead before freezing or that the larval membranes were damaged. It serves as a good nutritious food for most adult fish.
Feeding frozen Corethra is carried out either by preliminary thawing of a single portion in water or directly by thawing in the aquarium – for this, you simply immerse your hand with a piece into the water and wave it from side to side (Corethra gradually thaws and is eaten by fish) or gently rub the frozen piece with your fingers so as not to damage the membranes of the Corethra larvae.
Placing a whole piece into the aquarium for the purpose of its gradual thawing and consumption is not recommended to avoid an individual fish grabbing a large piece and swallowing it, which can lead to undesirable consequences (the probability of food not passing through the esophagus or overfeeding upon successful ingestion, the likelihood of a “cold” or associated disease). Frozen Corethra frozen at minus 18 degrees Celsius can be stored for a quite long time (at least 2 years) without losing its main nutritional properties.
Contraindications: – live Corethra can serve as a carrier of various infections; – “stale” live Corethra can cause gastrointestinal tract disorders; – live Corethra is not recommended for feeding very small fry, as the latter may be regarded as prey by Corethra and become a target of attack (Corethra is a predator feeding on plankton and small crustaceans); – uneaten and unremoved leftovers of frozen Corethra floating on the aquarium surface quickly become covered with “fungus” and can be eaten by fish along with “fresh” portions of food at the next feeding, which can cause undesirable consequences associated with digestive system damage;
When purchasing, the larvae should be transparent, shiny, and active, rather than greenish, and without a specific shrimp or fish odor. The fact is that aquatic hydrobionts used as food organisms accumulate various pollutants very easily. Fish, feeding on them day after day, concentrate toxins in themselves and soon begin to get sick. But there is another circumstance that must always be kept in mind. Along with all the above-mentioned live organisms, it is very easy to introduce pathogens of various fish diseases into the aquarium. When we buy live food, we usually know nothing about its origin, so we always take a risk.
Storage: Live Corethra, pre-disinfected, can be stored in the refrigerator in paper (regular newspaper) or fabric packaging, which must be periodically moistened with water to keep it damp (but not excessively). It is advisable to place the packaging with Corethra in a shallow plastic or metal tray. It can be stored in a jar with a small volume of water and a large surface area (otherwise aeration or frequent water changes are required): 0.5 liters of water per 1 tablespoon of Corethra. The jar is kept in the refrigerator, as it pupates at high temperatures. The water in the jar must be refreshed once a day.
Corethra is collected from the jar using a siphon made of a glass tube with an attached bulb, taking from the middle water layer so as not to capture floating and sunken dead larvae. Storing live Corethra is permissible until it moves actively and does not emit an unpleasant, putrid odor, the appearance of which can serve as a signal to stop using it. In any case, it is best not to store Corethra for more than 5-7 days. Some aquarists, upon the appearance of a “stale” odor, rinse Corethra with cold water, put it back into storage, and continue feeding fish, exposing them to completely unjustified risk.
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