Invertebrates in fish nutrition

The diet of inhabitants in the average aquarium is often based on one can of prepared food and occasionally on live or frozen “worms”. And that’s definitely not enough. Just imagine the variety of foods fish take in their natural environment. These are numerous species of insects, molluscs, crustaceans, fish, plants and algae, which they can find in their environment. These foods differ in terms of quality and quantity of nutrients. Invertebrates play an important role in fish nutrition. They are used in live, frozen, freeze-dried and dried form or as an ingredient in formulated foods. They provide different nutrients to fish, in different amounts and in different forms. None of these organisms alone will meet the nutritional needs of fish in the long run. However, invertebrates used together with other raw materials become a very good substitute for the natural diet of fish.

Antarctic krill (Euphasia superba)

It reaches 6 cm in length. It eats plant plankton and in winter it probably hunts for zooplankton. However, it has not been thoroughly explained how it manages to survive the harsh Antarctic winter. Krill is the source of easily digestible protein (about 70%) rich in essential amino acids. It is eagerly consumed by fish, especially in processed form – as an addition in flakes, granules and tablets. In breeding consumer fish, it is treated as a natural attractant. Its palatability is the effect of a specific combination of amino acids (glycine, proline), nucleic acids and trimethylamine N-oxide (TMAO). These substances, detected by the fish’s chemoreceptors, affect the taste and smell of the food. In addition to valuable protein, krill provide fish with unsaturated fatty acids, 40% of which are polyunsaturated fatty acids (PUFAs). Tropical foods with a high krill content include Krill Gran and Marine Power Krill Formula Granules.

Polyunsaturated fatty acids play an important role in moulting process of crustaceans, hence the use of krill as a good source of PUFAs in Shrimp Sticks, which is intended for freshwater and marine shrimps and other crustaceans kept by hobbyists.

Common squid (Loligo vulgaris)

The common squid (Loligo vulgaris) is an equally rich source of protein for fish as krill. They are predatory marine animals that inhabit the open sea with full salinity, where they are caught in large numbers for consumption. They contain about 75% protein and 9% fat, which is rich in PUFAs. Their very low fat percentage and high protein content makes them an interesting food for aquarium fish. Studies have shown that feeding fish squid and krill is very good for reproduction. This ingredient can be found in the Cichlid Color food, among other products.

Bloodworm

One of the most popular invertebrates used as food for fish in our latitude is bloodworm – a red mosquito larva. Although fish undoubtedly love chironomid, the use of this crustacean carries certain risks. This also applies to any live food, which involves the possibility of introducing pathogens and often also toxic substances into the aquarium. However, bloodworm obtained in a clean environment is a valuable, high-protein food, which in dry matter contains: about 60% crude protein, 10% fat and 11% ash. It contains relatively small amounts of unsaturated fatty acids (about 14%). Therefore, overuse of this food can lead to fatness and PUFA deficiency in fish. However, due to its high nutritional value and attractive taste for fish, bloodworms are a very good food for spawners and fish weakened by disease or long transportation.

Tubifex

Another controversial food is Tubifex. It is also found in bottom sediments, where it feeds on organic matter, algae and bacteria. Tubifex is a very energy-rich food due to its high fat content (about 20%). As with bloodworm, Tubifex is credited with an important role in introducing parasites into the aquarium, so using freeze-drying to prepare both of these invertebrates is a particularly desirable method of preservation. The freeze-dried food obtained from Tubifex is Tubi Cubi. Tubifex has been also used as an ingredient of flake food in the product Tubivit.

Daphnia pulex and Gammarus pulex

Daphnia pulex and Gammarus pulex are low-energy fish foods that are rich in roughage. The protein content of these crustaceans is about 50%, the fat content about 2.5% (but can also exceed 20%). Note that these values vary quite substantially in the diet of the crustacean, which depends on the season and habitat. Due to the relatively low protein content daphnia is absolutely not suitable as a staple food, especially for young fish. When comparing the chemical composition of consumer fish fed on standard feed and daphnia, it was found that the latter contained less fat and more ash and phosphorus. Similar results were obtained for fish fed with krill. It is a hint for aquarium hobbyists that crustaceans, apart from mechanical support of digestive tract due to high content of chitin, also protect fish against getting fat and keep them in good condition due to their chemical composition.

Artemia salina

An excellent and highly valued food by aquarists and breeders is the brine shrimp (Artemia salina). The fish are fed both larval forms – nauplii, which are hatched at home, and adults in the form of frozen, freeze-dried, live or ready-made food. Because artemia nauplii contain about 58% protein, 20% fat and 10% ash on a dry weight basis, they are an excellent hatchery food for many fish species. However, it is recommended to vary the diet of young fish also with other foods. It is worth noting that adult artemia has half the fat content.

Comparison of nutritional values of invertebrates

Crude protein, fat and ash content of selected aquatic invertebrates, [%] dry weight.

Raw material	Crude protein	Crude fat	Crude ash
artemia – adult form	60	13	12
artemia – nauplii	58	20	10
squid	74.8	8.8	3.4
krill	70	10.4	12.6
bloodworms	60	10	11
daphnia	50	2.4	19
Tubifex	47.8	20.1	4.5

Despite the apparently very similar protein and fat contents of these invertebrates, you must remember about the differences in their quality. Undoubtedly, the quality of protein is determined by the content of individual amino acids and their mutual proportions. In fat, in turn, we are interested, among other things, in the proportion of UFAs or PUFAs. All this means that the diet of fish should be as varied as possible.

Content of unsaturated fatty acids in selected raw materials, [%] of total fat content

	Σn-3	Σn-6	20:4n6 arachidonic	20:5n3 EPA	22:5n3	22:6n3 DHA
beef	0.6	3.1	–	–	–	–
poultry	1.0	19.6	–	–	–	–
fish	17.8	1.4	0.3	8.4	0.8	14.9
krill	24.2	9.9	–	14.7	0.4	8.3
squid	25.7	14.8	1.6	10.2	1.5	11.8
Gammaridae	18.9	6.0	1.4	11.8	0.6	1.7
Chironomidae	9.6	3.3	0.3	5.6	–	0.03

The crustaceans discussed in this article are a good source of natural colourants from the carotenoid group, which intensify the coloration of fish and have a beneficial effect on health, reproduction and general condition of the animals.

Carotenoid content in selected raw materials

raw material	carotenoids [mg/kg]
krill meal	80-120
krill oil	700-800
artemia – nauplii	90
daphnia	700-800
gammarus	670-800
Spirulina platensis	3 500
Haematococcus pluvialis	15 000 – 30 000
carrot	65

Invertebrates in fish nutrition are an important source of protein, carotenoids and chitin. In addition, invertebrates can act as natural attractants in fish nutrition such as krill. Read what nutritional properties algae have for fish.

dr inż. (Ph.D.Eng) Aleksandra Kwaśniak-Płacheta

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