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Hidden worlds in our aquaria

caridina - epaper ⋅ Ausgabe 4/2019 vom 15.10.2019


The most common small crustaceans, from cyclops to Mexican scuds

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In our aquaria we have a rich accompanying fauna, which we usually do not put in on purpose, but which is sti ll there, alive and kicking. These ti ny animals form an incredibly interesti ng world of their own, and the vast majority is benefi cial. There are lots of diff erent types, and all of them play an important role in the complex ecosystem inside an aquarium.

The small crustaceans that form part of this accompanying fauna in our tanks usually simply appear there all of a sudden, too. In an invertebrate aquarium without fi ...

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... sh you will see a lot more accompanying fauna, since fi sh would eat the ti ny critters. In additi on, the presence of potenti al predators causes them to live much more secretively.

Copepods in an aquarium.


Cyclops are frequent guests in invert tanks. Whitish to light beige in color and only 0.3 to 1 mm long (0.01 to 0.04 inches), the tiny crustaceans belong to the subclass copepoda, or copepods. They look a bit like a tadpole, with a large head and a very slender abdomen. Their long, pronounced antennae on the head are used for locomotion. Between them, there lies one single red or black eye, which has given them their name: Acyclops is a one-eyed giant in Greek mythology.

FemaleCyclops carry two clearly visible egg pouches on the right and left of their abdomen aft er mating. From these eggs, nauplius larvae hatch, which need to undergo a total of eleven molts, and go through several stages before they metamorphose into their adult form.

Cyclops can easily be identified by their typical way of swimming: It looks like they are hopping through the water. They also sit on the glass from time to time, where they munch on biofilms. Basically,Cyclops eat organic rests: dead or decaying plant matter, micro-organisms, left over fish or shrimp food and dead animals.

In nature, many copepod species live in temporary water bodies, and their life strategy takes that into account: When their biotope dries out, they can cover themselves in a hull of highly viscous mucus, forming a tiny, light-weight cyst. Inside these cysts they wait for better conditions. Quite oft en these cysts drift off with the wind and become part of the natural dust. This also explains why copepods tend to appear in aquaria all of a sudden, they can literally come out of thin air. Of course they may also sit in plants, and it is even discussed whether frozen food may contain viableCyclops cysts.

In an aquarium, copepods are not really a problem. If you feed powdered food on a regular basis or if you overfeed in general, their numbers may increase considerably, however, even then these peaceful tiny crustaceans are absolutely harmless. Small fish love to eat them. If you want to reduce theCyclops population nevertheless, increase your water hygiene and revise your feeding routine.


Seed shrimp are distributed in all the world. Their more or less oval form and small size of 0.5 to 3 mm (0.02 to 0.12 inches) has given them their name: They truly resemble a grain of seed. Ostracods have a bivalve-like shell consisting of two parts, between which they stick out their legs, which they use for swimming and crawling. Their movement is smooth, not saltational, and they oft en swim in circles. When in danger, they withdraw their feet and close their hard, calcareous shells, which are indigestible for some fi sh. The closed-off seed shrimp passes the digestive tract, is excreted, opens its shells and keeps swimming. Therefore they are not a great live food for some fish.

Ostracods of the Asian speciesTanycypris centa on a piece of cucumber .

Black and white AsianTanycypris sp. are frequent aquarium guests. They probably came with imported shrimp or crayfish. Please make sure the ostracods from your tank will never get out into nature - chances are that they are a foreign species that may endanger native seed shrimp.

Most freshwater ostracods are detrivores that live off left overs, there are only very few species that eat snails or other small aquatic animals. There are more predators among marine ostracods. If food is abundant, the numbers of seed shrimp inside the aquarium will explode

If you weren’t so unlucky to catch one of the rare predatory species, seed shrimp are harmless in any aquarium. The only creatures that may have a problem with them are snails with a lid: If an ostracod happens to wander inside, it can irritate the snail so much that it closes its lid, thus trapping the seed shrimp, never to come out any more. Then the snail may starve to death.

Apart from that, most ostracod species we usually have in the aquarium hobby are, in fact, a beneficial part of the accompanying fauna since they eat left over fish and shrimp food and dead organic matter, thus fulfilling the task of a clean-up crew. All the species are dioecious. Some reproduce through parthenogenesis, others prefer sexual reproduction.

The females carry the eggs under their shell or lay them onto substrates like rocks or plants. The eggs of some species can even survive a dry phase, so local species may therefore find their way into our tanks with the natural dust in the air if their habitat dries out.

This is just one way how ostracods may get into your tank, though. You can of course also introduce them with aquarium plants, animals, decoration or live food caught in nature.

Once you have them in your tank, ostracods are really hard to get rid of, but since they are useful scavengers that will eat practically any kind of left overs they needn’t be totally eradicated. You can control their numbers by cutting down on feeding, by only feeding high-quality food and by a good aquarium hygiene.

Ostracods and copepods directly under the water surface.

The pointy dorsal scales and the curved posture of the “fighting scud” are visible here .


In the 1980s, Bernd Posseckert found some up to 7 mm (0.3 inches) longHyalella azteca in bags with imported fi sh. He bred them as live food, and gave quite a few starting populations to fellow fish keepers. Some of the parcels were damaged during transport, so he started writing “Attention, Mex. fighting scuds!” onto the boxes. This helped a great deal, he says. To this day, the amphipods are known as “fighting scuds” in the German-speaking aquarium world. They are very robust and quite competitive, so the name has stuck.

They look very similar to theEuropean Gammarus , but don’t grow as large. Their dorsal scales are distinctly pointy, though. Mexican scuds are surprisingly fast. In the aquarium you may see them flit across the ground, or take a swimming dash down from the water surface.

Mexican scuds are opportunistic eaters and will also eat dead animals (like this fly that fell into the water).

Mexican amphipods are no picky eaters. They will eat anything from rotting plant parts and soft-tissue plants, algae and leftover fish food, spawn, dead animals and so on. They are an efficient cleanup crew and tend to reproduce in very high numbers if they find lots of food. Scuds strongly compete with shrimp for food, and especially with shrimplets. The females carry their eggs under their abdomen for 11 to 21 days. When mating, the male holds the female close to his belly, and the pair swims like that, sometimes for days.

Mexican scuds often travel in leaf axils, but also with decoration, gravel or water from infested tanks. They are especially hard to get rid of in shrimp tanks since every remedy also has highly adverse effects on the shrimp.Hyalella belong to the same class as them, so poisons are out of the question, and predators will also eat shrimplets. Your best bet is remodelling the entire tank.

The scuds don’t seem to tolerate very soft water in the long run, though.

When mating, Mexican scuds form a “double decker”.

The forking appendages on the last segment and the strikingly long last leg pairs are typical for aquatic sowbugs.


Asellus aquaticus is a rare guest in home aquaria, but still quite interesting. These light to dark brown isopods have a dark backline and grow to a length of about 1-2 cm (0.4 to 0.8 inches). They have seven leg and two antennule pairs, and a pair of forking appendages on their hindquarters. Aquatic sowbugs cannot swim. All these characteristics make them easy to identify

They belong to the peracarids, and are counted among the crustaceans. They originate from temperate climates on the northern hemisphere, where they live in stagnant or very slow-flowing fresh water. Water sowbugs are quite tolerant of low oxygen levels and pollution.

They eat detritus and leftover fish or shrimp food. Some shrimp breeders swear by them as a cleanup crew.

The females carry up to 100 eggs in a brood pouch under their abdomen for 3-6 weeks. You can get water sowbugs with plants, decoration or live food from nature or from tanks populated by A. aquaticus. They needn’t be eradicated since they don’t tend to take over the tank.

Thanks to its appendages, the large last segment looks much more like a head than the actual head of the water sowbug.

Info box

Scientific name :Hyalella azteca
Tank volume : starting at approx. 4-5 | (1-1.3 US gal)
Water hardness :medium to hard
Food: algae, dead or soft plants, flakes, tabs, vegetables, detritus, leaves …


You definitely don’t wantHyalella azteca in a shrimp tank since they compete for food with the shrimp off spring. The “fighting scuds” are interesting to watch and they are great algivores, though. Some aquarium hobbyists use them to clean away algae from moss – you just need to make sure you take out the moss before theHyalella start munching on it

Immerse plants that were put into a tank with Mexican scuds in order to get rid of the algae on them in strongly carbonated water for around 30 seconds in order to kill off remaining scuds before you put them back in your shrimp tank to avoid accidental introduction.

Small filterless tanks with a coarse substrate are a good home for the amphipods. They tend to crawl into the filter, so motor-driven filters are not a good choice. Air-driven sponge filters may work, though. You needn’t do a weekly water change, topping off the tank is sufficient. A partial water change every month or two is recommendable, though. The amphipods don’t need a lot of oxygen, so you don’t even have to install an air stone in their tank.

A Mexican amphipod looks a lot like aGammarus scud, but doesn’t grow as large.

Hyalella azteca are a great permanent source of live food in fish tanks – especially lurking predators like gobies just love eating the quick-swimming scuds and will even go hunting for them between the rocks of a coarse gravel substrate and under pieces of driftwood.

Photo: Tamara Stamm

Photo: Oliver Mengedoht

Photo: Oliver Mengedoht

Photo: Oliver Mengedoht

Photo: Frank Teigler