Phylum — Mollusca.
Class — Bivalvia.
Superfamily — Acroloxoidea.
Family — Acroloxidae.
Genus — Acroloxus.
— Acroloxus arachleicus [Kruglov & Starobogatov, 1991] — Acroloxus snails. Family Acroloxidae. The smallest aquarium snails are acroloxids. There are many species and forms of acroloxids, which is noticeable by the shape of their shells. Their shell looks like a low cap, with the body of the mollusk located immediately below. The largest known acroloxids reach a length of 7-8 mm. The remaining representatives of the family are even smaller. When viewed from the side, their shell looks like a low cap with a pointed apex.
These little ones lead a rather secretive lifestyle and prefer to act at twilight and in the dark. They look like small white spots on plants and aquarium glass. An aquarist might be very surprised one night when turning on the lights in their aquarium. Many hundreds of these snails will cover the leaves of plants and the glass.
But do not be alarmed: acroloxids are considered harmless, they do not harm higher plants. However, conclusions will have to be drawn, as their high numbers will indicate the presence of an adequate food base. These snails feed on organic residues. You will need to check if there is chronic overfeeding of the fish?
With a decrease in food input into the aquarium, the population of these snails will also decrease. They can serve as indicators of aquarium pollution from excess food. Fish usually do not eat acroloxids; if you try to pull them off the glass yourself, you will be surprised how firmly this tiny creature holds on.
Superfamily — Unionoidea.
Family — Unionidae.
Genus — Anodonta.
— Anodonta cygnea piscinalis [Linnaeus 1767] — Swan mussel. Family Unionidae. Subspecies exist: Elongated swan mussel (A. cygnea celensis) and Duck mussel (A. cygnea anatina). Present in many European countries. It is a shell formed by two elliptical valves tightly adjacent to each other. The shell expands towards the front end and narrows somewhat towards the back.
The shell opens due to a strong, horny, elastic ligament running along the dorsal side of the body from the inner surface of one valve to the other. As long as the swan mussel is alive, the valves are mostly closed. But when the animal dies, the muscles relax, and the shell opens.
Usually, they sit in the water not far from the shore, burrowed into the sand with the blunt front end of their body, and only the rear pointed part protrudes from the sand. The animal itself and its life differ little from the painter’s mussel. It differs from Unio by a larger shell size; the valves of the shell have teeth on the dorsal side from the inside, forming a hinge which the swan mussel lacks.
The shell is fragile, 12 cm long, 6 cm high, convex in the center up to 3.5 cm, elongated-oval, rounded at the front edge, evenly curved on the belly side, rounded corners, the hinge edge (inside the shell) is linear, without teeth.
The shell color is yellow-gray, sometimes greenish, brownish-chestnut or cream, lighter in the front than in the back. These mollusks are living biofilters: per day they pass about 40 liters of water through themselves, destroying the smallest biological turbidity that cannot be eliminated by any filters. The swan mussel also destroys small mechanical turbidity from suspended water-insoluble particles.
They need to be kept in large aquariums with good filtration and water current (good oxygen saturation is necessary). Water quality must be very high, with high hardness and neutral or slightly alkaline pH. The aquarium substrate should be sandy or muddy. This is a filtering species that feeds on microparticles through a siphon (hence the importance of a large volume and good current in the aquarium). In large aquariums with an abundance of plants and fish, it will have no trouble finding food in the form of microparticles.
They are semi-stationary and partially buried in the substrate, leading a predominantly sedentary lifestyle. When moving, you can notice that the shell valves part slightly, and a thick muscular outgrowth protrudes through the gap. This is the animal’s foot. It can increase significantly in volume. Having protruded its foot, the swan mussel begins to slowly shovel sand under itself and gradually deepens into the dug pit with its front end.
Using its foot, this animal moves along the bottom. When moving barely perceptibly, they leave a very characteristic furrow in the sand. They crawl no more than 25-30 centimeters per hour. The temperature can range from 10 to 24°C. Its reproduction in aquariums is very complex, practically impossible. They reproduce by glochidia (parasitic larvae). The larvae attach to the fish’s skin and parasitize it, but after forming into a mollusk, they fall off.
Genus Anodonta, Genus Corbicula, Genus Dreissena, Genus Sphaerium.
Unio, Unio pictorum, Unio continentalis. 
