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Dytiscus marginalis, , 19
Predacious diving beetle and larvae(Water Tiger)


Both the water tiger and the predacious diving beetle are fierce predators, The only way to remove them is to catch them, although the larvae are really hard to catch. In community tanks with larger fish the larvae usually die when fish try to eat them. Both can be found in ponds and small canals, and can be caught when collecting live food. The larvae pose the biggest threat to aquariumfish, especially to fry. The adult is hard to miss, but could infest a gardenpond.

The larvae

The larvae can wipe out complete batches of fry if introduced to a fry tank.They are already hard to spot when small, since they are extremely good at hiding. With their greyish color they can be found in the darker regions of the tank, usually between floating plants or in the corners of a tank.The larvae do not move much, they hang below the watersurface looking for prey. Breathing is done through the tail which at the end split into two featherlike structures that use watertension to hang below the watersurface. If alarmed it quickly dives to hide somewhere. It swims jerkely with rapid movements from it's hindlegs.They are really fast, so that makes it even harder to catch them. Still, that's the only way to get rid of them. Take a 20 cm small mazed net, remove a lot of the decoration, place the net in the tank and wait for them to surface for air. You can also remove all of the decoration, and try to catch them under water. There is no chemical treatment to get rid of them. Checking live food prior to feeding is still the best remedy.

The adult

The adult form can reach up to 5 cm in length, although Janze reports 3,5 cm as maximum size. The adult insect has a yellow bottom, and a yellow line at the side of the dekshields. The rest of the shields are olivegreen. They are also able to fly, and then make quite a loud noise, which sounds a bit like a wasp against a window. If you want to keep them in a tank for study, cover it. They like a well planted tank with lots of hiding places, preferably some driftwood reaching up to the surface or above. Although no sexual difference can be observed in the larvae, the adults do differ. Shields of the male are smooth, shields of the female are ribbed, and the male has suction cups on its forelegs(Godefroy,1957). They swim using their legs, which have hairs. They can move remarkebly fast under water. To feed them can only be done with live food, like daphnia, small fish, or mosquito larvae. On every foreleg it has 2 claws, which it uses to catch and eat prey.A prey is bitten with the jaws, and a brownish fluid is injected into it(Nes, 1952). This poisonous fluid killes the prey, and predigests its insides. Through the jaws the fluid with nutrients are reabsorbed. The parents make a nest in which the eggs are layed. After the larvae reach a certain size, they hatch on land. Innes describes how a full grown specimen was added to a goldfishtank for study. When introduced "It so quickly attacked a goldfish that the scales fell in a small shower and the fish died before it could be rescued."

References:Godefroy, J. (1958): Het verwarmd aquarium. N. V. Uitgevers Mij. Kosmos, AmsterdamNes, van, J. G. Th. (1952): Het Nederlands zoetwateraquarium. Thieme, Zutphen.Innes, W. T.(1936): The complete aquarium book. Halcyon house, New York, U.S.A..

Comments on this page:

The two larger photos on the Dytiscus marginalis site are showing Cybister lateralimarginalis. In Germany we call it the "Gauckler". It is a more atlantomediterranen species becoming more and more common in Middleeurope the past two decades.

The small photo on the left of the same site shows a Graphoderus cinereus, the most common species of this genus (4 species in Middleurope) in the Netherlands.

Both species of the family Dytiscidae are good flyers and could live in the same tank, pond or other larger standing water bodies with rich aquatic vegetation. The smaller Graphoderus cinereus is not hunting young or smaller fishes, it´s just feeding on carrion in the water or mosquito larvae or other insects.

Best wishes

Lars Hendrich

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