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Gammarus is a crustacean which is very common in British streams and rivers all over the country. I have also found them in ponds. It's a little like a sea shrimp, but if you've ever looked closely at the sandhoppers you get by the thousand when you disturb a pile of dried up seaweed on a sandy beach, then you'll know exactly what Gammarus looks like, for the sand hopper is another type of gammarus closely related to the freshwater one.
There are about 250 species of Gammarus, living in fresh, sea or brackish water or even in damp conditions. They are very difficult to tell apart - probably the habitat if the best indication. Most are excellent as food for fish. The largest get to about 30mm long though Gammarus Pulex, the one I cover here, is smaller.
Gammarus Pulex is quite easy to catch and it is very easy to keep alive over long periods - in fact is will do more than stay alive: it will readily breed and the young gammarus are small enough (about the size of a daphnia) to be eagerly accepted by most fish.
There are in fact three types of gammarus in British fresh waters, according to John Clegg in the Wayside and Woodland series 'The Freshwater Life of the British Isles'. The following is extracted:
Gammarus Pulex: the males can grow to about 20mm long (though 10-12mm is commoner). The females are slightly smaller and probably less common, for the shrimps are often found in pairs with the male tightly holding the female to his abdomen. They are usually to be found under stones, in the mud or amongst the plants and detritus. If you disturb their habitat they swim very rapidly, in bursts, away from the disturbance to find other shelter. In fact their surprising turn of speed in an aquarium makes them quite interesting to watch - and very good exercise for your otherwise not very busy fish!
They are largely scavengers feeding on dead plant material and other decaying organic matter. They are not however averse to attacking and eating other small living creatures - so it would be unwise to keep them with fish eggs or fry! If they survive in you tank and the food supply is low, they have a habit of making their homes in the crowns of some aquarium plants, where they can eat the fresh growth. This is not a problem I have noticed as they do not survive long enough to do noticeable damage before the fish eat them! However if they ate too large for your fish to eat, they will live and breed quite happily in a tropical aquarium.
They are particularly common in chalk spring-fed streams such as are favoured by watercress: they may be found by the thousand in these watercress beds. They also like streams where starwort grows.
They may not however be common in watercress farms: when it is damaged watercress gives off a chemical, phenethyl isothiocyanate (PEITC) which may have adverse efects on the poplation. Read International Journal of Zoology Volume 2011 (2011), Article ID 328749: Watercress and Water Quality: The Effect of Phenethyl Isothiocyanate on the Mating Behaviour of Gammarus pulex
I am fortunate enough to live within a few hundred metres of Burwell Moats which is just such a chalk spring fed stream abounding in watercress, fools water cress and of course Gammarus! Since this is spring fed it rarely ices up and gammarus can be caught on most days of the year.
The problem catching them is that you will tend to also pick up a lot of foreign material. However, I catch more than I need and keep them with the 'rubbish' so that this material becomes their food! This works very well: I use an old metal flour sieve to dig in under the watercress. The mud goes through the sieve and I end up with leaves, watercress shoots, bits of twig - and plenty of shrimps.
You can simply put weed, shrimps and detritus in a polythene bag with only enough water to keep it damp. However I use a wide bucket to carry them in, in a shallow depth of water, say about 50mm. The main danger is that you get so many they use all the dissolved oxygen and suffocate, so the method depends on how far they need to travel.
At home I split the catch and 'rubbish' between several wide buckets - the sort that emulsion paint is sold in is excellent, and easy to acquire! In each bucket is about 30-50mm of water plus shrimps, watercress, other water weeds, dead leaves etc. Keep the buckets in a place which is as near the temperature of the stream as you can, it must not get too hot in summer - for the shrimps need a good oxygen supply. It must not freeze in winter - though the shrimps can stand freezing: I've had ice form on the top of the water, trapping shrimps. I though 'shrimp lollies' for the fish, so put the ice plus shrimps into the tank (it's a large tank, so the small amount of ice didn't change the temperature). The ice melted and the shrimps sprang to life: they are very hardy and can certainly stand an immediate temperature change from ice at 0°C to 26°C.
However the shrimps do not like foul water: this will be low on oxygen and the shrimps will suffocate. So keep the water shallow and do not have an too much dead material: plenty of growing plants would seem best. Ideally some plant such as starwort or watercress that the shrimps can eat. If you find plants where they flourish, then they are probably eating it!
The shrimps breed quite readily in these conditions.
Since the shrimps live in the weeds rather than in the water, you need to keep them in a mass of vegetation (both dead and alive) and you will not want this detritus in your tank, so the problem is how to extract the shrimps from the culture and leave the unwanted rubbish behind.
The best way I have found is to place a piece of old netting on top of the culture. The shrimps burrow into and through this and cling to its meshes. So when you lift the material out, the shrimps, not being very speedy at recognising danger, are still clinging to the material. If you suspend this over a bucket or other container the shrimps will then drop off the material and you have a clean culture of shrimps.
However: this method of separating the shrimps does mostly rely on the water getting stale: as this occurs the oxygen gets depleted and the shrimps come to the surface. So catching them this way is a delicate balance between keeping the water fresh enough for thim to breed and stale enough for them to come to the surface to be extracted!
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Last modified: Fri, 24 Nov 2017 12:09:59 GMT
Page first published 11th December 2002.
Page written and © by Richard Torrens