Culturing Mexican Gammarus
by
Colin McCourt

(Hyalella aztec)

Introduction
I first came across this little species of shrimp whilst discussing live food sources with a fellow longstanding killifish enthusiast. He imparted to me that this species would remain (even when adult) small enough for average sized killi species to consume. They would also proliferate at such a speed that a 24x12x12 tank set aside for such a purpose would feed a whole fish-house weekly. What more could I want. He offered to send me a culture and from that contact, this article was spawned. I hope you enjoy this new editorial and be encouraged to seek and find more natural and obscure food resources for your fishes. Hyalella aztec as the second part of the latin name suggests hails originally from South America (Mexico). They are maintained in many laboratories around the world as they are extensively used in toxicological studies when assessing the environmental health of rivers, streams and other watercourses.

Life Cycle
This little amphipod will mate several times per year. If you observe the courtship closely you will notice that the male will carry the female on his back, for about a week, whilst swimming, during the copulation process. Females will normally produce around 40-50 fertilised eggs in her brood pouch which show up as being orange in colour and are conspicuous through her semi-transparent body. The tiny shrimp hatch within the egg pouch of the female and emerge as microscopic, fully developed young.

They will then develop an exo-skeleton and will shed this (moult) several times as they grow. The shrimp have no natural defences against predators so they will hide in any dark areas of the tank. They will however become more active under low light conditions or after dark.

Hyalella aztec, as with many other species of Gammarus Shrimp have a body which is laterally compressed, they are thicker in their length rather than width. The body is divided into eleven segments, one for each set of appendages. There are two long antennas on the head , followed by two pairs of grasping legs, half way down their body they have five pairs of legs for walking, these are followed by another pair of hind legs which assist the shrimp when feeding, curled head to tail. The tail section has two small protrusions.

Size
This little species is fairly small with adults perhaps reaching only 10-15 mm in length. This is perfect for feeding to tropical fish species of average size. (Males are larger than females)

Colour
This depends entirely on their diet within their habitat. They are semi-transparent and their entire digestive tract is visible through the walls of their shell. They will scavenge on all manner of materials, but most of the time they will adopt plant material as their primary food source, of which algae will have an effect on their colouration, showing up in the shrimp as various shades of green. Environmental surroundings can also effect the colouration. If subjected to a habitat with lots of silt and mud as a substrate then the shrimp will naturally appear to be brownish and blend into the surroundings which help to camouflage them and protect them from becoming prey. You will however always be able to spot the pregnant females by the colour of her eggs being carried within her brood pouch.

Movement
When you have a population of these going, you will notice that they will swim for around 2 to 6 seconds, and then they will take a rest and breathe. If they are not holding onto something within the tank they will sink toward the bottom in a curled position after a few seconds they will uncurl and swim another short distance. They need to have constant circulation of water over their gills, to achieve this their legs are constantly in motion.

Habitat
Gammarus Shrimp are primarily scavengers they mostly feed on feed plant and animal material that have settled to the bottom. Sometimes they will adopt a predatory role and attack other injured or stressed aquatic life. For the most part they tend to hide among plant matter and under rocks or debris when not travelling or mating. Shrimp need calcium to build their shells or exo-skeleton. So an environment which is non acidic and high in dissolved salts and calcium suit this little critter best.

Home Aquaculture
When first acquiring my initial starting culture of this shrimp, I asked many questions regarding their aquaculture and husbandry, all of which were met by the same reply.

Discussion
I'm not one for keeping anything that needs a lot of maintenance so don't worry about temp.,p.H etc.If the water gets cloudy I just do a water change with tank water. Drain off and fill up as quick as that. In nature they have to put up with flood and drought conditions as well as temperature fluctuations and a lack of food so I just feed them literally anything which in nature you might find in a stream, they are after all scavengers just like most crustaceans. They must think every day in captivity is Christmas, not having to rummage round for a meal! That said I just knew these little buddies were going to be a cinch. Looking at the evidence above I would have said that this species were nearly impossible to wipe out and you know that's not too far removed from the truth.

I initially set the shrimp up in a 3ltr ice cream tub. Rainwater buffered with a little Bicarb of Soda and some crushed coral sand (in a small tub). Added to this set-up a large Juwel brand coarse filter sponge was added. Juwel Coarse Filter Sponge - Excellent Gammarus Habitat Material It needs to be the course type as the shrimp utilise this for habitation purposes much the same as being under stones in the stream or river. Being habitants of moving waterways suggested that they like a lot of oxygenation in their water so I added an airline which provided plenty of agitation to their new home, all be it temporary. I feed the little beggars anything that's lying around but predominately they get Tetra Pond Pellets. (must try trout pellets from the local angling shop). They grasp on to these pellets and feed, 100's at a time, its so comical to watch. In a tub that size I would feed 15-20 pellets a day and water-change every other day. The population explosion of this species has to be seen to be believed. I have only had this new form of livefood a short while now, and its truly amazing how these things reproduce so quickly. I can see how it is possible to feed a whole fish-house on these and not deplete you initial stock.

Harvesting the shrimp is very easy too; all you need is a small container with a lid. Punch some holes in the lid and load the container with some delicacy (pond sticks),replace the lid with a length of string attached (so as you can retrieve the pot from the culture aquarium) After a while draw up the pot and it should be filled with Gammarus ready to be fed to your charges.

Summary
If you are looking for a small sustainable form of live food to be able to feed your fish on a regular basis on, then look no further than this little species. Simplicity in itself to set-up and maintain. The fish seem to love these critters and take them with gusto every time they are offered. Note!

One downside is that these critters should only be fed to breeding aquaria and aquariums devoid of (or folks not worrying about their greenery) plants. Due to the nature of things these shrimps will strip plant matter from a tank very good for hair algae but will relish aquarium plants as desert when that is used up. Sharing a tank with Cherry or other small species is also a bad idea due to being out competed for food. Those small points aside and looking at things from a breeders point of view then this shrimp species offers a lot of pluses when trying to keep a fully stocked larder, especially over the winter months when live food becomes scarce.

I hope some of you will give them a go.