Freshwater Amphipod - Gammarus
Not really a freshwater shrimp
Last Sunday, it was pretty warm, and S_ suggested that it would be a good day to take Sam down to the stream[1] to turn over some rocks to see what was underneath[2]. So, I grabbed this rock that was in the middle of the stream, turned it over, and found about 20 of these clinging to the bottom:
This was a surprise. I’ve turned over a lot of rocks in streams before, and never seen anything like this. It turned out that the most straightforward way to photograph them was to put some water in the petri dish that I normally use, put three of them in, and just let them swim around. And they did swim around — they zoomed around in circles pretty much without a break. Every now and then, one would stop long enough for a photograph, but then one or both of the others would pile into it and they’d be off again.
Given how much they looked like shrimp, it was pretty obvious that they were some type of crustacean. It was clear that Bug Guide wasn’t going to be any help, because they explicitly exclude fully-aquatic non-insect arthropods, so they’ve got nothing.
S_ did a bit of rummaging around, and found out that they were freshwater amphipods, in the genus Gammarus[3], also commonly known as “scuds”.
They are frequently mistaken for shrimp, but shrimp are a different crustacean order (shrimp are decapods, not amphipods).
For something that lives under rocks and comes out at night, they have reasonably well-developed compound eyes (you should be able to see the individual cells of the compound eyes here)
You can also see the bases of the antennae here. Unlike insects, which have two antennae, crustaceans evidently have four. The function is similar (smell, taste, touch), the crustaceans simply have more of their head appendages devoted to the task.
Gammarus[4] are detrivores. They eat small bits of organic debris - decaying plants, algae, fungus, animals smaller than themselves, each others’ corpses . . . pretty much anything they can get into their mouths. They have a set of legs on their abdomen that never seem to stop moving, they just keep paddling along, sweeping water along the underside of their bodies, and grab anything edible that flows past. They don’t really seem to swim much up in the open water, they are more likely to scoot rapidly along the bottom while lying on their sides. They evidently need well-oxygenated water (which generally means cold water, because cold water dissolves more oxygen than warm water), and do poorly if there are any pollutants present. They are doing really well in our little stream, so I’d say our water is probably pretty unpolluted.
These guys look so very shrimpy, that I keep thinking that it’s too bad they aren’t bigger - about 2 inches long would be big enough to peel and eat. I bet they’d be pretty tasty if they were, say, steamed, peeled, and dipped in melted butter. Mmmm. Butter. Maybe I could selectively breed them to be bigger. A lot bigger.
They are also sharing the stream with a lot of other little aquatic arthropods, I’ll have pictures of one of their neighbors next week.
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[1] Just off the NE corner of our house, there is a small year-round spring that produces a continuous stream of water. Technically, this could be considered a “creek”, even though at the points where it has a well-defined channel, it is maybe 6 inches wide and 3 inches deep, and runs maybe 200 feet from the spring to the drainage ditch by the road. It’s too small for fish, but it is a paradise for small aquatic arthropods.
[2] Getting to the creek is much more of an undertaking than it sounds like. Even though it is only about 50 feet from the house, the direct route there involves getting through a barrier of brush and Virginia Creeper vines while descending down a very steep slope with a drop of about 10 feet. At this time of year, there is also still about 3 feet of snow on the slope, because it is protected from direct sunlight and takes a long time to melt. Then, once at the bottom, it is unwise to stand in one spot for very long, because right beside the spring is a patch of very sticky, clayey muck that your feet gradually sink into. It’s a good place to lose boots. It is much easier to get out by following the stream all the way to the road, and then walking up the road back to the house, which is probably 10 times further than the direct route but far easier walking.
[3] Oddly, for these beasts the best sites to find pictures are not pages written by biologists, but rather pages written by fishermen. Gammarus evidently makes very good fish bait, you can buy them commercially. I gather that they are also popular in aquariums, and it looks like dried Gammarus pellets are a good food supplement for pet turtles (it keeps their shells from getting soft).
[4] I keep looking at this name and thinking of “Gamera”, the giant Japanese monster turtle that flies by pulling in its legs and shooting out jets of flame.
K T Cat:
What a great blog. I learn things every time I stop by. Thanks for sharing this with us.
18 April 2008, 7:53 amgarden:
Would you be willing to ship some of these to me? I’ve searched in many creeks and ponds near my house, (SC) and haven’t been able to find any. Many years ago, I had a culture of them that I would raise and feed to my aquarium fish.
26 April 2008, 9:51 amTim Eisele:
Well, I suppose I could try shipping some to you, but I have no idea how well it would work. I’ve never tried mailing live specimens, would they have reasonable odds of surviving if I just put maybe a dozen of them in a closed plastic jar with lots of water, and shipped them regular mail? It’s kind of a long way from here to SC, do you think they’d survive for 4-5 days in the mail?
garden:
I’ve ordered freshwater shrimp through the mail (priority mail) several times. They usually come double-bagged in fish bags. Duct taped double-bagged ziplock bags work too. There’s usually an aquatic plant or something in the bag that they can cling on to.
Actually, now that I think about it, I had always ordered shrimp when the weather was a bit cooler. It’ll be in the 80s here this week. I don’t think I want to subject the little guys to an unnecessary death. I guess I’ll keep searching streams and lakes. Thank you though.
I also forgot to mention earlier that I really love the site.
26 April 2008, 5:15 pmtim:
At this moment, I have about 100 shrimp swimming in front of me. I’ve been raising them as acquarium fish food. I’d caution “garden” against feeding them directly to fish, as they may be a carrier of parasites/disease. It’s best to culture them for a couple months so the parasites (the ones that depend on multiple hosts) die off. I’m trying to selectively breed some to a large size (the largest are nearly a cm), but I’m doubtful if I’d ever get enough to make even a decent shrimp omlette.
4 June 2008, 2:35 pmTim Eisele:
Well, 1 cm isn’t half bad, it’s certainly a good start. Maybe in 20 years or so they’ll hit eating size. Do you have any cultivation tips?
tim:
I spent most of my childhood studying my backyard arthropods (grasshoppers in elem school, then spiders in high school). I wish I had documented everything I found, but alas, I didn’t. I love the pics and commentary.
Back on topic, whenever I siphon off the water from my aquarium, I just let it sit in its bucket for about 10 minutes. I pour out the mostly clear stuff on top, and the detritus on bottom (fish and snail poop, decaying plants) goes into various containers I have around the house which house the gammarus shrimp. Most of them are clear plastic containers to make easier collection/observation, but also to give some photosynthesis to whatever algae are in the water (backup food for the shrimp). I also have float duckweed (skimmed off from my aquariums) in these containers as an additional source of backup food. Using this method, I’ve never had to actually feed the shrimp. They are all indoors, but next to a west facing window. The only time I experienced a die-off was when I put some in a used plastic water bottle, but capped it for a few days. The lack of oxygen sufficated them all. But generally, they’re relatively hardy creatures, tolerating even a hot summer afternoon sun.
4 June 2008, 8:58 pmRoland:
Finding alot of interesting information. I would like to culture scuds for the science class. Have access to lots in the lake behing the school.Could I feed them boiled lettuce because this is what I feed some tadpoles and worked well. They eat alot of lettuce. Would appreciate some information. Thank alot.
Roland
29 August 2008, 5:42 pmTim Eisele:
Boiled lettuce sounds close enough to what the commenter above was using, that it should work. Since tadpoles are detrivores too, I would expect than anything that worked for raising tadpoles should be fine for raising scuds too, but I haven’t tried it myself. Please let us know how it comes out!
Hayduke:
I just collected some plants for an aquarium but saved a few along with a lot of hair algae for a ten gallon tank to see what critters I might have collected along with. The plants and pond scum were collected about a quarter mile from some springs feeding a stream. There were some mosquito fish living in the stream.
There are large amounts of daphnia, scuds, isopods, hydra, insect larvae, snails, mites and all sorts of other things that I’ve found in the tank now. I wonder how succesfully the amphipods will survive in the tank as it is? I’m considering moving some of the scuds to another container for cultivation… or maybe turning this 10 gallon into a tank for them.
I really enjoyed this article you wrote. You have a very nice website and I am about to browse the rest of it.
21 December 2008, 2:02 pmCAB:
Last winter I collected a few amphipods from a stock pond in Texas, and put them in a jar next to an east facing window. (At the same time, I also gathered some water plants and grabbed a handful of pond mud to complete their new environment.) They are still going strong 9 months later, slowly increasing in population. At first I did some partial water changes, but even this seems unnecessary.
Just out of curiosity, I put about a 1/4 inch of this pond mud in a 6oz.jar of pond water, sealed it, and put it next to the first batch of amphipods. In a few days, plants started emerging, and a few days later, so did amphipods. This jar has remained sealed since July. The only input has been indirect sunlight. It still has a small population of amphipods. The water is crystal clear. Biosphere 4.
22 December 2008, 3:38 amVictor Carrasco:
Hi!
Thanks for the blog!
I have to say that seawater amphipods Gammarus setosus really looks like this ones, and that feeding behaviour is pretty the same. They even eat freswater worms.
I have a question, maybe if someone know better than me: What would the freshwater amphipod Pallaseopsis quadrispinosa eat in normal conditions? I am feeding them now with tetramin but I would like to know if they would eat algae or other smaller organisms! Particularly, a smaller organisms would be great to know.
2 February 2009, 8:41 amthanks
felix:
Our company supply various of aquarium fish feed.
18 March 2009, 2:27 amEvery year,export about 200tons gammarus.