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 to turn over some rocks to see what was underneath. 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 at the time they explicitly excluded fully-aquatic non-insect arthropods (this has changed since 2008 when I first wrote this page, by the way. As of 2013, BugGuide now includes freshwater crustaceans, and also any saltwater crustaceans that are likely to be found walking around on beaches).
S_ did a bit of rummaging around, and found out that they were freshwater amphipods, in the genus Gammarus, 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 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.
 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.
 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.
 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).
 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.