2/13 Gammarus



Poor Gammarus is not the most well known of creatures. This tiny aquatic invertebrate spends its life crawling along the bottom of streams and ponds, eating leaf litter, algae and critters smaller than itself. Only by chance did I discover these tiny shrimplike animals living in two of this forest’s spring fed streams, and though I’d seen small clusters of them when lifting sticks or debris, they didn’t seem very abundant. But then I got lucky…

My luck was not shared by the small herd of Gammarus I observed a few weeks ago, who were likely fleeing a soon-to-be dry streambed. A long line of these tiny (1/2 inch) amphipods clambered upstream, scooting on their sides where the water was shallowest. Like salmon battling their way to the spawning grounds, they had to navigate miniature rapids and waterfalls. This was an “upstream movement”, a seldom observed event triggered by crowding, food scarcity or other circumstances. Such migrations allow amphipods to recolonize stretches of a stream that may have better resources and less competition.

Gammarus emerging at the top of a steep drop-off in the streambed

Mostly, Gammarus just goes with the flow, moving downstream little by little as the current catches it. This is how I first noticed these little amphipods, curled up like tiny shrimp and tumbling along. Which makes it easy to tell them apart from their rather sluggish stream companions the isopods (aquatic versions of the familiar sowbug).

Two hundred species of Gammarus are found in the Crustacean order Amphipoda, and I had to parse through quite a number of research papers (and solicit the help of an inaturalist expert) to narrow it down to the species Gammarus minus. But it was worth it – the literature was chockful of fascinating life history tidbits. One in particular has opened my eyes to the complexity of their lives.

The lower, hunched up Gammarus male is carrying its mate underneath

In my photos some of the tiny (1/2 inch) amphipods appeared to be clutching an even smaller one as they dragged themselves along. Was this a hapless prey item being saved for lunch? Later, searching for more papers to read, I came across this one:

“Energetically costly precopulatory mate guarding in the amphipod Gammarus pulex: causes and consequences  

…..When female receptivity is predictable, but limited to a brief time window during each reproductive bout, precopulatory mate guarding (PCMG) may evolve as a male competitive strategy (Parker 1974). It is particularly common in crustaceans because females can generally be fertilized only for a short period after the moult (Ridley1983)…….”

Quoting another study, “In many amphipods, females can only mate immediately after they have moulted, when their outer carapace has been shed and their new shell below is still soft. Males of some species, including arctic Gammarus, can recognize females that are getting ready to moult, and seize them with their legs, holding on for up to a week in order to ensure that they have first chance at fertilizing her eggs.”

So he was carrying his mate along, maybe for many days, so they could be together at that one brief moment!

Female amphipod with recently hatched young. Image: Monterey Bay Aquarium Research Institute

Not only this, but once the female’s eggs are fertilized she carries them in her marsupium, or brood pouch, till the live young are hatched.

Learning such intimate details gave me great sympathy for little Gammarus, and I wanted to help them. As my readers well know, my way of helping this forest is habitat improvement of every sort I can devise. In fact, I suspected Gammarus and other aquatic invertebrates were already benefiting by my efforts over the years to widen the spring fed streams and encourage water retention in drier periods.


This is a section of stream near post #20 with abundant Gammarus – but the quantity of large and small organic debris in the stream channel didn’t just happen to be there. Because these small streams are impacted by the excess runoff from paved roads and parking lots on top of the ridge, they used to be scoured regularly by swift water during heavy rains. The result was a straight, deep and narrow channel that dried out quickly. The solution, as I saw it, was something I learned from people studying the pristine salmon streams of northern California. “Put the wood in the water” is the mantra there. But on a much smaller scale couldn’t it also help very degraded waters like the ones in this little fragment?

The benefits of woody debris are detailed in this great link:   https://www.deschuteslandtrust.org/news/blog/2016-blog-posts/woody-debris

I’ve copied their list, and note #5 in particular. Amphipods are important as food for many fish, though the ones in our little streams don’t have to worry about this.

1. Large woody debris helps slow the flow of water, making it easier for adult fish to move upstream and for juveniles to rear. This in turn helps them expend less of their limited and valuable energy fighting strong currents.

2. Gravel is an important material for salmon and steelhead to build redds and lay eggs. Adding wood debris to streams helps slow water flow allowing larger sediment like gravel to fall to the streambed instead of continuing downstream.

3. Large woody debris can help decrease water temperature by providing shaded areas along streams and creating pockets of cooler water for cold-water loving species.

4. Large woody debris creates places for fish to hide and seek refuge from predators.

5. Woody debris helps trap organic material like leaves and twigs that provide nutrients for insects and invertebrates (critters without spines), which in turn provide food for fish.

6. Large woody debris helps reinforce streambanks and channels by preventing erosion of soil along banks.

7. Pools of water and “steps” created by woody debris can also provide habitat for fish during periods of low water flow.

Recently enhanced stream habitat at bridge near #20

Like brush piles on land, little debris clumps in streams are habitat hotspots. So I’ve extended my favorite activity of brushpiling to the water, tossing in sticks, bark, armloads of leaves and rotten wood chips. Though amphipods are omnivorous, one of their more important roles is debris cycling in streams, so this should give them plenty  to do.





















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