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xenxes 07-23-2012 02:02 AM

Predatory Scuds Hunting My Baby Shrimp?!
Just read this old study on introduction of gammarus into lakes to raise fish production.. these 2 particular species breed faster and can be predatory in nature.


Both Gmelinoides fasciatus and Gammarus lacustris are omnivorous animals that are able to feed on detritus, plant debris and periphyton, but they prefer to prey on small invertebrates
Gmelinoides fasciatus

Gammarus lacustris

I think most of us have Gammarus pulex in our tanks, a detrivore that breed slower, but may also exhibit predatory behavior towards young shrimp. All I could find was this: There is mutual predatory behavior between Gammarus pulex and Gammarus duebeni celticus on the newly molted ( It only makes sense that they would predate on newly born shrimp.

I can't really tell any of them apart, trying to Google out more pics. Main worried because I have TONS of scuds in my 20L and never see any baby shrimp around. I removed my platies, corydoras habrosus, so all that's left are the scuds and a few CPDs. Previously I thought they were harmless, maybe not so much...

mordalphus 07-23-2012 02:17 AM

By small invertebrates, I'm pretty sure they mean copepods and smaller.

10gallonplanted 07-23-2012 02:30 AM

A baby shrimp isnt really that much larger than a copepod, maybe twice the size but that isnt much. Just throwing out some possibilities. But in my opinion a scud is much too slow to catch a shrimplet that can shoot backwards super fast, but if the shrimp was used to the scuds it could be a different story.

Anyways, they will eat your moss for sure!

stevenjohn21 07-23-2012 02:31 AM

I had scuds in a tank over a year ago , since then I've had cichlids, crayfish and angelfish (not at the same time) I scooped up some of the substrate from that tank into a 10 gallon for my cherry shrimp and a week after setting it up I see scuds !!!! They are impossible to get rid of so I hope they are not killing the baby shrimp .

xenxes 07-23-2012 03:09 AM


The predatory ‘clumping’ behaviour of both species is described and shown to be a form of feeding frenzy upon congenerics. Clumping behaviour also results in significantly higher rates of congeneric predation on G. d. celticus females than on G. pulex females. Support is thus given to the hypothesis that differential predation by males on moulted females may be the primary mechanism by which the introduced G. pulex has displaced the native Irish species G. d. celticus.
That sounds like adult female scuds--after molt--the other species clump onto it and eat it? I haven't seen them eat moss though, just hide under my fissidens.
Looks like I have the g. pulex? Idk.

I just trapped ~100, turkey bastered out ~100, and probably have 1000 left...

Threw about 5 tubs of these out so far, they're still visible on the substrate swimming all over the place. AGHHHH. I guess they've always been around, but after removing my platies and habrosus they are much more active.

WestHaven 07-23-2012 03:17 AM

How do they get into a tank in the first place?

xenxes 07-23-2012 03:27 AM

I think mine came from a LFS, I thought it was a baby shrimp... stupid me. They carry/hatch eggs much the same way shrimp do.

Their mating is interesting.. I saw quite a few of them attached (one on the other's back)


The shrimp will mate several times per year. During this process the male carries the female on his back as they are swimming. The female carries about 50 fertilized eggs in her egg pouch and these are orange in color and show through her semi-transparent body. The young shrimp hatch within the egg pouch and emerge as fully developed young shrimp but microscopic in size. The young develop an exo-skeleton but molt this a number of times as they increase in size. Sometimes these discarded exo-skeletons can be seen floating on the lake surface. Since the shrimp have no natural defense mechanisms, except camouflage, they usually hide in the Chara weeds and are more active during periods of low sunlight or after dark.

xenxes 07-23-2012 02:54 PM

I left a bottle trap in last night and came out this morning to find everything inside but the scuds were dead, casualties: 7 sakura/pfrs, 1 cbs, 2 c. pygmaeus. I have tiny needle poked holes all over the top of the bottle trap so I don't think it's stagnant water.

I had previously trapped c. pygmaeus with rcs before for the same amount of time ~6hrs, and never had a casualty. Only difference is the hundreds of scuds. So they either (1) maliciously clawed the victims to shreds, or more likely they (2) scared/harassed them to death wiggling all over the bottle.

(1) if we look at the anatomy, the first 2 gammarus species is capable of shredding into prey with its large gnathopods/claw

(2) whereas the pulex only has prehensile legs

The ones that weren't mating were feasting on the corpses.

I'm not certain which species I have, the macro shot above was the best I have and the front pods are tucked in.

On a happier note, my filter box is blooming

Geniusdudekiran 07-23-2012 03:57 PM

Darn you Xenxes, I was eating breakfast when I opened this thread. WAS. Lol

xenxes 07-23-2012 04:04 PM

Lol sorry, I'm at war with scuds! Hope this doesn't lead to a tank tear down.

This is what I have. It looks much more vicious than the benign g. pulex. I'll post if I get a positive ID. Getting more and more suspicious this is the reason I have no baby shrimp in a large heavily planted well fed tank.

Bottom pic is mine.

Perhaps even the common pulex is not as harmless as I thought, here a pic of one that caught a blood worm:

acitydweller 07-23-2012 04:11 PM

yep. these are the guys i have in one of my tanks.

I was about to bridge my two 5.5 gallon tanks together but luckily i noticed these buggers before doing so. These guys are prolific breeders...

When these guys swarm, you better watch out. Hard to tell in the photo but were the legs and eyes missing on your sakura/pfr's?

xenxes 07-23-2012 10:09 PM

Eyes weren't missing, but the fish was ripped open at the gut. I'm still have trouble believing they're capable... can't tell if they have claws or not from these pics. It almost seems like some do and some don't? Maybe I have 2 species.

Wish I knew what species these are. Maybe instead of fighting it, I should just go from being a shrimp keeper to scud keeper. I mean, I still have hundreds.

WTS: blue rili scuds $1 each. Lol.

EntoCraig 07-23-2012 10:17 PM

Awesome Thread! I have scuds in my bucket ponds for the fish out back. I have always considered bringing some inside... I do keep terrestrial Isopods in my frog tanks.

hedge_fund 07-23-2012 11:41 PM

people actually sell these on aquabid

hedge_fund 07-23-2012 11:49 PM

...also, since you have so many scuds it's probably good if you run some experiments to see whether they actually do attack any tank inhabitants. maybe purposely injure a small shrimp and see if they go for's not cruel if it's in the name of science!

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