Copepods, seed shrimp and scuds in a shrimp tank
Planted shrimp tanks are full of tiny life, and three small crustaceans turn up more than anything else: copepods, seed shrimp (ostracods) and scuds (amphipods). All three usually arrive as hitchhikers on plants, moss or wood, and all three can seem to appear overnight once there is spare food. They are relatives of your shrimp, not worms or parasites, and the first two are almost always harmless.
Telling them apart
| Copepods | Seed shrimp (ostracods) | Scuds (amphipods) | |
|---|---|---|---|
| Size | Almost microscopic to about 1–2 mm | Usually about 1 mm or less, often smaller than a poppy seed | About ¼–¾ inch (6–20 mm) as adults |
| Shape | Teardrop body, long antennae, often a forked tail; females may carry two egg sacs | A tiny two-part shell like a clam or seed, so the legs barely show | Curved, shrimp-like body flattened from side to side, many legs |
| Movement | Jerky hops and darts in open water and on the glass | Steady scurrying over glass, leaves and substrate; some swim fast | Swims on its side; hides from light under leaves and wood |
| What it eats | Varies: some filter tiny particles, some scrape surfaces, some catch tiny prey | Biofilm, algae, bacteria and dead material | Mostly dead plant material, algae and leftover food |
| Harmful to shrimp? | No, in normal numbers | No | Usually not to adults; competes for food, may eat soft plants |
Under a magnifying glass, the seed shrimp's shell is the giveaway: the University of Florida describes ostracods as having a bivalved carapace, which is why they are also called "mussel shrimp," and to the naked eye they look like dark dots swimming around. Scuds are easy to mistake for baby shrimp at first; look for the side-on swimming and the flattened body. If what you're seeing is a worm rather than a crustacean, see white worms in a shrimp tank.
Are they a problem for shrimp?
- Copepods: shrimp keepers treat them as a normal, harmless part of a planted tank. Texas Parks and Wildlife notes that some copepods catch and bite tiny prey, so if you breed shrimp and copepods are in the thousands, it is one more reason to cut the food that feeds them.
- Seed shrimp: harmless grazers. The University of Florida describes one common species, Cypridopsis vidua, feeding on the film of algae, bacteria and detritus that covers plants, the same biofilm your shrimp graze, and hiding when it senses fish. A huge population mainly means competition for that food.
- Scuds: the one to watch. They are mostly scavengers and detritivores, but they get much bigger, breed quickly, and in large numbers can strip soft plants and moss and outcompete shrimplets for food. They don't hunt healthy adult shrimp. Many keepers are happy with a few as cleaners; breeders often remove them.
Why there are suddenly so many
- Overfeeding. By far the most common reason. Leftover food and powder that sinks into the substrate feed all three, and their numbers rise to match. See how long shrimp can go without food and what shrimp eat.
- No predators. A shrimp-only tank has nothing that eats them, which is good for shrimplets but means the critters keep whatever food they find.
- They breed fast. The University of Florida notes that some seed shrimp can reproduce without males and produce drought-resistant cysts. Scud females brood their eggs in a pouch, and the young hatch fully formed, with no larval stage (Missouri Department of Conservation; Minnesota DNR). A few hitchhikers become a colony quickly.
How to cut them back without chemicals
- Feed less. Feed only what your shrimp clear within a few hours, skip a day or two a week, and remove uneaten food. This alone usually brings copepods and seed shrimp back to unnoticeable levels within a few weeks.
- Siphon the substrate surface and pick out decaying leaves during your normal water changes.
- Trap scuds. Put a small piece of blanched vegetable or a pellet in a shrimp-safe trap or a small dish overnight, then lift it out with the scuds in it. Check the catch for shrimplets before you discard it. A turkey baster or fine net works for strays; see how to catch shrimp in a tank for gentle netting tips.
- Clean the glass. Wiping the front glass removes the copepods and seed shrimp you can see, though it doesn't change their numbers much.
- Be patient. Populations fall with the food supply; they rarely vanish completely, and that's fine.
Treatments to avoid
These animals are crustaceans, like shrimp, so anything strong enough to kill them is a serious risk to your shrimp. Don't use insecticide-style or copper-based products in a shrimp tank (see is copper toxic to shrimp). Betel-nut planaria treatments and fenbendazole dewormers are aimed at worms, not crustaceans, and they typically kill snails; see planaria in a shrimp tank and hydra in a shrimp tank for when those make sense. For copepods, seed shrimp and scuds, cutting food is the treatment.
Keep them out of a new tank
If you'd rather not have them, especially scuds, rinse new plants and moss well and inspect them, or hold them in a separate container for a couple of weeks before they go into the tank. Buying tissue-culture plants avoids most hitchhikers. See how to quarantine shrimp and pest snails in a shrimp tank, which arrive the same way.
Track it
Note what and how much you feed on the free Water Parameter Log printable, alongside a rough count of critters on the glass, so you can see the link between food and numbers.
Frequently asked questions
What are the tiny white bugs on my shrimp tank glass?
If they hop in jerks, they are almost certainly copepods. If they are bean-shaped and scurry steadily, they are seed shrimp (ostracods). Both are harmless to shrimp.
Do copepods eat baby shrimp?
Copepods are generally treated as harmless in shrimp tanks. Some types catch tiny prey, so in a breeding tank it makes sense to keep their numbers down by feeding less.
Are seed shrimp bad for shrimp?
No. Seed shrimp graze biofilm, algae and detritus. A very large population mostly means there is too much food, and they compete with shrimplets for it.
Will scuds kill my shrimp?
Scuds don't usually harm healthy adult shrimp. In large numbers they compete for food and can damage soft plants and moss, so many breeders trap them out.
How do I get rid of copepods and seed shrimp?
Feed less, remove uneaten food and siphon the substrate surface. Their numbers fall with the food supply over a few weeks. Avoid chemical treatments.
Can I use No Planaria to kill copepods or scuds?
It isn't made for them, and betel-nut and fenbendazole treatments typically kill snails. Save them for planaria or hydra outbreaks that don't respond to feeding less.
How did copepods get into my tank?
Usually as tiny hitchhikers on plants, moss, wood or in water from a shop or another tank. Rinsing new plants, a short holding period or tissue-culture plants keep most of them out.
Sources: University of Florida IFAS EDIS, Cypridopsis vidua, a seed shrimp (IN1260) (about 8,000 ostracod species; called seed shrimp because they are often smaller than a poppy seed, up to about 1 mm; bivalved carapace, "mussel shrimp"; Cypridopsis vidua feeds on periphyton of algae, bacteria and detritus and hides when it detects fish; can reproduce by parthenogenesis; some produce drought-resistant cysts; seen as dark swimming dots); Missouri Department of Conservation, Scuds (sideswimmers, amphipods) (amphipods, "shrimplike sowbugs"; bodies flattened side to side; about ¼–¾ inch; mostly detritivores; most avoid light; swim on their sides; females brood eggs in a marsupium; no larval stage); Minnesota DNR, Minnesota Conservation Volunteer: freshwater amphipods (amphipods up to about 2 cm; eggs carried in a brood pouch; feed on dead plants and algae); Texas Parks and Wildlife, Common freshwater organisms (copepods nearly microscopic, with filter-feeding, prey-seizing and scraping types; seed shrimp pinhead-sized, clam-like, fast swimmers eating dead material; scuds up to ¾ inch, scavengers). Feeding and control advice is hobby practice compiled from owner reports.