Scuds (Gammarus Amphipods): ID & Control Guide

Side-swimming amphipods with laterally flattened bodies that scud through the tank. Mostly harmless detritivores and fish food, but big populations can nip eggs or fry.

Overview & Identification

Scuds, also called sideswimmers, are freshwater amphipods, most often in the genus Gammarus, a species-rich crustacean group of over 200 species. They have laterally compressed bodies with no carapace, sessile eyes, and 13 body segments split into head, thorax and abdomen. Most are under 10 mm long. Their distinctive scudding motion, with the animal tilted onto one side, is the easiest way to tell them apart from upright-swimming critters.

Body flattened side-to-side, like a comma or shrimp
Usually under 10 mm long
Swims tilted onto its side ('scudding')
No shell or carapace; sessile (stalkless) eyes
Front legs (gnathopods) grasp food

Where They Come From

There are roughly 2,250 freshwater amphipod species, and Gammarus pulex is purely freshwater while others are estuarine. Scuds arrive in aquariums hidden among live plants, in substrate, or with pond water and wild-caught live foods, where they quickly establish among leaf litter and detritus they feed on.

Harmful or Beneficial?

Most amphipods are detritivores or scavengers, and some graze algae, acting as mesograzers that help control algal growth, so scuds are largely beneficial cleanup-crew members and excellent fish food. The honest downside: some are omnivores or predators of small invertebrates, so very large populations may nip at eggs or weak fry and compete with shrimp, which matters mainly in breeding or shrimp tanks.

Control & Population Management

Add fish that actively hunt scuds to graze the population down
Cut feeding and siphon detritus so there is less to scavenge
Trap them with a sinking veggie slice removed after dark
Gravel-vacuum during water changes to pull out hidden individuals

Prevention

Quarantine, rinse and inspect new plants and decor before adding them, since scuds hitch in on live plants. Avoid wild pond water and untreated wild live foods, and feed conservatively so detritus does not build up into a food source for an exploding population.

Common Mistakes

Treating all scuds as pests when they are mostly beneficial cleaners and fish food
Letting numbers explode in a fry or shrimp tank
Skipping plant quarantine, the main entry route
Using harsh chemicals instead of fish predation and detritus control