Marine Gammarus (Gammarus oceanicus) may grow up to 27 mm in males and 22 mm in females. A rather uniform grey, yellow or greenish-brown, often darker in the female, no distinct colour markings. They live in coastal marine habitats, from mid-tide level to shallow subtidal, occasionally to depths of 25 meters; usually amongst algae; tolerates a wide range of salinity and temperature. Gammarus oceanicus can be bred in brackish water. They are largely scavengers feeding on dead plant material and other decaying organic matter.
You can keep them outdoors in a bucket or any water container. Make water level about 50 – 100 mm, put Gammarus, watercress, other water weeds, dead leaves, etc. Keep the buckets outdoors in cool conditions, it must not get too hot in summer. They will survive under ice. They are very hardy and can certainly stand an immediate temperature change from ice at 0°C to 22°C. Gammarus oceanicus do not like foul water: this will be low on oxygen and the Gammarus will suffocate.