Seed Shrimp (Ostracods): ID & Control Guide

Tiny bean-shaped crustaceans in a hinged bivalve shell that zip across glass and substrate. Harmless detritivores that bloom on overfeeding and are hard to fully eradicate.

Overview & Identification

Ostracods, popularly called seed shrimp, are small crustaceans whose body is enclosed in a bivalve-like valve or 'shell' made of chitin and often calcium carbonate, hinged along the back. Most are around 1 mm across, though the class ranges from 0.2 mm up to 32 mm; the largest freshwater species, Megalocypris princeps, reaches about 8 mm. In a tank they look like fast-moving beans or sesame seeds darting over glass and substrate, swimming with two pairs of well-developed antennae.

Rounded, bean- or seed-shaped hard shell that snaps shut
Typically ~1 mm, rarely larger in freshwater
Jerky, fast scooting across glass and substrate
Some freshwater species cannot swim and stay benthic

Where They Come From

Non-marine ostracods inhabit fresh water of all kinds, and around 2000 freshwater species in 200 genera are known. They hitchhike into aquariums on plants, driftwood, substrate or with live foods. Some species are partially or wholly parthenogenetic, so a single individual or its eggs can found a population, and certain ostracods tolerate extreme conditions such as acidic water near pH 3.4.

Harmful or Beneficial?

Most ostracods are deposit feeders, so they consume detritus, leftover food and decaying matter and are generally harmless. The class also includes herbivores, scavengers and filter feeders. They are essentially part of the cleanup crew; the main concern is cosmetic, and in very large numbers they may pester shrimp eggs, so heavy blooms are worth managing in shrimp tanks.

Control & Population Management

Cut feeding hard so leftover food no longer fuels the bloom
Siphon detritus and gravel-vacuum the substrate during water changes
Manually remove shells you see on the glass at water-change time
Accept that they are hard to fully eradicate and aim to keep numbers low

Prevention

Because they molt several times and reproduce repeatedly as adults, prevention is easier than removal. Quarantine and rinse new plants and decor, avoid moving substrate between tanks, and feed sparingly so detritus does not accumulate.

Common Mistakes

Dosing harsh chemicals that stress fish and shrimp instead of cutting food
Assuming they harm healthy fish when they are detritivores
Expecting total eradication rather than ongoing population control
Ignoring the overfeeding that drives the bloom