European bitterling

Identification

The European bitterling is one of the smallest fish species in Dutch waters — with an average length of just 5-7 cm. The body is deep and strongly laterally compressed, with conspicuously large scales for such a small fish. Normally the fish is silvery-grey with a blue-green iridescent stripe along the rear flank. During spawning season, males color spectacularly: the flanks turn pink to purple, the belly orange-red and the eyes bright red. Females develop a long ovipositor tube used to deposit their eggs inside freshwater mussels.

Behavior & ecology

The bitterling has one of the most remarkable reproductive strategies of any Dutch fish. The female lays her eggs via the ovipositor into the gill cavity of large freshwater mussels (Unionidae). There, the eggs develop protected from predators. In return, bitterlings transport the mussel larvae (glochidia) that temporarily attach to the fish's gills — a perfect symbiosis.

Bitterlings live in small groups in shallow, still or slow-flowing waters with soft, muddy bottoms. They feed on algae, plant material and small invertebrates. The presence of bitterling is a strong indicator of healthy waters with living freshwater mussel populations.

In the Netherlands

The bitterling is native and protected under the European Habitats Directive (Annex II). The species occurs scattered across lowland waters throughout the Netherlands, but depends on the presence of freshwater mussels. Due to declining mussel populations (water pollution, dredging, loss of suitable substrates), the bitterling has become rare in some regions. Stream restoration and water quality improvements benefit both the mussel and the bitterling. The bitterling is not a sport fish — its angling significance is nil, but its ecological importance is great.