Rose bitterling

The rose bitterling (Rhodeus ocellatus) is a small, peaceful East Asian fish with one of the strangest breeding habits in fresh water. For most of the year it is an unassuming silvery fish a few centimetres long. Then, from spring into summer, breeding males flush a deep rosy red, sometimes purple, to court females, which is where the name comes from. It is a subtropical, cool-water species that ranges from the Amur basin down to the Pearl River, and it does fine in an unheated tank, comfortable somewhere around 18-24 °C in neutral, moderately hard water. Keep a small group in 80 litres or more and feed it as the omnivore it is, on algae, soft plant matter, and small live or frozen foods.

The remarkable part is how it breeds, because it cannot do so without a partner of a completely different kind: a living freshwater mussel. A ready female grows a long egg-laying tube, about as long as her own body, and uses it to place two or three eggs at a time onto the gills inside a mussel. The male then releases milt over the mussel's intake so the inflowing water carries it in and fertilizes the eggs. The young develop safely inside the mussel for roughly two to four weeks, then swim out of its siphon already shaped like tiny adults, around 7 to 8 mm long. A single female repeats this every week or so throughout the season.

So if you want to watch the full life cycle, you need healthy mussels in the tank, which is a real challenge of its own. Without them you can still enjoy a hardy, active fish that colors up beautifully at spawning time; you just won't get fry.