Invertebrates
Triops
Triops · also called tadpole shrimp, dinosaur shrimp, shield shrimp
A triops looks like something dredged from deep time, and in a sense it is: a low, shield-backed crustacean paddling across the floor of a rain puddle, its body plan barely altered since before the dinosaurs. It lives fast — hatched, grown, and breeding in a fortnight — because its pool will soon be dust. Its real trick is the egg it leaves behind, which can wait dry in the soil for decades.
Overview
A triops, or tadpole shrimp, is a freshwater branchiopod crustacean with a broad shield-like carapace over its head and front, a long segmented tail, and dozens of leaf-like legs. Its name means “three eyes” — two compound eyes and a simple light sensor between them. It is an omnivore and scavenger of temporary pools on every continent except Antarctica, absent from lakes and rivers only because fish there eat it.
A living fossil
Fossils of Triops cancriformis from the Triassic, ~250 million years ago, are almost identical to living triops, making the genus one of the oldest surviving animal genera — it has kept essentially the same form through every mass extinction since the Permian.
Life at speed
A triops hatches under a millimetre long and reaches adult size and breeding age in about two weeks, then lives only a month or two. Many populations are all-female and self-fertilising, so one stray egg can found a new population. Survival rests on cysts — drought-proof eggs that lie dormant in dry soil for years, hatching within a day when rain returns, and only partly, as a hedge against false starts.
As a kit animal
Triops are sold like brine-shrimp sea monkeys — a packet of cysts, food, and a bowl — but grow far larger and faster, up to several centimetres, which makes them a rewarding short-term project. They must be kept fish-free and often alone, since they eat snails, shrimp, and each other.
Status
Mostly not evaluated and often abundant, but temporary wetlands are vanishing to drainage and development. Triops cancriformis is legally protected in the UK, where it clings on at just one or two sites.
Triops: common questions
Are triops the same as sea monkeys?
No, though they are sold in similar kits and both are branchiopod crustaceans with drought-proof eggs. Sea monkeys are brine shrimp, genus Artemia, which are small, delicate, filter-feeding animals that swim upside down and live in very salty water. Triops are tadpole shrimp, much larger and sturdier, with a hard shield over the front of the body, dozens of legs, and a scavenging, sometimes predatory habit, and they live in fresh water and spend most of their time on or near the bottom. Triops hatch from cysts the same way, grow much faster, and reach a size, up to several centimetres, that brine shrimp never approach, which makes them a more visibly rewarding kit animal, but they also foul a small bowl faster and will eat each other.
How long do triops live?
Not long. A triops is built for a pool that will disappear, so it hatches, matures, and breeds as fast as it can, reaching adulthood in roughly two weeks and starting to lay eggs soon after. Even with a constant water supply and good care in an aquarium, most triops live only about one to three months, with the female often outliving the male. The species is effectively an annual animal whose real continuity is in the egg bank in the soil, where cysts from one wet season wait, sometimes for many years, for the next good rain. So a triops kit is a short project by design, and the way to keep the line going is to let the tank dry out and save the substrate with its eggs.
What does it mean that triops are living fossils?
It means the animal looks almost exactly like its ancient fossil relatives and has changed very little over an immense span of time. Fossils of Triops cancriformis from the Triassic period, around 250 million years ago, show the same shield-shaped carapace, the same paired eyes, and the same general body plan as triops alive today, which is why it is often called the oldest living animal species, although modern genetic work shows the living populations are not literally unchanged and the picture is more complicated. The broader point holds, the tadpole shrimp body plan is extremely old and has survived every mass extinction since the Permian, probably because life in unpredictable temporary pools, with a bank of tough eggs in the mud, is a very robust survival strategy.
Can triops live with fish?
Generally no, and that is why you never find them in lakes, rivers, or established ponds. Triops evolved in fish-free temporary pools and have no defences against fish, which eat them readily at every life stage, so a triops tank has to be fish-free. They are also risky with other invertebrates, as an adult triops will hunt and eat snails, shrimp, small worms, insect larvae, and smaller or moulting triops, so they are usually kept in a species-only setup. The classic pairing is triops with a few ramshorn or bladder snails as clean-up crew, accepting that some snails will be eaten, or triops kept entirely alone in a simple tank with sand, a gentle filter or none, and a warm spot in indirect light.