By Chip Hannum (updated by Stuart Halliday).
Triops, sometimes called tadpole, dinosaur, or shield shrimp, are a small group of crustaceans that are in the Branchiopoda group that are found in temporary bodies of fresh or brackish water. Sometimes they are sold in ‘Sea-Monkey’ like kits in toy shops, and they are often billed as ‘living fossils’.
This name is well earned. This truly ancient group has survived virtually unchanged for roughly three hundred million years. The oldest Triops fossils date back to the Carboniferous Palaeozoic, and the world’s oldest known living animal species is the European triops, Triops cancriformis.
T. cancriformis fossils date back 200 million years ago to the Upper Triassic age. There were triops swimming around in pools of water when the first dinosaurs walked the Earth, and now 65 million years after the last dinosaur took a breath, the Triops are still here!
In the time that the triops have been here, the Earth has undergone countless changes. The land has gone from a single super continent, Pangea, to the seven continents of today.
The climates have cycled hot and cold and back again many, many times. Almost every animal species alive today has evolved since the Triops appeared on the scene. Thousands of more species that evolved within that time, thrived for a bit, and were driven extinct for one reason or another.
The triops have been there through it all. They are, in every sense of the word, biological marvels of survival and niche adaptation. They have done more than just survive, though. Today, triops are found on every continent except Antarctica, and there are at least 15 known species.
The most common species of triops you can buy over the counter is the American Triops longicaudatus (though other species are often sold on online auction sites). This is a more golden coloured animal and usually grows slightly smaller than Triops cancriformis and requires a touch warmer water temperature to live in. It is, however, a lot younger in the fossil records. Dating back to the Cretaceous-Maastrichtian period (about 70 million years).
Triops cancriformis has been seen in the wild up to 11cm (4.5″) in length (not including the tail). But it’s more usual to see this species in a tank grow up to 8cm (3″) in length. The average Triops longicaudatus will grow up to 6cm (2.5″) in a tank.
- Kingdom: Animalia
- phyla: Arthropoda
- subphyla: Crustacea
- class: Branchiopoda
- order: Notostraca
- family: Triopsidae
- genera: Lepidurus, Triops
Currently, 15 superspecies are recognised, with an additional seven or so subspecies belonging to them. Future genetic analysis will likely expand this number significantly.
The branchiopods are a primitive order of crustaceans. Lacking true gills, they breathe with leaf-like extensions on their legs. Scientists, being so original when it comes to these things, named them for this; branchiopod means ‘lung or gill foot. The alternative name for the branchiopods, phyllopods, is equally original; it means ‘leaf foot’. There are four extant orders of branchiopods: Anostraca, Cladocera, Conchostraca, and Notostraca. The anostracans contain the brine shrimp and numerous species of fairy shrimp. The cladocerans are composed of the daphnia species (water fleas). The conchostractans contain the lesser-known clam shrimp. Finally, the notostracans are composed of various species of tadpole shrimp, often known as triops. Generally, biologists group the anostracans, conchostracans, and notostracans as the (also originally named) ‘large branchiopods’.
The notostracrans contain only a single family, Triopsidae, and two genera, Lepidurus and Triops. All triops share a common body plan that looks like a cross between a horseshoe crab and the extinct trilobites. Commonly billed as ‘living fossils’, their appearance certainly fits this description. Species from both genera look very similar except for the key distinguishing characteristic of Lepidurus species: in between the two tail spines, Lepidurus species have a distinct supra-anal plate. In Triops species, this plate is either absent or extremely reduced. Sizes vary from species to species, but none are very large. T. cancriformis holds the size record, with one wild-caught specimen reported to measure 11 cm (almost 4.5 inches!).
Lepidurus packardi is pictured on the left. Note the presence of the supra-anal plate between the two tail spines (known as rami or furca). L. packardi is only found in certain areas of California in the U.S. Limited to an ever-shrinking habitat, it is on the endangered species list.
Triops live in temporary water pools (also known as vernal, astatic, or ephemeral pools) that generally occur during seasonal rains or flooding. As a rule of thumb, Triops species tend to be found in areas where warmer, shorter-lasting pools are common, and Lepidurus species are found where cooler, longer-lasting pools are the rule. Not only might a given pool only last a few weeks before completely drying out, but it may also be years, or even decades, before the water comes again if rainfall is erratic. How a creature that breathes with primitive gills is able to survive these conditions is why they have lasted through the aeons when so many other species have not.
Now read how and why the triops have been so successful.
- Klaus-Peter Kelber – Kelber, K.-P. (1999): Triops cancriformis (Crustacea, Notostraca): Ein bemerkenswertes Fossil aus der Trias Mitteleuropas.- In: Hauschke, N. & Wilde, V. (eds.): Trias – Eine ganz andere Welt, III.16: 383-394; (Verl. Dr. F. Pfeil), München.