Ten facts about triops
I gasped the first time I saw live Triops. A friend at University was holding a glass jar with a handful of squirming, large horseshoe crab-looking animals. It was love at first sight. When 20 years later the opportunity presented itself to do some research on Triops, I didn’t think twice. Most researchers will say their particular pet organism is unique, but Triops are fascinating in multiple ways and I hope that these ten facts convince them of the awesomeness of triops.
1. Triops belong, together with the genus Lepidurus to the Order Notostraca, a group of crustaceans distantly related to fairy shrimps and water fleas. Triops live all around the world, except from Antarctica. There are about 30 species in the genus, which live in ponds, playa lakes, floodplains and rice fields which dry seasonally. Species are hard to identify morphologically, but they can be told apart using DNA barcoding.
2. Fossil Triops virtually identical in their morphology to living triops have been found in ancient rocks, as old as 300 million years. Their lineage survived the end-Permian, end-Triassic and end-Cretacic mass extinction events with a remarkably conserved body plan, and likely, a similar life cycle. They are often referred to as ‘living fossils’, but Triops haven’t stopped evolving and radiating, if not in their outward morphology, their mode of reproduction and physiology seems to be very flexible.
3. Triops are very fast developers, they need to be, as their habitats can dry before they have chance of reproducing. As other crustaceans, they hatch into a nauplius larvae with has a single eye, and move jerkily with their pair of antennae. They moult several times in quick succession until, after a couple of days they look like tiny adults. They their antennae almost disappear and they swim using their legs. When adults, they moult every few days, and can become quite large. Triops live fast and furious, with a lifespan from 4 weeks to 3 months. Desert species are shorter-lived than temperate ones.
4. Triops reproduces in diverse ways. Many populations consist of males and females, but Triops are one of just a few animals that reproduce by androdioecy: some populations are made of hermaphrodites and a few males. Hermaphrodites can’t cross-fertilise each other, they can either self or mate with males. It is an intriguing reproductive system that appears to be linked to reproductive assurance. Hermaphrodites are better colonisers than either males as a single hermaphrodite is able to colonise a new habitat, therefore androdioecious or hermaphrodite only populations are more common in northerly latitudes.
5. As other invertebrates living in temporary ponds, like copepods, water fleas, rotifers and fairy shrimps, Triops survives dry or otherwise adverse periods through cysts. Cysts are embryos in suspended animation produced by a range of aquatic invertebrates. They are incredibly resistant to extreme temperatures, dryness and even high levels of radiation. Experiments carried out in the International Space Station show that they can survive 18 month exposure to outer space. A single female can lay several thousand cysts in her life.
6. In contrast to their short active life after hatching, cysts are extremely long-lived and can survive in a dormant stage in sediments for decades, allowing Triops to time-travel. Only when suitable conditions return, they hatch. And then, many cysts remain dormant even when conditions are right, waiting to hatch in a subsequent flooding period. This is an insurance survival mechanism, as often ponds dry before individuals have reproduced and this means mass mortality - and would mean population extinction - if not for the remaining cysts in the sediment. Using this ‘bet-hedging’ hatching strategy, Triops can survive in very ephemeral rain-pools in the desert, which only fill in rare rainy years.
7. Cysts are the way Triops colonises new habitats. Triops cysts can disperse in the guts of birds that feed on the egg-carrying females or hermaphrodites or, mixed with soil, in the feet of mammals that drink on the pools they inhabit. Triops has colonised remote islands, like the Galapagos, and no doubt this is due to the resistance and long-term life of their cysts. Their biogeography and phylogeny suggests that the ancestor of American Triops species travelled from Australia some tens of million years ago, possibly with migrating birds.
8. Triops dig nests to lay their cysts. They will readily eat their own cysts if they find them, something that happens when they are not provided with substrate. Nest-building might therefore be a behaviour to protect their cysts from conspecific egg predation (i.e. cannibalism), as there are few if any other sediment diggers in their habitats.
9. Triops are omnivorous, but their diet changes dramatically as they grow. When small they filter feed and scrape algae, bacteria and other unicellular animals, as they grow they become detritus and plant feeders, scavengers and increasingly predators, and will hunt planktonic invertebrates, aquatic insects and anything smaller than them that they can subdue, including other Triops. They spend much of their time ploughing the sediments and swimming just under the water surface in search of floating food.
10. Triops have a song! do you know many invertebrates that do? Go and listen to the awesome They Might be Giants’ Triops has three eyes.