The Secret of Their Survival

The success of triops hinges upon several key adaptations. Many of these adaptations involve their eggs. The eggs undergo an extreme form of diapause. Diapause is the halting of embryonic development, and in the case of triops eggs, they can survive completely dehydrated with no metabolic activity for years until exposed to favorable conditions again.
There are eggs that were proven viable after 27 years, and they are suspected to be able to survive much longer than that. The eggs are capable of surviving being frozen and exposed to extreme heat (although not boiling). They can even pass unharmed through the digestive tract of another animal and come out the "other end" ready to hatch. These tiny eggs can blow around with the wind as well, ready to land in a ditch puddle and colonize new territory with another generation of triops.

As if these adaptations are not enough, through a mechanism that is not yet understood, triops avoid putting all their eggs in one basket, er, puddle. Specifics vary from species to species, but the general strategy is that a small percentage of eggs can hatch a short period after being laid. This way, if the pool remains for an extended period of time, they can raise more than one generation in a single season. The rest of the eggs require at least one period of drying before they can hatch. Some of these eggs will hatch after just one such drying, but others will not hatch until two or more cycles of drying and wetting have passed. In this way, the triops hedge their bets against any given pool not lasting long enough for their children to reach maturity.

Further, the eggs have more tricks to avoid hatching in unsuitable situations. It's not just water which triggers an egg to hatch, it's also the osmotic pressure of that water as well as temperature and light. In a typical triops environment, the pool fills rapidly with rainwater or runoff from seasonal flooding. In this situation, the water is relatively pure, there are few dissolved minerals or organics - it has very low osmotic pressure.
Conversely, water in an established body has a high level of dissolved minerals and organics - it has relatively high osmotic pressure. Why is this important?
Triops, especially when young, lack the mobility, size, and biological weaponry to avoid any sort of predator their size or larger. An established body of water may contain fish, aquatic insects, amphibians, or even other triops which will make short work of the hatchlings. Further, an established body of water may be an older pool which will not last much longer. So, unless water conditions indicate a new freshwater body, the eggs won't hatch.

Just as important as not hatching into a pond full of hungry minnows, is not hatching into 20 cm of mud. The shell of triops eggs has an alveolar layer - this means it is a spongy matrix of hundreds of tiny, interconnected chambers, all in an egg less than 0.5 mm! When the eggs are first laid, the alveolar layer is filled with fluid and the egg sinks. Here, the eggs can be buried in sediment where they can begin their wait and are less likely to be eaten by the adult triops.
After drying, though, the alveolar layer is empty of fluid and instead filled with air. When the eggs are hydrated, if they aren't buried in sediment, they will float to the surface and be exposed to sunlight. The presence of light is what tells the egg it isn't buried and it is safe to hatch. After a period of floating, water again fills the alveolar layer and the eggs sink again; if they don't hatch, they will again be buried to wait and avoid being eaten by their brethren that do hatch out.

The final trick is sensitivity is to environmental temperature. Triops eggs will generally hatch out within a temperature range of 15° - 30°C (59° - 86°F). Still, this is indicative of a wide range of environmental conditions and triops growth rate is directly tied to temperature right up it becomes life threatening. As a consequence, there is a narrow range of temperature for each species that is indicative of ideal conditions, neither too cool, nor too warm. If the environment is cooler than optimal, growth rate will be impacted and hatchlings may not have enough time to develop before the pool dries. If the environment is too warm, the hatchlings may die once the pool reaches its maximum temperature. Hence, the closer to this ideal range, the higher the percentage of eggs that hatch. Taken together, the adaptations involving their eggs make the triops into wonderfully cautious opportunists.
22°C (71.6°F) appears to be the optimum temperature for hatching.

The second set of key adaptations is where they make their home: temporary water bodies. With the lone exception of Lepidurus arcticus, which is known to sometimes coexist with a single species of fish in a handful of deep Norwegian lakes, none of the larger branchiopods are found in permanent bodies of water unless there are extreme conditions which keep out predatory fish. Clam, fairy, and tadpole shrimp (triops) have adapted to life in temporary fresh or brackish water pools. The brine shrimp lives in lakes so salty that almost nothing else can live there and are often temporary as well.

This is the sort of pleasant environment Triops longicaudatus and newberryi might call home in the American southwest. The naturally formed potholes in the stone fill from desert rains for just a short time

All of the large branchiopods suffer from some common liabilities. Although called the large branchiopods, none of them are more than several centimeters in size. They are unable to move very rapidly, and are uniformly delicious to the animal world. Last, their reproductive strategies do not allow for the attrition of the sort active predation that occurs in a permanent body of water. Long ago, triops adapted to a niche environment, that of temporary pool where there can generally be no predators larger than themselves. Many have further adapted to extremes of pH, surviving in places from acidic peat bogs to alkaline pools. The triops have taken unpredictable and variable niche environments, where it seems nothing larger than freshwater plankton should live, and made them uniquely their own paradise.

The third category of adaptations is what makes it possible for them to live their entire lives in a few liters of rainwater that may only last a few weeks: an incredible metabolism and development rate. Although they may have lain dormant for ten years or longer, once returned to proper conditions the eggs hatch within a very short period. Some species can hatch within a few hours to a day. Others may take longer, but none wait more than a couple of weeks - that's precious time a wasting. The tiny naupilis larvae, initially only 1/20 of a centimeter or so in length, can grow to a few centimeters in just one week - if a human baby grew at the same rate, they would be 40 foot giants at the end of that first week. Particularly in this first week, they grow and metabolise as absolutely fast as they can based upon temperature, oxygen levels, and food availability.

As you might imagine, triops need a lot a food to achieve this phenomenal growth rate. Every day they need to eat roughly 40% of their body mass in food. When you hatch out into a new and temporary body of water, you can't be picky about where that food comes from. To a triops, if it's organic and it fits in their mouth, it's food. They readily consume whatever sort of dead plant and animal matter they find. The aquatic microfauna that forms in the pool, such as bacteria, algae and rotifers, is added to their menu. They nibble at plants growing in the pool. If a mosquito lays her eggs in a pool with triops, she will find that she has provided wriggling snacks to these swimming mouths.
Fellow crustaceans, such as daphnia or fairy shrimp, may find themselves as triops snacks if they have the misfortune to hatch out in a pool with triops. Amphibians that lay their eggs in a triops inhabited pool will find their eggs and young are devoured by the voracious triops. They will even eat one another if other food levels become low, picking off the smaller, weaker triops one by one.

\This incredible growth rate combined with the willingness to eat anything and everything that's smaller than they are allows them to out compete everything in the pool with them. They go from dust speck sized larvae to large creatures capable of predating other creatures in under a week. Always threatened by the specter of their pool drying up, there is no time to waste. They reach adulthood within a few weeks or less: when conditions are right, many species can reach sexual maturity and begin laying eggs before two weeks have passed.

The fourth category of adaptations is in the matter of sexual, or lack thereof, reproduction. Probably nowhere in nature does a single group of species possess such a diverse range of reproductive strategies. There is the obvious division of males and females with "normal" sexual reproduction in some populations. In some, such as T. granarius, sexual reproduction is obligatory. The extreme result of obligately sexual populations is that some of them have become male biased, as high as 70% males. However, most triops can reproduce by parthenogenesis. In parthenogenesis an egg develops into an embryo without having been fertilized by the spermatozoa of a male, sort of a virgin birth. This has allowed many populations to become female biased where males occur infrequently, sometimes less than one male for every one hundred females. This strategy is taken to the extreme with some populations becoming unisexual, composed entirely of females.
Still other triops are hermaphroditic, possessing male and female sex organs. This is even more complex in that some are capable of fertilizing themselves, still others must cross-fertilize with another hermaphrodite, and both types may or may not be able to reproduce parthenogenetically. Some populations are technically hermaphroditic, but appear to only reproduce parthenogenetically, perhaps an evolutionary intermediate to a unisexual population.
The truly bewildering thing about these markedly different strategies is that most recognized triops species utilize more than one reproductive strategy among geographically isolated populations. T. longicaudatus is particularly interesting in that it is known to exhibit all major categories: bisexual (normal distribution and both male and female biased), unisexual, and hermaphroditic. The apparent reproductive incompatibility, as well as recent genetic analysis, suggests that these differently reproducing populations will one day be recognized as subspecies of their current classification.

Taken together, these adaptations have allowed for this tiny group of animals to dominate a particular class of niche environments for three hundred million years. In the time that they've been here on this Earth, no insect, no fish, no amphibian, no reptile, and certainly no mammal, has displaced them as the rulers of the transitory environment of the ephemeral pools.
When one thinks about how much has changed on Earth in that period of time, it is staggering that a life form evolved so long ago is trucking along nearly the same way it always has. Their strategy has served them well so far. If man can keep from destroying their habitats, it is likely they will be here in another three hundred million years.