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.
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.