by Chip Hannum.
Triops may not be as well studied as Escheria coli or Pan troglodytus, but for such a small order of animals they receive their fair share of attention from scientists. There are five principle reasons that triops are or have been significant both scientifically and often economically. Most of the study centers on T. longicaudatus although T. cancriformis and other species are studied as well.
I. Crop Pests
As anyone who has kept triops as pets can attest to, triops are both voracious eaters and incessant diggers. Triops are considered crop pests in rice farming in the U.S. and Europe. In these countries, rice is seeded directly into the mud of freshly flooded paddies. By the time the rice is sprouting, so are the triops. Depending upon the exact timing, the triops may dig up seeds, chew the roots and leaves off of sprouting seeds, uproot small seedlings and leaving them to float about and die. Older seedlings may not be totally destroyed but still be heavily damaged by the triops. It is not uncommon for a farmer to return and find a recently seeded paddy totally decimated by triops infestation. The problem is further compounded in that populations are persistent for years once established.
As a result of this, much study has gone into controlling triops in rice farming, both with pesticides and organic methods. Much of what we know about the precise effects of drying period and humidity on egg hatching have come out of these studies. As an example, one of the proposed techniques for controlling triops damage involves careful manipulation of flooding. It was discovered that the eggs required a two week drying period as well as soil moisture levels below 25% before the maximal percentage of eggs will hatch.
This is exploited by farmers by first flooding the paddies after a prolonged dry period for a few days and then drained. The eggs set to hatch on that hydration do so and the hatchlings are killed when the paddy is dries out. The paddy is not allowed to dry below a 25% moisture level and is reflooded within a two week period. This results in few triops hatching on the subsequent flooding and the damage is within acceptable limits.
An example of the damage triops cause in rice farming. The vegetation floating in the bottom-right corner of the picture are the rice seedlings uprooted by triops activity.
II. Crop Helpers
On the other side of the world, triops are seen as beneficial to rice farming in Japan. This change from pest to helper is due to differences in the farming techniques of Japan. In Japan, rice is grown into small plantlings and then transplanted to the paddies. These rice plants are too large for the triops to uproot or damage. As such, the triops help the rice in two ways. One, they eat fungus off of the plant stems. Two, the same behavior that makes them pests in the West controls weeds in the Japanese rice paddies.
Where U.S. and European scientists have expended time and money to learn how to keep triops out of the rice paddies, the Japanese have expended the time and money to learn how to get triops into the paddies. The lethal range of pH for T. longicaudatus was learned from such research. It was noticed that triops were never found in organic rice farms versus ones using chemical fertilizers. Study revealed that the the soil pH in the organic farms was in the pH 4.0-6.0 range and triops larvae could not live past hatching in these paddies. The addition of garden lime to raise the soil pH in these organic rice farms makes them hospitable to triops again.
III. Food Source
This subject appears to have been largely abandoned, but at one point African scientists were looking at triops as a potential food source that could be aquafarmed in the African environment. A lot of information regarding the specifics of T. granarius lifecycle and reproduction came out of these studies. In the end, though, it was concluded that two things made triops unsuited as an aquaculture subject. One, their aggressive and cannibalistic nature under intense competition made getting large numbers difficult. Two, the chitin to protein ratio in triops makes them rather inedible except to the hungriest of men.
IV. Mosquito Control
One of the general movements since the 1970s is the attempt to move away from chemical control of pests where possible. Mosquitos are a leading vector of spreading disease in human society and their control is of paramount interest. In areas where there is permanently standing water it is possible to introduce species of fish which preferentially prey upon mosquito larvae to control mosquito populations. These programs have had some considerable successes. Some of the limitations for this model of solution are in flood irrigation farms or in catch basins for flood runoff. These waters are inherently temporary and you cannot control the mosquito populations with fish, enter the possibility of using triops as a biological vector of mosquito control.
Triops are attractive for this role because their life cycle exploits the same environments the mosquitos are using for breeding. The triops hatch as soon as the waters come and remain for up to two months. Afterwards, they naturally renew their stock for the next flooding. In controlled laboratory experiments, triops were able to reduce the number of mosquito larvae present by 95%. Further, they reduced the number of eggs layed in the first place by disturbing female mosquitos trying to oviposit. Their attractiveness is increased in that it is extremely easy to establish populations of triops in areas where the general climate is supportive.
In California where these experiments are taking place, they’ve found that after introduction by just a small amount of eggs the triops go onto establish renewable populations in well over 90% of the sites. It is so easy, in fact, to establish triops populations in these environments that they have had to determine pesticide regimens that can keep the triops out of pools without affecting mosquito larvae where they want to grow mosquitos for study. As an added bonus, there is no danger of the triops infesting permanent waters, and they’re not disturbing native populations either because these are man-made irrigation ditches and water basins.
In spite of these promising factors, the debate is whether they are truly effective in a natural setting. The first problem is that although triops are very fast growing and hatch nearly simultaneously as the ditches and basins flood, there is still a delay between hatching until they reach a size sufficient to effectively eliminate any mosquito larvae. The concern is that in this interval, an entire generation of mosquitos may successfully grow and emerge.
In this scenario, although you eliminate subsequent generations in these waters, there is still a huge increase in adult mosquito numbers in the area. Secondarily, triops populations peak at about 3 weeks. If the waters remain longer, it is unknown precisely how efficacious the shrinking triops populations will be at controlling mosquito larvae. The most significant critique is related to the fact that triops are lazy when it comes to getting food. Although classified as a “cruising raptorial predator” (I love that), like many so-called predators, they would much rather get an easy meal than chase down something. Preliminary evidence suggests that when there is vegetation in the pool the triops are ineffective for controlling mosquito larvae because they spend their time eating the easier to catch vegetation. Since vegetation is not exactly a rarity in this world, more research needs to be done to evaluate their effectiveness in the natural environments they would be used in versus fiberglass basins at a research center.
V. They Are Cool
Scientifically, triops are interesting research subjects for a number of reasons. Among them:
- Their antiquity as a species suggests that their physiology and genetics can give us insight into some of the earliest complex life forms.
- Particularly with Lepidurus species, very little specific information is actually known about their natural biology in the wild. T. longicaudatus, cancriformis, and granarius have received the most study so far and the picture is far from complete even with them.
- There are probably dozens of cryptic species currently misclassified and only identifiable with advanced genetic analysis. That’s a lot papers and and small grants for researchers and graduate students.
- Their life cycle presents many fascinating mechanisms for which there is currently only poor understanding. The factors controlling diapause with their eggs, exact reproductive patterns, is their death a programmed cell cycle or simply a factor of no maintenance and repair mechanisms, etc.
- They are easy to work with. Dried eggs can be left on a shelf for years until needed and adults raised in a matter of weeks with very little equipment or space requirements.
- Parthenogenetic reproductive lines make for good research candidates. You automatically get a homogenous population and you can maintain it for years at little expense.