Life in a Puddle
The dormant embryos of “Sea-Monkeys” and other branchiopods can survive being frozen, thawed and even eaten by a bird.
By Dyanne Fry Cortez
Some critter sightings leave lasting impressions. Javier Cortez was working with a survey crew in the 1970s when he found a colony of tadpole shrimp in a pool of rainwater on a Kerr County ranch. At the time, he had no idea what they were. “I thought they were tadpoles at first,” he says. “They were brown and had that teardrop shape, round in front, narrow in back.”
Close inspection changed his mind. A tadpole has a tail, but this animal had two lengthy projections at the back end. The rounded front was a shell-like structure that covered most of the body. A co-worker flipped one over to look at the underside and found dozens of tiny, wriggling legs. Cortez wondered if his crew had found the last remnant of a prehistoric life form.
In fact, tadpole shrimp are alive and well in Texas. They belong to a group of freshwater crustaceans that also includes fairy shrimp and clam shrimp. Collectively known as branchiopods, these animals have a knack for living in waters that are too salty, too small or too uncertain to support most forms of aquatic life.
Sharp-eyed observers can find branchiopods in the Panhandle’s playa lakes, the vernal pools of Enchanted Rock SNA and Hueco Tanks SHS, stock ponds, roadside ditches and other spots that hold water at certain times of year.
Branchiopods are distant relatives of the saltwater shrimp sold in seafood restaurants. Like all crustaceans, they have segmented bodies with compound eyes, chewing mouthparts and two sets of antennae. It may take a magnifying lens to discern those features. Most branchiopods are no more than two inches long, and some are considerably smaller.
Fairy shrimps (order Anostraca) make up the largest group, with close to 300 species worldwide. They’re the only branchiopods that actually look like shrimp, though they lack the tough outer shell of their seagoing kin. North American species have stalked eyes and 11 pairs of legs that are used for swimming and gathering food. They tend to swim belly up.
A few species qualify as giants, with adults measuring three to six inches, but most fairy shrimps are tiny. The 16 species found in Texas range from half an inch to an inch and a quarter when full grown.
This order includes brine shrimp (Artemia sp), which were packaged and sold as “Sea-Monkeys” in the 1960s and ’70s. Found in dime stores and comic-book ads, Sea-Monkeys were marketed as “instant pets,” good for hours of fun. Brine shrimp are also sold as fish food for the hatchery and aquarium trades. In nature, brine shrimp prosper in the Great Salt Lake and in other highly saline inland waters.
Clam shrimps, as the name suggests, resemble tiny clams, with a bivalve shell or carapace that encloses the whole body. They are usually classified as order Conchostraca, but some scientists divide them into two orders based on shell characteristics. About 30 species are known in North America; several are found in Texas. Clam shrimps don’t get much bigger than a half inch. Depending on the species, they have 10 to 32 pairs of legs. The legs aid feeding, but are seldom used for locomotion; clam shrimp row, using their second antennae as oars.
Tadpole shrimps (order Notostraca) have 25 to 44 pairs of legs, which are mostly hidden under the horseshoe-shaped carapace. Six species of tadpole shrimp exist in North America. One of these, Triops longicaudatus, is common in Texas. A large specimen might measure an inch and a half from the tip of the carapace to the end of the “tails,” which are properly called cercopods.
Branchiopods find plenty to eat in their small ponds. Most fairy shrimp are omnivorous filter feeders, using their waving legs to collect bacteria, free-floating algae and microscopic animals. A few larger species are carnivorous, dining exclusively on small animals (including smaller fairy shrimp). Clam shrimp move slowly, bumping through mats of algae, feeding on detritus or plankton. Tadpole shrimp cruise the bottom of the pond, sometimes plowing through the mud. They also have been known to eat fairy shrimp, as well as algae, amphibian eggs, tadpoles and insect larvae.
Casual observers usually find branchiopods by accident. The creatures seem to come from nowhere, sometimes appearing in landlocked pools that weren’t even there a few weeks earlier. One summer, after an unusual wet spell, a West Texas family contacted the Texas Parks and Wildlife Department with photographs of tadpole shrimp in a stock pond. “This pond hasn’t held water in 15 years,” the landowners wrote. “Did these things fall out of the sky?”
They may have.
Dried branchiopod eggs can be carried on the wind. It’s more likely, however, that the eggs were there in the dry pond for 15 years, lying dormant in the dust, waiting for the moment when conditions were just right to hatch. If branchiopods had a class motto, it might be Carpe diem, or Seize the day.
When the rainy season arrives, they don’t mess around. The “eggs” left over from the previous season are not, strictly speaking, eggs. They’re cysts: partially developed dormant embryos. When conditions are right, these come to life. Larvae appear a few days after a pool fills with water. They grow and develop quickly, reach maturity and mate. Females carry their fertilized eggs for a few days, perhaps providing some protection for the developing cysts. Some species drop cysts over several days in a series of clutches; some release them when the mother dies. The cysts sink to the bottom of the pond and settle in to wait.
In a good year, the shrimp get it all done before the pond dries up. The active phase of the life cycle typically takes three to four weeks. Some species require more time, some less. One type of clam shrimp has been known to complete the whole process in 15 days.
Branchiopods typically breed one generation per wet season, but some will do more if time permits. These species can produce two types of cysts: “summer eggs,” which hatch almost immediately, and “winter” or “resting eggs,” designed for the long haul. An embryo in the resting state can survive freezing, thawing, desiccation, dust storms, heat waves and extended droughts. Laboratory samples have hatched after sitting on a shelf for 16 years. They even survive being eaten by birds; in fact, this may be one way a species extends its range. Researchers have hatched and raised fairy shrimp, tadpole shrimp and clam shrimp from the droppings of mallard ducks.
Because “blooms” are short-lived and unpredictable, branchiopods are somewhat difficult to collect and study. Brine shrimp, the salt lake dwellers, are an exception. Brine shrimp cysts float, making it practical to harvest them for research and commercial production. Much of what is known about branchiopods in general began with studies of Artemia, or brine shrimp.
The unusual life cycle offers several advantages. By living where other organisms can’t, branchiopods limit competition for resources and avoid many potential predators. By developing in a hurry, they get ahead of tadpoles, predaceous insect larvae and other animals that colonize some ponds later in the season.
They also know how to hedge their bets. A female may produce more than 1,000 cysts in her short life, and they won’t all hatch the next time the pond fills. Over time, the cyst bank builds up. Pools inhabited by branchiopods may have hundreds of thousands of dormant embryos lying in wait.
The same traits that make them successful in their environment occasionally turn branchiopods into agricultural pests. Tadpole shrimp can be a problem in flooded rice fields, biting off shoots where they emerge from the mud. Fairy shrimp are a recurring problem at the Texas Parks and Wildlife Department’s Jasper fish hatchery, where largemouth bass rearing ponds are drained in winter and refilled in spring. That’s just the kind of environment that fairy shrimp like.
As hatchery biologist Lee Hall explains it, before stocking bass fry, the staff fertilizes the pond to stimulate growth of phytoplankton (microscopic plants). These provide food for zooplankton (microscopic animals), which serve in turn as food for the young fish. If the shrimp arrive first — and they usually do — they eat the phytoplankton.
“They hatch out, and three days later, you can see the bottom. You don’t want that, because the zooplankton don’t have anything to eat,” says Hall. The shrimp, she adds, “will outgrow the bass, so you can’t utilize them as a food source.” So far, the best available control method is to fill a pond, give the fairy shrimp a couple of weeks to hatch and grow, drain it and start over. This practice doesn’t completely eliminate the problem (remember that cyst bank), but it helps.
With all their built-in survival techniques, the only threat branchiopods can’t handle is habitat destruction. This has become an issue in California, which is home to an endangered tadpole shrimp and several endangered and threatened types of fairy shrimp.
No Texas species are on the endangered list as yet. If we continue to make good conservation decisions, perhaps they never will be.