Creature Feature

Fairy Shrimp: The Ephemeral Enigma
by Ray Stewart

Many animals use vernal pools when the opportunity for feeding, shelter, or other needs presents itself, but they can fulfill these needs without the use of vernal pools. However, some animals must have vernal pools to survive; they are obligated to live or breed in vernal pools. These organisms are referred to as obligate vernal pool species. These are the often overlooked invertebrates. I am as guilty as anyone of thinking mainly of vertebrates like myself. Most of us are vertebracentric (I invented this word to mean: narrowly compassionate toward vertebrate life forms) by nature. But we need to consider that it is the invertebrate world that makes this planet tick.

One Vernal Pool invertebrate is the Fairy Shrimp. Most elementary students know what ‘Sea Monkeys’ are. Actually the ‘Sea Monkeys’ that kids enjoy are a close relative more accurately called ‘brine shrimp,’ which are the salt water cousins of Fairy Shrimp. I nominate the Fairy Shrimp to represent a true obligate Vernal Pool species.

What is a Fairy Shrimp anyway? In a fundamental biological sense, calling it a ‘shrimp’ is just wrong. Common names often apply familiar handles to the unfamiliar (consider ‘seahorse’ or ‘starfish’).

Fairy Shrimp are among the most developed invertebrates, namely, the phylum Arthropoda (animals with exoskeletons - shells - and jointed appendages - legs). Fairy Shrimp are a kind of arthropod known as crustacean. All crustaceans have a pair of antennae extending from the forward part of their heads, but that is where the similarity of Fairy Shrimp to true shrimp ends. Crustaceans are one of the major groups of animals that inhabit vernal pools, and are main pillars of their mainly detritus-based food webs. In such a system the bottom level of the food chains is not living plants, but the nutrients present in the mass of fallen leaves, twigs, flowers, bud scales, pollen, fruit, and other plant and animal parts that has accumulated in the vernal pool during the previous autumn and winter and that continues to be added during spring tree bloom.

Fairy Shrimp are generally about 2 cm (3/4 inch) long and are found in subarctic, mountain, glaciated and unglaciated, wetland and desert environments where short wet periods occur. Fossils of this diverse and primitive order of beings date back to the Cambrian Period (more than 500 million years ago), long before the first jawless fish introduced simple vertebrate anatomy to the world. Originally populating the world's oceans, Fairy Shrimp were eventually forced by evolving predators into shallower and more protected waters, including fresh water habitats.

Branchiopods, the class of crustaceans to whih fairy shrimp (order Anostraca) belong, are filter feeders. They have a pair of stalked eyes and have a strong tendency to move toward the light (very remarkable during a night visit with a flashlight to a vernal pool). Females develop a brood sack on their abdomen which contains from 10 to 150 egg-like structures technically known as cysts.

Fairy Shrimp, as well as tadpole shrimp, produce cysts rather than eggs which are fully developed embryos enclosed in a hard, spongy shell. This is a great advantage when an organism lives in a quickly disappearing habitat like a drying vernal pool. The embryo can hatch out of its shell as soon as there is water in the vernal pool. Ideal conditions can produce a single generation in just 16 days. Cysts hatch, grow, mature into adults and reproduce in less than three weeks. Not all cysts will hatch in a single season. There is always a risk that a dry spring would not fill the pool long enough to complete a reproductive cycle. One study of vernal pools reported 1000 cysts per square foot and that only 3% hatched during any given flooding event. It would take a long series of false starts and unfavorable conditions to deplete the store of cysts that rest below the leaf litter.

Larvae develop in stages by molting. Hatching and early development may begin beneath an ice covered pool. The young larva, called a nauplius, periodically sheds its exoskeleton to grow, increasing gradually its pairs of legs from 3 pair to 11 pair. The nauplius stage of crustacean development is thought to be the most abundant form of multi-celled life on Earth!

A report of brine shrimp from the Great Salt Lake found viable cysts in a core sample dated to 10,000 years ago. How do you spell s-u-c-c-e-s-s? These creatures with an active life span measured in weeks have a dormant endurance measured in millennia.

Dry or warm conditions end the reproductive season. Their seasonal cycle ends once water temperature rises above 20 C, usually by early May. Even if water is still present in the Vernal Pool, conditions are less favorable at any rate because by this time predation has increased and dissolved oxygen levels likely have decreased.

How do Fairy Shrimp survive in such a temporary habitat? They are in the middle of the trophic pyramid, a mid-level predator. They feed on organisms that we don’t even see. Early spring augments biological activity with an increase in light and temperature. Leaf litter and detritus from the preceding season are decomposing, releasing their energy and nutrients to other organisms. Algae - mostly single-celled forms -use the nutrients and the best light of the year. Longer days and little canopy blocking the sun deliver a large dose of solar energy to the Vernal Pool. Algae also release oxygen to the water which helps increase supplies for higher level aquatic predators. Single and multi-celled organisms quickly respond to this bonanza in a bloom of microscopic life.

The vertebrate life to which we are so partial is not able to exploit this early life force. Fish that might filter-feed are absent. Amphibians are beginning to lay eggs. The adults aren’t interested in eating and the young are not yet ready to feed on the fairy shrimp. This is where the invertebrate population excels. In a vernal pool fairy Shrimp, the other arthropods, mollusks, worms of all kinds consume the invisible, microscopic abundance and concentrate this simple goodness into their own bodies. Food is then available for the vertebrate populations, including the later stages of salamander larvae, with whom we more readily relate. This is really neat stuff, this realization of how food and energy webs work - all through the patience and blind genius of evolution.

It takes many parts of an ecosystem to make a whole. Many of the parts of vernal pools are small and microscopic. Without the contributions of the diverse populations of invertebrates there would be a deadly rip in the fabric of life. Fairy shrimp are one of the mysterious creatures that weave that fabric together.

Watch fairy shrimp in action.

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Originally published in the Friends of Wetlands Newsletter, February 2006. Available online at www.fowl.org/news.htm