(written by Scott Green)
Fairy shrimp are easily identified in vernal pools. They appear as 1/2 to 1 1/2 inch crustaceans swimming upside down (ventral side up). The adult fairy shrimp have stalked compound eyes, two sets of antennae, and 11 pairs of leaf-like swimming legs. Coloration is usually red-orange due to the hemoglobin in the shrimp, but can range from translucent whitish to gray, blue or green. Because coloration is determined by the contents of the food supply in the pool which the shrimp inhabit, it is usually constant among the individuals of the pool.
Male shrimp possess an enlarged second antenna used to clasp the female during mating. Female fairy shrimp often have a brood sack on their abdomen. Female fairy shrimp usually outnumber males. They are capable of three states of mobility. Resting at the bottom of the pools, darting rapidly and drifting slowly. The shrimp propel themselves with a wave-like anterior-posterior beating motion of their legs. This action is complemented by the propeller motion of the outermost part of the legs (the "exopodites". By changing the angle of these exopodites the speed of motion can be changed.
Fairy shrimp reproduction is initiated when the male clasps the female with its second, clasping antennae. Though the male and females swim clasped together for several days, the process of copulation takes minutes. Hours after copulation the male fairy shrimp dies. The female carries both fertilized and parthenogenetic eggs externally in its brood sack for several days before being released to fall to the bottom of the pool, or the eggs may remain attached until the female dies. The number of eggs a female produces in a clutch varies from 10 to 150. Several clutches can often be produced during the life of a female.
Females can produce two types of eggs, thin shelled "summer" eggs and thick shelled "winter" eggs. The type of egg produced is determined by the number of males in the community; summer eggs will be produced if there is a shortage of males in the population. Summer eggs hatch rapidly; the young form while still inside the brood sac. The young from these eggs will populate the pool during the same season they are laid. The winter eggs remain in the mud at the base of the pool and dry out with the pool. The eggs will hatch in the spring when the pools refills. Though the resting period usually varies between 6 to 10 months, eggs have been hatched in a laboratory after 15 years. Eggs have been subjected to temperatures of as high as 99C and as low as -190 C and remained viable. Winter eggs usually hatch 30 hours after being exposed to water.
Typically, one generation inhabits each wet period of the pool. Fairy shrimp usually hatch as nauplius. The young will develop in a series of instars. Each instar involves molting the exoskeleton to grow more segments until they reach the 20 segments of adults. Development is often rapid in the spring, but can be slowed by unusually low temperatures. The speed of development usually reflects the amount of time water will remain in the pool, or the arrival of predators in the pool. Young which have hatched from winter eggs develop more slowly than those that have hatched from summer eggs. Fairy shrimp can complete their life cycle in 16 days. This allows for rapid reproduction.
Winter eggs can be carried from pools to pool by traveling animals, or, in the case of pools that dry out completely, picked up in the wind and be blown to other pools. For reasons currently unknown to scientists, there is an uneven level of population in a pool from year to year. In a single pool, fairy shrimp may be abundant for several consecutive years and absent the next.
DISTRIBUTION: There are three known types of fairy shrimp in Massachusetts. The most common fairy shrimp is Eubranchipus vernalis and any fairy shrimp caught in the state is likely to be this species. Eubranchipus intricatus is found in several locations but its distribution is not well known. Nationally this species is uncommon, but widely distributed in vernal pools with low salinity. Eubranchipus bundyi has been found in Massachusetts but there are few recorded occurrences of the species in the state. Microscopic examination of the antenna base or mouth parts is necessary to distinguish among these three species.
ECOLOGY: The leg movements serve the purpose of collecting algae, bacteria, protozoa, rotifers and floating detritus from the water. Food is then filtered from the water and scraped by sets of appendages to be eaten using a mandible mouth. Fairy shrimp have been observed gnawing on larger matter such as dead tadpoles, mollusks and amphibian eggs. The leg movements of the fairy shrimp also serve the purpose of taking the oxygen the animal needs from the water.
The ephemeral nature of the fairy shrimp reduces the limiting factors on their population. Fairy shrimp have few natural predators. They are unlikely to be heavily preyed upon by other vernal pool inhabitants because they utilize the pool before the majority of carnivorous insects have colonized the pool. Also, the wood frogs and mole salamanders breeding in the pools have not regained their regular appetite after winter hibernation and, thus, are not major predators. However, these amphibians, caddis fly larvae, dytiscid larvae, other insects, and, especially, waterfowl who utilize the pool, often do prey upon fairy shrimp. Because fairy shrimp live in temporary wetlands there are no predatory fish. The abundance of food is less of a factor in the population of fairy shrimp than in other organisms. The need of one part per million dissolved oxygen is the limiting factor in the size of fairy shrimp populations.