Friday, April 9, 2010
On March 28, my wife D’Ann and I visited a vernal pool on the Conant Land near Castle Rock. When I had been there a few days earlier, I heard Wood Frogs calling from the pool, and we returned for a closer look. Out of curiosity, I dipped my net into the pool and discovered three Fairy Shrimp, one about ½ inch long and two about ¼ inch long. I have only seen Fairy Shrimp one other time, at the first Vernal Pool walk led by Christine Kavalauskas in April 2002 at Conant Land (but not the same pool).
Encouraged by this, I dipped my net about a dozen more times, but found no other Fairy Shrimp. We also found about 50 Wood Frog egg masses and two Spotted Salamander egg masses at the north of the pool. There were probably millions of springtails (insects, order Collembola) drifting on the surface of the pond, driven into a light gray “scum” on the downwind side. We netted a predaceous diving beetle, a water boatman and saw water striders on the surface.
There are two, and possibly three, species of Fairy Shrimp in Massachusetts. The most common species is Eubranchipus vernalis; the other two species are rare. Immature Fairy Shrimp are orange or salmon colored; adult males tend to have a greenish cast, adult females bluish. Adult Fairy Shrimp can reach ¾ inches in length. Fairy Shrimp are identified based on the appearance of the male’s antennae. Males have much larger heads than females and special “antennal appendages” that are used to grasp the female during mating. Females of different species look very similar. (We did not see a male Fairy Shrimp, so its likely identification is based on E. vernalis being much more common.) Juvenile Fairy Shrimp look similar to the adults. The tips of the tail of E. vernalis are bright fluorescent white, making the tail more visible against the brown color of dead leaves in the pool.
Fairy Shrimp (distant relatives of the well-known supermarket shrimp) are filter feeders, and swim upside down while using their feathery legs to push food into their mouths. Recent studies have shown that they are omnivorous, eating anything from algae to small crustaceans. After mating, the female lays eggs that lie on the pool bottom until hatching is stimulated by flooding the following spring. The eggs resist desiccation when the pool dries up, and some eggs may survive for several years.
Only a small percentage of the eggs hatch the following year, leaving some eggs to hatch in later years. Fairy shrimp are only found in temporary waters that do not contain fish. However, Fairy Shrimp are eaten by predatory insects such as diving beetles found in vernal pools.
Fairy Shrimp do not tolerate warm temperatures, and are usually not found in pools when water temperatures reach 68-72 degrees F. This strategy helps shrimp avoid predation by salamander larvae and invertebrate predators that appear later in spring. If you want to find Fairy Shrimp, you need to look for them before the middle of May while pool water temperatures are cool. Even so, Fairy Shrimp may be present in a pool one year and then disappear, only to magically reappear several years later. Both Fairy Shrimp and their eggs can be carried on the feet and feathers of birds, thereby colonizing new pools.
Certain vernal pool animals known as “obligate species” require vernal pools for all or part of their life cycles. Examples of obligate species are Fairy Shrimp, Wood Frog and Spotted Salamander. Wood Frogs and Spotted Salamanders mate and lay their eggs in vernal pools in early spring. Fish that eat frog and salamander eggs and young cannot survive in pools that dry up. Wood Frogs and Spotted Salamanders also need undisturbed forested areas near vernal pools, where they spend most of their adult lives foraging under leaf litter and logs.
If you are interested in learning about the animals of vernal pools, I recommend the field guide by Kenney and Burne below. It includes excellent photographs of many vertebrates and invertebrates found in or around vernal pools.
Colburn, Elizabeth A. Vernal Pools: Natural History and Conservation, McDonald & Woodward Publishing Co., 2004.
Kenney, Leo P. and Matthew R. Burne, A Field Guide to the Animals of Vernal Pools, Massachusetts Natural History & Endangered Species Program and the Vernal Pool Association, 2000.
Calhoun, Aram J. K. and Phillip G. deMaynadier (editors), Science and Conservation of Vernal Pools in Northeastern North America, CRC Press, 2008. ∆
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