Fairy shrimp perfectly adapted to vernal pools

What was snow-covered just last week is now generally snow-free. Spring wildflowers are beginning to emerge and polka dot the forest floor with green, the ice is out of most ponds, and on warmish afternoons and evenings we are starting to hear that iconic sign of spring in New England - spring peepers and wood frogs calling from vernal pools. Vernal pools are seasonal wetlands that fill with water from snowmelt and spring rains, only to dry up by mid-summer. They provide a handy fish-free environment in which some of our local amphibians and many invertebrates (dragonfly and damselflies for example) love to lay eggs, thus avoiding hungry fish.

One night this past week I was out with friends exploring some vernal pools. We had our flashlights focused on a spring peeper, rudely interrupting its piercing trill, when we noticed fairy shrimp swimming through the water and forgot all about the peeper. Fairy shrimp are, for me, one of the most exciting animals to find in these parts. They only live in vernal pools. They are only active for a few weeks each spring. And, they can disappear from a pool for years before magically reappearing just when you thought they had vanished forever.

Fairy shrimp are freshwater cousins of sea monkeys (brine shrimp). They look like comic book creatures. Large and orange with big eyes, they swim upside down through the water, beating their numerous abdominal appendages with an undulating rhythm that resembles a curtain blowing in the wind. Fairy shrimp complete their entire life cycle in just a couple weeks - hatching, maturing and laying eggs before would-be predators emerge. Unlike frogs and salamanders and most of the invertebrates that spend some part (usually as eggs and juveniles) of their life cycle in these pools and then leave (usually as adults), fairy shrimp are permanent residents of vernal pools. They can’t leave when the pool dries up, they are stuck there. But that’s fine. Fairy shrimp have been around for millions of years, plenty of time to perfect a life cycle suited to temporary pools.

Here’s how they do it. I can’t say which came first, the shrimp or the egg, but let’s start with the egg. Fairy shrimp eggs are called cysts. They aren’t a typical egg that contains a developing embryo, instead these cysts contain fully-developed embryos able to hatch out and develop into mature adults within a couple weeks of hatching. In the cyst form they are resistant to drying out and can remain in a dried-up vernal pool for years if need be. As soon as conditions are good for hatching the embryos emerge. Some studies indicate that the cysts of some species of fairy shrimp must dry out (and possibly freeze) before they can hatch. This is a great adaptation for the quirky wet-dry cycle of a typical vernal pool, some of which can remain dry for years before once again filling with enough water to support life.

Vernal pools also sometimes dry out too quickly in the spring for fairy shrimp to complete their life cycle. Luckily, fairy shrimp have a back-up plan - only a portion of cysts hatch in a given year, some remain dormant. So, if one generation of fairy shrimp succumbs to a particularly early dry season, their encysted siblings will survive until the next wet season. It appears that these cysts can remain viable for decades or longer.

How did fairy shrimp end up having to live in vernal pools? According to the Vermont Center for Ecostudies (www.vtecostudies.org/wildlife/invertebrates/fairy-shrimp): “Fossils of fairy shrimp date back to the Cambrian Period (more than 500 million years ago), long before the first fish. Originally populating the world’s oceans, fairy shrimp were eventually forced by evolving predators into shallow, temporary freshwater habitats.”

Fairy shrimp are relatively large and slow-moving, so it makes sense that adaptations that have allowed them to grow and breed in a few short weeks in early spring, before most predators are on the move, have resulted in a species that is uniquely and perfectly adapted to live in New England's small, ephemeral vernal pools.

Susan Pike, a researcher and an environmental sciences and biology teacher at St. Thomas Aquinas High School, welcomes your ideas for future column topics. She may be reached at spike3116@gmail.com.