Vernal Pools and Fairy Shrimp

Vernal pond on May 23, 2013. Fairy shrimp and wood frogs use this pond.
Vernal pond on May 23, 2013. Fairy shrimp and wood frogs use this pond.

There are two woodland vernal pools that I have identified on my property. There may be others but I will not know until I do a more thorough inventory in my west woods. Vernal pools are a type of wetland formed in shallow basins with slowly or poorly draining soil. They have no inlets or outlets and seldom have any direct contact with ground water although. Because they temporarily hold melt water and runoff vernal pools recharge groundwater and can slow down the incidence of flash floods.

Vernal pools are often not noticeable during most of the year and may resemble dry basins or small meadows in the woods. There are clues one can use to locate them. One is landscape position which is lower than the surrounding forest land. Another is the soil which is usually a muck or sedge peat in some cases. If the upland soils are reddish then soils in the basin will be paler red or even gray at depth. There may be small patches of different colors in the soil caused by periods of oxygen depletion. Fallen leaves from the surrounding forest in the basin are usually blackened from long exposure to water.

There is a distinctive vegetation in the basin that differs from most of the surrounding forest plants including the trees. Vegetation in woodland vernal pools is a mixture of plant species tolerant of prolonged submersion and alternating wet and dry conditions. The more water tolerant species are found in the deeper parts of the pool. Those tolerant of alternating wet and dry conditions tend to be found near the edges.

Most vernal pools have one or several sedges such as Carex rosea, C. intumescens, and C. deflexa along the edges and C. tuckermanii or C. lupulina forming low tussocks in shallow water. In pools with peaty soils lake sedge (C. lacustris) or beaked sedge (C. rostrata) may occur. Where the soil pH is circumnuetral sedges such as C. cryptolepis and C. flava may be present (as in Cook County, Minnesota for example). Common grasses in and around vernal pools include marsh bluegrass (Poa palustris), Canada bluejoint (Calamagrostis canadensis), and fowl manna grass (Glyceria striata).

There may be water parsnip (Sium suave), blueflag (Iris versicolor), marsh speedwell (Veronica scutellata), bittercress (Cardamine pensylvanica), yellow watercrowfoot (Ranunculus gmelinii), and clustered bur-reed (Sparganium glomeratum) submerged in or emerging from the shallow water. Along the damp edges meadow horsetail (Equisetum ), small bedstraw (Galium tinctorium) and ferns such as marsh fern (Thelypteris palustris), sensitive fern (Onoclea sensibilis), and oak-leaf fern (Gymnocarpium dryopteris).

In forested areas the pools are often fringed by black ash (Fraxinus nigra) and red maple (Acer rubrum). Meadow willow (Salix petiolaris), dwarf alder (Rhamnus alnifolia), tag alder (Alnus rugosa), red currant (Ribes triste), winterberry (Ilex verticillata), and meadowsweet (Spiraea alba) are common shrubs of vernal pools in northern Minnesota.

There are often mosses in or around vernal pools. Some common species are tree moss (Climacium dendroides) and species of Dreplanocladus, or Sphagnum.

In the early spring vernal pools fill with water from snow melt and rain but by mid-summer they may be only damp areas or even completely dry. The brief time they are water-filled is important as it does not allow fish or the predacious larva of many insects like dragonflies to survive. But in most years there is enough water in them for a sufficient time so that certain species of amphibians like the wood frog (Rana sylvatica) and blue-spotted salamander (Ambystoma laterale) can lay eggs and mature larva before the pond dries up. They are also habitat for fairy shrimp (Eubranchipus spp.) a crustacean related to brine shrimp. These three species are found almost exclusively in vernal pools and are considered obligate vernal pool species. There are other small animals make use of vernal pools but they can also use more permanent bodies of water with some insect and fish predators present. Among these are spring peepers (Pseudacris crucifer) and chorus frogs (Pseudacris triseriata). However, it appears that these two small frogs use vernal pools more than lakes and permanent ponds. I have often found them trilling in the spring in vernal pools and in swamps where there are small pools and ponds.

I had suspected that fairy shrimp might be in one or both of my vernal pools but it wasn’t until this year that I found them here. So after seeing them in many other places across northeastern Minnesota I was excited to find them here. At first I thought I was looking at newly hatched salamander larva (another “to find” on my list) which I was searching for that day. But when I saw the pulsing legs on the underside of these little animals I knew they were shrimp.

Fairy shrimp in the genus Eubranchipus are widespread across the eastern US in vernal pools and other temporary bodies of fresh water. The vernal fairy shrimp (E. vernalis), which is what I think the species in my vernal pools may be, is a small crustacean measuring about 1 inch long. The body plan of fairy shrimp consists of three parts: head, thorax and abdomen. There is no carapace, a calcified part of the exoskeleton covering the head and thorax, as in other crustaceans. The head has a pair of stalked compound eyes, two pairs of antennae, and a mouth. In males the second pair of antennae grasp the female during mating. On the underside of the thorax are paired appendages, one pair to a segment, called phyllopods (“leaf feet” because of their leaf-like appearance) that serve to propel the shrimp, to collect particles of food, and move oxygenated water over the gills. The last two segments of the thorax are fused and have appendages specialized for reproduction.

Male fairy shrimp die soon after mating. Also, the female fairy shrimp outnumber males so not all females will mate. They can still lay eggs as fairy shrimp lay both fertilized eggs and parthenogenically produced eggs. The eggs come in two forms. One is thin-shelled and will hatch soon after being layed. The other is thick shelled and can remain dormant in the dry soil of a vernal pond for years waiting for water.

Fairy shrimp from one of the vernal pools on my property.
Fairy shrimp from one of the vernal pools on my property.

There may be other interesting animals in these vernal pools. One type I am looking for is another crustacean, an ostracod. I have seen these before in some vernal pools near Duluth, Minnesota and noticed that each pool had a differently colored species. Maybe I’ll find ostracods like these here, too.

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