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Fairy shrimp swimming upside down in spring woodland pools

March to May

Fairy shrimp are inch-long crustaceans that swim on their backs in the shallow woodland pools that fill each spring and dry out by summer. They hatch from tough eggs that sat in the bare pool bottom through all the dry months, and the adults are gone again within a few weeks.

Why this time of year

They hatch in cold water soon after ice-out, ahead of the predators, and an adult lives only one to three weeks. Populations fall away as the pool warms, so the window opens with the pool and closes well before summer.

How to notice it

  1. Find a shallow woodland pool with no stream running in or out and no fish in it, and go on a bright day when you can see the bottom.
  2. Kneel at the edge without stepping in, and hold still for half a minute while you watch the flooded leaf litter for slow drifting movement.
  3. Look for a pale forked tail on a nearly clear body about an inch long, belly upward, with a ripple of tiny legs running head to tail.
  4. Hold a light-coloured lid or clipboard just under the surface for contrast. Polarised sunglasses cut the glare.

Try this with kids: They swim upside down, so their belly is on top. Can you watch one long enough to see the wave of little legs run from front to back?

cysts are tough drought-proof eggs that wait in the dry pool bottom, sometimes for years, until it refills

Safety

  • Stay on the edge and do not wade in. The bottom is deep mud and leaf litter and the water is very cold in early spring.
  • Look but leave everything in the pool. Certified vernal pools and their animals are protected habitat in Massachusetts.
  • Check for ticks afterwards.

Sources

  • massaudubon.orgMass Audubon, Massachusetts-specific: fairy shrimp hatch and develop early in the season before most predators arrive, their hardy eggs sit in the pool's dry sediment resistant to drought and cold, and the shrimp shelter under submerged leaves.
  • vtecostudies.orgVermont Center for Ecostudies: Eubranchipus vernalis is widespread in southern New England; eggs hatch in late winter or early spring and shrimp are most often seen shortly after ice-out; adults live only 1 to 3 weeks; populations decline rapidly once water approaches 60F, which it dates in Vermont to mid- to late May; field marks of about three quarters of an inch, stalked eyes and upside-down swimming.
  • cdnsm5-hosted.civiclive.comMassachusetts Natural Heritage and Endangered Species Program, Guidelines for the Certification of Vernal Pool Habitat, March 2009. Town-hosted copy, confirmed on opening to be the state document itself, title page naming the Commonwealth and the NHESP director. Fairy shrimp are vernal pool obligates and count on their own as certification evidence; vernal pools are fishless temporary waters; certified pools are protected under the Wetlands Protection Act at 310 CMR 10.00; and the field method of viewing shrimp against a light-coloured background. Its 'March through early April' figure describes the mean annual high water period used to delineate a pool's boundary, not when shrimp are present.
  • vernalpool.orgVernal Pool Association, based in Belmont MA: two Massachusetts species, E. vernalis common and E. intricatus state-listed; shrimp active in winter and early spring and dying off as the pool reaches 50F, with no date given; 0.5 to 1.5 inches long; cysts must dry and be re-submerged before they will hatch; and the white forked tail as the mark visible from above.

Checked against these sources on September 4, 2026.

In the guide for March, April, May

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