Vernal Crayfish Life History and Habitat Use

Southern population sample

What to call a freshwater crustacean that resembles a small lobster? USDA Forest Service scientist Zanethia Barnett has a clever answer: “I study crayfish, but I eat crawfish.”

adult-vernal-crayfish
Vernal crayfish, like this adult, were collected in minnow traps. Photo by Susie Adams, USFS.

More than half of the nation’s 357 species of crayfish — also known as crawdads, mudbugs, or yabbies — can be found in the Southeast.

Crayfish break down plant materials, are an important food source for larger aquatic fauna, and can be an indicator of good water quality.

The vernal crayfish (Procambarus viaeviridis) spends part of the winter and early spring in shallow ponds, sloughs, or ephemeral streams. When these habitats begin to dry, they dig and retreat into burrows.

“These seasonal pools occur in bottomland hardwood forests and are valuable habitat for crayfish and other species: juvenile fish, tadpoles, and other amphibians,” says Barnett. “Because pools are more common at times when there are fewer visitors to the forests, most people don’t realize how important they are.”

In general, there are few chronicles of native crayfish. This information gap makes it difficult to develop effective management or conservation plans.

Barnett led a study of the vernal crayfish’s life history and habitat use near the southern extent of its range. She worked with SRS aquatic ecologist Susie Adams and Rebecca Rosamond, a wildlife biologist with the US Fish & Wildlife Service, to collect more than 3,500 vernal crayfish in the Dahomey National Wildlife Refuge in the Mississippi Delta.

vernal-pool
Seasonal wetlands on the Dahomey National Wildlife Refuge include vernal pools ringed with bottomland hardwoods like sugarberry and hackberry. Photo by Susie Adams, USFS.

Their research results were published in the Journal of Crustacean Biology.

The researchers collected samples from January to May 2012, November 2013 to June 2014, and February and March 2015. They found crayfish during every sample month.

They tallied the number of crayfish per trap, along with other crayfishes and fishes. Data about crayfish habitat on the refuge included the number and size of pools, amount of leaf litter, water quality, and air and water temperature.

Sample data also included species, sex, and reproductive condition – juvenile or adult, along with adult male form. Adult males molt between two forms over the course of a year. Form 1 males are reproductively active, while form 2 males are not.

Vernal crayfish in the study area had slightly different reproductive anatomy than other populations. In females, the annulus ventralis, or sperm receptacle, differed. In males, the gonopods differed. Gonopods are the first one or two pairs of swimmerets that are used for sperm transfer.

Such differences are often used to identify crayfish species and may indicate that the vernal crayfish population on the Dahomey are distinct.

Listen to a brief audio clip by author Zanethia Barnett describing this publication. • Text Transcript

Different numbers of juveniles and adults in each survey month gave the scientists clues about the timing of reproduction. Data pointed to a peak in breeding during May. Form 1 males were abundant, making up about 70 percent of the adults captured in May. Far fewer females were found during this month, suggesting that they began burrowing in April or May.

No females were found that had hatchlings, eggs, or glair – a substance the female uses to attach fertilized eggs that becomes visible at the onset of egg laying.

However, females kept in tanks for observation did have glair by May. By October, these same crayfish had more than a hundred ovarian (unfertilized) eggs each. Typically, about 40 percent of Procambarus eggs die. “That means each female could produce between 41 and 55 young. This is the first reported estimate of fecundity for this crayfish species,” says Barnett.

The scientists estimated that vernal crayfish in the study area live as long as two to three years.

vernal-crayfish-upclose
Vernal crayfish in the study area reached maturity at the end of the first or beginning of the second year. Photo by Susie Adams, USFS.

The greatest numbers of vernal crayfish were caught in shallow pools that persisted for at least three months, when water temperatures exceeded 50 degrees F and other crayfish species were present. There was no significant difference in water quality across the different pools.

Adams and Barnett are conducting more vernal crayfish sampling across the Lower Mississippi Alluvial Valley and neighboring parts of Tennessee, Mississippi, and Alabama. Adams is leading a related taxonomic study to further understand morphologic and genetic differences among populations.

“Research on the status of the vernal crayfish is ongoing,” adds Barnett. “The Dahomey population may be a new species.”

In any case, more information about its specific habitat and potential management needs will be crucial.

Read the full text of the article.

For more information, email Zanethia Barnett at zanethiabarnett@fs.fed.us.

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