More than 1,000 earthworm species exist, organized into three general groups. Epigeic worms live aboveground in crop and leaf litter. Endogeic earthworms live in the 2 to 3 inches of topsoil directly beneath the surface. All other earthworm species are anecic subsoil dwellers. These worms live up to 6 feet underground and spend their entire lives there, rarely if ever venturing to the surface. Earthworms benefit agriculture by helping to aerate the soil to enhance crop production, but they all require particular soil temperatures to thrive.
Because worms are coldblooded, they can't regulate their own internal temperature like mammals can. Higher environmental temperatures increase their rates of metabolism, requiring them either to feed more or to burn their own fat stores. The reverse is true when the soil temperature is too cold. Worms can't survive in temperatures below freezing or above 95 degrees Fahrenheit. At temperatures greater than 77 degrees Fahrenheit, worms eat and digest their food faster. Commercial earthworm operations maintain their beds at temperatures between 60 and 80 degrees Fahrenheit, which are considered ideal for the growth and activity of earthworms.
Although all worms have both male and female reproductive organs, they need a partner to reproduce -- they can't fertilize themselves. The worms lay side-by-side to mate and exchange sperm with each other. The sperm is stored in special chambers in front of the worms' egg-producing organs. After mating, worms produce an egg case called a "cocoon" that contains between one and 10 fertilized eggs. Worms produce between 20 and 30 of these cocoons per year on average. The reproduction process depends on the surrounding soil temperature; temperatures between 60 and 70 degrees Fahrenheit are ideal for cocoon production and hatching. For this reason, peak cocoon production for earthworms in the temperate zone occurs during spring and early summer months.
Like all coldblooded animals, earthworms have shorter lives at higher environmental temperatures, because the heat increases the speed of their bodily functions. Heat-sensing neurons in some earthworms enable them to detect higher temperatures and slow down body functions despite the heat so they can live longer than they would otherwise. Still, earthworms in their natural habitats typically live only a few months because of temperature changes and various environmental pressures. In a protected environment with controlled temperatures, earthworms can live as long as 10 years.
Temperature can indirectly affect other factors that pressure earthworm populations. Earthworms thrive in soil with adequate ventilation and drainage. Because they both absorb and lose moisture through their skin, the soil in which they live must be moist, but not overly so. Worms move to the surface after heavy rains that saturate the soil to keep from suffocating. The ideal environment for earthworms who live underground is a loose, loamy soil with a neutral pH.
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