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Biology/Natural History: This is the most common Echiuran in California but it does not get quite as far north as Washington State. Most Echiurans directly swallow mud or detritus but this species lives in a permanent U-shaped burrow through which it pumps water, collecting bacteria and other fine particulates on a net of mucus. From time to time it swallows the mucus net along with the attached food. This species often lives in high hydrogen sulfide conditions in the mud. Entrances to the U-shaped burrow are 40 to 100 cm apart and the burrow depth is 10 to 45 cm. The animal pumps water through the burrow by peristalsis of the body. A distinctive pile of castings may be found at the end of the burrow the animal's posterior is facing. The animal accumulates fecal pellets plus burrow excavations near its posterior end in the burrow, then periodically squeezes its body hard to send a blast of water out the anus, blowing the pellets out to form the castings. Many other species may be found sharing the burrow, which is the origin of the name. Species found living in the burrow include the pea crabs Pinnixa franciscana and Scleroplax granulata, the scaleworm Hesperonoe adventor, the clam Cryptomya californica, and one to several individuals of the goby fish Clevelandia ios, which goes in and out of the burrow.
Sexes are separate in this species. Gametes are produced in the coelomic fluid and ejected into the water through modified nephridia. Sperm are white and eggs are pinkish, pale yellow, or pale olive. Larvae are pelagic for about 60 days, then settle. A chemical from the castings of a Urechis burrow is a powerful stimulus to settle.
Predators include flounders, diamond turbot, leopard sharks (which may suck them from their burrows), sea otters, and humans (for bait). The parasitic protozoan Echiurocystis bullis may be found in the gut.