The feeding activities and excretory products of the marine decapods Callianasa and Upogebia and the marine annelid Onuphis have been studied in the natural environments of the southern Atlantic and eastern Gulf of Mexico coasts and in controlled aquarium experiments. These organisms produce depositionally significant quantities of argillaceous fecal pellets which are transported with hydraulically equivalent sand grains and deposited as viable, granular clays in relatively high-energy environments. The organisms also significantly alter clay minerals taken into their digestive systems and the organic-rich, argillaceous fecal pellets serve as sites for postdepositional alteration of the clay minerals.

The fecal pellets are more than 90% mineral matter, largely clay minerals. The decapods produce rod-shaped fecal pellets about 2 mm long and 0.75 mm in diameter. The specific gravity and water content of these pellets yield a fall velocity in seawater equivalent to that of 0.25 mm quartz grains. The annelids produce ovoid fecal pellets, about 0.5 mm in diameter, with fall velocities in seawater equivalent to coarse silt and fine quartz sand.

The argillaceous fecal pellets exhibit a clay mineralogy significantly different from that of the related suspended sediments. In fecal pellets the crystallinity of muscovite is disordered, the crystallinity of interlayered minerals is reduced, chlorite is largely destroyed, and the illite/montmorillonite ratio is reduced. There is a clay mineral difference between fecal pellets produced by decapods and annelids in the same environments. The argillaceous fecal pellets also are recycled in the coprophagic chain and the clay minerals are further altered.

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