Epibionts are uniquely valuable in their ability to constrain paleoecological hypotheses about their own as well as their host's behavior and environment. Rarely preserved epizoic bryozoans are here reported on fossil crabs from the Miocene Mishan Formation in the Zagros Basin of southwestern Iran. One-hundred-thirty-eight decapod crustaceans were recovered from the upper marly member of the Mishan Formation. Of those, seven decapods (5%) were fouled by bryozoans. Of these seven decapods, five had bryozoans growing attached to the cuticle of the hard outer surface of the exoskeleton of the host crabs and thus fouled while the host crab was potentially alive. Forty percent of the bryozoan colonies occurred on the host crab's dorsal carapace, and 60% were found on chelipeds. On average, 30% of the surface area of the host crabs' fouled skeletal components were covered by bryozoan colonies. The brachyurans were mostly leucosiids, including Leucosia persica and Myra sp. The bryozoans were all cheilostomes and included Acanthodesia sp., Thalamoporella sp., and an indeterminate ascophoran. These bryozoans are all the first reported occurrences in the Mishan Formation. The low incidence of crabs fouled by bryozoans is attributed to preservational bias. The results from this study are compared to those of fossil and extant host crabs reported in the literature. The bryozoan-crab relationship documented here is best described as commensalism.

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