Cheilostome bryozoan epibiosis on brachyuran crabs in the Paratethys Sea during the late Badenian (middle Miocene)

Abstract This study quantifies the prevalence of a rarely documented occurrence of bryozoans encrusting fossil brachyuran crabs. Over 500 crab fossils were examined from the reef facies of the Rákos Limestone Member of the Leitha Limestone Formation in the quarry at Diósd, Hungary. They were deposited in the upper part of the regional Badenian stage (i.e., lower part of the international Serravallian stage) of the middle Miocene. Nine bryozoan colonies were found encrusting five dorsal carapaces of three different crab species: Panopeus wronai , Dromia neogenica , and Cancer styriacus . The bryozoans were restricted to the exterior surface of the preserved carapaces of the host crabs, so the epibiosis was most likely syn-vivo . The prevalence was calculated at 1%. The bryozoans were all cheilostome gymnolaemates identified as a cribrilinid, a calloporid, Onychocella ? sp., and two indeterminate membraniporiform species. The low prevalence of bryozoan-encrusted crabs is consistent with other fossil-based bryozoan-crab studies but much lower than similar studies in today’s faunas. This discrepancy was attributed to both colony spalling-induced preservational bias and differences in carapace size. The bryozoan-crab symbiosis was described as phoretic hitchhiking.