FORAMINIFERA AS HARDGROUNDS

Small epibiont foraminifera encrusting very large benthic foraminifera were dredged from ~160 m water depth on the Oregon shelf. The host taxa are the large (up to 4mm) agglutinate foraminifera Ammodiscus pacificus, with fewer Haplophragmoides canariensis, Rhabdammina linearis, and Bathysiphon filiformis. The epibionts are Cibicides pseudoungerianus, C. refulgens and Tritaxis fusca and frequently two or three individuals were found attached to one host. Epibionts were considered fully attached when a ring of “glyco-glue” or “puffermasse” was evident around the periphery of the test. Isolated instances of smaller foraminifera using larger foraminiferal tests as hardgrounds have been illustrated in a few publications, however, this study of over 100 host specimens documents a persistent taxonomic association. In addition, several traces can be attributed to foraminifera that became detached from the host, leaving a distinct impression where the epibiont was dislodged. One specimen of H. canariensis displays a hole, rimmed by a distinct lip, bored completely through the test. Since C. refulgens is sometimes carnivorous, it is possible this hole is the result of predation by that species.

The samples studied were collected from two mapped sites, 1) a Miocene mélange that is diapirically intruding the Pliocene to Recent sediments, and 2) an adjacent silica-sponge reef. The foraminifera were extracted from very fine-grained sediment. The large hosts apparently provided a stable platform for the epibiont taxa, and the motility of the host prevented the smaller foraminifera from being smothered in the settling sediment. Thus, these larger foraminifera acted as convenient benthic islands for their epibionts.