SELECTIVE COLONIZATION BY GASTROCHAENID BIVALVES IN THE PLIOCENE PINECREST BEDS OF FLORIDA

Shells of the highly spinose, sessile, epifaunal Arcinella cornuta in the Pliocene Pinecrest beds of Florida were preferred habitats of boring gastrochaenids, suggesting a previously undescribed adaptive relationship between the two bivalves. Some species of the family Gastrochaenidae dwell in fully endolithic boreholes with calcareous linings that are completely hidden except for small conjoined siphonal openings, some in semi-endolithic boreholes partially covered by secreted calcareous envelopes or crypts, and some in soft sediments enclosed in freely constructed agglutinated crypts that completely enclose the gastrochaenid. All of these adaptive strategies were employed by gastrochaenids inhabiting shells of A. cornuta. Crypts were constructed along or through one or more spines of A. cornuta, and the relationship between borer and host appears to have been linked to the host's spinose ornament. The distribution of gastrochaenid boreholes and crypts suggests that juvenile gastrochaenids colonized and grew within the shells of A. cornuta, possibly initially settling inside the host's spines as they developed at the growing margin of the shell. This perhaps afforded the gastrochaenid shelter, protection from predators, and access to nutrient-rich currents passing across the surface of the host. As the gastrochaenid matured and outgrew the space available within its endolithic borehole, it was forced to break the surface of the host's shell and build a crypt, incorporating the remains of its previous domicile. This suggests an ontogenetic trajectory from a fully endolithic juvenile stage to a crypt-building, semi-endolithic mature stage. Preferential colonization of A. cornuta may reflect an adaptive habitat shift as gastrochaenids of the Pinecrest selected this species as an armored nursery for juveniles and a structurally sound, secure foundation for adults.