GALL-FORMING PARASITISM ON CIDAROID ECHINOID SPINES: RECENT BEHAVIORAL INNOVATION OR TAPHONOMIC ARTIFACT?

Eulimid gastropods (Caenogastropoda: Eulimidae) are specialized echinoderm parasites, and exhibit high degrees of host-specificity at the genus level. Eulimids in the genus Sabinella specifically target the cidaroid echinoid Eucidaris tribuloides, and are the only known eulimid parasites to form galls on the distal parts of the primary spines on living echinoids. These galls often act as domiciles, with the eulimid male, female, and eggs coexisting together within the gall. A dredge-collected population of E. tribuloides sampled from the eastern Gulf of Mexico, off the coast of St. Petersburg, Florida, produced many individuals with galls on primary spines formed by Sabinella. Interestingly, the spines of these cidaroids also exhibited frequent infestation by other epibionts (annelids, barnacles, bryozoans, oysters). Despite the ubiquity of Sabinella galls on E. tribuloides spines in these modern Florida populations, fossil specimens of echinoids with eulimid galls have not been reported. To assess this issue further, we surveyed more than 1000 individual E. tribuloides spines from the Florida Tamiami Fm. (Pliocene). We failed to identify any spine alterations that could be interpreted as galls. This negative outcome suggests that either galls weaken fossilized spines negatively or eulimid-echinoid interactions is very recent in evolutionary terms. Micro-CT scans of the modern galls revealed that the gall-forming material was less dense than the spine cortex. Also, in cases of well-developed galls, the inner cavity of the gall encroached into the spine shaft. This gall-induced weakening of spine material likely contributed to the high degree of breakage through the center of the gall in modern individuals. Compromised structural integrity of gall-bearing spines may thus contribute to their absence in the fossil record through preferential breakage or disaggregation of the gall material. However, the absence of gall material (broken or otherwise) on fossil spines, as well as the dearth of extensive encrustation on the fossil spines as compared to their modern descendants, points to the recent origin of Sabinella infestation in regional populations of Eucidaris tribuloides. The cidaroid host specificity and gall-forming behavior of Sabinella may represent a newly developed ecological interaction.