SYMBIOTES ON ECHINOID SPINES; ESTABLISHING BASELINES OF ENCRUSTATION INTENSITY

Echinoid spines can host a variety of symbiotes, and the ecology of these biotic interactions span the spectrum from mutualism and commensalism, to parasitism. Mature spines of cidaroid echinoids lack an epithelial layer and instead have a thick outer cortex which is thought to encourage symbiote attachment and growth. Present-day populations of the cidaroid Eucidaris tribuloides in the western Atlantic and Eucidaris thouarsii in the eastern Pacific are notable in experiencing high frequencies of encrustation on their spines. While moderate levels of encrustation have been interpreted as providing beneficial camouflage and other forms of protection for the host, high density of encrusters is instead likely to reduce host fitness and qualify as parasitism. The high density of encrustation found in present-day populations may reflect human-driven disruptions of local food webs. However, this cannot be confidently established without understanding pre-anthropogenic baseline levels of symbiotic encrustation in these regions. Here, we present a survey of present-day populations of E. tribuloides and E. thouarsii using a semi-quantitative method to establish encrustation intensity. We explore encrustation intensity of biomineralizing and non-biomineralizing symbiotes, as well as the specific case of the gall-forming gastropod parasite Sabinella. We then compare the intensity of spine encrustation in present-day populations to levels observed on the spines of the same taxa from the Plio-Pleistocene. We find significantly lower levels of encrustation on Plio-Pleistocene spines compared to their modern descendants, and no evidence of fossil parasitic galls. Taphonomic experiments suggest that the observed differences are unlikely to reflect preferential preservation of spines affected by encrustation or galling. We propose that the encrustation and parasitism may have intensified recently in these taxa in response to human impacts.