THE ROLE OF CRINOID ANAL TUBES IN MINIMIZING THE NEGATIVE EFFECTS OF INFESTING PLATYCERATID GASTROPODS

Camerate crinoids served as hosts for platyceratid gastropods from the Ordovician through the Permian. Platyceratids typically attached firmly over camerate periprocts where they remained during the lives of their hosts. Previous research indicates that at least some platyceratids were true parasites: Infested camerates are significantly smaller than uninfested individuals from the same species and populations.

The evolution of long anal tubes in camerates significantly minimized infestation by platyceratid gastropods. In fact, we are not aware of a single example of a tubed camerate with a platyceratid attached over the periproct. However, there are numerous examples of tubed camerates with platyceratids attached to the tegmen at the base of the anal tube. These gastropods accessed the interior of their hosts via drilling.

Despite the observation that camerate anal tubes effectively minimized platyceratid infestation, did they also minimize the negative effects of gastropod infestation observed in tubeless camerates? One hypothesis is that the increased length of the digestive system in tubed camerates facilitated greater nutrient absorption and thereby reduced the negative effects of platyceratid infestation.

To test this hypothesis, we examined 198 specimens of the tubed camerate crinoid Globocrinus unionensis (Worthen, 1890) from the late Mississippian (Chesterian) Monteagle Limestone, Madison County, Alabama. All specimens were collected from a single lens and represent a single population. Thirty-seven (18.7%) of the specimens showed evidence of infestation, including the presence of platyceratids and drillholes at the base of the anal tube. Based on radial plate height and width, in addition to the diameter of the basal circlet, infested and uninfested Globocrinus show no significant differences in body size. This suggests that the anal tubes of camerate crinoids may not have only minimized rates of infestation, but also the negative effects of infestation resulting from the loss of nutrients to platyceratids.