To determine whether differential use of mangrove structure by snail species mitigates predation rates by the whelk Thais kiosquiformis, we conducted a field experiment. We used T. kiosquiformis because it was the only predator we observed eating Cerithideopsis californica and Cerithideopsis montagnei snails (both according to direct observation and forensic evidence) during preliminary tethering trials in the forest.

Cages were constructed using an outer shell of welded stainless steel wire 1/2” mesh to ensure rigidity and an inner lining of chicken wire with smaller (1/4”) mesh size to prevent snail escape and prevent predator invasion. We built circular cages that enclosed 0.5 m2 and were 40 cm high. Cage height was reduced by no more than 5 cm when buried and cages had no lids because in lab trials, snails did not climb the metal cage materials.

Six cages were assigned to each of six treatments (36 cages total) that crossed predator presence/absence with three types of structure availability. The three structural manipulations were mangrove tree trunk only, both a trunk and pneumatophore roots, or no climbing structure. We used naturally existing tree trunks for these treatments, requiring haphazard but non-random cage placement. The appropriate root treatments were sometimes created by pruning preexisting pneumatophores to eliminate them. Each cage was cleared of all macrofauna and stocked with 50 snails of each Cerithideopsis species and, if appropriate, five predatory T. kiosquiformis whelks. All snails were collected on-site, within 50 m of where cages were installed. Cages were monitored as often as the tide and weather permitted; almost daily.

Over the course of the experiment, we documented T. kiosquiformis and Cerithideopsis spp. climbing behavior and all T. kiosquiformis predation events (recorded in the dataset accessible on this webpage) that occurred inside each cage. We did not count or record the number of uneaten prey snails, but all prey snails in a cage not recorded as eaten were assumed to be alive. Over time, a number of cages developed gaps through which both predator and prey snails could escape as the tide redistributed shell rubble in and around the cages. We stopped collecting data on a cage after a gap was seen since the number of predator and prey snails inside the compromised cage was no longer guaranteed to be equal to that in other cages.