The genera Dissodactylus and Clypeasterophilus are commensal or parasitic crabs that live in association with irregular sea urchins. Only a single pinnotherid species is expected to occur on a sand dollar; however, during field observations, D. crinitichelis and Clypeasterophilus stebbingi were found co-occurring on a single host (Clypeaster subdepressus). Laboratory experiments and in situ observations were conducted to explore mechanisms of host selection and use by C. stebbingi, as well as the possibility of competition with D. crinitichelis. Analysis of 213 specimens of Encope emarginata and 33 C. subdepressus revealed a high frequency of infestation (90 %) by brachyurans. The number of D. crinitichelis was positively correlated with the size of E. emarginata. During field observations and experiments, D. crinitichelis was able to colonize both echinoids, while C. stebbingi was found mainly on C. subdepressus. No exclusion was reported between individuals of the same sex for both crab species. However, D. crinitichelis may be territorial, since a single male occurred when adult females were added to the host. D. crinitichelis and C. stebbingi co-occurred on the same individual of C. subdepressus on rare occasions during field observations. C. stebbingi may exclude D. crinitichelis from the echinoid to avoid competition, as observed during the experiments. We suggest a species-specific association between C. stebbingi and C. subdepressus, while D. crinitichelis associated mainly with E. emarginata, but also with C. subdepressus. Both crabs may harm the echinoids by consuming their spines, but additional research is necessary to determine whether this is a parasitic association.