We investigated Ophiodaphne formata, an exceptional sexually dimorphic ophiuroid, in situ using scuba, and in the laboratory. Solitary males, solitary females, and pairs of coupled individuals were found associated with the irregular echinoid sand dollar Astriclypeus manni. We observed that coupled females, solitary males, and solitary females always hold their aboral surface against the oral surface of the host. Coupled males and females with disk diameters > 0.63 mm and > 3.7 mm, respectively, are considered to be ripe for reproduction. In order to couple, a solitary male approaches the margin of a female's disk, quickly inverts, interdigitates its arms with the female's, and keeps its mouth pressed against that of the female. At first, solitary males and females attach to their host singly and, secondly, couple with each other in order to engage in reproductive behavior. Solitary and coupled individuals are capable of changing their positions on the host, and transferring from one host to another. Coupled individuals separate after a period of attachment on the host.