The first record of symbiosis between a palaeonemertean (Nemertea) and echinoderms (Echinodermata)

The associations between Cephalotrichella echinicola Chernyshev et al., 2019, and the heart urchin Metallia spp. described here represent the first report on the symbiotic relationship between palaeonemerteans and echinoderms. In addition, it is the first report of a symbiotic relationship between nemerteans and heart urchins, and the third known association between nemerteans and echinoderms. Numerous specimens of C. echinicola were found associated with heart urchins Metallia spatagus and M. sternalis in four localities of the Bay of Nhatrang, Vietnam, South China Sea, at 6–10 m water depth buried in the coarse sand to a sediment depth of 10–20 cm. The infestation prevalence and mean intensity were higher in M. spatagus, than in M. sternalis, 77% and 68%, and 10 and 3 nemertean individuals per heart urchin host, respectively. Each infested host harbored from 1 to 49 nemertean individuals, and multiple infestations with three or more individuals per host were very common. Based on the high level of prevalence, absence of the nemertean on the other sand-dwelling heart urchins in the area, the complete absence of the nemerteans in the sandy bottom of the bay and in the sediment surrounded infested host individuals, and no reaction by the host to the nemertean’s movement on the host’s surface we consider this a symbiotic association.