An association between a hesionid polychaete and temnopleurid echinoids from Singapore

The hesionid polychaete Oxydromus cf. angustifrons (Fauvel, 1953 non Grube, 1878) occurred on the peristomial membrane and on the test between the aboral spines of two echinoid species Salmacis sphaeroides (L., 1758) and Temnopleurus toreumaticus (Leske, 1778) in Singapore. Each S. sphaeroides was associated with 1–5 polychaetes, whereas almost all T. toreumaticus individuals each carried only one polychaete. There was no linear relationship between host size and the number of polychaetes, although larger echinoids sometimes carried more polychaetes. In echinoids carrying a single polychaete, the polychaete was almost always located on the peristomial membrane (80% in S. sphaeroides and 62% in T. toreumaticus), and sometimes on the test between the aboral spines. In echinoids with multiple polychaetes, the largest individual was always on the peristomial membrane while the smaller ones were on the test between the spines. The polychaetes displayed aggressive territorial behaviour, attacking each other with their proboscis when there was more than one individual on a single echinoid. Laboratory observations also showed that the polychaetes had strong affinity for the peristomial membrane – they returned to the host within 7 minutes if deliberately displaced and individuals remained on the peristomial membrane of living echinoid over a period of more than two weeks. The polychaetes neither caused damage nor provided any benefit to their hosts, suggesting that the association is of commensal nature.