In the recent past the invasion of two new sponge pests Cliona margaritifera Dendy and C. lobata Hancock on the molluscan beds of the southwest coast of India is reported. These two new pests made their first appearance on cultured pearl oysters on raft at Vizhinjam in 1980 and thence started spreading to the economically important molluscan beds in and around Vizhinjam. The spreading of these pests along the southwest coast of India was rather fast and from this coast C. margaritifera could migrate to the raft-cultured pearl oysters at Tuticorin and C. lobata to the chank beds off Thiruchendur (southeast coast) within two years i.e. by 1982.

When these two new pests started spreading to molluscan beds off Vizhinjam there was a sudden spurt in the general incidence and infection pattern in every bed either through their own activity or through triggering the activities of others (conventional species) that are in the various beds. The initial hike in their incidence subsided gradually in natural beds and reached a level very close to that seen prior to invasion. But such a regulatory trend was not at all discernible in the culture systems indicating that the ecological equilibrium which is at play in the natural beds is no longer in operation in the man-made system viz. culture rafts.

The conventional boring species competed with the new invaders in all the beds and culture 1} stems and in this competition the new invaders proved to be more adaptable. But in final analysis when species from each group (conventional and new invaders) is considered individually, it could be noticed that C. celata Grant from the conventional and C. margaritifera from the new invaders are more adaptable, In some beds the adaptability shifted from C. celata to C.vastifica Hancock which is another widespread and coextensive native species. The reason for this behaviour though not clear, may be attributed to their historical dominance in that particular bed.

It is also suggested in this paper that C. margaritifera is capable of devastating any molluscan bed as was seen in the Ceylon (Sri Lanka) pearl banks in 1902. The reappearance of C. margaritifera at Tuticorin after a lapse of several years might pose a serious threat to the entire molluscan population in the Gulf of Mannar. A continuous monitoring of the activities of C. margaritifera, hence, is necessary on a long term basis.