We do not currently know of any antonyms for serpula.
The noun serpula is defined as:
Any of numerous species of tubicolousannelids of the genus Serpula and allied genera of the family Serpulidae that secrete a calcareous tube, usually irregularly contorted, but sometimes spirally coiled, with a wreath of plumelike and often bright-colored gills around its head, and usually an operculum to close the aperture of its tube when it retracts.