Antiquity of the scleractinian-sipunculan symbiosis

Extant corals symbiotic with sipunculans, i.e., the caryophylliid Heterocya thus and the dendrophylliid Heteropsammia, develop corallum. modifications (in comparison with 'ordinary' representatives of these families) that seem to meet the needs of the coral's worm partner. We distinguish two types of corallum. modifications, designated the monoporous and the polyporous type s. In the adult monoporous type, the shell inhabited by the sipunculan is u sually overgrown only in part by the coral base. There are two orifices: th e main one and a smaller pore in the upper part of the corallum. In the pol yporous type the shell inhabited by the sipunculan is entirely overgrown an d the coral produces a spiralled sipunculan housing. In addition to the mai n orifice there are several pores in the lower part of the corallum. Hetero cyathus priscus sp. n. from the Early Cretaceous (Albian) of France is the oldest example of symbiosis, in which the monoporous-type corallum was modi fied in the same way as in extant monoporous Heterocyathus. We speculate th at the monoporous type was ancestral, as only this type is known to occur a mong Cretaceous corals. Morphological similiarities between Heteropsammia a nd certain species of Heterocyathus, such as the Pourtales plan of septal a rrangement and skeleton porosity, may point to a close phylogenetic relatio nship.