A COMPARATIVE STRUCTURAL STUDY ON BACTERIAL SYMBIOSES OF CARIBBEAN GUTLESS TUBIFICIDAE (ANNELIDA, OLIGOCHAETA)

The bacterial symbiosis in 13 different species of the gutless phallod riline genera Inanidrilus and Olavius (Tubificidae, Oligochaeta) from Carrie Bow Cay (Belize), Florida and Bermuda is structurally compared. The result is an essentially consistent symbiotic pattern regarding t he position and ultrastructural design of the prokaryotic partners and their integration in the anatomy of the animal hosts. Based on numero us micrographs, it is calculated that the endobacteria comprise about 25% of the host's volume. This corresponds to at least 0.6 x 10(6) bac terial cells per worm of (fixed) 6 mm length, a figure that underlines the high nutritive importance of the bacteria in this symbiotic assoc iation. This importance is also endorsed by the percentage of bacteria in digestion (on average 8-10%, up to 24%) by the phagocytotic epider mal cells of the host. The symbiosis always involves two different mor phological types of bacteria, apparently without intermediate stages. Detailed biometrical analyses demonstrate the significantly different size ranges of the two morphotypes, even though in two of the species, the normally rod-shaped Type S-bacteria are much longer than usual. A bout 1% of the larger, oval 'Type L' is regularly found in division; i n some sections, this value can reach 11%. Formation of sulfur-contain ing globules in this type was proven by histochemical analysis. The tr ophic relevance of the symbiosis is considered. Zoogeographical and ev olutionary implications of the observed consistency in the symbiotic p attern in all the species studied are discussed.