A Comparative Structural Study on Bacterial Symbioses of Caribbean Gutless Tubificidae (Annelida,Oligochaeta)

The bacterial symbiosis in 13 different species of the gutless phallodriline 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 the position and ultrastructural design of the prokaryotic partners and their integration in the anatomy of the animal hosts. Based on numerous micrographs, it is calculated that the endobacteria comprise about 25% of the host's volume. This corresponds to at least 0.6 × 10⁶ bacterial cells per worm of (fixed) 6 mm length, a figure that underlines the high nutritive importance of the bacteria in this symbiotic association. This importance is also endorsed by the percentage of bacteria in digestion (on average 8–10%, up to 24%) by the phagocytotic epidermal cells of the host. The symbiosis always involves two different morphological 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. About 1% of the larger, oval ‘Type L’ is regularly found in division; in some sections, this value can reach 11%. Formation of sulfur-containing globules in this type was proven by histochemical analysis. The trophic relevance of the symbiosis is considered. Zoogeographical and evolutionary implications of the observed consistency in the symbiotic pattern in all the species studied are discussed.