Zoological Studies

A Coral-Killing Sponge, Terpios hoshinota, Releases Larvae Harboring Cyanobacterial Symbionts: An Implication of Dispersal

A. Terpios hoshinota, an encrusting sponge, overgrows hard corals on a relatively large scale, raising concerns for coral survival in the Indo-West Pacific region. However, mechanisms of dispersal of this sponge remain unknown. This study examined the ultrastructure of parenchymella larvae collected from T. hoshinota to infer potential mechanisms of dispersal and outbreaks of this threatening sponge. The ovum-shaped parenchymella larva has negative buoyancy and a limited swimming capability even though cilia cover its entire surface. Furthermore, larvae settled within 1 d in an aquarium, indicating a larval stage of short duration. These characteristics suggest that dispersion distances of Terpios larvae are short. An ultrastructural examination also indicated that larvae are filled with cyanobacteria, and lack spicules and a mesophyl structure as found in other sponge larvae. Most cyanobacteria in the larvae appeared intact and dividing as are found in adult Terpios, but some found within sponge cells had disintegrated. Cyanobacteria being engulfed in Terpios cells implies a trophic function of the microbes during larval development.