A new species of the sponge genus Thrombus (Porifera: Demospongiae: Astrophorida) is described from a single specimen collected at a depth of 21.5 m on a hard bottom off Monos Island, located off the northwest coast of Trinidad, W. I. The sponge lives symbiotically with a species
of vermetid gastropod, Siliquaria modesta Dall. The associated organisms form a mound measuring 2 m in diameter, covering an area of about 3.2 m2. Named Thrombus sphaeroidocladus, the sponge is the only species of the genus with representatives of all three categories
of spicules known for the genus, namely, simple acanthotriaenes, acanthotrichotriaenes and amphiasters. Also diagnostic is the abortive nature of the clads of the simple acanthotriaenes.
The Bulletin of Marine Science is dedicated to the dissemination of high quality research from the world's oceans. All aspects of marine science are treated by the Bulletin of Marine Science, including papers in marine biology, biological oceanography, fisheries, marine affairs, applied marine physics, marine geology and geophysics, marine and atmospheric chemistry, and meteorology and physical oceanography.