Infaunal soft-bottom invertebrates benefit from the presence of sedime
nt, but sedimentation is potentially harmful for hard-bottom dwellers.
Most sponges live on hard bottom, but on coral reefs in the Red Sea,
the species Biemna ehrenbergi (Keller, 1889) is found exclusively in s
oft-bottom lagoons, usually in the shallowest part. This location is a
sink environment, which increases the deposition of particulate organ
ic matter. Most of the sponge body is covered by sediment, but the chi
mney-like siphons protrude from the sediment surface. The sponge is at
tached to the buried beach-rock, which reduces the risk of dislodgment
during storms. Dye injected above and into the sediment revealed, for
the first time, a sponge pumping interstitial water (rich with partic
les and nutrients) into its aquiferous system. Visual examination of p
lastic replicas of the aquiferous system and electron microscopical an
alysis of sponge tissue revealed that the transcellular ostia are most
ly located on the buried surface of the sponge. The oscula, however, a
re located on top of the siphons; their elevated position and their ab
ility to close combine to prevent the filtering system outflow from cl
ogging. The transcellular ostia presumably remain open due to cellular
mobility. The sponge maintains a large population of bacteriocytes, w
hich contains bacteria of several different species. Some of these bac
teria disintegrate, and may be consumed by the sponge.