Sponges are the simplest forms of multicellular animal life and represent a bridge of sorts between unicellular animals and higher forms. They are basically a conglomeration of cells with no tissues, nervous system or internal organs. Sponges are non-moving creatures which filter feed by drawing water into tiny pores in the body and filtering out minute plankton and bacteria. Most simply attach to a rock or other substrate and grow by increasing the cells of the colony. Some are boring.
The term "boring sponge" doesn’t mean that these sponges are dull, although I’ll leave that for you to decide. In this case it refers to a group of parasitic sponges that actually bore holes in their molluscan host’s shells.
Without getting into a lot of the anatomical or reproductive aspects of these primative animals, which I’ll do in another diary, I’ll describe here how a boring sponge works. Although sponges are famous for their ability to reproduce asexually by budding off small bits of the colony, which then grows into another sponge, these animals also reproduce sexually by releasing gametes into the water where chance fertilization occurs. Most sponges are hermaphrodites, but they release eggs and sperm at different times to prevent self-fertilization.
The larvae are planktonic and eventually settle to the bottom and cling to a hard object. Here they grow into the adult sponge by adding cells which differentiate into various types, some structural, some digestive and others designed to move water through the body (flagella). The larvae of a boring sponge seeks out calcium, leading them to the shells of bivalves, snails and coral colonies.
As the baby sponge grows it begins to chemically etch away at the host shell by chipping off tiny pieces of calcium. These chips are then engulfed by the sponge and released through excurrent pores known as oscula. This process continues, forming passageways and tunnels within the shell structure. The colony grows into the hollows it creates and spreads throughout the shell. Periodically the sponge will bore a hole back to the surface of the shell, creating a link to the water in order to bring food in and expel wastes and shell fragments. The photo below shows a close-up of an early stage of the colony and you can clearly see the papillae that have protruded back through the shell wall.
When boring into a live animal, this process will eventually kill the mollusk. The weakened shell can no longer offer protection and the host will either die outright or be vulnerable to predators. Once the shell’s inhabitant is gone the sponge will continue to grow and dissolve until the original shell is completely gone. The sponge colony then becomes free-living. Below is a dried shell that had been partially dissolved by a boring sponge.
So, adult boring sponges actually occur in two different forms; the excavation stage, which is characterized by an embedded colony in a partially dissolved shell, and the gamma stage, which is the form taken by the sponge once the shell is gone. This last stage continues to grow, in the case of my local species (Cliona celata) up to a foot wide. The first two photos above are examples of gamma stage boring sponges. When this stage is reached the sponge does not attack other shells.
In some areas boring sponges can impact fisheries negatively by weakening and destroying mussel or oyster beds. In the case of the oyster industry, weakened shells of still alive oysters cause problems in the shucking line by crumbling and contaminating the meat. But these sponges don’t only attack live mollusks and corals. They will just as readily grow on the shells of animals that have been long dead. So overall this animal is beneficial to the marine environment by recycling calcium carbonate back into the water system. Mollusk shells that would otherwise take years to dissolve by natural erosion are quickly reduced to their basic chemical components by the sponge. This process of wearing away of shell or rock by animals such as sponges is known as bioerosion.
Fun Fact: In some areas bridge supports are no longer constructed of limestone because of the bioerosive effect of these animals.
Other diaries in this series can be found here.