03. SPONGES & CORALS
3(c) Why are sponges so toxic?
1. Chemical defence – Chemicals are produced to combat predators such as sea cucumbers.
Orange sponge Haliclona sp., with a synaptulid holothurian (sea cucumber) predator feeding on it. Image: Dr. John Hooper, QM.
2. ‘Space wars’ – there is a lot of competition for space on the reef and chemicals are produced to stop other sponges from settling down too close to another one.
In highly crowded encrusting marine communities, where space and other resources are limited, toxicity provides a competitive advantage. Image: Dr. John Hooper, QM.
3. Bio-erosion – Many sponges excavate their calcitic surroundings, such as on coral reefs, breaking down and recycling the calcium carbonate back to the reef system. This is an important process as the shells of many marine organisms and the skeleton surrounding coral polyps are made of calcium carbonate. Sponges use chemicals to remove calcium ‘chips’ which subsequently form a lot of the sediment on the seabed.
Yellow sponge Aka n.sp., excavating a coral with only the oscules (exhalant pores) exposed. The main ‘body’ of the sponge lives within the dead, excavated coral. Image: Dr. John Hooper, QM.
4. Chemical recognition – a chemical mechanism to help commensal and mutualistic associations. This is an important adaptation that increases survival in crowded ‘reef cities’.
A sponge pistol shrimp Synalpheus sp., in its host sponge Lissodendoryx colombiensis. Many small commensals, such as minute crustaceans and worms, live inside the sponge water canal system, giving rise to the term ‘sponge hotels’. Image: Dr. Arthur Anker, Florida Museum of Natural History.
5. Anti-parasitic – toxins repel unwanted, toxic ‘free-loaders’ from settling on, or in, the sponge.
Pink or white parasitic zooanthid coral (Palythoa) growing on the reddish sponge (Trikentrion flabelliformis). The sponge produces chemicals called ‘trikentrins’. The concentration of this chemical increases during the season when sponges are infested with zooanthid corals. ‘Trikentrins’ have been found to be effective against gram positive bacteria (like Bacillus subtilis). That is, they act like antibiotics.
Image: Dr. John Hooper, QM.
Sponge (Mycale sp.), and parasitic scyphozoans and hydroids living on the surface of the sponge, which in turn is encrusting on a gorgonian (sea fan) coral.
Image: Roger Steene.
6. Antibiotic - Sponges were models for ‘self’ versus ‘non-self’ recognition and were pivotal to the development of our current understanding of immunology.
7. Symbionts in 'sponge hotels' - Cyanobacteria are found both inside and outside the cells of sponges, particularly those of the mesohyl section. These symbioses, between the bacteria and the sponge, may produce unique chemistries. Lots of other ‘tenants’ check-in and check-out of ‘sponge hotels’, and many even get locked in permanently, never able to leave. For example: many commensal barnacles.
Filamentous cyanobacteria Oscillatoria sp., symbiotic in the sponge Dysidea, as seen under scanning electron microscopy. Image: Dr. Monika Schlacher, QM.
Cyanobacteria Oscillatoria sp., living in the sponge mesohyl. Image: Dr. Monika Schlacher, QM.
Ancient Origins – Sponges are one of the simplest multicellular organisms and have been around for a long period of time. Lots of evolutionary ‘experiments’ have occurred over this time. That is, they have had millions of years to experiment with biosynthetic pathways and produce new chemicals.
‘Living Fossils’: This living colonial ‘reef-building’ sponge Vaceletia n.sp., has its closest ancestor extinct by the end of the Cretaceous period, 65 million years ago. There is very little evident structural change over this vast time scale. This new species lives under the coral reef caves and cavities, significantly binding together the dead coral reef infrastructure.
Image: Dr. John Hooper, QM.