Bioerosion describes the breakdown of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, gastropods and fish; it can occur on coastlines, on coral reefs, and on ships; its mechanisms include biotic boring, drilling, rasping, and scraping. Overall, the species that cause this phenomena are highly diverse coming from several kingdoms of life. On dry land, bioerosion is typically performed by trees, pioneer plants, or plant-like organisms such as lichen. Terrestrial bioerosion is performed by either chemical (e.g. by acidic secretions on limestone) or mechanical processes (e.g. by roots growing into cracks). In marine systems bioerosion occurs on nearly all coastlines, and all reef systems.

Bioerosion by endolithic organisms is also well known in the fossil record on shells and hardgrounds, It is also important to note that while not traditionally considered within the term bioerosion larger prey fish such as sharks can break off larger chunks of reefs when in pursuit of prey or by humans by causing direct physical damage to a reef.

Internal Bioerosion

Internal bioeroders (borers) are organisms that break down the calcium carbonate coral skeletons while living within them and include two categories: Chasmoliths and endoliths. This occurs at various depths within the reef from a few micrometers to several cm and via mechanical, chemical, or both processes. Internal bioeroders such as algae, fungi, bacteria (microborers), protists, sponges (Clionaidae), bivalves (including Lithophaga), sipunculans, polychaetes, acrothoracican barnacles, and phoronids, generate extremely fine sediment with diameters of 10 to 100 micrometres while eroding corals.

Porifera

Sponges in the genera Cliona, Anthosigmella, and Spheciospongia are the most destructive and predominant bioerosive sponges. All three genera are endolithic, mechanical, excavators that usually only bore into the top 2 cm of the coral skeleton (microbioerosion) and branch significantly similarly to the roots of some plants; however, some species in the order Siphonodictyon bore large holes up to 12 cm into the reef (macrobioerosion). The microerosive sponges mechanically etch uniformly shaped 0.05 mm flakes of skeletal structure out of the holes leaving them with a pitted internal surface; these are also mechanically removed by the sponge via the sponges canal system. Due to the increased amounts of dead corals without their replacement by new live corals, bleached reef systems have been overrun with boring sponges that are actively breaking down reef systems removing habitat for fish, crustaceans, corals, and various other organisms. There is alternative evidence to suggest that increasing temperatures are affecting the sponges similarly to how they affect corals.

Annelid Worms

Two classes within phylum Annelida primarily contribute to reef bioerosion: Polychaeta and Sipuncula. Polychaetes are more prominent in specific locations and environments, and these worms are capable of boring 0.5-2 mm holes up to 10 cm into the reef skeletal structure via both mechanical and chemical means. While polychaetes are significant reef borers the cycle and relationship between which families occupy which reefs and when is yet to be understood.

External Bioerosion

thumb|A parrot fish (Bolbometopon muricatum) eating algae off of a rock in [[Borneo.]]

External bioeroders (grazers) are organisms that are visually identifiable on the surface of or around the reef structure. External bioeroders include sea urchins (such as Diadema), chitons, and marine fish, all of which are dubbed epiliths. Sea urchin erosion of calcium carbonate has been reported in some reefs at annual rates exceeding 10&nbsp;kg/m<sup>2</sup>. Fish also erode coral while eating algae. Parrotfish cause a great deal of bioerosion using well developed jaw muscles, tooth armature, and a pharyngeal mill, to grind ingested material into sand-sized particles.

Parrotfish

Parrotfish (Scaridae) are one of many families of fish that feed upon algae that grows on reef systems capable of extensive excavation of the reef skeleton. These fish primarily bite portions of the structure that stick out from the surface of the reef smoothing the overall terrain. These fish are also occasionally known to predate upon live corals which can have a deleterious affect. However, the benefits these fish provide to reefs in the form of increased growth, fecundity, and removal of algal competitors aiding in coral recruitment outweigh the negatives of their predation upon coral species. In one study, bioerosion of coral reef aragonite by an individual parrotfish was estimated to occur at a rate of 1017.7±186.3&nbsp;kg/yr (0.41±0.07 m<sup>3</sup>/yr) for Chlorurus gibbus and 23.6±3.4&nbsp;kg/yr (9.7*10<sup>−3</sup>±1.3*10<sup>−3</sup> m<sup>3</sup>/yr) for Chlorurus sordidus. Additionally, parrotfish have a preference for grazing upon certain species of live coral (Orbicella annularis in the Caribbean), and are reducing those species ability to survive and compete in reef ecosystems.

Fungi

The primary mechanism of fungi bioerosion in reef systems is through chemical dissolution of the calcium carbonate structure.

Fungi, specifically Aspergillus sydowii, are known to infect corals causing disease. While this in itself doesn't directly attribute to fungal bioerosion, reefs need to be actively producing more carbonate skeleton mass than is deteriorated through erosive processes to grow. Through the combined efforts of all bioeroding organisms disease in corals (both fungal and bacterial) diminish the overall growth of a coral reef which is their largest effect on bioerosion. One study found the affects of microbioerosion as a whole to be between 0.15 and 0.35 kg m<sup>-2</sup> y<sup>-1</sup> but noted that fungi only make up a relatively small portion of that value.

Additionally, as eutrophication and acidification of the oceans increases due to the anthropogenic factors causing climate change fungal presence in marine ecosystems becomes more diverse and abundant. Ocean acidification specifically is accentuated by fungal bioerosion during the dissolution of calcium carbonate by releasing carbon dioxide as a biproduct.

Further reading

  • Bioerosion Website at The College of Wooster