Why do corals release mucus?
Corals are known to release mucus from their body into the water. But why do they release mucus?
Look at the above picture. When I expose corals to air, they release slimy mucus.
In a natural environment, corals can be exposed to air during extremely low tides, experiencing high temperature and dryness under strong sunlight for a couple of hours.
You will see a copious amount of mucus released from the corals, which is a defense mechanism against desiccation. Corals coat their body with mucus, keeping in moisture to withstand severe environmental conditions.
Corals also release mucus under submersed conditions for several reasons. Here, I summarized the reasons (see Table 1).
Generally corals release mucus under stressed conditions such as defense against biofouling, pathogens, UV radiation, sedimentation, pollutants, and desiccation.
Even water currents and temperature or salinity changes can be a cause of mucus release.
Speaking of UV, coral mucus contains substances called mycosporine-like amino acids that can absorb UV light, which can protect corals from strong UV light.
When sediments (for example, silt and sand) fall down onto the coral surface, corals use mucus to clean up their surface.
Table 1. Summary of mucus release factors by various coralsSource: Nakajima & Tanaka (2014)1.
|Under stressed condition|
|Biofouling, pathogen||Porites astreoides||2|
|UV radiation||Lobophyllia hemprichii||5|
|Water current||Montipora digitata||15|
|Heterotrophic feeding||Siderastrea siderea||18|
Corals also release mucus under non-stressed conditions.
They use mucus as a tool to capture prey items such as bacteria and small zooplankton with their sticky surface. Corals transport the food items trapped by mucus into their mouth using ciliary movements18.
Corals are also known to release mucus as excretory pathways for excess organic matter21.
Appearances of coral mucus
The coral mucus is primarily consisted of sugar-protein, called mucin, and polysaccharides and lipids. The origin of mucus comes from zooxanthellae, a symbiotic algae23.
Studies show that corals release approximately half of the photosynthetic products (organic matter) provided by zooxanthellae into the water in the form of mucus, while they use the rest of the organic matter for growth and respiration24-26.
Coral mucus used to be defined by various terms such as fluid mucus, string, web or flocs, and mucous sheet (see Fig. 1).
Nowadays, however, mucus is commonly separated by the difference in size from a quantitative perspective, that is POM (particulate organic matter) and DOM (dissolved organic matter).
DOM is technically defined as organic matter that can pass through filters with a pore size of 0.7-1.0 micrometer.
The size of organic matter strongly affects subsequent pathways. For example, DOM in reef waters is mainly taken up by heterotrophic bacteria and incorporated into the microbial food web.
On the other hand, the larger POM is utilized by reef fishes, zooplankton, benthic animals; it also partially sinks down and is mineralized in the sediments.
Therefore, information about the sizes of the organic matter produced by corals is important for understanding the subsequent biological availability and carbon pathways.
Mucus production rates by corals
Now let’s look at DOM and POM production by corals. Look at Fig. 2.
This figure is the summary of release rate of DOC and POC by various corals (as of 2014)1. The x-axes indicate reference sources.
As you can see, both DOC and POC release rates vary greatly, ranging from -120 to 680 nmol/cm2/h for DOC, and from 3 to 170 nmol/cm2/h for POC. It’s not even easy to estimate the averaged values!
These large ranges of DOC and POC production rates could be caused by various factors, such as species differences, the surrounding environmental, and experimental conditions.
Here, I would like to discuss further about the differences in the experimental conditions.
When you measure the production rates of organic matter released by corals, usually the corals are incubated in a closed system like beakers or bottles for a certain period of time (such as several hours).
Now, look at Table 2. Many conditions differ among these types of cultural experiments, including incubated medium volumes, incubation times, type and intensity of provided lights, seawater temperature, and the stirring conditions of the medium.
For example, higher light intensity could increase the photosynthetic rates of zooxanthellae in corals, which increases the release rates of DOC and POC from the coral colony28.
Higher nutrient concentrations in seawater might reduce DOM release rates because the production of zooxanthellae cells is promoted with nutrients and less organic matter is available for release34.
These differences in experimental conditions among studies might cause the large ranges of POC and DOC release rates1.
Table 2. Different methods for measuring mucus release ratesModified from Nakajima & Tanaka (2014)1.
|Incubation method (volume of incubation medium, L)||Incubation time (hour)||Lighting||Stirring||Source|
|Bottle (<1)||3-4||Fluorescent lamp||No stirring||19|
|Flow through chamber||3||In situ||Flow through||31|
|Beaker (0.5-2)||4-6||Shading sunlight||No stirring||35|
|Bottle (8)||96||Direct sunlight||Continuous||32|
|Plastic bag (2-8)||24||In situ||No stirring||29|
|Bottle (0.7)||5||Halogen lamp||Continuous||29|
|Beaker (0.8-1)||6||Shading sunlight||No stirring||28|
|Bottle (1.8)||5||Direct sunlight||Every hour||30|
You can see a pattern of DOM and POM release by corals, however, when you look at the studies measuring both DOM and POM release simultaneously.
Generally, corals release more DOC than POC into the water. Approximately, 60-90% of organic carbon is released as dissolved form28.
So the majority of organic matter released by corals would be mainly utilized by bacteria.
Microbial decomposition of coral-derived DOM
The DOM produced by corals is rapidly decomposed and mineralized by heterotrophic bacteria.
A previous study from New Caledonia showed that only 10-20% of bacterial carbon demand was met by DOC derived from phytoplankton36. This result suggests that benthic organisms other than phytoplankton play a major role in providing DOM to pelagic bacteria.
A number of studies have reported that coral mucus enhances the growth of bacteria.
For example, my previous studies observed that the bacterial abundance in seawater increased much more quickly with the experimental addition of coral mucus than in the control seawater (Fig. 3)29,37.
So coral mucus functions as a good substrate for bacterial growth.
The rapid bacterial growth can subsequently lead to the emergence of bacterivorous protists such as heterotrophic nanoflagellates, which are capable of transferring carbon to the higher trophic levels through the microbial food web.
To test if coral mucus enhances the growth of bacterivorous protists, I examined bacteria and protists in the sea-surface microlayer over coral reefs.
The sea-surface microlayer is the thin boundary layer between the atmosphere and ocean, with a typical thickness of 10-250 µm. The sea-surface microlayer is generally enriched in both DOM and POM and microbes.
Coral mucus often includes air bubbles that provide buoyancy, which slowly ascend to the sea-surface and accumulate.
Passing through the water column, its sticky surface traps various organic particles such as bacteria.
So coral mucus contributes to the formation of enriched organic matter and microbes in the air-sea interface.
Also, higher coral coverage in an area can mean higher organic matter or coral mucus input, which results in a more stimulated microbial community at the air-sea interface compared to areas with lower coral coverage.
In my previous study, the abundances of bacteria and bacterivorous protists were higher in the sea-surface microlayer than in the subsurface water.
Moreover, microbes in the microlayer increased with increasing coral coverage, suggesting that higher organic matter or mucus released by corals enhanced production of bacterivorous protists38.
How great is the nutritional values of coral mucus?
Coral mucus is utilized by various reef animals including fish, zooplankton and various benthic animals.
Here, I summarized the list of animals that have been reported to feed on coral mucus (Table 3), which are either isotopically or visually confirmed.
Table 3. Summary of organisms that feed on coral mucusModified from Nakajima & Tanaka (2014)1
|Taxonomic group||Species||Method for feeding experiments/observations||Source|
|Blue sprat||Spratelloides delicatulus||behavior||39|
|Damselfish||Chromis sp.||behavior (no feeding on mucus was observed)||40|
|Surgeonfish||Acanthuridae||behavior (no feeding on mucus was observed)||40|
|Butterfly fish||Chaetodon ornatissimus||behavior||41,42|
|Copepod||Acartia negligence||14C incorporation||43|
|Copepod||Acartia tonsa||neutral red dyeing/[methyl-3H]-thymidine incorporation||44|
|Mysid||Mysidium integrum||neutral red dyeing/[methyl-3H]-thymidine incorporation||44|
|Crown-of-thorns starfish (COTS) larvae||Acanthaster cf. solaris||13C, 15N incorporation||45|
|Coral crab||Trapezia sp.||behavior||46|
|Coral crab||Tetralia fulva||behavior/carmine dyeing||47|
|Coral gall crab||Hapalocarcinus marsupialis||behavior||48|
|Coral gall crab||Utinomia dimorpha||behavior||48|
|Shrimp||Coralliocaris superba||behavior/carmine dyeing||47|
|Shrimp||Philarius imperialis||behavior/carmine dyeing||47|
|Shrimp||Jocaste japonica||behavior/carmine dyeing||47|
|Crab||Mithrax sp.||behavior/neutral red dyeing||49|
|Bivalve||Lithophaga lessepsiana||14C incorporation||50|
|Softcoral||Pseudoplexaura porosa||14C incorporation||51|
|Acoelomorph worm||Waminoa sp.||15N incorporation||52|
Then the question arises as to how great is the nutritional value of coral mucus.
In order to figure out the nutritional values of coral mucus, we can use the protein/energy ratio. The protein/energy ratio is often used to estimate nutritional values of food items53,54.
Energy (or calories) can be calculated using the following assumptions55:
- carbohydrate = 4.2 kcal/g
- protein = 5.65 kcal/g
- lipid = 9.45 kcal/g.
Here, I summarized the basic composition of carbohydrates, proteins and lipids of coral mucus from various corals (see Table 4), and calculated the protein/energy ratios of coral mucus.
Table 4. Summary of basic composition (%) of coral mucus and the protein/energy ratioAFDW, ash free dry weight. The values of AFDW indicate percentage composition to dry weight (DW). The values of carbohydrate, protein and lipid indicate percentage composition to either aAFDW or bDW. NA, not available. cglucose equivalent. Modified from Nakajima & Tanaka (2014)1.
|Coral||Mucus collection/generation method||Mucus form||AFDW (%)||Carbohydrate (%)||Protein (%)||Lipid (%)||Protein/energy ratio (mg/KJ)||Source|
|Lobophyllia corymbosa||air exposure (natural tidal exposure)||fluid||21||11b,c||57.7b||31.3b||20.6||57|
|Faviidae||air exposure (natural tidal exposure)||fluid||22||31.7b,c||23.8b||44.4b||8.3||57|
|Fungia scutaria||air exposure + seawater wash||fluid||83||62a||35a||3.4a||17.1||59|
|Acropora formosa||extraction with toluene||fluid?||91.4||55.6b||30.4b||4.2b||16.3||23|
|Pachyseris speciosa||extraction with toluene||fluid?||76||29b||34b||2.5b||24.1||23|
|Fungia fungites||extraction with toluene||fluid?||89||8b||72.2b||4.4b||35.7||23|
|Porites astreoides||air exposure + distilled water wash||fluid||87.3||39a||24.6a||2.8a||17.6||60|
|Porites furcata||air exposure + distilled water wash||fluid||87||51.6a||14.2a||0.2a||11.4||60|
|P. astreoides||field collection (underwater)||sheet||26.6||14a||27.5a||2.3a||27.9||60|
|P. furcata||field collection (underwater)||sheet||34||11.3a||34.7a||4.8a||28.7||60|
|Porites divaricata||field collection (underwater)||sheet||22.7||15.5a||30.5a||0.04a||30.7||60|
|Porites australiensis||flow-through aquarium||sheet||33.1||17.4a||30.7a||NA||NA||60|
|Porites lutea||flow-through aquarium||sheet||31.1||14.4a||25a||3.8a||25.1||60|
|P. lobata||flow-through aquarium||sheet||37.3||15.4a||24.2a||1.8a||26.5||60|
|Porites murrayensis||flow-through aquarium||sheet||24.3||11.6a||28.5a||0.6a||31.6||60|
|Porites sp.||flow-through aquarium||sheet||36.5||20.1a||22.7a||NA||NA||60|
Accordingly, the protein/energy ratio of fluid mucus ranges from 8.3-35.7 mg/kJ (average 19.8 ± 9.4 mg/kJ ), while that of a mucous sheet was 25.1-31.6 mg/kJ (average 28.4 ± 2.5 mg/kJ). Because mucous sheet can trap a lot of organic particles while they stay on the coral surface, the nutritional values tend to be high.
What if I compare these nutritional values of coral mucus with other food items in coral reef environments? Well, here is the summary (see Fig. 4).
The nutritional values of coral mucus are comparable or even higher than those of reef benthic algae, reef algal detritus, reef fish feces, phytoplankton and zooplankton.
That’s why coral mucus is one of the best nutritional sources!
I thank Adi for providing her beautiful drawing (above) and checking the English of this page.
Nakajima, R. & Tanaka, Y. The role of coral mucus in the material cycle in reef ecosystems: biogeochemical and ecological perspectives. J. Japanese Coral Reef Soc. 16, 3–27 (2014). (in Japanese, but with English abstract, figures and tables)
Ducklow, H. W. & Mitchell, R. Bacterial populations and adaptations in the mucus layers on living corals. Limnol. Oceanogr. 24, 715–725 (1979).
Cooney, R. P., Pantos, O., Le Tissier, M. D. A., Barer, M. R. & Bythell, J. C. Characterization of the bacterial consortium associated with black band disease in coral using molecular microbiological techniques. Environ. Microbiol. 4, 401–413 (2002).
Ritchie, K. B. Regulation of microbial populations by coral surface mucus and mucus-associated bacteria. Mar. Ecol. Prog. Ser. 322, 1–14 (2006).
Teai, T., Drollet, J. H., Bianchini, J.-P., Cambon, A. & Martin, P. M. V. Occurrence of ultraviolet radiation-absorbing mycosporine-like amino acids in coral mucus and whole corals of French Polynesia. Mar. Freshw. Res. 49, 127–132 (1998).
Drollet, J. H., Glaziou, P. & Martin, P. M. V. A study of mucus from the solitary coral Fungia fungites (Scleractinia: Fungiidae) in relation to photobiological UV adaptation. Mar. Biol. 115, 263–266 (1993).
Hubbard, J. A. E. B. & Pocock, Y. P. Sediment rejection by recent scleractinian corals: a key to palaeo-environmental reconstruction. Geol. Rundschau 61, 598–626 (1972).
Schuhmacher, H. Ability of fungiid corals to overcome sedimentation. in Proc 3rd Int Coral Reef Symp 1, 503–509 (1977).
Mitchell, R. & Chet, I. Bacterial attack of corals in polluted seawater. Microb. Ecol. 2, 227–233 (1975).
Neff, J. M. & Anderson, J. W. Response of marine animals to petroleum and specific petroleum hydrocarbons. (1981).
Bastidas, C. & Garcia, E. M. Sublethal effects of mercury and its distribution in the coral Porites astreoides. Mar. Ecol. Prog. Ser. 267, 133–143 (2004).
Daumas, R. & Thomassin, B. A. Protein fractions in coral and zoantharian mucus: possible evolution in coral reef environments. in Proc 3rd Int Coral Reef Symp 1, 517–523 (1977).
Krupp, D. A. Mucus production by corals exposed during an extreme low tide. Pacific Sci. 38, 1–11 (1984).
Wild, C. et al. Coral mucus functions as an energy carrier and particle trap in the reef ecosystem. Nature 428, 66–70 (2004).
Wild, C., Laforsch, C., Mayr, C., Fuß, R. & Niggl, W. Effect of water currents on organic matter release by two scleractinian corals. Aquat. Ecol. 46, 335–341 (2012).
Marcus, J. & Thorhaug, A. Pacific versus Atlantic responses of the subtropical hermatypic coral Porites spp. to temperature and salinity effects. in Proc 4th Int Coral Reef Symp 2, 15–20 (1981).
Niggl, W., Glas, M., Laforsch, C., Mayr, C. & Wild, C. First evidence of coral bleaching stimulating organic matter release by reef corals. in Proceedings of the 11th International Coral Reef Symposium’.(Ed. B. Riegl.) pp 905–910 (2009).
Lewis, J. B. & Price, W. S. Patterns of ciliary currents in Atlantic reef corals and their functional significance. J. Zool. 178, 77–89 (1976).
Herndl, G. J. & Velimirov, B. Microheterotrophic utilization of mucus released by the Mediterranean coral Cladocora cespitosa. Mar. Biol. 90, 363–369 (1986).
Goldberg, W. M. Feeding behavior, epidermal structure and mucus cytochemistry of the scleractinian Mycetophyllia reesi, a coral without tentacles. Tissue Cell 34, 232–245 (2002).
Tanaka, Y. et al. Net release of dissolved organic matter by the scleractinian coral Acropora pulchra. J. Exp. Mar. Bio. Ecol. 377, 101–106 (2009).
Meikle, P., Richards, G. N. & Yellowlees, D. Structural determination of the oligosaccharide side chains from a glycoprotein isolated from the mucus of the coral Acropora formosa. J. Biol. Chem. 262, 16941–16947 (1987).
Meikle, P., Richards, G. N. & Yellowlees, D. Structural investigations on the mucus from six species of coral. Mar. Biol. 99, 187–193 (1988).
Crossland, C. J., Barnes, D. J. & Borowitzka, M. A. Diurnal lipid and mucus production in the staghorn coral Acropora acuminata. Mar. Biol. 60, 81–90 (1980).
Davies, P. S. The role of zooxanthellae in the nutritional energy requirements of Pocillopora eydouxi. Coral Reefs 2, 181–186 (1984).
Muscatine, L., Falkowski, P. G., Porter, J. W. & Dubinsky, Z. Fate of photosynthetic fixed carbon in light-and shade-adapted colonies of the symbiotic coral Stylophora pistillata. Proc. R. Soc. London B Biol. Sci. 222, 181–202 (1984).
Levas, S. J., Grottoli, A. G., Hughes, A., Osburn, C. L. & Matsui, Y. Physiological and biogeochemical traits of bleaching and recovery in the mounding species of coral Porites lobata: implications for resilience in mounding corals. PLoS One 8, e63267 (2013).
Naumann, M. S. et al. Organic matter release by dominant hermatypic corals of the Northern Red Sea. Coral Reefs 29, 649–659 (2010).
Nakajima, R. et al. In situ release of coral mucus by Acropora and its influence on the heterotrophic bacteria. Aquat. Ecol. 43, 815–823 (2009).
Nakajima, R. et al. Release of particulate and dissolved organic carbon by the scleractinian coral Acropora formosa. Bull. Mar. Sci. 86, 861–870 (2010).
Crossland, C. In situ release of mucus and DOC-lipid from the corals Acropora variabilis and Stylophora pistillata in different light regimes. Coral Reefs 6, 35–42 (1987).
Tanaka, Y., Miyajima, T., Koike, I., Hayashibara, T. & Ogawa, H. Production of dissolved and particulate organic matter by the reef-building corals Porites cylindrica and Acropora pulchra. Bull. Mar. Sci. 82, 237–245 (2008).
Haas, A. F. et al. Effects of coral reef benthic primary producers on dissolved organic carbon and microbial activity. PLoS One 6, e27973 (2011).
Tanaka, Y., Ogawa, H. & Miyajima, T. Effects of nutrient enrichment on the release of dissolved organic carbon and nitrogen by the scleractinian coral Montipora digitata. Coral Reefs 29, 675–682 (2010).
Wild, C., Woyt, H. & Huettel, M. Influence of coral mucus on nutrient fluxes in carbonate sands. Mar. Ecol. Prog. Ser. 287, 87–98 (2005).
Rochelle-Newall, E. J., Torréton, J. P., Mari, X. & Pringault, O. Phytoplankton-bacterioplankton coupling in a subtropical South Pacific coral reef lagoon. Aquat. Microb. Ecol. 50, 221–229 (2008).
Nakajima, R. et al. High inorganic phosphate concentration in coral mucus and its utilization by heterotrophic bacteria in a Malaysian coral reef. Mar. Ecol. 36, 835–841 (2015).
Nakajima, R. et al. Enrichment of microbial abundance in the sea-surface microlayer over a coral reef : implications for biogeochemical cycles in reef ecosystems. Mar. Ecol. Prog. Ser. 490, 11–22 (2013).
Johannes, R. E. Ecology of organic aggregates in the vicinity of a coral reef. Limnol. Oceanogr. 12, 189–195 (1967).
Coles, S. L. & Strathmann, R. Observations on coral mucus ‘flocs’ and their potential trophic significance. Limnol. Oceanogr. 18, 673–678 (1973).
Hobson, E. S. Feeding relationships of teleostean fishes on coral reefs in Kona, Hawaii. Fish Bull 72, 915–1031 (1974).
Reese, E. S. Coevolution of corals and coral feeding fishes of the family Chaetodontidae. in Proc 3rd Int Coral Reef Symp 1, 267–274 (1977).
Richman, S., Loya, Y. & Slobodkin, L. The rate of mucus production by corals and its assimilation by the coral reef copepod Acartia negligens. Limnol. Oceanogr. 20, 918–923 (1975).
Gottfried, M. & Roman, M. R. Ingestion and incorporation of coral-mucus detritus by reef zooplankton. Mar. Biol. 72, 211–218 (1983).
Nakajima, R. et al. Crown-of-Thorns Starfish Larvae Can Feed on Organic Matter Released from Corals. Diversity 8, 18 (2016).
Knudsen, J. W. Trapezia and Tetralia (Decapoda, Brachyura, Xanthidae) as obligate ectoparasites of pocilloporid and acroporid corals. Pacific Sci. 21, (1967).
Patton, W. K. Distribution and ecology of animals associated with branching corals (Acropora spp.) from the Great Barrier Reef, Australia. Bull. Mar. Sci. 55, 193–211 (1994).
Kropp, R. K. Feeding biology and mouthpart morphology of three species of coral gall crabs (Decapoda: Cryptochiridae). J. Crustac. Biol. 377–384 (1986).
Stachowicz, J. J. & Hay, M. E. Mutualism and coral persistence: the role of herbivore resistance to algal chemical defense. Ecology 80, 2085–2101 (1999).
Shafir, A. & Loya, A. Consumption and assimilation of coral mucus by the burrowing mussel Lithophaga lessepsiana. in Proc Int Conf Mar Sci Red Sea 9, 135–140 (1983).
Coffroth, M. A. Ingestion and incorporation of coral mucus aggregates by a Gorgonian soft coral. Mar. Ecol. Prog. Ser. Oldend. 17, 193–199 (1984).
Naumann, M. S., Mayr, C., Struck, U. & Wild, C. Coral mucus stable isotope composition and labeling: Experimental evidence for mucus uptake by epizoic acoelomorph worms. Mar. Biol. 157, 2521–2531 (2010).
Wilson, S. K., Bellwood, D. R., Choat, J. H. & Furnas, M. J. Detritus in the epilithic algal matrix and its use by coral reef fishes. Ocean. Mar Biol Annu Rev 41, 279–310 (2003).
Wilson, S. Nutritional value of detritus and algae in blenny territories on the Great Barrier Reef. J. Exp. Mar. Bio. Ecol. 271, 155–169 (2002).
Henken, A. M., Lucas, H., Tijssen, P. A. T. & Machiels, M. A. M. A comparison between methods used to determine the energy content of feed, fish and faeces samples. Aquaculture 58, 195–201 (1986).
Ducklow, H. W. & Mitchell, R. Composition of mucus released by coral reef coelenterates. Limnol. Oceanogr. 24, 706–714 (1979).
Daumas, R., Galois, R. & Thomassin, B. A. Biochemical composition of soft and hard coral mucus on a New Caledonian lagoonal reef. in Proc. Fourth Int. Coral Reef Symp. II. Manila 59–67 (1981).
Pascal, H. & Vacelet, E. Bacterial utilization of mucus on the coral reef of Aqaba (Red Sea). in Proc 4th Int Coral Reef Symp 1, 669–677 (1981).
Krupp, D. A. The composition of the mucus from the mushroom coral, Fungia scutaria. in Proceedings 4th International Coral Reef Symposium, Manila 2, 9–73 (1982).
Coffroth, M. A. Mucous sheet formation on poritid corals: an evaluation of coral mucus as a nutrient source on reefs. Mar. Biol. 105, 39–49 (1990).
Bailey, T. G. & Robertson, D. R. Organic and caloric levels of fish feces relative to its consumption by coprophagous reef fishes. Mar. Biol. 69, 45–50 (1982).
Renaud, S. M., Thinh, L.-V. & Parry, D. L. The gross chemical composition and fatty acid composition of 18 species of tropical Australian microalgae for possible use in mariculture. Aquaculture 170, 147–159 (1999).
Ventura, M. Linking biochemical and elemental composition in freshwater and marine crustacean zooplankton. Mar. Ecol. Prog. Ser. 327, 233–246 (2006).