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.

Mucus sheet

Corals use mucus to remove sediments.

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 corals

Source: Nakajima & Tanaka (2014)1.
Release factorCoralSource
Under stressed condition
Biofouling, pathogenPorites astreoides2
Diploria strigosa3
Montastrea annularis3
Colpophyllia natans3
Acropora palmata4
UV radiationLobophyllia hemprichii5
Fungia scutaria5
Fungia repanda5
Acropora danai5
Pocillopora eydouxi5
Pocillopora meandrian5
Fungia fungites6
Fungia repanda6
SedimentationDiaseris distorta7,8
Fungia scutaria7,8
Cycloseris costulata8
Cycloseris doederleini8
Cycloseris marginata8
Fungia actiniformis8
Fungia danai8
Fungia echinata8
Fungia fungites8
Fungia granulosa8
Fungia horridae8
Fungia Klunzingeri8
Fungia scruposa8
Fungia sommmervillei8
PollutantsPlatigyra sp.9
M. annularis10
P. astreoides11
DesiccationF. scutaria12
Palythoa sp.12
F. scutaria13
Acropora spp.14
Water currentMontipora digitata15
Euphyllia sp.15
Temperature/salinityPorites porites16
Porites compressa16
Acropora sp.17
Non-stressed condition
Heterotrophic feedingSiderastrea siderea18
Agaricia agaricites18
Madracis mirabilis18
Montastrea cavernosa18
P. porites18
Eusmilia fastigiata18
Mussa angulosa18
Favia fragum18
Colpophyllia natans18
D. strigosa18
Cladocoral cespitosa19
Mycetophyllia reesi20

 

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).

Fig. 1. Various forms of coral mucus.

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!

Fig. 2. Summary of release rate of DOC (dissolved organic carbon) and POC (particulate organic carbon) by corals. Modified from Nakajima & Tanaka (2014)1.

 

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.

An example of coral incubation

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 rates

Modified from Nakajima & Tanaka (2014)1.
Incubation method (volume of incubation medium, L)Incubation time (hour)LightingStirringSource
Bottle (<1) 3-4Fluorescent lampNo stirring19
Flow through chamber3In situFlow through31
Beaker (0.5-2) 4-6Shading sunlightNo stirring35
Bottle (8)96Direct sunlightContinuous32
Plastic bag (2-8)24In situNo stirring29
Bottle (0.7)5Halogen lampContinuous29
Beaker (0.8-1)6Shading sunlightNo stirring28
Bottle (1.8)5Direct sunlightEvery hour30

 

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.

 

Microbes on coral mucus, stained with SYBR Gold.

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.

Development of bacterial abundance in seawater after addition of coral mucus

Fig. 3. Development of bacterial abundance in seawater after addition of coral mucus. Modified from Nakajima et al. (2015)37

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.

Sea-surface microlayer can be collected using this kind of mesh screen (mesh width: 1mm)

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 mucus

Modified from Nakajima & Tanaka (2014)1
Taxonomic groupSpeciesMethod for feeding experiments/observationsSource
Fish
Blue spratSpratelloides delicatulusbehavior39
DamselfishChromis sp.behavior39
DamselfishChromis sp.behavior (no feeding on mucus was observed)40
SurgeonfishAcanthuridaebehavior (no feeding on mucus was observed)40
Butterfly fishChaetodon ornatissimusbehavior41,42
Plankton
CopepodAcartia negligence14C incorporation43
CopepodAcartia tonsaneutral red dyeing/[methyl-3H]-thymidine incorporation44
MysidMysidium integrumneutral red dyeing/[methyl-3H]-thymidine incorporation44
Crown-of-thorns starfish (COTS) larvaeAcanthaster cf. solaris13C, 15N incorporation45
Benthos
Coral crabTrapezia sp.behavior46
Coral crabTetralia fulvabehavior/carmine dyeing47
Coral gall crab Hapalocarcinus marsupialisbehavior48
Coral gall crab Utinomia dimorphabehavior48
ShrimpCoralliocaris superbabehavior/carmine dyeing47
ShrimpPhilarius imperialisbehavior/carmine dyeing47
ShrimpJocaste japonicabehavior/carmine dyeing47
CrabMithrax sp.behavior/neutral red dyeing49
BivalveLithophaga lessepsiana14C incorporation50
SoftcoralPseudoplexaura porosa14C incorporation51
Acoelomorph wormWaminoa sp.15N incorporation52

 

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 ratio

AFDW, 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.
CoralMucus collection/generation methodMucus formAFDW (%)Carbohydrate (%)Protein (%)Lipid (%)Protein/energy ratio (mg/KJ)Source
Fungiaair exposurefluid21NANA93aNA12
Platygyraair exposurefluid9416b59b0b35.256
Acroporaair exposurefluidNANA22bNANA56
Fungiaair exposurefluid402.5b5b42b2.756
Lobophyllia corymbosaair exposure (natural tidal exposure)fluid2111b,c57.7b31.3b20.657
Faviidaeair exposure (natural tidal exposure)fluid2231.7b,c23.8b44.4b8.357
Platygyraair exposurefluid512a,c4.3a5.7a11.958
Fungia scutariaair exposure + seawater washfluid8362a35a3.4a17.159
Acropora formosaextraction with toluenefluid?91.455.6b30.4b4.2b16.323
Pachyseris speciosaextraction with toluenefluid?7629b34b2.5b24.123
Fungia fungitesextraction with toluenefluid?898b72.2b4.4b35.723
Porites astreoidesair exposure + distilled water washfluid87.339a24.6a2.8a17.660
Porites furcataair exposure + distilled water washfluid8751.6a14.2a0.2a11.460
Portites lobatacold-treatmentfluid6838.5a24.1aNANA60
Porites sp.cold-treatmentfluid7137.5a24aNANA60
P. astreoidesfield collection (underwater)sheet26.614a27.5a2.3a27.960
P. furcatafield collection (underwater)sheet3411.3a34.7a4.8a28.760
Porites divaricatafield collection (underwater)sheet22.715.5a30.5a0.04a30.760
Porites australiensisflow-through aquariumsheet33.117.4a30.7aNANA60
Porites luteaflow-through aquariumsheet31.114.4a25a3.8a25.160
P. lobataflow-through aquariumsheet37.315.4a24.2a1.8a26.560
Porites murrayensisflow-through aquariumsheet24.311.6a28.5a0.6a31.660
Porites sp.flow-through aquariumsheet36.520.1a22.7aNANA60

Mucous sheet

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).

Nutritional values (protein/energy ratio) of coral mucus and other possible food items

Fig. 4. Nutritional values (protein/energy ratio) of coral mucus and other possible food items. Data from Nakajima & Tanaka (2014)1

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!

Acknowledgments

I thank Adi for providing her beautiful drawing (above) and checking the English of this page.

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