Written by: Rebecca Gibbel
Edited by: Miranda Altice
Corals sometimes have a bizarre jumble of anatomic and physiologic features similar to vertebrates but with entirely different arrangements and functions. Their immune system is no exception! Understanding corals’ immunity can be difficult since they do not have blood, bone marrow, or antibody-producing lymphocytes as we do. However, corals have complex immune systems that share characteristics with vertebrates, especially when examined at the level of enzymes and other immune proteins.
Vertebrate immune function is divided into innate, adaptive, and passive categories
- The innate category consists of those immunological defenses inherently present in an organism from birth. It does not require past exposure to defend against foreign invaders but is not very specific.
- The adaptive immune response develops as a vertebrate animal encounters pathogens and foreign proteins during its lifetime. Lymphocyte cells circulate through the blood and produce specific antibody proteins that encounter and fight foreign microbes and then replicate to create an army in waiting. Antibodies allow an animal to have a memory of prior infections and rapidly recognize and neutralize a threat that is reencountered. Other circulating immune cells include macrophages, neutrophils, and eosinophils.
- Passive immunity consists of cells and antibodies produced by another animal and given to an individual. Examples are the maternal antibodies shared with a developing fetus or an injection of antibodies that a doctor gives to a patient.
Coral immune function
Invertebrates like corals have long been thought to lack adaptive immunity, at least by its original definition. However, cnidarian colonies have well-adapted innate immune processes suited to their aquatic lifestyle as communities of interdependent organisms. This partnership is known as the holobiont and consists of animal polyps, symbiotic microalgae, and an essential microbiome of bacteria and other microorganisms.
Like all animals, corals have non-specific defense mechanisms that act as barriers to invasion. These structures include nematocyst stinging cells, venom-producing glands, and the surface mucus layer, which entraps pathogens and continually sloughs off (Schmidt et al., 2019; Tardent, 1995). Corals are not as helpless as they may appear! The immune system of all animals provides additional, more specific defensive measures that protect against infection and aid in wound healing. Targets of the immune response include microbial pathogens, tumor cells, parasites, and other non-self cells.
In corals, immunity also plays an essential role in governing which symbiotic algae species make it past the velvet rope to be accepted to live within the cells and which are excluded (Jacobovitz et al., 2021; Nyholm & Graf, 2012). This recognition and processing ability requires pinpoint suppression of the immune response, which allows corals and their Symbiodiniaceae algae (commonly called zooxanthellae) to coexist. Reefs as we know them are only possible with this symbiotic relationship since corals derive most of their nutrition from their photosynthetic algal partners and can only grow and build reef structures by maintaining that partnership.
A Pacific coral colony with colorful pigments from symbiotic microalgae.
Coral’s innate immunity
- The surface mucus layer:
This superficial coating represents a significant part of corals’ innate immunity, since it is colonized by beneficial bacteria which produce antibiotics directed against non-colony microbes. This effectively outsources immune protection to a holobiont community member. Mammals also have mucous surfaces in the orifices that connect the outer and inner body environments. These moist membranes entangle and remove pathogens and irritants, and IgE antibodies are produced by the animal specifically for these mucoid surfaces. In this way, mammals have a function that is similar to corals’- just produced by a different mechanism.
- Amoebocytes:
The cellular part of the innate immune response of corals is performed by specialized cells known as wandering amoebocytes or phagocytes, which migrate through the gelatinous mesogleal connective tissue to capture and envelop foreign microbes. Cnidarian amoebocytes correspond to the macrophage cells of vertebrates but with the additional useful ability to respond to heat stress (Mydlarz et al., 2008). For the first time, Snyder et al. (2021) visualized live coral amoebocytes in engulfing and digesting damaged cells, fungi, and bacteria. This confirmed the cells’ phagocytic role, which had previously been suspected by histopathology.
A graphic representation of a phagocyte cell recognizing and engulfing a foreign microbe.
- Biochemistry:
Now for the trickier part- fortunately, this will not be on the test! Invading pathogens are recognized by the coral’s receptors, including Toll interleukin proteins, lectins, and integrin proteins. These receptors also identify cells that are identified as non-self or degraded host cells. Then, signaling pathways are activated, resulting in effector responses that neutralize the threat. These responses include the production of peroxidase enzymes, which are protective antioxidants, the melanization process to entrap pathogens, and programmed cell death, which knocks out compromised host cells as a necessary step of wound repair (Palmer & Traylor-Knowles, 2012). When immune-related genes are upregulated, their protein transcriptional activity can be measured. The enzymes that reflect immune activity are always active, showing that constituent immunity is present to maintain body integrity and defense preparedness. When a pathogen or injury is encountered, the level of these transcribed proteins is amplified (Bisanti et al., 2024). These immune responses are depressed in elevated water temperatures, which may help explain the relatively recent proliferation of coral diseases in oceans that are warming under anthropomorphic impacts.
- Melanization:
Corals use two physiologic processes that play minor roles in vertebrate immunity but are major components of the immune response of invertebrates. One is the melanization pathway, which produces insoluble melanin pigment that traps foreign particles and walls them off (Mydlarz & Palmer, 2011; Palmer et al., 2008). Melanization is associated with an inflammatory-like response in corals, particularly to fungal and parasitic challenges. It can be seen in the diseased coral below, showing the darker pigmented areas of reaction to disease (Bisanti et al., 2024). In vertebrates, melanin is produced in the skin in response to damage from solar radiation and fungal pathogens, and it can be used to heal wounds. Its function as a component of innate immunity in vertebrates like humans has only recently been explored (Tapia et al., 2014).
Left: A Siderastrea siderea coral showing melanin-pigmented diseased areas.
Right: A common sea fan (Gorgonia ventalina) with melanin surrounding Aspergillus fungal lesions.
- Biomineralization:
Corals may also employ their skeleton-building calcifying ability to entrap invading organisms. A diverse biome is normally assimilated into the skeletal layers, composed of bacteria, endolithic algae, viruses, fungi, and nematodes (Mueller et al., 2013; Pernice et al., 2020). To keep these populations from proliferating into the soft tissues of the living polyp, the calicoblastic cells that produce the coral calcium carbonate skeleton are postulated to have a selectively antagonistic immune role (Levy & Mass, 2022). Increased tissue calcification is also a feature of some vertebrate immune responses, particularly in response to proliferating tumor cells and encysting parasites.
Do corals have adaptive immunity?
Although it differs from vertebrate adaptive immunity, corals have an “ecologic memory” of past exposures to environmental stressors, which helps them recover from subsequent disturbance (Vompe et al., 2024). Heat stress alters the microbial populations of the coral microbiome, sparing thermotolerant bacterial members and eliminating more heat-sensitive ones. This makes corals better prepared for the next thermal event. In an ocean world where one of the biggest threats to survival is elevated temperature, improving tools to combat heat is one of the most valuable advantages a coral can have.
Another study that suggests that cnidarians have immune memory was conducted by Brown & Rodriguez-Lanetty (2015) using the model organism Exaiptasia diaphana. This work demonstrated that sea anemones, closely related to corals, respond to “immunological priming”. The authors describe this as stimulation of the immune system by exposure to pathogens, which results in long-lasting effects in response to subsequent encounters. Experimental anemones were experimentally treated with pathogenic Vibrio bacteria in a first “priming” exposure and were compared to unexposed subjects. When Vibrio bacteria were encountered a second time, the previously exposed anemones had 5 to 7-fold better survival, suggesting the existence of transient defense priming. These positive findings with closely related cnidarians suggest that adaptive immunity is possible with corals.
Do corals have passive immunity?
If the definition is expanded to encompass immune strategies that invertebrates employ, then the answer is… probably. Experimental treatments such as those performed by Ushijima et al. (2023) might be considered donations of passive immunity. The investigators developed probiotic treatments for corals, which provide specific commensal bacteria from healthy corals. These beneficial bacteria are part of the normal microbiome and produce antibacterial compounds that help control disease- likely by suppressing the overgrowth of pathogenic bacteria.
Another example of passive immunity may be with corals that have a brooding reproductive strategy. The parent colonies supply their larvae with their own Symbiodiniaceae before spawning, which helps prepare the offspring for the environmental conditions the parent has experienced. Different endosymbionts can convey some protection against heat stress and disease to their coral host, so consider this a donation of immunity- the best possible gift that parent corals can give their offspring!
A spawning colony of Acropora, which has a broadcast spawning strategy. Acropora larvae must assimilate their first population of Symbiodiniaceae from the water column or sediment.
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