Let’s Decode the Defense System of Aquatic Animals

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Efficient disease management and mitigation strategies are not possible without understanding the immunological aspects of the cultured animal. During the larval stage, fishes are easily susceptible to a wide range of stressors and diseases; hence improving survival during the earlier stage is crucial and beneficial to aquaculture production. Stress is the primary disruptor of homeostasis of all living organisms, including finfish and shellfishes.

Prolonged stress decreases the defense ability of the animal. A better understanding of the defense mechanism of aquatic animals will pave the way for the immunological control of diseases. Usage of functional ingredients and additives in feed, can offer a hand in achieving the healthy animal with better growth and profitable revenue to the farmer community.

Aquatic animals consist of two categories which include vertebrates likes fishes and invertebrates like crustaceans and mollusk. These organisms also possess a unique immune system like the mammals but with some exceptions in the former.

Pandemic is the recent heated argument of humanity, whereas epidemic is the daily life crisis of every aquaculture farmer. Efficient disease management and mitigation strategies are not possible without understanding the immunological aspects of the cultured animal. Let’s dive deeper into the defense system of both finfish and shellfish.

Overview

Immunology is derived from the Latin word immunis, meaning ‘exempt from’, which is the study of the defense system. Every aquatic organism can fight against the disease known as Immunity. Many cells and processes are involved in activating the immunity, which is termed as immune system/ defense system. Finfishes like seabass, catfish, possess both innate and adaptive immunity, unlike shellfishes like shrimp which lack adaptive immunity.

Substances that are foreign or alien to the body which can provoke an immune response known as an antigen. Antibodies or Immunoglobulins are a group of globular glycoproteins which binds explicitly with antigens and neutralize them, thereby protecting the body against various diseases.

The defense system of Finfishes Anterior kidney and thymus are the major primary lymphoid organs, newly hatched out larvae will possess maternal immunity till it starts exogenous feeding. Non-specific components like lectins and Hemagglutinins are present even during the egg and fry stage, which is evident that nonspecific immunity is the central backbone of the finfish defense system, which develops first in fishes followed by cell-mediated, humoral and specific immunity.

During the larval stage, fishes are easily susceptible to a wide range of stressors and diseases; hence improving survival during the earlier stage is crucial and beneficial to aquaculture production. Maternal antibodies have a significant role against the vertical transfer of pathogen during the spawning stage.

Non-specific defense mechanisms

Surface Barrier: Mucus

• First chemical and Physical barrier against the pathogen.

• Antimicrobial activity.

• Cells involved in the production of mucus include club cells, sacciform cells, goblet cells.

Surface Barrier: Skin

• Malpighian cells promote sloughing of infected cells to eliminate foreign matter.

• Rapid wound healing.

Surface Barrier: Gill

• The primary pathway for entry of pathogen.

• The protection is offered through the mucosal barrier and with the help of the gill epithelium.

Surface Barrier: Gastro-intestinal tract

• The protection is offered through the Mucosal membrane and by lowering the gut pH in gastric fishes like salmon.

Non-specific humoral factors

Growth inhibitors.

• The transferrin-Bacteriostatic and fungistatic effect through their binding affinity for iron is an essential nutrient for microbial growth.

• Interferon-Antiviral defense.

Enzyme inhibitors

• Anti-proteases, Alpha 2 macroglobulin acts by neutralizing the pathogenic enzymes.

Lysins.

• The complement system, which consists of more than 30 protein components they promote phagocytosis.

• Anti-microbial peptides-defensins, cathelicidins, piscidins.

• Lysozyme- present in fish ova, mucus, serum, phagocytic cells.

Precipitins and agglutinins

• Pentraxins are compounds involved in acute phase response which includes,

• CRP (C-reactive protein which binds to phosphorylcholine of the microbial cell wall)

• SAP (Serum amyloid P, which binds to the phospho-ethanolamine glycans and DNA)

• Lectins-Pattern recognition receptors for detecting infections in fish.

Non-specific cellular factors

Phagocytes (Macrophages and Neutrophils)

• Kupffer cells are absent in fish liver.

• Bactericidal chemotaxis, involved in phagocytosis

Cytokines

• Intercellular signaling molecules induce the movement of phagocytes to the site of infection.

Eosinophils, basophils and mast cells

• Significant role in inflammation and defense mechanisms.

Specific defence mechanisms

• Specificity and memory are the intrinsic property of this system.

• T cells-Cell mediated immunity.

• B cells-Antibody production.

Defense mechanism in shellfish

Shrimp is one of the globally traded commodities; epidemics is the neverending crisis in the culture industry, lot of vaccinations are available only for finfishes, but when it comes to shellfish like shrimp, mollusk, vaccination is impractical because of the primitive defense system this animal possesses which includes the lack of true antibodies, specificity, and memory. Cell-mediated and humoral immunity is present, whereas adaptive immunity is absent in crustaceans.

From this, one should have a complete idea about the immunological aspects of this animal when considering culture.

Innate immunity of shellfish

Hemocytes play a central role in the innate immune defense of the animal; different types of hemocytes present are classified based on the presence of cytoplasmic granules inside the cells.

• Hyaline cells: Phagocytosis and clotting.

• Semi-granular cells: Encapsulation and nodule formation.

• Granular cells: Prophenol-oxidase system and cytotoxicity.

Cellular defence mechanisms

Phagocytosis

• The first line of defence present in the crustacean.

• Engulfing and exclusion of pathogen by killing through the release of antimicrobial substances.

Nodule formation

• Isolation and melanization

• Occurs mainly in gills and hepatopancreas.

Encapsulation

• Mainly for an organism that are too large which cannot be engulfed through phagocytosis, example nematode parasites.

Cytotoxicity

• Direct interaction of hemocytes with pathogens.

Apoptosis/Programmed cell death

• Infected and damaged cells are eliminated through this process to maintain homeostasis.

Humoral defense mechanism

Lectins/Agglutinins

• Immune recognition factor and microbial phagocytosis through opsonization.

ProPO activation

• Associated with melanization, sclerotization and wound healing.

• Counterpart to the vertebrate complement system.

Anti-microbial compounds

• Hemocyanin which is the copper containing respiratory pigment of crustacean having anti-viral property.

• Peneidin isolated from P. monodon having anti-bacterial properties.

Stress and Immunology

Stress is the primary disruptor of homeostasis of all living organisms, including finfish and shellfishes. There are many factors called stressors that bring a change in a normal physiological response of an animal; being a cold-blooded animal, temperature is the biotic master factor that triggers changes in the animal behavior.

Prolonged stress decreases the defense ability of the animal. Stress can be acute or chronic, which depends on the duration of the stressor. Cortisol and catecholamines are the indicators of stress. Blood glucose level increasesas the result of secondary stress response to compensate for the increased energy demand. Hence as a result susceptibility of the animal to diseases increases due to the impaired immune system.

Conclusion

A better understanding of the defense mechanism of aquatic animals will pave the way for the immunological control of diseases. Usage of Functional ingredients and additives in feed, Nutraceuticals, immunostimulants, along with the OMICS studies, can offer a hand in achieving the healthy animal with better growth and profitable revenue to the farmer community. Proper nursery management, selection of healthy PL, and biosecurity measures are the keys for the mitigation of outbreaks of diseases, and further studies in the immunological aspects of aquatic animals can provide a different dimension in the future.

This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “LET’S DECODE THE DEFENSE SYSTEM OF AQUATIC ANIMALS” developed by: ATSHAYA. S, SOWMIYA. C. The original article was provided by the authors.

 

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