**3. External physicochemical barriers of invertebrates as first line of immune defence against pollutants**

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External physicochemical barriers of the invertebrates act as the first line of defence against the invasion of parasite, pathogen and toxin in a polluted aquatic environment. External system may lead to opportunistic invasion of environmental pathogens and parasites into the

the concentration of toxin, type and span of exposure and route of entry to the host.

Immunotoxins are difficult to identify as they can cause a wide magnitude of adversity on the

immune status of organisms. Immunotoxicology deals with the assessment of toxicological

response in an organism by estimating the responsiveness and reactivity of its immunological

parameters. Immunotoxicology of invertebrates, in recent times, has been gaining a special

scientific attention for its efficacy in monitoring the health of environment – both aquatic and

terrestrial. Accuracy and precision of selected toxicological responses of aquatic invertebrates

enabled a few species to function as suitable biomonitoring agents of water pollution [6]. Several

effective immunomarkers of aquatic pollution have recently been established in model

The nature and magnitude of immunotoxicity depend on multiple parameters including

body of host and increase the vulnerability of these biofilter species in polluted environment.

physicochemical barriers of the invertebrates include shell, tunic, test, cuticle, carapace, pinacoderm and others. In mollusc, hard calcareous shell or valves provide the primary protection against entry of pathogens and toxins. However, in shell-less molluscs, thick external cuticular sheet known as mantle or pallium are assumed to play a significant physiological role against toxin and pathogen entry. Soft body wall of molluscs consists of cuticle, epidermis and muscles, which are thought to take an active part in the innate immune defence against environmental pathogens and toxins. In a polluted environment, where environmental pathogens and toxins are present in sufficient quantity, the two valves of the mussel are kept closed to minimize the entry of undesired agents. In addition to external shell, mucus secreted from the internal viscera provides another line of defence against the invasion of pathogens and toxins. Mucus acts as a protective barrier prevent‐ ing the direct contact of toxins to epithelia. Secretion of mucus is reported to be an important detoxification and evasive mechanism of invertebrates. Pathogens are trapped within the mucus secreted by the organism leading to elimination. Calcareous shell made up of calcium carbonate and mucus is considered as external physiochemical barrier and plays an important role in the immune defence of aquatic invertebrates. Carapace, the external physiochemical barrier of crab provides the first line of defence in crustaceans. Corrosive toxins including mineral acids, alkalis, pesticides and detergents appear to be the poten‐ tial threats of the invertebrate. Contamination of water bodies by these pollutants often results in breaches of the physicochemical barriers of external body surface, which leads to facilitation of invasion of toxic microorganisms and parasites into the viscera of target invertebrates. Components of innate immune response comprise physicochemical barrier against external pathogen and parasite entry. Cuticle covered with waxy material serves as a mechanical barrier of crab against parasitic infection. This chitinous exoskeleton or cuticle helps in the process of wound healing by preventing the fatal loss of haemolymph from body, maintenance of tissue architecture and prevention of opportunistic invasion of pathogen. Whenever this cuticle is damaged by injury or infection, the wound is rapidly sealed by clotting of immunocytes, preventing blood loss and pollutant entry. Once the clot is formed, wound is darkened and accumulation of melanin occurs. Melanin is reported to be involved in sealing of wound and synthesis of new cuticle. On the other hand, pinaco‐ derm made up of flattened pinacocytes forms a continuous layer on the external surface of freshwater sponges and acts as a first line of defence against foreign invaders.

are difficult to identify as they can cause a wide magnitude of adversity on the immune status of organisms. Immunotoxicology deals with the assessment of toxicological response in an organism by estimating the responsiveness and reactivity of its immunological parameters. Immunotoxicology of invertebrates, in recent times, has been gaining a special scientific attention for its efficacy in monitoring the health of environment – both aquatic and terrestrial. Accuracy and precision of selected toxicological responses of aquatic invertebrates enabled a few species to function as suitable biomonitoring agents of water pollution [6]. Several effective immunomarkers of aquatic pollution have recently been established in model invertebrates [7].

152 Emerging Pollutants in the Environment - Current and Further Implications

**Figure 1.** Attributes of cell-mediated immune responses of invertebrate immunocytes exposed to various aquatic pol‐

External physicochemical barriers of the invertebrates act as the first line of defence against the invasion of parasite, pathogen and toxin in a polluted aquatic environment. External

**3. External physicochemical barriers of invertebrates as first line of**

**immune defence against pollutants**

lutants.

invertebrates [7].

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An experiment was performed to investigate the possible role of mucus as a primary barrier in removal of external particles. In this context, the shell of freshwater gastropod, *P. globosa* was drilled to create a micropore and a solution of cultured yeast (*Saccharomyces cerevisiae*) suspended in sterile phosphate buffered saline was injected into the body through a syringe (Figure 2). The mollusc was under scientific surveillance and the activity of the mollusc was photodocumented. After two hours of injection, hypersecretion of mucus was recorded by the experimental *P. globosa* within a time span of approximately thirty minutes. The expelled mucus contained more than ninety-five per cent of yeast particles, indicating the efficacy of mucus to act as external physicochemical barrier.

syringe (Figure 2). The mollusc was under scientific surveillance and the activity of the mollusc

was photodocumented. After two hours of injection, hypersecretion of mucus was recorded by

the experimental P. globosa within a time span of approximately thirty minutes. The expelled

mucus contained more than ninety-five per cent of yeast particles, indicating the efficacy of

Figure 2. Rapid elimination of foreign particulates entrapped in the mucus of aquatic

<H1>Xenobiotic-induced shift in haemocyte density and morphological damages of blood

Haemocytes are the immunocompetent cells which are functionally responsive to various

xenobiotics present in the aquatic environment. Homeostasis of total haemocyte density of

aquatic invertebrates within the permissible physiological limit may be considered as an

important immunological parameter [8] of cell-mediated immune response of molluscs [2].

Chakraborty et al. (2008) reported suppression in the total count of haemocytes of the freshwater

edible mollusc, L. marginalis under the sublethal exposure of sodium arsenite, an aquatic

pollutant [9]. Mukherjee et al. (2006) reported modulation in the total cell density of the same

specimen under the sublethal concentrations of azadirachtin, a neem-based pesticide, a common

contaminant of pond water [10]. According to them, toxin-induced alteration in the total cell

dynamics may lead to a gradual decline of this species in its natural habitat. The total count of

haemocyte of Villorita cyprinoides was found to decrease under the exposure of copper [11]. Ray

et al. (2013b) reported dynamics of the total haemocyte density of B. bengalensis and L.

marginalis exposed to environmentally realistic sublethal concentrations of cypermethrin and

fenvalerate, respectively [2]. Authors reported a significant increase in total haemocyte count

following exposure to experimental concentrations of cypermethrin and fenvalerate to B.

invertebrate, P. globosa.

**Figure 2.** Rapid elimination of foreign particulates entrapped in the mucus of aquatic invertebrate, *P. globosa*.
