The Chemistry of Pistol Shrimp and Antarctic Krill

Alaia Krishnan Claveras, Celina Moukarzel, Chloe Chan Lam, Julia Purdue, Lina Benchekor

Abstract

This paper investigates the chemistry behind the adaptive design solutions of krill and pistol shrimp. First, the exoskeleton that these crustaceans rely on for protection, structural support, and survival will be analyzed. Composed of organic substances such as chitin and inorganic minerals like calcium carbonate and fluoride salts, exoskeletons have been optimized for both flexibility and rigidity. Then, we will examine acetylcholine, a neurotransmitter that induces muscle contractions, facilitating the pistol shrimp’s rapid claw snap. Next, a discussion of omega-3 phospholipids will dive into the importance of maintaining membrane homeostasis and supporting cellular functions in cold environments. Finally, we will analyze the bioluminescence used by krill to camouflage themselves from predators. Through the luciferin-luciferase enzymatic reaction, krill photophores produce light that minimizes the silhouette effect created by solar light.

Introduction

Pistol shrimp and krill share a common ancestry as members of the subphylum Crustacea, which are characterized by their hard exoskeletons, segmented bodies, and jointed limbs. Antarctic krill play a pivotal role in the Southern Ocean’s ecosystem, thriving along the continental shelf and in deeper oceanic regions (Grinnell, 1988). Despite their small size, their combined biomass is immense, making them one of the most abundant animal species on Earth alongside nematodes, insects, and fish (Shao et al., 2023). Krill, a keystone species, is the primary food source for a wide variety of marine animals, from fish to large whales. Pistol shrimps (Kingston, 2022), known for their asymmetrical front claws capable of producing powerful cavitations, inhabit tropical and temperate coastal waters, often dwelling in coral reefs and sponge cavities (Bohnenstiehl, 2016). This paper explores the key chemical components and processes that allow for the existence of krill and pistol shrimp. How does the pistol shrimp exoskeleton allow for the strength and flexibility it requires for both protection and growth? How are pistol shrimp able to snap its claw shut with so much force? How do krill manage free radical accumulation to avoid damage caused by oxidative stress? How are they able to camouflage themselves from predators during the daytime? By answering these questions, we aim to deepen our understanding of the complex interplay between chemistry and survival strategies in these remarkable crustaceans.

Chitin exoskeleton: the structural foundation

The most fundamental chemical component of crustacean exoskeletons is chitin, a long-chain polysaccharide (a type of carbohydrate or sugar) composed of repeating units of N-acetylglucosamine, a derivative of glucose with an additional nitrogen group. The long chitin molecules are organized into fibrous layers that provide structural support through interactions with other components (Horst, 1983; Peng, 2022). See Figure 1A for a planar view of chitin fibers and Figure 1B for a cross-sectional view showing the layered arrangement. This arrangement displays the Bouligand, or “twisted plywood,” structure, where linear chitin fibers form thin sheets that stack helically across multiple layers. Each layer is slightly rotated relative to the one below it, creating an anisotropic (directionally dependent) assembly that enhances toughness and fracture resistance by redistributing mechanical stresses. This structure provides resilience against impacts and compressive forces in natural environments (Peng et al., 2022). Figure 1B highlights the helicoidal stacking of chitin fibers, crucial for maintaining structural integrity and protection against mechanical stress (Horst, 1983).

Fig.1. Microscopic structure of the chitin surface of the Antarctic krill cuticle treated with sodium hydroxide and hydrochloric acid (A plane structure of chitin; B structure of cross section) (Peng, 2022).

Although Figure 1 illustrates this structure in krill, the Bouligand arrangement of chitin fibers is also common across crustaceans, including pistol shrimp, where it serves a similar function in supporting exoskeletal strength and durability under environmental pressures (Peng et al., 2022).

Protein reinforcements

Chitin is combined with specific cuticular proteins, including proteins that contribute to structural reinforcement called Rebers-Riddiford proteins and stabilizing proteins called obstructor proteins, which bind to chitin to reinforce the exoskeleton (Bentov, 2016; Skinner, 1992). These cuticular proteins cross-link with chitin fibers, forming covalent bonds between amino groups on chitin molecules and specific side chains on the protein molecules, creating a stable, interconnected network that greatly strengthens the structure. By chemically bonding with chitin, these proteins provide additional rigidity and durability to the exoskeleton (Bentov, 2016; Skinner, 1992).

In krill, proteins involved in exoskeleton formation, such as gastrolith matrix proteins (proteins that assist in calcium storage and exoskeletal calcification), play a critical role during molting by helping rebuild the exoskeleton after it is shed (Nicol, 1992; Skinner, 1992). Additionally, proteins in the exoskeleton undergo sclerotization—a hardening process that chemically strengthens proteins through cross-linking reactions. This reaction begins with the enzyme phenoloxidase, which catalyzes the oxidation of phenolic compounds into reactive quinones. These quinones then react with specific amino acids, such as lysine and tyrosine, on adjacent protein molecules, forming covalent cross-links between the proteins. This cross-linking transforms the softer, flexible protein matrix into a rigid, durable network that enhances the exoskeleton’s resistance to mechanical stress and damage (Terwilliger, 1999). This chemical hardening enables the exoskeleton to provide robust protection under environmental stress (Bentov, 2016).

Mineralization

Another essential part of the exoskeleton’s chemistry is mineralization, the process of depositing minerals to enhance hardness and durability. In both krill and pistol shrimp, calcium carbonate is the primary mineral that crystallizes within the exoskeleton, providing the hardness and mechanical support necessary for protection and structural integrity (Bentov, 2016). Krill exhibit an additional adaptation: their exoskeleton incorporates elevated levels of fluoride, which combines with calcium and phosphate to form fluorapatite, a calcium phosphate mineral. This fluorapatite mineralization significantly increases the rigidity of the exoskeleton, as fluorapatite is known for its extreme hardness and durability. In Antarctic krill, this calcium phosphate mineral consolidates the cuticle, making it highly resistant to mechanical wear and predation, ensuring the krill can effectively avoid predators and maintain its protective structure (Peng, 2019). In contrast, the pistol shrimp relies primarily on calcium carbonate to harden its snapping claw. This mineralization process involves embedding calcium carbonate within the claw’s chitinous matrix, creating a reinforced composite structure. Concentrated at the claw’s tip, the calcium carbonate provides compressive strength, which helps the claw resist deformation under high pressure. The crystalline structure of calcium carbonate also disperses impact forces across the claw, reducing the likelihood of fractures and enabling repeated, forceful snaps without compromising structural integrity (Amini et al., 2018).

Molting and growth

Molting is a critical process for both krill and pistol shrimp, allowing these organisms to shed their old exoskeleton and form a new, larger one that accommodates growth. The molting cycle includes distinct phases: pre-ecdysis, ecdysis (shedding), and post-ecdysis, each involving unique physiological and chemical changes.

In the pre-ecdysis stage, specific enzymes, including chitinase and phenoloxidase, begin breaking down the inner layer of the old exoskeleton’s chitin-protein matrix. Chitinase hydrolyzes chitin chains, while phenoloxidase generates reactive quinones, which help to weaken protein bonds within the matrix. This enzymatic breakdown facilitates the detachment of the old exoskeleton, creating space for the new, flexible structure underneath to expand (Peng, 2022; Bentov, 2016).

At ecdysis, the krill or shrimp sheds its old exoskeleton, revealing the soft, newly formed cuticle underneath. This cuticle is initially fragile and requires rapid mineralization during the post-ecdysis phase. In krill, the new exoskeleton absorbs calcium and fluoride ions from hemolymph (internal fluids) and seawater, allowing these minerals to be incorporated as calcium carbonate and fluorapatite, which harden the cuticle. Fluorapatite provides additional rigidity, enhancing the exoskeleton’s ability to withstand environmental pressures and predation (Nicol, 1992; Peng, 2022).

Pistol shrimp also undergo molting, though less frequently than krill. During their post-molt phase, calcium carbonate is deposited across the exoskeleton, with significant focus on the snapping claw. This deposition provides the claw with the durability necessary for hunting, enabling it to perform high-impact snaps repeatedly without compromising structural integrity. The calcium carbonate concentration at the claw tip enhances its resilience under mechanical stress, ensuring that the pistol shrimp retains an effective predatory mechanism (Amini, 2018; Bentov, 2016). This multi-stage molting process not only allows for growth but also plays a vital role in maintaining the exoskeletal properties that are essential for krill and pistol shrimp survival.

Astaxanthin: oxidative stress defense

Astaxanthin (AX), also known as 3,3′-dihydroxy-β,β-carotene-4,4′-dione, is a lipid-soluble pigment that is part of the xanthophyll carotenoid group. Its chemical formula is C40H52O4, its molar mass is 596.84 g/mol and it consists of two oxygenated β-ionone-type ring systems linked by a chain of conjugated double bonds (polyene chain). The ionone rings each carry hydroxyl (-OH) and a carbonyl-C=O) groups. AX is primarily found in the exoskeleton and imparts the vibrant orange-red hue to shrimp and krill because of the 13 conjugated double bonds in its molecular structure. This structure lowers the energy gap between the ground state and the excited state, making it possible to absorb the lower-energy (longer wavelength) light of the blue-green region and reflect the higher-energy (shorter wavelength) light of the red-orange region. However, AX can form aggregates when bound to other proteins like crustacyanin that cause it to exhibit different optical properties, hence the appearance of colors such as brown or yellow. In shrimp specifically, three optical isomers of AX are found in the lipid bilayer of cells: (3R,3′R), meso, and (3S,3′S) (Figure 2) (Nishida, 2023).

Fig. 2. The three stereoisomers of astaxanthin found in shrimp (Nishida, 2023).

AX has a unique molecular structure with polar groups at both ends and a nonpolar middle section. This structure allows it to span the entire width of the cell membrane. The two ionone rings at the ends of the molecule bear polar groups which can interact with the hydrophilic heads of the phospholipids, while the non-polar polyene chain in the middle, containing methyl groups, interacts with the hydrophobic fatty acid tails. Crustaceans obtain AX directly by consuming algae and other marine organisms that produce and accumulate this pigment naturally or produce it from β-carotene. They can convert β-carotene into AX through a series of enzymatic reactions that is also often obtained from algae. The proposed pathway of β-carotene to AX found in krill and shrimp is demonstrated by the orange arrows in Figure 3. After obtaining or producing AX, Crustaceans then store it in their tissues, exoskeletons, hemolymph, and eggs (Šimat et al., 2022). This is important for their survival, as AX plays a vital role in protecting these organisms from oxidative stress, particularly from reactive species like singlet oxygen and free radicals generated by UV exposure.

Fig. 3. Conversion pathways of beta-carotene to astaxanthin. The orange arrows display the chemical pathway for most crustaceans, including shrimp and krill. The yellow and blue arrows indicate a less common, alternative pathway (Šimat et al., 2022).

AX has strong antiangiogenic (inhibition of the formation of new blood vessels), anti-inflammatory, and antioxidant activities. In fact, it is a stronger antioxidant compared to β-carotene and vitamin E. AX can gather and neutralize reactive chemical species, including reactive oxygen species (ROS), effectively deactivating these highly reactive molecules. This is useful for krill and shrimp, since it prevents cellular damage caused by metabolic processes and UV radiation exposure in polar waters.

Singlet oxygen

AX’s antioxidant properties are particularly important in combating harmful reactive species like singlet oxygen (¹O₂), which form when sunlight is absorbed and can pose a threat to the cellular integrity of these organisms. Singlet oxygen is an excited state of O₂ in which two unpaired electrons become paired, resulting in electrons with opposite spins (Figure 4). The formation of singlet oxygen can occur through the absorption of sunlight by photosensitizer molecules (such as chlorophyll) which then transfer this energy to ground state molecules to excite them (Ossola, 2021).

Fig. 4. Molecular orbital diagram of ground state triplet oxygen and excited singlet oxygen (Ormond, 2009).

The generation of singlet oxygen can be detrimental to biological tissues. Since the molecule has a high reactivity, it can react within an organism and modify DNA, proteins, and lipids. Nevertheless, carotenoids are amazingly effective at quenching singlet molecular oxygen, especially AX. Once the molecules make Van der Waals contact, the oxygen binds to the center of the carotenoids. They do this by forming a weakly bound complex with singlet oxygen through the donation of electron density from the highest occupied molecular orbital (HOMO) to the pi orbitals of the singlet oxygen (Figure 5).

Fig. 5. Process of singlet oxygen (red spheres) quenching by carotenoids (orange & white spheres). Orbitals are represented by the red and orange lines (Tamura, 2020).

This is called a Dexter-type super exchange mechanism, where the intermediate, in this case, involves a radical positively charged carotenoid and a radical negatively charged oxygen. This intermediate state is significantly higher in energy compared to the initial state (Figure 6), making it inherently unstable. Consequently, the system is driven to proceed to form new products. These products are triplet oxygen, the ground state of molecular oxygen, as well as triplet AX, meaning there are two unpaired electrons with parallel spins. The larger the length of the conjugated double bonds and the higher the HOMO level, the lower the energy gap between the initial and intermediate state, meaning a faster quenching of the singlet oxygen. While AX does not have the highest HOMO level, it efficiently quenches singlet oxygen because of the strong electronic coupling that occurs between both reactants (Tamura, 2020).

Fig. 6. Energy diagram of the reaction of singlet oxygen quenching. As seen, the intermediate state is significantly high in energy (Tamura, 2020).

Free radicals

AX is also important in mitigating the harmful effects of free radicals, including lipid peroxyl radicals, which can cause significant cellular damage in shrimp and krill. Free radicals are defined as molecular species with unpaired electrons, which can exist in different atomic orbitals. This makes these species highly unstable and reactive. For instance, singlet oxygen is not a free radical because it has all its electrons paired. In shrimp and krill, lipid peroxyl radicals could be damaging to the cells. They both have a high amount of polyunsaturated fatty acids (PUFAs) in their lipids, which are important for energy storage. However, when subjected to external stressors like fluctuations in temperature and light, these lipids can change to form free radicals which can cause cellular damage, particularly membrane damage (Ormond, 2009). While the exact mechanisms by which carotenoids scavenge free radicals are not fully understood, it’s possible that some may act as pro-oxidants (Krinsky, 2003), potentially generating even more reactive oxygen species and free radicals in the process. Nonetheless, AX has demonstrated very little pro-oxidant activity (Nishida et al., 2023).

In summary, AX is essential for the survival and general health of shrimp and krill as it provides substantial antioxidant effects. Hence, not surprisingly, shrimp fed AX for 4–8 weeks had a 91% survival rate, while shrimp fed no antioxidant had a survival rate of just 57% (Yamada et al., 1990). Interestingly, even microscopic organisms like cyanobacteria (Bai et al., 2023) and tardigrades (Jabbour et al., 2023) exhibit similar antioxidant mechanisms to prevent DNA damage from UV radiation.

Omega-3 phospholipids: extreme temperature survival

In the icy waters of the Southern Ocean, where temperatures plunge to near freezing, Antarctic krill showcase one of nature’s most remarkable chemical adaptations for surviving these extreme conditions. At the core of their adaptation is the intricate chemistry of their cell membranes, which rely on omega-3 PUFAs such as eicosatetraenoic acid (EPA) and docosahexaenoic acid (DHA) (Figure 7). This ingenious design allows krill to maintain membrane fluidity and ensure proper cellular function in a sub-zero environment.  

Fig. 7. Structural components of phospholipids found in cell membranes (Alberts, 2002). The phospholipids, with their glycerol backbone, polar head, and long-chain omega-3 polyunsaturated fatty acids (EPA and DHA), introduce kinks in the fatty acid chains due to double bonds, enhancing membrane fluidity. These kinks are essential for maintaining flexibility and ensuring proper cellular function in the cold.

Krill membranes contain omega-3 phospholipids (Burri, 2012), which are molecules perfectly suited for cold adaptations. The chemical components of a phospholipid include a glycerol backbone, shown in Figure 6, which forms the scaffold of the molecule, two fatty acid chains, often EPA (C₂₀H₃₀O₂) and DHA (C₂₂H₃₂O₂), both of which are long-chain omega-3 PUFAs, and a phosphate group with a polar head. The polar head of the phosphate is hydrophilic (water-attracting), allowing interaction with the aqueous environment outside the cell (Burri, 2012).

Membrane flexibility

What differentiates omega-3 fatty acids such as EPA and DHA from saturated fats is their degree of unsaturation. Unsaturation refers to the presence of double bonds between carbon atoms in the fatty acid chain. The double bonds create bends or curves (kinks), shown in Figure 8, in their H-C chain, which prevents the fatty acids from packing tightly together. This molecular disorder maintains membrane fluidity at low temperature, which is critical for maintaining the function of membrane-bound proteins and enzymes. 

Fig. 8. Chemical structure of EPA and DHA. EPA has five double bonds along its 20-carbon chain and DHA has six double bonds along its 22-carbon chain (Chan Lam, 2024).

The double bonds in EPA and DHA typically adopt a cis configuration, where the hydrogen atoms attached to the carbon atoms at the double bond are on the same side. The cis configuration creates a pronounced bend in the hydrocarbon chain. The spatial arrangement in cis-configuration disrupts Van der Waals forces, preventing a rigid, crystalline-like structure in the membrane. Additionally, the cis double bonds reduce intermolecular forces between adjacent phospholipid molecules, maintaining lateral mobility of the lipids within the membrane bilayer. The cis-kink structure that is found in the krill cell membrane is crucial because it prevents the fatty acids from aligning perfectly, therefore increasing fluidity of the membrane.  

The chain length and the number of double bonds present in DHA further enhance flexibility. DHA contains 22 carbons and six double bonds, and is more unsaturated than EPA, shown in Figure 11. This degree of unsaturation leads to more kinks in the chain, enhancing membrane fluidity at even lower temperatures. The flexibility introduced by omega-3 fatty acids can also affect the movement of cholesterol and proteins within the membrane (Falch, 2023). Cholesterol normally acts to modulate fluidity, but in frigid conditions, its effects are mitigated by the high concentration of unsaturated fats. 

The hydrophilic nature of the phosphate head allows it to interact with the water molecules surrounding the cell, stabilizing the membrane in two ways: 

The negative charge of the phosphate group can form electrostatic attractions with positively charged ions in the aqueous environment, further stabilizing the structure. 
The interaction with water creates a hydration layer that provides a physical buffer, protecting the membrane from mechanical stress caused by changes in external conditions (e.g., ice formation).

In freezing environments, membranes with high levels of saturated fats would effectively “freeze,” compromising cellular processes like nutrient transport, energy production, and signal transduction. Antarctic krill do not synthesize these essential omega-3 PUFAs on their own, but ingested through their diet of phytoplankton, microscopic marine plants that are rich in EPA and DHA. Phytoplankton, through photosynthesis, synthesizes omega-3 fatty acids, which krill consumes and incorporates into their own cellular membranes. This creates a biochemical cycle in which omega-3s, derived from primary producers, fuel the cold-resistant survival mechanisms of organisms higher up in the food chain. 

The survival of Antarctic krill in freezing water demonstrates the ingenious design solution of nature. The incorporation of omega-3 polyunsaturated fatty acids like EPA and DHA into their membranes is a finely tuned chemical strategy, enabling krill to maintain the flexibility and fluidity in their membranes under extreme cold. This structural adaptability ensures that vital cellular processes, driven by membrane-bound proteins and enzymes, continue without interruption, even in extreme conditions. The chemical adaptation displays how Antarctic krill can flourish in the Southern Ocean through complex and elegant chemical solution.

Chemical camouflage: bioluminescence in krill

Bioluminescence is the production of light from living organisms. One third of species in the Arthropoda phylum, as well as most marine dwellers, are bioluminescent, including jellyfish, krill, copepods, Radiolaria, and dinoflagellate. The purposes of bioluminescence vary, ranging from attracting mates or prey to evading predators through camouflage and blinding tactics (Tsarkova, 2021). In this paper, we will be specifically discussing bioluminescence as a design solution for krill’s need to camouflage itself from predators.

This form of bioluminescence, used by some marine animals to blend into their environment and avoid detection by predators, is known as cryptic bioluminescence. It is often achieved through a method called counter-illumination, where the animal produces light from its underside to match the ambient light from above, effectively erasing its silhouette when viewed from below (Warner et al., 1979).

Photophores

The production of this light takes place in organs called photophores. In the transparent euphausiid shrimp, there are 10 photophores widely spaced along the ventral surface of the body as seen in Figure 9. There is one on each eyestalk, two pairs placed laterally on the thorax, and four arranged along the mid-line of the abdomen under the cephalothorax and the gut, the least transparent structures (Grinnell, 1988).

Fig. 9. Photographs of M. norvegica (Fregin, 2002). In the left photo, the photophores can be seen in relation to the eyes, thorax, and abdomen. In the right photo, the photophores can be seen brightly luminescing in the dark.

The general structure of krill photophores is similar between species. Light-producing cells and refractive rods are positioned in the center of the light organ, forming a striated structure called the lantern. The lantern functions as a sinus and about 30 capillaries drain oxygenated hemolymph into it. A filamentous cuff wraps around the opening of the capillaries and functions as a sphincter (Kronstrom et al., 2009). Sphincters are circular muscles that open and close passages in the body to regulate the flow of substances, thus the filamentous cuff expands and contracts to control how much oxygenated hemolymph drains into the lantern. Electron microscopy shows that the filamentous material consists of thick and thin muscle filaments arranged in perpendicular blocks around the opening of each capillary (Figure 10).

Fig. 10. a. Cross-section of photophore from M. norvegica. (Kronstrom, 2009) The lantern (la) is composed of processes from light-producing cells (lc). Light is reflected against the inside of the organ wall (r reflector) and passes through a lens (le) before leaving the photophore. b. Representation of a capillary emptying into the lantern (la). The sphincter cells (sc) that act as cuffs lay at the ends of the capillaries and contains filamentous material (hatched area).

Luciferin and luciferase enzymatic reaction

Inside the light-producing cells of the lantern, a chemical reaction occurs to produce bioluminescence. The enzyme luciferase catalyzes the oxidation of the substrate luciferin in the presence of oxygen, adenosine triphosphate (ATP) (Xia, 2021), and either magnesium or calcium ions. In the oxidation process, luciferin loses some of its electrons to oxygen as it is a strong oxidative agent that is very electronegative. Within the luciferin molecule, electrons are forced to rearrange at higher energy levels, but not all of them are lost to oxygen. Instead, some of the electrons jump to higher orbitals (energy states) but then return to their lower initial energy state. The difference in energy between the two energy states is released as light. Thus, the luciferin-luciferase reaction releases the molecule oxyluciferin in an excited state in addition to other products like carbon dioxide and pyrophosphate. Light in the wavelength region of 530 to 640 nm is emitted when the excited luciferin relaxes to its normal state. It is important to note that the luciferin utilized by krill is structurally similar to the luciferin found in dinoflagellates (Tsarkova, 2021), one of the many types of phytoplankton that krill feed on. This suggests a dietary dependence on dinoflagellates; krill obtain the luciferin and luciferase they need for bioluminescence from the dinoflagellates that they eat.

Counter illumination

Animals that live in the mesopelagic region, the region of the ocean that extends from 200 meters to 1000 meters below the surface, like krill, spend the daylight hours at depths where the light-level is extremely low, $10^{−4} \frac{uW}{cm^2}$ and less. However, even under those low-level light conditions they are not completely hidden by the darkness of their environment. Deep-sea fish have functional vision at light-levels as low as $10^{−9} \frac{uW}{cm^2}$ (Clarke, 1963), which is far below the levels of light found in the environment of the deep scattering layers. Other predators associated with these low-level light layers likely have functional vision too. Under the down coming light, krill, when seen from below, would appear as a dark spot to predators. This detection from below would be the optimal situation for predators as they could catch the krill off guard and eat a large amount of them at once.

However, krill adapted and developed the ability to minimize the silhouette effect by matching the light produced by photophores on their ventral surface to the down-coming solar light as shown in Figure 11 (Warner, 1979). This process, known as counter-illumination, was proven by an investigation into the spectral composition of the luminescence in two species of krill (Clarke, 1963). The investigation concluded primary peaks of krill luminesce in the 470 to 480 nm range, which is very close to the 478 nm peak of blue light in the mesopelagic region, demonstrating that the purpose of the photophores is to produce light that is the same colour as the down-coming solar light.

Fig. 11. Demonstration of photophores eliminating body silhouette (Fregin, 2002)

The mechanism that controls bioluminescence in krill is not completely understood, though some believe that regulating the supply of hemolymph to the light-producing cells may be part of it. Relaxation of filamentous cuffs that encircle the base of the capillaries leads to an increased supply of the hemolymph, and thus oxygen, to the lantern, which may be sufficient to stimulate light production. Serotonin or 5-hydroxytryptamine (5-HT) is suspected to be the substance that relaxes or triggers another substance to relax the cuff structures. 5-HT is a monoamine that functions both as a neurotransmitter and a hormone and is synthesized from L-tryptophan by the enzymes tryptophan hydroxylase and aromatic L-amino acid decarboxylase. For krill, serotonin functions more as a neurotransmitter as it is likely carrying a chemical signal to the filamentous cuff that initiates its relaxation. Just as light exposure triggers release of serotonin in humans, the increase in light exposure that krill experience in the daytime may also trigger the release of serotonin and allow for continuous bioluminescence to be produced. As a result, krill are able to camouflage themselves from predators during the day through counter-illumination.

Conclusion

In conclusion, there are many chemical characteristics of shrimp and krill that allow them to overcome various challenges and thrive in their respective environments. We first discussed how crustacea’s need for a flexible, yet tough exterior, is solved by the chitinous exoskeleton, which minimizes dehydration while providing vital structural support. Next, we demonstrated how the oxidative stress caused by UV rays is addressed by the brilliant antioxidant astaxanthin, a molecule that protects shrimp and krill through the quenching of singlet oxygen and the mitigation of free radicals. The freezing conditions of the Antarctic also present a challenge to krill; however, PUFAs cleverly allow krill to maintain cell membrane integrity, growth, and reproduction. Lastly, the essential need of krill to evade predators during the day was addressed by the ability to produce bioluminescence and counter illuminate their silhouette. Collectively, these chemistry-based design solutions enable shrimp and krill to succeed in diverse marine environments.

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