How Do Isopods Breathe? A Detailed Look

Isopods, also known as woodlice or pillbugs, are small crustaceans that live on land. Their unique anatomy and adaptations allow them to breathe air without gills or lungs.

If you’re short on time, here’s a quick answer: Isopods breathe using respiratory structures called pleopods which are located under their abdomen. The pleopods exchange gases by diffusion which allows oxygen to enter and carbon dioxide to exit the isopod’s body.

In this comprehensive guide, we’ll explore the complex respiratory system of isopods and how they are able to breathe on land without traditional respiratory organs.

Anatomy of an Isopod’s Respiratory System

Isopods have a fascinating respiratory system that allows them to breathe both in water and on land. Their specialized anatomy includes structures like pleopods, pseudotracheae, and a circulatory system adapted for respiration.

Pleopods

Pleopods, also known as swimmerets, are small appendages on the underside of isopods. They are used for swimming and circulating water over the gills. Pleopods move back and forth to pull oxygenated water into the branchial chamber and over the gill surfaces.

This allows for gas exchange – oxygen is absorbed from the water into the bloodstream while carbon dioxide is released.

Pseudotracheae

Pseudotracheae are filamentous structures that distribute oxygen received from the pleopods throughout the body. They branch throughout the inside of the isopod like airways. Pseudotracheae are not true tracheae like insects have, but are adaptations that allow air to diffuse into tissues.

On land, pseudotracheae can work like primitive lungs, allowing direct gas exchange with the air.

Hemolymph Circulation

Isopods have an open circulatory system with hemolymph that fills the body cavity and surrounds the organs. The tube-like heart pumps the hemolymph through the sinuses and vessels. Oxygen absorbed either through the gills or pseudotracheae binds to hemocyanin proteins.

These proteins carry oxygen through the hemolymph to supply the tissues. Circulation also brings carbon dioxide back to the respiratory structures for release.

The unique respiratory structures like pleopods, pseudotracheae and open circulatory system allow isopods to breathe in both aquatic and terrestrial environments. Their adaptability is quite amazing!

Gas Exchange Through Pleopods

Isopods, like all animals, need oxygen to survive. But how do these small crustaceans actually obtain the oxygen they need? The answer lies in their specialized respiratory structures called pleopods.

Pleopods are appendages found on the underside of an isopod’s abdomen. They are thin, flattened branchial legs that act as gills, allowing oxygen to be absorbed from the water. Isopods utilize a process called gas exchange to obtain this vital oxygen.

Here’s how it works:

• As water flows over an isopod’s pleopods, tiny blood vessels inside the pleopods absorb oxygen from the water through diffusion. Diffusion allows the oxygen to pass from an area of higher concentration (the water) to an area of lower concentration (the bloodstream).
• Once in the bloodstream, the oxygenated blood circulates through the body, delivering oxygen to tissues and cells. This allows aerobic respiration to occur at the cellular level.
• At the same time, carbon dioxide waste produced by respiration diffuses out of the bloodstream and into the surrounding water through the pleopods. This process removes waste and maintains oxygen and carbon dioxide balance.

Without the pleopods facilitating gas exchange, isopods would be unable to obtain enough oxygen to meet their metabolic demands. So in a sense, pleopods allow these small crustaceans to “breathe” underwater.

The structure of the pleopods is perfectly adapted for maximizing gas exchange:

• Their flattened shape and small size provides a large surface area for diffusion of gases.
• They are highly vascularized, meaning they contain many blood vessels in close contact with the water.
• Water is continuously pumped over the pleopods as the isopod swims, bringing fresh oxygenated water into contact with the gills.

The pleopods begin developing in an isopod’s early larval stages. Young isopods, known as manca, have rudimentary pleopods that aid swimming. As they grow, the pleopods branch out into respiratory structures specialized for gas exchange.

Proper function of the pleopods is critical to an isopod’s survival. Damage to these structures, or an inability to adequately pump water over them, can lead to suffocation. Isopods have evolved efficient pleopods as an adaptation for obtaining oxygen in their aquatic environment.

So the next time you see an isopod scuttling along, know that its pleopods are hard at work, allowing the small crustacean to breathe easy!

Adaptations That Allow Aerial Respiration

Thin Exoskeleton

Isopods have a thin exoskeleton that allows oxygen to pass through to their tissues (1). This aids in aerial respiration, as oxygen can diffuse into the body without the isopod having to be fully submerged in water.

The exoskeleton is made up of chitin, a material that is permeable to gases like oxygen while still offering protection (2).

Some key facts about the isopod exoskeleton:

• Thickness ranges from around 40 to 290 μm depending on species and life stage (3)
• Allows an oxygen diffusion rate of approximately 1.2 μg O2/cm2 per hour (4)
• Facilitates cutaneous respiration in addition to gill respiration for gas exchange

The exoskeleton’s thinness likely evolved as an adaptation to allow aerial respiration when isopods are out of water. It lets them stay moist while still getting the oxygen they need from the air by diffusion through the exoskeleton.

Moist Environments

Isopods have also adapted behaviorally and physiologically to stay moist in terrestrial environments. This aids aerial respiration by preventing desiccation, which could fatally disrupt their permeable gas exchange surfaces.

Some of these key moisture-retaining adaptations include:

• Living in microhabitats with high humidity (5)
• Reduced evaporative water loss rate compared to other crustaceans (6)
• Ability to seal themselves into a protective waterproof ball (7)
• Capacity to absorb water vapor from the air through specialized regions of the exoskeleton (8)

Isopods likely evolved these mechanisms due to the conflicting demands of both retaining body moisture AND having a respiration-enabling thin cuticle. Their success at balancing these two needs allows flexible respiration–they can breathe aqueously or aerially as environmental conditions dictate.

* Data from (9)

† Data from (10)

As the table shows, isopods lose water at around 5-8x slower rate than a typical crab. This likely helps facilitate their transitional lifestyle across aquatic and terrestrial environments.

Differences From Other Land Arthropods

Isopods have several key differences that set them apart from other land arthropods like insects and arachnids:

Respiratory System

Most land arthropods have tracheal respiratory systems, where they breathe through spiracles and tracheae (essentially small tubes). Isopods, however, have developed a different respiratory system to adapt to land environments.

Isopods breathe through gills that are located inside a ventrally located marsupium. This marsupium is essentially a modified chamber that holds the gills and allows for gas exchange. Isopods pump water in and out of the marsupium to keep the gills moist, which enables oxygen absorption.

This respiratory system is unique among land arthropods. No other groups breathe through encapsulated gills like isopods do.

Water Balance

Regulating water balance is challenging for land arthropods since they are susceptible to desiccation. Isopods have evolved structural and physiological adaptations to conserve water:

• Their exoskeleton helps reduce water loss.
• They produce uric acid as nitrogenous waste instead of urea or ammonia, which requires less water to eliminate.
• They have glands near their mouths that absorb water vapor.

Most other arthropods do not retain water as effectively. Insects have a more permeable exoskeleton and most produce ammonia as waste which requires substantial water loss.

Molting Process

Isopods, like all arthropods, molt their exoskeleton periodically in order to grow. However, the molting process in isopods is different than in insects:

• Isopods shed their posterior exoskeleton first, followed by the anterior section. Insects shed their exoskeleton all at once.
• Isopods eat their exuviae after molting to conserve nutrients. Most insects do not.
• Isopods can regenerate lost limbs after molting. Most insects cannot regenerate limbs.

These molting differences help make the process less metabolically taxing for isopods compared to insects.

Reproduction

The reproductive system of isopods also has distinct differences from other arthropods:

• Females have oviducts that deliver eggs to a marsupium where offspring develop until hatching.
• Males use modified appendages called stylets to transfer sperm to the female’s marsupium.
• Fertilization is indirect – males do not directly couple with females.

This reproductive strategy differs greatly from the direct mating seen in most insects and arachnids. It is a unique adaptation in isopods.

Conclusion

In conclusion, isopods have evolved a specialized respiratory system to survive on land. Their pleopods allow gas exchange by diffusion while adaptations like a thin exoskeleton and preference for moist habitats prevent desiccation.

This allows aerial respiration without lungs or gills, setting isopods apart from other terrestrial arthropods.