Molluscs: Respiration | Zoology Optional Notes for UPSC PDF Download

Aquatic Respiration

Primary Respiratory Structures:
The primary respiratory structures are the ctenidia, also known as gills. These gills are located within the mantle cavity and are attached to the mollusk's body by membranes.

Structure of Ctenidia (Gills)

1. Ctenidia Arrangement: Ctenidia are paired, symmetrical structures, featuring two rows of flattened gill filaments. These filaments are arranged on either side of a long, flattened axis, which is penetrated by afferent and efferent vessels, allowing the flow of hemolymph (the mollusk's circulatory fluid).

2. Gill Filament Spacing: Narrow spaces are maintained between the gill filaments to allow free water flow while still keeping them close enough for adjacent filaments' cilia to work together in generating a water current.

3. Hemolymph Circulation: Hemolymph circulates through the gill filaments, facilitating gas exchange.

4. Supportive Structure: Skeletal rods provide structural support to the gill filaments.

5. Water Flow: Cilia on the gill filaments generate an inhalant water current, drawing water below the ctenidia. An exhalant current expels water above the ctenidia, completing the circulation.

Types of Ctenidia Based on Topography

• Holobranchiate: The ctenidia extend throughout the body, with the number of gills varying from 14 to 80 pairs. (Example: Polyplacophora)

• Merobranchiate: Ctenidia are limited to a particular area of the body. Merobranchiate gills are further subdivided based on the arrangement of leaflets:

  • Plicate: Simple, flat, transversely folded gills.
  • Monopectinate: Flattened gill filaments arranged in a single row.
  • Bipectinate: Flattened gill filaments arranged in two rows, which can be either equal or unequal.
  • Feathered: Gills assume a feather-like shape. (Example: Cephalopods)

Ctenidia in Different Mollusk Groups

• Monoplacophora: These mollusks have five to six pairs of unipectinate gills located in the pallial groove.
• Polyplacophora (Chitons): The number of ctenidia varies from 14 to 80 pairs, and the gill rows may be holobranchiate, except for two exceptions bearing the merobranchial type.
• Aplacophora: In this group, gills are reduced to a paired, feather-shaped structure located near the cloacal cavity.
• Gastropoda: The arrangement and number of gills vary widely within this group, with different subclasses exhibiting different patterns.
• Bivalvia (Clams and Mussels): Bivalves have bipectinate gills that are usually large and serve not only for gas exchange but also for food collection.
• Cephalopoda (Squid and Octopus): Cephalopods have large, paired, bipectinate gills, with water currents being generated from the afferent to the efferent side. The gill filaments may be non-ciliated and can be structured in primary and secondary folds to increase the respiratory surface.

The efficiency of aquatic respiration in mollusks is a testament to their adaptation to life in water, enabling them to extract oxygen from their aquatic surroundings.

Evolution of Ctenidia

1. Primitive Ctenidia: The most primitive ctenidia are found in zeugobranchiate prosobranchs. In these species, ctenidia are simple outgrowths of the body.

2. Bivalves: In bivalves, ctenidia have evolved to become more elaborate. They have grown in length and developed new tissue connections to enhance their functions. Three types of tissue connections have evolved in bivalves:

   • Interfilamentar Junction: Connections formed between adjacent filaments of the gills.
   • Inter-lamellar Junction: Connections between the lamellae of the gills.
   • Connections with Tips: The tips of the gill filaments are connected to the mantle or foot.

3. Food Collecting Organ: In bivalves, the branchial apparatus not only serves as a respiratory organ but also functions as a food collecting organ. Mucus secreted by the hypobranchial gland traps and strains out large, non-food particles. The osphradium also plays a role in assessing water quality.

Adaptive or Secondary Respiratory Structures

1. Anal Gills: Some mollusks, like Doris, have delicate leaflets forming a rosette around the anus. In Chaetoderma, symmetrical lateral gills are present on each side of the cloaca.

2. Cerata or Dorsal Appendages: In many opisthobranchs, dorsal appendages called cerata are highly vascularized and play a role in respiration. Cerata can be simple and club-shaped (as in Aeolis), dendritic (Dendronotus), or multi-lobed, resembling a bunch of grapes (Dotochica).

3. Pleural Gills: Some mollusks, like Pleurophyllida, have lateral rows of branchial leaflets located beneath the mantle.

4. Pallial Gills: Certain basommatophore pulmonates develop secondary external gills by enlarging the pallial lobe, just outside the pneumostome. However, these gills lack cilia. Examples include Planorbidae and Ancylidae.

5. Integument: In some Scaphopoda, specialized respiratory structures are absent, and respiration occurs through the internal surface of the mantle, particularly the anteroventral side. In nudibranchs (Gastropoda), the entire dorsum of the body acts as the site of gas exchange. Integumentary gas exchange also occurs in some parasitic species.

Terrestrial Respiration

1. Nuchal Lobe: Terrestrial prosobranchs like Monotocardia have a well-developed left nuchal lobe, which forms a long respiratory siphon.
2. Pulmonary Sac: Some amphibious prosobranchs, such as Pila, Ampullarius, and Siphonaria, have a pulmonary sac in the left pallial cavity. This sac functions as an aerial respiratory structure, allowing respiration during aestivation and when the oxygen concentration in the water is low.
3. Lung: In pulmonate mollusks (both stylommatophores and basommatophores), the respiratory structure is a true lung of independent origin. This lung occupies a significant portion of the roof of the pallial cavity. In some species, the lung extends to the walls and floor. The pallial cavity opens to the exterior through a large aperture called the pneumostome, which can be opened or closed by muscular contractions.

Respiratory Pigment

These diverse adaptations for respiration reflect the wide range of habitats and lifestyles among mollusks.

The mechanism of respiration and oxygen uptake in mollusks varies depending on whether the mollusk is adapted to aquatic or terrestrial environments. Here's an overview of how these mechanisms work:

Aquatic Respiration: In aquatic mollusks, such as many marine and freshwater species, respiration relies on the exchange of gases (mainly oxygen) dissolved in water. The mechanisms involved in aquatic respiration include:

1. Ciliary Beating or Muscular Pumping: Water containing dissolved oxygen is driven over the ctenidial (gill) surfaces by either ciliary beating or muscular pumping. Cilia on the gill filaments create water currents for gas exchange.
2. Branchial Hearts (Cephalopods): Cephalopods, like squids and octopuses, have specialized branchial hearts located at the bases of the gills. These hearts pulsate to drive haemolymph (mollusk blood) through afferent vessels, enhancing the oxygen exchange process.
3. Efficiency Compensations (Nautilus): Some cephalopods, like Nautilus, lack additional pumps like branchial hearts. To compensate for this, Nautilus has evolved two pairs of ctenidia (gills) to enhance gas exchange efficiency.
4. High Oxygen Utilization: Mollusks like gastropods and cephalopods exhibit high oxygen utilization from pallial (mantle) water. The efficiency of oxygen utilization varies among species, with some, such as Haliotis, Triton, and Octopus, having significantly higher oxygen uptake compared to sedentary lamellibranchs (bivalves), which typically have lower rates of oxygen uptake.

Terrestrial Respiration: In terrestrial mollusks, such as land snails, the mechanism for respiration and oxygen uptake is adapted to air rather than water. Here's how it works:

1. Muscular Movements: Contraction and relaxation of specific muscles within the pulmonate mollusks (terrestrial snails) cause the inflow and outflow of air into and from the pallial cavity (a space within the mantle).
2. Diaphragm Action: In a relaxed state, the diaphragm (the floor of the pallial cavity) arches upward, reducing the space within the cavity.
3. Inspiration: During inspiration, the pneumostome (an external opening) opens. Muscular contraction flattens the floor of the pallial cavity, creating a pressure drop inside the cavity, which allows air to rush in. After this, the pneumostome is closed.
4. Oxygen Uptake: There is a slight delay in the opening of the pneumostome, which facilitates the uptake of oxygen by the haemolymph. This delay ensures that oxygen is absorbed efficiently.
5. Respiratory Movements: The rate of respiratory movements increases with lower oxygen concentrations and higher temperatures. Some terrestrial snails, such as Arion, exhibit increased respiratory movements when environmental conditions necessitate higher oxygen uptake.
6. Prosobranch Respiration: In prosobranchs, such as some aquatic snails, the mechanism involves the alternate dilatation and contraction of the mantle and the wall of the pulmonary chamber. This helps facilitate the inflow and outflow of air through the left siphon, which can enlarge above the water level when the snail is submerged.

Overall, mollusks have evolved a variety of respiratory mechanisms to suit their specific environmental conditions, whether in aquatic or terrestrial habitats. The efficiency of these mechanisms varies among different mollusk groups, reflecting their diverse evolutionary adaptations to various ecological niches.