Table of contents | |
Aquatic Respiration | |
Evolution of Ctenidia | |
Adaptive or Secondary Respiratory Structures | |
Terrestrial 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.
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).
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.
Hemolymph Circulation: Hemolymph circulates through the gill filaments, facilitating gas exchange.
Supportive Structure: Skeletal rods provide structural support to the gill filaments.
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.
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:
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.
Primitive Ctenidia: The most primitive ctenidia are found in zeugobranchiate prosobranchs. In these species, ctenidia are simple outgrowths of the body.
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:
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.
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.
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).
Pleural Gills: Some mollusks, like Pleurophyllida, have lateral rows of branchial leaflets located beneath the mantle.
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.
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.
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:
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:
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.
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