Torsion and Detorsion in Gastropoda

What is Torsion?

• Torsion, in biological terms, refers to the rotation of the visceral organs in the larval stage of gastropods, occurring in an anticlockwise direction through a 180° angle. This twisting process is a pivotal developmental event that transforms a symmetrical larva into an asymmetrical adult form. It is particularly prominent in the veliger stage, which is the free-swimming larval phase of gastropods.
• The process of torsion is primarily driven by the contraction of specialized larval retractor muscles and differential growth patterns. This movement takes place rapidly, often within a few minutes. As a result of torsion, asymmetry begins to manifest early in the veliger stage, as the mesodermal bands, which contribute to muscle and tissue formation, develop unevenly. More specifically, the mesodermal band on the right side of the larva becomes larger than the left.
• The right mesodermal band consists of five mesodermal cells that elongate to form muscle tissue. As these cells differentiate into muscle fibers, they play a crucial role in displacing the visceral hump, the part of the body containing the organs, to the left side of the larva. This shifting of the visceral hump is a key feature of the torsion process.
• Simultaneously, the muscle cells on the right side of the larva begin to converge, transforming into the larval retractor muscles. These muscles are responsible for generating the force necessary to initiate and sustain torsion. Interestingly, no muscle cells are present on the left side, which is critical to the torsion process. As soon as the muscle cells on the right side gain the ability to contract, torsion begins, resulting in the characteristic twisting of the visceral organs.
• Torsion is a crucial stage in the development of gastropods because it leads to the transformation of the larval body into a more complex and asymmetrical adult form. This process ensures the proper alignment of the internal organs and sets the stage for further development in the transition from larva to adult. Thus, torsion is not merely a physical twisting of the body but a sophisticated mechanism that plays a central role in the overall developmental biology of gastropods.

Conditions before Torsion

The following points outline the key features and conditions that are present in the veliger larva prior to the onset of torsion:

• Mantle Cavity Location: The mantle cavity, which contains the pallial complex, is located at the posterior side of the larva. This cavity is crucial for housing important structures such as the gills (ctenidia) and nephridiopores.
• Ctenidia and Nephridiopores: Located posteriorly, the ctenidia (gill structures responsible for respiration) and two nephridiopores (openings for the excretory system) are positioned near the posterior end of the larva. These structures are essential for the larva’s respiratory and excretory functions, maintaining internal homeostasis.
• Straight Alimentary Canal: The alimentary canal in the veliger larva is straight, with the mouth positioned at the anterior side and the anus located at the posterior side. This simple, straight arrangement of the digestive system is typical before torsion and reflects the symmetrical nature of the early larval stage.
• Auricles Behind the Ventricle: The auricles, which are part of the heart, are located posteriorly, behind the ventricle. This positioning reflects the basic organization of the circulatory system in the early larva.
• Bilateral Symmetry in the Nervous System: The nervous system of the veliger larva exhibits bilateral symmetry, with nerve ganglia and other nervous components positioned symmetrically on either side of the body. This symmetry is characteristic of the larval stage before torsion alters the body’s structure.
• Embryonic Bilateral Symmetry: Initially, the embryo of the gastropod larva is bilaterally symmetrical. During the veliger stage, the larva possesses a foot and a planospiral shell, both of which contribute to the early development of the organism’s structure. However, the shell is not coiled during this stage and remains in its pre-torsion form.

How Torsion Occurs

Torsion in gastropods is a complex, stepwise process that leads to the twisting of visceral organs in a counterclockwise direction, resulting in a significant anatomical transformation. This process occurs as a morphological phenomenon characterized by the rotation of various body structures. The following points explain how torsion unfolds in gastropod larvae:

• Bending in the Antero-Posterior Sagittal Plane: The process of torsion begins with the bending of the animal’s body along the ventral side in an antero-posterior sagittal plane. This bending occurs about a transverse axis, which facilitates the rotation of specific body parts.
• Displacement of the Mantle Cavity: Initially, the mantle cavity shifts towards the right side of the body. This movement continues as the mantle cavity progresses toward the anterior end. During this shift, the head and foot remain relatively fixed in position, which ensures stability while the torsion process takes place. This step is essential in initiating the rotation of other body components.
• Looping of the Digestive Tract: As the mantle cavity moves, the digestive tract undergoes a looping motion. This causes the mouth and anus to become closer together, significantly altering the alignment of the alimentary system. The looping of the digestive system is a key component of the torsion process.
• Transformation of the Visceral Mass and Shell: The visceral mass, originally saucer-shaped, begins to transform into a cone shape. Simultaneously, the shell, which was also flatter in earlier stages, becomes spirally coiled. This change in shape marks a crucial phase in the development of the gastropod.
• Simultaneous Coiling: The visceral mass and shell coil together in a coordinated manner, resulting in what is known as an exogastric coil. This coiling is essential for the further morphogenesis of the gastropod, helping establish the asymmetry that is characteristic of the adult form.
• Ventral Rotation of the Visceral Mass and Mantle: The ventral portion of the visceral mass and the mantle rotate through an angle of approximately 180°, or sometimes slightly more. This rotation is a critical component of torsion, as it repositions the visceral organs within the developing organism.
• Twisting of the Dorsal Mass: The dorsal mass of the larva undergoes twisting such that organs on the right side, such as the right gill and right auricle, remain intact and are positioned toward the posterior. Meanwhile, the corresponding structures on the left side are often lost or repositioned, leading to further asymmetry in the larva’s anatomy.
• Completion of Metamorphosis and Lateral Torsion: As the metamorphosis progresses, a lateral torsion occurs. This is a secondary rotation that follows the primitive ventral plexus. The original coil of the visceral sac and the shell, which was once positioned dorsally (exogastric), eventually becomes ventral (endogastric). This shift completes the torsion process and results in the final anatomical configuration of the gastropod.

Cause and Significance of Lateral Torsion

Lateral torsion is a critical aspect of gastropod development, arising from differential growth between the left and right sides of the organism. This twisting of the body is essential for the organism’s survival and functional adaptation. The following points explain the causes and significance of lateral torsion:

• Cause of Lateral Torsion: Lateral torsion results from the arrested growth on one side of the body, typically the right side, coupled with the active expansion of the other side, often the left. This asymmetrical growth pattern leads to the rotation of the body structures, with the mantle cavity and pallial complex moving toward the right and anterior sides. This shift is primarily driven by the more robust growth of the visceral mass on the left side of the larva. The unbalanced growth between the sides causes the twisting that characterizes torsion.
• Significance of Lateral Torsion: Lateral torsion is not merely a random developmental event; it plays a vital role in the protection and organization of the gastropod’s body. One of its main functions is to provide compactness to the organism, which is crucial for efficient space utilization within the body. The reorganization of internal organs, particularly the mantle cavity and pallial complex, enables a more efficient arrangement for growth and function.Moreover, torsion is important for the continuous growth of the gastropod. The ability to compact organs and structure them more efficiently ensures that the organism can continue its development without spatial constraints. This compact organization is essential for the animal’s long-term adaptation and survival.
• Adaptive Value: The lateral torsion is a response to the needs of the organism’s life and survival. It reflects an adaptive strategy that optimizes internal organization, enhances protection, and facilitates ongoing growth. These changes are critical for the gastropod’s survival in its environment, contributing to the species’ overall adaptability and success.

Effect of Torsion and Shuttling of Pallial Complex

Torsion in gastropods is a transformative process that results in significant changes to the organization of internal organs and their relative positions. The torsion leads to the displacement of several anatomical structures, affecting both the exterior and interior arrangements of the organism. Below is a detailed explanation of the effects of torsion, particularly focusing on the shifting of the pallial complex and related organ systems:

• Displacement of Mantle Cavity: Initially, the mantle cavity is located at the posterior end of the body. However, following torsion, the mantle cavity shifts to a position just behind the head. This change results in the associated parts of the pallial complex being relocated forward, altering the overall body plan of the gastropod.
• Changes in Relative Position of Organs: Prior to torsion, the anus and ctenidia are positioned towards the posterior end of the body, and the auricles are located behind the ventricle. After torsion, the anus and ctenidia move forward, and the auricles shift to a position in front of the ventricle. This rearrangement is crucial for the reorganization of internal systems that support the gastropod’s life functions.
• Twisting of Alimentary Canal: The alimentary canal, initially straight, undergoes twisting, forming a loop. This twisting brings the mouth and anus closer together, facilitating a more compact digestive system. The looping of the digestive tract also contributes to the rearrangement of other body systems, furthering the effects of torsion.
• Origin of Chiastoneury: One of the key neurological consequences of torsion is the crossing of the pleuro-visceral connectives, forming a structure known as chiastoneury. This results from the pallial complex shifting from a posterior to an anterior position. The right connective passes over the intestine (supraintestinal), while the left connective passes beneath it (infraintestinal). The crossing of these nerve connectives is essential for the reorganization of the nervous system.
• Endogastric Coil Formation: Prior to torsion, the visceral sac was arranged dorsally or exogastric. However, after torsion, this coil becomes ventral or endogastric. It is important to note that the coiling of the shell is a separate evolutionary event, not directly associated with torsion, and the shell retains its symmetrical spiral configuration.
• Loss of Symmetry: The torsion process leads to the loss of symmetry in the gastropod’s body. The anus, which was originally located at the posterior end, is displaced towards the right side of the mantle cavity. This, combined with the reduction or loss of paired structures on the left (or topographically right) side, results in the asymmetry characteristic of adult gastropods.
• Stages of Torsion: Torsion in gastropods occurs in two stages:
  • Stage I: The initial 90° rotation of the visceral hump is driven by the contraction of the larval retractor muscles. This stage typically takes only a few hours. By the end of Stage I, the mantle cavity, which was originally ventral and posterior, is now positioned on the right side of the body, while the foot projects on the left side.
  • Stage II: The remaining 90° of torsion is primarily driven by differential growth, which takes place over a longer period. The exact mechanism of torsion is not fully understood, but this stage is characterized by slow, ongoing developmental changes in the body structure.
• Different Mechanisms of Torsion: According to Thomson (1958), there are several ways torsion can occur in gastropods:
  • Muscular Contraction Alone: In some species, such as Acmaea, a 180° rotation of the visceral hump occurs through muscular contraction alone. This mechanism is considered the original form of torsion.
  • Two-Stage Torsion: In common gastropods like Haliotis and Patella, torsion occurs in two phases. The first 90° rotation is achieved by the contraction of larval retractor muscles, while the remaining 90° results from differential growth. The initial phase occurs more rapidly, followed by slower growth.
  • Growth-Only Torsion: In some gastropods, such as Vivipara, the full 180° rotation is achieved through growth processes alone, without muscular involvement.
  • Torsion with Halting: In species like Aplysia, torsion results from differential growth, but the repositioning of the anus is halted at a stage appropriate for the adult form.
  • Unrecognizable Torsion: In species like Adalaria, torsion of the viscero-pallium is not clearly recognizable. The organs appear to be in the post-torsion position from the beginning.
• Effects on Organ Systems: After the completion of torsion, several organ systems are significantly affected. The most notable changes occur in the pallial organs and nervous system. The formation of loops in the alimentary canal and the crossing of pleuroparietal connectives are commonly observed, especially in groups like the protobranchia. The shifting of the pallial complex and related organs profoundly influences the function and organization of the gastropod’s internal systems.

Views on the Significance of Torsion in Gastropods

Torsion is a defining characteristic of gastropods that significantly affects their anatomy and behavior. However, the precise significance of this process remains a topic of debate, and several theories have been proposed to explain the evolutionary advantages of torsion. Below are the key perspectives on the significance of torsion in gastropods:

• Garstang’s View (1928):
  • Torsion as Adaptive for Larvae: Garstang suggested that torsion is an adaptive feature that primarily benefits the larval stage of gastropods, particularly for protection. Before torsion, the larval form (veliger) swam in the sea with the mantle cavity located posteriorly. In this configuration, the larva was vulnerable to predators, as there was no protective space into which the delicate head and velum could be withdrawn when threatened.
  • Protection after Torsion: Following torsion, the mantle cavity moves to the anterior end of the larva, effectively providing a space where the head and velum can retract during danger. In this way, the larva could withdraw into its mantle cavity, stop ciliary movement, and fall to the sea bottom, evading predators. This view is widely supported by several researchers, including Yonge (1947), Barnes (1980), and others.
  • Objections: Despite widespread support, the theory has faced criticism:
    1. Some pelagic larvae, such as lamellidens, are not twisted yet still survive in their larval form.
    2. In some gastropods, the cilia are under nervous control and can be stopped without requiring the mantle cavity retraction.
    3. In species like Haliotis, the shell rotates in two phases, but the larva is only pelagic during the first 90° rotation. The head cannot retract into the mantle cavity until after the full 180° torsion is completed and the larva settles at the bottom.
• C.M. Yonge’s View (1947):
  • Disadvantage of Non-Twisted Primitive Gastropods: Yonge suggested that primitive gastropods, which were not twisted, faced disadvantages in both respiration and locomotion. The gills, located posteriorly inside the mantle cavity, produced respiratory currents that conflicted with the animal’s forward movement and the surrounding sea currents, impeding respiration and locomotion.
  • Torsion for Ventilation and Streamlining: Once torsion occurs, the animal’s internal currents align, improving the flushing of the mantle cavity with water. Torsion thus becomes advantageous for efficient ventilation. Additionally, the repositioning of the anus to the anterior reduces the possibility of waste products interfering with the respiratory current.
  • Adaptations for Waste Management: Several adaptations have evolved to prevent interaction between the respiratory current and fecal matter:
    1. Some gastropods, such as Haliotis, develop folds in the shell to reorient the anus and prevent contamination of the respiratory flow.
    2. In some species, one of the gills and its corresponding auricles are lost, allowing the respiratory current to pass laterally through the mantle cavity.
    3. In other gastropods, the gills may be reduced or lost entirely, with alternative pallial respiratory surfaces developing, as seen in Patella.
• Morton’s View (1958):
  • Advantages of Anterior Mantle Cavity: Morton emphasized the importance of an anteriorly positioned mantle cavity for both larvae and adults. In adult gastropods, the anterior placement of the mantle cavity allows the animal to interact with incoming water more efficiently. This placement improves the detection of chemical cues and the capacity for gaseous exchange, offering a significant advantage in terms of sensory perception and respiration.
• Ghiselin’s View (1966):
  • Evolutionary Adaptation of the Shell: Ghiselin proposed that primitive gastropods developed a conical shell for protection, but this shell configuration was disadvantageous for locomotion. The anteriorly prolonged shell caused balance issues while crawling. Torsion helped resolve this by rotating the shell through 180°, placing it in a more advantageous posterior position. This change allowed gastropods to move more effectively with a balanced body structure.
  • Support for Adult Torsion: Similar views were echoed by Stasek (1972) and Purchon (1977), who supported the notion that torsion offered advantages for adult locomotion and balance, aligning with Ghiselin’s explanation.
• Coiling and the Head-Foot Complex:
  • Improved Locomotion, Feeding, and Sensory Functions: Coiling, which occurs as a result of torsion, enhances the gastropod’s ability to withdraw its head-foot complex into the mantle cavity. This withdrawal improves the efficiency of locomotion, feeding, and sensory functions. By allowing the gastropod to retreat into the mantle cavity, torsion provides protection while facilitating smoother, more controlled movement.
  • Symmetry and Volume Economy: The head-foot complex maintains its bilateral symmetry, while the visceral hump, along with the shell, becomes coiled, helping the gastropod conserve body volume and providing greater compactness.

Detorsion in Gastropods

Detorsion is a process observed in some gastropods, particularly within the order Opisthobranchia, which results in a reversal of the torsional changes that initially occurred during development. This process leads to a secondary symmetry in species that experience detorsion, restoring a more bilateral structure. Below is a detailed explanation of the mechanism, significance, and variation of detorsion in gastropods.

• Definition and General Concept:
  • Detorsion refers to the reversion or unwinding of the torsion that occurred during the larval stage of some gastropods. This leads to a return to a more symmetrical body structure.
  • During detorsion, the pallial complex (which includes structures such as the mantle cavity, gills, and auricles) shifts back towards the posterior part of the body, traveling along the right side.
  • As a result, the ctenidia (gills) are positioned to face backward, and the auricles move behind the ventricle, contributing to a more symmetrical appearance.
• Effects on Anatomy:
  • The visceral loop, which was previously twisted due to torsion, becomes untwisted, allowing the body to regain symmetry.
  • Detorsion results in the restoration of secondary external symmetry in gastropods. This is significant for organisms that have undergone extensive modification from their primitive torsional condition.
  • The shell, which may have originally been involved in the torsional process, often becomes absent in species that undergo complete detorsion. As a consequence, the gills (ctenidia) are liberated and become exposed to external currents, which aids in respiration.
• Association with the Loss of Shell and Gills:
  • In species undergoing detorsion, there is often a loss of the shell, along with the release of the gills from their protective case.
  • The gills, no longer enclosed, are now directly exposed to the surrounding environment and can interact with external currents for efficient gas exchange.
  • This exposure is particularly advantageous in species where the protective shell is no longer present, and the gills must function more independently to facilitate respiration.
• Gradations of Detorsion:
  • Different gastropod species exhibit varying degrees of detorsion. In some cases, detorsion is partial, while in others, it is complete:
    1. Partial Detorsion: In species such as Acteon and Bulla, detorsion is only partial. These species retain some aspects of their original torsional structure, but undergo sufficient changes to exhibit a degree of symmetry.
    2. Complete Detorsion: In species such as Aplysia, detorsion is complete, leading to a fully untwisted, symmetrical body plan.
    3. Complete Loss of Shell and Mantle Cavity: In certain nudibranchs like Doris and Apolidia, both the shell and mantle cavity are absent. These species become secondarily bilaterally symmetrical, representing a higher level of detorsion.
• Ecological and Functional Implications:
  • The process of detorsion is significant for the ecological adaptations of the gastropods that undergo it. By untwisting the body, these gastropods are able to achieve a more symmetrical arrangement, which may benefit locomotion and other physiological processes.
  • The exposure of the gills to external currents in detorted species enhances respiratory efficiency, especially in species that have lost their shell for protection.
  • Detorsion allows some gastropods to become more streamlined and better adapted for their environments, especially those that may not require the protective advantages of a coiled shell.