Molluscs: Feeding Habits | Zoology Optional Notes for UPSC PDF Download

Introduction to Molluscs and Snail Feeding Habits

Gastropods, a class of molluscs that includes snails, are of particular interest due to their diverse feeding habits and behaviors. Some species of gastropods can be parasites of other invertebrates, including mites and nematodes.

Feeding Behaviors and Adaptations in Gastropods

• Snails have evolved a wide range of feeding behaviors, and their choice of food can vary significantly based on the species and environment.
• Most snail species are herbivores, feeding on plant material. Terrestrial snails consume leaves, bark, seedlings, and fruit, while marine snails may scrape algae from rocks on the sea floor.
• Some snails are scavengers, feeding on decaying plant matter or animals, while others are carnivorous, using their radula (a feeding organ) to tear apart prey.
• Herbivorous snails can be grazers, browsers, or planktonic feeders. Grazers and browsers may feed directly on plants in their surroundings.
• Ciliary feeders, such as some sessile gastropods, use gill cilia to draw in particulate matter, which they roll into mucous balls and carry to their mouths.
• Most pulmonate snails (air-breathing snails) are herbivores, but some also feed on earthworms and other snails.
• In bivalves, which include clams and oysters, most are suspension feeders. They use gill cilia to create mucus traps that entangle particles suspended in the water for consumption.
• Cephalopods, such as squids and octopuses, are predaceous and feed primarily on small fishes, molluscs, crustaceans, and worms. They have complex feeding mechanisms and use their arms to capture and handle prey.

External Factors Influencing Snail Feeding Behavior

• External factors, such as the abundance and distribution of food, presence of competitors, risk of predators, season, and time of day, can influence snail foraging behavior.
• Snail food preference can be determined by nutrient requirements, digestive limitations, and factors like food attractiveness and palatability.
• Different snail species may have narrow and distinct dietary preferences, while others may be more generalist feeders.
• Seasonal variations in food preference and feeding behaviors are observed in snail populations.
• The spectral composition of light in water plays a role in snail attraction, with the presence of different photoreceptors in snail eyes contributing to their responses to light stimulants.

Effect of Substances on Snail Behavior and Physiology

• Substances like molluscicides, amino acids, carbohydrates, and attractants can influence snail behavior and physiology.
• Sub-lethal concentrations of molluscicides can impact snail fecundity, hatchability, and survival.
• Attractant-based bait formulations containing specific substances can alter biochemical parameters in snail tissues, such as the gonadal/nervous tissue.
• Snails have the ability to recognize and choose preferred food sources, and their responses to bait formulations can vary based on the presence of different substances.

Structure of the Feeding System in Gastropods

The feeding system in gastropods, including snails, involves a combination of muscular movements, specialized feeding structures, and neural control. Here's an overview of the structure and functioning of the feeding system in gastropods:

• Food-Finding Movements: Gastropods, like other mollusks, employ specific muscular movements to locate food. These movements are typically controlled by muscles in the head and foot, which may also produce unrelated movements.

• The Radula: The radula is a crucial structure in gastropod feeding. It is a ribbon-like structure covered with rows of chitinous teeth. In herbivorous gastropods, it plays a central role in ingesting food.

• Radula Movements: The radula is manipulated by the buccal muscles. These muscles control the protraction (extension) of the radula out of the mouth towards the food, as well as its retraction, which helps rasp the food.

• Hinged Radula: In some gastropods, such as Aplysia and Pleurobranchaea, a fold in the center of the radula acts as a hinge, allowing the two halves of the radula to open and close. This adaptation enhances its effectiveness in food manipulation.

• Neural Control: The nervous system in gastropods plays a crucial role in coordinating feeding movements. The buccal ganglia control the radula and buccal cavity's movements. These ganglia are innervated by the cerebral ganglia.

• Cerebral Ganglia: The cerebral ganglia innervate the anterior portion of the gastropod, including structures related to feeding, such as the head, lips, and mouth. They also communicate with the rest of the central nervous system.

Role of Octopamine in Feeding

• Octopamine is a neurotransmitter in gastropods, and its distribution in the central nervous system has been described in Lymnaea species. Octopamine-immunoreactive neurons are found in the buccal, cerebral, and pedal ganglia.
• Octopamine concentration is notably high in the paired buccal ganglia, which contain key elements of the feeding network in gastropods. This suggests that octopamine is involved in regulating feeding behaviors and related neural circuits.

Role of Chemo Receptors in Locating Food

Chemoreception, primarily olfaction, plays a significant role in gastropods' ability to locate food. Gastropods use specialized chemosensory organs to detect chemical cues in their environment:

• Anterior and Posterior Tentacles: Terrestrial pulmonates have two pairs of tentacles, the anterior and posterior tentacles, which are important chemosensory organs. The tips of the tentacles bear densely packed ciliated sensory cells.
• Mantle Tentacles and Osphradium: In prosobranch gastropods, the mantle tentacles and osphradium (a specialized sensory organ) are implicated in chemoreception. These structures appear to play a sensory role in detecting food and other environmental cues.
• Rhinophores: Opisthobranch gastropods, such as sea slugs, use their rhinophores, located dorsally on the head, for chemoreception. Rhinophores are well-innervated and contain specialized epithelium for detecting chemical cues.

Behavioral Responses to Chemical Stimuli

Gastropods exhibit various behavioral responses to chemical stimuli, including those related to food. These responses can be influenced by factors such as hunger, habituation, sensitization, satiation, the quality of food, and associative learning.

In summary, the feeding system in gastropods, including snails, involves complex interactions between muscles, specialized structures like the radula, and neural control. Octopamine plays a role in regulating feeding behaviors, and chemo receptors, such as tentacles and rhinophores, help gastropods locate and respond to chemical cues in their environment, particularly those related to food. Gastropods' ability to sense and respond to chemical stimuli is crucial for their survival and foraging behavior.