Slugs are terrestrial mollusks, closely related to snails, yet they lack the protective, external shell that defines their cousins. This absence leaves their soft, moist bodies highly susceptible to the environment, making their survival a delicate balance of biological necessity and environmental access. As gastropods, slugs are essentially bags of water living on land, their ability to thrive is dictated by a few fundamental requirements that govern where and when they are active.
Essential Water and Humidity Requirements
The single greatest threat to a slug’s existence is dehydration, a consequence of having permeable skin and no shell to seal in moisture. Slugs constantly lose water through their skin via evaporation, making high environmental humidity necessary for survival. The mucus they excrete, which is approximately 91% to 98% water, serves multiple functions, including defense and locomotion, and its production necessitates a high moisture intake.
The secreted slime trail is a viscoelastic substance that acts as both a lubricant and an adhesive, allowing the slug to glide across surfaces and even up vertical walls. This mucus enables movement by becoming liquid under pressure and sticky when relaxed, providing grip. The constant need to replenish this water-rich mucus means the slug must continually absorb moisture from its surroundings, often through its skin, to compensate for the significant loss.
Diverse Dietary Needs
Slugs are generalist feeders, consuming a wide array of organic materials and playing a significant role in nutrient cycling within their environment. While often seen as garden pests, their diet is not limited to living plants. Many species are primarily detritivores, feeding on decaying plant matter, fallen leaves, and decomposing wood, or are fungivores that consume various types of fungi.
Food consumption is accomplished using a specialized feeding structure called the radula, a ribbon-like organ unique to most mollusks. The radula is covered in thousands of microscopic, backward-facing, tooth-like projections. This structure moves over a cartilaginous base to scrape, rasp, or cut food particles before they enter the digestive tract.
Maintaining a Safe Habitat
Because of their physiological vulnerability, slugs require a microhabitat that addresses their need for moisture and thermal regulation. They are highly sensitive to rising temperatures and air currents, which increase the rate of water loss from their bodies. Slugs seek out mild ambient temperatures, with optimal activity often occurring around 17 to 18 degrees Celsius. Direct sunlight and temperatures above 21 degrees Celsius can be rapidly fatal, leading to nocturnal activity where they maximize exposure to cooler temperatures and higher humidity levels.
During the day, slugs retreat to damp resting sites, often under logs, rocks, leaf litter, or burrowed into cool, moist soil. This substrate provides a stable, humid refuge where they can rest, absorb water, and avoid the lethal conditions of a dry environment.