The Biology of Slugs
from buttemer 1997
Student # __
Sensory Perception: A Slug's eyes are on the tips of the upper (long) pair of tentacles! The eyes have a lens and a retina and can probably see only major patterns of light and darkness. The long tentacles are also olfactory (smell) organs, used for long distance detection of food (a meter or so). The lower (short) tentacles are for close up smelling and tasting. Slugs find their way "home" at night by using a scent trail they secrete!
Breathing (Respiration) Slugs breathe through their pneumostome, on the right side of the mantle. It leads to a simple air cavity, or lung. Slugs get about 1/2 their oxygen this way. They get the rest by absorbing oxygen via their moist skin. (Kinda how you do in your lungs!)
Water Balance - a real problem! Slugs have very little control over the amount of water they take in and lose. Slugs get water from the food they eat, and presumably by drinking. However, having either too little or too much water can be a problem. Lets look at the case of too little water first.
Water lose: Slugs lose water: by making mucus to move, getting it "sucked out" by dry soil and dry air, and by evaporation through their skin. So slugs tend to operate best at dusk, dawn, and periods of high humidity. Slugs hide in waterproof, but moist shelters during dry spells. If a slug loses 1/4 of its weight in water it won't be able to move. A lose of 1/2 means certain death!!!
Why is having too much water a problem? Well, a slug dropped into water can't prevent the water from soaking through its skin until the concentration of its body tissues becomes too low to sustain life. Large slugs die first by drowning (lack of oxygen to its tissues. Smaller slugs can pull in more oxygen from the water (because they have a large surface area per unit volume) so they don't drown. They die through dilution of body tissues! Umbrellas anyone?
Transportation: Slugs glide along at a top speed of about 0.007 miles/hr on a muscular foot that never leaves the ground. Its foot is constantly lubricated by a trail of mucus dribbled from its chin. Waves of muscular contraction, beginning from the tail and going to the head, propel it forward. A slug can't reverse these contractions, so it can't back up! Crawling with mucus has its advantages. Slugs can go up horizontal surfaces or even crawl upside down! Gliding on a soft layer of slime has two disadvantages. 1. Slugs must overcome the resistance to movement the mucus provides. 2. The energy cost of producing mucus (laying your own road ahead of you!) is very high, much more than the energy cost a mammal or reptile pays. Thus slugs are limited in the area they can go to find food and mates.
Feeding: As mentioned before, slugs gnaw, or rasp, away at their food with a 27,000 tooth radula. They eat plant foliage, fungi, lichens, decaying vegetation, and some slugs are carnivorous or even cannibals!! An average slug needs about 1 calorie/day to survive. (Sidebar -actually 1 calorie = 1000 calories, but that's another story! People will do anything to seem like they are eating less!) More on food in ecology and native bananna slug articles later...
Reproduction - Slugs are hermaphroditic and have elaborate courtship rituals. (They can have it both ways, hey watch out what you're gnawing on buddy!) Slugs can actually fertilize their own eggs if they can't find a partner. Slug eggs are moisture and temperature sensitive. Clutches are laid once or twice a year, usually in fall, and hatch out in the following spring.
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