Snails and slugs can eat practically everything. Some eat fresh plants, to the despair of gardeners, me included. Others eat live animals, such as earthworms or even other snails, animal waste, rotting vegetation or fungi. Some species eat one type of food, others are less picky, but practically all lands snails and slugs use their tiny teeth when feeding.
Snails* don’t chew their food before “swallowing” but scrape it up and rasp it using a structure called radula. It is a somewhat similar to our tongue, but covered with many rows of tiny teeth. The snail sticks its radula out to scrape the food. If the food is in a larger piece, e.g., a leaf, snails can cut it with their jaw (snails only have an upper one) before rasping it.
The number, size, shape and distribution of the teeth on radulae differ between species. Some snails have more than 100 000 teeth. The properties of the teeth depend on the type of food a snail eats. The shape of the teeth can also vary across the radula.
The teeth are made of chitin – the same biopolymer from which the external skeleton of insects is made. They can also be hardened by minerals like iron, silicon, calcium or magnesium. the teeth at the front of radulae are used the most and thus wear out the fastest. But new teeth are constantly growing at the back, to replace the worn ones.
Snails grazing on algae from a hard surface (like glass) leave a characteristic trail:
While on a soft fruit or mushroom, they leave less regular marks:
The text above refers mainly to land gastropods (snails and slugs) as all of them (with one known exception) have radula. But this structure is present in many other molluscs, for example aquatic gastropods and cephalopods like squids and cuttlefish and in reduced form in octopuses.
* For a better flow of the text I only write snails, but I mean snails and slugs unless specified otherwise.
While working in the lab, behavioural ecologist dr. Alex Schnell noticed a curious behaviour of Franklin, one of the cuttlefish. In the morning when Alex would enter the lab to start experiments, Franklin would drench her with water she ejected through her syphon. However, in the evening when Alex would come to the lab to feed the cuttlefish, Franklin would not “attack” her. Alex started wondering whether this behaviour was a sign of simple associative learning of mornings with experiments that the animal didn’t like and evenings with a meal. Or maybe it showed something more: self-control and resistance of the temptation to drench the scientist in the evening.
In children self-control (the ability to delay gratification) can be tested using the so-called marshmallow test. A child is seated in a room with just a table and a chair. In front of them the experimenter leaves a marshmallow (or another treat) and tells them that they can eat it straight away or wait for a while when an experimenter leaves the room. They are told that if they wait, they will get another treat later. In the original test children waited on average 3 min before they eat the treat, but self-control increases with age.
… and for cuttlefish
Although one can’t just tell cuttlefish to wait, the researcher devised an experiment that could test for self-control in these animals. First*, they taught six cuttlefish that one compartment in their tank provides less-tasty food directly, while another compartment opens only after a delay, but contains their favourite food. Both compartments were transparent, so the cuttlefish could see both food items. Further, whenever the animal ate from one compartment, the other one was emptied of food.
Once the cuttlefish seemed to understand the rules, the real test started. Both compartments were placed in the tank and an individual was positioned at an equal distance to them. The delay for opening the compartment with favourite food was increased between experiment, causing some animals to give up and just eat the less-tasty prey. All cuttlefish were willing to wait at least 40s for their favourite food. However, many cuttlefish waited for more than a minute and one even two minutes – close to young children’s score. Interestingly, like children, chimpanzees, dogs and parrots, some cuttlefish seemed to try to distract themselves from the temptation of the direct reward, by turning away from the directly available food item.
A high degree of self-control has been shown for example in chimpanzees, corvids (birds from the crow family) and parrots. This was usually explained by the animal’s highly social lifestyle, long life and ability to use tools. After all, to keeps social bonds healthy sometimes it’s better to let others eat and delay one’s own gratification. Tool use also demands waiting for all the components to be assembled before it can be used. However, a cuttlefish lives for only two years, is not particularly social and does not use tools – so what could explain its self-control? Dr. Schnell speculates that it improves the foraging success of cuttlefish. They often lie motionless, camouflaged, on the sea bottom. Waiting for their prey to get close enough not only increases their attack success, but also reduces the chances of being seen by predators.
For a long time, people though that only humans have self-control, but this is definitely not the case; instead, it seems to be widely spread in the animal kingdom.
* Here I explained only the core of the experiment. The whole experiment was more complex and can be found in the original paper.
I often battle slugs and snails, because they eat the vegetables in my garden. However, recently I decided to take a closer look at them, especially the way they move. I have read that land snails have two basic gaits – adhesive crawling and loping. I decided to check that with my own eyes, which resulted in the photos and videos in this entry (taken with a phone so excuse the quality).
Land slugs and snails move by contracting the muscles in their foot. But contractions alone are not enough. They also need mucus, which acts partly as a lubricant – aiding sliding – and partly as glue – to stick to the ground so that the snail doesn’t slide backwards*.
On smooth, non-absorbent surfaces such as glass or plastic, snails practically always move by adhesive crawling – their entire foot sticks to the ground, and after passing, the trail of mucus is continuous.
When a snail/slug moves on a rough, absorbent surface (wood, bricks, concrete) it often lopes. It regularly lifts its head up and then lowers it back to the ground. As a result, the foot forms arches and only partly adheres to the ground. The position of each arch relative to the ground is constant and it looks a bit as though the snail is sliding on an invisible bridge. This type of gait causes the mucous trail to have breaks in it.
Snails generally move more slowly over rougher surfaces, with approximately the same speed for both gaits. Research suggests that loping is a way to save mucus when crawling on such surfaces.
Much of the research on the movement of snails has concerned the crawling gait, because the experiments were conducted on smooth surfaces – glass and plastic that allow observation of the snail from below. Therefore, it is not certain whether all land snails and slugs can lope. However, the snails that have been specifically tested could.
I decided to check the gait of snails and slugs in my garden. I found various species and watched them on different surfaces. I observed mostly adhesive crawling on glass and loping on wood. So, my observations were in line with expectations. Unfortunately, I also saw great grey slugs on the floor in my house – on laminate flooring they moved using adhesive crawling. On concrete and old metal, I often saw slugs and snails raising their heads, but I was unable to clearly identify the foot arches, and the trace of mucus was not clearly visible. More research is needed …
I encourage you to observe slugs and snails moving on various surfaces. You will surely find different species in your garden, or a nearby park, forest or meadow. I am curious what you’ll see and I invite you to share your observations in the comments section below this post.
* Snails’ mucus is a non-Newtonian fluid – under low stress it is very sticky and helps the snail adhere to surfaces. Under stress (caused for example, by contractions in sections of the snail’s foot), the mucus liquifies, allowing the slug to slide.
You can find more of my snail and slug videos here.