Marion is getting to know her new research organisms
Posted: September 14, 2014
Upon arriving in the Tooker lab about three months ago, one of my first tasks was to get familiar with my new model organism. While I would not say that my young career has been organism driven (I have worked with crab spiders and kissing bugs during my Master, and grasshoppers for my PhD) I always thought of myself as an entomologist, someone who studies insects, or at the very least arthropods.
“Entomology (from Greek ἔντομος, entomos, "that which is cut in pieces or engraved/segmented", hence "insect"; and -λογία, -logia) is the scientific study of insects, a branch of arthropodology. In the past the term "insect" was more vague, and historically the definition of entomology included the study of terrestrial animals in other arthropod groups or other phyla, such as arachnids, myriapods, earthworms, land snails, and slugs. This wider meaning may still be encountered in informal use.”
I guess informally I am still an entomologist since I now work with slugs. Thank you Wikipedia: identity crisis averted. So what are slugs? They are molluscs (or mollusks) and belong to the class Gastropoda which also include snails. Slugs and snails, in addition of being delicious (at least for the latter: I can’t say that I have ever eaten a slug), are very bizarre creatures. This is coming from someone who has collected grasshopper poop for the past 5 years. For instance, one of the characteristic of gastropods is that they experience an anatomical event called “torsion” early in their life. Their shell (for snails), mantle, and visceral mass (aka internal organs) rotate by 180° with respect to the head and foot. Practically speaking, it means that after the torsion their anus is located above their head. Since hearing about that as an undergrad, I always had a hard time figuring how this could be practical… I am not alone here by the way, this is Wikipedia takes on the matter:
“A particular problem gastropods had to overcome was the location where wastes were excreted: above the head, which can potentially lead to fouling of the mouth and sense organs.”
That said, this is not even remotely the weirdest thing about them. I have since experienced the orange defensive slime of the dusky slug (Arion subfuscus). I feel fortunate because this is truly a unique experience. I highly recommend it. (Might take a while to get rid of that stuff, fun times.) There is also their odd mating habits, described as “pugnacious actions” in the very serious Transactions of the American Microscopical Society (Karlin and Bacon, 1961). Being a nutritional ecologist, a lot of my work in the Tooker lab will be concerned with what slugs eat, and how food/specific nutrients influence their performance. For that I have to understand how they eat. One of my first discovery was that slugs have… way too many lips. The mouth of the gray garden slug Deroceras reticulatum -one of the species I am currently working with- is surrounded by 3 anterior lips, 5 pairs of lateral lips (yes 5 PAIRS of lips), and one single posterior lip (a little bit disappointing here, I know) (South, 1992).
More recently, I became curious to know whether or not slug benefitted from aggregating on plants, as it seemed to me that they were doing so in the field. I wanted to measure if it had anything to do with the plants themselves. For example, when aphids are present at high density, they are able to divert resources from distant plant part and create nutrient sinks, which allow them to feed on tissue that would otherwise be sub-optimal (Way et al., 1970). Slugs could also be aggregating to overcome plant defenses, as seen for caterpillars of the pipevine swallowtail (Battus philenor) which grow 25% faster at high density, most likely because they are able to deal with plant trichomes more efficiently (Fordyce and Agrawal, 2001). Aggregation could also influence plant-induced defense. The jasmonic acid pathway (JA) is involved in induction of direct and indirect plant defense against herbivores, and can act in a density-dependent fashion, for example against spider mites (Gols et al., 2003). While this is bad news for aggregated spider mites, a different scenario is possible for slugs. It has been recently shown that the locomotion mucus of Deroceras reticulatum activates the salicylic pathway (SA) (Kästner et al., 2014a; Kästner et al., 2014b). SA induces resistance against bacteria and some phloem feeding insects. Now, the interesting thing about that is that SA and JA are antagonists… So slugs could actually be shutting down plant defense against chewing herbivore with their slime. That would be cool right? Well, we will know only if Rosie (the high school student who is training me) to extract jasmonic and salicylic acid does not lose patience with me.
Another discovery made by Maggie (PhD student by day, slug expert by night) is that slugs are con artists. You can put slugs on a plant, and cage the plant, leaving no chance for potential escape, but there is not guarantee that you will find any slugs at the end. NONE. A likely explanation is that slugs have found a way to deviate the rules of time and space that apply to most organisms and teleport themselves at will. Another possibility: they are very sensitive to desiccation and shrivel in the soil when dying, making it impossible to recover any traces of their presence. To alleviate this problem I decided to tattoo my slugs with fluorescent dyes. This would have the triple advantage of 1) as already mentioned, being able to find them after a week on a plant/soil; 2) being able to mark them individually instead of losing time training myself in slug facial recognition; 3) improve my tattooing skills. Honestly? I just saw Kevin, a postdoc in the lab, work with fluorescent dies for his research on invasive stink bugs and I was jealous because fluorescent stuff is cool.
Fordyce, J. A., and A. A. Agrawal, 2001, The role of plant trichomes and caterpillar group size on growth and defence of the pipevine swallowtail Battus philenor: Journal of Animal Ecology, v. 70, p. 997-1005.
Gols, R., M. Roosjen, H. Dijkman, and M. Dicke, 2003, Induction of direct and indirect plant responses by jasmonic acid, low spider mite densities, or a combination of jasmonic acid treatment and spider mite infestation: Journal of Chemical Ecology, v. 29, p. 2651-2666.
Karlin, E. J., and C. Bacon, 1961, Courtship, mating, and egg-laying behavior in the Limacidae (Mollusca): Transactions of the American Microscopical Society, p. 399-406.
Kästner, J., D. von Knorre, I. T. Baldwin, and S. Meldau, 2014a, Salicylic acid-dependent gene expression is activated by locomotion mucus of different molluscan herbivores: Communicative & Integrative Biology, v. 7, p. e28728.
Kästner, J., D. von Knorre, H. Himanshu, M. Erb, I. T. Baldwin, and S. Meldau, 2014b, Salicylic Acid, a Plant Defense Hormone, Is Specifically Secreted by a Molluscan Herbivore: PloS one, v. 9, p. e86500.
South, A., 1992, Terrestrial slugs. Biology, ecology and control, Chapman and Hall Ltd.
Way, M. J., M. Cammell, and A. Watson, 1970, Aggregation behaviour in relation to food utilization by aphids, p. 229-247.