Condition‐dependent immune function in a freshwater snail revealed by stable isotopes (2022) Seppälä et al., Freshwater Biology, link to article

Image credit: Bj.schoenmakers, CC0, via Wikimedia Commons

The Crux

There are myriad factors at play when it comes to parasitic infections, but the primary physiological barrier for the parasite is the immune function of host organisms. Despite its importance and usefulness, the immune function is costly to maintain. Building and effectively using immune defenses relies on the host being able to secure enough food to properly fuel its defenses. As a result, individuals in poor condition are more susceptible to parasites. Building off of that, if the conditions in a given area are poor/worsening, then an entire population of organisms may be vulnerable to disease outbreak.

Many studies have investigated the dependence of immune function, including one of my own, but many of those studies take place in lab settings where the food given to a host is carefully controlled. While there are obvious benefits to controlling experimental conditions, it can be hard to generalize the findings of a lab study to the natural settings that organisms actually live in. Today’s authors utilized an observational study of a freshwater snail (Lymnaea stagnalis) to better relate host condition in nature to immune function.

What They Did

The authors collected 101 snails from a small pond in Zurich, Switzerland. Each snail had its size measured, and then the authors extracted two samples of haemolymph (see Did You Know?). One sample was used to quantify PO-like activity (phenoloxidase-like activity, used to mobilize melanin as a defense against parasites), while the second sample was used to quantify the anti-microbial properties of the haemolymph, itself.

Did You Know: Haemolymph

Blood is an incredibly useful tool used by the body to transport oxygen and nutrients around the body, but not all organisms have the same kind of blood that we do. Invertebrates like insects and snail have haemolymph, which is similar to blood in purpose, if not in appearance. Unlike blood, haemolymph is (in my experience) clear like water, but it still contains many useful proteins and other molecules that are key to the immune system. For example, the haemolymph in the snails used in this study can be used to not only mobilize defenses against parasites, but to also directly kill microbes.

After the authors collected the haemolymph samples, they collected feces from the snails for stable isotope analyses. Stable isotope analysis is a technique that allows researchers to understand the composition of an organism’s diet. The authors also conducted a stable isotope analysis on the tissue of the snails. Unlike the feces, which would tell the authors what the snails had recently consumed, analyzing the tissue would give the authors an idea of what the snail had consumed over the past few weeks.

After collecting all of these data, the authors then related the resource use of the snails (both in the short and long term) to variation in immune defenses (both the PO-like activity and antimicrobial activity).

What They Found

Snail immune function was condition-dependent, and was related to their diet both in the short-term and the long-term. Specifically, snails that had recently consumed resources with a potentially higher proportion of animal protein had stronger immune defenses. Additionally, snails with low lipid reserves had high PO-like activity, which is an indication of how costly utilizing such immune defense is.

Problems?

A weakness of the study is the limited sampling. Though the authors were thorough with their design, they only sampled a single population at a single point in time. However, they correctly point out this limitation in the paper itself, highlighting the need for further study. Studies are often limited in scope by how much money or help a researcher can acquire for the project, and today’s study was no exception.

So What?

The novel results show that even within a small pond there is considerable variation in resource consumption, host condition, and multiple aspects of immune defense. Further, the importance of recently-consumed resources for host immune function demonstrates that there could be substantial variation in host immune function on a very fine scale, which has interesting implications for the outcome of host-parasite interactions in natural communities. Now all I want to do is go back to that pond in Switzerland and see how much parasitism changes over very small scales.

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Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who thinks about food in the short and long term. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.