Gut microbiota of a long-distance migratory bird demonstrates resistance against invading environmental microbes

By Elana Rusnak, SRC intern and MS student

Just as with humans, animals have many kinds of bacteria in their gut that helps them with acquiring the nutrients from the food they eat, as well as boosting their immune system (Khosravi & Maxmanian, 2013). Many species that migrate from breeding to non-breeding grounds during the year may have an increased risk of transmitting harmful bacteria that they pick up from sites they stop at along the way. If migrant guts have a “colonization resistance” to harmful microbes, they will be less likely to carry them along their migratory routes. However, it is also possible that gut communities may become unstable when continually exposed to new strains of bacteria. Migratory species are faced with changes in geography, diet, and physiology, which are all factors that contribute to changes in gut bacteria composition.

Figure 1

Red-necked Stint (Calidris ruficollis) By Patrick_K59 – https://commons.wikimedia. org/w/index.php?curid=45909879

In a study by Risley et al. earlier this year, they looked at the invasion resistance of the gut of the Red-necked stint (Calidris ruficolis) to microbes they ate at various sites along their migratory route (Siberia to Australia). They chose this study species for a few reasons: they migrate to their non-breeding grounds and stay there for at least a year and a half afterward, so it’s easy to track birds that have recently migrated and those that have been there for a long time. These shorebirds also eat by sifting through coastal sediments with their bills, so their food source is readily available to study at known locations. The researchers believed that recently migrated birds would have a different gut community than resident birds, and that their gut community would become more like that of the resident birds over time.

Red-necked stint distribution, with non-breeding areas in blue. By Nrg800 – Own work, https://commons. curid=11336387

They caught and sampled 71 total stints in Australia in two groups: recently migrated birds, and resident birds. They isolated and analyzed gut community DNA from each individual, and also sampled sediments from along the migratory route, and compared the bacteria in these sediments to what was found in the guts.

Contrary to what they predicted, less than 0.1% of the gut community in migrating birds came from the local foraging sediments. They found almost no difference in the gut microbes of the recently migrated birds and the resident birds, suggesting that this particular species does not incorporate sediment microbes into their gut as they migrate. However, they did find a high abundance of a single strain of bacteria in the recently migrated birds that decreased over the course of the non-breeding season. This strain may be related to an immune response to compensate for being exposed to so many different kinds of bacteria on their migratory route.

These results show that the Red-necked stint is highly resistant to microbial invasions that are taken in with their food as they migrate. The researchers suggest that this resistance decreases their susceptibility to infection as they visit new locations on their route. Future research could elucidate if this resistance is only found in migratory species, and to further understand the relationship between microbe invasion and infection risk.

Works Cited

Khosravi, A., & Mazmanian, S. K. (2013). Disruption of the gut microbiome as a risk factor for microbial infections. Current opinion in microbiology, 16(2), 221-227.

Risely, A., Waite, D., Ujvari, B., Klaassen, M., & Hoye, B. (2017). Gut microbiota of a long‐distance migrant demonstrates resistance against environmental microbe incursions. Molecular ecology.

Nocturnal migration reduces exposure to micropredation in a coral reef fish

By Josh Ratay, SRC intern

The French grunt was used as the organism of study, though gnathiid isopods feed on many different species of reef fish. Image from Wikimedia Commons.

The French grunt was used as the organism of study, though gnathiid isopods feed on many different species of reef fish. Image from Wikimedia Commons.


Nocturnal migration reduces exposure to micropredation in a coral reef fish is a new study examining daily migrations of French grunt (Haemulon flavolineatum) and the exposure to parasitic isopods. French grunts are known to move off the reefs and into seagrass beds at night. Such behavior in animals is usually attributed to increased availability of prey or decreased exposure to predators. However, this study investigates the idea that the fish’s movements may serve as a method of avoiding isopods, which are far more common on the reefs than in seagrass communities.

The French grunt served as an ideal model organism for this study since it is both extremely common and well-studied on Caribbean reefs. However, many other reef fish are known to undertake similar nightly migrations away from the reef. Gnathiid isopods are tiny parasitic crustaceans which attach to fish and feed on blood. Though they usually hide in reef sediments, the larvae emerge to find a short-term host fish directly before molting and growth. Though far smaller than their hosts, these parasites can have many negative health impacts on fish, including decreased concentrations of red blood cells, tissue damage and infection, and even the death of juvenile fish which are exposed to a high isopod load.

Larval gnathiid isopods (bottom right) feed on the blood of fish before molting. Adult males (left) and females (right) tend to remain in the sediment. Image from Wikimedia Commons.

Larval gnathiid isopods (bottom right) feed on the blood of fish before molting. Adult males (left) and females (right) tend to remain in the sediment. Image from Wikimedia Commons.

In this study, several tests were performed at different locations in the Caribbean to test the hypothesis that leaving the reef at night results in fewer numbers of isopods infesting French grunts. Cages containing 5 to 8 fish were deployed overnight at both reefs and nearby seagrass habitats where grunts had been observed. Also, the exact arrival times (around dawn) of the wild grunts to the reefs were observed at different sites, and cages of grunts were subsequently placed at the reefs for both the 30 minutes preceding the arrival time and 30 minutes following the arrival time. The goal of this test was to investigate whether or not the fish’s arrival time coincided with lessened isopod activity. Additional cages were also deployed to test day versus night isopod levels, and the whether or not juvenile grunts were also susceptible to isopod infestation. All fish from the cages were placed in seawater tanks, where the isopods naturally detached and were then counted.

Box and whisker plot showing the decrease in isopods in the time after the fish have returned to the reef vs. immediately before. Figure from Sikkel et al 2017.

Box and whisker plot showing the decrease in isopods in the time after the fish have returned to the reef vs. immediately before. Figure from Sikkel et al 2017.

Statistical analysis of the data revealed that grunts which spent the night on the reefs contracted 3 to 44 times as many isopods as those which were left in the seagrass bed. Fish which were placed on the reefs during the 30 minutes before the return time of the wild grunts contained twice as many isopods as those which were exposed to the reef after the return time. Nighttime isopod load on the reefs was also much higher than the daytime load, and isopods were detected on juveniles from the reefs but not the seagrass beds. Overall, these results were significant and suggest that moving to seagrass beds at night is an effective method of isopod evasion for both adult and juvenile grunts, and that the fish return in the morning as isopods become less active. The results from this study suggest that further research is needed into the potential for parasites to drive movements of coral reef organisms.

Works cited
Sikkel, P. C., Welicky, R. L., Artim, J. M., McCammon, A. M., Sellers, J. C., Coile, A. M., & Jenkins, W. G. (2016). Nocturnal migration reduces exposure to micropredation in a coral reef fish. Bulletin of Marine Science.

Evidence for collective navigation in salmon for homeward migration

By Hanover Matz, RJD Intern

The long migrations undertaken by Atlantic and Pacific salmon to reach their spawning grounds are known by many. Salmon are anadromous; they spend their adult lives foraging at sea, and then return to the freshwater rivers of their births to spawn and reproduce. How this remarkable feat is accomplished by the salmon with such accuracy is believed to be a combination of geomagnetic and olfactory cues. However, Berdahl et al. examined evidence that salmon may also be relying on social interactions and collective behavior in order to navigate to their natal homes. The authors hypothesized that salmon migrating in large groups could more accurately navigate to their spawning grounds by reducing the effect of navigational errors made by individual salmon. In order to determine whether this was the case, Berdahl et al. compiled results from previous studies that indicated a decrease in straying with an increase in abundance, proposed potential benefits of collective navigation for salmon, and used catch size data to better understand how salmon aggregate in the wild.

The basis for collective navigation in migrating animals is that by traveling in relatively large groups, animals can reduce the effect of navigational errors made by individuals in the group by distributing those errors across the decisions of the group as a whole. A school of salmon returning to their home river can more accurately find that river by relying on the collective decisions of the group rather than an individual salmon can by reading cues on its own. Berdahl et al. synthesized results from multiple studies that showed salmon more effectively returned to their natal grounds when they were in higher abundance, indicating the possible role of collective behavior. Figure 1 shows the results taken from several studies, all of which demonstrate that as the run size or abundance of salmon increased, the straying rate of salmon migrating back to their natal rivers decreased. The more salmon present, the less likely individual salmon were to stray from the correct migration path, suggesting the possible role of collective navigation.

Figure 1 salmon paper (1)

Results of several studies indicating a negative relationship between straying rate and run size in salmon

With this evidence for collective navigation in mind, the authors postulate several potential benefits to salmon through such behavior. Not only would collective navigation allow schools of salmon to properly orient themselves when traveling from the high seas to coastal environments, but it would also allow them to pinpoint the proper estuaries and river mouths necessary to reach their natal grounds. When presented with multiple rivers to migrate towards, collective navigation would also allow the group to effectively decide on the correct path, rather than relying only on the abilities of the individual fish. By making orientation and migration decisions based on the collective choices of the group, the salmon can reduce the effect of errors made by individual fish and the school can reach their spawning grounds based on the decisions of only a few informed individuals. Schooling offers many other potential benefits besides collective navigation, such as protection from predators, and it is likely that salmon rely on both visual and pheromone cues in order to migrate as a group. In order to further provide evidence for collective navigation in salmon, the authors collected catch size data to determine the degree to which salmon school in the high seas.  The data revealed that the catch sizes for multiple species varied from the expected Poisson distribution, indicating the salmon likely aggregate in the open sea. It was overall rare to find fish, and when they were found, they were often found in large numbers, suggesting a degree of schooling at sea. This schooling may increase with increasing migratory behavior. Figure 2 shows the results of the catch size data.

Figure 2 salmon paper (1)

Catch size data for multiple salmon species, shown in solid lines, does not follow the expected Poisson distributions, shown in dashed lines, indicating aggregation of the salmon

What does this evidence for collective navigation indicate? The possibility that salmon may rely on collective navigation in order to reach their spawning grounds implies that populations require enough individuals to aggregate and migrate as a group. What this means for salmon fisheries is that even if a fishery is managed to allow for the future reproduction of the salmon, if enough individuals are removed from the fishery so that the remaining fish cannot effectively aggregate, they may not be able to reach their natal rivers to reproduce. The population may be numerically fished so that there are enough individuals remaining to continue reproducing, but they may be distributed in such a way that they cannot school in large enough numbers to migrate. This could potentially lead to detrimental overfishing of salmon stocks. In order to fully understand the degree that social interactions may play in salmon migration and reproduction, future studies will have to be conducted on the role of collective navigation in salmon and other organisms that migrate over long distances.



1. Berdahl, A., Westley, P. A., Levin, S. A., Couzin, I. D., and Quinn, T. P. 2014. A collective navigation hypothesis for homeward migration in anadromous salmonids. Fish and Fisheries.