The picture above shows four u-shaped burrows (the right arm of a short one overlaps the left arm of another broken one). I am not sure who took this picture but its quality makes me think it might be Jim Wolford. The leftmost and rightmost burrows are complete and easily transcribed. If we look at the left narrow u-shaped burrow, we see that the end of the u dips into some black muck: this is the anoxic layer. In this photograph the anoxic layer is probably 6 cm below the mud surface (the edge of which can be seen 4/5ths the way up the photo).
If we look at the left arm of the 2nd burrow, we see a female mudshrimp (Corophium volutator). I think the generic name might have changed recently. The females actually excavate the burrows. The larger males will cohabit with receptive females for the purposes of mating, which one of my past PhD students Dean McCurdy might even have on film. A lot of what I talk to below is largely from Dean’s work.
I guess the first question of interest is what makes the intertidal mudflats. Sort answer: erosion of sandstone cliffs that are found all around the Bay of Fundy. This erosion occur in part from tidal currents. The world-celebrated Bay of Fundy has some of the highest tides in the world and there can be considerable force in the movement of water. The silt-laden waters likely act like a scrub brush causing further erosion and when the water velocity slows the mud and silt particles are deposited.
The mudshrimp show curiously biased sex ratios. Among adult shrimp, females can outnumber males anywhere from 3 to 1 to 9 to 1. Although estimates vary, the mudshrimp can number into the tens of thousands per square meter of substrate. This species just by its sheer biomass is seen as a key species. It is food for ~ million+ migratory shorebirds that stage on some Bay of Fundy mudflats before flying southwards to central and south America in the late summer. It is also food for several species of intertidal fish and many invertebrates. Mudshrimp also act as first or second intermediate hosts for nematodes and trematodes. The diagram below shows the life cycle of a trematode parasite which encysts in the mudshrimp.
Dean demonstrated that mudshrimp parasitized by trematodes show more crawling behavior than unparasitized controls and that the altered behavior tends to occur a few weeks after experiment infection when the trematodes have encysted and are likely to survive ingestion by shorebirds which feed on parasitized shrimp. Further, this behavioral alteration, thought to be a form of parasite manipulation, is only seen during the daytime and not at night (shorebirds do feed at night but they tend to probe the mud rather than peck at its surface). Very few mudshrimps are parasitized by trematode metacercariae, which gives credence to the logic of manipulation to increase the parasitized host’s risk taking behavior: else, those trematodes might simply miss being ingested. This system might lend itself to more detailed biochemical studies of who is in control: the parasite or host.
I’d like to return now to the problem of biased sex ratios: a problem two other people have helped with immensely: Dr. Selma Mautner and kathy Cook. When the tide is ebbing, they feed on crawling mudshrimp which are principally males. We initially thought the birds could account for the sex ratio bias. However, birds are only abundant at staging areas for ~ 1.5 months and female biases occur all year round. Additionally, female biases occur at intertidal flats not visited by shorebirds, and biases are observed among juvenile mudshrimp (before males start crawling).
We decided to examine whether there might be microparasites known as microsporidians in our populations. These microparasites have been associated with sex ratio biases in other invertebrates. As a quick check, we dissected out ovaries and testes of females and males. With used diagnostic genetic techniques where we amplified DNA using Microsporidian 16S rDNA primers. Using this technique, we could tell which individuals were parasitized and which were not. In short, approximately 26% of females carried microsporidians in their ovaries whereas no males carried the parasite in its testes. The graph does not show error bars per se, but rather bars that denote the limits of what prevalence of infection could be, given our samples. The result is clear. There is a huge sex bias in parasitism.
We then brought females into the lab and reared their young from them for three months, at which time the young could be sexed. We asked, what was the sex ratio of offspring from infected versus uninfected females? We found tremendous variation. Some females produced all male broods (despite having over 20 young). Some had all daughters. Most females had a mix of sons and daughters. And even some had intersexes (~2% of young) with both male and female characters. On average though, more sons were produced from uninfected than from infected mothers. We do not believe sons dying in infected broods can explain these results. Rather, we expect (as a working hypothesis) that the parasite feminizes males to become daughters, which then transfer the parasite to the original mother’s granddaughters.
Using a technique for staining DNA known as DAPI staining, we have found that Microsporidians transferred in eggs; eggs are big enough to harbor such parasites (sperm are not). Vertical transmission means parasite encounters new hosts (daughters and then granddaughters), but parasites should be relatively benign. We suspected that intersexes might be incompletely feminized males but we have no proof for this assertion yet. Importantly, infected females actually produced 25% larger broods than uninfected females, which relates to the premise suggested by others that these parasites should be relatively benign (this is the first time this result has been recorded for microsporidian parasites).
Finally, sex ratio distortion appears to have evolved several times independently. To address this question, we excised strong bands from gels and used genomic approaches to sequence the 16S rDNA. Using these techniques, we retrieved the exact same sequence suggesting we were dealing with a single species (97% similarity in this gene region is taken as a minimum to consider that you have the same species). This species was not closely related to other microsporidians known to distort sex ratios.
We can now ask: are males important? If you think about it, males may not be that important from the viewpoint of population stability or population growth. Just as you need one rooster (perhaps a backup) in the barnyard, you might not need a lot of males to ensure egg production. But ensuring egg production is not a goal, or even a consideration, of blind selection. Especially when you have parasites involved. You may get sex ratios that are not optimal for population stability or growth, but which are perfect for the shorter-term success of parasites.
Notwithstanding, we can ask how are shrimp males doing? To address this question, we examined the factors associated with the availability of reproductively active males. Those factors include the distribution of receptive females (no big surprise here, males are doing a good job of positioning themselves near females). We also found that time of season was tremendously important. Reproductively active males were about half as abundant later in the season as they were early in the season: this work was based on a 4th year honours thesis by Keiko Lui. And later in the season, fewer females were carrying eggs. In the lab, not only are proportionality fewer females carrying eggs when there are few males around, but those females that are carrying eggs are not carrying many eggs. In other words, we expect that males might become limiting and affect population stability or potential of shrimp populations to grow or to rebound following depredation by shorebirds and fish.
We are still somewhat uncertain as to how limiting males might become. But we do know that shrimp are very important. As an example, shorebirds arrive at mudflats from arctic breeding sites relatively lean (carcass ~30 g). At Bay of Fundy intertidal sites, individual birds eat constantly over 2-3 weeks feeding on mudshrimp, which are rich in omega-3 and omega 6 fatty acids. The shorebirds then depart relatively fat for 4,500 km non-stop flight to NE South America. Their fuel is mudshrimp (> 80% of their diet). When we consider not just shorebirds but all other species using the mudflats, we come very quickly to the conclusion that shrimp are a key species. In fact, if you remove shrimp, the mudflat starts to erode. All the intertidal predatory worms, bottom feeding fish (many of which are commercially important) and the shorebirds would be in jeopardy if mudshrimp populations declined.
Professor & student