Continuing our series of posts from first year MA students, here is a student in my lab, Molly McCargar (@MollyMcCargar ) is writing about her really cool work on looking at gut microbes in herbivorous coral reef fish.


Herbivores and Connectivity

Herbivores in most systems aren’t traditionally the stars or most charismatic of fauna. However, in reefs, almost all conservation programs now focus on the importance of herbivorous fish as a means to prevent algae dominance in these systems, giving them a much higher profile than most groups of herbivores. These fish, such as parrotfish and surgeonfish, help trim the fleshy macro-algae that are particularly prone to settlement in nutrient rich areas so that the reef’s symbiotic phytoplankton can have access to sunlight. This is critical, as their ability to receive sunlight drives the sustenance of the coral reefs they live within, and thus all the organisms that live in reefs, one of the most fantastically bio diverse ecosystems known to man. However, we know very little about how these reef fish came to specialize on algae, and how they accomplish the task. This is important for conservation planning and prioritizing what makes one species more valuable than the other, an important factor in prioritizing conservation resources.

To explore possible ways in which we could prioritize these species we need to look into the natural history of herbivory in fish. Despite their essential role in stabilizing one of the most bio diverse ecosystems known , it turns out that fish do not possess cellulase, the enzyme that breaks down plant cell walls, indicating that the incidence of herbivory in fish is counter intuitive or disadvantageous. Regardless of this, herbivorous fish have evolved several digestive adaptations to help extract nutrients from plants. These range in complexity from simply having an elongated intestinal tract, to actually trading out the stomachs of their carnivorous ancestors and forming new physical structures that assist in mechanically grinding the plant matter until the cells burst.

While the elongated intestinal tract, which serves to increase contact between the plant matter and gastrointestinal juices, appears to be the most basic and initial adaptation, current herbivorous fish almost invariably feature an additional more complex adaptation as well. The most common of these are: an extremely acidic intestinal tract that serves to chemically lyse the plant cells, symbiotic partnerships (including some with the largest species of bacteria discovered so far) with microbes harbored in their intestines, and the previously mentioned structures that mechanically grind the plant matter. Specifically, these are the “crop,” a thick walled structure that holds sand and sediment that grind to break plant cells, and the “pharyngeal mill” in parrotfish, which feature inward rows of teeth that grind together, also breaking plant cells, and leads directly into the intestinal tract. Due to the advent of these evolutionary adaptations, any differences in intestinal length between species with these advanced adaptations can act as a proxy for differences in efficiency.

My thesis research as part of the M.A. in Conservation Biology program at Columbia University will primarily focus on creating this potential hierarchy of efficiency and a phylogenetic comparison of the intestinal microbial communities across different adaptations. However, another central issue of reef conservation that reef herbivores represent is reef connectivity. What is connectivity? Connectivity as it applies to conservation practice refers to biotic and abiotic pathways between different habitats facilitating dispersal of organisms, larvae, and nutrients. In coral reef systems, connectivity is particularly critical as individual reefs are often separated from each other by vast spans of differing habitat at different depths. This concept is important to understanding reef health because it explains how one reef patch may rebound in the event of bleaching or local extinction of species, by using the other reefs in their network to reseed their communities. As such, the idea of connectivity is crucial to marine protected area (MPA) planning, which is moving away from large reserves to several small reserves connected to each other by corridors coinciding with known dispersal pathways of the reef system.

As such, to plan effective marine reserves a lot of research has focused recently on dispersal pathways for commercially important species of fish between their different life history stages. While this is highly effective for assessing how to best conserve fish and coral reef biodiversity, these are not the only indicators of reef health, although they are often billed as such due to their highly visible presence. However, more and more it seems that the foundation for which these two groups flourish from is a healthy diversity of microbial life, both in sediment and acting symbiotically with other organisms. However, almost no research has been done on how reef microbes disperse across reef systems despite their vital role in reef health and resilience.

This is where these reef herbivores may step in to help us fill these gaps in our knowledge of reef connectivity. The fact that these fish are not only harboring different symbiotic microbes, but also ingesting sediments (as in the parrotfishes and the Surgeonfishes) and incorporating microbes into their digestive system suggests that they may be potential dispersers of important reef microorganisms. However, my effectiveness in uncovering useful information about microbial connectivity will be primarily determined by my ability to get the most out of a single field season, coming up this summer. As anyone who has done fieldwork knows, this could have highly variable results! Much of fieldwork can go wrong very quickly, simply due to logistics and environmental factors. Even if all goes according to plan, it may take several passes through your experimental design and sampling method before you hit your stride, and are performing at maximal efficiency. As part of my attempt to assess microbial connectivity I will be trying to take sediment cores wherever I catch wild specimens, a field technique I have no previous experience with which could mean a lengthy learning curve!

Fortunately, thanks to some foresight by my PI*, I may have found a way to get a head start on these issues. Over this past summer, before I began my M.A. work, my advisor and his lab spent their field season in Fiji and brought me back some treats! These treats consisted of a whole parrotfish (important reef herbivores), sea grass it feeds on, and a sediment core collected from the same site as the parrotfish, and a water sample from the same site. This “whole picture” assembly gives me the opportunity to determine the level of environmental connectivity an herbivore has with its immediate surroundings. To get started on this, I spend part of this past winter break at a collaborators lab in Delaware** extracting and amplifying the microbial DNA from each component of the “whole picture,” after which the samples were sent off to the University of Maryland to be sequenced. Hopefully, having already completed this one trial will give me an edge going into the upcoming field season this summer. As a mechanical grinder, we expect this parrotfish to have at least some level of connectivity with its environment. However, the species I will be working with this summer will all be Surgeonfishes, which includes the other advanced adaptations of acid digestion and digestion with microbial symbionts as well. This means that even if this pre-trial goes well, the same treatment could generate completely different results for roughly two thirds of my data. Even if this trial was unsuccessful or of too poor a quality to be useful, the knowledge of what doesn’t work will be just as useful when preparing for collection this summer. Want to find out the fate of this parrotfish trial? Stay Tuned!


* I totally didn’t pay her in brownies to say that -JAD

** This work was done in Jen Biddle‘s lab. Jen is awesome and I’m excited she’s participating. Follow her on twitter (@subsurface_life). -JAD