Archer, S.K., E.W. Stoner, and C.A. Layman. 2015. A complex interaction between a sponge (Halichondria melanadocia) and a seagrass (Thalassia testudinum) in a subtropical coastal ecosystem. Journal of Experimental Marine Biology and Ecology, 465(2015):33-40. http://dx.doi.org/10.1016/j.jembe.2015.01.003
Relationships can be complicated. This is equally true for the interactions between species in an ecosystem as it is for humans. Part of what makes some species interactions so complicated is that they can be “context dependent.” This means that the relationship between two different types of organisms will be different depending on the condition of the environment that they are in. Furthermore, the interaction may even change from mutually beneficial to both organisms to harmful to one of them if the conditions change! I will use the experiments by Archer et al. in this paper to describe an example of a complex, context dependent, relationship between turtle grass (Thalassia testudinum) and a sponge (Halichondria melanodocia) in near-shore habitats in the Bahamas.
Turtle grass is very common in the Caribbean, and like all seagrasses, it is a foundation species because it forms the foundation of some of the most productive and valuable ecosystems on the planet. The “foundation” of the ecosystem is formed because seagrasses are photosynthesizers, serving as the base of the food chain by making food from sunlight, and providing habitat for other organisms to attach to or hide from predators. Unfortunately, human disturbance has led to a decrease in seagrass abundance around the world.
Marine sponges are commonly mistaken as plants because they do not move. However, they are actually very simple invertebrate animals that filter feed on small organisms in the water. Because they do not move, sessile invertebrates like sponges need a place to attach themselves. The particular sponge in this study fancies the base or “shoots” of turtle grass as a place to settle down. Like us, sponges host a wide range of bacteria that live inside their bodies. Luckily for turtle grass, some of the bacteria living inside of sponges are capable of producing “bioavailable” nutrients for plants. This forms the base of the hypothesized relationship between turtle grass and the sponge that the researchers wanted to test – the seagrass gives the sponge a structure to attach to, and the sponge provides nutrients for the seagrass. But there is more to it than that…
This experiment was conducted at 6 different sites around the Bahamas. Each site contained 10 square meter study plots where the amount of seagrass and sponges were counted. Each of the 60 plots were visited multiple times to observe changes and measure rates of growth. The researchers wanted to test three hypothesized interactions: 1. Seagrasses benefit from nitrogen and phosphorus provided by sponges, 2. Seagrasses provide sponges with structure to attach to, and 3. Sponges prevent sunlight from reaching some portion of the seagrass, thus slowing their growth. All three hypotheses were tested by observing naturally occurring seagrass and sponge plots. Additionally, controls were set up to test how sponges and seagrasses responded to non-living physical representations of their counterparts. Fake seagrass beds were built out of mesh rugs and polypropylene ribbon to test the response of sponges to a non-living structure that is physically much like turtle grass, while dead sponges were placed around seagrass shoots to mimic the shading effect of live sponges, without the potential benefits of a live sponge.
Results and Significance
The results of this study are somewhat preliminary. Many of the results are suggestive of particular hypotheses, but more research needs to be done in order to sufficiently confirm these hypotheses.
Do sponges benefit from structure of seagrasses?
Yes, but only to a point. Sponges do not prefer overly dense clusters of seagrass (Figure 2). This trend was observed in both naturally occurring seagrass beds and the artificial controls. Overly dense seagrass beds are responsible for trapping a large amount of particles floating through the water and depositing them at the base of the plant. This process (known as sedimentation) reduces the ability of sponges to pump water through their bodies, which is how they feed. Therefore, it is hypothesized that overly dense seagrass beds are not good for sponges because excessive sedimentation decreases sponge feeding.
Do sponges limit the growth of seagrass by shading them?
Yes. Turtle grass growing with dead sponges placed around their shoots did not grow as well as turtle grass without any sponges around it (Figure 3).
Does seagrass benefit from nutrients provided by sponges?
Probably. This question is the most difficult to answer. If seagrass shaded by dead sponges do not grow as well as seagrass that is growing without any sponges around, then why does seagrass growing with live sponges seem to grow at about the same rate as seagrass without any sponges? Live sponges shade seagrass just as much as dead sponges – about 12-59% of the light-capturing region of seagrass may be shaded by a sponge. In order to have a net-neutral effect on seagrass growth, live sponges must be providing some sort of benefit to the seagrass that makes up for their shading. It is hypothesized that the benefit provided is nutrients from the sponge, but this was not explicitly detected by this study. More experiments would need to be done in order to conclusively determine this.
Why does this matter?
Because the interaction between seagrass and sponges is complex and context dependent, the relationship can shift as the environment changes. How much the seagrass benefits from the nutrients provided by the sponge depends on how much bioavailable nutrients are available in the water. How much the sponge shades the seagrass depends on the depth and clarity of the water and the height of the plant. These factors and more are all subject to change in the face of global climate change and continued coastal development. In order to understand how to protect an important organism like seagrass, it is important to understand the sometimes complex relationships between organism interactions and the mechanisms that drive these interactions.
Derrick is pursuing a Ph.D. in the Organismic and Evolutionary Biology Program at the University of Massachusetts Amherst. He is interested in anadromous fish migrations, how aquatic organisms interact with their physical environment, and the impact of human development on natural systems.