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Biology

Turtle manure and eelgrass seeds make for an aquatic garden!

Article: Diane C. Tulipani and Romuald N. Lipcius. 2014. “Evidence of Eelgrass (Zostera marina) Seed Dispersal by Northern Diamondback Terrapin (Malaclemys terrapin terrapin) in Lower Chesapeake Bay,” PLoS ONE 9(7): e103346. doi:10.1371/journal.pone.0103346

 

How are baby marine plants made?

 Plants, like animals, depend on reproduction to survive from generation to generation. Instead of giving birth or laying eggs, most plants depend on spreading their seeds either abiotically or biotically.

Figure 1: A young diamondback terrapin sea turtle. Credit: http://seaturtleexploration.com/wp-content/uploads/2014/06/diamondback-terrapin-1.JPG

Figure 1: A young diamondback terrapin sea turtle. Credit: http://seaturtleexploration.com/wp-content/uploads/2014/06/diamondback-terrapin-1.JPG

Abiotic dispersal uses physical methods such as wind and water currents to transport seeds to new places. This is something many of us have witnessed: just think about a dandelion blowing in the wind or an acorn falling from a tree. Biotic dispersal uses living organisms to move seeds around. A common method for biotic dispersal is through feces. Many species of reptiles accidently ingest seeds while foraging. Later on when they excrete waste, a few of those seeds remain viable. Just think, a seed got moved to a new location and even has some manure to kick start its growing process!

Eelgrass is one of the most common sea grasses in Chesapeake Bay and is a common habitat for young diamondback terrapins. In May 2009, researchers noticed eelgrass seeds in terrapin feces. This was an unexpected finding that prompted a study to answer the question: Are diamondback terrapins a biotic dispersal method for eel grass?

In this study, Tulipani and Lipcius aimed to discover how often seeds were found in terrapin feces, if they were capable of germination (growing), and the overall potential for seed dispersal in the lower Chesapeake Bay.

The Method

 

Figure 2: Regions in Chesapeake Bay where the eelgrass samples were collected in May 2010. Note that SAV stands from submerged aquatic vegetation, such as eelgrass. Image is modified from Orth et al., 2010.

Figure 2: Regions in Chesapeake Bay where the eelgrass samples were collected in May 2010. Note that SAV stands from submerged aquatic vegetation, such as eelgrass. Image is modified from Orth et al., 2010.

A total of 118 diamondback terrapins were ethically caught and uniquely marked in lower Chesapeake Bay in 2010 and 2011 (Figure 2). The researchers also had fecal material from the 2009 study which ignited this study. Once captured, the turtles were measured for various metrics (such as head width and shell length) and gender was determined by tail and claw size. (Male turtles, for example, have longer claws than females).

 

Terrapins were placed separately in fresh water buckets and not fed to induce excrement. The feces was sieved, air dried, and then searched for eelgrass seeds. Seeds were considered viable if they were firm when gentle squeezed with tweezers. Eelgrass seeds from the field were also collected for comparison.

 The Findings

 Of all the diamondback terrapin feces collected, 92% contained pieces of eelgrass. This means that the turtles were prowling in eelgrass beds to search for food. Interestingly, the probability that eelgrass seeds would be in the collected feces depended on the turtle’s head size. Smaller turtles had more seeds in their feces than larger turtles; eelgrass seeds were found in 33% of small male feces, 35% of small female feces, yet only 6% in large female feces. (No large males were captured.) Smaller terrapins typically prefer to reside in shallow waters, such as eelgrass beds, so there is an understandable link between turtle size and accidental seed ingestion. On average, the researchers found 2.4 seeds per terrapin.

In the 2010 turtle collection, 39.3% of the seeds found were considered viable while 26.3% were called viable from the 2011 sample. This translates to 1,341 to 1,677 eelgrass seeds being spread in this region each year. While this is a small fraction of the overall eelgrass seed dispersal, it is still important and has the potential to spread seeds to new eelgrass beds.

 Significance

 This is the first study to demonstrate the diamondback terrapin as a biotic dispersal mechanism for eelgrass seeds. While younger turtles, who contained more seeds per fecal sample than larger turtles, have a smaller travel range, this study explains a new mechanism of spreading eelgrass seeds. Small turtles have been measured to travel up to 5.7 km, suggesting that this biotic dispersal method could spread eelgrass seeds within established eelgrass beds and to new beds. This helps increase the genetic diversity of eelgrass.

The diamondback terrapin and eelgrass is a wonderful example of a mutualistic relationship. Turtles gain nutrients and shelter within the eelgrass beds while the eelgrass gets to increase its genetic diversity by having the turtles carry its seeds to new and exciting places.

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