Week 3: Some Background Information

This past week we caught 12 new diamondback terrapins at Queens Creek, which branches off of the York River.  We followed the same procedure as the previous week, measuring and marking each of them.  Since nothing too eventful happened, I thought I’d give you all some background information on diamondback terrapins.

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This guy didn’t want to come out of his shell for the picture

Diamondback terrapins are the only North American turtle that lives exclusively in brackish water (which is slightly salty but not as salty as an ocean, for example).  They have a relatively long lifespan, with the oldest individuals over 20 years old.  There are various populations of subspecies found from Cape Cod, Massachusetts to Corpus Christi, Texas.  The subspecies in this area is Malaclemys terrapin terrapin.  They feed on a variety of invertebrates, such as snails, clams, mussels, crabs, worms, and small fish.

The growth rings are visible on this terrapin's shell

The growth rings are visible on this terrapin’s shell

Adult females lay 10-30 eggs per year, depending on their age.  The younger breeding females lay one clutch of about 10 eggs per year, and the older breeding females lay two or three clutches of roughly 10 eggs each in a year.  However, only about 10% of the eggs that are laid survive to hatch.  This is an example of r-selection, in which an organism produces a large number of offspring but gives them little care, hoping that some survive.

Terrapins are sexually dimorphic, meaning that the females and males look different.  The females are larger, growing to a maximum of 6-8 inches in length.  They also have rounder shells and heads, and short, skinny tails.  The males grow to a maximum of 3-5 inches and have long, wide tails.

Terrapins were considered an inexpensive pest until the 1800s.  There were so many of them that they’d get caught up in fishing nets and weigh the nets down till they broke.  In colonial times, a wagonload of terrapins, used to make meals such as terrapin soup, could be purchased for $1.  As they became more and more rare, terrapins began to be treated as a delicacy.  And by the early 1900’s they could cost as much as $125 per dozen.

Diamondback terrapins are a keystone species, playing a large role in determining the structure of their communities.  As I mentioned above, they eat invertebrates, including snails of the genus Littorina and the genus Melampus.  These snails in turn consume the Spartina marsh grasses, such as those present around the creek where I’ve been conducting my fieldwork.  In the absence of the terrapins, the snail populations would increase dramatically and eat all of the marsh grass, destroying the habitat of numerous other organisms.

The marsh grass around Queens Creek

The marsh grass around Queens Creek

We’re not just working with these terrapins because of how cute they are.  We’re trying to make sure the terrapin populations aren’t declining far enough for this to happen, and we’re identifying factors in the mathematical model that would cause their populations to decline to dangerous levels.  Hopefully, our results will show that the terrapins in this area are doing just fine, unlike some other areas along the Atlantic coast.

–          Sarah Gilliand

Sources:

—1. Ayers, C., P. Tian, and R. Chambers.  “A Matrix Model of Human Impacts on Diamondback Terrapin Populations.” 2001. Unpublished.
—2. Brennessel, B. Diamonds in the Marsh: A Natural History of the Diamondback Terrapin. Hanover: University of New England, 2006. Print.
—3. “Diamondback Terrapin: The Brackish Water Turtle.” Vimeo. UAB Media Studies, n.d. Web. <http://vimeo.com/23662117>.
—4. Dorcas, M. E., J. D. Willson, and J. W. Gibbons. “Crab Trapping Causes Population Decline and Demographic Changes in Diamondback Terrapins over Two Decades.” Biological Conservation 137.3 (2007): 334-40. Print.