Crab Count: The Results are In

After a summer of hard work pulling up traps, turning over rocks, and measuring fiesty crabs, it's now time for the fun part: putting all of our data together and looking at the results!

First, some basic numbers:

• In total, we caught and measured 493 individual crabs this summer!
• 270 were Asian shore crabs
• 130 were green crabs
• 89 were red rock crabs
• 4 were spider crabs
• 300 were male and 193 were female
• They ranged in size from a tiny 4mm to a whopping 12.04 cm

From our total numbers, it certainly appears as if invasive Asian shore crabs and green crabs are the dominate species in Boston Harbor, but let's take a closer look at our individual sites.

Asian shore crabs, the most dominant species in our total, are actually only the most common species at two of our six sites: Camp Harbor View on Long Island and the Harry McDonough Sailing Center in South Boston. These sites are both rocky and gravely beaches where we searched for crabs in the intertidal zone (the area between the high and low tide marks which is sumberged for only part of the day). Asian shore crabs do very well in the intertidal zone because they are better than other crab species at tolerating the dryer conditions of this habitat.
Black's Creek in Quincy is also a gravely beach where we searched for crabs in the intertidal zone, but we found almost exclusively green crabs at this site. Why no Asian shore crabs?
Black's Creek is a Salt Marsh Estuary where fresh water mixes with ocean water and lowers the salinity. Green crabs are very tolerant of a wide range of different salinities, but Asian shore crabs can't survive as well in the low salinity of Black's Creek.
Green crabs are dominant at Piers Park Sailing Center in East Boston, Courageous Sailing Center in Charlestown, and in Fort Point Channel at the Boston Children's Museum as well. These crabs were introduced to New England in the early 1800's; their quick reproduction and adaptability to a wide range of conditions have allowed them to easily outcompete native crabs, so it is no surprise that we found so many of them.
At Piers Park, Courageous, and the Children's Museum, we caught crabs from the subtidal zone (the area that is always submerged, even at low tide) using traps. At these sites, we saw two additional spcies of crabs that are not as tolerant of the dryer intertidal zone. Red rock crabs were common at Piers Park and Courageous, and spider crabs were found at Piers Park and the Children's Museum. Although we never saw the sea floor at these locations, the species of crabs that we found at each of them can actually tell us a little bit about what the bottom is like! Red Rock crabs (as you can probably guess from the name) like habitats with lots of rocks, while spider crabs (a type of mud crab), prefer muddy bottoms. Based on the data we collected, Courageous probably has a more rocky bottom, Fort Point Channel has a more muddy bottom and Piers Park likely has a mixture of both.
This is only a small example of the kinds of things we can learn from the crab count information we've collected this summer. We'll be sending all of our data along to Dr. Judy Pederson at MIT who will use it to study the spread of invasive species. Hopefully, she'll keep us posted on what she finds!
-Paula

Citizen Scientists at Save the Harbor/Save the Bay

Many hands make for light work, as the saying goes—but in the case of researchers studying invasive species, the many hands and eyes they borrow from Citizen Scientists make it possible for a few researchers to study huge stretches of coastline. Dr. Judith Pederson, a professor with MIT’s Sea Grant College Program, has enlisted the help of Save the Harbor/Save the Bay’s Boston Harbor Explorers togather data on the abundance, density, size and gender of invasive crabs in Boston Harbor. Dr. Judy and her fellow researchers put this data to use in several important ways, making our Boston Harbor Explorers part of a scientific pursuit that’s very important and widespread.

First, field observations by citizen scientists often yield new and unexpected observations. Citizen scientists, by virtue of the sheer volume of data they can collect, are a wonderful resource for tracking the spread of invasive organisms. In a study of invasive crabs conducted in 2005, a volunteer Citizen Scientist recorded a 40-mile northward range expansion by Asian Shore Crabs in Maine (Delaney et al. 2008). In layman’s terms, an amateur scientist discovered an Asian Shore Crab 40 miles farther north than anyone had ever seen one before! 40 miles is an incredible distance for a crab that rarely grows larger than a few inches across. The fact that Asian Shore Crabs were able to spread so quickly is a testament to how adaptable and prolific these invaders are.

Second, each additional Citizen Scientist gives researchers another pair of eyes and hands to observe and record new data. Scientists can then use the data that Citizen Scientists collect to produce models of how invasive organisms reproduce and establish themselves in their

new habitats; these detailed computer models take into account features like water temperature, salinity, and the direction of ocean currents to estimate how quickly an invasive species will establish itself in a new environment. Understanding the population dynamics of an invasive species is an essential step before the invader can be contained or eradicated.

Together, these two facets of Citizen Science—novel observations combined with data collection—make this approach ideally suited to tackling the challenges presented by invasive species. The earlier that scientists notice that an invasive species has spread to and established itself in an area, the easier it is to eradicate the invader. The longer that government agencies take to act on an invasive species threat, the more time the invader has to reproduce and expand its range. The fact that Citizen Scientists can be trained to recognize new invasive species means they can assist researchers and public agencies in stopping invaders before they become too plentiful in their new habitats. To that end, the “Hitchhikers” guide that MIT Sea Grant publishes includes several anticipated invaders in addition to the introduced species that are already commonplace on New England’s shores. Our Boston Harbor Explorers learn how to recognize not only the invaders that are already here, but also any new species that have yet to gain a foothold here in the Boston Harbor.

Knowledge is power. Learning how to recognize and prevent the spread of invasive species enables our Boston Harbor Explorers to take an active role in preserving the harbor’s native species for future generations.

Cheers and Calipers,

Aaron

For more information, you can check out the following papers and publications:

Delancy, DG et al. Marine invasive species: validation of citizen science and implications for national monitoring networks." Biological Invasions: V10, pp. 117-128, 2008.

You can also check out Dr. Judith Pederson's "Hitchhikers Guide to Exotic Species," available through the MIT Sea Grant Bioinvaders Page.