Blue claw crabs on Martha’s Vineyard

Eva Faber (left) and Luanne Johnson 
prepare to measure water quality —  including temperature, salinity, clarity  — in Tisbury Great Pond.
File photo by Dan Cabot

Eva Faber (left) and Luanne Johnson
prepare to measure water quality —  including temperature, salinity, clarity — in Tisbury Great Pond.

Eva Faber reached a gloved hand cautiously into the trap, where half a dozen blue claw crabs clung tenaciously to the wire framework. She was trying to coax a large female to release its hold, which it finally did — in order to flip itself backwards and give her hand a sharp nip with its powerful pincers. Blue crabs are surprisingly agile, and a pinch hurts.

Eva, who will be a junior at Martha’s Vineyard Regional High School, has won a grant from the Marjot Foundation of Falmouth to study blue claw crabs in Edgartown and Tisbury Great Ponds. She will work with MVC water expert Bill Wilcox and Vineyard wildlife biologist Luanne Johnson through the summer and into the fall.

The Marjot Foundation’s mission is to support independent research in environmental science by students in public high schools. Grant winners must compete in state science fairs and must present their work to trustees, donors, and members of the Marjot scientific advisory board. Eva’s is the first Marjot grant to a Martha’s Vineyard student.

Blue claws in the great ponds

Although they have been here as long as anyone can remember, not much is known for certain about the Vineyard’s blue claw crabs. Writing in a report funded by the Riparian Owners of Tisbury Great Pond, Mr. Wilcox observed in 2004: “From anecdotal evidence, the population of blue crabs in the Great Ponds is highly variable over time. Unfortunately, there are no hard local statistics available on the population or harvest.”

Although the life cycle of crabs is in general well known, there are mysteries about how that plays out in the Vineyard’s south shore ponds, which are artificially opened to the sea.

Eva explains that female crabs store sperm two to nine months after mating, fertilizing eggs from stored sperm, and releasing them in water with high salinity. The eggs require a salinity of 20 parts per thousand, or more, which is why females are observed to gather inside the barrier beaches of Edgartown and Tisbury great ponds, where the water is saltiest, perhaps waiting for the pond to be opened. The males, Mr. Wilcox believes, never leave the ponds, staying near the northern shores where the water is least salty. But, he said, no one knows whether the females need to exit the ponds to the sea or whether they just release their eggs on the outgoing tide. Edgartown and Tisbury Great Ponds are opened at unpredictable times, and once opened, they may stay open for weeks or only for a few days, depending on winds and storms.

After hatching, according to Mr. Wilcox’s report, the crab larvae spend 30 to 45 days at sea, where they pass through a number of life stages as one of many kinds of plankton. When only about one millimeter wide, they return to ponds on an incoming tide, seeking brackish water. Mr. Wilcox told The Times that it is unlikely that the miniature crabs that enter the ponds are the offspring of the pond that produced them, but he speculates that the variations in blue claw crab populations from year to year are proportional to the number of days the ponds remain open to the sea. However, it is not known whether a long pond opening is required or only that the opening be at a propitious time in the crabs’ life cycle. Does the pressure of time sometimes force females to release their eggs inside the ponds if no ocean channel is available? Could the eggs hatch and the larvae survive within a pond? Can females produce a second crop of eggs?

In her grant proposal to the Marjot Foundation, Eva wrote: “I would like to get a better understanding of how mechanical pond openings influence blue crab abundance, distribution, and recruitment…. A significant food source for other animals, such as otters, and a vital part of pond ecology on Martha’s Vineyard, it is important to know how blue crabs are influenced by pond openings.”

The study will run from May to November of this year. Working with Ms. Johnson, Eva will chart water temperature, salinity, and dissolved oxygen in various parts of the two ponds; she will also trap, sex, and measure crabs in areas of the ponds with different salinity, both before and after the ponds are opened to the sea. Working with Mr. Wilcox, she will inspect plankton nets towed in the ponds to look for crab larvae, both before and after the ponds are opened. The Marjot Foundation provides significant funds for materials and equipment, as well as stipends for young researchers and their mentors.

If Eva can answer some of the questions about Vineyard blue claw crabs, it will be worth a few pinched fingers.

The genesis of a grant

Ms. Johnson, who is completing a PhD in environmental studies at Antioch University New England, has worked for many years as a biologist and educator on the Island. A wildlife biologist generally supports herself with grants, and Ms. Johnson’s current grant involves, among other things, a study of the Island’s river otters. Because blue claw crab swimmerets show up in otter scat, she knows that otters eat crabs, and so she was led to a report on the crabs written by Mr. Wilcox. From reading that report she learned that much research needs to be done on blue claw crabs.

A chance encounter with a member of the Marjot Foundation on the ferry sparked a chain of relationships. Ms. Johnson learned about the foundation and its mission to high school students. Because she has often worked with Island elementary school science teachers, she has long known David Faber, a middle school science teacher at the Edgartown School, and his daughter, Eva. She worked with Eva last year when Eva won the MVRHS science fair with a project on the effect of smoke on grass-plain seed germination. She thought that Eva, a hard worker and an ingenious scientist, might be the Vineyard’s first Marjot grant recipient. Ms. Johnson approached Mr. Wilcox about the project and persuaded Eva to apply.

Ms. Johnson noted that last year’s science fair project was a lab study, in which Eva planted wild-collected grass plain seeds in a terrarium and introduced smoke from a beekeeper’s smoker. The blue claw study is a field study and has a different set of challenges, including getting pinched by one of the subjects.

Eva told The Times that she plans to pursue some kind of science in college and beyond, but she has no idea what kind of science she’ll wind up doing, considering everything from medicine to research. It will definitely be science. “Science has always been my love,” she said.