Crabs in the Cove: 25 Years of Snail Shells

Measuring Shells at Kestrel Cove

1Ms. Folasade Wexley's eighth grade class took a marine biology field trip to the tide pools at Kestrel Cove, where scientists from a small marine lab have studied the rocky shore for more than 30 years. At low tide, the students found hundreds of small sea snails grazing on seaweed. Dr. Wren Ashdown, a lab scientist, showed them how to measure the thickness of each shell at its opening, called the lip, with a fine caliper before putting the snail back where it was found. Student Arjun Petrakis noticed that most of the shells he measured felt heavy and thick.

2Dr. Ashdown explained that green crabs, which are not native to this coast, were first seen in Kestrel Cove in 1995. A green crab eats a snail by crushing its shell with its claws. Most of these snails live a few years and start to breed at about one year old, so a new generation hatches about every year. The lab calls a snail thick-shelled when its lip is 1.0 mm or more.

3Every five years since 1990, the lab has measured randomly chosen snails in Kestrel Cove and in Gannet Cove, a similar rocky cove 40 km up the coast where green crabs have not been found. The snails were measured the same way and at the same time of year each time. Table 1 shows the results. The histograms show every measurement from Kestrel Cove in 1990 and in 2015.

4The lab has also run two tank tests. In the first, green crabs were kept in tanks with a total of 20 thin-shelled snails and 20 thick-shelled snails. After two days, the crabs had eaten 14 of the thin-shelled snails and 3 of the thick-shelled snails. In the second test, the lab raised young snails in tanks with no crabs and no water from crab tanks. Most young from thick-shelled parents grew thick shells, and most young from thin-shelled parents grew thin shells.