Horseshoe Crabs Return to East Harbor, Cape Cod

East Harbor in Truro, Massachusetts, a former salt marsh lagoon in Cape Cod National Seashore, partly restored to the tides since 2002. NPS photo.

Driving Route 6 toward Provincetown, you cross a narrow strip of land beside a shallow lagoon called East Harbor. Under its calm, bird-dotted surface lives an ancient animal that people, birds and other sea life all depend on: the horseshoe crab.

A harbor cut off

East Harbor was naturally connected to Cape Cod Bay for hundreds of years, until a dike built for the railroad to Provincetown in 1868 cut it off. Its marine life died as it turned into a freshwater lake, later renamed Pilgrim Lake. The lake was not healthy: water quality declined, and in 2001 there were fish kills and insect outbreaks.

An 1868 map of East Harbor, showing the original inlet at the west end and salt marsh behind the barrier beach at Beach Point.

Alewives and other fish killed by oxygen loss in the water, summer 2001. NPS photo.

In 2002, Cape Cod National Seashore and the Town of Truro teamed up to let the tides back in through a culvert connecting the lake to Cape Cod Bay. Flushed daily by the tides, the lagoon became brackish again, its water quality improved, marine animals returned as salinity rose, and it took back the name East Harbor.

No one knew how much sea life would find its way in through a narrow 700-foot tunnel. Then, in 2010, park natural resource staff found molted horseshoe crab shells on East Harbor's beach.

Why horseshoe crabs matter

• To people: they are used as bait in the whelk (sea snail) fishery, and biomedical companies harvest their blood for Limulus Amebocyte Lysate (LAL), used to test the safety of all vaccines and implantable medical devices. There is not yet an approved synthetic substitute, though work toward one continues.
• To ecosystems: their eggs feed migrating shorebirds such as the federally threatened red knot, and adults are important coastal predators that keep worms, mollusks and other prey in check.

They are also extraordinary survivors. Horseshoe crabs first appeared about 440 million years ago, are not true crabs but close relatives of spiders and scorpions, are the last of their lineage, and have come through all five mass extinctions. Even so, many populations are declining from overharvesting and habitat loss. Cape Cod National Seashore is protected, so they cannot be harvested there.

Tracking the crabs

Their arrival was a surprise, since the only way in is through the culvert: at high tide, crabs from Cape Cod Bay can crawl in and make the 700-foot trip. To learn how the population uses a place free of harvesting, park staff and interns ran a two-year tagging project with researchers and graduate students from Antioch University New England in Keene, New Hampshire.

Park staff and interns tag horseshoe crabs in East Harbor. NPS photo.

The team waded the shallows to find crabs, recorded each one's size, sex and mating status along with the conditions and GPS location, fitted trackers suited to its age, and released it. They asked how many crabs live in East Harbor, how they use it through their lives, and whether they move in and out.

Tags are key to following the crabs' movements. NPS photo.

What they found

• A large population: around 890 crabs were tagged. The park's aquatic ecologist, Dr. Sophia Fox, had expected to be lucky to find 200.
• Every life stage, from larvae to adults, was present. Juveniles stayed mostly in the northeast of the lagoon; most adults and mating were in the southwest. Radio tracking found females throughout.
• Some crabs leave: despite the long culvert, at least a few moved out into Cape Cod Bay — three were detected by a receiver outside East Harbor — so the crabs keep the two waters ecologically connected.
• A longer mating season: body size matched other populations, but East Harbor's crabs were found paired well into October.

Crabs detected at each receiver; three pinged a receiver outside East Harbor after crawling out through the culvert. NPS map.

The crabs' success suggests they can rebound and thrive where they are protected. Monitoring at East Harbor will continue.

Sources

• National Park Service, Cape Cod National Seashore: "The Return of the Horseshoe Crabs!" by Charlotte Hohman.