Sometime in the 1970s, the muddy bottom of Sandy Hook Bay, New Jersey became home to dense mats of tubes built by the amphipod Ampelisca abdita, a tiny crustacean. Local fishermen, who had found only small quantities of quahogs living around the sand edges of the bay, now found abundant quahogs to harvest throughout the bay’s roughly 4,500 acres of muddy bottom.
The amphipod tubes were about an inch and a half long and extended just above the bottom in a dense “meadow”. Each was built by an individual animal, but they were so close together they covered nearly the entire central bay bottom. Long-time shellfish researcher Clyde Mackenzie of the Northeast Fisheries Science Center’s (NEFSC) James J. Howard Marine Sciences Laboratory at Sandy Hook says about 30 boats with a single raker would each harvest six or more bushels of littlenecks, cherrystones and larger chowder-sized quahogs each day from the late 1980s through the early 2000s. That was unusual, since quahogs are typically found in sandy or muddy-sand sediments and are generally scarce in soft mud.
“In 2000, the population density of quahogs was about 15 times higher in the mud habitat than in the sand habitat around the edges of the bay,” said MacKenzie. “The mud habitat also had a dense population of the amphipod Ampelisca abdita (about 24,000 per square meter) associated with it, which the sand habitat did not have.” In addition, the principal quahog predators – the longwrist hermit crab, Atlantic oyster drill, and xanthid mud crabs – were scarce in the mud habitat, but seven times more abundant in the sand habitat.
Field surveys and laboratory studies from 1999 to 2005 were conducted by Mackenzie and colleagues Robert Pikanowski and Donald McMillan from the NEFSC’s Sandy Hook Lab to compare the characteristics of mud and sand habitats in relation to the abundance of the northern quahog (Mercenaria mercenaria). The researchers suggested that the tube mats encouraged settlement of larval quahogs and reduced water flow that prevented the tiniest quahog seed from being buried in the mud surface by the bay’s swift currents. The mats also reduced water siltation allowing the juvenile quahogs to feed and reduced predation, and made a once unsuitable habitat for quahogs a good place to be.
Then something happened.
MacKenzie has surveyed the bay for the presence of the Ampelisca amphipods and their tubes since 2005, using the same grab sampling equipment. While dense mats were present each year through 2010, they disappeared sometime in 2011 and have not returned.
“This species, which once covered the full extent of the bay in a nearly continuous meadow, has become scarce,” MacKenzie said. “I found only a small trace in the northeast corner of the bay this year, but nothing in the remaining areas.”
The once muddy bay bottom now consists of silt, clay and benthic algae. Since 2013 fishermen have found little or no quahog seed in the area, and in 2014 and 2015 almost no littlenecks. The handful of fishermen still harvesting in the bay find only the large chowder quahogs, the same size they have been returning to the bottom for years due to low consumer demand.
“We cannot explain why the amphipods disappeared during 2011 and have not come back,” Mackenzie said. “We know it isn’t from the effects of superstorm Sandy because that occurred in October 2012. It seems clear that the quahogs were able to live in the bay’s mud because of the presence of the dense mats of Ampelisca tubes, and once the animals and their tubes left so did the quahogs.”
As to why they are no longer there, MacKenzie has some ideas. “It may be a consequence of the changing climate and weather affecting marine organisms. A new generation of Ampelisca had been appearing every January; there are three generations a year, spaced four months apart. Records show that January 2015 was very cold and January 2017 was very warm. Perhaps the temperature extremes affected them adversely. Only future tests will tell for sure.”
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