Sound controls

Scientists use sound to lure baby oysters to reef | Summary TS

IIt’s been two difficult centuries to be an oyster. Although archaeological evidence suggests humans have been actively eating and farming shellfish for thousands of years, oyster farming really accelerated with the industrial revolution. Their meat was one of the cheapest sources of protein in rapidly growing 19th-century coastal cities around the world, while their shells were made into concrete and other materials for building roads and buildings. Oyster populations plummeted, the fishery collapsed and the once overabundant staple became a rare delicacy in less than 200 years.

The flat oyster (Ostria angasi) is one such victim. Endemic to the waters off southern Australia and Tasmania, the species was decimated to near extinction during the colonial period. Where there were once vast reefs formed from the shells of generations growing on top of each other, now only scattered living individuals remain. With so few adults in the wild, initial restoration efforts — in which researchers build the beginnings of reef structures on rocky terrain favorable to oysters — have been hampered by the slow recruitment of larvae to these areas. But a new study points to hope from an unexpected source: sound.

A team from the South Seas Ecology Laboratories at the University of Adelaide recently placed speakers on the restoration project reefs, hoping that the sounds of a healthy reef ecosystem would attract oyster larvae. There had been studies suggesting that oyster larvae in the waters off North Carolina could drop vertically onto a reef in the presence of navigational cues such as sound, even if they lack ears or any obvious sound detection device. But the question remained: Could they actively swim towards a robust sound reef?

Marine life uses sound like humans use vision.

—Jennifer Miksis-Olds, University of New Hampshire

To create a potentially engaging oyster playlist, the team recorded sounds from Port Noarlunga Reef, a nearby marine protected area with healthy reefs, as well as ambient sound from a sedimentary seabed with no oysters as a control. . Then, “we took this kind of leap of faith that we could put these speakers down and attract the oysters,” says study co-author Sean Connell, a restoration ecologist in Adelaide. “What we’re seeing is that we can actually increase the recruitment of oysters to these reefs a thousandfold.”

Specifically, the team found that Olympia oyster larvae, which are only 170 to 189 μm long (about the width of a human hair), could and did orient themselves based on reading sound from Port Noarlunga Reef, swimming horizontally in the direction of the responders both in the field and in laboratory tank experiments set up to allow more precise monitoring of the behavior of individuals.

Oyster reefs are formed as molluscs grow on a rocky underwater surface, each generation attaching itself to the shells of the previous one.

The result has Connell excited about the implications for oyster reef restoration, not just in Australia but around the world. However, he notes that there are still many unanswered questions about the technique’s potential. A clear impediment to the recruitment effect is the presence of anthropogenic noise. Although some of the reefs used in the study were remote and calm, one was located near a busy seaport. “Very high background noise, with motorboats, pile driving, even road noise, seems to attenuate the signal, and we don’t get an increase in recruitment,” says Connell. On such sites, “there is no difference between our sound processing and our controls”.

Jennifer Miksis-Olds, director of the Center for Acoustics Research and Education at the University of New Hampshire, who was not involved in the work, is not surprised that even such a small creature, devoid of ears or organs equivalents, uses sound for orientation. . “Marine life uses sound like humans use vision,” she explains. “Acoustics is the main sensory modality for the majority of marine life, because acoustic signals travel so fast and so far underwater compared to other [signals] such as light or chemicals.

Miksis-Olds praises the Australian team’s study on two points. “First, they did lab studies that informed their field studies,” she says. “I also really liked the combination of basic science and applied science involved in this study. . . . Understanding that these larvae can swim both horizontally and vertically has really contributed to our basic science knowledge, while the use of a speaker on the reef is a very good applied use of these findings that will improve conservation efforts and restoration.

Stephanie Westby, who leads the National Oceanic and Atmospheric Administration (NOAA)-run oyster reef restoration program in the Chesapeake Bay, is also interested in the technique’s potential to increase oyster recruitment. Like the flat oyster, the oriental oyster originating from Chesapeake (Crassostrea virginica) has been brought to the brink of local extinction over the past 200 years, with disastrous consequences for other species that share the largest estuary in the United States.

Divers take part in a project designed to bring oysters back to Australian reefs.

Divers take part in a project designed to bring oysters back to Australian reefs.

COURTESY OF SEAN CONNEL

“The estimates we have for pre-colonial times in the Chesapeake Bay put us at about 1% of our historical level. And if you lose ninety-nine percent of your oysters and your oyster reefs, you’re going to lose ninety-nine percent of the ecosystem services that go with it,” says Westby, who doesn’t did not participate in Connell’s study. These services include water filtration – a single adult oyster can filter up to 50 gallons of water each day, removing plankton and excess algae – and the habitat that the shells accumulated by oysters provide. later to fish, crabs and other marine life.

Westby says she thinks the Adelaide team’s results could have an impact in the Chesapeake if they can be replicated with Oriental oysters. “Anyway, we’ve seen promising results on the south Atlantic coast of the United States,” says Westby, referring to the North Carolina oyster study. Similar work in the Chesapeake by other groups is likely in the near future, she adds. “Promising all around, that’s how I would describe it.”