Sea cucumbers may save Great Barrier Reef

Tropical sea cucumbers and their faeces could save coral reefs from the harmful impacts of climate change, scientists have found.
Scientists at One Tree Island, the University of Sydney’s research station on the Great Barrier Reef, say sea cucumbers reduce the impact of ocean acidification on coral growth.
“When they ingest sand, the natural digestive processes in the sea cucumber’s gut increases the pH levels of the water on the reef where they defecate,” Tree Island director professor Maria Byrne said.
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This works to counter the negative effects of ocean acidification.
One of the by-products when sea cucumbers digest sand is also calcium carbonate (CaCO3), which is a key component of coral.
“To survive, coral reefs must accumulate CaCO3 at a rate greater than or equal to the CaCO3 that is eroded from the reef,” Professor Byrne said.
“The research at One Tree Island showed that, in a healthy reef, dissolution of calcium carbonate sediment by sea cucumbers and other bioeroders appears to be an important component of the natural calcium carbonate turnover.”
The ammonia waste produced when sea cucumbers digest sand also serves to fertilise the surrounding area, providing nutrients for coral growth.
Sea cucumbers are among the largest invertebrates found on tropical reefs.
About 30 species are commercially harvested by the fishery industry along the Great Barrier Reef and throughout the tropics.
“We urgently need to understand the impact of removing sea cucumbers and other invertebrates on reef health and resilience at a time when reefs face an uncertain future,” Professor Byrne said.

Oyster shell dissolution rates in estuarine waters: effects of pH and shell legacy

Oyster shell is a crucial component of healthy oyster reefs. Shell planting has been a main component of oyster restoration efforts in many habitats and has been carried out on scales from individual and grassroots efforts to multiagency efforts across entire estuaries. However, the cycling and lifetime of the shell that makes up the bulk of an oyster reef has only recently received attention, and most of the work to date has focused on the role of epi- and endobionts on shell degradation. Here we report findings from a laboratory study in which we manipulated pH in a flow-through control system using water from the mesohaline mouth of the Patuxent River to measure dissolution rates of intact oyster shell. Shells from the Eastern oyster (Crassostrea virginica Gmelin 1791) with three different legacies were exposed to 4 levels of pH that encompass a range typical of the mesohaline waters of the Chesapeake Bay (7.2–7.9 on the NBS scale). Mass loss over a 2-wk period was used to measure dissolution rate on 3 shell legacies: fresh, weathered, and dredged. We found that pH and shell legacy had significant effects on shell dissolution rate, with lower pH increasing dissolution rate. Fresh shell had the highest dissolution rate, followed by weathered then dredged shell. Dissolution rates were significantly different among all 4 pH treatments, except between the lowest (7.2) and the next lowest (7.4); however, shells lost mass even under noncorrosive conditions (7.9). We discuss the implications of our findings to ongoing efforts to understand shell budgets and cycling in oyster reef habitat, the interaction of biological and geochemical agents of shell degradation, and the complexity associated with shell carbonate cycling in the unique milieu of the oyster reef.