Benthic animals ride out innovative lab experiment 

Dog cockles and sponges are widely distributed around our coasts, where they are important in structuring communities and ecosystems. Recent laboratory experiments show that some populations of these invertebrates are resilient to high levels of suspended sediment.

It’s important work because there’s generally a poor understanding of the biological responses of benthic creatures to sea floor sediment stirred up by storms, seabed dredging and fishing. 

Malcolm Clark and Vonda Cummings shared these results in a recent webinar.  

Technically challenging

An innovative chamber system was developed especially for the project as the team needed to keep high and consistent levels of sediment suspended for several weeks. The experiment also maintained water flows, oxygen, temperature and pH levels similar to the waters from which the animals were collected.

Two species (one a cockle and one a sponge) from the west and south coast of central New Zealand were placed in 16 sea water chambers with suspended sediment and monitored for 6 weeks. There were no strong negative effects observed and survival rates were high.  

Sixteen 37-litre chambers were filled with sea water and concentrations of sediment ranging from none to very high. Cockles and sponges were placed in the chambers and sampled after one, three and four weeks. They were then given a two-week break in clean water. The researchers measured condition and body health – looking for evidence of sediments accumulating internally and externally. 

Only one cockle died during the six-week experimental period. And that was after four weeks exposure to the highest concentration. Cockles were observed puffing out the sediment. Vonda says despite sediment accumulating on gills, there was no evidence of respiration being affected. 

Three sponges died in the low to intermediate sediment concentration range. Evidence of internal sediment accumulation emerged during the first week. But at the end of the experiment after two weeks in clean water it had been fully expelled in a third of the animals. Some morphological changes were noticed.  

“Both species had mechanisms to clear the sediments, especially once the sediment source was removed. The conditions in the environments they had been collected from, where suspended sediment levels can become elevated for various reasons, may have predisposed these individuals to coping, at least over the timeframes and conditions we investigated in these experiments,” says Vonda. 

Malcolm says the findings have given researchers fresh insight into the resilience of these two deep sea species. Equally as important, there’s now a robust and well-tested lab system for maintaining concentrations of suspended sediment 

“Our experience [running this experiment] has already been picked up by NIWA research projects investigating tolerance of juvenile scallops to suspended sediments and the ROBES Endeavour Fund project looking at the resilience of deep sea sponge and coral species from the Chatham Rise.”

WATCH THE WEBINAR 

Find out more about the Sediment tolerance and mortality thresholds of benthic habitats project