Marine Lab Finds Sea Slugs May Restore Seagrass

17 Jun 2020

A few weeks ago, the Marine Lab provided an update on annual seagrass surveys in the J.N. “Ding” Darling Wildlife Refuge. In the report, staff explained that last year during surveys they discovered dense concentrations of sea slugs called Oxynoe. 

“In fact, they were the highest density of that type sea slug ever found,” said Research Associate Mark Thompson.

Those sea slugs were known to live exclusively on the Caulerpa algae, which had overgrown and eliminated seagrass which had previously been abundant within the western and eastern impoundments in the refuge. The sparse research that has been done on the Oxynoe sea slug suggests that if they reached high enough numbers they could actually reduce the amount of Caulerpa algae in an area. 

Surveys this year showed major changes in survey areas around the impoundments. First, the widespread blankets of Caulerpa algae were much reduced inside the impoundments – in fact, they found only a few patches there. But on those few patches, they found very high numbers of Oxynoe again. Possibly the Oxynoe had eaten the algae into scarcity. Although the algae was gone, the seagrass had not yet come back. Their research has led them to believe that they may find seagrass there next year since the algae has been reduced so much.

This year, they also noticed Caulerpa algae was more abundant outside and adjacent to the impoundments. The algae was covering seagrass there and now they wonder if it will soon reduce the health of that seagrass community. But they also found the Oxynoe sea slug crawling all over the algae outside the impoundment. 

Last year, they only found a few scattered sea slugs outside the impoundments but now there are very dense concentrations there. 

“It seems like a pattern. Where the Caulerpa algae blooms – sea slugs follow,” said Thompson. “We will soon see if the sea slugs can reduce the algae enough to prevent the disappearance of the seagrass there.”

As a reminder, widespread algae blooms are caused by an elevated supply of the nutrients nitrogen and phosphorus to our waterbodies. 

“We are feeding the algae – and possibly the Oxynoe sea slug is benefiting from the mess we are making,” said Thompson. “At least they are cute and cuddly.”