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Marine Lab finds sea slugs may restore seagrass beds

By Staff | Jun 24, 2020


Recently, the Sanibel-Captiva Conservation Foundation Marine Lab provided an update on annual seagrass surveys in the J.N. “Ding” Darling National 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,” Research Associate Mark Thompson said.

The sea slugs were known to live exclusively on Caulerpa algae, which had overgrown and eliminated the seagrass which had previously been abundant in the western and eastern impoundments of the refuge. The sparse research conducted on Oxynoe suggested that if they reach 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 refuge’s impoundments. First, the widespread blankets of Caulerpa algae were much reduced – in fact, there were only a few patches. On those few patches, very high numbers of Oxynoe were found again. Possibly, the Oxynoe had eaten the algae into scarcity. Although the algae was gone, the seagrass had not yet come back. The SCCF team’s research has led it to believe that it may find seagrass there next year since the algae has been reduced so much.

This year, the team also noticed that Caulerpa algae was more abundant outside and adjacent to the impoundments. The algae is covering seagrass there, and now they wonder if it will soon reduce the health of that seagrass community. However, they also found Oxynoe crawling all over the algae outside the impoundment. Last year, the team only found a few scattered sea slugs outside of the impoundments, but now there are very dense concentrations.

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

The SCCF noted that widespread algae blooms are caused by an elevated supply of the nutrients nitrogen and phosphorus that flow as runoff into local waterbodies.

“Too many homeowners, golf courses and communities are feeding the algae – and possibly the Oxynoe sea slug is benefiting from the mess they are making,” he said. “At least they are cute and cuddly.”