Giant Purple Sea Slugs Spotted in Oakland


California Sea Hares - large purple sea slugs - have invaded Lake Merritt in Oakland. The video was captured from an OpenROV by a group who was testing out structure from motion techniques with the underwater drones.

This is a companion discussion topic for the original entry at


I’m just amazed anything can live in Lake Merritt!


So these are not native to the lake? I’m a bit unfamiliar with the area.


The tidal lagoon is surrounded by the city of oakland which unfortunately terminated many of its storm drains here. Visibility is often murky with Algae, tons of bird droppings, etc. The history on Wikipedia is actually interesting.

Because storm drains in downtown Oakland and surrounding areas drain directly into the lake, trash and nutrient pollution have become the largest problems affecting Lake Merritt. Increased levels of chemicals like nitrogen and phosphorus cause algae blooms, which deplete the water of oxygen. The fountains provide oxygen to about an acre around each one. A proposed aeration system was never approved by the City of Oakland. Trash removal is coordinated by the non-profit Lake Merritt Institute under contract with the City of Oakland. Regular Saturday and Tuesday volunteers, the Peralta Service Corporation (part of the Unity Council), St. Paul’s Episcopal School, and HandsOn Bay Area have been cleaning the trash since 1997. Schoolchildren and their science teachers volunteer by cleaning the lake with nets.

The Institute sponsors clean ups five days a week during the school year and four days a week during the summer. Individuals can also work alone using the four “U-Clean-It” boxes maintained by the Institute. About 1,000 to 5,000 pounds of trash are removed monthly.


How are the two ROVs diving together? The new neutrally buoyant tethers?