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Grand Side-Gilled Slug Takes a Day Trip
As large as a dinner plate, the Grand side-gilled slug delights divers during night dives. But during the last day of 2025, one crawled across the bottom in daylight.
A Nocturnal, Giant Sea Slug
Most divers are familiar with nudibranchs. They have feathery gills exposed on the top of their bodies. Side-gilled sea slugs are different. Their large, plume-like gills hide between the mantle and the foot. They are usually tucked along the right side of the body. This unique arrangement gives the group its common name.
Grand side-gilled sea slugs live primarily on shallow sand and rubble bottoms. They are almost exclusively nocturnal. This behavior makes them ideal targets for sightings during night dives. After dark, they crawl across the sea floor, actively hunting tunicates, anemones, and other invertebrates. With strong jaws and a wide mouth, this species can even consume sleeping fish.
Like other members of this group, the Grand side-gilled slug has a pair of rolled rhinophores. These detect chemical cues and water movement, helping to locate prey in the absence of light. When threatened, it can secrete sulfuric acid as a chemical defense. As a result, most predators avoid this giant slug.
The Grand Side-Gilled Slug
The Grand side-gilled slug is the most commonly seen species of its kind in Gorontalo. It can grow to an astonishing 21 centimeters. It may be the largest sea slug divers are likely to encounter. Its scientific name is Pleurobranchus grandis.
Its coloration varies widely. However, the species usually displays three dark bands that contrast sharply with the rest of its body. When moving, the slug can arch its mantle, forming a raised spout toward the rear. This structure helps channel water and waste away from the body as it travels across the seabed.
Despite its size, P. grandis moves deliberately and calmly. Notice its subtle movements in our video.
Surprising Sighting in Gorontalo
Imagine our surprise and delight at finding a Grand side-gilled slug crawling in broad daylight. Fortunately, one of our dive masters had an underwater camera ready to capture this moment. Given that the date was December 31st, we speculated that it was on its way to a New Year’s Eve party.
For your chance to make an unexpected encounter in Gorontalo, make a booking with us.
