Slugs that Fly? The Great Winged Sea Slug.

By The Marine Detective on April 2, 2020

Here’s a species that deserves the descriptor “Great” without doubt – the GREAT Winged Sea Slug.

I will never forget the first time I saw one of these tiny sea slugs “flying” underwater.  My brain came close to exploding. I did not know of their existence prior to one flapping past my mask.

Gastropteron pacificum is usually no bigger than your thumbnail. Maximum length is ~2 cm long and with “wingspan” to 4 cm. The species is also referenced as the Pacific Wingfoot Snail and the Pacific Batwing Sea Slug. But, as mentioned, I prefer the reference to their greatness.

Just marvel at how they can propel themselves, as captured in this video.

I will ALSO never forget the first time I saw them spawning, so many of them on the sandy ocean floor, their egg masses expanding to be bigger than they are.

I try to document this every year, looking in areas with sand in from late March into May. I have found them, and their eggs, as shallow as 2m depth.

And sure enough, on March 31st, there they were again. They are gathering to mate!

The photos below show you what the peak of the spawn looks like. Photos are from May 26th, 2019. Just look at the number of them! How do they find one another? How many eggs in an egg mass? So many questions!

I bet you also want to know how it can be that their masses of fertilized eggs are bigger than the sea slugs themselves. I presume the masses must expand with seawater but  .  . .  I do not know.

As is the case for most terrestrial and sea slugs, Great Winged Sea Slugs are simultaneous hermaphrodites whereby both parents become inseminated and lay eggs. It’s a great strategy to maximize chances of reproductive success when finding a mate is particularly challenging and your babies hatch into the planktonic soup of the ocean.

Among my many wonderings about this species is: Why have I never seen Great Winged Sea Slugs swimming during the time they are aggregating to mate?  I learned from research by Claudia Mills in Friday Harbour (published in 1994), that only sexually mature animals swim AND that they were only observed doing so between September and February i.e. not while mating.

Why swim? In may work well to escape annoying divers and/or bottom feeding fish like Ratfish. The timing suggests that it allows for population dispersal – spreading out for food and/or mates. You would think that the fact that hatch as plankton would spread them out enough. Also, HOW do they then assemble in numbers like this? Is it possible that these sea slugs smell one another’s scent trails even in the ocean?

Please know that this species IS a sea slug but it is NOT a nudibranch. Great Winged Sea Slugs don’t have naked gills and adults do have an internal shell when adults. Great Winged Sea Slugs belong to the group of sea slugs known as “bubble shells” of the order “Cephalaspidea”. You can even see the bubble shell in some of these images.  Ronald Shimek creatively described these sea slugs as having “an internal shell that looks quite like a soap bubble and is about as durable.”

The wing-like structures are called parapodia. When the sea slug is not swimming, these “wings” wrap around the body forming a water-filled cavity. See what looks like a siphon? Part of the “head-shied” folds into a siphon directing water into the cavity. There’s also an exhalant siphon.

The photo above is from the first time I ever noted this species. I was able to follow one as it drifted to the bottom and then saw the siphon appear. This added to the sensation that my brain was going to explode with awe. I shared the photos with experts and learned that, at that time (2007) it was not known what any members of the family feed upon. This added to my appreciation / understanding of how little is known about marine species that are even common and in the shallows. Bill Rudman responded with “I suspect they may feed on small flatworms or other invertebrate with no hard parts – but that is just a guess.” Apparently Gastopteron are known to feed on detritus and diatoms but it a laboratory setting, To my knowledge, there has not been confirmation of the diet of the species when in the wild.

More about the eggs via Jan Kocian: ”  Anne Hurst (1967) described the egg masses and veligers of Gastropteron pacificum. She considered the egg mass to be of “Type C,” that is, “in the form of an ovoid or globular jelly bag attached by a jelly string. Ths is common amongst cephalaspideans” (Hurst, 1967: 256). The egg mass of G. pacificum “is almost globular and of clear jelly. It contains widely separated rounded capsules containing spherical pink eggs. The smooth-walled capsules each have a short string-like protrusion from one point on their surfaces and this does not appear to be attached elsewhere. As the eggs develop to form a ball of cells, the pink colour becomes concentrated and at one side of it is a group of yellowish cells, the whole being surrounded by a narrow layer of greenish cells” (Hurst, 1967: 268) . .  . Egg capsule dimensions range from 181-220 µ, and the animals take 14-15 days to hatch.”

I hope, dear reader, that these words and images offer an additional chance to get lost in the natural world for a little bit. It offers me such comfort to see the steady flow of the natural world around me – from the courting of song birds, to the emergence of plants, and the mating of sea slugs.

Know that, right below the surface, there’s a world or greatness  .  .  . where slugs fly.

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Note that if you see similar egg masses in the intertidal zone,I believe they are more likely to be from one of two other sea slug species that are also “bubble shell” sea slugs (order Cephalaspidea).

#1) Diomedes’ Aglaja (Melanochlamys diomedea to 1.5 cm long ): A fabulously wicked little sea slug that crawls under the sand looking for other sea slugs to snack on.

#2) Spotted Aglaja (Aglaja ocelligera to 3 cm long): Usually also under the sand and prey to Diomedes’ Aglaja.

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Sources:

• Kocian, Jan Gastropteron pacificum – Swimming animals
• Mills, Claudia E. Seasonal swimming of sexually mature benthic opisthobranch molluscs (Melibe leonina and Gastropteron pacificum) may augment population dispersal. The Johns Hopkins University Press, Baltimore, 1994
• Simek, Ronald, March 31, 2014, A Swimming Blob . . .

For an additional blog about another bubble shell sea slug in the NE Pacific Ocean see – “Shelled Sea Slug! A small mystery solved.”

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Classification of Sea Slugs 

Regarding the photo below:
The Opalescent Nudibranch is a nudibranch.  Nudibranchs DO have external gills (hence “nudi” = naked and “branch” = gills). Adults do NOT have an internal shell.
The Great Winged Sea Slug is a “bubble shell” sea slug (Cephalaspidea). They do NOT have naked gills and adults DO have an internal shell.
There! Now don’t you feel better knowing that: (1) Not all sea slugs have naked gills and hence not all sea slugs are nudibranchs; (2) However, all nudibranchs are sea slugs.