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....but just for a few days. I won't see them again til next year. It's a silent drama of individual risk and species survival in this quiet bay, especially in the face of new competition.

The Daily Bucket is a regular feature of the Backyard Science group.  It is a place to note any observations you have made of the world around you.  Snails, fish, insects, weather, meteorites, climate, birds and/or flowers.  All are worthy additions to the bucket.  Please let us know what is going on around you in a comment.  Include, as close as is comfortable for you, where you are located.
Barlow Bay, Salish Sea, Pacific Northwest                                    Early September, 2013
Haminoea vesicula (White Bubble Snail)
Peering down into the eelgrass bed from my kayak a few days ago it was like deja vu - the eelgrass blades were swarming with mating nudibranchs and their eggs. I'd witnessed a similar gastropod orgy in the early spring, of the swimming Hooded Nudibranch, in this same quiet bay.

At least I thought they were nudibranchs until I got a close look. Unlike their fellow gastropod, these carried small transparent shells on their backs, which means they are snails instead. In the photo above, the snail is climbing up the leaf, and the more orangey part in the middle is the shell. Needless to say, it's too small for the snail to fit its entire body into. These are Bubble Snails.

Here's an empty shell, and a living Bubble Snail on my hand, attempting to withdraw into its shell as much as it can (I put it back after the photo). The shells are so delicate they usually crumble into sand before they wash up on the beach.

on hand
bub shell
How does such a lightly armored creature protect itself from predators? Fully-shelled snails withdraw and close off entry. Nudibranchs produce noxious toxins, taunting predators with bright colors and patterns. Bubble Snails have neither - instead, they live burrowed in or on the mud, browsing algae and benthic diatoms, out of sight.

Except at this one time. They climb up onto the eelgrass in their thousands to cavort and lay their eggs, throwing caution to the "winds". They leave these bright yellow coils behind, each containing about 500 eggs.

What's so special about Eelgrass (Zostera marina)?

Unlike algae (kelp, seaweed, what have you), eelgrass is actually a flowering plant with vascular tissue, the fibrous tubes that carry food and water in almost all our land plants. Makes it less useful as a garden fertilizer than the algae I use, which dissolve straight into the soil. But it's a sturdy support for the White Bubble Snails (Haminoea vesicula) who were choosing it today for their massive mating event.

More below the coiled gastropod egg case...

Eelgrass beds are filled with life, a three-dimensional "forest" of gently moving water, tons of organic material (including dead eelgrass blades) and a complex ecosystem. The grass is a substrate for crawling things and protection for swimming things. It gathers sediment and holds it with rhizomes anchored in the sandy mud. Eelgrass meadows moderate the force of waves, protecting shorelines.

Almost always it's under water and a casual observer won't even notice it. Here's a view across the bay at a low tide. Directly offshore the beach is some open water, then a "grassy" zone. This is the top of the eelgrass.

bb bpeng
From water level you aren't likely to notice it at all unless you look down.
across bed
Even at the lowest of the low tides, when the short nearshore seaweeds lie exposed and drying in the sun, the eelgrass is still mostly submerged. But just barely. It needs light too. In the next photo you can see the same section of bay from the beach this time, at an extreme minus 3 foot tide (taken at the solstice in June). Below the dune plants, we see the sandy beach, the bottom which marks the average low tide. Below that is a zone of sea lettuce (Ulva spp.), a vivid green algae, and below that is the eelgrass zone. I tried walking out into the eelgrass to check on our buoy that day (it's on the far edge of the eelgrass zone) but it was over the tops of my tall rubber boots there. Nevertheless, you can understand why we need to move the Blue Penguin out into deeper water on such occasions.
low tide narrow
In temperate latitudes, eelgrass is seasonal, like all plants, growing rapidly in spring and summer. In fall, seeds germinate and overwinter, while summer blades die, break off, sink or wash away, and decompose. This is why I am surprised these Bubble Snails are mating right now. As you can see in the photo below (and note how big the egg case is relative to the snail), some eggs are fastened to green blades, others to dead brown ones. The dead eelgrass will fall apart soon.
Sexually, Bubble Snails are not like other marine snails. Bubble Snails are hermaphroditic, and can both deliver sperm and receive it, though not simultaneously. One of these snails is acting as a male at this moment, the other as a female. They may or may not exchange roles in immediate reverse copulation. After completing the act, over the next 20 minutes, the "female" one will produce the fertilized egg mass, glueing it securely to the surface of the eelgrass. It takes a month for the eggs to mature, so you can understand why it's critical the Bubble Snails take this risky journey up out the safety of the mud onto the strong grassy blades of eelgrass.

(As a side note: I feel a little weird hanging over the side of the kayak spying on creatures having sex, taking their picture and posting it on the internet. Snail pron. The things we find ourselves doing in the name of science...)

veliger larva
And the species threat to this native Bubble Snail?

I was able to positively identify these as Haminoea vesicula by the shape of their head: posterior more like a short cape than folded back bunny ears (these pics don't show that very well but I took lots just for ID purposes). It is wonderful see native Bubble Snails in Barlow Bay, rather than the very similar introduced species, the Japanese Bubble Snail Haminoea japonica, first identified in Washington waters in the 1980s, possibly brought over with Asian shellfish.

H. japonica has a survival advantage over H. vesicula: while the eggs of both hatch into planktonic larvae that can drift afar (see the microscopic veliger larva in the photo), some Japanese Bubble Snail eggs can also hatch as crawling youngsters who have a head start in a friendly habitat like this one. There are very few species that have this capability, and our native Bubble Snail may have some serious competition in future.


Time for your observations. Anything unusual you're seeing as we slip from summer into fall? Or are your regulars coming and going as you've come to expect?


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Originally posted to Backyard Science on Tue Sep 10, 2013 at 07:00 AM PDT.

Also republished by Community Spotlight.

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