by Joseph DeSisto
When I begin a tangent I like to see it through to fruition, so why not continue our journey into the strange and wonderful world of snails and slugs, class Gastropoda? Recall that the last two articles were on carnivorous, terrestrial slugs and toxin-wielding sea slugs. Today I’m going to tell you about some more amazing gastropods I’ve been reading about: several lineages of holoplanktonic, pelagic snails. In other words, tiny snails that live and swim about in the open ocean, among plankton. The inspiration for this article comes from an extensive treatment of the biology, ecology, and evolution of the pelagic snails and slugs by Lalli and Gilmer (1989).
The term “holoplankton” refers to organisms that spend their entire lives as members of the planktonic soup that nourishes the oceans of the world. This is in contrast to meroplankton, which includes animals that spend only part of their lives as plankton. For example, crabs, lobsters, sea stars, and sea urchins all have free-swimming larval stages that are microscopic and can be considered plankton. Plankton can also be divided into photosynthetic, “plant-like” phytoplankton, which includes algae and diatoms, and zooplankton, which includes tiny animals that persist by eating either phytoplankton or other zooplankton. Phytoplankton and zooplankton form the base of virtually all marine food webs, and so are critically important to the survival of pretty much all marine life, from lobsters to mussels to whales.
Many of the large, sea-floor-dwelling snails are meroplanktonic, and have swimming, microscopic larval stages. There are however, holoplanktonic snails as well: these are diverse and often bizarre, and many appear as if they belong in another world. There are the janthinids, which live on rafts which they construct out of air bubbles bound by hardened mucous. These snails are usually blue or violent, and spend their entire lives drifting about on their flotation devices, snatching their jellyfish prey from the drifting current. Then there are the heteropods, active predators that use large, well-developed eyes to search for prey. Like the predatory slugs we discussed earlier, heteropods use a long radula to capture their food, which consists mostly of zooplankton, including other pelagic snails.
Among their prey are some of the most beautiful and ecologically important of the planktonic snails, the sea butterflies belonging to the order Thecosomata. Their shells, often thin and transparent, range from spiral-shaped to long and needle-like, but all thecosomes have a pair of large, paddle-shaped “wings” which they use to swim rapidly through the water. This unusual strategy among snails encouraged French fishermen to refer to these snails as papillons de mer, or “butterflies of the sea,” and the common name of sea butterflies stuck.
I say rapidly, but this all relative — while most zooplankton can hardly swim at all, instead simply floating along with the current, two common genera of thecosome, Limacina and Creseis, can fly through the water at 8-12 cm per second. This is about the same as most sea butterflies that have been studied. The fastest we know of is Gleba cordata from the northern and equatorial Atlantic — this Michael Phelps of the snail world can soar at a show-stopping 45 cm per second (Lalli and Gilmer 1989). Keep in mind G. cordata has a meager shell length of 1.5-2 cm, although the wings can be much longer. They need to be able to move fast to out-swim their predators, among which are the gymnosomes or sea angels, planktonic slugs that feed exclusively on sea butterflies.
Sea butterflies are difficult to maintain in captivity and, as a result, not much is known about their behavior. We do know that they are generalist plankton feeders, preferring phytoplankton and bacteria, although they do occasionally take zooplankton such as copepods, which are tiny, fast-swimming crustaceans. Other organisms that have been found in the guts of thecosomes include single-celled protozoans, immature heteropods (the predatory snails from earlier), and the microscopic larvae of crustaceans, snails, and other sea-floor invertebrates (Lalli and Gilmer 1989).
We also have a pretty good idea of how they harvest prey, which was shown in detail by Gilmer and Harbison (1986). When a sea butterfly gets hungry, it secretes a sticky, mucous “net” from its mouth; plankton that floats into the net becomes trapped in sticky mucous. This net can be enormous — Cavolinia tridentata, with a shell length of only 1.5 cm, can secrete a net up to 20 cm in diameter. When the snail has harvested enough food, it simply pulls the net back into its mouth and digests the contents.
Although thecosomes usually don’t make up a large portion of the zooplankton in any given location, except perhaps in the subantarctic seas (Hunt et al. 2008), they do occasionally undergo population booms or outbreaks. This can lead to a nuisance: in 2013 in Jacksonville Beach, Florida, surfers and swimmers began to report “stings” from small, sharp animals. The culprit: a needle-shaped sea butterfly, Creseis acicula, whose narrow shell helps the snail glide through the water. You can read the full story from First Coast News here.
Sea butterfly “outbreaks” can be a problem for bathers, but more often they simply mean lots and lots of fish food. Limacina is a common, worldwide genus of sea butterflies, many of which are found in the cold waters of the North Atlantic. Here they can form a substantial part of the diet of fish, including some commercially important species: Hardy (1924) estimated that Limacina form about 2.2 percent of the yearly diet of North Sea herring, for example. This may not sound like much, but the North Sea herring fishery is worth an annual 20 million euros (22.7 million U.S. dollars) to Scotland alone, 2.2% of which is pretty significant.
There is plenty more to say about the natural history of pelagic snails, but I’m going to save the rest for another time. I hope I have shown you that the sea butterflies and their relatives are amazing and beautiful animals. Unfortunately, due to recent changes in ocean chemistry, sea butterflies aren’t doing so well lately.
Shells are made of calcium carbonate (CaCO3). During shell formation, snails and other marine invertebrates combine two forms of calcium carbonate: calcite and aragonite. Unlike most snails, which construct their shells mainly out of calcite, the shells of sea butterflies are mostly made of aragonite. Aragonite is found near the ocean’s surface, where it normally occurs in abundance. As you travel deeper underwater, the dissolved aragonite concentration decreases, to the point where there is not enough for sea butterflies to make their shells.
This usually isn’t a problem for sea butterflies, who spend most of their time near the surface anyway — that’s where the phytoplankton is most abundant. But in recent years the aragonite concentration in surface waters has been declining. In many locations there is not enough to support healthy populations of aragonite-based snails such as sea butterflies. While sampling for Limacina helicina in the Scotia Sea between Antarctica and Chile, Bednaršek et al. (2012) discovered several populations where the snails were unable to form complete shells — these occurred in areas where aragonite concentration had dropped below levels necessary for shell growth.
In large part due to human activities, the amount of carbon dioxide in the atmosphere has increased rapidly in the last few centuries. So has the amount of dissolved carbon dioxide in surface waters of the world’s oceans; this process is called ocean acidification. Ocean acidification has many negative effects on marine life, and will likely see its own story in the future of this blog. Just one of these is that dissolved aragonite concentrations are going down, and the oceans are quickly becoming inhospitable to sea butterflies and other organisms that incorporate large amounts of aragonite into their bodies. Orr et al. (2005) estimate that the Southern Ocean surface waters will be undersaturated with respect to aragonite by 2050, and hence be unable to support populations of sea butterflies and other pelagic snails. This area will then continue to grow, and more and more of the world’s marine habitats will become aragonite-starved.
Whether their models are correct, only time will tell. Meanwhile, ocean acidification is happening now, and is a very real threat not only to sea butterflies but to countless other marine species and, ultimately, the humans that depend on them.
Before signing off, I would like to thank Dr. Christian Sardet for graciously allowing me to use some of his photographs of thecosomes and other planktonic gastropods. Dr. Sardet and his colleagues maintain The Plankton Chronicles, a superb collection of videos, photographs, and information on the immense diversity of animals and protists collectively known as plankton. If you are interested in plankton at all, I strongly recommend visiting the site.
Bednaršek, N., G.A. Tarling, D.C.E. Bakker, S. Fielding, E.M. Jones, H.J. Venables, P. Ward, A. Kuzirian, B. Lézé, R.A. Feely, and E.J. Murphy. 2012. Extensive dissolution of live pteropods in the Southern Ocean. Nature Geoscience 5: 881-885.
Gilmer, R.W. and G.R. Harbison. 1986. Morphology and field behavior of pteropod molluscs: feeding methods in the families Cavoliniidae, Limacinidae and Peraclididae (Gastropoda: Thecosomata). Marine Biology 91: 47-57.
Hardy, A.C. 1924. The herring in relation to its animate environment. Part I. The food and feeding habits of the herring with special reference to the east coast of England. Fishery Investigations, Series 2 7(3): 53.
Hunt, B.P.V., E.A. Pakhomov, G.W. Hosie, V. Siegel, P. Ward, and K. Bernard. 2008. Pteropods in Southern Ocean ecosystems. Progress in Oceanography 78(3): 193-221.
Lalli, C.M. and R.W. Gilmer. 1989. Pelagic snails: The biology of holoplanktonic gastropod mollusks. Stanford, California: Stanford University Press.
North Sea herring fishery recertified as ‘sustainable.’ 25 July 2013. BBC News. Retrieved from: http://www.bbc.com/news/
Orr, J.C., V.J. Fabry, O. Aumont, L. Bopp, S.C. Doney, R.A. Feely, A. Gnanadesikan, N. Gruber, A. Ishida, F. Joos, R.M. Key, K. Lindsay, E. Maier-Reimer, R. Matear, P. Monfray, A. Mouchet, R.G. Najjar, G. Plattner, K.B. Rodgers, C.L. Sabine, J.L. Sarmiento, R. Schlitzer, R.D. Slater, I.J. Totterdell, M. Weirig, Y. Yamanaka, and A. Yool. 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437: 681-686.
Turner, L. 29 August 2013. Needle-like creatures wash ashore Jacksonville Beach. First Coast News. Retrieved from: http://www.firstcoastnews.com/