Section of Mollusks

August 7, 2020 by carnegiemnh

Mobilizing Millions of Marine Mollusks: Seashells by the Eastern U.S. Seashore

“What a pretty seashell, where did it come from?”

Perhaps the most important information that natural history museums keep about their specimens is where they came from. For many researchers, locality information is more important than the specimen itself. The specimen is useful to verify correct identification, but you can’t look at a specimen to determine where it came from.

As more and more museums share their specimen databases on-line, locality information is being used to document changes in distributions of organisms, including new occurrences of invading species, range shifts due to climate warming, and the disappearance of species becoming locally extinct.

To facilitate uses of locality information, museums are scrambling to georeference their specimens. This term refers to the electronic pairing of the historic recorded location for each collected specimen with an established system of latitude and longitude coordinates. Georeferencing can be tedious and time-consuming, what with interpreting messy handwriting, dealing with misspellings, and tracking down obscure names, some of which have changed over time. Once I spent over an hour and got only two specimens georeferenced.

The US National Science Foundation (NSF) recognizes the importance of georeferenced specimens to facilitate understanding of where species occur and how their distributions change over time. Consequently, NSF has awarded $58,762 to Carnegie Museum of Natural History as one of 14 collaborating museums on a $2.3 million grant for a project titled: Mobilizing Millions of Marine Mollusks of the Eastern Seaboard. The project is spearheaded by Rudiger Bieler at The Field Museum in Chicago.

The main goal of the project is to georeference, and make available online, 535,000 lots representing 4.5 million specimens of marine mollusks (snails, clams, etc.) from the eastern USA. Only 15% of Eastern Seaboard mollusks in museums are currently reliably georeferenced. To facilitate georeferencing and promote standardization, each collaborating museum will focus on georeferencing all lots from particular geographical areas. Notably, for the first time, these museum records will distinguish between live- and dead-collected specimens, important information given that shells of dead mollusks sometimes persist for hundreds of thousands of years, and can be moved by currents and other animals such as hermit crabs. Whether or not a shell was collected alive is therefore crucial information for studies of biotic change using mollusks.

Two lots of East Coast USA seashells ready to georeference.

For CMNH, this award primarily means support for georeferencing our 11,436 lots of marine mollusks from eastern USA. In addition, we will catalog the eastern US part of our backlog, image relevant type specimens, create an exhibit, and, the aspect I am most excited about is creating an IPT, or integrated publishing toolkit, which will allow automatic updates from our in-house database to our web presence in the InvertEBase Symbiota portal.

The grant-funded new public display will interpret our biologically, commercially, and recreationally important marine mollusks from the Eastern Seaboard, and showcase mollusk diversity. The display will appeal to anyone who has beachcombed shells. Labels will describe how scientists use modern and historical specimens to study change in marine ecosystems over time. My hope is that visitors will learn that mollusks are diverse and beautiful, that museum collections are useful, and that evidence-based studies show ecosystem changes.

The Eastern Seaboard region includes 18 states, nearly 6,000 km of coastline, and about 3,000 molluscan species. Boundaries, from Maine to Texas, stretch from the shore outward to the edge of the U.S. Exclusive Economic Zone. The 14 collaborating U.S. collections contain 85% of all Eastern Seaboard marine mollusk museum holdings. These museum holdings average 8 specimens per lot – a lot is one species from one place at one time.

One hundred million mollusk specimens have been documented in natural history collections across North America. Each mollusk species in these collections average 1100 individuals, revealing geographic and morphological variation, and making mollusks among the best sampled group of metazoans, or multi-cellular animals. So far, freshwater and terrestrial mollusks have dominated digitization efforts of mollusks. This project is the first to focus on marine mollusks.

Shells are bio-archives. Shell skeletons record information about the animal and its environmental conditions throughout its life cycle. Shell material can be used to infer past ocean temperatures, seasonal fluctuations, and growth rates. Shell testing can reveal presence of trace elements and pesticides, allowing detection and identification of marine contamination and pollution.

In addition to their use in documenting what lived where and when, mollusks are important in other ways. Shells bring us joy when we find them on the beach. And many of us eat them. In 2016, three of the top 10 most valuable fisheries in the US, worth hundreds of millions of dollars, were mollusks: scallops, clams, and oysters.

Timothy A. Pearce, PhD, is the head of the mollusks section at Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.

The Zebra Mussel and the Shopping Cart

Zebra mussels (Dreissena polymorpha) are eastern European freshwater bivalves that invaded North America. Something unusual about their biology facilitated this invasion.

In marine waters, many benthic (living on the bottom) animals add their babies to the plankton, the mix of small and microscopic organisms largely adrift in the water column.

The situation is different in freshwater where almost all benthic animals lay their eggs on the bottom. (Freshwater plankton exist, but the organisms that compromise it spend their whole lives as plankton.) I don’t know why marine and freshwater animals differ that way, but they do. Zebra mussels are a major exception to this rule; they live in freshwater, but they put their babies (larvae) in the plankton.

How did zebra mussels invade North America? Partially loaded ships require ballast to safely navigate at sea. Decades ago, ships were loaded with rocks and dirt (and slug eggs) as ballast, and when they reached their intended port these materials were removed and replaced with cargo. That is why so many invasive slugs (essentially all your garden slugs are non-native) arrived first in seaports and spread from there. Ballast tanks that can be easily filled with water and drained are a design feature of modern ships, and depending upon some ship’s departure points, their ballast water sometimes contains larval zebra mussels. For many years, ships were slow enough that zebra mussel larvae arrived in North America dead, but eventually reductions in ocean crossing time worked in the invaders’ favor. In 1988 some larval zebra mussels arrived alive in the ballast water pumped out into Lake St. Clair near Detroit. By 1990, zebra mussels had infested all the Great Lakes and now they occur in more than half of the 50 United States.

Fig. 1. Freshwater snail (*Elimia livescens*) colonized by zebra mussels (left) and uncolonized (right). From Douglas Lake, Michigan 30 Aug. 2015 (photo by T.A. Pearce).

The economic and ecological devastation caused by zebra mussels is legendary. Zebra mussels make threads (byssal threads) for attaching to hard objects. They clog intake pipes of city water supplies and power station cooling pipes, requiring costly removal. They compete with native mussels and young fish for food and can smother or hinder movements of our native mussels, snails (Fig. 1), and crayfish when they settle in large numbers.

Fig. 2. Replica of shopping cart covered in zebra mussels.

A noteworthy item that became encrusted with zebra mussels is a shopping cart that was dredged out of Lake Superior in 2012. A replica of the shopping cart was on display during the We Are Nature exhibit at Carnegie Museum in 2018 (Fig. 2).

Lest you think I am biased against zebra mussels, I will note two possibly positive things you can say about them. First, they filter water efficiently and because they pump up to a liter (quart) per day, they cleaned up the formerly polluted water in Lake Erie. But even that can be negative, because they removed so much plankton from the water that our native species now have a hard time finding enough to eat. Second, because zebra mussels selectively concentrate certain toxic metals, including uranium, they have potential to be used in bioremediation efforts to clean water of this radioactive pollutant (Immel et al. 2016). But those are the only good things you can say about them. Mostly, they wreak havoc.

Literature Cited

Immel, F., Broussard, C., Catherinet, B., Plasseraud, L., Alcaraz, G., Bundeleva, I. & Marin, F. 2016. The shell of the invasive bivalve species Dreissena polymorpha: biochemical, elemental and textural investigations. PloS One, 11(5): e0154264. https://doi.org/10.1371/journal.pone.0154264

What’s So Good About Being a Slug?

When lifestyles or forms evolve multiple times, we often think they must have some benefit. For example, flying creatures evolved at least 4 separate times: birds, bats, insects, and pterosaurs. These 4 separate origins of flight support the idea that there must be an advantage to flying. (You can probably think of some advantages.)

Slugs evolved from snails more than a dozen separate times. By that logic, there must be an advantage to being a slug, but compared to flight, it’s harder to think what the advantage might be. Slugs evolved from snails by reducing the size of the shell and internalizing it (yes, most slugs have an internal shell), and there are likely to be consequences of reducing the shell.

A snail with an external shell large enough for the body to pull back into. *Webbhelix multilineata* from Ann Arbor, Michigan.

Furthermore, in the lineages leading from snails to slugs, an intermediate stage occurs called a semi-slug (not a slug the size of a semi-truck). In contrast to snails that have an external shell large enough to accommodate the body, or slugs in which the shell is completely internal or absent, semi-slugs have an external shell, but the shell is too small to accommodate the animal’s entire body. Many semi-slugs live on our planet today (for example, Vitrinizonites latissimus lives in the Great Smokey Mountains). Curiously, semi-slugs evolved from snails at least 22 times.

A semi-slug whose external shell is too small for the body to fit into. Family Urocyclidae from Mount Kenya

Let’s consider some possible advantages and disadvantages of these body forms: protection from predators, protection from desiccation (drying out), need for calcium, and fitting into tiny hidey holes. As shown in the table, shells protect snails from predators and from drying out, but the snails still need lots of calcium to build shells, and the rigid shell prevents them from squeezing into tiny hidey holes. Snail score: 2 advantages, 2 disadvantages. Slugs, on the other hand, are not protected from predators or drying out, but have less need for calcium and can fit into tiny hidey holes. Slug score: 2 advantages, 2 disadvantages. However, semi-slugs seem to have all disadvantages: no protection from predators or drying out, a need for calcium, and can’t fit into tiny hidey holes. Semi-slug score: 0 advantages, 4 disadvantages.

	Predator protection	Desiccation protection	Need less calcium	Fit in tiny hidey holes
Snail	+	+	–	–
Semi-slug	–	–	–	–
Slug	–	–	+	+

In evolution, every form in a lineage must have at least a limited track record of survival, so how did slugs evolve from snails if they had to go through a life form having so many disadvantages, and how could that evolution have happened so many different times?

A slug with an internal shell (not visible). Ariolimax cf californicus from the Santa Cruz Mountains, California.

Although we don’t know the answer for sure, my studies suggest some possible answers. I examined locations where slugs and semi-slugs evolved from snails. I discovered that many of those events seem to have happened on oceanic islands (40%) and within 35° of the Equator (80%). Islands often have fewer predators and tropical and subtropical islands often have regular moisture inputs (daily rain or fog), so on islands there might be less need of shells or hidey holes for protection from predators or desiccation. I’m not sure what to predict about calcium because many islands are volcanic, with calcium-poor soils, but calcium carbonate would be available from empty seashells washed up on the shore. If calcium were difficult to find, that might favor forms needing less calcium. Evolutionary biologists use the term “relaxed selection” to refer to a situation in which changes to an organism’s environment cause less need to maintain certain forms or behaviors.

It seems likely to me that relaxed selection on tropical islands allowed the evolutionary transition from snails to semi-slugs to slugs by reducing the disadvantages of having, or not having, a shell.

Where better to be sluggish than on a tropical island?

September 18, 2019 by carnegiemnh

Is It a New Species? Wish They All Could Be California Snails

Discovering a new species is exciting but determining whether it’s a new species can take some doing.

photo of possible new Trilobopsis species snail — Fig. 1. *Trilobopsis* species from Santa Cruz Island. Note scale-like hairs on shell. Diameter approximately 8 mm (1/3 inch). (Photo by T. Pearce)

During our project on land snails in the California Borderlands, team member Jeff Nekola discovered a population of the land snail genus Trilobopsis on Santa Cruz Island (Fig. 1). That genus does not occur on any other California Channel Islands; in fact, the closest known mainland locality for that genus is 335 km (210 mi) north in the Salinas Area. We recognize that conditions on the northern Channel Islands tend to be cooler than the adjacent mainland, due to the California Current, and some other typically northern plant and animal taxa (or close relatives) also occur surprisingly far south on the northern islands. The Santa Cruz Island population of Trilobopsis represents a serious range extension to the south for the genus (Fig. 2). Is it merely a range extension of a known species, or could it be a new species?

Fig. 2. Trilobopsis on Santa Cruz Island are 335 km south of the nearest other population near Salinas.

Peculiarities about the distribution of Trilobopsis on Santa Cruz Island make us wonder if it is a long-established native species or a recent introduction from the mainland. Its localized occurrence on Santa Cruz Island spans only a couple of hectares (a few acres) near an area where humans have been active over the past century or so. Small ranges, near human activity, often hint that a population was introduced. In contrast, if the snail had been on the island for thousands of years, we would expect it to have spread to other parts of the island that have suitable habitat.

Fortunately, team member Barry Roth is an expert on Trilobopsis. He is a very careful worker, scrutinizing shell features and internal soft-part anatomy before drawing conclusions. His impression is that the Santa Cruz snail is different from any described species. The next question could be, to what mainland form is it most closely related?

These days, DNA can supplement evidence from shell and internal anatomy features to help elucidate relationships. To get DNA, we usually need live-caught individuals. While museum collections contain libraries of snail shells, and sometimes soft parts, rarely do they contain all the species needed, or fresh enough DNA for the comparison. So, it was time for a field trip.

In August 2019, team member Charles Drost organized an expedition to northern California to seek live specimens of Trilobopsis species for DNA. Jeff had annotated numerous maps with known locations, compiled from some of my past field work (when I was a student at Berkeley in the mid-1980s) and extensive field work by Barry. Fortunately, despite the normal late summer drought conditions, we were able to find living specimens of Trilobopsis at nearly all the target sites we visited.

Fig. 3. Charles showed me where to find live *Trilobopsis* snails in a log. (Photo by C. Drost)

We were struck by the differing habitats of some populations. We think of typical Trilobopsis species living in talus rock piles (as does the one on Santa Cruz Island), but we found some populations living in leaf litter, and one population we found was living inside of rotting logs (Figs. 3-4).

Fig. 4. Living *Trilobopsis* in a cavity in rotting log. (Photo by C. Drost)

One likely side benefit of this research will be a revision of the genus; there might be more species of Trilobopsis than currently recognized, or there might be fewer species than currently recognized if some forms simply look different by growing in different environments.

We await results of the DNA comparisons, so we can learn which mainland populations are most closely related to the Santa Cruz Trilobopsis. Gotta love those California snails.

May 14, 2019 by carnegiemnh

Horizontal Gene Transfer, the Placenta, and Velvet Worms

Everyone reading this (except you robots out there) inherited genes from their mother and father. This is the predominant way, in multicellular animals and plants, for genes to be transferred from one organism to another, from parent to child, and is called vertical gene transfer. But less commonly, genes can be transferred from an individual unrelated to you, possibly from a different species, and is called horizontal gene transfer. Viruses accomplish horizontal gene transfer naturally, while in the lab, genetic engineers use viruses to transfer genes horizontally to create genetically modified organisms.

The gene syncytin-2, which produces an essential membrane between the mammalian placenta and the developing fetus, appears to have come from retroviruses, who use the gene to produce a membrane around their virus capsule. If our ancestors had not acquired this retrovirus gene, you and I would not be here today. We have to be grateful for horizontal gene transfer.

Now for the speculative part of this article. Velvet worms (Onychophora) are a whole phylum (major group) of animals most people have never encountered. They look kind of like a cross between an earthworm and a millipede.

A velvet worm of the genus Oroperipatus. [image from Wikipedia]

Nowadays, they are tropical and terrestrial, but their marine relatives once occurred 500 million years ago. Unusual for their bizarre habit of shooting strings of glue at their prey, some (not all) velvet worms have placentas. That leads me to two questions, the answers to which I do not know. (1) Did retroviruses transfer this essential membrane-producing gene to the velvet worms, as they did for mammals? (2) Do the velvet worms that have a placenta also have a belly button?

To address the question whether retroviruses transferred the gene, researchers could examine whether the syncytin-2 gene occurs in velvet worms, and if so, determine whether the gene’s DNA in velvet worms matches that of the retroviruses? Finding a close DNA match for the syncytin-2 gene in both groups of organisms would be a strong case that the retroviruses are responsible. To determine whether they have a belly button, let’s get some velvet worms and scrutinize their bellies with a microscope.

May 3, 2019 by carnegiemnh

Is Jabba the Hutt a Slug?

Jabba the Hutt action figure — Image credit: Tomasz Mikołajczyk from Pixabay.

The movie Star Wars introduced us to Jabba the Hutt, with a slug-like body form. In the movie, he was an unsavory character and notorious Crime Lord with a fondness for Princess Leia.

Was Jabba the Hutt a real slug, or did he just look like one? In biology, we know that creatures can look similar either by descent or convergence. Two individuals that look similar by descent implies that their past common ancestor also looked similar. An example of similarity by descent is crows and canaries, that fly and look similar because their common ancestor could fly and looked similar. On the other hand, two individuals that look similar by convergence implies that their past common ancestor looked different, but they acquired their similar features independently. An example of similarity by convergence is birds and bats, that both fly and have wings, but their common ancestor did not fly or have wings.

It is easy to notice that Jabba the Hutt has a body shape like a slug, but I also noticed that he has features of other groups of creatures, for example, he has arms with fingers, as many tetrapod vertebrate animals have.

California banana slug (*Ariolimax cf californicus*), photo by Tim Pearce.

To evaluate whether Jabba the Hutt is slug-like because he is a real slug by descent or due to convergence, let’s compare Jabba the Hutt’s features with those of slugs and tetrapod vertebrates.

Table comparing 10 features of Jabba the Hutt to those of slugs and Tetrapoda

table comparing Jabba the Hutt to slugs vs. tetrapod vertebrates

The table shows that Jabba the Hutt’s features match those of Tetrapoda in 9 out of 10 features (checked off in the table), suggesting he belongs to Tetrapoda vertebrates.

I conclude that Jabba the Hutt was not a slug.

I note that one can find suggestions on the internet that Jabba the Hutt had a skeleton, which is further support for my conclusion that he was not a slug.

Finally, I want to note that slugs can be very nice creatures. Comparing the villainous Jabba to a slug is disrespectful to slugs.

Mobilizing Millions of Marine Mollusks: Seashells by the Eastern U.S. Seashore

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The Zebra Mussel and the Shopping Cart

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What’s So Good About Being a Slug?

Is It a New Species? Wish They All Could Be California Snails

Horizontal Gene Transfer, the Placenta, and Velvet Worms

Is Jabba the Hutt a Slug?

Table comparing 10 features of Jabba the Hutt to those of slugs and Tetrapoda

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