(Not So) Boring Clams

by Tim Pearce

Some clams, in the families Teredinidae and Pholadidae, bore holes in wood or rock that is immersed in seawater. We humans often think of wood and stone structures as relatively permanent, but these clams force us to challenge that idea. In fact, the wood-boring clams, known as ship worms, are a centuries-old scourge to shipping activities because they weaken wooden ships and pilings.

Wood bored by shipworm, Lyrodus pedicellatus

The wood-boring clams are highly modified from the clams that normally come to mind. Their shells are reduced to a pair of abrasive cutting tools at the end of a long, worm-like body. The clam twists the shells back and forth, breaking off chunks of wood as it burrows through the wood. The clam eats the wood, aided by symbiotic bacteria that digest the wood. As the clams burrow, they somehow seem to know when they are near another clam’s tunnel and they avoid breaking into it, but how they know is a puzzle.

Rock bored by clam, Penitella penita, from Washington State

Human efforts to prevent shipworms from destroying wooden ships and pilings included coatings containing tributyl tin (TBT). While paints containing TBT did protect against shipworm damage, the chemical was toxic and caused reproductive problems in aquatic organisms. In particular, TBT causes masculinization of female fish, snails, and other aquatic species. So, other methods to protect wood are now used instead.

Rock-boring clams also have shells adapted for abrasion at one end, but they differ from the ship worms because the shells of the rock-boring clams are not as reduced as in the ship worms, and the rock boring clams do not derive nutrition from the rock particles. As the clams bore into the rock, they grow, so the burrow tapers wider inward, so the clam shell cannot get out. However, the clam gains great protection from predators. The clam siphons protrude through the rock opening to bring in water and food and to expel wastes.

Rock-boring clam, Zirfaea crispata, from England

Other clams specialize in boring in calcium carbonate. These clams are important in the destruction of limestone, reefs made of coral skeletons, and even shells of other mollusks.

Timothy Pearce is the Head of the Section of Mollusks at Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.


Weighing up to 30lbs the Black Sea Hare is the largest slug in the world and is found in abundance along the Pacific Coast of the United States. 

Aptly named Sea Hare due to their rabbit ear looking appendages, these creatures are truly a primordial wonder of the ocean. Who would have ever thought a Sea Slug could grow to the size of a small dog!?

FEEDING! The chambered Nautilus is a mollusk, related to the octopus, squid, clam and snail. A nautilus, along with the cuttlefish, squid, and octopus, are all cephalopods, meaning “head-foot,” so named because the feet (tentacles) are attached to the head.
The nautilus is the only cephalopod that has a fully developed shell for protection. The nautilus has more than 90 suckerless tentacles. Grooves and ridges on the tentacles are used to grip prey and deliver food to a crushing, parrot-like beak. NEWS: This fascinating animal is now on the list of protected species. Quite rightly of course. Of all the squid-species the “Chambered Nautilus” is the only one with a beautiful outer shell. This shell is used for jewelry which is a popular souvenir for tourists. The Nautilus lives at great depth (200 m) but must go to the surface to eat. This makes it extremely vulnerable to its main predator: humans.


The Biology and Evolution of Mollusks

From tiny snails to the giant clam (Tridacna gigas), mollusks are the most diverse and widely distributed family of marine invertebrates. Professor Gonzalo Giribet, Curator of Invertebrate Zoology at Harvard’s MCZ, discussed how scientists are decoding the Mollusca genetic family tree to learn how they’ve adapted, survived, and thrived since the pre-Cambrian era, and to explore the potential benefits of mollusks from medicine to human health, and other fields. 

Presented by the Harvard Museum of Natural History

Shopping cart symbol

by Patrick McShea

The shell-encrusted shopping cart in We Are Nature would get lots of visitor attention even if it weren’t suspended from the ceiling. Hundreds of zebra mussels coat the familiar contraption, creating an eerily appropriate symbol for human-altered natural systems:  An empty icon of consumer culture armored by hitchhiking organisms of global trade.

Zebra mussels, a freshwater species native to the Caspian Sea and Black Sea, were unwittingly introduced into the Great Lakes during the 1980s via ballast water dumped by ocean-crossing cargo ships. The creature’s rapid dispersal since then has been attributed to the passive drifting of tiny larvae and the ability of mature zebra mussels to attach to boats moving between the lakes and adjacent river systems.

As invaders, zebra mussels have profound effects on ecosystems. They feed by filtering tiny organisms from the water, and by sheer numbers can out-compete fish larvae and native mussel species dependent on the same food source. Zebra mussels attach to any submerged hard surface. Their profusion attracts attention when it results in clogged water in-take pipes, but not necessarily when thousands of the striped fingernail-sized creatures occupy physical positions atop existing beds of native freshwater mussels.

At Carnegie Museum of Natural History, concern for the health of our region’s diverse population of native freshwater mussels has a long history.  In 1909, Arnold Ortmann, then Curator of Invertebrate Zoology, termed the disappearance of mussel species “the first sign of pollution of a dangerous character in a stream.” His observation was based upon biological surveys in rivers and streams throughout Western Pennsylvania, fieldwork performed during a time of rapid industrialization that garnered the museum an irreplaceable collection of local mussel shells.

Shells of  Potamilus alatus, or pink heelsplitter, a native freshwater mussel  in the Carnegie Museum of Natural History Section of Mollusks.

Patrick McShea works in the Education and Visitor Experience department of Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences of working at the museum.


Voyage of the Giant Squid - Shelf Life #8

Getting a giant squid from New Zealand to New York is no easy feat. Curator Neil Landman tells the tale of a sizable specimen’s journey to the collections at the American Museum of Natural History, and Curator Mark Siddall explains why this giant cephalopod has a new name.

From the American Museum of Natural History

How Scallop Eyes Relate to Human Uniqueness

by Timothy A. Pearce

We humans like to think we are special among all creatures. To support that notion, we claim unique traits such as language, tool use, consciousness, etc. Oops, all of those traits have now been shown to occur in other species. Do not fear, though, for I have found a trait that seems to be unique to humans: a fondness for 90 degree angles (aka right angles). You heard it here first! I don’t know where on the evolutionary lineage to modern humans we acquired this fondness for right angles, but evidence of this fondness is all around us in the modern built environment.

What does fondness for right angles have to do with scallop eyes? First let me tell you about the amazing eyes of scallops. They have up to 200 eyes along the mantle margin, and those eyes contain concave mirrors. Instead of being similar to cameras (as our, and most, eyes are), scallop eyes are similar to reflecting telescopes, and each eye has two retinas so they can see clearly in both narrow and peripheral views at the same time.

New research published this week in Science (and described in the New York Times ) demonstrates that the concave mirror of each scallop eye is tiled with more than 100,000 square mirror tiles. Did you get that? They are squares! Outside of the human built environment, right angles are scarce. So to find squares in the eyes of scallops is remarkable. The properties of the tiles making up the mirror has implications for the scallop’s ability to see in the particular wavelengths of light in its surroundings and can inspire improved human optical devices. Future studies will have to examine why a scallop needs to have such amazing vision. But for now, I am amazed to know that scallop eyes contain square mirrors.

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.

anonymous asked:

Is the divide between slugs and snails really as clean cut as one having a shell and the other not, or is it much more complicated than that. You'd be surprised (well maybe not you, since your Mollusc Facts after all) how hard finding good snail/slug info is.

From my understanding the terms “slug” and “snail” aren’t scientific classifications, but generic terms referring to whether they have a large, external shell or not. Gastropod taxonomy is quite confusing - some gastropod families contain only slugs, some contain only snails, and some contain both - so unfortunately there’s no way to simply divide the class into slugs and snails.

What we generally refer to as snails are gastropods that have a large enough shell that they are able to withdraw their body into it completely. Gastropods that appear shell-less (but actually have an internal, vestigial shell) are referred to as slugs. However it’s not even this clear cut either, because there are also “semi-slugs”, which are gastropods with a small, external shell that they cannot withdraw into but is not quite small enough to be considered vestigial. These often look like regular slugs, as their small shell might be hidden beneath the mantle.

So no, unfortunately it’s not as simple as shells vs. no shells. Taxonomy can get pretty complicated, and it’s being constantly being revised as we get more and better genetic information, so we often find that old classifications that were made based on physical characteristics don’t apply anymore. So while initially it might seem to make more sense to classify all the gastropods with shells together, we’re discovering that actually that doesn’t represent their evolutionary relationships. 

Even though it is confusing, these new taxonomic classifications can end up revealing some really interesting things. For instance, with gastropods, we think that having a shell is probably basal - meaning this is something the common ancestor of all gastropods had. The fact that slugs show up in so many different gastropod families, each with their own separate lineages, means that gastropods must have lost their shells many different times throughout their evolution (possibly over 10 different times). When we see the same trait evolving in so many separate instances, we know that it must be fairly advantageous for some reason. The reduction of shells in slugs was probably due to several different reasons, depending on the challenges they faced:

  • Terrestrial slugs probably lost their shells when they colonised the land as there is much less calcium than in an aquatic environment. While terrestrial snails obviously didn’t evolve this way, instead they evolved to have much thinner shells than those of marine snails.
  • Not having a shell could also be advantageous in a terrestrial or an aquatic environment, as it allows slugs to burrow and crawl into tighter spaces in search of prey and to escape predators.

  • In sea slugs, the loss of their shell allows them to be more streamlined  in the water, so they can have a pelagic or swimming lifestyle.

  • Also, for some sea slugs, not having a shell allows them to expose their whole body to sunlight in order to photosynthesise - either using symbiotic algae in their body, or chloroplasts from algae they have consumed.
Obviously by losing their shell, slugs may be more vulnerable to predation, but many slugs have evolved measures to protect themselves as well - by secreting toxins, by camouflage, by mimicking dangerous or distasteful species, and even by incorporating stinging cells form their prey into their own body.
This went off on a bit of a tangent from the original question I know, I just find taxonomy and evolution really interesting! It’s thought that the common ancestor of cephalopods also had a shell, so cephalopods probably went through similar events for some of them to have lost their external shell, but I think I’ll save that for another post.