The fan mussels (Pinna nobilis) are a species of enormous mussel which live in seagrass beds of the Mediterranean Sea. They can grow to nearly 4 feet long (though most are 1-2 feet in size at maturity), and live with most of their bodies protruding straight up out of the sediment, anchored down into the sand with long rootlike byssal threads which grow out of their rear hinge.
These mussels grow up to 20 cm per year, almost entirely in the vertical direction. As they gain in mass, their bodies start to sink in the sand beneath them, so it is believed this extremely fast growth rate evolved in order to stay above the sediment. It also helps them to remain elevated above the seagrass around them, where they can access passing phytoplankton and organic particles in the current.
Because they are exposed to the current like a giant fan, they need a very strong anchor. So they create huge quantities of byssal threads which root them down in the sand. These byssal threads are known as as “sea silk” and communities around the Mediterranean have used the silk to sew clothing for thousands of years. The material is extremely fine but strong, and has historically been of immense value as a result. Sea silk or sea wool is mentioned in writings of the ancient Egyptians, Greeks and Romans.
Unfortunately, the fan mussels are considered critically endangered due to overharvesting, pollution, climate change and destruction of their native seagrass habitats. However, they are now protected and active conservation efforts are underway. When the cruise ship Costa Concordia ran aground off of Italy in 2012, a community of fan mussels were rescued from a seagrass bed next to the wreck and moved to another nearby site. I hope someday to study the fan mussels because I find them to be a truly charismatic bivalve with many interesting mysteries still waiting to be uncovered about their unique lifestyle.
You might think of clams as rather pacifistic creatures. Most of them are; the majority of bivalves are filter-feeding organisms that suck in seawater and eat the yummy stuff being carried by the currents. This mostly means phytoplankton, tiny single-celled photosynthetic plankton which make up most of the biomass in the world’s oceans. Most bivalves could be considered exclusively herbivorous, but as I’ve learned happens throughout evolutionary biology, there are exceptions to every rule. We already talked about parasitic bivalves that have evolved to hitch a ride on other hapless marine animals. But there is an even more sinister lineage of bivalves waiting in the sediment: yes, I’m talking about killer clams.
Carnivory in bivalves has evolved multiple times, but the majority of known carnivorous bivalves fall within an order called the Anomalodesmata. Within that order, two families of clams called the Poromyidae and Cuspariidae have a surprising number of species which are known to eat multicellular prey.
Now, you can rest easy because there are no clams that eat people. You’re safe from the Class Bivalvia, as far as we know. But if you were a small crustacean like a copepod, isopod or ostracod, you would be quite concerned about the possibility of being eaten by a poromyid clam in certain regions of the world. These clams lie in wait in the sediment like a sarlacc, with sensory tentacles feeling for passing prey and a large, overdeveloped siphon ready to suck up or engulf their helpless targets.
Because they spend their lives under the sediment, these clams aren’t very well studied, and the first video of them alive was only taken in recent years. In addition, many of these killer clams live in deeper water, where their murderous lifestyle provides an advantage because food supplies can be much more sparse than in the sun-drenched shallow coastal zone. Much like the venus flytrap and carnivorous plants have arisen in response to the low nutrient supply of boggy swamp environments, the ability to eat alternative prey is valuable to the killer clams in all sorts of unconventional environments.
The siphon which these clams use to suck up their prey is a repurposed organ. In most other bivalves, the siphon is usually a snorkel-like organ which enables the clam to safely remain buried deep in the sediment and still breathe in oxgyen and food-rich water from open water above. But for the poromyids, the siphon is instead a weapon which can be used like a vaccum cleaner hose, or even be enlarged to engulf hapless prey. The poromyids have also evolved to have a much more complex, muscular stomach than any other bivalves. It takes a lot more energy to digest multicellular food, while most other bivalves simply just feed from the single-celled food they catch on their gills, expelling the other un-needed junk as “pseudofeces.”
Hopefully soon we will have video of this predatory activity in action. But until then, you can imagine that somewhere on earth, tiny copepods foraging on the surface of the sediment pass by a strange field of squishy tentacles. Suddenly, out of nowhere a hellish giant vacuum hose appears in view and sucks them in like Jonah and the whale. Then it’s just darkness and stomach acid. What a way to go!
Oyster. Reading that word, you probably formed an image in your mind of a rough-shelled creature with a shiny mother-of-pearl (nacreous) inside that someone pulled out of some silt in an estuary. And yes, that’s what most oysters look like. Some oysters are of additional economic value through their creation of pearls. These pearl oysters have long, straight hinge lines and live in the tropics in and around coral reefs.
The hammer oysters are another sort of oyster, not of the Ostreidae family that includes most of the bivalves we think of as oysters, but still closely related and in its own family, the Malleidae. Malleus is the latin word for hammer, and the most distinctive genus of hammer oysters indeed look just like a hammer sitting on the seafloor.
What the…that thing’s alive? How does that even work? This is an oyster? That’s how I imagine the first scientist to discover the hammer oyster reacting. Because they are weird and rather incomprehensible-looking. But when you know the way they live, it makes more sense.
The hammerhead part of the oyster is just a super elongated hinge. The creature has a long, straight hinge like other oysters, but it has evolved to instead have a relatively narrow set of valves attached to that ridiculously overbuilt hinge. Like other oysters, they secrete byssal threads from their backside to attach themselves to the bottom. The narrow valves commonly poke up out of sandy bottoms in tropical waters nearby coral reefs. They do particularly well in seagrass beds, and often live in large colonies similar to other oysters.
The absurd hinge helps these creatures to stay anchored into the sediment, but also serves as “wings” that help it avoid sinking into the sediment over time. One thing us humans don’t realize sitting on sand is that it actually acts like a liquid. Over time, if we sat on wet sand, we would likely begin to sink in unless we spread out our arms and legs to increase our surface area. In the ocean, all sand is quicksand. Different organisms have different strategies to avoid being engulfed by the sediment they live on, and the hammer oyster has had good success with its strategy. It doesn’t care that you think it looks weird. It just sits there, filtering water for passing food particles and plankton. It’s very good at it, has been perfecting the strategy for over 250 million years, and doesn’t need your smartass remarks, thank you very much.