Mollusks, or Mollusca, are one of the earth’s largest and most diverse groups of animals. Scientists estimate that there could be anywhere from 50,000 to 200,000 different species living today.
In this guide to mollusks, we’ll latch onto basic biology facts of this group. We’ll take a look at various life cycles. Mollusks are affected by a warming world, which we will look at. And we can explore how you might forage them sustainably for an ancient and delicious snack.
The mollusk group contains within it many familiar and beloved creatures: snails, octopus, clams, squid, scallops, oyster, chitons, limpets. The list of species is vast and ever-evolving. As people, our interactions with this category of creatures are rich and diverse as well. We munch on salty calamari, sing along to the Beatles’ classic pop tune. People slurp down oysters as aphrodisiacs. Foragers hunt for shells to steam, share, and collect on the seashore.
The mollusk group holds some lesser-known rockstars as well: take the monoplacophorans for example. This group of mollusks was thought to be extinct for millions of years, an ancient ancestor until it was found in the ocean waters of Costa Rica in 1952. For how beloved, prevalent, and studied these creatures are, there are still plenty of mysteries to behold.
Here’s what we do know: mollusks are a clad of soft-bodied organisms, that exhibit a “head” region, and a “muscular foot” region. In many species, their soft bodies are clad and protected by hard exoskeletons. An example of this is the shells inside of which snails and clams live.
These creatures are found all over the world and play an impactful role in both the health (and sometimes destruction) of an ecosystem. We’ll get more into detail about this later on. Either way, if you really wanted to, you could travel to the summits of a mountain or the cool currents of the deep sea and find them, existing in a wide range of colors, shapes, and sizes.
Scientists organize mollusks, within the Lophotrochozoa clad, into seven systematic groups. Here’s a little about each one:
The Bivalvia group includes mussels, clams, scallops, and more. There are over 10,000 document species of this category and they are critical to most marine and freshwater ecosystems – not to mention regularly dangling from a queen’s neck as a string of pearls, or moving across the dock in Molly Malone’s wheelbarrow.
You can easily identify these bivalves by their two-halved shells and typically display bilateral symmetry. They often burrow into the sediment or live on the ocean floor. Some species are able to swim by opening and closing their shell. Not all bivalves have a shell though – some have evolved to maintain a reduced shell, others have no shell at all.
Except for oysters and scallops, there is usually a great deal of symmetry displayed in the soft bodies of bivalves in addition to their shells. In many species, there is something called a pallial cavity, which contains gills that capture food particles within a water current. They lack a head or jaw, but can extend a long siphon to the water’s surface as a means of breathing and filtering water.
We long thought that this group of mollusks was an ancient memory preserved via fossil. But then, a living specimen was discovered in Costa Rica in 1952, as mentioned above. Since that time, 20 other species have been discovered living in the waters of our planet today.
So why were these little guys so difficult to find for so long? Well, often thrive in deep water, allowing them to evade detection until our technology and oceanic exploration finally caught up with their tiny bodies. This has been an exciting thing in the scientific community, as many think that this group is the most primitive ancestor from which the other groups of mollusks eventually evolved.
Scientists have found species living on both soft bottoms and hard surfaces. There are small, and reminiscent in appearance to limpets. Their shells look like a small cap. At one point, scientists hypothesized that they were closely related to arthropods and annelids, though that theory has since been debunked. Their forms vary from circular to pear-shaped.
Gastropods make up about 80% of mollusks on planet earth. With over 62,000 species, we named these invertebrates for the Greek words “stomach” and “foot”.
The most common forms of gastropods in modern life are snails and slugs and whelks. They are the only group of mollusks to have invaded and inhabited land and are extremely diverse in size, shape, and shell morphology.
Because this group is so vast, the various species have wildly different feeding habits, which obviously depend greatly on their endemic habitats. However, for all their differences, most species use a radula to scrape up food from their environment. Some are grazers, some are scavengers, others are killer carnivores.
Physically, we characterize them by their procession of a distinct, single shell that contains their visceral mass of internal organs. Often, the shell coils – to the delight of humans collecting shells on the seashore through the decades. Their heads showcase developed eyes and tentacles, and their foot (often described as a belly) is used for crawling.
Cephalopods include the famed species of oceanic glory, including octopus, squid, nautilus, cuttlefish, and ammonites. Of all the organisms in phylum mollusca, we understand cephalopods to be the largest, the most mobile, and the most intelligent.
These creatures have an incredible propensity for locomotion, like jet propulsion and disguise – many exhibit complex learning behavior, tentacles with impressive suckers, and eyes like that of a camera.
There is an abundant fossil record of cephalopods that we have been able to track, revealing links and clues to extinction events and predatory adaptions. Human fascination has long been an element to this group, with folklore and mythology well-peppered with their presence.
Today, there are 800 named species of cephalopods, and in contrast, 17,000 in the fossil record. The earliest ones to be identified within the fossil record are the nautiloids, from the late Cambrian period.
Some species have a hard shell, others do not. Those without a shell are identified as endocochleate – sometimes the shell is internalized as cartilage, other times it is entirely absent.
There are plenty of cool individuals within this group. Two highlights:
There is a species of octopus, the Indonesian Coconut Octopus or Octopus marginatus, that biology students have observed moving across the ocean floor using the tips of its tentacles to propel its body forward. It looks remarkably as though it is walking like a bipedal hominid.
Secondly, everyone has heard of the giant squids of mythology – but did you know that Mesonychoteuthis hamiltoni, also known as the colossal squid is longer than a school bus? This is especially remarkable when compared to the pygmy squid, or Idiosepius notoides, which would be able to fit upon your thumbnail.
The common name for this group of mollusks is “tusk shells”. These “shovel foots” have conical, tusked-shaped shells that arc gently. Their curved shape is reminiscent of an elephant tusk or a boar’s. These organisms lack eyes, but are able to burrow into layers of sediment. These are creatures that, quite literally, live their lives with their heads buried in the sand – something we’ve all probably craved at one point or another. Male tusk shells will even keep their heads in the sediment as they search for a source of food.
Scientists believe that scaphopods’ closest relatives are the bivalves and they do share similarities with other mollusks when one looks closely at their various life stages.
Fun fact: People around the world have used these shells as a means of adornment via necklaces and other avenues. At one point in time, people used these shells were as currency in the Pacific Northwest region of North America. We have identified roughly 900 species.
Aplacophora, or the “naked mollusks”, are shell-less organisms that look a lot like worms. They are carnivorous, mostly deep-sea creatures with a mantle that secretes calcareous spicules, which gives them a shining appearance. There are about 320 species of this small group of mollusks.
According to the database at the University of California Berkely, this group is vastly understudied and some scientists argue that its placement within the mollusk tree is problematic.
We do not currently have a fossil record for this group, but all species live in deep-sea regions, which makes studying them that much more difficult.
Polyplacophora, or chitons, also known as eight-shelled mollusks, are marine creatures. Most species are intertidal, a term used to describe the liminal areas of an ecosystem where the ocean meets the land depending on high and low tide – a popular zone for other animals to forage.
This group of mollusks lives worldwide, and you can easily identify them by their eight-valved shells. Chitons are flattened and oval in shape. Spines or hairs often cover their shells. They typically cling to hard surfaces. Commonly found in rocky tidal zones, different species of chitons have been observed in depths up to 7000 meters, where they make a home on pieces of waterlogged wood.
These creatures have a heart, two kidneys, a simple nervous system, and sensory organs. While the head lacks tentacles or eyes, some of their sensory organs specialize as light receptors, with a lens and retina-like structure. They feed on organisms such as sponges and algae, which they scrape from hard surfaces with their radula. Some species even feed on crustaceans by trapping them with the anterior part of their bodies.
We’ve covered a lot of information so far here, but we’ve still barely even scratched the surface of mollusks. With such an incredible array of diversity, mollusks exhibit very different life cycles too. Diving into specific examples can be a fascinating way to understand the particular species that most excite you.
Let’s look at Loligo opalescens, the California market squid that mates off the coast of California’s channel islands. Sex is a once-in-a-lifetime event, with both males and females dying shortly afterward. These brainy cephalopods conduct a mating ritual – in which the male courts the female with an attractive display. In some species of cephalopods, this includes elaborate color changes, like a popstar with a wardrobe change – only way cooler.
If he succeeds in this form of foreplay, the male squid will then extend his hectocotylus, which is a specialized arm of his body, to deposit sperm packets. We call these sperm packets “spermatophores” and they land on or in the female squid. When this happens, the male’s hectocotylus will blush red as a warning to any competition nearby. After the adults mate, they produce benthic egg sacs, which then hatch into planktonic larvae and eventually develop into juvenile squid, which then mature into full adults. At which point, the life cycle begins again.
Elevations in atmospheric carbon dioxide lead to the acidification of ocean waters. This has a big impact on all animals on planet earth – including molluscs. Different mollusk species are particularly vulnerable at different stages of their life cycles. Especially during the embryonic and larval early-life stages, these organisms have very specific environmental needs. As the ecosystem changes due to a warming planet, many of these creatures will not be able to develop or survive. Because mollusks are such an integral aspect of many ecosystems, if they can’t reproduce, the web of life the depends upon them will be destabilized.
The predictions around this are dire.
And while mollusks are under threat, sometimes they are also the threat – in the form of invasive species aided by human habit and ignorance.
For example, zebra mussels, a mollusk native to Eurasia, are currently wreaking havoc on North American freshwater ecosystems. They arrived at the Great Lakes in the 1980s and have spread ferociously from there. As they move up riverways and along streams, they spread to Colorado and even Texas. Along the way, they filter out algae that other native species depend on. They also attach and incapacitate native species of mussels. Anyone who spends time in zebra mussel-infested waters known they can slice open the bottom of your foot like a piece of glass, while power plants spend millions of dollars trying to remove the mussels from clogged water intakes.
It is an evolving situation that will require dedicated activism and education to control. Something to think about and further investigate if you live in impacted areas.
Part of activating and understanding our role in climate change and protecting mollusks lies in a practice of deepening our connections to ecosystems and sustainably participating in the biological interactions within the landscape we call home.
For some people, that might look like using particular technology to remove zebra mussels from shorelines, interfering in their reproductive cycles, washing any boats with warm soapy water, and taking care not to transport water from one body to another. This will help to protect the native mollusks in your area that are under threat.
Other folks might get really excited about mollusk identification and sustainable foraging. As these areas of interest grow, so does our propensity for protecting ecosystems. We being shifting our own lifestyle habits towards more ecologically-minded alternatives.
Learning about what is possible, and sustainable, in your area will be a very specific area of research. It takes work but is well worth the trouble. Few things are as delicious as limpets cooked in butter on a sunshiny seashore with loved ones.
The mollusks of the world continue to evolve, ornament, feed, and mystify human beings. Observe them in the wild and continue to learn about their lives and relationships. You might just gain a better understanding of this colorful planet we call home. From the tiptop point of a mountain to deep-sea caverns, from Atlantic conch shells to land snails to the iridescent beauty of the abalones, mollusks have a lot to teach us about ecology and point us towards the creative possibilities of adaptions, change, and reciprocity.