Types of Mollusk: Snails, Bivalves, Squid, and More
Most people think of mollusks as slow-moving creatures, like snails, or animals that hardly move at all, like mussels. This is only part of the picture.
The oceans of the world were once dominated by schools of predatory mollusks called ammonites with sharp beaks and a surprising turn of speed. Those kinds of mollusks died out at the same time as the dinosaurs but, in the present day, squid and octopuses are formidable ocean hunters with an intelligence that outshines any fish.
This page explores all types of mollusks, from the humble slug to the compelling Giant Pacific Octopus.
What Exactly is a Mollusk?
Mollusks rank towards the bottom of the evolutionary tree appearing over 500 million years ago. They are, never-the-less, sophisticated creatures with:
- a heart and blood circulation,
- stomach and digestive tract
- special organs for breathing air (primitive lungs) or extracting oxygen from water (gills).
- a nervous system
- male and female reproductive organs
Mollusks are found in most places on Earth, from deep oceans to mountain tops. They share a similar level of overall characteristics but can appear very different at first glance.
So What is a 'Mollusc' (with a 'c' not a 'k')?
Pretty much every English speaker outside of North America calls these creatures 'molluscs'.
Many of the important shellfish that people eat are bivalve mollusks. This includes clams of all kinds, scallops, mussels and cockles.
Bivalves have a hard chalky shell to protect them from predators which is hinged so that it can open. Most suck water into their bodies through siphons, filter out food floating in the water and pump the water out again. You can see this in the video below.
Bivalves take oxygen from the same water using gills in a similar way to fish.
The internal organs are suspended in a central cavity and are bathed in blood that is rich in food and oxygen. A simple heart keeps the blood circulating.
Burrowers and Clingers
Many bivalves live in turbulent water near to the seashore. Waves and tides can quickly take them to places where they will die, like deep water or the land.
Some bivalves, like mussels, use very strong hair-like filaments called byssus to hold onto rocks.
Other bivalves like cockles and razor shells avoid rocky shores and live in sandy places. These creatures burrow into the sand as the tide retreats, or if predators threaten to eat them. They push a muscular 'foot' into the sand, spread it out wide, and then pull themselves down.
The picture below shows just how many cockles can live on a sandy beach.
Squid, Octopus and Nautilus Family
'Cephalopoda' is the scientific name given to the group that includes squid and octopuses.
These creatures are more sophisticated and intelligent than most other animal groups, including many animals with backbones. Their large eyes and brains make them excellent hunters.
'Cephalopod' means 'head and foot' in Latin and the name was chosen because these animals lack what early scientists thought of as a body. The brain and internal organs are found together in a single muscular mass (the head), with a 'foot' or 'feet' (sometimes arm-like tentacles) directly attached.
Camouflage and Bioluminescence
Many cephalopods can change color to camouflage themselves (useful for ambush predators that hide in wait for a meal to come by) or produce dazzling light shows to confuse larger predators, like sharks.
The color changes are achieved by skin cells called chromatophores that can shift different colored pigments around at speed. There are also light reflecting chemicals that brighten or dull down the effect.
Bioluminescence is a separate phenomenum involving the wonderfully named enzyme 'luciferin'. This interacts with oxygen to produce light under the control of the nervous system.
Octopuses are solitary creatures that live on the sea floor, especially liking reefs and rocky shores.
Strong suckers on the arms allow octopuses to cling onto prey animals like fish or get around in underwater burrows, rock crevices, and caves.
When an octopus wants to swim (usually in emergencies only), it uses pulses of water from a special, muscular tube called a siphon, to propel it in the right direction.
The Astounding Octopus Brain
Octopuses have the largest brains of any invertebrate (animals without backbones).
There is a large central brain near to the eyes and individual brains in each of the arms (or tentacles). Often these brains act independently. Tentacles can search for food, avoid danger and make sure they do not get tangled up, all on their own. The central brain overrides the smaller brains when there is something important to be done.
They are quick learners, often learning difficult tasks simply by watching other octopuses.
They also have long memories and seem to bear grudges. One aquarium worker was repeatedly sprayed whenever she went near 'Truman the Octopus' in the New England Aquarium (Boston).
Truman never sprayed anyone else and even remembered the worker after she returned from a break of several months. Truman, apparently, was also a very good shot.
Recent studies suggest that all octopuses are poisonous to some degree. They kill prey by jabbing their sharp beaks into their victims and injecting venom.
The Blue Ringed Octopus found in Australian reefs is the only species that can kill a human being. It is well worth avoiding.
The North Pacific Giant Octopus pictured below, can reach 5 meters (16 feet) across and weigh in at over 70 kg (150 lb).
They are found in coastal seas around the entire North Pacific, from California to Korea.
Lobsters, crabs and fish are their favorite food. One of these huge creatures was once seen eating a four-foot long dogfish.
Like octopuses, squid can change their color with special skin cells called chromatophores. The change can be almost instant as the squid adapts to changing backgrounds.
Squid can also produce light displays with bioluminescence to attract mates or confuse predators.
Unlike octopuses (that prefer a solitary life on the sea floor), squid are strong swimmers and often move around together in large shoals.
They have eight arms like octopuses but also a pair of special arms that are used to seize prey and deliver it to the mouth.
Sailors had plenty to fear in older times, and stories of many-armed sea monsters engulfing ships were common.
This is understandable when you consider that the Colossal Squid (scientific name: Mesonychoteuthis hamiltoni), for example, is about the same length as the ship that Christopher Columbus first sailed to America in (around fifty feet or 15 meters).
The Colossal Squid also has ferocious hooks like grappling irons for catching prey and even teeth on its tentacles.
One of its favorite foods is the Patagonian Toothfish which can grow to be bigger than human beings.
Very large squid are not really a serious threat in reality, however. The real giants live at great depths.
Those encountered at the ocean surface are usually dead or dying (but still scary enough to start a myth, or two).
Another kind of squid that spooked early mariners was the vampire squid (Vampyroteuthis infernalis).
The creature has impressive-looking spines inside the webbed tentacle array. It was believed that the creature could attach itself to a man's face and suck the victim dry.
This has been shown to be untrue but it could be the inspiration for the creature in the classic SciFi movie 'Alien' which did something similar (and looked very squid-like)
Squid perform beautiful dances as part of their mating rituals. These dances are all about showing other squid that they are:
- the right species (different species perform different dances)
- in the right life stage to mate (only mature squid dance like this)
- fit and healthy (the complex moves demonstrate this)
After mating, the females lay eggs in clusters on the sea bed.
Sadly, most squid die after mating.
There are only 6 species of the Nautilus family still alive today, although the fossil record tells us that there were once many.
Nautilus family members are similar to other cephalopods in their body layout but have a hard shell surrounding them body almost completely.
They possess many more tentacles than their squid and octopus cousins (around ninety) which are used to catch prey and also sense chemicals in the water that might mean food or danger is near. The tentacles lack suckers but have ridges which give a firm grip of any prey.
The Ancient Greeks were fascinated by these creatures and believed that they used sails to get around the world's oceans.
Ammonites are one of the most animals in the fossil record. Almost anyone can find one in almost any country on the planet, if they know where to look.
They swam in the worlds' oceans for over three hundred million years, dying out at the same time as the dinosaurs, around 66 million years ago.
Some were large creatures with thick shells that would defy the most ferocious of enemies. The largest fossil individual found so far is 8 feet, 6 inches (2 meters) in diameter.
Chiton are a type of mollusk that live in seawater near to coasts. They are not so very different to snails in their overall body arrangement. Whereas snails have a single shell, though, chiton have an upper shell composed of eight interlocking pieces. This protects them from predators but also allows free movement. When disturbed they can roll into a ball like a woodlouse.
Most are small (fingertip size) but some like the Gumshoe Chiton are roughly the size of a human foot.
This group includes snails and slugs.
Gastropoda is the only mollusk group with species that have made the transition to full time land dwelling.
These land dwellers (the snails and slug) are not as well adapted to dry conditions as some animals, however, and most like to stay in wet places or only venture out when it is damp or raining.
Sea-living gastropods include limpets, sea snails, conches, abalone, whelks and periwinkles,
They are a very successful group around 70,000 species.