Mollusks: Nudibranchs, Sea Slugs, Snails, Limpets, Abalone, Sea Hares, Clams, Scallops, Oysters, Mussels, Chitons, Octopuses, Squids and Nautiluses

Phylum: Mollusca

(from the Latin words for “soft body”)

Did you know that the hundreds of species of rainbow colored nudibranchs we so greatly admire as divers are closely related to snails, even those that are considered annoying garden pests? Did you know that many specialists consider octopuses, squids, cuttlefishes and nautiluses the most intelligent of the invertebrates? Many snails are smaller than the fingernail of your little finger while giant squid can grow up to 68 feet (21m) long. Giant squid inhabit the waters of the deep ocean while chitons, limpets, sea hares and many mussels inhabit tidal zones. And despite some obvious differences, all creatures described in the phylum Mollusca are closely related.

Mollusks, a collection of invertebrates that includes nudibranchs, slugs, snails, chitons, clams, oysters, scallops, mussels, cuttlefishes, octopuses, squids and many other familiar creatures, are a successful group of animals. In comparison with other invertebrates, mollusks are highly developed creatures. They possess well-defined organ systems and have sophisticated sensory abilities.

Species of mollusks are widespread, inhabiting marine, freshwater and terrestrial ecosystems. In the animal kingdom, only the Arthropoda, a phylum which includes lobsters, crabs, shrimps and insects, and possibly the phylum which describes worms known as nematodes (not particularly relevant to divers) contains more species. Scientists have described more than 110,000 species of mollusks and other species are constantly being discovered. Of the known species, more than 75,000 occur in saltwater or freshwater environs, and as divers we see one or more species almost every time we dive.

The number of classes in the phylum is six or seven depending on the school of thought of the evolutionary specialist. Of these, only members of four classes are commonly witnessed by sport divers and beachgoers. These classes are (1) Gastropoda (the gastropods) which includes all nudibranchs, sea slugs, sea hares, snails, limpets and more; (2) Bivalvia (the bivalves) which includes clams, oysters, mussels and scallops; (3) Cephalopoda (the cephalopods) which includes octopuses, squids and nautiluses, and (4) Amphineura which includes the chitons.

The Body Plans of Mollusks

The bodies of mollusks display bilateral symmetry, meaning they are organized front and back. All mollusks have a brain or concentrated bundle of nerves, and all have an organ known as a mantle which surrounds the body and is responsible in many species for secreting and maintaining a hard shell.

Although the many species of mollusks look quite different at first, the bodies of all species are separated into the visceral mass, head and foot. All of the major organs of the respiratory, digestive and excretory systems are contained in the visceral mass. The brain or nerve concentration is located at one end of the body in a recognizable head. In its most recognizable form such as those found in snails, nudibranchs and sea hares, the foot looks like a muscular hook used to pull the animal across the sea floor and for digging and attachment. While to the naked eye it looks as if snails use their foot to crawl across the substrate, many actually glide across the sea floor on a slippery secretion. Clams use their modified foot to dig and burrow while in octopuses and squids the foot is highly modified into eight tentacles which assist in swimming and grabbing prey. The tentacles are lined with sucker-like discs.

The mantle is a thin and often handsome flap of flesh that covers the body. As divers, we tend to notice the colorful mantles of creatures such as flamingo tongue, fingerprint and chestnut cowries and those in many species of scallops, clams and oysters. Some conch and other snails possess a colorfully decorated foot, and you will want to distinguish between the foot and mantle when identifying species. In many species, the mantle can be retracted and folded into overlapping flaps as it is tucked away next to the visceral mass.

In octopuses and squids, the mantle has been modified significantly to assist in swimming. In these animals, the water-filled mantle can be quickly and forcefully contracted to help create thrust. While mantles in octopuses and squids are quite flexible, they are also strong and muscular. Aiming a movable, tube-like body part known as a siphon controls the direction of the thrust.

Mollusk Shells

Many species of mollusks such as scallops, snails, mussels and oysters possess an obvious external shell that helps protect the animal from predators. The shell also helps protect species that inhabit tidal zones from the threat of desiccation by trapping life-sustaining moisture and oxygen.

Not all shell-bearing marine species are mollusks, and not all mollusks have shells. It might seem as if the shell is missing in animals like nudibranchs, sea hares, squids and octopuses. In the case of sea hares and squids, they have internalized a shell which has been greatly reduced in size. Like nudibranchs, adult octopuses have eliminated the need for a shell. Chitons, a group of animals seen in tidal zones, are the only mollusks that have jointed shells. This flexibility enables chitons to readily fit into depressions in rocks. The shell of all shelled species enlarges and thickens as specimens age.

Mollusk shells occur in three general forms. Some species such as many snails have a single shell which is typically shaped in a spiral. Other species such as scallops and oysters have two shells hinged together. These species are often called bivalves in reference to the pair of shells which are also referred to as valves. The third group, the chitons, has eight overlapping plates arranged in a row.

The Radula

With the exception of the bivalves, another characteristic of most mollusks is the presence of a rasping, tongue-like organ called a radula. The structure of the radula varies from species to species depending on the animal’s diet. However, as a rule, the radula is hard to the touch and armed with several backward curving teeth. The organ can be moved at will and is used by grazing species to scrape algae and other food off of various seagrasses, sponges and corals. More predatory species of octopuses and squids use their radula to puncture their victims and remove tissue and fluids from their prey. In some species, the radula also assists in the injection of paralyzing toxins.

Bivalves are filter feeders, and they have no use for a radula. It is believed that ancient bivalves possessed a radula but that it has been lost over time.

The Gastropod

Mollusks: Nudibranchs, Sea Slugs, Sea Hares, Snails, Limpets and More

The gastropods make up the largest class of mollusks with more than 80,000 species described. This group includes the many species of marine animals called nudibranchs, sea slugs, sea hares, snails, abalone and limpets. So, within this large and seemingly diverse class you will find ornate species of nudibranchs such as Indo-Pacific’s Spanish dancer, California’s Spanish shawl and the dull-looking California sea hare. Other gastropods include navanaxes, chestnut, flamingo tongue, fingerprint cowries, queen conchs, cone snails and keyhole limpets.

A characteristic of most members of this class is their single shell. The loss of a shell as adults in the case of nudibranchs and sea slugs is well-supported by evolutionary specialists and is evidenced by the fact that these animals possess a shell as larvae.

Nudibranch is derived from the Latin words for naked (nudi) and gill (branch). Nudibranchs are said to be “naked” because they are considered snails without shells, and the prominent projections on the top and backs of many species are gills. Trying to describe a nudibranch to someone who has never seen one is almost impossible, but it can make an interesting party game or communication exercise.

Snails such as abalones and conchs and gastropods such as cowries which have retained their shells, use them for protection. They can pull inside their shell until danger passes.

The shells of many species ranging from the potentially deadly cone snails to colorful top snails are quite striking, and each species produces a unique design. Maneuvering a hatch-like plate called the operculum can shut the opening to the shell. The head of gastropods is easily discernible and it is quite common to see a pair of stalked eyes on the ends of protruding tentacles peering from a shell. In some species, the tentacles have been lost.

Nudibranchs have developed several very effective alternative means of defense. Some rely on secretions that prove noxious to predators. Amazingly, in some other species the animals are able to steal the stinging cells of corals, hydroids and other cnidarians on which they prey and put them to good use. (For more information on cnidarians, see “Cnidaria: The Stingers,” Dive Training, April 2001.) The nudibranchs take the unfired stinging cells and transfer them to projections of their own backs where they provide a formidable defense.

Some sea hares are quick to release clouds of dark “ink” to repel or confuse predators. Like their close relatives the nudibranchs, individual adult sea hares possess functioning male and female sexual organs simultaneously, and like their terrestrial namesake, sea hares are incredibly prolific. They lay their eggs in large, entangled masses that often resemble huge balls of yarn. Incredibly, studies have shown that some California sea hares produce as many as 95 million eggs a month for months on end. Scientists with too much free time on their hands have made this calculation: if just one adult produced its normal load of eggs and all survived and reproduced without being eaten, and all their offspring did the same and the process repeated for one year, the earth would be 6 feet deep in sea hares. Like I said, way too much free time on their hands.

Collectively, gastropods feed on a variety of sources and display diverse feeding habits. Many graze on a range of algae and other food found along the sea floor. Some are scavengers, and still others are predators. The radula of predatory snails such as southern California’s Kellet’s whelk use their radula to bore holes into their prey which can include other shelled mollusks and the leathery eggs of horn sharks and swell sharks. Once the hole is bored, the radula is used to rasp the desired tissues.

In cone shells, the radula has been modified into a harpoon-like device which can be shot rapidly into prey to inject a highly toxic poison. Though it is extremely rare, cone snails can kill humans. It is wise to leave them alone.

A Caribbean animal called a lettuce leaf nudibranch is actually a close relative of nudibranchs known to scientists as a saccoglossan. These remarkable creatures carry their own greenhouse. They are algae feeders, but they only digest part of their take at any given feeding. They store undigested cytoplasm which they suck out of algae in the lettuce-like frills on their backs. The cytoplasm continues to photosynthesize, providing the saccoglossan with nutrition.

Bivalve Mollusks:

Clams, Scallops, Oysters and Mussels

Clams, oysters, scallops and mussels are known as bivalves because all possess two shells hinged by a very strong ligament. Either one or two strong abductor muscles can be flexed to draw the two shells together for protection, and it is these same muscles that are highly sought as food. The commercial value of some oysters results from the production of pearls, the result of accumulated secretions over foreign objects that lodge in the body tissues.

The mantle of bivalves secretes the shells and the connecting ligament and covers the vital internal organs. The mantle also encloses a pair of tubes known as siphons, which are used to draw in oxygen-rich water and expel oxygen-depleted water. Bivalves are filter feeders and the water also contains their food.

Bivalves differ from other mollusks in that they do not possess a distinct head and radula. They do have a large, muscular foot that is used for digging and for attaching the animal to the substrate. Some clams can rapidly bury themselves in sand and soft mud by digging into the sediment with their foot.

As adults, some scallops can crawl rapidly, burrow and even swim. To swim, they close their shell quickly, forcefully pumping water to create thrust. But most bivalves are immobile. They cement themselves to the bottom with strong threads secreted by the byssal gland or they burrow into one place in a reef.

Bivalves are prodigious spawners and often release millions of eggs during a single spawning. Fertilization is external; it occurs in the water column. Currents carry fertilized eggs and the resulting larvae to areas where the larvae settle out of the plankton and into a reef community.

The giant clam found in the South Pacific can live several hundred years and weigh up to 600 pounds (272 kg). In recent years they were sought so much that Australian officials moved to protect them from extinction.

Many bivalves are reclusive or heavily encrusted and as a result, often go unnoticed by divers. However, in some species such as the thorny oyster, the Indo-Pacific clam and the flame scallop, a Caribbean species also known as a file clam, the colorful mantle of a feeding specimen often catches attention. If you encounter a colorful bivalve and want a closer look, move slowly. Do not create a pressure wave or shadow. The mantle of scallops is lined with many primitive eyes that help warn these animals of predators. They react by closing their shell.

Despite the strong abductor muscles found in bivalves, some sea stars readily eat these mollusks. It is debated whether sea stars pull apart the shells by brute force or force the bivalves to open their shells to get needed oxygenated water.

The Cephalopods:

Octopuses, Squids, Cuttlefishes and Nautiluses

Worldwide, there are more than 650 species of mollusks described as cephalopods. Inhabiting every imaginable undersea habitat, this group includes octopuses, squids, cuttlefishes and nautiluses.

The word cephalopod is derived from the Greek words for “head” and “foot.” So it should come as no surprise to learn that the head and foot of cephalopods are prominent. The foot is highly modified into arms lined with suckers, hooks and other adhesive structures. The arms assist in snaring and holding prey and other objects and in locomotion.

In most cephalopods, the shell is greatly reduced or missing. In squids and cuttlefishes, the shell is relatively small and internal. Octopuses have given up their shells altogether, but they have developed several other highly effective means of self-defense. However, the shells of chambered nautiluses are large, ornate and elaborate.

Squids and cuttlefishes tend to live up in the water column where most feed on a variety of fishes. Most octopuses are bottom dwellers and prey primarily on worms, crustaceans and other mollusks. The largest species is the rather docile Giant Pacific Octopus which can weigh as much as 600 pounds (273kg). Despite the legends of sea lore, the tiny blue-ringed octopus found in the waters of southeastern Australia poses a much more serious threat to humans as members of this cryptic species are equipped with a highly poisonous toxin that can be injected when they bite.

The tentacles of octopuses and squids possess one or more rows of sucker-like discs. Squids possess 10 tentacles, octopuses have eight, and those animals known as nautiluses have as many as 90 tentacles. The tentacles of nautiluses are adhesive, but lack suction cups. A cephalopod usually grabs its prey with its tentacles, drawing it close to its mouth. Then the predator bites its victim with a strong parrot-like beak. A powerful immobilizing toxin is often injected by the bite to subdue the prey.

The nervous systems of cephalopods, especially the senses of vision and smell, are well-developed, enabling fast responses to a variety of stimuli. Elaborate mechanisms, the eyes in cepha-lopods bear strong resemblance to human eyes. In fact, common squid (also known as market squid), which as their name implies are quite common in California waters, are routinely used in eye research and teaching.

Sophisticated chemoreceptors are found on the end of each tentacle in octopuses. The receptors help these mollusks avoid predators, interpret their surroundings, and locate and capture their prey. When threatened, some octopuses are known to eject ink which dulls the olfactory senses of predators such as moray eels and creates a smoke screen allowing the octopus to escape.

Octopuses are remarkably adept at changing the shape of their bodies to maneuver through extraordinarily small openings. Their bodies are somewhat like a large supple muscle as they lack a spine and shell.

Perhaps the most awesome ability in octopuses is their ability to alter their shape, color and texture to match their surroundings. Vision plays a key role, as do pigmented cells called chromatophores. Octopuses, squids and cuttlefishes are also believed to use their ability to rapidly change their color and to cause their bodies to pulsate and ripple in astonishing displays as a means of communicating with other members of their species. Vivid color displays are common during courtship and when the animals seem threatened.

Like squids, octopuses create thrust to swim by forcing a powerful stream of water through their excurrent siphons. This natural jet engine design functions by first expanding the mantle, causing it to fill with water. Then powerful muscles rapidly contract the mantle, forcing a powerful jet of water through the excurrent siphon. They are highly directable, allowing octopuses and squids to swim equally well in all directions.

The sexes of cephalopods are separate. Males possess a tentacle called a hectocotylus which transfers sperm packets to the females.

The price of sex is often quite high. Some squid live only long enough to reach maturity, mate and lay their eggs. Many female octopuses lay their eggs in grape-like strands and then wither away from exhaustion until they perish as they guard their nests without leaving to feed.

Many species of squid play vital roles in several oceanic food chains. A variety of fishes including rays, sharks, tunas, jacks and billfish, as well as dolphins, whales, sea lions, seals, and many other marine animals depend on a variety of squids as a primary food source. In central and southern California, massive gatherings of mating squid attract a host of scavengers, predators and divers to witness a stunning natural phenomena. The squid often gather in uncountable numbers and are soon joined by a variety of rays, sharks, pilot whales, sea lions, seals, fishes, lobsters, sea stars, birds and other animals that prey on the dead and dying squid.

An estimated three to five species of cephalopods that occur in the waters of the Western and Indo-Pacific are known as chambered nautiluses. They have appeared in the fossil records for at least 200 million years. Some specialists think 400 million years is a more accurate estimate. While the living animal occupies the outermost chamber in the animal’s shell, a tube leading from the inner chambers helps enable chambered nautiluses to regulate the flow of gases which control buoyancy. These cephalopods are normally reclusive and prefer deep water.

Mollusks are a varied, yet fascinating group of animals. Encountering any number of species will add pleasure to your dives.

Sex In Mollusks

In most species of mollusks the sexes are separate. However, it seems as if there are always exceptions to the general rule when it comes to sex in the animal kingdom. In bivalves as well as snails, nudibranchs, and sea slugs, a single adult simultaneously possesses male and female reproductive organs. Animals that fit this description are hermaphrodites. In some cases male and female organs are functional simultaneously. In other cases only one organ or the other is functional at any given time. But despite all the explanations, you can rest assured that when two nudibranchs get together, it is truly a case of boy meets girl meets boy meets girl.

It is somewhat reassuring to learn that even in hermaphrodites cross-fertilization with another adult is the rule, and self-fertilization is rare. However, it can make you scratch your head as you try to figure out Mother Nature’s grand plan when you discover that in many species of oysters a single animal changes sex several times during a single spawning season. Even with this feat, the mollusks are not through amazing us. When mating, the snails described in the genus Crepidula form a pile and the snail on the bottom becomes a female.

Story and Photos by Marty Snyderman