Tuesday, October 23, 2012

Marine Snails of Singapore

A snail (Phylum Mollusca, Class Gastropoda) is a soft-bodied animal with a single coiled shell that is usually large enough for the animal to retract into.

"Gastropoda" means “stomach-foot", as gastropods often appear as if they are crawling on their stomach, which is actually their foot. They usually have one to two pairs of tentacles, and most have a radula, which is a rasping tongue-like structure for feeding. Gastropods can be snails or slugs - with the latter having either a much reduced shell, an internal shell, or no shells at all. Snails and slugs are not two different genetically distinct groups though, and a particular slug may be more closely related to a snail rather than another slug.

Marine snails refer to snails that live in the marine environment, and Singapore certainly has a wide variety of marine snails! To differentiate one species from another, it is important to know the parts of a snail shell first. I have a diagram here highlighting some of the parts of a snail shell that is important to help with identifying the species:

You can click on the above picture to for a bigger image, and a clearer look at the anatomy of a snail shell.

And here are some of the marine snail families that I have seen in Singapore. For families that I have photographs of more than one species, I will a separate page with more photos and details. Thanks to Siong Kiat for helping with the identification!

Family Nacellidae

Nacellid limpets (limpet is a common name for any marine snail with a flattened conical shell) are characterised by having a mother-of-pearl iridescence on the interior of their shells, which can be seen sometimes on the exterior surface too on well-eroded shells. Also, they have folds on the mantle edges acting as secondary gill leaflets, and a long radula (tongue-like structure for feeding) several times longer than the shell. The nacellid above is a Rayed Wheel Limpet (Cellana radiata). It is usually found on rocky shores, and like other limpets, it does not move much during low tide to avoid dehydration and also, the rocks will be baking hot under the sun. During high tide, it will move around, grazing on algae on the rock surface.

Family Lottiidae

Lottid limpets are characterised by the interior of their shells being porcellaneous. They have feather-like gills. The shells are bilaterally symmetrical - very obvious for species with prominent ridges (or ribs) on the shell exterior. Like the nacellids, they clamp tightly to the rocks during low tide, sealing the shell edges with the rock surface, protecting them from desiccation. This adaptation, together with their flattened conical shell also allows them to survive the strong waves pounding against the rocks. The lottids and nacellids are also called true limpets, being part of the clade Patellogastropoda. The above is a Sweet Limpet (Patelloida saccharina). 

Family Fissurellidae

Despite their flattened conical shells, fissurellids are not true limpets, and in fact are not very closely related to them. They can be seen under rocks or in shady areas during low tide, sometimes with their mantle exposed. Their flattened shells allow them to easily creep under rocks and into cracks and crevices to seek food and also to avoid the hot sun. The species with a hole at the top of the shell are commonly called Keyhole Limpets (Diodora spp.), and are believed to feed on sponges. The ones without the hole, and their mantle encloses the edge of the shells are commonly called Shield Limpets (Scutus spp.). The Shield Limpets feed mostly on algae .

Family Haliotidae

Abalones (Haliotis spp.) have ear-shaped shells that are loosely coiled with a row of small openings on the left side for respiration. They are seldom encountered in local waters, and are usually much smaller than the temperate species - the biggest I have seen in Singapore is about 6-7cm long. They feed on macroalgae, and colours of their shells are said to reflect their diet. Abalones are widely considered - not just among the Chinese - to be a delicacy, and there are many farms around the world rearing the bigger species. Their highly iridescent shells after polishing are used to make into decorative items and jewelleries. Wild abalones are regarded to be highly threatened with extinction - not because they are collected for food - but more because they are easily affected by acidification of the ocean (due to increasing levels of carbon dioxide) which erodes their shells.

Family Trochidae

The trochids are sometimes called top shells as the shells of many species resemble toy tops. They have a thin and round operculum (the trap door) which allows them to retreat deep into their shells when threatened. Most species feed on algae and detritus (tiny decaying matter), but some smaller species may filter plankton and other edible particles from the water column. The bigger species are collected for food, and as their shells have a nice iridescence when polished, some are collected for making decorative items or cut into buttons. The picture above shows: (left main) a Radiate Top Shell (Trochus radiatus); (left inset) the thin and flexible operculum of a Toothed Top Shell (Monodonta labio); (left bottom) a Toothed Top Shell; and (left top) a population of Button Shells (Umbonium vestiarum).

Family Chilodontidae

The chilodonit snails are a family of small snails previously classified as top shells, but molecular genetic studies elevate them to a separate family of their own. They are generally turban-shaped, with a corneous operculum. Their shells have a nice iridescence when polished, and some are collected for making decorative items. Most species graze on algae.The main picture features a Four-keeled Euchelus (Euchelus quadricarinatus), while the inset shows the underside of a Black Euchelus (Euchelus atratus).

Family Turbinidae

The turbinids are sometimes called turban snails for the turban-like shells of many species. Unlike the rather similar-looking top shells which have thin and flexible opercula, Turban snails have thick and hard opercula which protect them from predators. These pretty opercula come in different colours and patterns, and are sometimes called "cat's eye". They are used in shell jewelleries and bigger ones are used as paper weights. Turban snails feed on algae. Some species may come with spines. The left photo shows a Turbo sp.; right top is an Astralium calcar with protruding spirally arranged spines; and right bottom shows the thick and hard cat's eye of a Turbo intercostalis, which is very smooth in the middle.

Family Angariidae

The Dolphin Snail (Angaria delphinus) is the only species from this family that I have seen in Singapore. It was previously classified as a trochid or turbinid, but recent studies place it in its own family instead. With some imagination, the coiled shell appears like a jumping dolphin, and hence the common name. Its shell is very thick, though the operculum is thin and flexible, much like the trochid's, allowing it to retreat deeper into its shell when threatened. The shell is often overgrown with algae, allowing it to blend exceptionally well into the surrounding. It feeds on algae.

Family Neritidae

Nerites are snails with relatively thick and somewhat hemispherical shells, making them difficult for predators to grip and crack. They have a semi-circular aperture (i.e. the doorway) that for some species may come with a few teeth on the inner side, and a tight-fitting operculum with a peg that can "lock" the operculum within the aperture. The spire usually appears somewhat depressed. They feed on algae as well. Some examples from this family include: (left) the Undate Nerite (Nerita undata); (right top) the Oualan Nerite (Clithon oualaniensis); and the Articulate Nerites (Nerita articulata).

Family Cerithiidae

The cerithiids are one of several families of horn shells with a long and pointed spire. The operculum is not as circular as those of other horn shells, and is marked by a rapidly expanding spiral pattern with the middle of the spiral not in the centre of the operculum. There is usually a long and open siphonal canal at the front end extending from the aperture. They are often gregarious, feeding on algae and detritus. Some of the species found in Singapore include: (left) the Chinese Horn (Rhinoclavis sinensis) with the operculum mark by an expanding spiral pattern, and the long and open siphonal canal; (right top) the Pellucid Horn (Clypeomorus pellucida); and (right bottom) Clypeomorus batillariaeformis.

Family Batillariidae

The batillariids are represented by just one member,the Zoned Horn Shell (Batillaria zonalis), in Singapore. They are commonly called Creeper Snails locally as they are often seen in huge numbers "creeping" slowly over the sandy/muddy substrates, leaving long and narrow trails behind them. This species has a short siphonal canal and has a somewhat circular operculum marked with concentric circles on the surface. The shell has distinctive triangular patterns repeating round and up the spire. They are gregarious, and thousands and thousands of them can be seen on the shore, feeding on algae and detritus. They are often mistaken with other horn-like snails.

Family Potamididae

The potamidids are yet another family of horn shells (or creeper snails). Unlike the cerithiids, the operculum is usually circular and marked with concentric circles on the surface. They have a shorter siphonal canal. They feed on algae and detritus. Some of the bigger species are collected for food. Some local examples include: (left) the Mud Creeper (Terebralia palustris); (right top) Small-winged Creeper (Cerithidea microptera); and (right bottom) the Belitong (Terebralia sulcata).

Family Planaxidae

The planaxids are small snails with thick shells and a pointed spire. The spire occupies about half the length of the shell. I only have photos of one of the species found in Singapore, Planaxis sulcatus. They usually gather in large numbers on rocky upper shores, and feed on algae. They are sometimes confused with the periwinkles, but they have much thicker shells.

Family Calyptraeidae

The calyptraeids, or slipper snails, resemble limpets with their flattened or conical shells. They are consecutive hermaphrodites, starting out as males but eventually become females later. The two species found in Singapore, Calyptraea extinctorium and Crepidula walshii, are usually found attached to the shells occupied by hermit crabs or to the undersides of horseshoe crabs. The former has conical shells, while the latter has flattened shells - the bigger ones are the females, while the smaller ones on top of the females are the males. They are filter feeders, and hence living with the hermit crabs means opportunities of food particles sent to the water column as the hermit crabs feed. They are sometimes mistaken for other flat conical snails .

Family Cypraeidae

The cypraeids, commonly known as cowries, have smooth and glossy egg-shaped shells. Some may come with interesting patterns, while others may be very plain. They keep their shells smooth by enclosing them in their mantle, which prevents other organisms like algae or barnacles from growing on them. At the same time, the mantle protects the shell from abrasion, and repairs the shell if there are damages. The smooth shell makes it hard for predators to have a good grip, and the snail will retract into the shell when threatened. They have a narrow aperture, with teeth along the sides, which makes it difficult for predators to reach the snail within the shell. The mantle often also has protrusions or dull colours to enable the snail to camouflage. Due to their smooth and pretty shells, many cowries are collected to make into decorative items, resulting in the over-collection of some of the more attractive species in some localities.

Family Ovulidae

The ovulids, or egg cowries, are closely related to the cowries. Like the latter, they also have smooth and glossy shells, and they can enclose their shells with their mantle. They, however, lack the teeth along their apertures. Depending on the species, they can be egg-shaped or spindle-shaped. They are usually found living on corals and gorgonians, feeding on the polyps of these animals. Many of these snails have colours or patterns similar to their hosts for camouflage purposes. They are sometimes mistaken with the closely related cowries.

Family Ficidae

The fig snails got their common name from their shapes, which resemble the shape of the edible fig. They have a very short spire, a wide and long aperture, and a long siphonal canal. They are usually found in sandy areas. Some studies suggest that Fig Snails feed on other invertebrates, such as polychaetes or echinoderms, but little further studies have been done to confirm these. So far I only have photos of the Variegated Fig Snail (Ficus variegata).

Family Littorinidae

The periwinkles are a diverse group of snails commonly found on the upper shores, either on rocks, trees or seaweed. They generally have a smooth columella (the inner side of the aperture or doorway) and thin operculum. They are usually inactive, being exposed to more heat on the upper shore, and hence they are very heat tolerant. The species on high rocky shores feed on algae but usually only during the high spring tide. The ones on the trees usually feed on the film of algae growing on the leaves and bark. They are able to breathe air with their simple lungs, modified from the mantle cavity. Some species can be confused with the planaxids, but their shells are thinner.

Family Naticidae

The naticids are commonly called the moon snails due to the round shell, nocturnal habits and many species are whitish in colour. The body, when fully extended out of the shell, is proportionally much bigger than the shell as the animal inflates itself with sea water. Part of the mantle covers the shell, protecting it from abrasion and preventing other organisms such as algae or barnacles from growing on the shell. Hence, the shell usually appears smooth without encrusting organisms such as algae or barnacles. Moon snails hunt for small clams and snails in the sand to feed on. They are often seen digging just beneath the surface of the sand. As they burrow, they will spread their foot out wide to detect the prey. Upon detecting the prey, they will enclose the prey with the huge foot, and secrete an acidic to soften the shell, after which they will slowly create a hole on the shell with the radula, and feed on the prey in the shell. Moon snails lay their egg capsules in sand collars, which are basically a flattened and hardened ribbon of egg capsules, sand and mucus, spirally coiled up.

Family Assimineidae

The assimineids are commonly called berry snails locally, as they are very small (0.5cm or smaller), usually red, orange or brown in colour, and gregarious - hence appearing like a handful of berries dropped on the ground. They breathe atmospheric air, with part of their mantle cavity modified into simple lungs. They feed on detritus in the mangroves and mud flats. The species found on our mudflats is usually the Red Berry Snail (Assiminea brevicula).

Family Strombidae

The strombids, or conches, are a diverse group of snails with thick shells, especially the outer lips. The operculum is thin and long with serrated edges, and is used like a "walking stick" to anchor the animal as it moves by dragging the shell along, sometimes in leaps. They have a pair of eyes on long stalks, allowing them to peep out of their shells to check for threats before they move. Many species are collected by human for food, and the pretty shells collected for making decorative items. The shells vary widely from species to species - some may come with long spines while others may not, while some may have a much longer spire. Strombids are mostly herbivorous, feeding on algae, and occasionally detritivorous. Their shells are usually very well camouflaged, covered with algae, silt and even corals sometimes. The main picture above features a Spider Conch (Lambis lambis); right-top features a Vittate Conch (Doxander vittatus); and right-bottom features a Gong Gong (Laevistrombus turturella).

Family Bursidae

The bursids are commonly called Frog Shells, as their nodulose shell surfaces resemble the warty skins of some frog species. They are characterised by the thickened ridges (varices) running along the sides of the shells, and the deep canals at the front and back end of the aperture. They are believed to feed on segmented worms, which they stun with their acidic saliva. The only species I managed to photograph is Bufonaria rana, which has spines along the varices.

Family Ranellidae

Commonly called the tritons or trumpet snails, the ranellids are characterised by thick shells with a long siphonal canal extending from their backends. Larger species were hence used as trumpets in olden days by blowing into the siphonal canal. Some species may come with not more than two strong ridges (varices) running along the sides of the shell. I have only photographed two species in Singapore so far - Linatella caudata with the hairy appearance, and the small Gynerium natator with the strong varices. The former has been observed to feed on Fan Shells (Pinna bicolor). Trumpet Snails generally can secrete a paralyzing saliva to stun their prey.

Family Cassidae

The cassids, commonly called helmet shells, have helmet-shaped shells with short spires. The aperture is long and narrow. The outer lip is thickened and recurved outwards to form a helmet-like rim. They feed on echinoderms, usually sea stars or sea urchins. Some species first secrete a paralysing saliva to prevent the sea urchin prey from releasing its venom, then secrete an acid to dissolve the shell so that they can feed on the prey. The main photo and right-top show a Japanese Bonnet (Semicassis bisulcata), while the right-bottom features a Grey Bonnet (Phalium glaucum).

Family Vermetidae

The vermetids, also commonly called worm snails, build tube-like shells which resembles those of some segmented worms, such as the sabellariids. The interior of the tubes of worm snails, however, is cleaner and glossier, due to a layer of mother-of-pearl (nacre). The ones I have seen in Singapore so far come with an operculum, but there are species which lack an operculum as well. When they are young they are actually free-living, but settle down and cement themselves to hard substrates as they mature. To feed, they produce mucus strands to stick and trap plankton. Some species have been observed to come out of their tube to feed on tiny animals. The photo above features a Vermetus sp., while the inset shows its operculum.

Family Columbellidae

These tiny snails are also called dove snails, though I have no idea why they are so named. The small shells are thick and strong, the outer lips are especially thickened and they have a small and elongate operculum. The Lightning Dove Snails (Pictocolumbella ocellata) are very common on rocky shores, easily recognised with the pale "lightning" patterns on a black background. The inset features a Dotted Dove Snail (Euplica scripta). Dove Snails appear to have a rather diverse diet across species, with some feeding on algae, some scavenge, and others feed on sessile animals like sponges and sea anemones.

Family Nassariidae

The nassariids, also called dog whelk or just whelk locally, are characterised by a tall spire, and a long siphon. They are mostly scavengers, and the long siphons allow them to sniff out dead animals. Some Dog whelks have small sea anemones growing on their shells, which get a free ride to food - decaying particles from the carrion the dog whelk is feeding on.

Family Melongenidae

The melongenas typically have a drop-shaped or oval operculum and strong foot, and some species has been observed to use its operculum as an anchor to drag and move around. Most species feed on clams and oysters, but the Spiral Melongena (Pugilina cochlidium) feeds on barnacles. Another species which I have photographed in Singapore is the Ternate Melongena (Hemifusus ternatanus) with a longer shell, shown in the inset. The egg capsule are arranged like little dominoes.

Family Muricidae

The muricids are often called rock shells as they are found on rocky shores, or drills as they bore holes on barnacles and other shelled molluscs to feed on them; or just murex for the ones with spines. The shells vary from species to species, but are generally thick and strong. Some species feed on clams by prying the shells open; some species secrete an acid to soften the shells of their prey, and use their radula to scrape a hole through the shells, taking hours in the process, so as to access the meat; yet other species may just feed on other small invertebrates. The ones on our shores generally lay masses of yellow egg capsules which turn purple when they hatch. Since ancient time, a valuable reddish purple dye had been extracted from various species of murex.

Family Mitridae

The mitres generally have obvious spiral sculptures on their shells, and narrow apertures. Small winding ridges (or plaits) can be found on the columella. They are carnivorous and scavengers. The common name and family name come from the resemblance of the shells of some species to the headgear, the mitre worn traditionally by religious leaders. I have only photographed one species so far, the Undulate Mitre (Pterygia undulosa) shown above.

Family Volutidae

The volutes are large snails with a short spire and most species have no operculum. The large foot usually comes with obvious patterns and colours, and they have a long siphon. They are carnivorous, and the ones in Singapore feed on other snails and clams. They can burrow to seek for their prey with their long siphon. Upon finding the prey, they will wrap their foot around it, and wait for the clam or snail to open up or emerge. They will then use their radula to feed on the prey. Unlike many marine snails, the eggs of the volutes hatch directly into small snails and not planktonic larvae. Many of these snails are collected for food and for making decorative items. The snail of the left is a Baler Shell (Melo melo), which can grow to more than 20cm long. The right top is an adult Noble Volute (Cymbiola nobilis), while left bottom is a juvenile which is only of a single shell colour - the patterns have not formed yet.

Family Olividae

The Olive Snails are so named for their olive-shaped shells. They are characterised by their smooth and glossy cylindrical shells, and a long and narrow aperture. Most species have no operculum. Like other snails with highly polished shells, they protect the shells by enclosing it in their mantle flaps. Olive Snails are carnivorous sand-burrowers, feeding on other small animals or carrion. I have only photographed one species in Singapore - the Weasel Olive (Oliva mustelina).

Family Conidae

I have not seen a live cone snail in Singapore, though others have seen them and we often see eroded shells on our shores. They generally have conical shells, a long and narrow aperture, and either a small operculum or no operculum at all. All cone snails are venomous and carnivorous, their prey range from snails to crustaceans to fishes, depending on the species. The radula tooth, attached to a long proboscis, works like a harpoon. The snail can fire the tooth loaded with venom into its prey, inject the venom to paralyse it, and then draw back the proboscis with the prey attached. The venom from some species is sufficient to kill humans, and hence they must not be handled.

Family Turridae

Turrids are characterised by their turret-shaped spire and long siphonal canal. The outer lip is thin, with a characteristic notch at the upper part. They are carnivorous - some species rasp at prey with their radula, while others use their venom-loaded radula teeth like a harpoon, similar to cone snails. I only have photographs of one species, the Javanese Turrid (Turricula javana), which is found in sandy areas.

Family Cancellariidae

The cancellariids are also called nutmeg snails, as the shells of some species resemble nutmeg seeds. Many of the species have a turreted spire, somewhat resembling a spiral stairway. The aperture is relatively wide with no operculum in adults, though the young of some species are known to have an operculum. Some scientists believed that they feed on microorganisms, while others believed that they feed on the body fluids or egg cases of marine animals - some parasitise other marine animals. The only species that I have seen in Singapore is the Scalariform Nutmeg (Trigonostoma scalariformis).

Family Aplustridae

The aplustrid snails, often called paper bubble shells, have thin and round shells, hence the common name. They have a depressed spire, and the bodies of the snails are usually paper thin and brightly coloured. They have a wide aperture, and do not have an operculum. I have only photographed one species in Singapore so far - the White-banded Paper Bubble Snail (Hydatina albocincta). The soft parts of this snail cannot retract completely into the shell, and hence it avoids predation by swimming. Other paper bubble snails may avoid predation by camouflage. They feed on cirratulinid polychaete worms.

Family Architectonicidae

Sundials (Architectonica spp.)
Architectonicid snails are also called sundials, for their resemblance with the timekeeping sundials. The shell is circular and depressed, with a flat or convex base, appearing like a circular coil. They are usually sand-burrowers, though some are found on hard substrates too. They usually feed on sea anemones and other cnidarians, and have a tough cuticle covering the mouth region to protect the snail against the stings from the cnidarian prey. I only managed to photograph two species in Singapore - the Clear Sundial (Architectonica perspectiva) in the main picture, and the Partridge Sundial (Architectonica perdix) in the inset.

Family Siphonariidae

The siphonariids have flattened, conical shells, and hence are sometimes mistaken for the true limpets. Unlike the true limpets that breathe with gills, siphonariids breathe with simple lungs. In addition, siphonariids are hermaphrodites, meaning each animal possess both male and female reproductive systems, while true limpets have separate sexes. It is possible to differentiate some of the species by looking at the shells - many of the true limpets’ shells are bilaterally symmetrical, if you look at the patterns and sculptures. The siphonariids are also called false limpets, and so far I have photographed three species - on the left is the Flattened Siphon Shell (Siphonaria atra) which, right top is the Javan Siphon Shell (Siphonaria javanica) and right bottom is the Guam Siphon Shell (Siphonaria guamensis) .

Family Ellobiidae

Technically, the ellobiids are classified as terrestrial snails, and are considered by many to be the most primitive of terrestrial pulmonates (snails which have lungs instead of gills) found along our shores. They breathe air instead of water, are hermaphrodites, and do not have an operculum. The inner shell walls of most species are partially or completely resorbed, and hence empty shells are very buoyant. Species that I have photographed include: (left) Midas' Ear Shell (Ellobium aurismidae); (right top) Singapore Hollow-shelled Snail (Melampus sincaporensis); and (right bottom) Trigonal Pythia (Pythia trigona).

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Zainul Usman said...

good references to identification Seashells mollusk, where will we buy the book about how to identification marine mollusk? Thanks for information and nice blog...

Anonymous said...

thanks for putting this one up. this served as a very good guide in my study regarding mollusks associated with mangroves