Micromollusk

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Numerous shells of the freshwater snail Potamopyrgus antipodarum compared to an American dime, which is 18 mm in diameter Mudsnail2.jpg
Numerous shells of the freshwater snail Potamopyrgus antipodarum compared to an American dime, which is 18 mm in diameter
Two shells of the freshwater snail Gyraulus crista, about 2 or 3 mm in width Gyraulus (Armiger) crista.JPG
Two shells of the freshwater snail Gyraulus crista , about 2 or 3 mm in width
Six shells of the land snail Columella edentula, the scale bar is in mm Columella edentula.jpg
Six shells of the land snail Columella edentula , the scale bar is in mm

A micromollusk is a shelled mollusk which is extremely small, even at full adult size. The word is usually, but not exclusively, applied to marine mollusks, although in addition, numerous species of land snails and freshwater mollusks also reach adult size at very small dimensions.

Contents

These tiny mollusks or their tiny shells are easy to overlook, as many of them are not very noticeable to the naked eye, and thus many people are not aware that they even exist. Nonetheless there are large numbers of families and vast numbers of mollusk species, in particular marine gastropods or sea snails, which are minute enough to be considered micromollusks.

Considerable numbers of marine gastropod species are only about 5 or 6 mm in adult size; many others are only about 2 or 3 mm in adult size; and a few have adult shells which are as small as one millimeter or even smaller still. Despite their tiny size, many of the shells have a good deal of elaborate sculpture. A fair number of them are even quite colorful, although many others are colorless and translucent.

Certain species of micromollusks are very common in the right habitat, and can on occasion be present in huge numbers. However, because of their minute size, micromollusks often go unnoticed by beachcombers, shell collectors and even more serious conchologists.

Micromollusks are not very popular as a subject of study, even among professional malacologists, primarily because these minute species can be very challenging to work with. It can often require great care, patience and persistence to find micromollusks, sort them, store them, and identify them correctly. Working with them usually also requires special techniques and special equipment compared with that needed for most of the larger shelled species. Discriminating the features necessary for successful identification of micromollusks to the species level almost always requires a stereo or dissecting microscope. Identifying, or adequately photographing, the smallest species may sometimes require a scanning electron microscope. Access to a first rate scientific research library is also often necessary, since many of the popular shell identification books and field guides either omit micromollusks completely, or only include a very few species for any particular area.

Because of all these various challenges, micromollusks are poorly known compared to their larger relatives, and thus there are undoubtedly numerous species which have yet to be discovered and described.

Shells of some micromollusks: from left, Truncatella bahamensis (holotype), Truncatella bilabiata bilabiata, Truncatella pulchella. CaribbeanTruncatella1970.jpg
Shells of some micromollusks: from left, Truncatella bahamensis (holotype), Truncatella bilabiata bilabiata, Truncatella pulchella .

Defining the size

There is currently no universally acceptable definition for the upper limit of the size range for micromollusks. Because of this, the exact use of the word varies from one expert to another; however, the maximum size of the shell of a micromollusk species is usually 5 mm to 7 mm, around one quarter of an inch or less.

The shells of the very smallest micromollusks are less than one millimeter in adult size, and thus they are truly microscopic, smaller in fact than some sand grains. Many other micromollusks are from 2 to 4 mm in maximum size; even for people with small hands, this means that the shells are far too small to be picked up with the fingers using the normal grasping action.

Techniques used

Micromollusks are most often found by the careful searching of sediment samples which have been taken from "promising-looking" areas. Once sediment samples are clean and dry, they are searched under the microscope. Minute shells are picked out using a very small sable-hair paintbrush which has been dampened at the tip. Soft entomological tweezers are also sometimes used. Shells this small are usually stored in small glass vials, or in paper micromounts.

Marine sampling

For dead shells of marine species on sandy beaches, these minute empty shells wash up in the lightest deposits of beach drift, in more sheltered areas where the very smallest particles of detritus are left behind by the retreating tide; this is often in a rather flat and level part of the beach. When at least some minute shells are seen on close visual inspection, a sediment sample taken at that spot may contain many more.

Underwater in a marine context, for example when scuba diving, a sediment sample is often taken from areas such as the surface layer of sand under rocks, or at the edge of a coral reef. Live micromollusks are also found by washing seaweeds or algae in fresh water in a bucket.

Non-marine sampling

Land micromollusks are often found by taking samples of leaf litter from rich areas, subsequently sieving or sifting the litter, and then searching it under a strong light and magnification.

Freshwater micromollusks which live on aquatic plants are often collected by passing a plankton net vigorously through and around water weeds so that minute mollusks end up falling into the glass tube at the end of the net. Small bottom-dwelling micromollusks such as Pisidium species are found by scooping a bottom sample of mud into a fine meshed long-handled net, and then agitating it and moving it through the water repeatedly, until only solid particles remain in the net.

Because most empty land snail shells and many empty freshwater shells float, another effective way to sample dead shells from an area can be to sort through river drift, the accumulations of small floating bits and pieces left behind by creeks and rivers after floods.

Excluded from the category

Juveniles or larval stages of larger species of mollusk are not considered to be micromollusks, even though these immature shells may sometimes be very small indeed, and may often be encountered in the same sediment samples where micromollusks are found.

Examples

The Bailey-Matthews National Shell Museum exhibit about micromollusks shows some species from Sanibel, Florida BMNSM micromollusks.jpg
The Bailey-Matthews National Shell Museum exhibit about micromollusks shows some species from Sanibel, Florida

The word "micromollusk" is used most often for marine shelled species, although a reasonable number of land and freshwater species are also small enough to qualify as micromollusks: for example, the land snail family Punctidae and the majority of species in the freshwater bivalve genus Pisidium .

Numerous families of marine gastropods are composed entirely, or almost entirely, of minute species:

Also see:

Fresh water and land species:

Related Research Articles

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Bivalvia Class of molluscs

Bivalvia, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. Bivalves as a group have no head and they lack some usual molluscan organs like the radula and the odontophore. They include the clams, oysters, cockles, mussels, scallops, and numerous other families that live in saltwater, as well as a number of families that live in freshwater. The majority are filter feeders. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment where they are relatively safe from predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. Some bivalves, such as the scallops and file shells, can swim. The shipworms bore into wood, clay, or stone and live inside these substances.

Diaphanoidea is a taxonomic superfamily of small sea snails, marine opisthobranch gastropod mollusks or micromollusks in the Cephalaspidea, the headshield slugs and bubble snails.

Conchology Study of mollusc shells

Conchology is the study of mollusc shells. Conchology is one aspect of malacology, the study of molluscs; however, malacology is the study of molluscs as whole organisms, whereas conchology is confined to the study of their shells. It includes the study of land and freshwater mollusc shells as well as seashells and extends to the study of a gastropod's operculum.

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A siphon is an anatomical structure which is part of the body of aquatic molluscs in three classes: Gastropoda, Bivalvia and Cephalopoda.

Sea snail Common name for snails that normally live in saltwater

Sea snail is a common name for slow-moving marine gastropod molluscs, usually with visible external shells, such as whelk or abalone. They share the taxonomic class Gastropoda with slugs, which are distinguished from snails primarily by the absence of a visible shell.

<i>Pisidium casertanum</i> Species of bivalve

Pisidium casertanum, the pea cockle or pea clam, is a minute freshwater bivalve mollusc of the family Sphaeriidae.

Pisidium moitessierianum is a species of minute freshwater clam. It is an aquatic bivalve mollusc in the family Sphaeriidae.

<i>Truncatella</i> (gastropod) Genus of gastropods

Truncatella is a genus of very small land snails with an operculum, terrestrial gastropod mollusks in the family Truncatellidae. These small and minute snails live on land, very close to seawater. They walk with a strange looping action, and the adults have truncated shells. Many of the species are small enough to be considered micromollusks.

Freshwater bivalves are one kind of freshwater mollusc, along with freshwater snails. They are bivalves which live in freshwater, as opposed to saltwater, the main habitat type for bivalves.

<i>Truncatella subcylindrica</i> Species of gastropod

Truncatella subcylindrica is a species of small land snail that lives at the edge of the sea. It has gills and an operculum and is gastropod mollusk or micromollusk in the family Truncatellidae.

<i>Anentome helena</i> Species of Gastropoda

Anentome helena, common name assassin snail or bumblebee snail, is a species of freshwater snail with an operculum, an aquatic gastropod in the family Nassariidae, most of which are marine.

<i>Truncatella caribaeensis</i> Species of gastropod

Truncatella caribaeensis is a species of a very small somewhat amphibious land snail with a gill and an operculum, a semi-terrestrial gastropod mollusk or micromollusk in the family Truncatellidae, the truncatella snails or looping snails. These tiny snails live in damp habitat that is very close to the edge of the sea; they can tolerate being washed with saltwater during especially high tides. These snails are sometimes listed as land snails and at other times they are listed as marine snails.

Truncatella clathrus is a species of very small somewhat amphibious land snail with a gill and an operculum, a semi-terrestrial gastropod mollusk or micromollusk in the family Truncatellidae, the truncatella snails or looping snails. These tiny snails live in damp habitat that is very close to the edge of the sea; they can tolerate being washed with saltwater during especially high tides. These snails are sometimes listed as land snails and at other times they are listed as marine snails.

<i>Aclophoropsis</i> Genus of gastropods

Aclophoropsis is a genus of minute sea snails with left-handed shell-coiling, marine gastropod mollusks or micromollusks in the family Triphoridae.

Truncatella californica, common name of the Californian truncatella, is a species of very small amphibious snail, a gastropod mollusk in the family Truncatellidae.

<i>Truncatella scalarina</i> Species of gastropod

Truncatella scalarina is a species of very small land snail that lives next to seawater, a gastropod mollusk or micromollusk in the family Truncatellidae.

Conotalopia ornata is a species of very small sea snail, a marine gastropod mollusk or micromollusk in the family Trochidae, the top snails.

References

  1. Ross L. T. (1970). "The anatomy of Truncatella Risso 1826 (Mollusca: Prosobranchia), and a revision of the genus in the Gulf of Mexico and the Caribbean Sea". unpublished, Florida State Univ. dissertation. Retrieved on September 2007.

Further reading