Micromollusk
From Wikipedia, the free encyclopedia
A micromollusk is a descriptive term for a shelled mollusk which is extremely small, even at full adult size. The word is usually, but not exclusively, applied to marine mollusks.
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 are only about 2 or 3 mm in adult size; and a few have 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 are often either ignored or overlooked by beachcombers, shell collectors and even some 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 mollusks.
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.
Contents |
[edit] Defining micromollusks by 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, but 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, and even for people with small hands, this means that the shells are far too small to be picked up with the fingertips using the normal grasping action.
[edit] Some techniques for finding and handling micromollusks
Marine micromollusks are most often found by the careful searching of sediment samples taken from "promising-looking" areas.
On sandy beaches, the empty shells of micromollusks may sometimes be washed 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 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, for example when scuba diving, a sediment sample may be taken from areas such as the surface layer of sand under rocks, or at the edge of a coral reef. Live micromollusks may also be found by washing seaweeds or algae in fresh water in a bucket.
Land micromollusks may be found by taking samples of leaf litter from rich areas and subsequently sieving or sifting the litter and then searching it with a good light and some magnification.
Freshwater micromollusks which live on pond weed can be 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 can be 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.
Once sediment samples are clean and dry, they may be searched under the microscope. Minute shells can be picked out using a very small sable-hair paintbrush which has been dampened at the tip, or instead soft entomological tweezers may be used.
Micromollusk shells can be stored in small glass vials or in paper micromounts.
[edit] Excluded from the category of micromollusks
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.
[edit] Examples of micromollusks
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:
- Aclididae
- Assimineidae
- Caecidae
- Cerithiopsidae
- Cingulopsidae
- Elachisinidae
- Epigriidae
- Eulimidae
- Falsicingulidae
- Fossariidae
- Iravadiidae
- Juliidae, mentioned in Sacoglossa
- Limacinidae
- Litiopidae
- Mathildidae
- Obtortionidae
- Omalogyridae, the smallest known gastropods, all species are less than 1 mm in adult size
- Orbitestellidae
- Peraclidae
- Pyramidellidae
- Retusidae
- Ringiculidae
- Rissoellidae
- Rissoidae
- Scaliolidae
- Scissurellidae
- Sequenziidae
- Skeneidae
- Skeneopsidae
- Tornidae
- Triphoridae
- Truncatellidae
- Vanikoridae
- Vitrinellidae
Also see:
- Acteonidae
- Acteonoidea
- Barleeia and others in the Barleeidae
- Hydrobiidae (a freshwater family)
- Truncatella
- Pisidium in the family Sphaeriidae, freshwater bivalves
[edit] References
- ^ Ross, Landon 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.[1]
[edit] Scientific papers
"Techniques for collecting, handling, preparing, storing and examining small molluscan specimens", May 2007, Daniel L. Geiger, Bruce A. Marshall, Winston F. Ponder, Takenori Sasaki & Anders Waren, Molluscan Research 27(1): 1-50
[edit] Books
- Rosenberg, Gary, 1992. The encyclopedia of seashells, Dorset Press, New York
[edit] External links
- An article about micromollusks at Conchologists of America
