Flexopecten flexuosus (Poli, 1795)
&
Neritina communis (Quoy & Gaimard, 1832)

(1st one has no English name. The other is the "Zigzag Nerite")

Class: Pelecypoda

Subclass: Pterimorphia

Order: Ostreoida - same order as oysters

Family: Pectinidae

Species: Flexopecten flexuosus (Poli, 1795)

English Names: None that i know about!!

Localities:  Mediterranean & NW Africa

Image: Taken by Ian Holden on a Sony Mavica MVC - F73

Class: Gastropoda

Subclass: Orthogastropoda

Family: Neritidae

Species: Neritina communis (Quoy and Gaimard, 1832

English Names: Zig-zag Nerite

Localities:  Western Pacific: most common in the Philippines

Image: No idea where it came from!!

These two wildly differnt species were chosen to represent "polymorphism" in the molluscan family. This is where there are many different color and/or pattern variations present in a single, interbreeding population, and is quite common in many families of molluscs - much to the delight of collectors, i might add!!

I personally love to gather together a large number of shells collected from a local population of a particularly polymorphic species, and arrange them into "variation sets" which display the variability of that "lot" - a biological sampling term meaning a group of individuals collected all at the same time, from one local population - when biologists collect samples, they always collect "lots", and it is most useful for scientific purposes to try and represent as much variability as possible within that population (within the overall framework of conservation, of course!!!).

So, whereas a shell collector will normally only collect the very best, most attractive, most mature and best quality specimens, a scientist will usually try to collect examples of all the developmental stages of the animal, from eggs to gerontic (very old specimens, which often show interesting deformations typical of what old age does to that particular species - baldness would be a good example of a normal gerontic feature in Homo sapiens L., for example!!) specimens, and as many "morphs" or variations in form, color, pattern, "sculpture" (the surface features of the shell - such as frills, bumps and the like) and even microsculpture (surface features too small to be seen with the naked eye: this is sometimes useful in distinguishing very similar species from each other - as in the boreal bivalve family Astartidae, where despite some articles in the literature which deny it, microsculpture is a very reliable means of separating several confusingly similar species).

But i digress... back to polymorphism!! All species of plants and animals in the wild, with the exeption of "top predators" like lions and polar bears, have to find ways to keep from being preyed upon (i.e. eaten!!) by predators on the prowl for a meal. Some use camoflage - they do their best to blend in with the background - chameleons and squid are the masters of this: they can change their color and sometimes even pattern, to match their environment and make them hard to find. Others use "chemical warfare" - they develop chemicals which make them distinctly unpleasant to eat or sometimes even touch, thus discouraging most predators. Good examples would be stinging nettles, poison ivy, skunks, those toads that have hallucinogenic poison in their warts, and certain "magical" mushrooms that produce effects most animals would not wish to repeat, having experienced them once!! Other species simply make themselves difficult to eat - with hard shells (turtles, molluscs) they can withdraw into, spines (think Muricidae!!!), or physical barriers such as lots of mucus (hag fish for example) which predators might find unpleasant or difficult to penetrate. There are lots of other predator avoidance and defence mechanisms out there (like simply hiding in cracks and crannies a lot!!) but again, let's get back on topic.

Polymorphyism seems to be a way of confusing predators so they end up only eating some individuals but not others - hence ensuring the survival of the population in the long run. I think it is most useful in situations where there are a number of predator species - each of whom preferentially eat some morphs, while leaving others alone. For example, imagine a polymorphic species of littoral (i.e., intertidal - the littoral or intertidal zone is the area between high and low tides) mollusc has 4 main predators - a bird which is more attracted to specimens on the violet end of the spectrum, perhaps because they can't see things at the red end as easily; a crab which is more likely to find and eat highly patterned specimens than ones with no pattern, a species of racoon that finds individuals on the red end of the spectrum easier to see, hence ends up eating them more often, and a fish which finds it easier to find and eat strongly colored specimens as opposed to paler ones. This predator combo will mean that there is selective predation-related mortality on all colors, as well as strong patterning and strong coloration. The end result will be a polymorphic population where you'll have all the colors of the rainbow which there are genetic variants in the prey population for, but there will be little patterning and the colors will be mostly pastels as opposed to brighter ones: you will have a high degree of color polymorphism, as opposed to pattern and color intensity variation.

// Selective predation can explain much of the polymorphism in shallow-water species, but the often-extreme variation found in deep-water species, where color and pattern are not very useful because of the kind and the intensity of light found at increasing depths (water selectively absorbs light at the red end of the spectrum, because of its high oxygen content. So, it is blue, and the deeper you go, the less red-end colors (red, orange, yellow) are visible: everything gets "bluer" as you descend into the watery deeps.). In these cases, it is highly likely that most polymorphyism is "residual" - left over from earlier times when the species lived at much shallower depths and a diversity of forms was more useful for survival.

Pectens - family Pectinidae - although numbering only about 300 species, are one of the most successful of all the bivalve families. They can be found in marine habitats in all corners of the world-ocean, from pole to pole, and in many places are exeptionally abundant.

Scallops (the normal English term for Pectens) are a major food item throughout the world, and are also a boon for shell collectors because they live on the ocean's bottom and when they are trawled, all kinds of other shell-bearing molluscs come up with them!! They are filter-feeders, which means they simply filter out tiny organisms (plankton) and detritis from the water which comes their way. Many species are also capable of changing gender - at one point in their life-cycle a given individual may be male but later on, female!

// An interesting footnote to the family, is their taxonomy at the genus level: it changes more often than some folks change their socks!! No universally accepted method of dividing the family up into genera has ever been devised, so most species of Pectinidae have been assigned to 3 to 5 or even more genera since their initial description. Many collectors and curators consequently find it difficult to keep up - result: moocho confusion!!

Nerites - family Neritidae - are a small (about 75 species) family of vegetarians (they are mostly algae-scrapers) who are unusually adept at surviving in extreme conditions, partly becuse of their exceptionally tightly-fitting operculum - the calcareous or chitonous "door" to the shell, which can be used not only for protection agains predators, but also to seal the animal inside the shell when it is out of water, preventing dessication.

Many of them are inhabitants of the upper intertidal zone, where they are only submerged for a few hours each day. Some genera live in brackish water, and are able to tolerate a large range of salinity. Still other genera, such as Theodoxus, the "river nerite", are fresh water denzions.

Many Nerite species are fantastically polymorphic - indeed, some of them, such as a small species of Neritina who's name escapes me at the moment (ok, i've been trying to find it again for years and years!!!), have literally hundreds of distinct color and pattern "morphs" (to "morph" means to change into another form). This extreme morphic diversity may be related to the marked ability of many Nerites to change their phenotypic expression (i.e., which genes and/or "alleles" (forms of a gene) are activated, resulting in different physical charactaristics) according to the environment they find themselves in.

For example, in a brackish water environment a given species may have a more elongate form with a much narrower aperature than its corresponding form in salt water. This is called "phenoplastic switching", which simply means that different genetic expressions - hence a different set of genetically-determined charactaristics ("phenotype") - occurs as a response to different environmental conditions.

Flexopecten flexuosus (Poli, 1795) is a species found at moderate depths in the western Mediterranean and NW African waters. It is known for its tremendous polymorphism: there are dozens of hues of color and about 15 different methods of pattern formation.

Neritina communis (Quoy & Gaimard, 1832) is a relatively common intertidal species found in a wide Indo=Pacific range but most abundantly in the Philippines, which actually should be called Neritina waigiensis (Lesson, 1831), since Lesson described it in the previous year to the one that Quoy and Gaimard's article was published ("describing" a new species or form consists of publishing an article detailing the its charactaristics in a journal recognized, either currently or retroactively, by the International Committee for Zoological Nomenclature (ICZN), and designating a representative or "type" specimen or specimens, which are usually deposited in a museum, where they can be accessed by other researchers).

Technically, communis (Quoy & Gaimard, 1832) is a "junior synonym" of waigiensis (Lesson, 1831), and should simply be resigned to the synonomy of that species. However, many people still use the old, familiar name since Lesson's initial description was not widely known in the conchological or malacological communities until long after Quoy and Gaimard's name had been accepted universally and used around the world.

n cases similar to this, where a common species has been known by one name for a long time before an earlier description in a valid but obscure publication comes to light, rather than declaring the well known name superseded by the new upstart it is "grandfathered" for the sake of convenience, saving the vast amounts of time and confusion which would result as millions of labels and data-sets around the world have to be changed to accomodate the new scientific name.

In the case of N. waigiensis the grandfathering option was not taken by the ICZN, so this is in fact the valid name now: i used the old, still widely-used name in order to introduce concepts of taxonomic synonomy to you, the Gentle Reader. (Ok - truth time: until just a few minutes ago i actually thought they were two different species that just looked remarkably similar..... when i found out otherwise i just left the designation as it was, and re-wrote the article to slip in the additional technical stuff!!)