Bivalve mollusks
Bare-gill mollusks
Cephalopod mollusks

    Bivalve mollusks are amazingly diverse in form. and their beauty  strikingly perfect. Necklaces and amulets made of bivalve shells can adorn the human body and household interiors, vases incrusted with their pearly fragments promote good mood and lofty inspiration, and  the poetic depiction of Venus by a famous artist from Florence also  associate her birth with mollusk shells, the bivalves, as you have already probably guessed.
    Bivalve mollusks are widespread in all seas of the World Ocean. However, the number of their species may vary depending on the geographic latitude and hydrographic
specifics. For instance, whereas there are about seventy bivalve species in the Arctic, their number in the coastal seas of Siberia normally does not exceed 30-35 species. In the Chukchi Sea, the number of bivalve species increases to 40-50, and in the Bering Sea to 60-65. The specific diversity of bivalves increases even more to the south. In the Sea of Okhotsk, they number from 80 to 90; in the northern part of the Sea of Japan, from 120 to 130; and in its southern part, to about 200 species. From 200 to 240 species inhabit the Yellow and East China Seas.  In the South China Sea, including the Pacific coast of the Philippines, there are more than 300 different bivalve mollusk species.
    With regard to diversity of bivalve species, the coast of Primorye is subdivided into two areas, the northern and southern; the boundary between them may be drawn at Cape Povorotnyi. In the northern area washed by the cold Primorye current, the bivalve specific diversity on the average normally does not top the 70-75 mark to increase in bays and decrease in open coastal sites. In the southern area, the number of species compared with that inhabiting the northern area abruptly rises to 110-120. Among other things, this is caused by the highly uneven shore line. Shallow inlets and deeply indented bays allow species of different geographic origin to exist here, including local or low-boreal species inhabiting both open shelf areas; those from cold water or high-boreal and arctic areas, inhabiting chiefly open shelf sites; and warm-water or subtropical species occurring in the most heated sections of bays and for whom Peter the Great Bay is  essentially a refugium.
    The body of a bivalve mollusk is enclosed in a calcareous shell consisting of two valves. The sizes of adult specimen shells of Primorye species vary from 2-3 to 200 mm. The thickness, form, sculptural decorations, projections (teeth) inside, the color outside and inside the shell and other morphological properties also vary widely, and in conjunction  with the anatomy  of the soft mollusk body is essentially an object of taxonomy, the science of classification of living organisms.
    The soft body consists of a mantle or fold lining the shell from inside, gills, oral blades Ц lamellate outgrowths that sort food particles; arms that help the shells to dig in, move and fasten to rocks by means of special byssus threads; and a visceral mass containing a digestive gland (liver analog), a stomach, an intestine, a pericardial bursa (heart), kidneys and reproduction organs.    Most species reproduce by external fertilization, when male and female gametes are simultaneously spawned to meet outside the organism. Fertilization results in a larva which lives deep in the water to grow from 120-150 to 300-400 microns. After a while, lasting from one week to one month and over, the larva settles on the sea floor to lead a lifestyle largely similar to that of adult specimens.
    Despite the fact that Primorye has long been an object of research,  the biology of many of its  bivalve mollusk species still remains insufficiently studied for a variety of reasons. Some species occur rarely in general, not only in Primorye, while others do not form regular populations and we are aware of their presence only on seeing empty shells. Again, basing on their distribution, we believe that some of their biological properties have been relatively well studied on populations from other areas. Yet, quite often it happens that we are speaking  of morphologically similar, albeit different species that vary in additional  properties studied neither in Primorye, nor elsewhere.
    After the mollusks die, the shells of species inhabiting small depths (of up to 20 m) normally surface to the sea floor. When the weather is windy or stormy, they are ejected onto gently sloping sand beaches to not infrequently form large accumulations that turn an empty coast into a motley carpet of colors.  At the same time, the life of empty shells on beaches is ephemeral. Under the impact of waves, high tides, gales and precipitation, some of them sink again to hardly accessible depths, while others (the most fine and brittle ones) are destroyed.  However, after a while, a new storm
bring new shells.
    So, when you bake in the sun on the yellow sand of a Primorye beach, noting all the variety of life, please take note of the shells lying around. Their appearance can help you distinguish bivalve mollusk species, the first essential step in learning the life of sea creatures. Having learned to determine the species, you could bring order to your initially modest shell collection (to possibly promote your future career!). Finally, a shell collection may help you make various articles and ornaments  to harmonize your surroundings.
    The following descriptions of Primorye bivalve species are almost similar to the  wording and style of scientific descriptions found in special taxonomic literature.  Taxonomy is the most ancient of sciences, and its terminology is simple and readily understandable. Once you learn it, it can, among other things, be your first bridge to the fascinating mysteries of bivalve mollusk life.
    Significantly, most species have neither Russian, nor local indigenous names. So shell lovers make good use of Latin names given by biologists to each respective species.

Zh. EVSEEV, Cand. Sci. (Biology), Institute of Marine Biology, Far East Branch, Russian Academy of Sciences.

Yu. YAKOVLEV, Cand. Sci. (Biology), Institute of Marine Biology, Far East Branch, Russian Academy of Sciences.