Here is an article about ocean quahog seen under a microscope. It is found to be extensive that the geographic range of the ocean quahog. This occurs along the east coast of North America north of Cape Hatteras to St. George Bay, Newfoundland, Canada, off the southern coast of Iceland, off the Faroe and Shetland Islands, and along the European coast northward from the Bay of Cadiz, Spain, around the British Isles, in the North Sea, and off the Norwegian coast to the White Sea in Russia Off the Middle Atlantic coast it is common at depths of 35 to 75 m.
Hermaphroditism may occur though sexes are separated. A period of intense spawning from August to November has been found for ocean quahogs, although minor spawning activity has been observed in earlier and later months. Sperm and eggs are released into the environment where fertilization and larval development occur. In samples from off Long Island, NY, the youngest ocean quahogs that had attained sexual maturity were an age 5 male and an age 6 female. Growth of ocean quahogs is fairly rapid during the first 20 years of life but lessens greatly thereafter. Ocean quahogs of about 100 years and older are common and a maximum shell length of 140 mm and a maximum longevity estimate of 225 years have been reported.
Early studies that presented largely unsubstantiated age and scientists reviewed growth observations for ocean quahogs. Earlier investigators interpreted dark concentric rings or bands found on the external valve surface of small quahogs as annual marks. Larger, older quahogs were not aged because the rings crowded together at the valve margin and became obscured by the thick, black periostracum.
At the NEFSC resent age determination, Woods Hole Laboratory has been based on enumeration of annuli in acetate peel preparations. Various experimental evidence including radiometric analyses suggests that annual age marks are formed in the valves of ocean quahogs. Validation of an annual periodicity for these marks has been supported by a marking experiment. Recovered individuals show the expected number of annuli formed during the period between marking and recapture.
In the examinations of acetate peels in light microscopy, outer and inner layers of the three-layered aragonitic ocean quahog shell are quite obvious, unlike the very thin prismatic pallial myostracum, which separates the outer from inner layers. Annuli occur in the relatively thick outer valve layer, curve toward the umbo from exit locations at valve surface bands, and seem to merge with the prismatic pallial myostracal layer. Annuli in peels appear as dark lines; growth increments form a lighter, textured background. Investigators at Princeton University that separated growth increments from predominantly homogeneous microstructures found definite prismatic microstructures, considered to be annuli. Although the microstructures are only visible by scanning electron microscopy, light microscope examinations of acetate peels clearly revealed the periodicity of annuli in small and large marked quahogs.
The left valves of ocean quahogs are prepared, since they have a single tooth that contains age marks, and correspondence in the number of marks in the tooth and valve adds confirmation to an age estimate for a specimen. A diarnond-impregnated blade on an Isomet slow peed saw machine sections the valves. A valve is oriented on the machine to make a cut through the umbo and to the ventral margin such that the broadest surface of the tooth remains in the anterior valve portion. This portion is immersed in common bleach to remove the periostracum, rinsed in tap water, and allowed to dry before embedding it in Epon 815 resin. After hardening, the embedded valve cut surface is uncovered by grinding off excess resin and polished to a high luster on a vibrating lap machine. Etching the cut surface of the valve for 1 minute with 1 percent HCl precedes application of sheet acetate and acetone. The sheet is peeled off after the acetone evaporates. The image produced in the peel is a necessary procedure, since the thin-age annuli are microscopically indistinct on the external valve surface, in the cut surface, or thin-sections of ocean quahog shells. Although annuli and growth increments are reproduced clearly in the peels, they must he examined microscopically. Optimal contrast between annuli and growth increments in examinations of peel preparations is possible under a compound microscope at low magnifications; low transmitted light intensity, and with the iris or diaphragm of the substage condenser closed down.
Problems in determining an age for an ocean quahog relate to the loss of the earliest-formed annuli in the valve from erosion of the outer valve layer, a condition not unusual in old individuals. Annuli formed during the first ten to fifteen years in the life of an ocean quahog may split into multiple lines at the valve surface exit locations. Careful observation will usually reveal that they merge at the pallial myostracum. These conditions can result in deviations in agreement between annuli counts of the valve and hinge tooth, and individuals have been found to have a confusing pattern of growth lines suggestive of deviant growth.
Original Text:
http://www.nefsc.noaa.gov/fbi/age-man/oq/oq.htm
