Marine Rhythms Some of the most dramatic examples of biological rhythms are found in marine organisms. The periods or lengths of the rhythms are rather diverse and include circadian, circatidal, circalunar, and circannual rhythms, and various combinations of them. Perhaps most famous is the rhythm of reproductive activity of the South Pacific marine worm referred to as the palolo worm. This species spawns at the last quarter of the moon in October and November (spring in the Southern Hemisphere). The worm lives buried in coral reefs and, at spawning, the last twenty-five to forty centimeters of the worm, which bears the gametes, breaks off and rises to the surface of the sea. The gametes are released into the seawater, where fertilization takes place. The spawning always occurs at daybreak. The exact timing of the spawning is an adaption that increases the chances for successful reproduction in this species. Similarly, the California grunion, a small smelt about fifteen centimeters long, spawns in the spring at about fifteen minutes after the time of the high tide each month. During the spawnings, or "grunion runs," the fish ride the waves onto the sandy beaches, where the females burrowthe posterior end of their bodies into the sand. The male curls around the female's body and releases sperm as the female lays her eggs. The fish return to the sea and the eggs continue to develop until approximately fifteen days later, when the high tide returns and uncovers the hatching young. During the grunion runs, the adult fish are caught by fishermen (legally only by hand) and are eaten. Neither the palolo worm nor the grunion has been sufficiently studied to determine what environmental factor-moonlight, gravity, magnetism, or another factor-synchronizes their rhythms so precisely.
Shells Shells are external coverings that are produced by an organism. As such, they require a process by which the constituents of the shell are deposited in a site-directed fashion. What this means is that calcium and carbonate cannot come in contact with one another when their concentrations exceed their solubility product; otherwise, they will precipitate (form a solid). Thus, these ions must be directed to the area where the preferred precipitation is to take place. For this reason, a matrix is needed to provide a negative attractive force for calcium or other bivalent ions, such as magnesium. Other parts of the matrix may be composed of or house the enzyme carbonic anhydrase for the conversion of carbon dioxide reversibly to bicarbonate. The bicarbonate will degrade to carbonate ion that can then react with the positively charged calcium ion. Calcium can be taken out of the seawater or diet and concentrated; likewise, bicarbonate ions can be formed in the gills of some of these organisms and transported to deposition sites. However, having a matrix containing the enzyme carbonic anhydrase ensures that the calcium will only precipitate at that matrix site and nowhere else. In this way, the organism can control the shape of the shell by laying down a fiber matrix, usually composed of protein, as in mollusks, or a protein-chitin mixture, as is found in arthropods. The dumping of bicarbonate outside of a tissue where calcium is present will cause precipitation on the tissue membranes, a consequence with detrimental effects on the cells of the tissue. How calcium and carbonate ions are brought together varies from phylum to phylum.
