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Troca
Troca I have been collecting shells in Reunión Island for 11 years. Recently, with other members of the Reunión Shell Club, I started a new inventory of marine molluscs found here. Since Deshayes' monograph was compiled in 1863, skin diving has opened horizons that were inaccessible in those days. We discovered that several species now are found on Reunión that were not included on Deshayes' list.
The most surprising was our discovery locally of three sea shells whose known geographical range, as reported by the Schilders and Dr. C. M. Burgess, is far from our island. They are Cypraea mariae Sch.-Sch., 1927; C. beckii Gaskoin, 1936; and C. contaminata Sowerby, 1832.
C. mariae is considered to have a Pacific range. The first specimen was collected in Reunión by Mr. Cornic while skin diving. Last June, July and August, my wife and I found seven specimens, and J. C. Martin found two. All were dead and empty, but seven were fresh and the others were quite recognizable. All were collected at Cap Champagne, St. Paul (eastern Reunión Island) on a sand bottom 10 to 15 meters deep, at the base of a living coral cliff, where we look for dead shells every time the sea has been rough. C. mariae has a particular look and cannot be confused with any other species.
C. beckii also seems to be reported from the Pacific Ocean only. We have not found any alive, but my wife and I have collected seven specimens on Boucan-Canot Beach, St. Gilles. Three are in good state. J. C. Martin, diving with us, in June 1971 found one on the sand bottom, 15 meters deep, below the coral cliff at Cap Champagne; it was empty, but in very good condition. Mr. Beneteau has collected two more at St. Gilles.
There is no doubt that our sea shells are real C. beckii. The anterior columellar teeth are fused in a crest, and labial teeth stained with brown. C. beckii differs from C. macandrewi from the Red Sea, with which I could compare it but which we have never found in Reunión.
I found my first recognizable C. contaminata at the base of the coral cliff at Cap Champagne a few days after cyclone "Hermine" passed in 1970. After that, we examined our stock of beachcombed sea shells and found several more mingled with worn C. owenii and C. punctata. Subsequently, we collected a few more specimens at Boucan-Canot Beach and two at Cap Champagne, 15 meters deep. Among our fellow divers, Mr. Beneteau alone has picked up 21 specimens, all on Cap Homard and Boucan-Canot Beach.
In addition, I have seen in a private collection in Mauritius a specimen found on that island.
Troca A gastropod, such as a littorine, after settlement on a preferred substrate, begins a rapid growth phase. As the mantle grows so does the shell which is formed by the mantle. The first portion of the shell deposited is the organic matrix (conchiolin). The pigment melanin is probably simultaneously bound to this protein matrix. Initially, the new shell is thin and lightly calcified although the pattern is fully developed. Additional deposition of calcium carbonate crystals occurs later. The pigmentation pattern of the shell depends on the localization of pigment-producing cells in the mantle edge and the temporal rhythm of transferral of this pigment to the shell matrix. The sculpture is dependent upon the areas of the mantle which fold, the size and extension of the folds, the resultant structure of the organic matrix and the temporal rhythm in the deposition of calcium carbonate, as well as in the production of the folds themselves. The basic temporal rhythm appears to be a 24-hr. interval. For example, in Philippine littorines, there is an endogenous 24-hr. rhythm in the formation of a new growth stria, in the production of pigment spots and the construction of sculpture nodes. During the maximmn growth phase prior to sexual maturity, L. pintado deposits a pigment spot in a spiral row every other 24-hr. period. Adjacent spiral rows are on an alternate rhythm so that the pattern resulting is a "checkerboard" (J). This species has no significant sculpture, but in a form of another, L. picta, each spiral row consists of a series of nodes, one node being produced per day during maximum growth.
Photos - uncredited
Strombus hawaiensis
Pilsbry, 1917
[ Images rotated to upright
for this display – B. Dayle ]
In many species, however, there are periodic elements in ornamentation that do not correspond to a 24-hr. rhythm. Some of these, for example the axial color in L. picta, the rhythm may be based on a lunar or semi-lunar period resulting from the interaction of diel and tidal components of rhythmic activity. In others, for example many Strombus species, the production of varices only at the shell lip may correspond to the cessation of growth at sexual maturity. In such animals as the Murex species, the regularity in the intermittent varices appears to be an inherent characteristic and relatively independent of environment, although in some cases it may be related to availability of food or seasonal change.
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