Molluscs selection

In molluscs the kidney was the only organ with measurable amounts of chromium. The level rose in molluscs selected for experimentation and exposed to chromium, but the levels were variable between organisms and different populations. This trend may reflect seasonal differences such as those observed by Bryan47, who suggested that variations in the concentrations of several elements in Chlamys opercularis and Pecten maximumus were due to changes in the availability of food. 

It is well established that the principal source of secondary metabolites in marine molluscs resides in the sometimes selective concentration of chemicals contained in their food. The chemical connection between molluscan predator and its algal or invertebrate diet has been confirmed as a result of many investigations of the chemistry of herbivorous [6] and carnivorous [3] molluscs and their dietary sources. However, the assumption often reported, as in a recent review [7], that marine molluscs invariably obtain their metabolites from dietary sources, should be regarded as an oversimplification. In fact, active biosynthesis of secondary metabolites has been ascertained in several cases, as documented here. 

The presence in molluscs of molecules structurally related to typical dietary metabolites could be ascribed either to selective accumulation of minor compounds acquired through the diet, or to an in vivo chemical transformation of major metabolites acquired from the prey. However, all reports on this topic have to be carefully evaluated before drawing hurried conclusions. In particular, interaction among molecules from different organs could favor formation of artifacts when the secondary metabolites are extracted from the whole mollusc and not from individual dissected tissues. Only some cases, where the ability of the molluscs to modify dietary metabolites seems to be well supported, are reported in this chapter. 

The half-time persistence (Tb 1/2) of zinc in whole molluscs is extremely variable, and reported to range from 4 days in the common mussel (Mytilus edulis) to 650 days in the duck mussel (Anodonta nutalliana) intermediate values were 23 to 40 days in limpet (Littorina irroratea), 76 days in the California mussel (Mytilus californianus), and 300 days in the Pacific oyster (NAS 1979). Zinc persistence in selected organs also shows considerable variability and may be significantly different from Tb 1/2 values seen in whole animal. For example, the Tb 1/2 of zinc in... 

The most famous halogenated bis-indole is Tyrian purple, the dibromo analog of indigo (1). This colorful mollusc metabolite, which was the major component of the ancient dye, continues to receive attention (1369-1374). Additional studies of Tyrian purple from various molluscs have revealed the presence of 6,6 -dibromoin-dirubin (1441) (1375), 6-bromoindigotin (1442) (1375, 1376), 6-bromoisatin (1443) (1377, 1378), 6-bromoindoxyl (1444) (1378), 1445 (1378), 1446 (1378), 6-bromoindirubin (1447) (1379), and 6 -bromoindirubin (1448) (1379). The bromi-nated indirubins are potent and selective kinase inhibitors (1379). A direct-exposure... 

Small sedentary grazers such as amphipods appear to select chemically defended seaweeds as host plants since they would otherwise be subject to intense predation by reef fishes however, they do not sequester metabolites as do other selective grazers such as sacoglossans and opistobranch molluscs.105 109 Grazing by amphipods induces increased concentrations of acutilol A acetate and acutilol B in Dictyopteris menstrualis and makes the seaweed less susceptible to attack by other predators 112 the same terpenes acted as antifoulants which prevented the settlement of bryozoan larvae.107 This evidence for multiple roles for algal metabolites may provide an explanation of previously documented differences in chemical composition in Dictyota.113... 

Avila, C. and Durfort, M., Histology of epithelia and mantle glands of selected species of doridacean molluscs with chemical defensive strategies, Veliger, 39, 148, 1996. 

Macromarines A Selective Account of the Potential of Marine Sponges, Molluscs, Soft Corals and Tunicates as a Source of Therapeutically Important Molecular Structures... 

Scheme 6.2 Selected Mollusc compounds sources and activity.

The results of sequential leaching experiments and the bioaccumulation studies reveal a selective assimilation of certain chemical forms of some trace metals by molluscs. There seems to be a relationship between seawater-soluble chromium levels and accumulation of the element in the kidneys of exposed bivalves. Particulate forms of chromium, while present in the digestive glands, do not appear to have been assimilated by the organisms, even though much of the total chromium was dissolved by a relatively mild extractant -25% acetic acid. Thus, an estimation of the bioavailability of particulate metals based on their solubility in dilute acetic acid can be erroneously high, at least for herbivorous filter feeders. 

Szefer, P., Ali, A.A., Ba-Haroon, A.A., Rajeh, A.A., Geldon, J., Nabrzyski, M. Distribution and relationships of selected trace metals in molluscs and associated sediments from the Gulf of Aden, Yemen. Environ. Pollut. 106, 299-314 (1999)... 

The turnover of stable and radioactive nuclides of trace metals differs greatly with animal species, element, time, and physicochemical form of the metal in the water. Concentration factors and turnover rates of a given element can range several orders of magnitude in value in different bivalve molluscs. The reliability of models developed to predict concentration change is dependent on the selection of input parameters appropriate for the situation under consideration. 

Equilibrium models are used to assess the environmental impact of power plant siting. The use of a single, maximum concentration factor for bivalve molluscs as input into the model in this situation is appropriate for screening purposes, i.e., to determine whether the maximum credible value would impact the environment. However, when more realistic estimates are required, selection of concentration factors applicable to the site, species, and situation is necessary. 

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