Molluscs, digestion

Murexine and related compounds have marked actions on the nicotine receptor as expected from choline esters (87-89). Toxins from the digestive glands of nudi-branchs have marked effects on the cardiovascular system of the rat (23). Antiviral and antibacterial substances have been obtained from molluscs (90,91). 

Labelled mevalonic acid dibenzylethylenediamine (DBED) salt (14 pCi) was injected into the digestive glands of D. limbata [90]. After 24 h, the animals were sacrificed and labelled polygodial (30) was recovered from the skin of the mollusc. After two subsequent chromatographic purifications, the dialdehyde was reduced to the corresponding diol (35) which was significantly labelled. The ability of D. limbata to synthesize de novo its own defensive allomone raised a series of questions linked to... 

Figure 5. Digestion strategies in molluscs. Modified from [68] Decho, A. W. and Luoma, S. N. (1996) Flexible digestion strategies and trace metal assimilation in marine bivalves in Limnol. Oceanogr., 41, 568-572. Reproduced with permission...

There are studies on the uptake of contaminant metals into terrestrial molluscs from contaminated sites [103] and from food to which metal salts (Ca, Co, Fe, Mn, Zn) had been added [104], The metals accumulate in the animal s digestive gland where histological and ultrastructural studies indicate that a major route of uptake is by phagocytosis [105,106],... 

The application of high-sensitivity ICP-MS detectors coupled to HPLC has enabled the detection of trace arsenic compounds present in marine animals. Thus, arsenocholine has been reported as a trace constituent (<0.1% of the total arsenic) in fish, molluscs, and crustaceans (37) and was found to be present in appreciable quantities (up to 15%) in some tissues of a marine turtle (110). Earlier reports (46,47) of appreciable concentrations of arsenocholine in some marine animals appear to have been in error (32). Phosphatidylarsenocholine 45 was identified as a trace constituent of lobster digestive gland following hydrolysis of the lipids and detection of GPAC in the hydrolysate by HPLC/ICP-MS analysis (70). It might result from the substitution of choline with arsenocholine in enzyme systems for the biogenesis of phosphatidylcholine (111). 

Molluscs visceral hump of limpet (Patella vulgata)86100,000 digestive juice of snail (Helix pomatia)87 150,000. 

The known sources which most closely approach the female-rat preputial gland in activity are the digestive juices of molluscs and of locusts. As shown for the rumen of the sheep,100 the contents of the mammalian large intestine probably owe most of their /8-glucuronidase activity to microbes, whilst in the small intestine the enzyme is probably mammalian in origin.40... 

Barium, however, is taken up not only by the kidneys of molluscs, but also is measurable in the adductor muscles and digestive gland. 

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. 

Mineralized tissues of skeletons and shells of molluscs, annelids, barnacles, and echinoderms may be affected by an enzymic dissolution (digestion) of the organic matrix which contains calcium carbonate crystallites (Carriker and Smith, 1969 Carriker et al., 1969). An ability to digest organic matter from within animal skeletons has been demonstrated for bacteria (DiSalvo,... 

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