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Environ. Toxicol. Chem. 20, 2001
O. Simon and A. Boudou
some values obtained in carnivorous fish, which reach values
as high as 90%, or even higher [3,35]. Assimilation efficiency
of MeHg in edible muscle of the shrimp Pandalus borealis
contaminated by mussel consumption was estimated at about
42% [36]. The particular features of the species studied, in
particular the structural and functional features of the biolog-
ical barriers of the digestive tract, may be the reason for these
low absorption capacities in crayfish. At the present time, our
knowledge of the structure of the digestive tract in decapod
crustaceans and the mechanisms involved in digestion show
that during the first stage, food is ground in the gastric mill,
mixed with enzymes produced by the hepatopancreas, and sort-
ed into two fractions. The coarse fraction, containing all par-
ticles greater than 100 nm in diameter, is passed directly to
the midgut; and the fine particles and liquids enter the hepa-
topancreas where peritrophic membranes prevent particulate
material from coming into contact with the epithelium. The
hepatopancreas is a complex tubular organ derived from the
midgut, the functions of which include absorption and storage
of nutrients, synthesis of digestive enzymes, and detoxification
of trace metals and xenobiotics [32,37]. During the second
stage, numerous mature B cells from the hepatopancreas are
extruded into the lumen, isolated from the epithelium by the
production of a new peritrophic membrane, transferred into
the midgut, and then voided. During the last stage, all the
material derived from the hepatopancreas is passed to the hind-
gut and then voided [10,11]. The crayfish’s method of pre-
dation and ingestion of food may also contribute to a consid-
erable underestimation of transfer rates. Most decapod crus-
taceans are macrophagous, feeding on large pieces of animal
matter cut off with claws and macerated by the array of mouth-
parts and maxillipeds [32]. This behavior may lead to a con-
siderable loss of Hg before the ingestion of food in the stom-
ach, via transfers to the water column (diffusion of metal and
nonabsorption of small fragments resulting from dilaceration).
Losses by diffusion may also be increased by the storage of
interface, especially at the branchial lamellae level. Ecophy-
siologic studies on the ventilatory activity of the crayfish (As-
tacus leptodactylus) have estimated, via the simultaneous de-
termination of the partial pressure of oxygen in the inspired
and expired water, the water flow rate in the gill cavity. These
results show marked differences between morning and evening
periods, related to the activity of the organisms. At 23ЊC, the
average flow rates were 98 and 254 ml/kg/min, respectively
[40]. On an average basis of 180 ml/kg/min and an average
weight of 15 g (fresh weight), the amounts of Hg brought by
the water flow over the 30-d exposure period would then be
29.8
metal burdens measured in the crayfish after deducting the
background level are 3.8 and 3.0 g, respectively. For Hg(II),
g for Hg(II) and 3.3 g for MeHg. The corresponding
the transfer rate would be 13% and for the MeHg, the transfer
rate would be 90%. Clearly, these figures must be considered
with certain reservations, given that difficulties can be linked
with extrapolating data, such as use of different crayfish spe-
cies, wide differences in the masses of the organisms (15 g
on average in our experiment; 30 g for the ecophysiologic
studies), and specific features of the experimental conditions
(which may have a strong influence on the ventilatory activity
of the decapod crustaceans and include stress, feeding, motor
activity, and so on).
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D and D
ϩ T.
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