4. Discussion4.1. Metal bioaccumula

4. Discussion
4.1. Metal bioaccumulation in eggs and hatchlings
At the end of embryonic development, the embryos
showed very low metal concentrations compared to
older cuttlefish (Table 2). Such concentrations are close
or even identical to those measured in the vitellus of the
newly spawned eggs, suggesting that the vitellus contains
a sufficient amount of essential metals (Cu, Fe, and
Zn) needed for the development of the embryos. Previous
studies with radio-labelled metals have demonstrated
that 65Zn, 109Cd and 57Co were mainly retained on
the eggshell (96% after 11 days exposure), acting as a
protective barrier limiting/hindering the incorporation of
waterborne metals. However, the thickness of the eggshell
changes during embryonic development. When
spawned, its thickness is of the order of 1 mm, (Lemaire,
1971) and is composed of albumin and other proteins,
that harden when coming into contact with seawater.
Then, it becomes thinner during embryonic development,
being almost transparent at the moment of hatching
(Wolf et al., 1985). Very low concentrations of Pb
and Vin the embryos also suggest a specific limitation of
their incorporation into the eggs, but this issue should be
addressed specifically in the future.
Immediately after hatching, the rapid increase of
Cu, Fe and Zn concentrations in cuttlefish tissues
(Table 2) suggests that the hatchlings are highly dependent
on the essential metals to fulfil their metabolic
demands. It therefore follows that the metals are rapidly
taken up once they are in contact with seawater. At
this stage, the accumulation only occurs through the
dissolved pathway by branchial absorption and tegumental
adsorption, the hatchlings still living only on
their vitellary reserves for a week (Mangold and Bidder,
1989). A high 65Zn accumulation rate has been
experimentally shown for juveniles exposed to radiolabelled
seawater (Bustamante et al., 2002b). Like the
essential elements, Ag concentrations are 10 times
higher in hatchlings than in the embryos (Table 2),
showing a very rapid bioaccumulation of the metal.
Similarly to 65Zn, exposure of hatchlings to 110mAg
via seawater leads to high concentration factors (i.e.
320) after a period of only 36 h (Bustamante et al.,
2004). Therefore, the dissolved pathway appears to be
a very important route for essential elements and Ag,
whereas this route appears rather limited for Cd (Bustamante
et al., 2002b), Pb and V (Miramand et al.,
1980, 1981) whose concentrations remained very low.
Further investigations on the uptake of V and Pb are
specifically needed to assess this hypothesis.