87
an order of magnitude of those found in the Kabelle goatfish
(Table 6), reveals an
apparent discrimination by the lagoon
biota between isotopes of the same heavy element.
apparently due,
This is
not to some sort of discriminatory ability
on the part of individual organisms, but rather to the physical separation of radioactive and stable isotopes of zinc in
the lagoon into two distinct phases.
The organization of the
lagoon biota is thus able to impose a
degree of
selectivity
dependent upon the ecological relationships of any particular
organism with its environment.
The relatively low concentration of zine in the G.
I.
tracts of the surgeonfish compared to the goatfish is probably
due to several factors.
fish G.
food
(1) Most of the mass of the surgeon-
I. tracts used in the analyses consisted of undigested
(algae), whereas in the goatfish
the G.
I. tracts were
nearly all empty except for relatively small amounts of uniden-
tifiable gurry, and the mass of the G. I. tract was nearly all
due to the tissues of the tract itself.
(2) Surgeonfish have
virtually no stomach,
the greater proportion of the digestive
tract being the long,
coiled intestine.
In goatfiash there is
a large stomach which constitutes a significant proportion of
the total mass of G.
I.
tract tissue.
Since the zinc enzyme,