AM. ZOOLOGIST, 8:495-506 (1968).
Aspects of Nutrition and the Metabolism of Copper in Isopods
WOLFGANG WIESER
Lehrkanzel fi'ir Tierphysiologie, Zoologisches Institut, Universitdt,
Innsbruck, Austria
SYNOPSIS. Terrestrial immigrants from the sea have to cope with two important
nutritional problems: a shift in pathways of absorption, and the altered availability of
nutrients. The more terrestrial species of littoral crustaceans switch to food as the
main source of water and salts. No serious difficulties accompany the oral uptake of
mobile ions, but the vegetarians amongst the immigrants are required to assume a new
attitude with regard to the assimilation or heavier metals. Some important heavy
elements, notably Cu and Zn, are soluble in sea water but form rather intractable
organic complexes in plant tissues. Herbivorous amphipods and isopods on land are
incapable of extracting copper directly from their primary food sources. To compensate for this shortcoming, help is enlisted from microorganisms, which render the
copper present in ill-digested fecal material available to the crustaceans. Moreover, in
terrestrial and intertidal herbivorous crustaceans — as compared with their marine
relatives — the storage capacity of the hepatopancreas, as well as the efficiency of
Cu-assimilation, is augmented; compartmentalization of Cu-storage is more rigorously
carried through; the movements of Cu and Zn within the body are more strictly
regulated, copper, for example, being exchanged between different compartments in
the course of endogenously or exogenously induced phases of the animal's life cycle.
Among the numerous problems that
faced marine animals trying to extend
their range of existence across the littoral threshold onto land was that of
how to replace the rich supply of nutrients given to their ancestors in the form
of a medium that is not only a means of
transport but also an excellent solvent of
inorganic and organic compounds. The
conquerors of terrestrial habitats found
themselves in the position of having to
obtain all body constituents (with the exception of oxygen, and occasionally some
water) from their food. This not only
shifted the port of entry for a number of
nutrients from gills and similar epithelia
to the gut, but also caused additional
complications in the case of elements that
abundantly occur in a dissociated state in
Part of the research described in this article was
supported by the Osterreichischer Forschungsrat;
the work on intertidal species by a grant from the
Zoologiska Station Kristineberg, Sweden. Thanks
are due to Dr. B. Swedmark, Director of the
last-named Institution, and to Dr. Lucia Wiest for
technical assistance. Presentation of this paper at
the Symposium on Terrestrial Adaptations in
Crustacea was made possible by grant GR-6613
from the National Science Foundation.
sea water but are present in the food
largely as organic complexes.
I will deal mainly with the second of
these problems, the problem of altered
availability of nutrients, but a few words
concerning the first, that of rerouting the
pathways of absorption, are also called
for.
REROUTING THE PATHWAYS OF ABSORPTION
Whereas marine crustaceans have the
chance of picking up ions through their
gills from the medium, intertidal forms
had to become less dependent on the sea
as a source of soluble body constituents.
We find species like Birgus latro or
Pachygrapsus crassipes (Gross, 1957) that
only occasionally return to the water in
order to drink or to absorb nutrients
along traditional channels by immersion.
A further stage is represented by species
that are independent of the sea in the
sense that they are capable of absorbing
water and dissolved salts from the interstitial spaces of sandy substrates. Bliss
(1963) has demonstrated this capacity for
Gecarcinus, which employs the pericardial sacs for assistance in the uptake of
495
496
WOLFCANG WlESER
water and ions from moist sand. The development of true terrestrialness, however, is accompanied by further reduction of
these and similar soft-skinned differentiations of the body surface, which are also
potential sites of water loss.
The next step, the shift from interstitial
water absorbed by external epithelia to
orally ingested food as the main source
of salts, has been illustrated by Remmert
and his collaborators for Orchestia
platensis and Ligia oceanica. The former
species is not as well adjusted to terrestrial life as the latter and still depends to
some extent on substrate salinity (Bock,
1967) whereas L. oceanica apparently can
satisfy all its mineral requirements
through algal food (Remmert, 1960,
1967a; Jons, 1965). By experimentally increasing the salinity of food, animals from
the North Sea could be induced to propagate on algae from brackish water of the
Finnish Gulf, which originally had
proven insufficient to ensure reproduction
of these animals (Remmert, 1965). Even
for the uptake of water, intertidal animals have to rely more and more on the
food as the primary source. For example,
Williamson (1951), showed that Orchestia and Talitrus extract the water they
need from algae and that the "amount of
food eaten is determined by the animals'
water requirements rather than their energy requirements." This is an important
observation.
The shift from gills to gut as the main
gateway for the common constituents of
body fluids has an immediate implication:
the intestinal wall rather than the body
surface represents now the important osmotic barrier between environment and
"milieu interieur." For this reason I cannot agree with Edney (1961) when he
says that terrestrial animals do not, in the
true sense, encounter osmotic problems.
All generalizations concerning osmotic
adaptation in intertidal animals — e.g.,
Jones' rule (Jones, 1941) that in intertidal species there is a tendency towards
hypotonic regulation — ought to be
weighed against the fact that it is now the
food that represents the saline environment and the intestinal wall that has to
provide the seat (if any) of osmoregulatory mechanisms.
These considerations probably apply to
all nutrients soluble in sea water, although for the rarer elements the osmotic
problem is not relevant. What is relevant, however, is that for all nutrients
absorbed passively, a gradient of some
sort has to be established in order to allow transfer from the environment into
the organism. This seems to be the main
reason for the development of molecular
storage forms, like ferritin, which sweeps
the interior of cells of free ions, thus
providing the conditions for the maintenance of gradients between the intestinal
lumen and the cytoplasm of absorptive
cells.
The shift from branchial to intestinal
absorption can be illustrated for copper,
by comparing marine decapods (Zuckerkandl, 1960; Kerkut, et al., 1961; Bryan,
1967) and terrestrial isopods (Wieser,
1967); decapods and a noncrustacean
marine animal, the octopus, which also
seems to satisfy its requirements for the
metal from food (Ghiretti and Violante,
1964); and, for zinc, a marine and a freshwater decapod (Bryan, 1967). The
ecological position of the fresh-water species is similar to that of terrestrial species,
since in both cases the role of the body
surface as an important absorptive organ
for soluble nutrients has been taken over
entirely by the intestinal wall.
ALTERED AVAILABILITY OF NUTRIENTS
From a quantitative point of view algae
represent an even better source of minerals than sea water since most, or at least
all important, elements are concentrated
by them (Black and Mitchell, 1952).
Quite generally, the cell sap as a medium
is not very different from sea water as far
as the availability of lighter elements is
concerned, since there is always a rich
supply of e.g. Na, K, Ca, Mg, Cl, either
in ionized or in easily dissociable form.
The situation is different for the heavier
497
METABOLISM OF COPPER IN ISOPODS
TABLE 1. Relationship between Cu-content of food (leaf litter and seaweeds) and efficiency of
Cu-assimilation (expressed as the ratio Cu-inpul/Cv-otitpiit) hi three species of isopods and
amphipods.
Quotient of Cu-assimilation (n)
Food-copper
(/ig/mg dry weight)
<0.05
0.05-0.09
0.09-0.14
0.14-0.20
0.20-0.50
>0.50
Porcellio scaber
0.36 (4)
0.43 (4)
0.76 (3)
2.5 (1)
8.98 (6)
Ligia oceanica
Orchestia
gammarella
0.44 (2)
0.53 (4)
1.34(6)
1.52(6)
1.15(1)
0.75 (2)
0.93 (3)
1.28 (5)
1.20(3)
1.27 (3)
elements, particularly for Cu and Zn, and
it is with the fate of these two elements
that I shall be concerned in the remainder
of this article.
Both metals occur in soluble form in
sea water, but in organisms they form
organic complexes. Cu may be found in
phenoloxidases, sulfurtransferases,
and
oxidases; Zn in carboxypeptidases, alcohol dehydrogenase, and carbonic anhydrase. In body fluids of animals, Cu may
occur as the proteid, hemocyanin, or in
the form of albumin and other protein
complexes; Zn may become adsorbed to
hemocyanin (Bryan, 1967). Free Cu and
Zn do not occur in organisms to a significant degree, although this statement will
have to be qualified to some extent, as
demonstrated below.
There appears to exist an important
difference between animals and plants as
sources of these metals. Whereas the Cu
and Zn complexes of the body fluids of
animals are relatively easily dissociated,
the organic complexes inside cells are
very stable (Lang, 1955; p. 65). Thus it
might turn out that herbivorous animals
in search of these elements could be confronted largely by tight organic complexes, whereas carnivores may count on
the more manageable forms in the body
fluids of their prey. There are some indications that Cu and Zn indeed are much
more difficult to extract from plant than
from animal tissues. Graham and Telle
(1967) report that in soybeans, sesame
seeds, and cotton seeds, Zn is so tightly
bound by phytic acids that it cannot be
used by animals. Leaching in sea water or
Natural copper content of most vegetable material
tap water will not liberate significant
amounts of Cu from seaweeds (Black and
Mitchell, 1952).
More direct evidence is presented by
feeding experiments with isopods (Porcellio scaber, Ligia oceanica) and the amphipod, Orchestia gammarella (Wieser,
1967). If fed pure leaf litter or sea weeds,
either untreated or artificially enriched
with CuSO4, these species are incapable of
extracting copper from their food unless
the latter has accumulated unphysiologically high amounts of the metal (Table
The loss of metals via the feces under
conditions of dietary deficiency is a common phenomenon; it occurs with iron in
mammals (Buddenbrock, 1956), with zinc
in Auslropotamobius pallipes (Bryan,
1967), and with copper in Helix pomatia
(Weischer, 1965). What is significant in
the case of terrestrial crustaceans is the
fact that they lose copper—at least under
laboratory conditions—while feeding on
what usually is considered their staple
diet, i.e., leaf litter in the case of P.
scaber, and Fucus in the case of O. gammarella. I consider this shortcoming an
important aspect of peracaridean physiology and one that provides an unexpected
view of the adaptative problems of nutrition experienced by these immigrants
from the sea.
The Cu-metabolism of herbivorous isopods and amphipods is shaped by three
factors: (1) the marine heritage of hemocyanin as respiratory pigment and main
blood protein, (2) the stable nature of
copper complexes in vegetable material,
498
WOLFGANG WIESER
(3) the peculiar digestive process during
which a nearly compact cylinder of
ingested food is attacked along a narrow
dorsal groove, the typhlosolis, through
which the digestive enzyme injected by
the two or three pairs of hepatopancreatic
tubules can sweep back and forth. The
mixing of digestive juice and gut content
is aided by peristalsis and by lateral
movements of two rows of pendulum-like
cells bordering the typhlosolis (Hartenstein, 1964), but it appears that the mixing process is not very efficient.
Shaped, so to speak, by these conditions,
the Cu-metabolism of terrestrial isopods (and amphipods?) has developed a
few highly characteristic, adaptative traits
which I shall now discuss.
Hyperphagy and selective feeding
The lack of a nutrient very often induces
animals to eat more of the potential
source of the deficient substance. This is
well established in mammals, for example,
with regard to NaCl, Ca, or vitamin B (but
not for vitamin D!) in rats (Donhoffer,
1960). The fact that feeding on overwintered leaves causes heavy Cu-losses via the
feces in P. scaber and O. asellus might be
taken as the starting point of an argument
through which the high feeding rates observed in these animals are rationalized.
High rates of ingestion, coupled with low
rates of assimilation, have often been
noted. The usual explanation offered is the
teleological one that isopods are among the
primary consumers of leaf litter and that
they have to ingest a lot in order to give
other organisms the chance of continuing
the job of working on the profuse but illdigested fecal material (Van der Drift,
1951; Balogh, 1958; Dunger, 1958). The
observations by the authors quoted above
were based on short-term laboratory experiments, and it is significant that when another author repeated the experiments under more natural conditions (Gere, 1962),
the feeding rates he obtained were only
about one-third of those of the laboratory
experiments.
The controversy on this point between
Dunger and Gere can be resolved, I think
satisfactorily, by comparing the quantities
of leaves eaten when the isopods are being
offered food in (a) a glass dish with a piece
of moist filter paper or with sterile sand as
a substrate, (b) the same kind of dish, but
with a layer of fecal material, taken from
an old culture of these animals, as a substrate (Wieser, 1965a/ It turns out that
under the former, more artificial, conditions P. scaber will eat about three times as
much from the same source of food
than under the latter conditions. One possible conclusion from this discrepancy in
experimental findings is that the fecal material, produced by the animals from the primary food source, contains something that
cuts down the need for leaf consumption.
That this something is actually exploited is
indicated by the observation that the feces
produced are regularly recycled by the animals. The best way of making this observation is by offering to isolated animals alternately food of different colors {e-g-, leaves
and carrots).
If the fecal substrate is removed, consumption of leaves by P. scaber goes up.
Hyperphagy in this species might then be
interpreted as the result of a metabolic deficiency, the inability to extract certain essential nutrients from pure leaf litter —
essential nutrients, however, that are contained in the feces produced. As shown in
Table 1, copper would be one of the nutrients meeting these qualifications.
It is obvious that hyperphagy in this case
offers no immediate solution to the isopod's problems. The deficient nutrient (s),
as far as one can tell, is present in sufficient
amounts in the primary food material
but cannot be extracted from it. Increasing
the rate of ingestion will not alter the negative balance. On the other hand, the rapid
degradation of primary vegetable material
permits with equal rapidity the establishment of a fecal substrate from which (as
the experiments alluded to above suggest)
the missing nutrient (s) can now be absorbed.
It seems that the high feeding rates observed in isopods are characteristic of an
METABOLISM OF COPPER IN ISOPODS
499
we may infer from the curve that the
isopods experience a net loss of copper via
their feces for 2-3 days, after which they
are able to extract again, by recycling the
feces, more copper from the substrate than
they give up to it. Thus, within a few days
the fecal substrate has become a new
source of copper. The lag period suggests
that microbial activity is involved in rendering the substrate more palatable to the
isopods, perhaps by breaking up macromolecular complexes liberating the metal.
I would suggest then that the presence of
a moist substrate with high nuicrobial
activity is a sine qua non for the assimilation of complexed nutrients in herbivorous
isopods. In other words, herbivorous
crustacean emigrants from the sea depend
FIG. 1. Changes in Cu-content of feces during experiments in which specimens oE Porcellio scaber
on routes where the accumulation of microwere put into glass dishes and fed leaf-litter of
bial substrate is possible. The wrack beds
Populiis niger. Each point denotes the mean of
of the sea shore, rotting sea weeds of salt
5-18 experiments; vertical lines represent standard
marshes, and rotting leaf mold of terrestrial
deviations.
habitats bordering the intertidal zone,
important phase in the creation of suitable represent important routes of this kind, inhabitats for these animals. Observations on habited by talitrids, oniscoids, and even by
Orchestra platensis point in the same direc- members of the genus, Sphaeroma. It
tion for amphipods (Remmert, I960). Dur- is via these and similar ports of entry
ing this phase metabolic efficiency is very that terrestrial habitats must have been inlow (Wieser, 1965a), but there is no premi- vaded by peracaridean crustaceans.
um on the efficiency of energy turnover at
this stage of the feeding cycle. Energy considerations demand that the high rates of Storage and compartmenlalizatiun
ingesiion and low rates of assimilation
If it is usual for terrestrial isopods to
claimed for isopods be true only for experi- pass through nutritional phases in which
mental situations, and that in nature they the extraction of copper from food is diffirepresent transitory phases in the degrada- cult, the presence of large stores of copper
tion of organic matter.
in the body would be advantageous. FigThe main question we have to ask now ure 2 demonstrates that there is indeed a
is the following: what are the changes tak- trend towards an increase in Cu-storing caing place in the fecal substrate that allow it pacity of the hepatopancreas with increasto become the unexpected source of ing terrestrialness of the species. A particuessential nutrients for animals that are re- larly interesting example is offered, by
Sphaeroma serrata, the high Cu-content of
sponsible for creating it in the first place?
Jf several specimens of P. scaber are put which suggests a terrestrial rather than an
into a glass dish provided with plenty intertidal way of life. Remmert (19676) has
of food in the form of overwintered leaves, discovered that another member of the
they start eating rapidly, producing within genus, S. hookeri, actually does lead a
a few days a thin layer of feces. If these semi-terrestrial existence. Not only the avfeces are analyzed daily for their Cu- erage amount of copper stored in the hepacontent, the curve shown in Figure 1 is topancreas increases in terrestrial isopods,
obtained. Keeping in mind the co- but the efficiency of assimilation of copper
prophagous feeding habits of these animals, increases also if the metal is offered in a
WOLFGANG WIESER
500
Comttra
Id o 11A
cylmdracra, Plymouth
Qt a nuto AA,
Gimmaru*
locusla,
Mtfinog*mm*ru»
Orchtilia
PI ym oul h
Plymoulh
m a r m u i , Plymouth
g i m m m l i , Plymouth
ii
a
•
PoretDio
ieab«r.
Ptymw-lh
FIG. 2. Cu-content of hepatopancreas o£ several
species of isopods and amphipods as a function of
habitat. Means and standard deviations are indicated. (From Wieser, 1967)
digestible form. Table 1 shows that P. scaber may absorb about 90% of the copper
present in leaves that have been soaked in
CuSO4 and thus carry enormous quantities
of the metal, whereas L. oceanica and the
amphipod, O. gammarella, will assimilate
not more than 20-30% of the copper in the
food under these unphysiological conditions. That isopods and amphipods occupy
a rather special position with regard to
their efficiency of assimilation is brought
home by comparing them with the snail,
Helix pomatia, which according to Weischer (1965) will assimilate up to 95% of the
copper present in its normal diet, apparently without the help of microorganisms (although Weischer is not explicit on this
point).
The large amounts of copper present in
intertidal and terrestrial isopods must have
caused special problems with regard to the
storage and the control of movements o£
this potentially toxic metal. Some of the
mechanisms involved will now be considered.
Movements of copper withi?i the body
The spider crab, Maja squinado, or the
shore crab, Carcinus maenas, may lose up
to 50% of their bodily copper to the medium with each molt (Zuckerkandl, 1960;
Kerkut, et. ah, 1961). The Cu-content of
the marine isopod, Conilera cyclindracea,
may vary over two orders of magnitude
(Wieser, 19656), and that of blue crab
serum may show an 18-fold variation
(Horn and Kerr, 1963). Obviously, in marine crustaceans, with plenty of free copper
available in the medium, there is little need
to regulate the movements of the metal
within the body.
In terrestrial species the situation is different. The hepatopancreas represents a
very efficient storage center, and the transfers between this and other bodily compart
ments seem to be guided by the necessity
of having to move large stockpiles of a
toxic metal, of which as little as possible
should get lost, through a maze of metabolic processes in hepatopancreas, blood, and
integument. A somewhat similar situation
exists with regard to zinc when the lobster,
Homanis vulgaris, and the freshwater
crayfish, Austropotamobius pallipes, are
compared. In the former species, Zn is
passed from the medium to all organs except muscle, gonad, and shell, and it is lost
via urine, blood, and body surface. In
Austropotamobius, however, Zn is concentrated by the hepatopancreas, and
it can be lost only when it is adsorbed
by food passing through the gut and into
the hepatopancreatic lumen (Bryan, 1967).
In P. scaber and O. asellus the hepatopancreas usually contains about twothirds of the copper in the body. However,
during molting and seasonal cycles, as well
as in special situations of stress, considerable portions of the metal may be moved
between hepatopancreas and other bodily
compartments (Wieser and Wiest, 1968).
For example, during the feeding experiments summarized in Figure 1, about half
the hepatopancreatic copper is transferred
into the blood and the remainder of the
body. For as long as the animals keep losing copper through their feces, the central
metabolic organ will contain only about
one-third of the total supplies of the metal.
After the third day, when the original
equilibrium between animals and fecal substrate is being restored, the copper in the
body follows suit by gradually establishing
the previous pattern of distribution (Fig.
3).
The usual explanation for the movements of copper between bodily compart-
501
METABOLISM OF COPPER IN ISOPODS
ments in invertebrates is based on assumptions about the synthesis of hemocyanin.
Thus the dynamics of copper is usually
supposed to be tied up with life stages that
might require a special supply of oxygen,
e.g., with molting stages (in Maja
squinado) or with the onset of hibernation
(in Helix pomatia). However, for the
movements of copper discovered in P. scaber, in connection with a situation that can
only be called "nutritional stress," the relationship with the synthesis of hemocyanin
is not at all obvious. It would seem more
likely that copper is transferred for the sake
of copper and not for the sake of hemocyanin. In keeping with this suggestion is
the observation (unpublished) that the
movements of copper between hepatopancreas and blood are not accompanied by
parallel changes in the concentration of hemocyanin in the blood. Could it not be that
during periods of high metabolic activity of the hepatopancreas, or when there is
danger of losing too much of the metal to
the alimentary canal, copper is fed into the
blood as a safety measure to keep the losses
down?
Cellular and intracellular compartmentalization
There are several known ways of storing
copper. In decapods it may be stored in the
form of pseudo-crystals or large refractive bodies in special copper cells of the
hepatopancreas (Ogura, 1959). In isopods
it may be found throughout the hepatopancreas in the form of small granules, which
represent the metal in easily dissociable
state that can be complexed with chelating
agents—at least after fixation of the tissues
with alcohol (Wieser, 1965c). As far as one
can tell there are small cells in the
hepatopancreas of isopods, in which copper
is concentrated when it becomes plentiful. These storing cells become more prominent as one goes from subtidal to intertidal
to terrestrial species. In Ligia oceanica
most of the copper occurs in small, conoid
cells (the "Speicherzellen" or S-Zellen of
Frenzel, 1894), but one can nearly always
o.ii
-
r
o.i* Lj
O.t2
L
1
O.O8
I
0.06
y
/
•
a.
0.0*
0.03
•
•
FIG. 3. Changes in Cu-content of hepatopancreas
(open circles) and blood and integument (filled
circles) in the course of the experiments described
under Figure 1. Means (of 6-17 experiments) and
standard deviations are shown.
observe small amounts of this "free" copper in the other cells of the hepatopancreas
as well (Fig. 4a). The same holds for
Sphaeroma serratum in which the hepatopancreas has become so crowded with copper granules (Fig. 4b) that it is difficult to
understand how this organ is capable of
performing any function beyond the storage of copper. In truly terrestrial oniscids,
copper is nearly always confined to the
conoid S-cells, which are squeezed in between the large, secretory B-cells (Frenzel's
terminology; Fig. 4c).
However, the development of specialized
and well defined cells for the storage of
copper in the hepatopancreas does not
solve all problems. How is one to understand the presence of large amounts of low
molecular, easily dissociable, and thus potentially toxic, copper in the central metabolic organ of these animals? A solution is
indicated by the behavior of hepatopancreatic copper in different incubationmedia. Incubating in distilled water will
extract an average of 20% of the copper
from the freshly dissected hepatopancreas.
This particular water-soluble form is loosely bound to an unidentified carrier and can
be set free by treatment of the extract
with GNT HC1. Eighty per cent of the copper remains in the hepatopancreas and can
502
WOLFGANG WIESER
be rendered accessible to chelation by ashing. Sections show the storage cells ruptured by the osmotic shock treatment but
the copper is still present in granules or
vesicles adhering to the broken cell walls.
On the other hand, carbon tetrachloride
will extract approximately 80% of the
hepatopancreatic copper, with the remaining 20% recoverable by ashing. It appears,
therefore, that a major portion of the hepatopancreatic copper is not really "free," but
wrapped up in vesicles or cytosomes.
FIG. 4. Series of histological sections through the
hepatopancreas of isopods in order to show the
distribution of copper, (a) l-igia oceanica and (b)
Sphaeroma serratum, showing S-cells packed with
copper (1) and diffuse copper in B-cells (2); (c)
Porcellio scaber, with large copper-free B-cells and,
between them, the smaller S-cells, which in the
three species represented here seem to be mainly
copper-storing cells; (d) P. scaber during molting
cycle, with copper-filled S-cells lining the lumen,
and two spent H-cells; (e) P. scaber during molt-
ing demonstrating the glucoproteid material (3)
synthesized at certain steps of this process (see Fig.
5); (/) P. scaber after a temperature-stress. The
same glucoproteid material has turned up and the
copper has become mobilized, occurring in a scattered way even in the large B-cells (2). The S-cells
(1; pointing at nucleus) appear to be nearly free
of copper. All preparations fixed in absolute alcohol and stained with rubeanic acid in N'aacetate
bulfer; counterstained with hcnialum. (From
Wieser, 1967)
METABOLISM OF COPPER IN ISOI-ODS
503
Lipid solvents will destroy the membranes
of these vesicles liberating the metal. By
fixing the tissues with absolute alcohol, the
lipid-soluble form can be made visible by
chelation (for example, with rubeanic
acid), without going into solution.
Nothing like this is known to occur in
marine crustaceans, or in any other animal
for that matter. Thus, the conclusion appears warranted that the occupation of land
by peracaridean crustaceans led to the
development of a complicated and unique
system of copper metabolism, involving—
apart from hemnryanin—at least two forms
of the metal, the one the main storage
form, the other perhaps a transport form.
FIC. 5. Half-schematic represenlation of cycle of
events taking place in hepatopancreas of Porcellio
scahcr during molt. 1, inlermolt stage; 2, beginning
of
histological
changes
during
proecdysis
(roughly D2). The two heavy arrows indicate the
two steps of ecdysis, first ecdysis posterioris, then
eceysis antcrioris. Heavy extramiclear grains, copper;
dashed bodies, glucoproteid material, turning
up twice during the cycle. (After Wieser, 1965c).
WOLFGANG WIESER
FIG. 6. The comparison of a hepatopancreatic tubule during intermolt (Jett) with one corresponding
to molting stage 3 in Figure 5 illustrates the
dramatic changes that take place in the central
metabolic organ of terrestrial isopods during the
molting cycle. 1, S-cells containing copper; 2, Bcells, either vacuolized or containing glucoproteid
grana. After stage 3 copper is gradually complexed
by proteins (?) and can no longer be chelated with
rubeanie acid.
METABOLISM OF COPPER IN ISOPODS
The problem of maintaining a gradient of
free copper between the lumen of the hepatopancreas and the Cu-rich cytoplasm of
hepatopancreatic cells is solved by enclosing the major part of the free copper by
membranes, thus removing the metal from
unimpeded circulation in the same way as
calcium is removed in the gastropod hepatopancreas (Hirsch, 1917), or hemoglobin
in the blood of many animals. A further
analogy, based on chelation rather than
"vesiculation," is the removal of free iron
by ferritin.
Mobilization
We do not yet know how hemocyanin is
synthesized and we do not know how the
movements of copper occurring in crustaceans and molluscs are controlled. The quantitative relationship between a particlebound, lipid-soluble and a carrier-bound,
water-soluble form of copper would suggest
that whenever the body wants to draw
on the resources of the metal in the hepatopancreas, it siphons off the water-soluble
—presumably
the
true
"transport"—
form of copper, which will then be
replenished by the lipid-soluble stock of the
metal. This would constitute a neat
transport system, by which the presence at
any time of large amounts of ionic copper
in the cells is avoided. The copper that is
"free" is really surrounded by membranes,
and the copper that moves around is bound
to a carrier and thus probably harmless.
However, there exist phases in the lives
of isopods in which the whole hepatopancreas seems to erupt and the constraints,
just discussed, of the copper transport system appear to break down completely. A
dramatic cycle of histological changes was
discovered in connection with the molting
process in Porcellio scaber (Wieser, 1965c),
and this is represented schematically in
Figures 5 and 6. Without going into
details the characteristics of the sequence of
events may be summarized as follows:
(a) The molting process involves the
mobilization of practically all copper in the
storage (S-) cells.
(b) Mobilization takes the form of a bi-
505
phasic cycle during which copper alternates
between
histochemically
"free"
and
"bound" states. That is, from a histochemical point of view, copper disappears twice
during the whole process.
(c) Mobilization of copper is always correlated with the appearance of glucoproteid
material in S- and B-cells, with which
the latter in particular are filled to the
bursting point (Fig. 6).
(d) Only under these conditions, i.e.,
during the presence of the glucoproteid material, does free copper spill from S- to
B-cells, or into the lumen of the tubule.
We are far from understanding the significance of these events taking place within the fragile confines of a tubular organ
bound by a thin muscularis, a basal membrane, and a single layer of epithelial cells.
The whole process may be correlated with
the synthesis of hemocyanin and of the
phenoloxidases required for the construction of the new cuticle, or it may reflect the
evolution of an elaborate step-function,
through which during the periods of
high metabolic activity the equilibrium between a water-soluble and a lipid-soluble
form of copper is suspended and the entire
copper stores of the hepatopancreas are
turned into stable organic complexes. This
could be a means of protecting sensitive
enzymatic reactions from the metal.
Whatever the significance of this process
it has no parallel in other animals, not even
in other crustaceans in which the fate
of the hepatopancreas during molt was
studied (see Skinner, 1965, 1966). This
leads us to the assumption that terrestrial
isopods, by adapting the old marine habit
of Cu-storage to the new problems facing
the landbound immigrant, were forced into
the elaboration of a highly complex
mechanism, involving compartmentalization, transfer, and mobilization of this metal. To the naive observer, this mechanism
appears to be much more complicated than
the physiological role the metal could
possibly play in the lives of these animals.
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