Nitrogenous excretion in aquatic and terrestrial crustaceans

13 Pages·1991·10.6 MB·

by  Peter Greenaway

NITROGENOUS EXCRETION IN AQUATIC AND TERRESTRIAL CRUSTACEANS
PETER GREENAWAY
Greenaway, P, 1991 090): Nitrogenous excretion in aquatic and terrestrial crustaceans.
Memoirs of the Queensland Museum 31: 215-227. Brisbane. ISSN 0079-8835.
For water-breathing animals the most economical nitrogenous excretory product in
enerectic terms isammonia(ium)and thisis generally excreted across the gills. In terrestrial
situations most animals produce less toxic materials, notably urea and purines, which are
eliminated by the excretory organs. Crustaceans are predominantly an aquatic group and
marine and freshwater species conform to the above pattern in excreting ammonia (ium),
Most amphibious species generally have frequent recourse lo water and show little change
in the basic aquatic pattern of excretion, The small terrestrial forms, notably Amphipoda
and lsopoda and the crab Geograpsns grayi, retain ammonia excretion but eliminate it as
a gas, The gecarcinid land crabs excrete NHq"* in the urine whilst Birgus latro excretes uric
acid in the faeces. Many terrestrial and some aquatic species store purines and may have
purine synthetic ability. The role of stored purine is unclear but it may act as a N Tesetve,
an jon store or as a non-toxic buffer when normal ammonia excretion is inhibited. Urea is
Nol an important excretory product and crustaceans appear to lack a complete urea cycle.
Exctetory pallerns are discussed in relation lo the evolutionary history of the group. (]
Nitrogenous excretion, Crustacea, ammonia, terresirial Crustacea, uric acid
Peter Greenaway, School of Biological Science, University of New South Wales, P.O. Box
I, Kensington, New South Wales 2033, Australia; 6 July, 1990,
The form in which animals excrete waste nitro- which can economically excrete their waste in
gen and the route, or mechanisms, by which it is the form in which it is produced.
eliminated are quite labile, but there arc several Other forms of waste nitrogen are also elimi-
underlying factors which strongly affect the nated by animals. chiefly purines and urea, bur
mechanism employed, Firstly, metabolism of these have several selective disadvantages. They
protein yields ammonia9, typically from trans- are more complex molecules and their synthesis
amination and deamination reactions, and the requires energy and also elaborate enzyme sys-
metabolically cheapest form of excretion ts am- tems (Hartenstein, 1968). This extra metabolic
monia. Secondly, ammonia is toxic to animals, cost is only outweighed under conditions con-
sidered inudequate for safe excretion of am-
especially higher vertebrates (Campbell, 1973)..
monia, i.e, on land, especially in mesic and xeric
although less so to crustaceans which may main-
habitats where water availability is limited.
tain ammonia concentrations appreciably higher
The generalised pattern seen in animals then is
than in vertebrates (Table 1). Thirdly, NH, is
as follows: water-breathers are ammonotelic ex-
highly diffusible and cannot be contained or
creting NH,/NH,° across suitable surfaces, usu-
concentrated by cells, but in the ionic form il can
ally the gills; terrestrial forms (particularly
be concentrated and transported by the usual ion
arthropods) excrete purines or, less commonly,
transport mechanisms. In body fluids most am-
urea, and any NH,/NH," excretion is small and
monia is protonated (NH,*) and within the usual is often primarily a function af acid-base regula-
range of pH only around 1% will be present as
tion.
NH,. Elevated [NH,*] however, leads to eleva-
The Crustacea are predominantly an aquatic
tion of [NH] with resultant toxic effects, so that proup with the major radiation in the sea but with
ammonia concentrations must be kept within a a substantial number of speciesi n freshwater and
tolerable range. Rapid excretion into Jarge only a few on land. Most species have spent their
volumes of water avoids any build up in concen- whole evolutionary history in water and am-
tration, For these reaSons, ammonia excretion is monotelism is expected to be the standard pat-
considered to be the preserve of water-breathers tern of excretion, The terrestrial forms would be
expected to excrete one of the more complex,
less. toxic, compounds, The extentt aw hich these
' In this review NH3 tefers to molecular ammonia.
NH4* to ammonium ions, and ammonia to the sum predictions hold is examined below, Whilst
af both (=total ammonia). water availability in the habitat is normally an
216 MEMOIRS OF THE QUEENSLAND MUSEUM
TABLE 1. The concentration of ammonia in the haemolymph of a range of crustaceans.
Species Ammonia
mmol.L!
Callinectes sapidus 0.39 Cameron and Batterton (1978)
Callinectes sapidus 0.011 NH3 Kormanik and Cameron (1981b)
0.82 NH4*
Carcinus maenas 0.9 Binns (1969)
Uca pugilator 20.0 Green et al, (1959)
Cardisoma carnifex 1.6 Wood et al. (1986)
Geograpsus grayi 2-3 Greenaway and Nakamura (in press)
Gecarcoidea natalis 2-4 Greenaway and Nakamura (in press)
Cherax destructor 0.1 Fellows and Hird (1979)
Notostomus gibbosus 217 Sanders and Childress (1988)
Porcellio scaber 1.5 Wieser and Schweizer (1972)
over-riding determinant of the excretory mecha- AMMONIA FORMATION
nism employed, there are certain other factors Ammonia for excretion originates overwhelm-
which may be important in determining ex- ingly from the catabolism of amino acids but
cretory rates and patterns in Crustacea. These minor amounts will result from the degradation
include the moult (at which time there is consid- of purines, pyrimidines, and urea of which the
erable catabolic and anabolic activity involving latter may originate from a high intake of ar-
proteins), the nitrogen content of the diet and ginine (Fig. 1).
metabolic rate (including effects of temperature Before the carbon skeleton of an amino acid
and season). Additionally, where ammonium is can be utilised in the citric acid cycle its amino
excreted in exchange for cations in the water the group must first be removed. Most commonly
salinity of the latter may be important as
this is achieved by transamination, the transfer
osmoregulatory requirements may conflict with
of the nitrogen containing a-amino group to an
those for nitrogenous excretion. This may affect
a-keto acid. The general reaction is as follows
migratory species such as Eriocheir sinensis and
prawns as well as species subject to fluctuating
amino acid
salinities (Regnault, 1987). The taxonomic in- L-a-amino + a-ketogluterate < >a-keto acid + L-glutamate
heritance in terms of available metabolic path- transaminase
ways and the length of time a species has spent These reactions do not release ammonia but
in a particular habitat can also exert a major
pass it on to an a-keto acid to form L-glutamate.
influence. This review is directed towards the
Certain amino acids possess other N groups and
effects of adaptation to terrestrial habitats on
these may be removed by deamination followed
excretory mechanisms.
by transamination of the amino nitrogen e.g. the
amide groups of asparagine and glutamine.
EXCRETION IN AQUATIC HABITATS
asparaginase
Asparagine + H2O > Aspartate + NH3
The formation, detoxification and elimination
of ammonia by aquatic crustaceans has been the glutaminase
subject of several recent reviews (Kormanik and Glutamine + H20 >L-glutamate + NHa
Cameron, 1981a; Evans and Cameron, 1986;
Regnault, 1987) and whilst the subject is far from
being totally understood available data are well These pathways are believed to be utilised in
documented. In consequence, nitrogenous ex- some crustaceans (Krishnamoorthy and Srihari,
cretion in aquatic crustaceans will be reviewed 1973; King et al., 1985). Despite some contro-
to provide a basis for the logical treatment of versy it is clear that glutamate itself can undergo
terrestrial groups. oxidative deamination in crustaceans (Chaplin et
NITROGENOUS EXCRETION 217
Aapiraypine bolism of other nitrogenous compounds. Thus
purine and pyrimidine nucleotides may be
Aral vetdy broken down to yield ammonia (Fig. 1), although
Aa Garden amp the amounts involved are likely to be very small
Porthe | given the presence of scavenging pathways. Di-
Synthesis
Advoosine =G ianavioe etary intake of these components may necessi-
t yad
b-wlatumale late some nitrogenous excretion, depending on
the animal's requirements. In actively working
Inpelne Coamardines
skeletal muscle, the deamination of AMP results
Hyposanihine in ammonia production. Whilst AMP-deaminase
has been identified in certain crustaceans its
we,
function is yet ta be established (Regnault, 1987)
Urle acid and the importance of the pathway in overall
ammonia production in crustaceans is unknown.
Albanian All these pathways lead to utic acid which may
then be excreted as urate, stored or degraded to
Alanine weal
urea OF ammonia.
Ure Arginine Ammonia may also be produced from urea via
Pyrimidines 4\ urease, but as a complete synthetic pathway for
| urea is lacking in the crustaceans examined (Har-
vitirew eyele
tenstein, 1970; Claybrook, 1983; Regnault,
NMG
1987) available urea is likely to be restricted to
FIG. 1. Metabolic pathways producing ammonta in that produced from dietary arginine via arginase
cTustaceans., which is present in the midgut gland and gills of
many crustaceans.
al., 1965; Bidigare and King, 1981; Batrel and
Regnault, 1965; Regnault and Batrel, 1987). arginase
Arginine + H20 urea + ornithine
GDH :
Arginine is an essential amino acid in
L-glutamate +H20 < ==> o-ketoglutaraie + NHy
crustaceans that have been investigated (Zandee,
Indeed as glutamate is the only amino acid 1966; Claybrook, 1983) so that urea from this
from which the a-amino group can rapidly be source represents a dietary excess rather than a
removed to release ammonia and the enzyme, controllable form of N excretion. Urea will also
glutamate dehydrogenase is very important in be formed as an intermediary compound in uri-
ammoniogenesis. In many animals, the direction colysis. Thus the possible generation of am-
of the reaction is determined by the co-factor so monia from urea is small,
that oxidative deamination (NH, release) is
stimulated by NAD* whilst the reducing reaction AMMONIA DETOXIFICATION
(NH, uptake) requires NADPH (Lehninger,
1982). In aquatic Crustacea, the major site of excre-
tion is the gills (Kormanik and Cameron, 1981;
GDH Evans and Cameron, 1986) so the ammonia gen-
Hi0+L-glutamate+NAD < > NHj+ NADH +H o-ketoglutarate
erated in metabolising cells must be transported
there for elimination. As the concentration of
o-ketoglulursie + NADPH +H + NHI GDH > NADP + 1.0 + L-glulamate ammonia in haemolymph is high (Table 1), con-
siderable amounts may be carried in this form.
However, if production was high or excretion
In crustaceans the relative concentration of was discontinuous, a non-toxic molecule would
NAD* and NADH, and substrate concentrations
be necessary for transport or temporary storage
may determine whether formation or deamina- of nitrogenous Waste. The obvious candidates
tion of glutamate occurs (Batrel and Regnault, are glutamate, the end product of transamination
1985; Regnault and Batrel, 1987). In most cases reactions, and glutamine which offers the advan-
N is transferred to glutamate and its oxidative tages both of readily crossing cell membranes
deamination will release ammonia into the cell, and of doubling the amount of NH, carried. In
Ammonia may also be generated by cata- vertebrates, glutamine synthetase forms glu-
MEMOIRS OF THE QUEENSLAND MUSEUM
monly seen in animals are de novo synthesis of
Oe ee Se 4 ee see ee LD
i purines and urea production. As the crustaceans
cient Nal
Jo nol appear to possess a functional urea cycle
3) te Made pludanate _ ol 4_4 naan (Claybrook, 1983), synthesis of this compound
from ammonia is unlikely. Although many crus-:
| METATINDISIAG FT RLES taceans show periodically high levels of urea in
the haemolymph (up to 28 mmol/L in Holthui-
He LETTE nt HOTT E sana transversa, P, Greenaway, unpublished)
and excrete some urea, it is generally a minor
component of total output. The ability to synthe-
TARTU Pen sise purines de nova is also thought to be lacking
in aquatic crustaccans. The midgut gland 1s re-
' ported ta form and excrete spherules of uric acid
ROPER! fF gta annpere huranine in several species of aquatic decapods (Fischer,
ullis
1926), The small amounts of uric acid excreted
or stored are generally thought to be derived
warra from purine catabolism rather than synthesis but
this may need to be examined more closely.
FIG. 2. A pathway for ammonia detoxification and
transport m aquatic crustaceans (aller King ef al. ELIMINATION OF WASTE NITROGEN
1985).
8amine in the peripheral tissues and in this {orm AQUATIC CRUSTACEANS
waste is transported to the site of elimination There is conclusive evidence from whole ani-
(liver). There is evidence for a similar glutamine mal studies that ammonia is the chief excretory
detoxificatlon pathway in the crabs Cancer and product of aquatic crustaceans and that the site
Carcinus (Fig. 2) (King ct a/., 1985), High levels of excretion is the gills. Gills provide a large and
of glutaminase in the gills of these crabs are well ventilated surface and the respired water
presumed to release NH,= [rom glutamine for carries. away excreted ammonia, preventing
elimination whilst at the site of ammonia genera- development of gradients adverse to excretion,
tion (muscle), the levels of glutamine synthetase Waste nitrogen may arrive at the gills as am-
and glutamate dehydrogenase are high. Glu- monia (usually 99% NH,= and 1% NH,) or in
tamine also appears to be an important vehicle detoxified forms such as glutamine and perhaps
for ammonia transport in Paratelphusa hy- other amino acids (see above).
drodromous in freshwater (Krishnamoorthy and The processes by which ammonia is excreted
Srihari, 1973). This pattern does not hold forall have altracted considerable attention, although
crustaceans, however, as King et al, (1985) much of it has been concerned with osmoregula-
found hitle evidence to indicate glutamine tory mechanisms rather than excretion (Evans
detoxification in cither Hamarus or Pandalus. and Cameron, 1986; Schoffeniels, 1976). Whilst
Evidence favours GDH as the controlling step in the possible mechanisms of elimination of am-
ammonia excretion (summarised in King er al., monia are now fairly clear, the actual mecha-
1985), nisms used by particular species are seldom so.
The formation of alanine as a detoxification itis often extremely difficult technically to dis-
and transport molecule is also possible as tinguish between potential excretory mecha-
crustaceans possess the necessary enzyme nisms, and it is frequently unclear whether
(alanine transaminase) and indeed show high apparent mechanisms are primarily osmoregula-
levels of alanine synthesis in vive (Claybrook, tory or excretory in function as the two systems
1983). There is at present no specific evidence may be closely linked. Available data for crusta-
indicating its general use for detoxifica- ceans are often fragmentary and patterns gener-
lion/transport purposes in crustaceans although ally have to be constructed from inadequate data
it is important in vertebrates (Lehninger, 1982), by comparison with better understood mecha-
Serine has been implicated as the main route of nisms in fish. Potential mechanisms of excretion
detoxification in the crayfish Cherax destructor are presented in Fig, 3 and discussed below.
(Fellows and Hird, 1979) and a serine cycle has The high diffusibility of NH, allows loss to the
been suggested in other animals (Bishop, 1976). water by diffusion down its partial pressure
Other major routes of detoxification com- gradient cither through or between the epithelial
NITROGENOUS EXCRETION 219
Haemolymph Gill Ex <IM ediu (Tansport mechanis9 m. Ammonia+ could then be
Epithelium
generated by removal of the amide N of glu-
tamine, deamination of serine and transamina-
NU, NAy4=Ni,! tion of other amino acids. NH,= could be
transported directly into the cell by Na/K acti-
vated ATPase with NH,* substituting for K~
(Towle and Holleland, 1987). NH, could enter by
diffusion if the gradient were suitable but with
he
active generation of ammonia in the gill
epithelium the gradient may well be adverse.
(dutamine The next step is the elimination of ammonia
from the gill epithelial cells. The mechanisms
wenn!
proposed to account for this rely on amiloride
Sertoy
sensitive entry into the cell of Na* fram the water
via Sodium channels in the apical membrane
(Kirschner, 1983). This is thought to create a
potential gradient which favours efflux of either
NH,* or H9, again through apical ion-selective
fg ene: 2ndcn an a a
channels or antiport mechanisms (Fig. 3) and
results ii n a coupled 1:1 exchange of Na* with
NH,= or H*. The ultimate driving force is the
operation of basolaterally located Na/K ATPase
FIG. 3. Diagram summarising possible mechanisms (Towle and Kays, 1986) which maintains a low
pf ammonia elimination in aquatic crustaceans, intracellular [Na] which allows continued apical
influx of Na. Where Na*/NH,* exchange occurs,
cells. The driving gradient will be small as NH, the ammonia would be eliminated directly. Inthe
forms only a small portion of total ammonia in case of Na8/H* exchange the loss of protons
the haemolymph (Kormanik and Cameron, would1 encourage the dissociation of NH,9 yield-
1981a). Only a small proportion of waste is ing H* and NH, and the latter would be lost to
likely to be lost in this way unless the gradient the water down its partial pressure gradient (Fig.
can be increased locally by active means (see 3). If osmoregulatory requirements for uptake of
below), Nevertheless, diffusive loss appears. to Na* exceeded the supply of H* excretory am-
be the major mechanism in the crab Callinectes monia, additional protons could be provided
sapidus in seawater (Kormanik and Cameron, from bicarbonate via the activity of carbonic
1981b). anhydrase.
Ammonium ions may also be lost directly from There is evidence for Na9/NH,* exchange (but
the haemolymph by diffusion down their electro- nat for Na*/H*) in the marine crabs Cancer and
chemical gradient. Passive, transmembrane dif- Petrolisthes although it docs not account for all
fusion may be impeded by the electrical charge the ammonia ¢xereted (Hunter and Kirschner,
of the ion and the pathway taken is probably 1986). In Uca tangeri, there is evidence for an
paracellular, i.e. through intercellular channels apical H* pump (Krippeit-Drews ev al., 1989)
and the apical junctions (Kormanik and whilst Bigalke (1986) has demonstrated a
Cameron, 1981a). There is no information on the Na8/H* exchanger in Eriecheir sinensis, A
importance of this route in crustaceans but it is Na*/NH,= exchange has also been suggested in
likely to be minor, freshwater crayfish (Shaw, 1960). In Eriocheir
As passive loss. of ammonia seems to explain sinensis, & small component of sodium influx
only a small portion of the total excretion of apparently exchanges for NH, (Gilles and
ammonia, the bulk must involve an active com- Pequeux, 1986) butin the absence of whole body
ponent of transport either into or out of the flux data for Na* and NH," it is not clear what
epidermal cell. Entry into the epithelial cells of proportion of total N excretion this could repre-
the gill could be as NH;, NH,", or detoxified sent, [f nitrogenous excretion is normally much
ammonia (amino acids). The neutral amino acids lower than Na influx perhaps all N could exit by
(glutamine particularly, serine and alanine if this mechanism. In seawater-adapted E. sinensis,
used) would cross. the cell membrane readily bul howewer, sodium influx was absent, so that am-
the negatively charged glutamate may require a monia in seawater animals must have been elim-
221) MEM@IRS OF THE QUEENSLAND MUSEUM
inated by other means (Gilles and Pequeux, ISOPODA
1986), presumably diffusion. In Callinectes The isopods are the most successful of the
sapidus, here isno evidence of Na*/H* coupling terrestrial crustaceans with approximately 1000
(Pressley efa/., 1981) and ammonia efflux is also species ranging {rom supralittoral to desert hab-
insensitive to amiloride indicating a lack of itats. Like their aquatic relatives, they are pri-
Na*/NH,° coupling (Kormanik and Cameron, marily ammonotelic but with the important
T9B1b), It is suggested thal ammonia outpul in difference that much of the ammonia excreted is
lhis species is by diffusion of NH,. in gaseous form (Dresel and Moyle, 1950; Har-
There are then, numerous possible routes by tenslein, 1968; Wieser et al, 1969; Wieser and
which waste ammonia can be removed, and con- Schweizer, 1970; Wieser, 1972b). Surprisingly,
siderable variability in the way these routes are there have been no comprehensive investiga-
actually used by particular crustaceans or even tions either on the partitioning of excretion be-
within a single species under different condi- tween the possible routes or the proportions of
lions. The osmoregulatory requirements for Na* various compounds eliminated by cach route.
transport may well determine which pathways Dresel and Moyle (1950) partitioned faecal non-
are used and, in curyhaline species, salinity may protein nitrogen in several species. Ammonia
determine which is used at.a particular time. A was the main nitrogenous product, uric acid
probable pattern for freshwater crustaceans is made up 5-10%, amino acids 1-6% with anly
described ubove utilising electrical coupling of traces Of urea, Faecal ammonia in Percellio
Na* and H*, but in marine crustaceans where scaber, however, comprises only 10% of total
there is no osmoregulatory requirement for Na®
ammonia excretion, the other 90% being re-
a different pattern might be expected,
leased as a gas (Wieser and Schweizer, 1970).
No data are available either for the urinary flow
OTHER FORMS OF EXCRETION rate nor the concentration of nitrogenous ex-
Although ammonia is the dominant excretory eretory products of urine. Nevertheless, the over-
product there are some data indicatinlogw levels all dominance of gaseous NH, excretion is clear,
of excretion of other materials by aquatic Interestingly, excretion of NH, is nol continu-
Crustacea. Thus uric acid spherules are formed ous but varies diurnally with peak excretion
in cells of the midgut gland and excreted into the coinciding with periods of minimal activity. It is
gui lumen in several species of crabs, the argued that this allows excretion whilst the ani-
anomuran Percellena and in Panulirus (Fischer, mal is resting in a moist microclimate thus min-
1926), In Callinectes sapidus exposed to low imising water loss during volatilization of NH,
temperature and salinity, crystals of uric acid (Wieser ef al., 1969; Wieser, 19725), This diur-
appear in the cells and lumina of the labyrinth nal pattern of excretion may be endogenous in
and bladder of the exeretory organs (Johnson, isopods as itisalso seen in aquatic species (Kirby
1980), Whether this uric acid is derived from an and Harbaugh, 1974), Excretion in terrestrial
excess of other purines or synthesised by the isopods also shows scasonal patierns and is af-
animals is unknown. fected hy (emperature, presumably via the effect
Urea appears to be produced in response to
on general metabolism (Wieser ef al., 1969;
elevated salinity in some crustaceans (Sharma, Wieser, 1972a, b).
1966, 1968, 1969; Krishnamoorthy and Srilvari, Although it has been known for 40 years that
1973) und Horne (1968) has suggested that Car- volatilisation of NH, is the principal method of
disama guanhumé may excrete urea, Further stu-
nilrogenous excretion in terrestrial isopods. the
dies are required on the metabolic capabilities of exact nature of the mechanism involved remains
jhese animals. obscure. Given that excretion of NH, is not
continuous, storage of ammonia between peri-
TERRESTRIAL CRUSTACEA ods of elimination must occur and some method
of detoxification of ammonia is indicated, How-
In ferresitial arthropods the major excretory ever, dala for routine levels of ammonia (e¢. 15
products are purines and clearly \here has been mmol.L7) andamino acids (¢. 2 mmol,L') iy the
strong selective pressure for purinotelism in the haemolymph are available only for P, seaber,
terrestrial habitats. {t might be expected that the While ammonia concentrations of 1-17 mmol.L
{erresttial crustaceans would conform to this pat- and glutamate and glutamine of 6-7 mmol-L*
tern but the limited information available indi- are reported in homogenates of the body wall of
cales thal thts is nol generally the case. P. scaber (Wiescr and Schweizer, 1972), Infor-
NITROGENOUS EXCRETION a2)
mation on detoxification mechanisms is lacking The jerrestrial isopods examined all have
for terrestrial isopods but the presence of glu- deposits of uric acid in the body (Dresel and
famine and glulaminase in the body wall ol P. Mayle, 1950), There are no specific issues im
seaber indicates # capacity for generation of which this is located, as in the freshwarer Aselles
ammonia from glutamine at this site (Wieser. (Lockwood, 1959), and in Onisews aselles
1972c; Wieser and Schweizer, 1972). Thus glu- deposits are reported as being located in the
tamine may be involved in ammonia detoxifica- 8body wall9 (Hartenstein, 1968). The highest
tion and transport as indeed may other level reported is in Armadi/lidium (c, Sumal.e!
mechanisms suggested above for aquatic crusta- wel weight) whilst levels in QO. asellus and P.
ecans. Further investigations are nceded on this scaber are almost an order of magnitude lower
matter, Hartenstein (1968, 1970) provided some (Drescl und Moyle, 1950; Hartenstein, 1968). Et
evidence for oxidative deamination in Oniseus is nol clear whether these deposits represent
asellus but.amino-oxidase activity was concen- excess dietary intake of purines or if they have
trated in the hepatopancreas which ts not suitably been synthesised de novo but there are no data
located for volatilisation of resultant NEL, suggesting the presence of synthetic pathways
As ammonia generated will be largely iomsed and Hartenstein (1968) considered synthesis an-
at normal pH levels, the next mater for con- likely. As uricolytic enzymes are present in OQ.
sideration is its conyersion to NH. One possi- asellus (Hartenstein, 1968) storage of uric acid
bility is that NH, is generated in the tissues, at cannot be considered obligatory and must per-
the point of climination, by local alkalinisation form some useful metabolic role.
and the NH, diffuses out into the air down its
partial pressure gradient. Alternatively, NH,= AMPHIPODA
may be eliminated into urine in the excretory There are numerous species of terrestrial am-
organs or across the body wall into fluid (urine phipods. in the Family Talitridae. ranging from
or pleopod fluid) which is then made alkaline supra-littoral to fully terrestrial, The latter are
{Wieser, 1972b) forming NH, which is lost to the best represented in the Southern Hemisphere.
air. This could be achieved by reabsorption of particularly Australia and New Zealand (Hurley,
protons, This would presumably require ion 1968) where they are extremely abundant in
{ransport across. the epidermis lining the chan- forest and grassland habitats well mland. Know!l-
nels. although it could also occur across. the edge of their nitrogenous excretion is pour. Ly the
pleopods which have an appropriate ion-trans- supra-littoral Grchresta, ammonia makes up the
porting epithelium (Kuemmel, 1981), major portion of faecal non-protein nitrogen
A potential mechanism for production of (Dresel and Moyle. 1950) bat it is not known
gascous NH, which utilises the latler form of What portion of Latal nitragea excretion this rep-
elimination was suggested by Hoese (1981). The resents. Neither ts it clear from this work if
terrestrial isopods possess a network of water- Orchesiia excretes gaseous NFL,, Further work
conducting channels which carry urine, Urine is necded on this group, particularly on the ter-
released into this system, from the miaxilliary resirial species,
In the freshwater cave dweller Niphargus,
glands, flows along the channels on the dorsal
spherules of uric acid are Stored in special cells
and ventral surfaces and over the pleopous
on the pericardial septum where the urates bind
before being re-ingesled af the anus (Hoese,
a variely of anions and cations (Graf and Mi-
1981, 1982) and it was suggested that volatilisa-
tion of NH, would be effected during this circu- chaut, 1975). These are considered to be reserves
of purine and ions for metabolic usage rather
lation, If release of urine occurred during periods
than deposits of waste nitrogen derived trom
of inactivity in retreats, water loss from the sys-
protein cajabolism {Graf and Michaut, 1975)-
tem by spillage and evaporation would be mini-
Data are lacking for terrestrial species.
mised, This hypothesis is based on observations
of micro-anatomy and fluid Now and requires
physiological evidence to substantiate it, Thus i} ANOMURA
must be shown that the urine released contains The Anomura are represented on land by the
adequate amounts of NH," to explain measured Coenobitidae, and include the Robber Crab, Bir-
rates of NH, release. Secondly, it must be de- gus latro, and twelve species of shell-carrying
monstrated that the {uid in the channels actually hermit crabs. Coenobia, some of which nccupy
becomes alkaline as gaseous NH, cannot be re- supra-littoral niches whilst others. range inka
leased otherwise. (Hartnoll, 1988), Nitrogenous excretion hats
MEMOIRS OF THE QUEENSLAND MUSEUM
ho
TABLE 2. The chief excretory products and their routes of excretion in Gecarcaidea
natalis, Geograpsus grayi and Birgus latro.
% Total % N Output % # Output as
N output as Ammonia Uric acid
Gecarcoidea natalis
Faeces
P
Gas
Total
Geograpsus grayi
Faeces
P
Gas
Total
Birgus latro
Faeces 96.2
P 0.0012
Gas 3.5
Total
been studied only in B. Jairo (Greenaway and or is contained in special 8urate9 cells as de-
Morris, 1989), scribed for the amphipod Niphargus (Graf and
B. latro is uricotelic and solid uric acid, ex- Michaut, 1975). White deposits have also been
creted in the faeces, comprises about 50% of observed in Coenobita brevimanus (P. Greena-
total non-protein nitrogenous excretory waste. way, unpubl. obs.). The functional significance
The remaining faecal excretory nitrogen is of urate reserves is unknown but, given that B.
largely ammonium (11.9%) with some free latro can excrete utic acid, their maintenance
amino acids and urea (Greenaway and Morris, must perform some useful metabolic role. This
1989). Loss of nitrogen in the excretory fluid or may be to act as a mobilisable nitrogen reserve
as gaseous NH, is insignificant (Table 1). for synthesis of amino acids or perhaps as an ion
Uric acid appears as separate white portions of store prior to ecdysis or modulation of
the faecal string, quite separate from the brown haemolymph ions during dehydration.
faeces comprised of food residues. Excretion is
episodic with uric acid released into the gut in BRACHYURA
distinct bouts of excretion. Homogenates of the The land crabs are drawn from some 16 differ-
midgut gland contain xanthine oxidase, the ent families and range from amphibious to
enzyme responsible for the final step in the con- highly terrestrial in habit (Bowman and Abele,
version of purine bases to uric acid and the 1982; Hartnoll, 1988).
midgut is presumed to be the active site in the GECARCINIDAB. Early work on Cardisoma
final stage of uric acid production. &. faire can guanhumi discounted the urine as a significant
synthesise purines de novo but nothing is known vehicle for excretion (Horne, 1968; Gifford,
of the metabolic pathways concerned. 1968). Subsequent work has confirmed low uri-
Starved B. fatro excrete uric acid at an un- nary nitrogen concentrations (Wolcott and
diminished rate and may metabolise protein Wolcott, 1987a; Wolcott D.L., pers. comm.;
during inanition. The observed output could also Greenaway and Nakamura, in press) but this has
result from excretion of uric acid stored within little relevance to excretion as urine in land crabs
the body (see below). is not the final excretory fluid and 1s passed into
Large amounts of uric acid are stored by B. the branchial chambers. Here it may be exten-
latro patticularly in laboratory-maintained ani- sively modified before being released as a final
mals (Greenaway and Morris, 1989). Extensive excretory fluid (P) (Wolcott and Wolcott, 1984,
white deposits occur in all tissues but it is not 1985; Greenaway et al., 1990) in which [NH,*]
clear whether the urate is free in the haemocoel, is considerably elevated (Wolcott and Wolcott,
NITROGENOUS EXCRETION
1987b; Greenaway and Nakamura, in press), in press). G, gray7 is also ammonotelic but differs
This NH,= must be added to the urine during its from the gecarcinids in excreting most of its
residence in the branchial chambers, presumably waste nitrogen as gascous NH, (Table 2). Re-
via the gills but the mechanism is unknown, Anv lease of gas is not continuous and the crab alter-
of the mechanisms described above for aquatic nates between bouts of excretion lasting several
crustaceans could be utilised with the urine pro- days and similar periods of minimal excretion. It
viding ions for exchange (Fig. 4). The maximum is probable that protein catabolism ts ongoing so
[NH,*] recorded in P of G. natalis was 73 ammonia must be detoxified and temporarily
stored between bouts of exeretory activily.
mmol.L!, but the average was much lower ate,
1] mmol.L! (Greenaway and Nakamura. in Possible metabolic pathways were outlined
press) and similar values are reported for other above. Exeretion of gaseous NH, by starved
gecarcinids (Wolcott, D,L., pers, comm), animals is similar to that of fed crabs indicating
In Gecarcoidea naralis, the chief excretory continued catabolism of protein during starva-
product is ammonia which makes up 92% of the tion (Greenaway and Nakamura, in press).
non-protein nitrogen excreted. Thisiseliminated Elimination of NH, probably occurs in the
largely in the exeretory fluid (70%) and the branchial chambers where the gills provide a
remainder exits a8 gaseous NH, and faecal nilro- large surface area of epithelium with an ultra-
Structure suiled for ion transport (Greenaway
gen (Greenaway and Nakamura,i n press; Table
2). The small amount of excretion of gaseous and Farrelly, 1990) and the urine again could
provide a source of ions to fuel any exchange
NH, (Table 2) may well have originated from
processes which may be involved. Ammonia
faeces and/or P by direct volatilisation. No sig-
could be lost directly in gaseous form ar passed
nificant excretion of gaseous NH, was reported
into the P as NH,° and lost as NH, following
from C. cariifex (Wood and Boutilier, 1985) and
alkalinisation.
while Horne (1968) reported considerable
The possibility af conflicts between ion regu-
gaseous excretion by C. guan/numi no supparting
latory requirements and nitrogen excretion
data were offered.
Routine excretion in gecarcinids is as NH,= in should be considered in brachyurans. The mech-
anisms of elimination of ammonia described
the final excretory fluid. Total nitrogen excretion
above (Fig. 4), depend on activity of basolateral
is relatively low (Horne, 1968; Gifford, 1968:
Na/K-ATPase and the entry of sodium ions from
Wolcott and Wolcott, 1987b; Greenaway and
the fluid bathing the gills (in this case urine), In
Nakamura, in press) and the [NH,"] of P is
salt replete animals, sodium is not reabsorbed
normally low enough lo avoid toxicity problems
from the urine (Walcott and Wolcott, 1985; Tay-
although blood levels are quite high (Table 1).
lor, Greenaway and Morris, unpubl. data) and
All species. of gecarcinids studied contain
this could block ammonia excretion, Greatest
deposits of uric acid (Gifford, 1968; Wood and
difficulty would be experienced by animals on a
Randall, 1981; Greenaway and Nakamura, in
diet rich in both salt and nitrogen, Dehydration,
press), Amounts are very variable but appear to
too, could cause exeretory problems as reduction
be related to diet and accumulation is greatest in
and cessation of flow of excretory fluid would
laboratory specimens (Wolcott and Wolcott,
first elevate [NH,=], perhaps ta toxic levels, and
1987b). Gifford (1968) considered that uric acid
finally eliminate excretion. Such reduction in
deposits exist free in the haemocuvel but histo-
urine flow occurs during desiccation in the ge-
logical evidence forthis is fucking. The origin of
carcinids (Kormanik and Harris, 1981). In Car-
the deposits is unclear but it is unlikely that they
disoma carnifex, ammonia excretion ceases
are derived solely from excess dietary purine
during desiccation but is elevated on rehydration
compounds and de nove synthesis must be a
sugpesting interim storage in non-toxic form
possibility. As with Birgus faire some useful
(Wood et al., 1986). Uric acid, amino acids or
metabolic function of the reserves must be as-
protein could be utilised for this purpose, These
sumed as most crustaceans possess urpoolylic
problents could also occur in isopods ifexcretion
enzymes and could degrade the deposits and relies on ammonium being excreted into the
excrete ammonia, urine or across the gills into a urine or water film.
GRAPSIDAE. The only other land crab in which
nitrogenous excretion has been studied is Gee-
grapsus grayi, @ small Carnivorous species with ASTACIDEA
a relatively high tate of nitrogenous excretion (¢. There are numerous species of senti-lerresirial
100 umot-ke'. bh!) (Greenaway and Nakamura, freshwater crayfish in Australia and N. America
24 MEMOIRS OF THE OLIFENSLAND MUSEUM
which may spend lone perods without tree Telyin the respiratory medium as a vehicle for
water, ¢.e. Engacus, Cherax, Nothing 1s known excretion by releasing gaseous NH, Air flow
of their excretory patterns during aerial expo- over (he body or through the branchial chambers
sure. effectively prevents the buildup of ammonia
next lO the excretory surface maintaining a
favourable gradiem for excretion, Until this
CONCLUSIONS
mechanism of excretion is better understood it
will not be possible to say how much inde-
In aquatic crustaceans, the main excretory pro-
pendence from water it allows. Actual elimina-
duct is ammonia which is excreted across the
tin of NH, may first require transport of NH,*
gills into the respiratory water streani and carried
into the excretory (uid for conversion to NH, or
away, This ensures that the extemal ammonia
may be direct from the excretory epithelium,
concentration is always low and there is a
Excretion of gascous NH, may be seen as an
favourable gradient lor excretion and no prob-
advance in terrestrial adaptation over that shown
lems of toxicity, By contrast terrestrial arthro-
by gecarcinids,
pods, other lhan crustaceans, ure mustly Exeretion in Birgus /atro shows a complete
urinotelic and the overriding selective fuctor
break from the aquatic pallern of ammmonolelism.
avouring this, is believed to have been the con-
The elimination of nitrogenous waste as uric acid
servation of body water, Purines may be excreted
and the associated ability to synthesise purine is
in crystalline form in the faeces with minimum
a major evolutionary advance ald places this
accompanying water loss, an important factor in
species al a similar level of adaptution to the
the maintenance of waler balance of smal! ani-
other major groups of terrestrial arthropods.
mals in a desiccating environment. Of the ter-
Why have terrestrial crustaceans retained am-
restrial crustaceans examined Only ane species is
monotelism whilst other terrestrial arthropods
clearly purinotelic (Birews faire) and, although
have not? The main considerations may be the
many more species seem Lo have an active purine
restricted metabolic pathways present at the
metabolism, their dominant exeretory product is
onset Of terrestrial life and the period of cvalu-
ammonia. Comparatively few terrestrial species
tion spent on land. As aquatic species are am-
have been examined and while further work may
monotelic and probably lack synthetic pathways
reveal a few additional purinotelic species, the
for both purines and urea, colonists would thus
group appears to be predominantly am-
monotelic, begin terrestrial life excreting ammoniunracross
[Lis of interest to consider how far the various the gills and this would be expected to influence
groups of terrestrial crustaceans have diverged both behaviour and habitat selection, Moist mi-
from the typical aquatic pattern and moved croclimates such as leaf litter and rainforest
towards the terrestrial system. In this we are would be favoured and activity restricted to
hampered by the relative lack of information on moist conditions so that adequate excretory fluid
excretion in terrestrial groups. would be available for elimination of ammonia.
The gecarcinid crabs appear to have diverged Such habitats are characteristic of terrestrial am-
least from the aquatic pattern. They continue to phipods, isopods and crabs, and although several
excrete ammonium across their gills and us res- species live in Xeric habitats, their activity is
piratory Water is not available they utilise the restricted and the animals have a humid retreat
excretory fluid from the antennal organs, The or burrow, It would be naive, however, to assume
volume of urine produced is limited, however, that limitations of the excretory system are re-
und this necessitates quite high concentrations of sponsible for the restricted habitats occupied e.g.
ammonia in the branchial chambers and may Birgus latro has a system well tailored for ter-
result in an adverse electrochemical gradient restrial life but occupies similar habitats to other
across the gill epithelium. Urine flow is tied to terrestrial crabs. Limitations of the reproductive
overall water balance rather than excretory re- and osmoregulatory systems under terrestnal
quirements and this may cause exeretory diffi- conditions could equally well affect distribution.
culties necessitating temporary storage of waste Given the habitat in which the animals are found,
nitrogen in negative water balance. exeretion of various forms of ammonia is
Geograpsus grayi and terrestrial isopods have feasible and may be advantageous as it carries a
departed further from the aquatic pattern und minimal energetic cost and requires only small
have solved the loss of the respiratory water as changes to existing systems whilst efficiently
an excretory vehicle in a different manner, They excreting waste nitrogen