Osmoregulation in Invertebrates

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Osmoregulation in
Invertebrates
Pat Willmer, University of St Andrews, Scotland, UK
Osmoregulation ensures that a correct balance of salts and water is maintained inside an
animal, both in the circulating fluids (blood), andwithin the cells. It plays amajor role in the
achievement of homeostasis, especially in freshwater and terrestrial invertebrates.
Introduction
Osmoregulation is essential for all animals, especially for
larger ormore complex animals living in relatively change-
able habitats. In the stable environment of the sea, ionic
and osmotic regulation is rather easy; but active regulation
is needed whenever external conditions are constantly
difficult (where the animal is not in osmotic equilibrium
with its surroundings), or when these conditions change on
a daily, tidal or seasonal basis. Hence osmoregulation, in-
volving controlled ionic exchanges at the skin, gills or kid-
neys, and adjustment of cellular solutes, is particularly
important (and often expensive in terms of energy) in en-
vironments such as estuaries, freshwater and on land.
Ion andWater Balance – Basic Principles
Faced with a change in the environmental availability of
ions or water (such as the tide going out on a beach, or hot
arid conditions arriving in summer), animals may have
several options. The first possibility is simple evasion;
avoiding the problem in space (migration, or simply hiding
locally) or in time (dormancy, or a reproductive cycle so
that resistant eggs or cysts occur during the most difficult
seasons). All these strategies are behavioural means of
avoiding the need to osmoregulate. They are cheap and
easy, and very common among invertebrates.
A second possible strategy is tolerance or endurance,
allowing the body fluids to change in step with the external
conditions and not bothering with regulation. Animals
that simply put up with the changes are described as os-
moconformers. This strategy is common where conditions
change cyclically, because there is a reasonable guarantee
that ‘normal’ conditions will be restored soon. Hence, os-
moconforming often occurs in animals on tidal seashores
(alternating seawater and air) and in estuaries (alternating
seawater and freshwater).
However, the third possibility is that of true osmoreg-
ulation, involving varying kinds of physiological (and
sometimes again behavioural) homeostatic mechanisms to
keep the internal concentrations constant despite changing
external conditions. Although this is themost complex and
expensive approach, it is crucial for most nonmarine in-
vertebrates (and all vertebrates). All animals ‘want’ to
achieve constant water content. Water is about 99% of all
free molecules on Earth, and on average about 75% of
most animals’ wet weight. Osmoregulation allows animals
in difficult habitats to maintain this situation against pre-
vailing salt and water gradients.
There are two possible levels of regulation for multicel-
lular animals, since they can use control processes either at
the skin surface (between outside world and extracellular
body fluids) or at the individual cell membranes inside the
body (between body fluids and cells). The first level is ‘op-
tional’, depending on habitat; the second level always
occurs, fine-tuning ion levels within cells.
Osmoregulation and Excretory Organs
Osmoregulation involves a number of different processes,
varying with habitat, but for most invertebrates much of
the active regulation of blood composition and concentra-
tion is concentrated at specific surfaces or organs. Many
aquatic invertebrates regulate across the skin, or localized
parts of it such as the gills, whereasmost land invertebrates
concentrate their ion transport systems at specific internal
surfaces. Since animals usually have an excretory ‘kidney’
to handle nitrogenous wastes, it is this kidney that is co-
opted for additional osmoregulatory functions; thus water
or salts that have to be lost are combined with nitrogenous
products as urine. Specialized ion transport epithelia are
therefore particularly localized in the excretory organs. In
addition to fundamental regulation of NaCl and water
balance, these may become the main site for regulating
particular ion levels; potassium regulation, the control
of pH via bicarbonate concentration, active transport
of magnesium and sulfate into the urine and selective
Article Contents
Introductory article
. Introduction
. Ion and Water Balance – Basic Principles
. Osmoregulation and Excretory Organs
. Marine Invertebrates – Ion Regulation and Buoyancy
. Littoral and Estuarine Habitats – Solutions to
Fluctuating Salinity
. Freshwater Habitats – Retaining Salts and Avoiding
Bursting
. Terrestrial Invertebrates
. Special Tricks
doi: 10.1038/npg.els.0003646
1ENCYCLOPEDIA OF LIFE SCIENCES & 2006, John Wiley & Sons, Ltd. www.els.net
resorption of calcium are common properties of excretory
systems.
Excretory organs that achieve these osmoregulatory
functions arehighly varied.They includeflamecells in some
worms, producing ammonia in an isosmotic urine; earth-
worm nephridia, with adjustable urine flow and concen-
tration; the gastropod kidney with many tubules folded up
together and producing a highly variable product accord-
ing to habitat; and the antennal or coxal glands found in
crabs, spiders, ticks and woodlice. The most unusual var-
iants are theMalpighian tubules of insects and myriapods,
which occur as clusters of simple blind-ending tubules lying
free in the haemocoel and opening into the gut, delivering
an isosmotic primary urine, which is then modified into
concentrated (hyperosmotic) excreta in the rectum.
Most taxa use an excretory mechanism based on ultra-
filtration, where hydrostatic pressure forces blood through
a semi-permeable membrane into the lumen of the osmo-
regulatory organ. The membrane acts as a filter, holding
back blood cells, proteins and other large molecules, but
allowing water, ions and small solutes (especially amino
acids and sugars) to pass through. In some cases (insects,
leeches), secretion replaces filtration as the initial step. In
either event, the primary product is a simple isosmotic ex-
tract of the body fluid, with very little change in compo-
sition. Most components of the body fluids would
therefore be ejected from the body if no further modifica-
tion of urine were to take place. However, inmany kidneys
up to 99% of the initial filtrate is taken back into the body,
involving substantial active transport of ions (with water
following passively). The ion uptake pumps in the distal
tubule walls particularly resorb Na+ from the filtrate,
usually coupled (via symport proteins) to recovery of glu-
cose and amino acids. Further modification may occur in
the last part of the tubule, or in the gut, to produce urine of
the required overall concentration, containing most of the
nitrogenous waste. A simple model of excretory organ
function is shown in Figure 1.
Marine Invertebrates – Ion Regulation
and Buoyancy
The seas represent constant osmotic conditions, being well
mixed by currents, so that salinity is fairly invariant world-
wide at about 3.5% salts (or 35 parts per thousand). This is
equivalent to an osmotic concentration of 1000–
1100mOsm, predominantly made up of common salt
(NaCl), with some magnesium and sulfate, plus smaller
amounts of calcium, potassium and bicarbonate. Cells and
organisms are (primitively) built to be roughly in equilib-
rium with seawater, because life evolved there. Thus the
cells and body fluids of marine invertebrates are in virtual
osmotic equilibriumwith the seawater around them (i.e. all
bodily fluids are also about 1000–1100mOsm), and there is
no problem of water moving osmotically in or out and
causing swelling or shrinking. These animals are isosmotic,
and they are also usually stenohaline.
Table 1 shows some examples of blood compositionfrom
a range of invertebrate phyla; the blood is always mainly
NaCland is ionically andosmotically very like seawater. It is
apparent that some animals do alter the blood a little, usu-
ally associated with buoyancy; the heavier ions (especially
Range of
urine
concentration
External
medium
Storage
bladder
Final
duct
Proximal tubule, may be
resorption/secretion
of some solutes
Collecting
area
Blood
C
on
ce
nt
ra
tio
n
± Minor changes in
osmotic concentration
(± Major ionic changes)
Distance
No further
change
Major changes in
osmotic concentration
of urine
Distal tubule, overall
nonisosmotic
resorption or secretion
Figure 1 Schematic design of an osmoregulatory/excretory organ, showing major concentration effects.
Osmoregulation in Invertebrates
2
sulfate) tend to be reduced in the pelagic animals. But the
majority of cells (especially their enzymes and membrane
proteins) operate best with contents very like the seawater
withinwhich theyevolved, andno ‘wholebody’ regulation is
needed.
However, even formarine invertebrates, the second level
of regulation is important, because while cells are osmo-
tically similar to seawater, ionically they are very different.
Cell function (especially cell excitability) depends on con-
tinuous ion gradients across membranes. Cytoplasmic
compositions involve high potassium, but low sodium and
calcium (the Na/K ratio being the reverse of that in sea-
water); and cells commonly contain high levels of organic
anions and relatively low chloride levels. So the cytoplasm
is a quite distinct and different environment from its sur-
roundings, despite net osmotic equilibrium with seawater.
Even though, strictly speaking, osmoregulation is not
required, allmarine invertebrates’ cellsmust be operating a
fairly high degree of ‘ionic’ regulation all the time, using
the classic Na/K exchange pump and calcium pumps to
maintain their ionic gradients.
Littoral and Estuarine Habitats –
Solutions to Fluctuating Salinity
Animal cells evolved to operate best osmotically with con-
tents like seawater, so that even cells living in much more
dilute surroundings tend to maintain a concentration at
least one-third that of seawater, perhaps 300–450mOsm.
If the cells’ contents fall below this range, for most aquatic
animals they cease to function properly. Animals that have
moved away from the seas therefore require continuous
regulation at the outer body surfaces as well as at the cel-
lular level, and invertebrates living in brackish water hab-
itats have serious problems with osmoregulation.
Brackish waters are osmotically intermediate between
saltwater and freshwater and are found in estuaries (rivers
joining the sea) and on seashores (where rain and runoff
can dilute rock pools and interstitial water when the tide is
out). Here avoidance may be the ideal solution for many
invertebrates – hiding in a crevice, burrowing deep into the
sand – as this takes them into essentially marine surround-
ings to survive until tidal seawater returns. But most or-
ganisms also need some capacity for surviving dilution;
they have to be euryhaline.
Patterns of osmoregulation and
osmoconformity
For all aquatic invertebrates, the basic patterns of osmotic
response can be appreciated by plotting blood concentra-
tion against the concentration of the external medium.
This (Figure 2) reveals the diversity of approaches, from
almost perfect osmoconformity to strong osmoregulation
where the blood concentration is kept well above the
Table 1 Ionic composition of marine invertebrate body fluids, as percentage of seawater
Na K Ca Mg Cl SO4
Cnidaria
Aureliaa 99 106 96 97 104 47
Mollusca
Pecten 100 130 103 97 100 97
Loligoa 95 219 101 102 103 36
Annelida
Arenicola 100 104 100 100 100 92
Aphrodite 99 103 100 101 99 92
Crustacea
Maia 100 125 122 81 102 66
Carcinus 110 118 108 34 104 61
Echinodermata
Marthasterias 100 111 101 98 101 100
Echinus 99 100 101 99 99 99
Urochordata
Salpaa 100 113 96 95 102 65
aIndicates a pelagic species.
Osmoregulation in Invertebrates
3
environmental concentration. But note that there is a con-
tinuum of strategies, not a simple dichotomy.
There are no perfect regulators, where the blood would
stay at marine concentrations regardless of the external
world; this would be extremely expensive. Instead, there
are varying degrees of keeping the fluid concentration
elevated above the surroundings, most brackish inverte-
brates maintaining the blood at around 25–50% of
seawater concentration. Crustaceans are particularly good
at euryhaline regulation (notably shore crabs such as
Carcinus).
By contrast, osmoconforming canbe almost perfect, and
it is cheap, using up little energy in active transport of ions,
so reducing the metabolic rate and the food requirement.
Here worms and molluscs are well represented. The
common mussels such as Mytilus are good euryhaline
conformers, often surviving down to 20% seawater with
their blood also just above 20% normal concentration.
Mechanisms of osmoregulation in brackish
invertebrates
There are three main mechanisms involved in adaptation
to osmotic changes: reduced external permeability; ion ex-
change between the blood and the outside world; and cel-
lular osmoregulation.
Reduced permeability
For any animal living in an uncertain or regularly changing
environment, it makes obvious sense to reduce the surface
permeability, thereby reducing the rate of change of ion
Sea-
water
Medium concentration (mOsm)
Brackish waters
Fresh
water
10000 500
1000
500
Hydra
Anodonta
Artemia
Myfilus
Daphnia
Nereis
Lumbricus
Aedes
Patella
Ga
mm
aru
s
Iso
sm
ot
ic 
lin
e−
ful
l c
on
for
m
ity
Carcinus
Uca
Hypothetical perfect regulation
Bo
dy
 fl
ui
d 
co
nc
en
tr
at
io
n 
(m
O
sm
)
Key
Fresh water
invertebrates
Brackish
invertebrates
Stenohaline marine
invertebrates
Figure 2 Patterns of osmoregulation and osmoconformity in aquatic invertebrates.
Osmoregulation in Invertebrates
4
concentrations, fluxes of water and changes in body vol-
ume, to a level where they can be tolerated or controlled.
This strategy is especially useful for ‘damping’ tidal
changes in brackish invertebrates, and many common
shore and estuary animals are ‘preadapted’ byhaving shells
or thick cuticles. Permeability to water is often lower in
crabs living in brackish estuarine habitats than in closely
related freshwater species. Table 2 compares values in some
familiar examples.
Lowered permeability is the first line of defence, and
water permeability (Pw) may be varied and controlled
through the lifetime of an animal (especially between suc-
cessive cuticular moults in crustaceans). But it is only a
delaying tactic; aquatic animals can never be totally im-
permeable and exclude the world outside. Hence, some
strategy is required to keep the internal fluids at acceptable
levels.
Ion transport mechanisms
For any animal in water more dilute than the sea, ions and
water will move passively down their gradients, water into
the body, ions outward. Hence, for brackish animals ions
have to be actively resorbed. This may be achieved in the
simplest animals over the whole body surface, but more
commonly it occurs at the gills (which have to be permeable
anyway, to allow respiratory exchange). Sometimes it may
be localized to the excretory organs, though this is un-
common except in truly freshwater or land animals.
The basic mechanism for active uptake of salt seems to be
rather constant, based on a classic sodium ion pump, at the
cell/blood interface. In some tissues this may be supple-
mentedbyananionpump,mainlyhandlingchloride.But salt
uptake cells also reduce the electrical cost of transportbe-
cause they use exchange systems, Na+ for NH4
+ at the cat-
ion pump, and Cl2 for HCO3
2 at the anion pump, thereby
getting the two ions theymostneed in exchange for twowaste
products (dissolved ammonia and carbon dioxide).
Cells operating these exchanges can be found in many
invertebrates (in gills, guts, excretory organs, skin sur-
faces), always with a characteristic structure: a highly in-
folded apical membrane to give a large diffusive surface
(the ‘brush border’), and a somewhat infolded basal mem-
brane on which the pumps are sited, with mitochondria in
abundance. This cellular architecture allows the pumps to
work at a very high fluid flow rate.
Brackish animals thus have a means of taking up the
extra salts needed to counter the inevitable passive outflow,
and blood concentration is elevated to an acceptable level.
Cellular regulation
The mechanisms described thus far help keep the cell
surroundings relatively constant, and less dilute than the
outside world. But the cells must perform additional reg-
ulation across their own membranes; above all, cells have
to hold on to ions, which affect internal cellular processes
dramatically, especially enzyme activities and deoxyribo-
nucleic acid (DNA)–histone interactions. So when faced
with the problem of amore dilute blood (thus potential for
water to enter the cells osmotically and dilute the cyto-
plasm), animal cells keep their ions but get rid of amino
acids instead, exporting them from the cell into theblood to
reduce the osmotic gradient. Amino acids are therefore
often termed ‘osmotic effectors’.
The key mechanisms that regulate amino acid levels are
often directly sensitive to [Na+]: for example, enzymes that
control interconversions of amino acids, and carriers that
regulate amino acid movements across membranes. Thus,
lowered blood [Na+] automatically causes loss of amino
acids from cells, lowering the osmotic concentration of the
cytoplasm to balance the osmotic gradients.
Table 2 Permeabilities (measured as water loss rates in air)
Water loss rate
mg cm22 h21 Torr21
Marine
Worm � 2500
Crab � 2000
Hypersaline
Brine shrimp 100
Estuarine/Littoral
Amphipod, isopod 200–300
Crab 80–200
Freshwater
Crab/crayfish 100–250
Insect larva 200–400
Insect adult 20–50
Terrestrial
Slug/snail (active) 2500
(inactive) 10–100
Earthworm 200–500
Woodlouse 80–160
Scorpion 10–80
Spider, mite 30–60
Centipede, millipede 40–270
Insect larva 150–700
Insect adult 20–80
Arid/Desert
Woodlouse 14–30
Cockroach 12–80
Ant, wasp 4–25
Beetle adult 3–15
Beetle pupa 1
Fly pupa 0.3
Scorpion, spider, mite 0.6–2
Osmoregulation in Invertebrates
5
Figure 3 summarizes the mechanisms involved in osmo-
regulation for littoral and estuarine animals. This built-in
physiological circuitry to maintain blood [Na+] and keep
the cells functioning at reasonable osmotic concentration
and relatively constant volume allows them to survive de-
spite regular and drastic changes in their environment.
Freshwater Habitats – Retaining Salts
and Avoiding Bursting
Freshwater is not really strictly defined, but just of very low
salt content, commonly between 0.01 and 0.5 parts per
thousand; that is, usually less than 1% of seawater. Only
about 0.1% of Earth’s water is free freshwater, but this
forms important habitats for invertebrates, as moving wa-
ter (rivers and streams), or as still lakes, ponds and
marshes. Freshwaters show tremendous chemical varia-
bility, varying from alkaline streams (pH 9–10) in chalk
uplands to acidic peat bogs (pH 3–4). But all these habitats
are extremely dilute, and all invertebrates there (often
dominated by rotifers, oligochaeteworms, crustaceans and
insect larvae) must osmoregulate continuously to keep
their blood, and especially their cell contents, at a high
enough concentration, while preventing excess water en-
tering osmotically and bursting the cells.
Figure 1 includes some freshwater animals, restricted to
the lower left-hand corner of the plot. Somemay be slightly
euryhaline and able to tolerate concentrations higher than
freshwater, (e.g. Gammarus), but others (e.g. the mussel
Anodonta, and the waterfleaDaphnia) are strictly freshwa-
ter inhabitants (being stenohaline) and will not survive
above about 10% seawater. All the freshwater animals are
osmoregulators, with their blood concentration above the
isosmotic line. Conformity is no longer an option, as it
would leave the cells too dilute for proper functioning.
Mechanisms
All threemechanisms outlined in the section on patterns of
osmoregulation and osmoconformity still apply. Surface
permeability is low, and ion uptake to offset the inevitable
outward leakage is intense and concentrated in more per-
meable areas such as the gill epithelia. Cellular amino acids
must be kept low to reduce gradients for water uptake;
Net effect of ↓ salinity
↓ Permeability of skin
↑ NaCl uptake to blood
↑ Amino acids in blood
Regulated
blood concentration
↓[Na+]I
Cell
↑ [Amino acid]I
Amino acid
symport
↑ Amino acid
permeability
HCO3
−
NH4
+
Na+
Cl−
↑ [NaCl]I
↓ Pw
I
↓ Concn
↓ Concn
Outside
medium
Skin
Blood
Krebs
cycle
 ↑ Amino acids
esp.glutamate
+ proline
o
Figure 3 Key mechanisms for regulating blood and cell concentrations during changes of salinity.
Osmoregulation in Invertebrates
6
intracellular osmoregulation and volume regulation are
mainly achieved by eliminating potassium ions instead.
However, the continuous presence of gradients forwater
uptake and salt loss usually necessitates some further reg-
ulation to blood concentrations, and most freshwater in-
vertebrates have additional ion uptake systems in the
excretory organs; the result is a strongly hyposmotic urine
(more dilute than the blood andmuchmore dilute than the
outside medium), thus removing excess water from the
body (Table 3 shows comparative values). This ion resorpt-
ion requires elongate kidney tubes with high surface area,
increasing the length of the excretory/osmoregulatory
organ.
Terrestrial Invertebrates
For terrestrial animals, the problems of osmoregulating
are rather different. It is not possible to show the responses
of terrestrial animals in Figure 1, as the external medium is
air and has no osmotic concentration. Nevertheless, since
land invertebrates evolved from littoral or freshwater an-
cestors, they too tend to have blood concentrations of 250–
500mOsm. Being surrounded by air rather than water, the
exchange gradients are always outwards. Salts will leach
out if not restrained, and even more seriously water will
always tend to evaporate from the body. Evaporation
occurs from any biological surface into air, but is made
worse by animal preferences for relatively warm conditions,
since evaporation rate increases exponentially with surface
temperature.Hence,maintaining a raisedbody temperature
(whether by ectothermy or endothermy) means that land
animals’ water problems are compounded. Only in excep-
tionally cool humid air – e.g. in a deep lake cave or sea cave,
with relative humidity (RH) in excess of 99.5% – would
there be no gradient for water to move out of the animal.
For land animals, then, active life requires a constant
osmoregulatory activity, to maximize water retention.
Size and scaling aremajor factors in osmotic physiology,
and small size has both pros and cons on land. Many very
small organisms are said to be land-dwellers but really
spend their lives covered with water films (nematodes,
mites and insect larvae in wet soils, and litter organisms);
all these are inactive when their habitat is too dry, and are
physiologically ‘aquatic’. Small- to medium-sized inverte-
brates (e.g. earthworms) may live in the same environment
and yet be ‘land’ animals, because they exceed the size
where water films seem substantial. Theseanimals have a
large surface area to volume ratio (SA/V), so water and ion
fluxes are large and rates of change of concentration are
high. But small size also gives a much greater choice of
microclimates, as the animals can shelter in humid crevices
or burrows where water loss gradients are much reduced.
Indeed, many small/medium invertebrates get by at the
‘behavioural’ level alone, being cryptic, nocturnal and liv-
ing in the wet tropics or on the fringes of water bodies.
Many earthworms, slugs and larval insects are effectively
like freshwater animals in their overall osmoregulatory
powers. Only the somewhat larger animals, and those liv-
ing in drier conditions, need more specialist adaptations.
These parallel the adaptations seen in aquatic animals, but
with some additional features and special tricks.
The osmoregulatory problems for land invertebrates
are 2-fold: firstly, salts are not directly available on land,
and must be obtained by eating or drinking, and sec-
ondly, water loss is inevitable so water must be obtained
and then carefully preserved within the body. Osmoreg-
ulatory ability on land is strongly linked with tolerance
of water loss. Dehydration inevitably leads to a propor-
tional increase in the concentration of osmolytes; if there
is no osmoregulation, losing x% of body water leads to
osmotic concentrations rising by 100/(1002x) (e.g. a
1.33-fold rise for 25% water loss). Most animals can
tolerate a few percent overall water loss. However, many
land invertebrates do much better: some slugs and snails
can tolerate 40–75% loss, causing substantial shrinkage
of the body, and many insects survive 30–50% loss. Such
animals must be osmoregulating intensely for their cells
to continue functioning. The land arthropods are partic-
ularly good osmoregulators, the adults’ body fluids stay-
ing fairly constant over a wide range of humidities and
water contents; this applies to cryptic or semi-terrestrial
crabs and woodlice as well as to the more obviously xeric
insects and arachnids. Some insects with highly protec-
tive cuticles can even live without any free water (e.g.
clothes moths, flour beetles) and still stay at constant
water balance throughout their lives.
Table 3 Urine concentrations in freshwater animals
Species Concentration Urine:blood ratio
(mOsm)
Annelids
Pheretima 45 � 0.2 (hyposmotic)
Crustaceans
Eriocheir 800 � 1.0 (isosmotic)
Pseudotelphusa 120 � 0.3 (hyposmotic)
Molluscs
Viviparus 30 � 0.3 (hyposmotic)
Anodonta 25 � 0.8 (hyposmotic)
Compare with
Marine
invertebrates
� 1100 � 1.0 (isosmotic)
Insects on land � 1000–6000 � 3–15 (hyperosmotic)
Osmoregulation in Invertebrates
7
Mechanisms
Reduced permeability
Terrestrial invertebrates face a conflict of interests, since
any ‘skin’ has several simultaneous functions: it has to let
some things in (oxygen, and also sensory information), and
some things out (CO2 and other wastes), while also and
critically not letting out water and ions and not letting in
pathogens. These functions are mutually incompatible,
and it is impossible to design the perfect multifunctional
skin. Instead, the land animals all have to compromise;
smaller invertebrates may opt for more protective skin,
with special valves to regulate exchanges, whereas larger
animals or those in more humid places may find a less
protective skin a better compromise. ‘Skins’ in land inver-
tebrates, therefore, vary from epithelia with nearly unre-
stricted permeation to very resistant nearly ‘waterproof ’
cuticles.
Several features of skins and their secreted coverings
potentially contribute to low Pw. The epidermal mem-
branes may be relatively waterproof. Then the bulk over-
lying material, whatever its constituents (e.g. chitin,
proteins, keratin, calcium salts) will usually be inherently
relatively impermeable, and its permeation properties may
alter with its molecular packing as it becomes more or less
hydrated. There may also be a specific layer conferring
greater impermeability due to a high lipid content, and this
is commonly at the outer surface. Examples of the resultant
Pw values, measured from cutaneous evaporative water
loss (CEWL), are included in Table 2.
Annelids have a good terrestrial representation as earth-
worms, which have a moderately thick cuticle but cannot
be very impermeable as they breathe through the skin.
However,Lumbricus stays active even in soils of 10%water
content, and can reduce Pw in low humidity to improve its
osmoregulation.
Among the molluscs, only pulmonate gastropods (fa-
miliar snails and slugs) do well on land. They mostly have
the advantage of a very impermeable shell, but their soft
parts are extremely permeable, moist and mucoid to assist
locomotion. They show marked behavioural adaptations,
retreating into the shell and aestivating through hot dry
spells. In a few such cases there is an ability to reduce
evaporative rates from the soft tissues, with the exposed
cells of the mantle edge having some (currently unclear)
mechanism to reduce water loss about 100-fold.
Arthropods are entirely covered by cuticle, and can
achieve an order of magnitude lowerPw than even inactive
molluscs. The lowest values ofPw to date come from pupal
stages (where there is no possibility of taking in water),
especially xeric ones such as tsetse flies. These arthropods
provide the very best examples of protective surfaces that
minimize CEWL. Their particular asset is an epicuticular
layer of high lipid content, made up mainly of C20–C37
fatty acid derivatives. These occur in a ‘liquid crystal’ form,
changing phase from more liquid to more solid according
to conditions (especially temperature). In the most xeric
insects and arachnids, the presence of this waxy epicuticle
ensures much slower water loss than in similarly sized iso-
pod or decapod crustaceans with the same general cuticle
design but little or no waxy component.
Osmoregulation by excretory organs and guts
The principle of ion regulation and water resorption in
land invertebrates is simple; first, as is the case in nearly all
animals, produce a fairly unmodified extract of the blood
in one specific collection area of the body, and then take
back from it the vital water plus any ions that are needed,
dumping the residue with the nitrogenous waste. In some
semiterrestrial animals such as earthworms, the tissues are
nearly always hyperosmotic relative to the (freshwater) soil
surroundings, so the animals take inwater osmotically and
always produce a hyposmotic urine to get rid of this excess
water. But for most land animals the main necessity is to
retain water, so they produce a very hyperosmotic urine,
preferably almost a dry paste.
There are four other important differences between
aquatic and land invertebrates:
(1) The initial extract of the blood is oftenmade by active
secretion rather than by passive filtration.Membrane
pumps move ions from the general body fluids into
the lumen of the excretory organ. Other oppositely
charged ions may then follow down the electrical
gradient created, or may be handled by separate
pumps, while water follows down the osmotic gradi-
ent and may carry with it other small molecules.
(2) Ideally, the faecal and urinary losses should be dealt
with together and ejected via a single opening: this is
what most terrestrial arthropods achieve, by putting
their urine into their guts via theMalpighian tubules,
and then doing all thewater resorption for both faecal
and urinary material via the rectum. Some earth-
worms also have nephridia opening into the gut.
(3) Osmoregulation is usually even more vital and very
energy consuming, so the excretory tubules are much
longer andmore heavily endowedwithmitochondria,
and usually have considerable architectural complex-
ity. This reflects a real problem with terrestrial ion
uptake systems: how can animals apparently remove
waterfrom their urine, back into their own more di-
lute body fluid compartment, when water itself can-
not be actively transported and the only active
process possible is ion uptake? There are two possi-
ble ways of producing a hyperosmotic fluid. The first
involves a complex cellular (or sometimes multicel-
lular) architecture, where a high fluid flow is created
into one compartment by ion pumping across highPw
membranes, and then ions are pumped out again in
another compartment where Pw is low and water
cannot follow the ions. By the time the fluid dis-
charges fromhere into the blood it is severely depleted
Osmoregulation in Invertebrates
8
of ions, and the system has in effect transported water
fromone side of the tubule (the urine) anddeposited it
on the other side (the blood). The ions are recycled,
with very little net ionic movement. Examples of this
‘water-transporting’ mechanism occur widely in in-
sect rectal epithelia.A secondpossibility for removing
water, not mutually exclusive, is ‘concentration by
countercurrent’; this is dealt with for desert inverte-
brates in the section on special tricks.
(4) Finally, the nitrogenous product changes; instead of
highly soluble and very toxic ammonia, which can
readily diffuse away from an aquatic animal, land an-
imals commonly switch to urea or better still uric acid,
which is nontoxic and can be excreted as a dry paste
with virtually no water loss. Insects, some arachnids
and gastropods produce uric acid, and only the humid
dwellers (isopods, earthworms, etc.) still use ammonia.
Ion uptake systems in terrestrial animals serve to keep the
blood reasonably osmotically constant, and thus the cells
of the rest of the body can function properly. It is worth
remembering that the best vertebrate examples, such as
desert rats, have ion transport systems that can only
achieve urine 2–3� seawater, while the best insects achieve
up to 6� seawater despite the added problems of small size
and large SA/V ratios.
Cellular regulation
In arthropods andmost other land invertebrates subject to
evaporative water loss, the water reserve is the haemo-
lymph or blood. Insects tolerate fairly substantial changes
in blood volume and composition (either due to water loss,
or dilution due to feeding), without excessive attention to
fine-tuned osmotic regulation, and instead have local
mechanisms to keep their cells osmotically balanced. This
situation is unlike vertebrates, where keeping the blood
constant is a major goal (perhaps because of the very strict
requirements of the respiratory haemoglobin).
In a dehydrated insect, the blood volume falls substan-
tially, and to avoid it becoming excessively concentrated
osmotic effectors are removed and tied up elsewhere. In
particular, this involves sequestering small organic mole-
cules into the fat body.Amajor strategy is to convertmany
hundreds of small amino acids (each of which is one os-
molyte ‘unit’) into large proteins that have little osmotic
effect. Thus, if the bloodvolumeof a locust falls by 85%the
blood concentration rises by only 20%. Most insects are
very good at this cellular-level osmoregulation, and may
tailor it according to need; it may be particularly pro-
nounced in neural andmuscle cells, where ionic levels must
be maintained for normal excitability to persist.
Uptake systems
Whereasaquatic animals showed the threebasicmechanisms
already discussed for osmoregulation, land invertebrates
have often evolved additional mechanisms. In particular,
on land supplies of water and salts are erratic, and animals
need specific mechanisms for acquiring both.
Drinking and eating (using ‘preformed water’) are com-
monly the main avenues for regulating the input side of
water balance and osmoregulation. The water content of
food is high for carnivores, but extremely variable for
plant-feeders, from dry seeds to exceptionally watery sap.
The nature of the food also affects the animal’s salt prob-
lems; eating animal flesh or blood naturally yields ions in
roughly the correct proportions, but eating plantsmay give
a high potassiumdiet, with inadequate sodium, and certain
plant fluids (nectar or xylem) give almost no salt input.
Many land animals therefore have sophisticated systems
for regulating food choice,with chemoreceptors (especially
for salts and sugars) on food-handling appendages or in the
mouth. But there are also cases of specific salt seeking; for
example, many butterflies seek out bird droppings to ac-
quire salts, or aggregate at natural salt licks.
‘Metabolic water’ is invariably another component of
water gain for land animals. This is water as a byproduct of
catabolic and anabolic reactions in the body. In relatively
dry terrestrial habitats it is an important component
of overall osmoregulation and water balance; for example,
clothes moths living at very low humidities may use
as much as 70% of their food intake primarily for water
production.
Many land invertebrates have systems to take up water
from outside the animal. For example, some springtails
and hoverfly larvae can take water from puddles and rain-
drops into the rectum, land crabs can take water into the
gill chamber from damp sand, spiders get water into the
pharynx by suction against soil capillarity and various
myriapods and wingless insects have coxal sacs that they
can evert on to a substrate to take up water, probably
osmotically (the surfaces of the sacs being salty) rather than
by suction. The most striking adaptation, though, comes
from those arthropods that have acquired mechanisms for
taking up water vapour from unsaturated air; this again is
mainly found in desert and xeric insects and is covered in
the section on special tricks.
Reducing water loss in respiration
All terrestrial animals must have a respiratory exchange
site permeable to oxygen; they therefore inevitably also
have a site permeable to water from which additional res-
piratory evaporativewater loss (REWL) is bound to occur.
There is a general trend to assist osmoregulation by inter-
nalizing the gas exchange surfaces, switching to the more
sophisticated invaginated systems of lungs and tracheae.
Most crustaceans, even the best land crabs, retain gills,
but enclose them in a nearly sealed chamber with their
surface area up to 4-fold decreased. However, parts of the
crab gill chambermay be well vascularized; in the Trinidad
mountain crab the chamber becomes lung-like, with no gill
Osmoregulation in Invertebrates
9
filaments left, giving maximum water conservation. True
invaginated lungs have humidity close to 100%at the point
of oxygen uptake, reducing evaporative loss. Most lungs
have tidal flow though, which means water loss remains
high because the entrance is always open and the system is
regularly flushed. The pulmonate lung of slugs and snails is
typical, highly vascularized and with a single opening (the
pneumostome).
Tracheal systems give the best possible savings on water
loss, and are found in some spiders, most other arachnids
and virtually all insects and myriapods. Insects also have
fully closable spiracles at the tracheal openings, with two
sets of muscles to control the size of the aperture; these are
sensitive to temperature, humidity and the state of hydra-
tion of the insect. At best, fully closed spiracles in a resting
insect can reduce REWL by 70–90%; but in flight the
spiracles have to be open to oxygenate the thoracic mus-
cles, and 60–70% of water is then being lost through the
respiratory system.Thus, evenwith thebest possible design
REWL has a significant impact on terrestrial osmoregu-
lation.
Special Tricks
Desert arthropods
Beetles, ants and cockroaches are particularly abundant in
deserts. They tolerate 40–60% loss of body water, and
commonly show Pw values considerably lower than ‘nor-
mal’ land animals (Table 2). But they also have some special
tricks for osmoticstability.
Some species achieve considerable concentration of
urine using countercurrent flow (where two parts of the
system run close together and in opposite directions), the
classic way to achieve maximum exchange of materials.
TheMalpighian tubules run forward, tightly pressed along
the outside of the backwardly flowing rectum, forming a
‘cryptonephridial complex’. Active transport of ions from
blood to tubules sets up standing concentration gradients,
allowing the rectum to resorb a high proportion of the
water; the final region of the rectum (and hence the excreta)
may reach concentrations of 5000mOsm.
Many desert insects have tricks to acquire water, often
from condensing dew or fogs. But taking up atmospheric
water in the vapour phase is perhaps the most spectacular
trick, allowing some xeric insects andmites to obtain water
without any liquid presence. Table 4 shows the occurrence
of this; all known cases are arthropods. Uptake happens
only above a specific relative humidity (the critical equi-
librium humidity, CEH), which is usually at least 70–80%
(but as low as 45% RH for Thermobia). Some of these
animals take water vapour into their rectum, condense it
into liquid water and then use the ‘normal’ water resorpt-
ion mechanisms in the rectal walls. Others use the front
end of their guts (various ticks, and the desert cockroach
Arenivaga). Here salivary sacs are everted and a very con-
centrated hygroscopic fluid secreted on to their surfaces,
which takes up water, the diluted fluid then being swal-
lowed. The sac surfaces are also highly sculpturedwith fine
furrows, indicating that physical capillarity helps water
uptake. But it is still unclear how any fluid could take up
water at 45% RH; this needs an osmolarity beyond the
scope of biological solutions.
Respiratorywater canalsobe saved.Manydesert beetles
have their abdominal spiracles opening to a cavity beneath
the wing covers, where water condenses to be recovered via
the anus. There may also be ‘discontinuous ventilation’,
where an insect breathes rapidly for 15–30 min then closes
its spiracles for many hours. Oxygen gradually gets used
up, but the CO2 produced is stored in body fluids as bi-
carbonate, so a slight vacuum occurs internally; this neg-
ative pressure allows more oxygen to leak in slowly at the
spiracles, and also prevents water vapour leaking out.
Eventually, the bicarbonate and CO2 levels get too high,
Table 4 Critical Equilibrium Humidity (CEH) values for ar-
thropods that take up water vapour from unsaturated air
Taxon CEH (as % RH)
Arachnids
Mites and ticks
Ornithodorus 94
Ixodes 92
Echinolaelaps 90
Acarus 70
Amblyomma 87
Insects
Bristletails
Ctenolepisma 48
Thermobia 45
Fleas
Ceratophyllus 82
Goniodes 60
Xenopsylla 50
Booklice
Liposcellis 70
Roaches and grasshoppers
Arenivaga 80
Caterpillars
Tinea 95
Beetle larvae
Tenebrio 88
Lasioderma 43
Onymacris 84
Crustacea
Woodlice
Porcellio 87
Osmoregulation in Invertebrates
10
the spiracles open and the system flushes out. This discon-
tinuous respiration, found in many pupae and in desert
insects such as ants, can give a 25-fold saving of respiratory
water.
Invertebrate parasites
Endoparasites are bathed in fluid that is already being
regulated by their host; they are probably in osmotic bal-
ance with their animal host’s tissues, and for any host of
reasonable size this will give the parasite a stable habitat.
Thus, they have very few osmotic problems, and the stages
living inside the host are usually highly permeable, with
weak or absent kidney function. However, the larval or
transmission phases may survive outside the host in very
different osmotic conditions (often air or freshwater) and
need more substantial osmoregulation. Furthermore, par-
asites ofmigratory aquatic hosts, for example, eels, salmon
and some crabs thatmove between seas and rivers, will also
have to be euryhaline, though the host does most of the
regulation and the parasite conforms with the resulting
osmotic concentration of the host tissues.
Many aquatic ectoparasites are surprisingly sensitive to
variation in salinity, and parasitic flukes are rare on hosts
in low-salinity environments; where the host is more
euryhaline than the fluke, the parasite may be absent from
part of the host’s range. Terrestrial ectoparasites are ex-
posed to water balance and ionic problems across their
external surfaces, but have the advantage of being plugged
in to a reliable regulated fluid source, so can afford more
water and ion loss than a free-living species.
Hypersaline invertebrates
Salt lakes occur when freshwater bodies with no outflow
gradually evaporate, in extreme cases being 6–8� seawa-
ter. Some have the same relative ionic composition as
seawater, but lakes produced mainly by evaporation from
springs can be very acidic or alkaline (‘soda lakes’). High-
salinity conditions also occur as ‘brine ponds’, where
pools and coastal lagoons become isolated above the
littoral zone. Here, concentrations of up to 4� seawater
are found. Salt lakes and ponds obviously pose a major
physiological problem to animals in terms of osmotic and
water balance.
The brine shrimp Artemia (see Figure 2) colonizes salt
lakes, as do larval brine flies, midges and mosquitoes. All
these insects and crustaceans are extremely good ‘hypo-
regulators’ maintaining relatively low blood concentra-
tions in very salty media. Their main adaptation is to
increase the drinking rate several fold to acquire the es-
sential water, but at the expense of a heavy salt load, which
must be excreted. In adult Artemia salt glands on the gills
perform this function, and in larvae it is achieved by a
specialized ‘neck organ’; both sites have very high sodium
pump activity, and probably also chloride pumps. Many
brine-dwelling animals are also able to excrete urine
strongly hyperosmotic to their own blood, with concen-
trations of up to 8700�mOsm; crustaceans use the antennal
glands, while insects use their rectum, which has large
numbers of specialized secretory cells. Hyposmotic regu-
lation is relatively much more costly than hyperosmotic
regulation in all animals so far investigated.
Further Reading
Dejours P, Bolis L, Taylor CR andWeibel ER (eds) (1987)Comparative
Physiology: Life in Water and on Land. Padva: Liviana Press.
Gaede K and Knu¨lle W (1997) On the mechanism of water vapour so-
rption from unsaturated atmospheres by ticks. Journal of Experimen-
tal Biology 200: 1491–1498.
Gibbs AG (1998)Waterproofing properties of cuticular lipids.American
Zoologist 38: 471–482.
Greger R (1988) NaCl Transport in Epithelia. Berlin: Springer.
HadleyNF (1994)WaterRelations of Terrestrial Arthropods.NewYork:
Academic Press.
Kinne RKH (1993) The role of organic osmolytes in osmoregulation;
from bacteria to mammals. Journal of Experimetal Zoology 265: 346–
355.
PequeuxA,Gilles R andMarshallWS (1988)NaCl transport in gills and
related structures.Advances in Comparative Environmental Physiology
1: 1–73.
Ruppert EE and Smith PR (1988) The functional organisation of filtra-
tion nephridia. Biological Reviews 63: 231–258.
Somero GN, Osmond CB and Bolis CL (eds) (1992) Water and Life.
Berlin: Springer.
Strange K (1994) Cellular and Molecular Physiology of Cell Volume
Regulation. Boca Raton, FL: CRC Press.
Willmer PG, Stone GN and Johnston IA (2005) Environmental Phys-
iology of Animals, 2nd edn. Oxford: Blackwell.
Osmoregulation in Invertebrates
11