How a low tissue O2 strategy could be conserved in early crustaceans: the example of the podocopid ostracods
Laboratoire d'Ecophysiologie et Ecotoxicologie, des Systèmes Aquatiques, UMR 5805, Université Bordeaux 1 and CNRS, Place du Dr B. Peyneau, 33120 Arcachon, France
* Author for correspondence (e-mail: jc.massabuau{at}epoc.u-bordeaux1.fr)
Accepted 17 September 2004
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Summary |
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Key words: respiration, evolution, crustacea, control of breathing, oxygen regulation, hypoxia
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Introduction |
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There was an explosion of life in the lower Cambrian and, based on both the
crustacean fossil record (Vannier and Abe,
1995; Waloszek,
1999
; Shu et al.,
1999
; Siveter et al.,
2001
; Horne et al.,
2002
) and molecular phylogeny
(Yamaguchi and Endo, 2003
),
the different species of the Ostracoda, one of the largest groups of
crustaceans, appears to have been established since that time. If one compares
present and early fauna, very few such primitive animals are still present and
so ostracods emerge as an outstanding group covering at least 500 million
years of aquatic life. Their carapace fossils are proven values for
interpreting the geological age, depth, salinity and other paleoecological
parameters of sedimentary rocks.
Remarkably, in water-breathing animals, there is a strategy of gas-exchange
regulation that consists of maintaining PO2 in
the arterial blood within an astonishingly low and narrow range (13
kPa, i.e. about 510 times lower than in homeotherms;
Massabuau, 2001). In mammalian
tissues, including the brain, the most frequently measured
PO2 is also at 13 kPa
(Vanderkooi et al., 1991
).
Based on the postulate that basic cellular mechanisms have been established
since the early stages of evolution, it has been suggested that this
similarity in oxygenation status could be the consequence of an early
adaptation strategy, which subsequently, throughout the course of evolution,
maintained cellular oxygenation in the same low and primitive range,
independent of environmental changes (Massabuau,
2001
,
2003
). The low arterial
oxygenation strategy observed in water-breathers could thus be a link in a
chain stretching back as far as the Proterozoic ages. The whole story,
including the elaboration of sophisticated gas exchange systems that work in
either water or air (and the use of respiratory pigments) plus
adaptations to increasing animal sizes, complexity and metabolic levels in
earth's changing atmosphere may represent a remarkable example of
homeostasis operating over a vast time scale.
In the present paper we have used podocopid ostracods to test this
hypothesis by gaining more insight into the O2-supply control
mechanisms in early crustaceans. Podocopid ostracods are minute crustaceans
(0.33 mm) with a laterally compressed body within a calcified bivalved
carapace that encloses a domiciliary cavity. They inhabit diverse benthic
environments both in fresh- and seawater ecosystems. The podocopids are the
most diverse and widespread ostracods today. It is important to note that they
lack gills and heart, which suggests a clearly primitive morphofunctional
structure. Interestingly, however, they all possess a pair of ventilatory
appendages, physiologically analogous to the scaphognathites present in
present Crustacea (Hughes et al.,
1969). These appendages beat rhythmically and bring water currents
into the domiciliary cavity where gas exchanges
(O2CO2) occur.
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Materials and methods |
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Animals were acclimated in the laboratory for at least 1 month before
experiments began (Massabuau,
2001) and then remained in the experimental set-ups for 38
weeks. As no significant mortality or statistical difference was observed as a
function of experimental duration, all the data are presented together. Note
that this illustrates the stability of our procedure and observations in our
experimental conditions. Altogether, a total of about 750 h of observation was
performed. For reference, 1 kPa=7.5 Torr or mmHg. In seawater
(salinity=30
) equilibrated with air, the partial pressure is 21 kPa,
and the oxygen concentration is 9.33 mg l1 (280 µmol
l1) at 10°C and 7.90 mg l1 (242
µmol l1) at 18°C.
Maintenance conditions
The animals, together with their natural sediment, were placed in open-flow
PVC tanks (40 cm x15 cm x15 cm) in a dark room thermostated at 10
or 18°C. They were all supplied with seawater from the Bay of Arcachon
(water PO22021 kPa; water pH
7.8;
salinity
30
). Given the animal size and the amount of organic
material and microfauna naturally present in the sediment, no external food
was added. When required, specimens were isolated under a binocular microscope
before experiments. To minimise external disturbances, the experimental tanks
were isolated from laboratory vibrations on anti-vibrating benches.
Ventilatory analysis by video recording
The experiments consisted of studying ostracod ventilatory activity when
exposed to various steady water PO2 conditions
at 10°C, by visual inspection after or during video recording (Figs
1 and
2). All video observations were
carried out under dim light using infrared light (=880 nm) to limit
any disturbance to the animals. Recordings were made using an XY driven
Leitz MZ12 binocular microscope (Oberkochen, Germany) equipped with a B/W
Ikegami camera (CDD Camera, ICD42B; Maywood, USA). Data were displayed on a
Sony TV monitor (HR Trinitron PVM 1453MD; Tokyo, Japan). They could be either
analysed on-line and/or stored on a JVC tape recorder (S-VHS, HRS75000MS;
Tokyo, Japan) or a Panasonic tape recorder (VHS, NV/SD45; Osaka, Japan). As
the animals were only motionless on exceptional occasions, no attempt was made
to use any automatic frequency counting device.
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Experimental procedure
Analyses were performed on groups of mixed species originating either from
the Bays of Arcachon or Biscay. One week before the experiment started, a
group of 715 ostracods were transferred to a wedge-shaped
micro-aquarium (Fig. 1; volume
500 µl; 2 cm x2 cm, thickness, 03 mm; water renewal rate
1020 µl min1). This was hand-made using a
microscopic slide fixed using SYLGARD (Down Corning; Michigan, USA) on a
thermostated glass plate (10 cm x6 cm x0.5 cm). It was provided
with in situ muddy sand and vegetal remains to mimic a `natural-like'
environment, in which the animal could move freely, dig and hide between sand
particles. This aquarium was part of a 11 closed recirculatory system with
constant entry and exit levels, set at 10±0.1°C for all experiments
using a laboratory-constructed thermoelectric device. During the experiments
PO2 varied from 221 kPa (27282
µmol l1). The partial pressure of CO2
(PCO2) was maintained at 0.1 kPa, a value
typical of water PCO2 in air-equilibrated
environments. The gas mixtures were bubbled through the reservoir of seawater
feeding bottles. The N2/O2/CO2 gas mixture
was obtained via mass flow controllers (Tylan General, model FC-260;
San Diego, USA) driven by a laboratory-constructed programmable control
unit.
Two sub-types of experiments were performed in this setup: (1) analysis of reference ventilatory pattern in normoxia and response to 3-day exposure periods at 3 kPa, and (2) ventilatory responses to 2 h exposure periods by decreasing the range from 21 to 2 kPa.
Long-term exposure at water PO2=21 and 3 kPa
Analysis of the reference ventilatory pattern to long-term exposure in
normoxia (21 kPa, 282 µmol l1) and 3 days in hypoxia (3
kPa, 40 µmol l1) was performed from April to June on two
podocopid species from the Bay of Arcachon and two species from the Bay of
Biscay. Animals were first studied for 3 days in reference normoxic conditions
(21 kPa), where the animals had adapted to the set-up for 1 week, then in
hypoxia for 3 days (PO2=3 kPa, hypoxic test)
and finally in normoxia after 3 days of recovery (recovery). When the analysis
started (day 1), the experiment consisted of focussing on individuals
(recognizable by their species, location in the aquarium, size and shell
marks) and studying them for 1 h periods in normoxia, hypoxia and, finally,
normoxia. Thus, a total of 3 h (3 x1 h) of analysis was performed on
each individual studied. Their ventilatory pattern was described during
reference days 1, 2 or 3, test days 4, 5 or 6 and recovery days 7, 8 or 9. For
each animal, the time spent actively ventilating during the studied hour (min
h1), the mean ventilatory burst duration (min), the burst
number (h1) and the mean ventilatory frequency within bursts
(min1) were determined. There were no statistically
significant differences as a function of time at each water
PO2 level, so all data were pooled. Comparisons
were performed using a paired t-test, as an individual was its own
reference.
Resistance to 24 h-anoxia
To obtain some insight into the potential ability of podocopids to survive
anoxic exposure, a group of ten specimens from the Bay of Arcachon
(Leptocythere and Cyprideis) were exposed to
PO2=0 kPa and
PCO2=0.1 kPa during a 24 h exposure period.
After recovery at PO2=21 kPa, ventilatory
activity and behaviour were studied during the following 72 h period.
Short-term exposure at various oxygenation levels
These experiments were done on three species from the Bay of Arcachon and
two from the Bay of Biscay. Each group was exposed to seven plateau levels of
different water PO2 presented in the following
order (21, 10, 6, 4, 3, 2 and 21 kPa). The duration of exposure was 2 h per
oxygen level, and ventilatory frequencies within ventilatory bursts
(min1) were measured during the last 30 min of exposure.
Again, each specimen was identified. Under each experimental condition,
measurements were performed twice per individual and both determinations were
pooled.
Behavioural regulation of organism oxygenation status
This experiment, performed in August, consisted of studying the ostracod
positioning into natural and experimentally manipulated O2
gradients at 18°C. Thirty five cores of muddy sandy sediment (sediment
depth, 15 cm; water column, 5 cm; N=35) were handle-collected with
glass tubes (diameter 5 cm, length 20 cm) in the Bay of Arcachon at Malprat
Island (day 0; Carbonel, 1978,
1980
). In an attempt to
minimize heterogeneity between cores, the size of the sampled area was 1
m2. They were transferred within 11.5 h to a laboratory
thermostated at 18°C and placed in experimental tanks fed with running
seawater (water PO2
21 kPa, water pH
7.8,
salinity=30
). Great care was taken not to disturb the sediment at the
interface. Five cores were randomly chosen 12 h later to determine
reference normoxic O2 profiles and ostracod location. Following
O2-profile determination, the cores were immediately frozen in
liquid N2 to fix the ostracod positions in the sediment. They were
kept at 20°C prior to analysis. One day after field sampling (day
1), 15 cores were exposed to hyperoxia (water column
PO2=40 kPa) to manipulate the anoxic zone. The
remaining 15 cores were kept in parallel under normoxic conditions.
O2 profiles and ostracod location were determined on five normoxic
and five hyperoxic cores at days 4, 7 and 11.
Oxygen profile determinations
O2 profiles were measured with O2 polarographic
microelectrodes (UNISENSE Microsensors; Aarhus, Denmark) driven by a PRIOR
micromanipulator (Cambridge, UK; steps, 0.2 mm). The microelectrode was
advanced through the central region of the core, recording the O2
changes with depth. Preliminary experiments, during which five profiles were
measured per core (1 central, 4 peripheral), demonstrated that in our
experimental conditions a single central measurement was representative of the
entire vertical O2-distribution, except when a burrow was present.
Following this preliminary observation, two cores were eliminated from the
full analytical process during the experimental run, as a burrow was detected.
Thus, the total number of analyzed cores for ostracod positioning was 33
instead of 35.
Ostracod position in the oxygen gradients
The analysis was performed on melting cores. Each core was sliced
(thickness, 400 µm) using razor blades and a precision micromanipulator.
The slices were obtained from +1 cm above the sedimentwater interface
to 1 cm. Ostracod number (410 per core) and species analysis
were determined for each slice after animal sorting under binocular.
Oxygen diffusion distances
The study was performed on a total of five Argilloecia and five
Cyprideis. Whole animals were immersed in a fixative for electron
microscopy (6% glutaraldehyde buffered with 0.4 mol l1
sodium cacodylate, pH 7.4, osmotic pressure 1100 mOsmol l1)
for 12 h at 4°C and subsequently rinsed in cacodylate buffer (0.4 mol
l1, NaCl 4%). They were then embedded separately in
Araldite. Serial sections were cut using a Reichert (Depew, NY, USA) automatic
ultra-microtome. Ultra-thin sections were taken from randomly distributed
areas of the Araldite block. Maximum diffusion distances were measured on
enlarged pictures (semi-thin preparations) after visual inspection using a
Leica TCS 4D microscope (Solms, Germany).
Statistical analysis
Values are reported as mean values±1 standard error
(S.E.M.) or 1 standard deviation (S.D.). Differences
were evaluated using a MannWhitney U-test, a two-tailed
Student's t-test, a Fisher test and/or analysis of variance (ANOVA).
P<0.05 was taken as the fiducial limit of significance.
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Results |
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In a next step towards determining the existence or absence of ventilatory adaptation mechanisms to face water PO2 changes, we then examined the effects of exposure in 2 hstages in the same five species (plus Loxoconcha elliptica, an extra genus from the Bay of Arcachon). These observations are presented in Table 2. When the hypoxic values are compared to the normoxic reference and recovery status, no hyperventilation could be detected in response to hypoxia. The relationship between fR and water PO2 was: fR=0.6(water PO2)+72.9 (r2=0.49, P<0.079). Note in addition that the percentage of apnoeic animals remains independent of water PO2 in the range 212 kPa [number of apnoeic animals=0.09(water PO2)+46.2; r2=0.015, P<0.79]. Fig. 4A,B, presents all pooled data to illustrate this absence of ventilatory adaptation ability.
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What could be then the adaptative solution, if any, developed by these animals? Are they using an alternative strategy to maintain their cellular oxygenation status, or are they exhibiting a total absence of tissue PO2 regulation? They naturally inhabit oxygen gradients in the upper layers of the sediment, so we studied their positioning in naturally occurring and experimentally manipulated O2 gradients. In addition, experiments were performed at 18°C to stimulate the animals' O2 requirements and O2 dependency. Fig. 5 presents the results of these experiments performed in natural cores from the Bay of Arcachon. As O2 penetration varied between cores, and was independent of exposure time, all data were grouped by core O2-profile characteristics (redox fronts were in the range 23, 34, 45, 56 and 78 mm, indicated by dotted lines in Fig. 5). Two types of podocopid species were found in the cores, Leptocythere castanea (N=120) and Cyprideis torosa (N=82). Interestingly, 2 h after field sampling, most animals were naturally found in the 35 kPa layer at 12 mm below the surface. They then stayed there during the 10-day experimental period in all cores supplied with normoxic water, independent of the acclimatory adaptation period to laboratory conditions. In cores where the O2 profile had been experimentally manipulated, the distribution also remained clearly linked to the same low oxygenation layers, and this was independent of depth and sampling time ranging from 411 days. Fig. 6A is a frequency distribution diagram summarizing these observations. It is clear that the PO2 in water where animals were most frequently found (N=202) ranged from 3 to 5 kPa, although data were not normally distributed and higher values of up to 1618 kPa were occasionally observed. The above data set consequently demonstrates the existence of a behavioural regulation mechanism of body oxygenation in ostracods.
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We next addressed the issue of O2 diffusion problems in these animals, which lack any blood circulatory system. Specifically, we measured maximum diffusion distances from ventilated water in the domiciliary cavity to the more central tissues. Fig. 7 presents a typical transverse section from a large 500 µm Argilloecia (Fig. 7A) and a 600 µm Cyprideis specimen (Fig. 7B), showing that the maximum diffusion distance for oxygen between ventilated water located between the valves and the body core ranged from 50100 µm. The cuticle thickness at soft body level was 2.4±0.2 µm (N=10 measurements).
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Discussion |
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Comparison with previous data
To our knowledge, very little data relating to respiratory problems in
ostracod podocopids have been published. In fact, the study of living
ostracods has been dominated by zoologists and micropaleontologists developing
geological applications (palaeoenvironment and stratigraphy). Consequently,
how the carapace is produced, together with valve morphology in relation to
habitat, has been extensively studied. By contrast, there is much less
information about the soft anatomy (Okada,
1982; Keyser,
1990
) and physiological data is scarce
(Van Morkhoven, 1962
;
Maddocks, 1992
). Hagerman
(1969
) reported on oxygen
consumption and anaerobic survival in the brackish water podocopid
Hirschmannia viridis (Muller) at 20°C. He reported that
Hirschmannia did not survive anaerobic conditions for 13 h
(LC50=7 h) and water PO2
2 kPa
for 160 h (LC50=55 h). Interestingly, he did not observe any
mortality at water PO2
4 kPa. These data are
consequently in good agreement with the present report and would suggest that
the presence of ostracods at water PO2
2
kPa, as shown in Figs 5 and
6A, could be only transitory.
Variability evidently exists between species, however, as Danielopol et al.
(1993
) reported that the
freshwater Limnocythere inopinata survived a 96 h exposure period at
0.2 kPa, but Metacypris cordata did not (at 11°C). It is also
noteworthy that Geiger (1990
)
studied the distribution of freshwater podocopids Cytherissa
lacustris in sediments from Lake Mondsee, Austria. He reported that
C. lacustris was most abundant 510 mm below the
sedimentwater interface and that the maximum O2 penetration
depth was 58 mm, depending on the O2 concentration in the
overlaying water. This evidently reflects the positioning in the sediment
O2 profile as described in the present study. Some information
about scaphognathite frequency in Metacypris cordata (ostracod
podocopid) was also reported by Danielopol et al.
(1993
). These authors worked
on two individuals immobilised upside down at different water
PO2 values. They reported that ventilatory
activity was irregular and infrequent at PO2=15
kPa (11°C), with a ventilatory frequency ranging from 010 beats
min1. Between
PO2=0.40.8 kPa, they reported a change
in the beating frequency from 0 to 50 min1 but the scarcity
of their observations evidently limited any conclusion.
O2 diffusion problems in tissues
The present report, together with Geiger's data
(Geiger, 1990) demonstrate
that ostracod podocopids live at low ambient
PO2, in the sediment, far from air-equilibrated
waters. This evidently raises the question of how oxygen diffuses in the
animal's soft body. The problem of the maximal body size of small animals in
which O2 can penetrate by pure diffusion has often been addressed
since the pioneer works of Harvey
(1928
) and Krogh
(1941
). Assuming a homogeneous
spherical body in which oxygen is consumed at a constant rate, and assuming
that PO2 at the centre is 0, the maximum
diffusion distance is
,
where KO2 is the Krogh's constant of diffusion
(µmol h1 cm1 kPa1),
PO2 is the O2 partial pressure in
water (i.e. in the domiciliary cavity in podocopids) and
O2 is the rate
of O2 consumption or flux (µmol h1
cm3). Paul et al.
(1997
), taking as an example
Daphnia magna, which is also a millimetre sized crustacean (14
mm; Kobayashi, 1982
), a
Krogh's constant of 0.378 x 103 µmol
h1 cm1 kPa1
(Bartels, 1971
) and a
O2 value of 39
µmol h1 cm3 (from
Kobayashi and Hoshi, 1984
),
calculated that the maximum diffusion depth is 340 µm for an external
PO2 of 21 kPa, 280 µm for 13.3 kPa, 200
µm for 6.7 kPa and 140 µm for 3.3 kPa. Similarly, Vannier and Abe
(1995
) calculated a critical
radius at water PO2=21 kPa of 1 mm, over which
O2 diffusion is not sufficient to supply a spherical ostracod. In
the present work, we studied animals having a maximum diffusion distance of
50100 µm from the domiciliary cavity to the deepest tissues, and
report their spontaneous positioning at inspired
PO2=35 kPa. Our results are thus
consistent with simple O2-diffusion capability, especially as
D. magna is a so-called active species, while ostracods are much more
sluggish.
Oxygen control mechanism in ostracods
The present results show that the ostracod podocopids we studied lack the
ventilatory regulation mechanism present in crustaceans
(Childress, 1971;
Massabuau and Burtin, 1984
;
McMahon, 2001
;
Pirow and Buchen, 2004
), fish
(Eclancher, 1972
;
Shelton et al., 1986
),
molluscs (Tran et al., 2000
),
amphibians and reptiles (Shelton et al.,
1986
), birds and mammals
(Dejours, 1981
;
Bouverot, 1985
). Note that the
six species we studied were randomly chosen among podocopids inhabiting
deep-sea and shallow waters and are thus representative of many, if not all,
endobenthic podocopids. Paul et al.
(1997
) and Pirow and Buchen
(2004
) also studied the
principles of respiratory physiology in the minute crustacean D.
magna. In brief, they reported the existence of cardio-circulatory
adaptations to hypoxia, and recently Pirow and Buchen
(2004
) demonstrated an
O2-ventilatory drive. Thus, in contrast to our findings, in
Daphnia a high ventilatory activity copes with a decrease in ambient
oxygen availability. There are, however, numerous fundamental differences
between cladocera and podocopids. First, Daphnia are planktonic
filter feeders, which lack scaphognathites but do possess thoracic appendages.
The beating of these thoracic appendages causes efficient ventilation within
the animal's filtering chamber because this region is well irrigated and
PO2 is lowered in the exiting water
(Pirow et al., 1999
). In
addition Daphnia possess a heart and a simple circulatory system
containing haemoglobin, whose concentration and affinity varies according to
ambient oxygenation status. Podocopids, on the other hand, possess a pair of
scaphognathites, as present in malacostraceans, but no circulatory system.
Finally, ostracod podocopids have been present since the lower Cambrian,
whereas Daphnia probably appeared more recently as they have only
been reported from the Permian (250300 million years old;
Schram, 1982
).
Importantly, we found that podocopids regulate their tissue O2
status by behavioural adaptation. Indeed, they escape both the more oxygenated
and anoxic pore waters by moving into the sediment and following O2
profile displacements, independent of time and sediment depth. The maximum
velocity reported was 1 cm min1 in Metacypris
cordata (Danielopol et al.,
1993) which is consistent with the O2 kinetics that we
imposed in sediments. To our knowledge, this is the first time that such a
chemotropism has been demonstrated in ostracods. It demonstrates in these
early crustaceans the existence of an O2 chemosensitivity that is
either of peripheral or central origin. In crayfish
(Massabuau et al., 1980
;
Ishii et al., 1989
) and fish
(Shelton et al., 1986
), the
presence of peripheral O2 chemoreceptors has been reported at gill
level. Importantly also, the ventilatory control loop appears incomplete in
podocopids, as the ventilatory frequency was definitively independent of any
change in water PO2. In their biotopes,
ostracod positioning is evidently not only driven by oxygenation problems but
also by feeding. Ostracods sweep bacteria, algae, protozoa and small particles
of detritus into their mouths with the fine, feather-like hairs attached to
their appendages (Elofson,
1941
; Horne,
2003
). In the Bay of Arcachon, organic material is homogeneously
present in the first centimetres of sediment, partly due to bioturbation
processes (Relexans et al.,
1992
). Thus, it is likely that in the present experimental
conditions, food availability in the layers where the podocopids are living
was not a limiting step that significantly interfered with ostracod
displacement.
Physiology of the crustacean respiratory system viewed from an evolutionary perspective
In Macrura and Brachyura, which are modern malacostraceans, the rhythmic
movement of each scaphognathite is controlled by five levator and five
depressor muscles, innervated by two motoneurons arising from a central
pattern generator (CPG; Simmers and Bush,
1983) located either in the suboesophageal or thoracic ganglion
(Pasztor, 1968
;
Young, 1975
). In ostracods,
despite the fact that they are early crustaceans, there is already a
well-developed cerebrum, a circumoesophageal ring of ganglia, a chain of
ventral ganglia and a network of motor nerves connected to the various muscles
of the oral and posterior regions (Rome,
1947
; Hartmann,
1967
). As we have reported, the activity pattern of the
scaphognathites is also perfectly well organised
(Vannier and Abe, 1995
) and
similar to that observed in modern crustaceans
(Young, 1975
). Their rhythmic
movement is controlled by four muscles innervated by nerves originating from
the circumoesophageal ganglia (Hartmann,
1967
). Because there is now considerable evidence from a variety
of different invertebrates that the major features of the motor patterns
underlying rhythmic behaviour are essentially determined by CPG within the
central nervous system (Harris-Warrick et
al., 1992
; Marder and Bucher,
2001
), it is thus very likely that such a respiratory CPG does
exist in podocopids. The idea of a central unique CPG is indeed reinforced by
the observation of a strong bilateral coordination, which suggests a unique
central neuronal connectivity. This strongly suggests that the existence of
central neuronal circuits (here respiratory centres) capable of producing a
rhythmic movement, possibly appeared very early in the course of
evolution.
Horseshoe crabs Limulus polyphemus are more closely related to
chelicerates than they are to true crustaceans, but they have also evolved
little in the past 250 million years and have probably existed since the
Silurian period (440410 million years ago). Interestingly, their
ventilatory pattern has also been reported as highly variable
(Watson, 1980;
Mangum and Ricci, 1989
), a CPG
displaying pattern motor outputs characteristic of rhythmic gill ventilation
has been described (Wyse et al.,
1980
), and an absence of ventilatory change from normoxia to
hypoxia has been reported (Mangum and
Ricci, 1989
). All taken together, this set of observations appears
to be similar to our findings in the podocopids.
Finally, it is worth noting that diverse features can be taken as signs of
immaturity and/or primitive status of the respiratory centres in podocopids.
First, in resting crustacean decapods such as the green crab Carcinus
maenas, the gill chambers are irrigated by regular rhythmic beating of
the bilateral pair of scaphognathites with short pauses (mean duration, 13 s;
frequency, 70 h1;
Jouve-Duhamel and Truchot,
1983), while in podocopids longer pauses (up to at least 60 min),
exhibiting an apparently erratic frequency are observed
(Fig. 3A,B). In
Carcinus, their frequency and duration were largely decreased by
exposure to hypoxia (Taylor,
1982
; Jouve-Duhamel and
Truchot, 1983
), which contrasts markedly with the situation
reported here. Second, most decapods are capable of periodically reversing the
direction of ventilatory current flow
(Arudpragasam and Naylor, 1964
;
Hughes et al., 1969
). One
motor programme, driven by a specific set of motoneurones and underlying
reversed beating of the scaphognathites, is responsible for this pattern
(Simmers and Bush, 1983
). Its
functional significance is not clearly understood, but it appears to be
important in cleaning detritus in the branchial cavities. In podocopids, it
could also be important to clean the domiciliary cavity, as the animals are
living between detritus in muddy sandy sediments. However, despite a total of
750 h analysis of breathing patterns, we never once observed any
reversal, which strongly suggests their absence or, at least low occurrence.
Finally, but importantly, the present data demonstrate that changes in water
PO2 had no regulatory effect on the breathing
rhythm, which is most certainly a major primitive characteristic. Indeed, the
ability of an organism to maintain the homeostasis of its internal
environment, independently of its external environment, has been one of the
fundamental keys of evolution. Based on the present work, podocopids appear,
by contrast, firmly subordinated to sediment layers containing low oxygen
levels.
In conclusion, the present data obtained in podocopids strongly suggest that the strategy of low tissue PO2 could have existed in early animals, even if they exhibited immature physiological ventilatory regulation mechanisms. This reinforces the hypothesis of an appropriate regulation of the cellular O2 status, strongly conserved throughout the evolutionary process. In this view, one can suggest that, once the oxygen concentration on the earth started to increase, podocopids used the hypoxic layers in the sediment as an ecological refuge while their ancestors could live in an open but hypoxic ocean. This is of course speculation, but it is obvious that, whatever evolutionary solution they developed, it has been exceptionally efficient as they are today among the oldest living animals present on our planet and one of the largest crustacean groups.
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Acknowledgments |
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