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First published online May 1, 2006
Journal of Experimental Biology 209, 1874-1882 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02200
Removal of the chorion before hatching results in increased movement and accelerated growth in rainbow trout (Oncorhynchus mykiss) embryos
Department of Integrative Biology, University of Guelph, 50 Stone Road East, Guelph, Ontario, N1G 2W1, Canada
* Author for correspondence (e-mail: patwrigh{at}uoguelph.ca)
Accepted 7 March 2006
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Key words: exercise, protein content, oxygen consumption, yolk protein, embryo, yolk-sac larvae, hatching, development, rainbow trout, Oncorhynchus mykiss
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). In addition, the chorion is involved in
respiration, excretion of metabolic wastes and ionic and osmotic balance
(Groot and Alderdice, 1985).
Mechanical protection offered by the chorion is especially important in
salmonid embryos as gravel movement and pressure changes within the redd can
result in physical damage to the embryo
(Groot and Alderdice, 1985).
This is apparent in the relatively thick chorions of salmonid embryos compared
with marine embryos, which are pelagic and must remain buoyant (reviewed by
Blaxter, 1988).
Given the restrictions of living within a sphere, are embryos capable of
movement? The development of locomotion varies widely between fish species
depending on the timing of ontogenetic events, as well as life history of the
species. In Danio rerio embryos [at 29°C hatching occurs at 48 h
post-fertilization (h.p.f.)] spontaneous contractions begin as early as 17
h.p.f. followed by a coiling behaviour in response to touch at 21 h.p.f. and
finally organized swimming in response to touch starts at 27 h.p.f.
(Saint-Amant and Drapeau,
1998). In salmonids, embryonic development is protracted, with
hatching occurring after several weeks, depending on the species and
environmental conditions. In Salmo salar embryos (at 6°C hatching
occurs after 81 days) weak sporadic contractions of the trunk were observed at
27 days post-fertilization (d.p.f.) followed by rhythmic movements at 32
d.p.f. (Johnston et al.,
1999). In S. trutta, occasional body movements at 35
d.p.f. that increased in frequency up to hatching at 54 d.p.f. have been
reported (Proctor et al.,
1980). Thus, it appears that fish embryos have the appropriate
`machinery' and are capable of movement while still confined in the chorion
before hatching.
Very few studies have considered the relationship between activity and
metabolism in embryonic or larval fish. Activity substantially influences the
metabolic rate, and consequently oxygen consumption in all organisms
(Fry, 1971), including adult
and juvenile fish (Brett, 1964;
Fry, 1971;
van den Thillart, 1986).
Davenport and Lönning (Davenport and
Lönning, 1980) reported that oxygen consumption was
significantly reduced in anaesthetized, and therefore inactive, Gadus
morhua larvae compared with unanaesthetized larvae. Weiser reported that
active yolk-sac rainbow trout increased their metabolic rate by 85–166%
over and above the routine metabolic rate
(Weiser, 1985). There is no
data, to our knowledge, on the influence of activity on oxygen consumption at
earlier developmental stages in salmonids, before the time of hatching. This
is particularly interesting because of the increase in body movements prior to
hatching (Proctor et al.,
1980).
In addition to changes in metabolic rate, activity or exercise is known to
influence many other physiological parameters, including changes in
whole-animal growth. Growth in adult fish is plastic and is influenced by a
forced moderate exercise regime over an extended period of time, also known as
exercise training. When subjected to exercise training, adult fish show much
more rapid growth (demonstrated by changes in weight gain) compared with
non-exercised fish, and this is especially true for salmonid species, e.g.
S. trutta (Davison and Goldspink,
1977); Salvelinus fontinalis
(Johnston and Moon, 1980);
O. mykiss (Houlihan and Laurent,
1987); S. salar
(Totland et al., 1987). By
contrast, very little is known about the effects of activity on growth during
early life stages. No relationship was found between activity and growth in
larval zebrafish (Bagatto et al.,
2001), and it was suggested that any extra energy obtained was
allocated to swimming and was unavailable for growth. To our knowledge the
effects of activity on growth have not been further considered in the
embryonic or larval stages of other fish species, including salmonids that are
more differentiated at hatch relative to zebrafish
(Blaxter, 1988).
Rapid growth in rainbow trout is associated with higher rates of protein
synthesis (Valente et al.,
1998). The same is true in adult fish subjected to exercise
training. The rate of protein synthesis must exceed the rate of protein
degradation for growth to occur. Swimming stimulated growth and protein
synthesis and, to a lesser extent, protein degradation in muscle tissue of
trained adult rainbow trout compared with controls
(Houlihan and Laurent, 1987).
The effects of activity on protein synthesis have not been explored in the
early life stages of fish. Protein is the major yolk constituent of embryonic
and early larval salmonids. In addition to supplying energy via
catabolic processes, yolk protein is broken down providing amino acids for
tissue growth in the developing embryo
(Heming and Buddington, 1988).
Thus, an increase in activity levels in embryonic or larval fish may lead to a
more rapid conversion of yolk protein into embryonic tissue.
The first objective of our study was to ascertain if the chorion restricts movement before hatching. We predicted that by removing the chorion several days before hatching, dechorionated embryos would exhibit more movement than embryos whose chorions remained intact (chorionated). Our preliminary findings indicated that dechorionated embryos did in fact show more movement. Our second objective, therefore, was to measure the effects of activity on metabolic rate in embryonic rainbow trout. We predicted that the more active, dechorionated embryos would consume more oxygen compared with chorionated embryos. Given that in adults, there is a positive relationship between activity and growth, our third objective was to measure the effects of activity on growth in embryonic rainbow trout. We predicted that because dechorionated embryos move more than chorionated embryos, they will grow faster. The last objective of our study was to test if dechorionated embryos convert yolk protein into embryonic tissue at a faster rate than chorionated embryos. We predicted that the protein content would be higher in the embryonic body of dechorionated embryos compared with chorionated embryos at any given time. To test these predictions, the number of movements, embryonic body dry mass, and protein contents were measured before, during and post-hatch in chorionated and dechorionated rainbow trout embryos.
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Experimental protocol
There were four series of experiments conducted. Series I: recording of
movement; Series II: recording of oxygen consumption; Series III: estimation
of growth rate; Series IV: estimation of total protein.
Separate batches of embryos were used in each series, and hatching time was
consistent between each batch of embryos (30–34 d.p.f.). For each batch,
embryos were obtained from three or more different females. In each series of
experiments, embryos were removed from the incubation trays prior to the time
of hatching and briefly placed in shallow Petri dishes. Embryos were either
dechorionated under a light microscope by manual removal of the chorion using
fine forceps (experimental group or dechorionated) or handled with fine
forceps without removal of the chorion (control group or chorionated).
Following this procedure, unless otherwise stated, embryos were placed into
mesh-bottom chambers (
25 embryos per chamber) and returned to the
incubation tray. There was no mortality associated with the dechorionating
procedure.
Series I: recording of movement
In order to measure the degree of activity or movement in chorionated and
dechorionated embryos, embryos were randomly chosen from the incubation tray
on 27 d.p.f. and were separated into chorionated and dechorionated groups as
described above. Following this procedure, chorionated and dechorionated
embryos were then immediately placed into one of eight wells (1.2 cmx1.8
cmx0.6 cm; widthxlengthxdepth) in a custom-built plexiglass
chamber (5 cmx18 cm) supplied with flow-through water. Embryos were kept
under a 12 h semi-dark (lights on at 50% normal intensity) and 12 h
darkphotoperiod for the duration of the experiment. A video camera mounted
above the chamber was used to continuously record movement (24 h
day–1) for 80 h. Videotapes were later analyzed and the total
number of movements per 5 h period was recorded to compare movement in
chorionated and dechorionated embryos. Values are reported as number of
movements h–1.
Series II: recording of oxygen consumption
Closed respirometry was used to measure and compare oxygen consumption in
chorionated and dechorionated rainbow trout embryos. On 27 d.p.f., embryos
were randomly removed from the incubation tray and one-half of the embryos
were dechorionated and the remainder were left intact (chorionated). Following
this procedure, embryos were returned to the incubation tray and left
undisturbed for 5 h (in order to recover from the dechorionating
procedure) after which time they were placed, in pairs, into one of six
temperature-controlled respirometers. Two respirometers were left empty as
controls. The chambers were filled with air-saturated (10 mg
O2 l–1), autoclaved water (9 ml), and the
temperature was maintained at 10°C. Embryos were left undisturbed in the
chambers for about 1 h before sealing each chamber. The chambers were sealed
for 1 h during which time the water oxygen content was continuously measured
and did not fall below 70% saturation. Measurements were made as described
previously (Ninness et al., in
press). Measurements were made once daily for 7 days, with
naïve embryos being used each day. During each trial, one to two
respirometers were randomly chosen and the number of movements made by the
embryos in the respirometer was recorded. Preliminary experiments were
conducted to estimate the capacity of our system to detect changes in oxygen
consumption (Ninness et al., in
press).
Series III: estimation of growth rate
To measure and compare growth rate in chorionated and dechorionated
embryos, embryos were randomly chosen from the incubation tray and one half of
the embryos were dechorionated 24 d.p.f. Dechorionation occurred early in this
series relative to other series in order to maximize the effects of activity
on growth, and 24 d.p.f. was the earliest that the chorion could be removed
without causing any ill effects to the embryo. Measurements were made 24, 28,
30, 32 and 33 d.p.f. Embryos (N=8) were anaesthetized in 0.15 g
l–1 ethyl 3-aminobenzoate methanesulfonate salt (MS-222) and
if present, the chorion was removed. The embryonic body was separated from the
yolk sac (N=6, each value represents a pooled dry mass of five
embryos in order to increase the accuracy of measurements). Embryonic body dry
mass was measured after a constant mass was achieved (48 h at 50°C).
A separate experiment was conducted to ascertain if the differences in growth
between chorionated and dechorionated embryos persisted into later life
stages. Embryos were dechorionated 24 d.p.f. and measurements were made 24,
30, 45, 50, 60, 75, 90 d.p.f. Fish were sampled as described above, except
that in addition to the embryo body, the dry mass of the separated yolk sac
also was measured. After 100% hatch (34 d.p.f.) fish were moved from the
incubation tray into a divided floating container in a 900 l recirculating
tank. Fish were `over-fed' starting 50 d.p.f., using an automatic feeder
(Sweeney Enterprises Inc., Boerne, TX, USA) set to dispense every hour for 12
h d–1 for the duration of the experiment.
Series IV: estimation of total protein
To measure total protein concentration (mg protein
individual–1) in chorionated and dechorionated embryos,
embryos were randomly chosen from the incubation tray and one half of the
embryos were dechorionated 27 d.p.f. Measurements were made 27, 30, 32 and 33
d.p.f. Embryos were anaesthetized in 0.15 g l–1 MS-222, the
chorion was removed if present, the embryonic body was separated from the yolk
sac, blotted dry on a Kimwipe and was frozen at –80°C for up to 4
weeks before analysis.
Tissue analysis
To measure total tissue protein, 0.01 g of frozen tissue was ground to
a fine powder under liquid N2 using a mortar and pestel, dissolved
in 10 volumes of ice-cold trichloroacetic acid solution (TCA; 10%) and
centrifuged at 14 000 g for 10 min. The supernatant was
removed, the pellet was washed with TCA and centrifuged again at 14 000
g for 5 min. The supernatant was again removed, the pellet was
dissolved in 500 volumes of 1 mol l–1 NaOH, vortexed and
rocked for 24 h. Samples were then diluted to 0.5 mol l–1
NaOH by the addition of water and centrifuged at 500 g for 10
min. Preliminary experiments were performed to ensure that the concentration
of NaOH and incubation time were sufficient to completely dissolve the
embryonic protein (data not shown). Total tissue protein was quantified as
described by Lowry et al. (Lowry et al.,
1951) with modifications outlined by Rutter
(Rutter, 1967). The protein
content (mg protein individual–1) was calculated as the
product of the protein concentration (mg g–1 protein) and the
wet mass (g individual–1).
Statistical analysis
All statistical analyses were performed using the general linear models
(GLM) procedure of the SAS system (version 8e; SAS Institute In, Cary, NC,
USA). Differences in movement between chorionated and dechorionated embryos
(Series I) were analyzed using a repeated-measures analysis of variance
(ANOVA). Differences in dry mass (Series II) and protein content (Series III)
were analyzed using a two-factor ANOVA (treatment and time). The Tukey test
was used to test for differences between chorionated and dechorionated embryos
(P<0.05). Values are presented as means ± s.e.m.
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1 s. In embryos not encased in the chorion (i.e.
dechorionated and hatched) a movement consisted of two body flexions with a
duration of 0.5 s. The magnitude and duration did not appear to differ
between the two groups.
The number of movements was 36-fold higher in dechorionated relative to
chorionated embryos before hatching (–15 to –5 h)
(Fig. 1). Similarly, the number
of movements was ninefold higher in dechorionated compared with chorionated
embryos during hatch (0 h). By 5 h post-hatch, the difference in movement
between the two groups had decreased, but dechorionated embryos continued to
move significantly more (1.5-fold) than embryos whose chorions remained
intact until hatching (chorionated). By 10 h post-hatch there was no
significant difference in movement between the two groups
(Fig. 1). Additionally we
analyzed, but have not shown, movement up to 50 h before hatch and 30 h after
hatch. It is worth noting that differences in movement between chorionated and
dechorionated embryos over the longer term were similar to
Fig. 1 (–15 h to –5
h). Differences in movement between chorionated and dechorionated embryos
15–30 hpost-hatch were similar to the difference reported 10 h
post-hatch in Fig. 1.
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Series IV
The majority of whole embryo (embryonic body plus yolk-sac) protein content
(mg protein individual–1) was present in the yolk fraction
(92%) of rainbow trout embryos around the time of hatching, as expected.
Only a very small portion (8%), of the whole embryo protein content was
present in the embryonic body (Fig.
5A,B). In the embryonic body of chorionated and dechorionated
embryos (highlighted in Fig.
5C), the difference in protein content between the two groups
increased with time so that at 30, 32 and 33 days post-fertilization the
protein content of the dechorionated group was 28–72% greater relative
to the chorionated group.
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60 movements
h–1). Furthermore, after natural hatching the number of
movements increased very rapidly (fourfold increase 5 h post-hatch). Thus, it
appears that the musculoskeletal system is sufficiently developed for frequent
activity before hatching. These results are consistent with previous
observations of spontaneous movements during late embryonic development in
both mammals (Suzue, 1996) and
fish (Proctor et al., 1980;
Peterson and Martin-Robichaud,
1983; Saint-Amant and Drapeau,
1998).
The fact that movement increases so rapidly after hatch in rainbow trout
embryos suggests that movement has an important physiological function. One
possibility is that early movement is linked to further muscle growth and
development. During larval growth (i.e. after hatch) in teleost fish, the red
and white muscle layers of the myotome continue to develop by an increase in
the number (hyperplasia) and diameter (hypertrophy) of muscle fibres
(Nag and Nursall, 1972;
Johnston, 2001). In birds and
mammals, body movements create muscle force, which is necessary for the
development of the muscle fibres
(Vandenburgh et al., 1991).
Hence, it is probable that the movement observed in rainbow trout immediately
after hatch is an important contributor to normal skeletal muscle development.
In contrast, van der Meulen et al. reported that the normal development of
several features of the axial musculature, including the development and
segregation of red and white muscle layers occurred in mutant immobile
zebrafish (van der Meulen et al.,
2005). It would be interesting to know if salmonids are similar to
zebrafish in this regard, or if indeed mobility is necessary for normal muscle
differentiation and development.
Movement also may be important for respiration. Immediately after hatching,
the skin is a major site of gas exchange with oxygen uptake occurring
primarily by diffusion (Rombough and Ure,
1991). Surrounding the embryo is a semi-stagnant region of water,
the boundary layer, where oxygen is depleted and metabolic wastes accumulate
(Rombough, 1988a). Once out of
the chorion, body movements may be an important mechanism for stirring the
water layer immediately surrounding the larva, thereby replenishing the
dissolved oxygen in the boundary layer. However, if movement was critical for
respiration, then intact embryos would be expected to move more because of
lower oxygen tensions surrounding the encapsulated embryo, although direct
measurements have not been reported. Our movement data does not support this
idea and it therefore seems unlikely that movement is related to
respiration.
An additional factor that may play a role in the increased movement in
dechorionated embryos relates to water currents. Rheotaxis is a behavioural
orientation to water currents (Arnold,
1974). It has been reported that water velocities as low as 1 cm
s–1 can be detected and induce subsequent orientation in
plaice (Pleuronectes platessa)
(Arnold, 1969). Furthermore,
positive rheotaxy has been documented in early stages of fish development,
including zebrafish larvae (Bagatto et al.,
2001). Although the water flow in the present experiment was low,
it is possible that the swimming movements demonstrated by dechorionated
embryos were a response to water flow over the embryo that would not be sensed
by chorionated embryos.
Oxygen consumption
Removing the chorion of trout embryos several days before hatching resulted
in an increase in activity but not an increase in oxygen consumption. This is
not consistent with what we predicted, nor does it agree with data in juvenile
and adult fish where an increase in activity is correlated with an increase in
oxygen consumption (e.g. Brett,
1964; Fry, 1971;
van den Thillart, 1986). These
unexpected results are probably not due to measurement error as the oxygen
consumption values for newly hatched larvae in the present study
(2.6±0.15 µmol g–1 h–1) are
comparable to those reported by Weiser
(Weiser et al., 1985) for
newly hatched rainbow trout larvae (4.8±1.1 µmol
g–1 h–1). Weiser's measurements
(Weiser et al., 1985) were
made at 12°C, compared to 10°C used in the present experiments,
possibly accounting for the slightly higher values reported by Weiser
(Weiser et al., 1985).
It is possible that at the stage of development (i.e. around hatch) used in
our study, differences in oxygen consumption between active (dechorionated)
and inactive (chorionated) embryos were difficult to resolve. Bagatto et al.
reported no significant differences in mass-specific routine oxygen
consumption between trained and untrained yolk-sac and swim-up zebrafish, but
at the free-swimming stage, trained zebrafish had a significantly higher
mass-specific routine oxygen consumption compared with controls
(Bagatto et al., 2001). It is
probable that around the time of hatching, the production of new tissue
constituted such a large proportion of the total energy expenditure that
differences in activity between the two groups did not make a measurable
contribution to metabolic rate. After hatch, larval fish often experience
rapid growth (Kamler, 1992),
associated with a high metabolic cost. Jaworski and Kamler reported that yolk
energy was used mainly for tissue growth, while less energy was expended to
fuel metabolism in several different species, including rainbow trout
(Jaworski and Kamler, 2002).
Indeed, growth in trout embryos accounted for more that half (59%) of the
total amount of energy consumed between fertilization and 90% yolk absorption
(Rombough, 1988b). Finally,
van der Meulen et al. assayed the expression of several genes involved in
energy metabolism in immobile zebrafish embryos
(van der Meulen et al., 2005)
and concluded the energy metabolism in immobile embryos was not greatly
affected by a lack of muscle activity (i.e. cellular energy metabolism was
similar in active and inactive, immobile zebrafish embryos). Taken together,
our study and others, suggest that, movement in fish embryos has a trivial
metabolic cost relative to the cost of growth.
It should be noted that there may be species differences. For example,
Davenport and Lönning reported significantly higher oxygen concentration
in active, unanaesthetized G. morhua larvae compared with inactive,
anaesthetized larvae immediately after hatching
(Davenport and Lönning,
1980). G. morhua larvae are pelagic, however and will
swim to the top of the water column immediately after hatching. In contrast,
trout will remain at the bottom of the water column for several weeks after
hatching.
Growth
We predicted that the increased movement of dechorionated embryos would
result in accelerated growth and yolk utilization. Indeed, the dry mass of the
embryonic body of the dechorionated embryos was significantly higher than that
of chorionated embryos. This is consistent with the improved growth observed
in adult salmonids during exercise training
(Davison and Goldspink, 1977;
Johnston and Moon, 1980;
Houlihan and Laurent, 1987;
Totland et al., 1987;
Bugeon et al., 2003). Forced
exercise is known to be a powerful stimulus for the hypertrophy of both red
and white fibres in fish (Johnston and
Moon, 1980; Totland et al.,
1987) and an increase in fibre number in red muscle
(Sänger and Stoiber,
2001). As a consequence, there is an increase in the total muscle
cross-sectional area, thus contributing to whole-animal growth. It should be
pointed out that in preliminary experiments, we found no significant
differences in length between chorionated and dechorionated embryos and so
length was not measured in subsequent experiments.
Only a few studies have considered the effects of movement on growth and
muscle development in the early life stages of fish and to our knowledge our
study is the first to report a positive relationship between activity and
growth. In contrast, van der Meulen et al. reported that muscle development
was not grossly affected by decreased muscle activity in immobile zebrafish
embryos (van der Meulen et al.,
2005). Two studies reported that growth was actually improved in
`immobile' S. salar larvae, compared with `mobile' larvae
(Hansen and Møller,
1985; Peterson and
Martin-Robichaud, 1995); however, movement was not quantified.
Swim training in larval zebrafish (D. rerio) did not improve growth
(Bagatto et al., 2001). The
differences between our findings and the above studies may relate to species
differences, as well as differences in experimental protocols.
Factors other than exercise are known to contribute to growth in fish, and
possibly contributed to the increased growth observed in dechorionated embryos
and larvae in the present study. Some of these include hormones
(Sumpter, 1992) and oxygen
tension (Matschak et al.,
1998). The lack of measured difference in oxygen consumption
between chorionated and dechorionated embryos in the present study eliminates
oxygen as a key factor. The most notable growth factors expressed during early
development include insulin, IGF-I (insulin-like growth factor I), IGF-II,
growth hormone and thyroid hormone (for a review, see
Johnston, 2001). It may be
possible that removal of the chorion initiates changes in the levels of one or
all of these hormones, thus affecting growth.
The accelerated growth in dechorionated embryos persisted post-hatch up to
45 d.p.f., but by 50 d.p.f. (first-feeding in the present experiment) there
were no differences in body mass between the two groups. These findings are
consistent with others (Geffen,
2002), who found that smaller, early hatching herring larvae
catch-up with larger, late hatching larvae by the time of first feeding. First
feeding has been described as a `critical period', associated with high
mortality (Blaxter, 1988).
Therefore, there is a high survival value placed on attaining the largest
possible size at this time because larger fish are stronger swimmers, are
better able to catch prey and are less susceptible to predation
(Blaxter, 1988). Indeed,
between 45 and 50 d.p.f. the growth rate was greater in the smaller
`chorionated' compared with the larger `dechorionated' fish (2.06 mg
day–1 vs 1.81 mg day–1,
respectively). It is well established that developmental rate is under strong
genetic control (Blaxter,
1988). It is therefore possible that once the activity level
between the two groups was similar, the growth rate of dechorionated fish
slowed relative to chorionated fish, resulting in an optimum body size at
first-feeding.
The findings of the present study support the prediction that dechorionated
embryos convert yolk protein into embryonic tissue at a faster rate than
chorionated embryos. After hatching, the protein content of the embryonic body
tissue steadily rises, whereas the protein content of the yolk decreases
(Rønnestad et al.,
1993; Terjesen et al.,
1997). This process was accelerated in dechorionated embryos in
the present study, as tissue protein content was significantly higher relative
to chorionated embryos. Rates of yolk reabsorption and protein synthesis are
influenced by extrinsic factors, such as temperature
(Mathers et al., 1993;
Peterson and Martin-Robichaud,
1995). The present study indicates that activity also influences
the rate at which yolk proteins are converted into body tissue.
It should be noted that there were no differences detected in the protein
content of the yolk between chorionated and dechorionated embryos over the
short time scale investigated (around the time of hatching). The yolk protein
content was 20 times greater than the protein content of the embryonic
body. It would be difficult, therefore, to resolve small differences in yolk
protein content if present, whereas it was much easier to distinguish small
differences in the protein content of the embryonic body.
One obvious question arising from the present study is why have a chorion?
Our results imply that early removal of the chorion may be beneficial given
that larger fish are stronger swimmers, are better able to catch prey, and are
less susceptible to predation (Blaxter,
1988). Salmonid embryos in the wild develop buried in gravel in
flowing rivers and streams, where protection against mechanical damage
provided by the chorion is especially important. In our laboratory study,
embryos were not subjected to the same extrinsic conditions relative to their
wild counterparts. It is possible, therefore, that if developmental time
within the chorion were reduced, increased mortality may occur as a result of
mechanical stresses in the natural environment. It is probable that the
developmental time spent within the chorion is a trade-off between optimizing
protection and attaining the largest possible size at hatch. Our results also
showed that although there were short-term effects on growth by early removal
of the chorion, these effects did not persist in the long-term (i.e. up to
first feeding). In the long-term, therefore, early removal of the chorion may
afford no advantage to the animal.
In summary, we have demonstrated that the chorion restricts movement before the natural time of hatching in rainbow trout embryos. The relatively high level of activity observed immediately after manual removal of the chorion indicates that trout skeletal muscle is sufficiently developed to sustain frequent exercise several days prior to hatch. Removal of the chorion and increased movement were correlated with increased growth rate and protein content, but not oxygen consumption. The influence of pre-hatch activity had a finite impact on larval growth, as the difference between the two groups (chorionated and dechorionated embryos) disappeared by 50 d.p.f. These findings demonstrate that early growth and development in trout is a complex interplay between intrinsic (e.g. genetic) and extrinsic (e.g. exercise) factors.
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References |
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