Syddansk Universitet
The consistent, non-destructive measurement of small proteiform aquatic animals,
with application to the size and growth of hydra
Levitis, Daniel; Goldstein, Josephine
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Marine and Freshwater Research
Publication date:
2013
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Citation for pulished version (APA):
Levitis, D., & Goldstein, J. (2013). The consistent, non-destructive measurement of small proteiform aquatic
animals, with application to the size and growth of hydra. Marine and Freshwater Research, 64(4), 332–339.
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CSIRO PUBLISHING
Marine and Freshwater Research, 2013, 64, 332–339
http://dx.doi.org/10.1071/MF12220
The consistent, non-destructive measurement of small
proteiform aquatic animals, with application
to the size and growth of hydra
Daniel A. Levitis A,B and Josephine Goldstein A
A
Max-Planck Odense Center on the Biodemography of Aging, Institute of Biology,
University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark.
B
Corresponding author. Email: [email protected]
Abstract. Hydra (Cnidaria), the basal metazoan most often studied in cellular, molecular and developmental biology, is
difficult to measure because it is small, proteiform and aquatic. To facilitate broader organismal and ecological study of
Hydra, we evaluated three methods, whereby a polyp’s body column can be measured by means of photomicroscopy. The
volume, cylindrical surface area and surface area corrected for changes in body shape were all highly repeatable methods
(r . 0.97) when shape varied little. However, shape changes altered volume and cylindrical surface area. Repeated
corrected surface-area measurements of the same individuals in markedly different positions yielded standard deviations
,5% of the mean measured area. This easy, non-lethal means of individual size measurement explicitly accounts for the
flexible morphology of a polyp’s hydrostatic skeleton. It therefore allows for the elucidation of how growth and size vary
over time, age and food intake. We found that hydra changed size dramatically from day to day, and that although the food
level influenced adult size, it had little effect on the early growth of recently detached buds. Finally, we discuss ecological
and biological applications of this method.
Additional keywords: food quantity, freshwater polyp, Hydra, individual size, photomicroscopy.
Received 15 August 2012, accepted 2 February 2013, published online 10 April 2013
Introduction
Size is a key variable in understanding the demography
(Kirkpatrick 1984), ecology (Peters 1986; White et al. 2007) and
physiology (West et al. 2003) of an organism. Accurate methods
for measuring the sizes of individuals, particularly of model
organisms, are therefore vital. For many organisms, size can be
measured simply, based on weight, the linear distance between
two fixed points, the area of a fixed surface, or the volume of
fluid displaced. However, many aquatic invertebrates, particularly within the basal metazoans, have no fixed distance points
or surfaces, being proteiform (of variable, dynamic morphology
as a consequence of lacking hard pieces). We, therefore, set out
to develop a non-lethal method for measuring body size of the
proteiform animal genus most often studied in laboratories,
namely Hydra. Hydra are common subjects in fields such as
biodemography (Martı́nez 2002), developmental biology
(Shenk et al. 1993) and ecotoxicology (Karntanut and Pascoe
2002), where proven, precise and non-invasive methods of size
measurement are called for. No such approach that allows for
consistent results despite change in shape, has been available.
The only available methods for measuring a hydra polyp
involve measurement of length (Massaro and Rocha 2008),
calculation of surface area (Kaliszewicz 2011; Kaliszewicz
and Lipińska 2011), autoradiographic cell counting from a
macerated and stained individual (Otto and Campbell 1977) or
Journal compilation Ó CSIRO 2013
weighing of dry mass (Stiven 1965). These methods each
present significant shortcomings. Hydra are completely lacking
in hard parts, and can change their body length several fold in
less than a second. Shape changes result in bunching or stretching of the body wall, thus changing the estimated surface area of
the hydra (Fig. 1). These animals quickly die if dehydrated and
are too small for volume measurement through fluid displacement. Moreover, changes in shape lead to changes in body
volume, because of fluid movement into and out of the body
cavity. Hydra possess minute dry mass, such that the weighing
of dried individuals is impractical for most laboratories. Cell
counting is lethal, time consuming and leaves open the question
of how an individual’s physical size is related to its number of
cells.
To have a simple, consistent way of measuring the size of a
hydra polyp would open many avenues of research not otherwise
practicable. For example, it would be easier to compare between
studies using hydra for ecotoxicology (e.g. Quinn et al. 2012) if
we knew precisely the sizes of individuals used in each population and whether size influences lethal dosage, as it does in many
other organisms. Studies looking at predation by hydra polyps
(e.g. Elliott et al. 1997) could benefit from knowing how large
the predators are. Even for field studies, where the same
individual cannot be found and measured repeatedly, monitoring the change in the size distribution of a population of polyps
www.publish.csiro.au/journals/mfr
Measurement of hydra
Marine and Freshwater Research
Surface area (mm2)
3.0
2.8
2.6
2.4
2.2
5
10
15
Elongation (length/width)
Fig. 1. Cylindrical surface area of an individual hydra increases as it
elongates itself. Measured body-column surface area (according to the
method of Kaliszewicz (2011)) of a single individual at different elongations
E (length/width) illustrates how cylindrical surface area changes with a
change in the body shape. Red dots are data points overlaid on silhouettes
from each photo.
would allow greater ecological understanding of what factors
influence the ability of these polyps to feed, grow and maintain
their physiology. Photomicroscopy makes this fast and easy for
the first time.
In the current paper, we present and evaluate a method for
measuring the cylindrical body-column surface area (excluding
the hypostome and reproductive protuberances) of hydra by
using photomicroscopy and freely available software, while
correcting for the bunching and stretching that occurs as hydra
change shape. Although the body of the individual is variable in
both length and radius, it is frequently a good approximation of a
deformed cylinder. A photograph meeting requirements discussed below allows measurement of both the length and crosssectional area of the cylinder, allowing for the calculation of the
surface area. When the radius shrinks, the length increases
enough so that the surface area increases slightly. The ratio of
the length over the mean width of the column (calculated from
cross-sectional area) provides a measure of the elongation of the
individual in a particular photo; a correction can then be applied
to remove the influence of elongation on surface area. This
correction is necessary, we believe, because folds and ridges in
the ectoderm appear as the polyp contracts, obscuring part of its
surface area. Such ridges are clearly visible when watching live
individuals change shape under the dissecting microscope or in
scanning electron microscope (SEM) photographs (Bolzer et al.
1994). Modelling the body column as a cylinder ignores this
unevenness, and therefore underestimates the true surface area
by an amount that increases as the bunching does. As such, our
corrected surface area is a standardised size measure designed to
stay relatively constant with changes in shape, even as measured
length, width, volume and cylindrical surface area co-vary.
Neither the cylindrical surface area nor the corrected surface
area is likely to capture the true body-column surface area of a
hydra individual exactly. Determining which of the two more
closely approximates a true value would require defining and
then measuring the exact surface area of every detail of the
333
polyp’s body column. This is distinct from our goal of producing
a precise measure robust to change in shape. Indeed, an absolute
surface area could not be determined even with the most
advanced three dimensional measurements, because the absolute surface area would increase with the resolution of the
measurements, and to a lesser extent, with change in shape.
Our initial tests of corrected surface area, described below, lead
us to believe it is both precise and accurate enough for most
applications in ecology and beyond.
Here, we describe the apparatus and procedure employed,
and compare the repeatability and robustness of this method to
change in shape with those of other photomicroscopic measures,
and apply the new method to the following two ecological
questions on hydra: how does the size of an individual vary
over time and age; and how does food intake influence the size of
adult and juvenile polyps? We also discuss probable sources of
error in our approach and then move onto what our initial
applications of this method reveal about the biology of hydra,
and what other questions it may be useful in examining.
Materials and methods
Polyp cultivation
We studied freshwater polyps of the species Hydra vulgaris
of hybrid backcross strain AEP (Martı́nez et al. 2010),
obtained from the laboratory of Dr Daniel Martı́nez in Pomona,
CA, USA, and followed culture procedures from that laboratory
(Martı́nez 1998; Martı́nez and Bridge 2012). We placed
randomly chosen individual adult polyps (body length
(mean s.d.) when elongate of 2.7 mm 1.0 mm) into separate
wells of six-well plates with 6 mL of medium each. Additionally, we tracked the growth of buds from those individuals from
first sign of budding on the mother polyp, until they had separated from the mother and started to reproduce (initiated the
formation of a bud or a gonad). Polyps were maintained in an
incubator at 188C and a 12 h light/12 h dark cycle. Each polyp
was fed three freshly hatched nauplii of Artemia salina three
times a week, except where otherwise noted.
Photomicroscopic analyses
For size measurement, we took dark-field photographs of polyps
daily under the stereomicroscope (Leica M125, Leica Microsystems, Wetzlar, Germany) with an integrated camera system
(Leica DFC295, resolution 2048 1536 pixels, full-frame HQ)
at the highest magnification that included the entire elongated
polyp in the photograph (generally 12.5 magnification for
adult polyps). On feeding days, photographs were taken before
feeding, so that the volume of ingested food did not directly
manipulate polyp sizes. So as to photograph all polyps with the
same focal length and make sure body columns were perpendicular to the line of sight, we pushed each polyp to the bottom of
the well with a custom-built perspex coverslip. This was cut
from the lid of a 12-well plate, affixed with cyanoacrylate glue to
a plastic-coated wire handle and then soaked in excess medium
to remove any soluble glue components before use. A protruding
circle on the underside of the well-plate lid was employed as a
0.32-mm-high foot around the exterior of each coverslip, to
make the animal lie flat without being squashed.
334
Marine and Freshwater Research
D. A. Levitis and J. Goldstein
(a)
(b)
1 mm
(c)
1 mm
(d )
Length I
Area A
1 mm
Fig. 2. Hydra is proteiform, which complicates measurement. Shape change can be described in terms of change in elongation (denoted E, length over
width, see Eqn (4)). (a) Contracted hydra individual (E ¼ 1.3); (b) hydra in elongate position (E ¼ 17.2); (c) semi-elongate hydra individual (E ¼ 12.4),
with bud and tentacles separated from the body by superimposed black lines to allow for automatic selection of pixels representing the body column for
measurement of area; (d) enhanced detail of measured body column from C, showing length and cross-sectional area, from which the size of the body
column was calculated.
Photographs of the polyps were analysed in the freeware
program ImageJ (Rasband 1997). To minimise measurement
error, photographs of polyps standing partially upright, those
whose body columns crossed over themselves and those otherwise obscured from view were excluded from analysis. Polyps
that contracted enough to be more spherical than cylindrical
(Fig. 2a) were also excluded, although this later proved to be
unnecessary. Only pictures of elongate (Fig. 2b) or semielongate (Fig. 2c) polyps were used. Hypostomes, including
tentacles, as well as reproductive protuberances (buds or
gonads) were separated from the body stalk by digitally drawing
black lines (strength 4 pixels) on the photographs. Thereafter,
the visual area of the body column (Fig. 2d) was measured
by automatically selecting and counting the contiguous lightcoloured pixels representing the body column of the hydra
(using a tracing tool in legacy mode, with tolerance value ¼
140). A 1-pixel-thick curving line was drawn to measure the
length in the centre of the body column. Sample sizes are given
in Table 1.
Table 1. Sample sizes by life stage and food treatment
Life stage
Adult
Adult
Adult
Adult
Adult
Bud
Bud
Bud
Food quantity
(nauplii per week)
Number of
individuals
Mean number of
measurements per
individual (s.d.)
0
6
9
12
24
0
9
30
5
5
15
6
3
8
29
5
15.8 (7.1)
31.8 (0.5)
7.0 (12.0)
28.3 (5.5)
31.7 (0.6)
22.3 (8.2)
20.2 (8.9)
29.0 (0.7)
Assuming that the body shape of a polyp is cylindrical after
tentacle crown and reproductive parts are excluded, the cylindrical surface area of the body column of an individual hydra
(Kaliszewicz 2011) can be expressed by the equation
Measurement of hydra
Marine and Freshwater Research
SA ¼ p l w;
ð1Þ
where SA is the calculated cylindrical body-column surface area
of the individual, w is the mean width (diameter) of the stalk on
the photo, and l is the length of the central line of the stalk. The
mean width, w, was calculated from photomicroscopically
measured cross-sectional area A and length l (Fig. 2d), as
follows:
w ¼ A=l:
ð2Þ
Volume V of the body column was then calculated as
V ¼ p ðw=2Þ2 l
ð3Þ
and body elongation E as
E ¼ l=w:
ð4Þ
We then introduced the corrected surface area, cSA, which
was calculated as
cSA ¼ SA þ SA=c E;
ð5Þ
where c is a correction factor that controls how big the effect of
elongation is on the calculated value of cSA. Larger values of c
lead to smaller differences between cSA and SA. The value of c
for any population should be set such that an initial series of
photos of the same individual on the same day taken at different
elongations yield no significant correlation between E and cSA.
Note that the value of c is likely to vary among populations, and
therefore no general value of c can be given. As described below,
we use a c of 5.
Comparison of measurements with change in shape
To evaluate the effect of changes in body shape on each size
measure, we measured nine individuals each in six very different
positions, from very elongate (E ¼ 7–20) to very contracted
(E ¼ 0.5–2). Series were taken over the span of minutes to
minimise the actual change in size. We then calculated the
coefficient of variation (CV, the standard deviation over the
mean) in V, SA and cSA for each individual and a mean and
standard deviation of these CV values for each measure.
Repeatability
To test investigated size measures described above (V, SA, and
cSA) for repeatability, 10 randomly chosen adult polyps were
each photographed five times within the course of 5 min in
similar positions. Polyp volumes were analysed in ImageJ and
repeatability was calculated as described by Lessells and Boag
(1987) from a one-way ANOVA.
Variation over time and age
We examined how individual cSAs change over time by
examining measurements on a set of individuals, each of which
had been measured on at least six separate days. Variation in
measured size can be caused by an actual change in size, but also
by measurement error. To model this process, we fit splines
335
(piecewise smooth polynomial functions) to the calculated cSAs
of each of eight individuals over time, by using the smooth.
spline function of package stats in statistical programming
language R (R Development Core Team 2011). The spline
output by this function depends on the smoothing parameter
(spar). A high spar value gives a relatively straight line, and a
loose fit to the data, whereas a low spar yields a spline that fits
the data points very exactly, which is only justifiable if measurement error is negligible. We used a two-step process to
choose a reasonable common spar to use for all individuals.
First, individual spar values were calculated for each individual
using the default method, which chooses spar on the basis of the
degrees of freedom. Second, the mean of these individual spar
values was used in refitting splines to each individual. The spline
fit to each individual’s sizes over time in this way was taken as a
rough estimate of how the individual’s size actually changed,
and the residuals were therefore taken as rough estimates of the
error in each measurement.
The effect of food on corrected surface area
Although it has previously been shown that an individual hydra
increases in cell number when fed more (Otto and Campbell
1977), the effect of food on the physical size is not well documented. All of our adult polyps were initially fed nine nauplii a
week, and then subgroups of individuals were switched to 0, 6, 9,
12 or 24 nauplii per week and their corrected surface area was
tracked over the following two months. Similarly, groups of
newly detached buds were fed with 0, 9 or 30 nauplii per week
(Table 1). We then tested whether buds grew faster and reached
a larger size when exposed to higher food availability, and
whether adults increased in size in response to more food.
Results
Photomicroscopy
With practice, a careful worker can obtain photos of Hydra
highly suitable for use with this method at a rate of roughly 1
polyp per minute. The majority of this time is spent waiting for
polyps to elongate fully, which is unnecessary when measuring
the corrected surface area (see below). The use of unscratched
well plates and coverslips greatly improves the usefulness of the
photos. Processing and calculating sizes from these photos also
takes time, although no more than a minute per photo should be
needed. Individuals measured varied from 0.1 to 6.2 mm2 in the
corrected surface area, with a mean adult size of 2.5 mm2.
Comparison of measurements with change in shape
Using series of photographs designed to represent the range
of elongations a polyp is capable of achieving (Emax . 20,
Emin , 1), a correction factor, c, of 5 (determined with a subset
of the data by calculating correlation coefficients for values
from 0 to 10 by steps of 0.1) yielded cSA values uncorrelated
with elongation. Using this value, cSA of each individual varied
little with shape (mean of CVs ¼ 0.049, standard deviation of
CVs ¼ 0.016) (Table 2). Uncorrected surface area was the best
alternative to cSA, performing slightly better for some individuals, but much worse for others. Body-column volume changed
markedly with elongation, suggesting fluid exchange either with
the tentacles or through the mouth. Changes in shape were
336
Marine and Freshwater Research
D. A. Levitis and J. Goldstein
Table 2. Summary measures of self-consistency of three methods of
size measurement
Repeatability (relongate) of size measurement when shape is consistent and
mean coefficients of variation for polyps with elongate shape (CVelongate)
and changing shape (CVchanging) are shown for volume (V), cylindrical
surface area (SA) and corrected surface area (cSA)
Measure
relongate
CVelongate
CVchanging s.d.
0.972
0.973
0.977
0.088
0.045
0.042
0.384 0.104
0.060 0.025
0.041 0.017
3
V (mm )
SA (mm2)
cSA (mm2)
area each day. The splines suggest that 52% of the apparent
variance in size over time is due to actual change, with the other
48% being due to measurement error. However, these splines in
providing very conservative estimates of daily change, will tend
to overestimate measurement error. The mean squared residual
from each measurement to the spline was 0.11, implying a
residual error of 0.33 mm2, or roughly 13% of the area of an
average individual. This is a much greater degree of measurement error than we would infer from the CVs calculated above,
which estimated less than 3% error. If we instead assume that the
measured values are correct (and measurement error is negligible), the mean daily change in surface area is 21%. The true
value is likely to be between 7% and 21%.
Corrected surface area (mm2)
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1
2
3
4
5
6
7
8
9
10
Polyp number
Fig. 3. Corrected surface areas of 10 adult hydra polyps measured
repeatedly (n ¼ 5) in elongate position. Many pairs of individuals are clearly
distinguishable on the basis of size, despite being from the same clonal line,
and receiving identical care.
observed to influence even cSA somewhat, although not in any
predictable manner.
Repeatability
Although cSA yielded the lowest coefficients of variation (Tab.
2) with both changing and unchanging shapes, we found all three
measures to be highly repeatable (r . 0.97) when using photographs of only extremely elongate individuals (i.e. when shape
did not greatly vary), all taken within the same 5 min by the same
scientist and with the same apparatus. Indeed, when shape varies
only slightly, simple methods such as measurement of length
will also be repeatable. Constant shape allowed for clear discrimination between individuals of only moderately different
sizes (e.g. polyps No. 6 and No. 9 in Fig. 3), but not very similar
individuals (e.g. polyps No. 8 and No. 9).
Variation over time
Splines fitted to the data (with mean spar ¼ 0.49, Fig. 4) give a
sense of how each individual is likely to be changing over time.
If we assume that the splines are good estimates of the actual
change over time in the individuals, we can calculate that the
average adult polyp gains or loses more than 7% of its surface
The effect of food on volume
Adult corrected surface area changed significantly depending on
food level (repeated-measures ANOVA, F ¼ 105.05, d.f. ¼ 754,
P ¼ 2.2e–16; for sample sizes see Tab. 1), with individuals
switched to more than 9 nauplii per week increasing in size and
those not fed shrinking dramatically (Fig. 5a). Each group took
roughly 20 days to reach a final mean size, with those fed 24
nauplii per week reaching 3.19 mm2. Hydra individuals raised in
lower food treatments, being fed 12, 9, 6 and 0 nauplii per week,
revealed an average size of 2.86 mm2, 2.54 mm2, 2.61 mm2 and
0.34 mm2, respectively. Starvation led to severe declines in the
corrected surface area, with most individuals less than a quarter
of their original size after 1 month.
In contrast, we observed significant growth after detachment
from the parent by buds of all food treatments, including the zero
food treatment (Fig. 5b). Indeed, growth in corrected surface
area showed no significant dependence on food availability in
buds in their first 3 weeks (repeated-measures ANOVA,
d.f. ¼ 468, F(feeding) ¼ 2.66, P(feeding) ¼ 0.104, F(age) ¼
767.60, P(age) ¼ 2.0e–16). Nevertheless, we observed a sharp
decline in size by unfed detached buds after 3 weeks.
Discussion
Corrected surface area and cylindrical surface area are far less
dependent on changes in the body shape than are other measures,
suggesting that the epithelium of the hydra stays relatively
constant in size, even as its length, width, shape and volume
change. Using corrected surface area minimises the measurement error associated with the change in the shape of an individual and furthermore allows for the detailed study of the size
of the individual and its dependence on factors such as age, time
and food. This provides for the low-cost, low-effort and noninvasive measurement of small, aquatic, proteiform and translucent organisms such as hydra.
Sources of error
In developing the present method, we made every effort to
reduce measurement error. Nonetheless, several sources of
within-individual variation remain. The cylindrical and actual
surface area of the body column of an individual may change
significantly, even within a few seconds, as the individual
contracts or elongates its hydrostatic skeleton. Our corrected
surface area is designed to correct for resulting changes in the
ectoderm of the hydra polyp; however, because the changes may
Measurement of hydra
Marine and Freshwater Research
2.93
3.86
1.40
2.15
Corrected surface area (mm2)
1
34
4.37
3.86
1.94
1.21
1
64
4.20
4.63
0.66
1.54
1
64
3.83
2.87
1.57
0.64
1
64
1
18
1
64
1
64
1
59
337
Days
Fig. 4. Corrected surface area of an individual hydra varies considerably from day to day. Eight individuals were measured
repeatedly (circles) over the course of 2 months and smoothing splines were fitted to these values (red lines). Two- to three-fold
changes in size over the course of short time periods were observed repeatedly, with no obvious relationship to the timing of
budding. Food intake was kept at nine nauplii per week before and during this period.
be non-linear, the correction is not perfect. Changes over longer
time spans may be attributable to reproduction, feeding, injury,
sloughing of cells or regrowth of lost tissue. Changes in the bodyposition may also impose measurement errors; we modelled all
body columns as cylinders; however, some elongated bodies
will fit that model more exactly than others. Similarly, a hydra
individual lying less flat or out of focus will appear to have a
smaller surface area. These errors can be minimised through
338
Marine and Freshwater Research
D. A. Levitis and J. Goldstein
Nauplii per week
(a) 5
24
12
9
6
0
4
3
Corrected surface area (mm2)
2
1
0
0
10
20
30
40
50
60
70
(b) 5
Nauplii per week
30
9
0
4
3
2
1
0
0
10
20
30
40
50
60
Days
Fig. 5. Size changes over time after changes in food level. Average corrected surface area (cSA) of (a) adults and (b) detached buds of
Hydra vulgaris subjected to different feeding treatments. All individuals were fed nine nauplii per week until Day zero. Where unfed
adults quickly begin to decrease in size, unfed detached buds grow for 3 weeks before beginning to decline in size. Food supply makes a
small but significant difference to adult size.
careful positioning under the camera. The correction factor
chosen to account for the effect of elongation may also impose
errors, if a particular correction factor is not well suited to an
individual, or if the surface area changes in ways inconsistent
with the model.
Further sources of error arise in processing photos. The
correct set of pixels must be selected in measuring the area,
and the line drawn to measure the length of the column must
smoothly follow the middle-line of the body. Black lines drawn
to separate the tentacles from the column must be placed so as to
exclude the base of the tentacles. Spots and scratches on the
apparatus, tentacles lying across the body and reproductive
protuberances must be separated from the body with additional
black lines, without obscuring the body column.
Initial applications of the method
Hydra do not show increasing mortality or decreasing fertility
with age, meaning that unlike many other species, older adults
are not more likely to die, or less likely to reproduce (Martı́nez
1998, 2002; Martı́nez and Bridge 2012) and it is therefore easy
to fall into the trap of thinking of adult hydra as unchanging over
time. However, our results demonstrated that each individual
polyp changes its body-column size considerably from one day
to the next, even when its environment is kept constant.
Although some of this change may be attributable to reproductive effort (production of buds, eggs and testes) or to time
since last meal, much of it seems to be stochastic. This implies
that Hydra avoids senescence not through stasis, but rather
through rapid change (Bosch et al. 2010). This is consistent with
models that explain the non-senescence of hydra in terms of
rapid cell turnover and selective loss of damaged or mutated
cells. An organism losing and gaining size as rapidly as a hydra
polyp does may thereby continuously erase any physiological
information by which age could influence demographic traits.
The method presented here makes it possible to integrate size
measurements into studies of hydra resource usage and
demography.
Although size was correlated with food intake in our adult
samples, we observed rapid growth in recently detached buds,
regardless of food level, which reversed only after 3 weeks in the
unfed group. That we detected significant growth even in buds
provided no food at all suggested that hydra buds, while still
attached, are provided with some concentrated resource analogous to yolk, on which they can draw to provide for growth for
Measurement of hydra
Marine and Freshwater Research
some weeks after detaching from the parent. Visual inspection
of buds at different stages of growth revealed no obvious
discoloration or anatomical feature explaining this phenomenon. This will require further investigation, both to better
document the phenomenon with increased sample sizes, and
to identify potential mechanisms. This also points to an additional hypothesis as to how adults lose and regain size so rapidly:
They may be shifting between concentrated and more diffuse
distributions of internal resources without gaining or losing dry
mass or nutrient content.
Further applications
Hydra is far from the only organism that can be measured in
this way, and our observations regarding hydra are likely to be
true for many other polyps. Our approach can be adapted to
study the distinct life stages of other Cnidarians (planula larvae, polyps and medusae) and could be extended to study other
invertebrate taxa possessing hydrostatic skeletons (e.g. annelids, nematodes or molluscs). Size is likely to be as important a
variable for proteiform animals as it is for arthropods and
vertebrates.
Hydra are common in freshwater bodies across much of
the world, and are a model system in a growing number of
fields, including subdisciplines of ecology such as ecotoxicology and biodemography (Galliot 2012); however, these
organisms have been the subject of surprisingly few field
studies (Bell and Wolfe 1985). Lack of knowledge about the
field ecology of our model organisms for molecular and
cellular biology is a common problem, Caenorhabditis elegans being perhaps the best example of this. Hydra has
become a common subject for studies at the suborganismal
level, providing tools for anyone studying hydra, and also
increasing the need to understand how it functions as an
organism in its community. One barrier to progress on this
front is the relative difficulty of finding ecologically informative ways to measure individual polyps. We encourage
ecologists to study hydra and, in doing so, to consider the
importance of size as a variable.
Acknowledgements
We thank Daniel Martı́nez, Iris Levitis, Annekatrin Friedrich, Beate
Proske, Maria Peix, Wiebke Jahn, Katja Krause, Paul Dunn, Beatriz
Cotarelo Muñoz, Uta Cleven, Antje Storek-Langbein, Ralf Schaible and
Felix Ringelhan for invaluable help and encouragement. We also thank
two anonymous reviewers for improving an earlier draft. This work was
funded by the Max Planck Society and the Max-Planck Odense Center
on the Biodemography of Aging, a collaboration of the Max Planck
Society for the Advancement of Science and the University of Southern
Denmark.
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