LIFE CYCLE, REPRODUCTIVE PATTERNS AND THEIR YEAR-TO-YEAR VARIATION IN A FIELD POPULATION OF THE WOLF SPIDER PIRATA PIRATICUS (ARANEAE, LYCOSIDAE)

9 Pages·2003·9.4 MB·English

by  Frederik Hendrickx

2003. The Journal of Arachnology 31:331-339
LIFE CYCLE, REPRODUCTIVE PATTERNS AND THEIR
YEAR-TO-YEAR VARIATION IN A FIELD POPULATION
OF THE WOLF SPIDER PIRATA PIRATICUS
(ARANEAE, LYCOSIDAE)
Frederik Hendrickx: Unit of Animal Ecology, Department Biology, Ghent
University, K.L. Ledeganckstraat 35, 9000 Ghent, Belgium. E-mail:
[email protected]
Jean-Pierre Maelfait: Institute of Nature Conservation, Kliniekstraat 25, 1070
Brussels, Belgium and Unit of Animal Ecology, Department Biology, Ghent
University, K.L. Ledeganckstraat 35, 9000 Ghent, Belgium
ABSTRACT. Patterns of growth, phenology and reproduction were studied in a field population ofthe
wolf spider Pirata piraticus from November 1997 until October 1998 and in June 1999 to unravel the
intrapopulation variation and co-variation ofthese traits. Individuals ofP. piraticus overwinterasjuveniles
of different instars while adults were found from the end of April until September. Strong year to year
variation in the age and size ofoverwinteringjuveniles was present, resulting in acorrespondingdifference
in adult size in the subsequent breeding season. The main period ofreproduction occurred from May until
August with larger individuals breeding earlier in the season. The size at which adults breed was also
significantly different in the successive years. Clutch mass (cocoon mass), clutch volume and fecundity
are dependent on the size of the female according to a weakly negative allometric relationship. The
differences in those reproductive traits between the succesive years are therefore proportionate to the
differences in female size. This was in clear contrast to egg size, a life history trait that shows much less
variation and appears to be independent offemale size. Therefore, egg size was not significantly different
between spring 1998 and spring 1999. There is, however, some variation in fecundity due to egg size and
number independent of female size. When corrected for female size, females with larger eggs produce
relatively fewer eggs indicating a trade-offbetween these two reproductive characters.
Keywords: Life history, reproduction, egg size, fecundity, year-to-year variation, Pirata piraticus
In wolfspider populations, considerable var- The life-cycle of a field population of this ly-
iation can be observed in phenology (life cycle cosid in Denmark has already been analysed
timing), growth rate, adult size and reproduc- by Toft (1979), who demonstrated that adults
tive output (e.g. Petersen 1950; Edgar 1972; of this species appear in spring and females
Kessler 1973; Humphreys 1976; Toft 1979; Al- produce one or (possibly) two egg sacs in
derweireldt & Maelfait 1988; Simpson 1993; summer. The hatched juveniles grow during
Maelfait & Hendrickx 1998; Samu et al 1998; summer and autumn and overwinter as sexu-
Buddie 2000). Most ofthese studies dealt with ally differntiated juveniles or subadults. Ju-
only one life history trait and did not look for veniles that are bom at the end ofthe summer
interrelations between these traits. However, overwinter a first time as juveniles (not yet
the variation and co-variation of these traits is sexually differentiated) and a second time dur-
ofparticular importance to understandthe costs ing the subsequent winter before they reach
and benefits of a specific life history trait the adult stage in May. Laboratory experi-
(Steams 1992; Roff 1992) and can be used to ments conducted by Schaeffer (1976a, b) re-
predict changes in life history patterns when vealed that temperature as well as photoperiod
environmental conditions change. are important factors that determine growth
In this study we analyzed the life-history of and development of this species. Data about
a field population of a common wolf spider intrapopulation size differences, year-to-year
Pirata piraticus (Clerck 1757) in Belgium. variation, and the variation and covariation of
331
332 THE JOURNAL OF ARACHNOLOGY
adult and reproductive traits are, however, its widest point (Hagstrum 1971; Alderwei-
&
lacking. This information might be of impor- reldt Maelfait 1988), to the nearest 0.03
mm
tance to understanding the evolution of the using a graticule eyepiece fitted to a Wild
life-history pattern of this species and the in- stereomicroscope —
terpopulation variation therein. Reproductive traits. Patterns of repro-
duction were analysed on females originating
METHODS
— from the sampling campaign conducted on
Collection of the animals. All animals 17June 1998 and on females from a second
were captured from the same locality, a tidal sample, conducted on 3 June 1999 during
marsh (Galgenschoor) situated north of the which only females with an egg sac were col-
city ofAntwerp (Flanders, Belgium) along the lected to study year to year variation in repro-
tidal river Schelde (SHIS' N, 4°18' E). The ductive characteristics. The number ofeggs or
vegetation consisted mainly of common reed young present in the egg sac was taken as a
(Phragmithes australis). Within this tidal measure of fecundity. To obtain a measure of
marsh, the same area of approximately 10 by clutch mass, the egg sac (including eggs and/
m
10 was used as the sampling site. All ani- or juveniles) was weighted to the nearest 0.1
mals were captured by hand picking. Pitfall mg on an Ohaus Galaxy 110 electronic bal-
traps are useless in these tidal marshes be- ance. The weight ofthe cocoon itselfwas neg-
cause of the frequent inundations and because ligible compared to its content (< 0.1%).
pitfall captures have a serious bias caused by Before measuring the size of the eggs, they
differences in activity between the different were stored for approximately one month in
developmental stages and sexes (Maelfait & ethanol 70%. After this treatment, the egg
Baert 1975; Maelfait 1996). After measuring shell becomes fully expanded and size differ-
life history traits of all captured specimens, ences between developing and undifferentiat-
they were deposited at the Department Biol- ed eggs can be neglected. As the shape of the
ogy at Ghent University (Belgium). Sampling eggs is ellipsoid, egg length as well as egg
was carried out every one or two months from width was measured to the nearest 0.01 mm.
November 1997-October 1998 resulting in a Egg volume was calculated according to the
total of 9 sampling occasions, more or less formula: egg volume = 0/6 x (egg length) X
evenly distributed throughout the year. Sam- (egg width)2.
pling took place in 1997 on 19 November and The product ofegg volume and egg number
in 1998 on 26 February, 1 April, 29 April, 15 was calculated to obtain a measure of clutch
May, 17 June, 10 August, 18 September and volume. Female mass and, if an egg sac was
21 October. An additional sample was taken present, clutch mass, of females captured on
3 June 1999. — 3 June 1999 were additionally weighed to the
Growth and phenology. For analysis of nearest 0.1 mg on an Ohaus® Galaxy 110 elec-
phenology and growth patterns in the field, tronic balance.
only the specimens captured in 1997 and 1998 To correlate reproductive traits with the size
were used. All animals were kept individually of the mother, carapace width was cubed to
in plastic tubes to avoid cannibalism and the make it proportional to volumetric measure-
exchange of egg sacs between females. Once ments like fecundity, clutch volume, clutch
transferred to the laboratory, animals were mass and egg size—.
kept in a freezer at — 10 °C before measure- Data analysis. Differences in proportions
ments were made. All animals were sorted by of adult versus juvenile spiders over the dif-
developmental stage and in the case of adult ferent sampling dates were analysed by an R
and larger juvenile spiders also by sex. The X C independence test (Sokal & Rohlf 1995).
criteria to determine a juvenile as a sexually To test for size differences between the de-
differentiated male was the presence of velopmental stages and the different sampling
(slight) swollen palps, while the presence of dates, ANOVA was used if assumptions for
two (sometimes very small) reddish dots in normality and homogeneity of variance were
the central part ofthe epigastric fold was used met. Otherwise, we resorted to non-parametric
to assign an individual as a sexually differ- (Kruskal-Wallis) ANOVA. Multiple compari-
entiated female. To determine the size of the sons of the different groups were performed
animals, the carapace width was measured at with Scheffe test.
HENDRICKX & MAELFAIT—LIFE HISTORY OF PIRATA PIRATICUS 333
0.2 0.6 1.0 1,4 1,8 2,2 Z6 3,0 0.2 0.6 1,0 1.4 1.8 2,2 26 3.0 0,2 0.6 1,0 1.4 1.8 22 26 3.0 0.2 0,6 1.0 1.4 1.8 22 26 3.0 0,2 0.6 1,0 1,4 1.8 22 26 3.0
Jumiifra Juvaiiemales Juvenilefmiales Adultmales Adultfemales
Carapacewidth (mm)
—
Figure 1. Frequency distributions of the different developmental stages according to carapace width
(mm).
The size distribution of a particular group veniles. A high peak in the number of juve-
was analysed by mixture analysis, in which niles was observed in August 1998 when ju-
model selection was based on the likelihood veniles emerged from the egg sac ofthe adult
ratio test (LR-test). The test was performedby females. After that a different growth pattern
the Mixture 1 program, developed to analyse in comparison with the previous year is ob-
patterns offluctuating asymmetry (Van Dong^ served, as small juveniles are already totally
ee et al. 1999). absent in October 1998. This difference in
RESULTS proportion ofjuveniles on October 1998 com-
— pared to November 1997 is significant (x^ ~
Phenology. -The numbers and the propor- 19.80; P < 0.0001)
tions of the different developmental stages
Adult spiders emerge from overwintering
over the different samples are depicted graph-
sexually differentiated and undifferentiatedju-
ically on Fig. 1. From November 1997 until
venile spiders, as the proportion undifferenti-
the beginning ofApril 1998, onlyjuvenile spi-
ders were found. From the end of April on- ated juveniles encountered during winter dif-
fered significantly from the proportion
wards, adult spiders appear whilejuvenile spi-
mdearlseswearnedstfilelmparleessentw.erSeexrueaclolryddeidffienrenatlimaotsetd n1o5n.-2a%d;ult s=pid0e.r1s5:inPM=ay0.101959)8.(29.6% versus
equal numbers over the different sampling The highest proportion of males (40.7%)
dates. The highest proportions of sexually dif- was recorded on June 1998 and almost all
ferentiated juveniles were recorded in spring died by August 1998. Females were present
and autumn, while they were almost totally until October 1998 with the highest propor-
absent in May and June 1998. From Novem- tions recorded on June 1998 (44.2%) and Au-
berto mid-June, the number ofsmall, sexually gust 1998 (—37.2%).
undifferentiated spiders present in the samples Growth. The distrubution ofthe carapace
showed a somewhat similar pattern compared width of the spiders of the different develop-
to the number of sexually differentiated ju- mental stages on the different sampling dates
334 THE JOURNAL OF ARACHNOLOGY
effect F,^ = 96.8; F < 0.0001; year x stage
,28
affect; F,, ,28 = — F = 0.87).
Reproduction. Females with an egg sac
were found in May 1998 (60.5%), August
1998 (71.4%) and September 1998 (20.0%).
In May 1998, all females had eggs in theiregg
sac while in August 1998, 18 out of 30 fe-
males with an egg sac had first instarjuveniles
in their egg sac. Female size is not signifi-
cantly different between females with and fe-
males without an egg sac (F,^73; F = 0.43).
As mentioned above, the analysis of the
—
Figure 2. Carapace width (mm) ofjuvenile fe- variation and the relationship of reproductive
males in their hibernating stage and their adult size traits and adult female size was conducted on
in the subsequent spring. individuals captured in June 1999. There is a
great variability for female size measured as
carapace width^ (C.V. = 12.5%); fecundity
are presented on Fig. 1. A significant bimo- (C.V = 17.2%) and clutch volume (C.V. =
dality in size distribution is observed in ju- 17.1%). This is in clear contrast with the var-
veniles captured in August 1998 {LR = 58.77; iation in egg volume (C.V = 3.1%), which is
d.f. = 3; P < 0.0001). Juvenile females are very consistent within the population.
on average significantly larger than juvenile The results of the correlation between fe-
males and significantly different over the dif- male size and female mass with the different
ferent sampling dates (two-way ANOVA; sex- reproductive traits are presented in Table 1.
effect: Fi^4,o = 5.07; P = 0.025; date effect: Clutch mass, clutch volume and fecundity are
Fg,410 = 10.3; P < 0.0001; interaction: Fg^4io; in all cases positively correlated with carapace
P = 0.39). Sexually differentiated juveniles widthL When female mass is taken as a mea-
captured in November 1997 are significantly surement of female size, only clutch mass in
smaller than those captured in April, Septem- the untransformed data, and clutch mass and
ber and October 1998 (F < 0.0002). This im- fecundity in the log transformed data are sig-
plies that sexually differentiatedjuveniles still nificantly correlated. No significant relation-
increase in size before they reach the adult ship is observed between the two measure-
size. ments of female size and egg size. Higher
A comparison of differences in carapace correlation coefficients are observed between
width between males and females could only carapace width-^ and the reproductive traits
be performed on adults captured in May, June than for female mass and the reproductive
and August, when a sufficient number of in- traits. A better fit of the data is also observed
dividuals of both sexes were present in the when both variables are log transformed
samples. Males have a significantly smaller (higher r-values for all reproductive trait var-
cPar<ap0a.c0e00w2i)d.thFetmhaanlefesmiazelessh(owAsNOaVsAig;niFfji^ca,3n4t; iaalblloems)e.trAyllwirtehprfoedmuaclteivseiztera(istlsopseho<w 1n)e.gHatoiwv-e
heterogeneity over the different sampling ever, this is only significant for female mass
dates (ANOVA; P = 0.003), with fe- compared to carapace width^, indicating that
105;
males captured at the end of April being sig- larger females have a relatively lower weight
nificantly larger than females captured in June compared to smaller females.
and August (F < 0.05). Although not signif- A significant negative correlation between
icant, the same pattern also appears to exist egg size and the residual values of log fecun-
for males (ANOVA; F F = 0.14). dity on log carapace width^ (r = —0.38; F =
The larger overwinte2,r4i2n:g juvenile females 0.045) reveales that a trade-off between egg
in October 1998 compared to November 1997 size and fecundity is present (Fig. 3). This
seems to result in a corresponding increase in negative relationship is not due to the fact that
size of adult females captured on June 1999 females with larger eggs have a lower clutch
compared to the size ofthe adult females cap- volume as shown by the lack of a relationship
tured in June 1998 (Fig. 2) (ANOVA; year- between egg size and the residual values of
HENDRICKX & MAELFAIT—LIFE HISTORY OF PIRATA PIRATICUS 335
—
Table 1. Regression equations between the untransformed and log transformed measurements of fe-
male size and some reproductive traits (n = numberofindividuals). (*) in the logtransformeddataindicate
significant negative allometry (slope < 1; F < 0.05).
n r P Intercept Slope
Carapace width^
Female mass 28 0.70 <0.0001 7.84 1.36
Cocoon mass 28 0.53 0.004 5.19 0.92
Number of offspring 28 0.40 0.034 18.79 2.51
Egg size 28 0.02 0.937 0.36 0.00
Reproductive output 28 0.45 0.016 5.86 0.96
Female mass
Cocoon mass 28 0.47 0.011 6.62 0.42
Number of offspring 28 0.33 0.089 25.40 1.05
Egg size 28 -0.10 0.622 0.38 0.00
Reproductive output 28 0.32 0.098 9.77 0.35
Log carapace width^
Log female mass 28 0.72 <0.0001 0.61 0.72*
Log cocoon mass 28 0.59 0.001 0.30 0.83
Log number of offspring 28 0.45 0.017 0.80 0.81
Log egg size 28 0.00 0.986 1.14 0.01
Log reproductive output 28 0.48 0.010 0.35 0.81
Log female mass
Log cocoon mass 28 0.52 0.004 0.17 0.75
Log number of offspring 28 0.38 0.044 0.73 0.70
Log egg size 28 -0.09 0.641 -0.36 -0.06
Log reproductive output 28 0.37 0.051 0.36 0.64
l-o0g.0c9l;utPch=ma0s.s65;onFilgo.g4)c.arapace width^ (r = 01.95968) a.nEdgg19s9i9ze(iAsNalOsVoAr;emFa,rk4a3b=ly0s.i2m7i;laPr i=n
When corrected for female size, no differ- 0.6; Fig. 7).
ences between the successive years are noted
DISCUSSION
for fecundity (ANCOVA; 49 = 0.39; P =
0.54; Fig. 5 and clutch volume (ANCOVA; The observed life cycle of Pirata piraticus
)
^1,47 0.64; P = 0.2; Fig. 6). Slopes ofboth for 1998 is in agreement with the patterns ob-
regressions are not significantly different (P > served by Toft (1979). From November 1997
— —
Figure 3. Therelationshipbetweenegg sizeand Figure4. Therelationshipbetweenegg sizeand
the residuals of the regression of log fecundity on the residual ofthe regression oflog clutch mass on
log female carapace width^. log female carapace width^.
336 THE JOURNAL OF ARACHNOLOGY
until end ofApril 1998, onlyjuveniles spiders
were found. The main period ofgrowth occurs
from the second half of June, the period in
which the spiderlings emerge until the begin-
ning of the winter period (second halfof Sep-
tember). In August, a clear bimodality is ob-
served in the size of the juveniles. This size
distribution implies the presence of two dif-
ferent cohorts or time periods at which juve-
nile spiders are released. As suggested by Toft
(1979), it is likely that the group of smaller 0.7 0.8 0.9 1.0 1.1 1.2 1.3
juveniles originates from a second egg sac Logcarapacewidth^
produced by the females. The production of a —
second sac has also been observed frequently Figure 5. The relationship between log female
in the laboratory (F. Hendrickx, pers. obs.). carapace width-"* and log fecundity in June 1998 and
June 1999.
Adults that appearearly in the breeding sea-
son (May) are larger compared to those found
in the central period of the breeding season individuals captured in autumn 1998 in our
(June-August). As suggested by Alderwei- study were already sexually differentiated, it
&
reldt Maelfait (1989), it is likely that these is likely that all these individuals completed
larger individuals overwintered twice before their life cycle in about one year. This means
reaching the adult stage. that the adult spiders captured in 1999 were
Perhaps the most striking result is the pro- larger than the spiders captured in 1998, al-
nounced difference in size and proportion of though their period of growth was shorter.
juveniles between the successive winters, Concerning the reported interspecific and
which demonstrates that growth rate might interpopulation differences in ratio of one-
show strong year-to-year variation. Although versLis two-year old individuals (Schmoller
to a lesser extent, this year-to-year variation 1969; Den Hollander 1971; Edgar 1972), it is
was also observed by Den Hollander (1971) in of crucial importance to compare data over
which ten populations of the Pardosa piiUata several breeding seasons. Edgar (1972) forex-
(Clerck 1757) group were studied. Good ample recorded interpopulation differences in
growth conditions in autumn, possibly due to the proportion of one and two year old indi-
high temperature or food availability (Schmoll- viduals in two populations of Pardosa lugub-
er 1970; De Keer & Maelfait 1987; Beck & ris (Walckenaer, 1802). As both populations
Connor 1992) might be responsible for the ob- were sampled in different years, these results
served differences. It is also important to note have to be interpreted with caution. Addition-
that density dependent cannibalism is observed ally, it is not clear whether the two popula-
in wolf spiders (Wagner & Wise 1996; Samu tions contain the same species, as P. lugubris
et al. 1999) in which larger individuals of the comprises a complex of related species (Top-
population prey on smaller individuals. Such fer-Hoffmann et al. 2000) that have distinct
cannibalism could also be the cause ofthe ab- distribution patterns and habitat choices (Hen-
sence of smalljuveniles and the dominance of drickx et al. 2001).
largerjuveniles in autumn of 1998. The larger Size of the adult females showed consid-
size ofthejuveniles in autumn 1998 compared erable variation within a breeding season with
to autumn 1997 is also reflected in the larger larger individuals breeding earlier in the sea-
adult size in the subsequent spring. This im- son. According to the results obtained by Toft
&
plies that growth conditions in the previous (1979) for P. piraticus and Alderweireldt
year are largely responsible for the ultimate Maelfait (1988) forPardosa amentata (Clerck
size of the adults. Beck & Connor (1992) ob- 1757), it is likely that those larger individuals
tained similar results for the crab spiders Mis- are descendants of the second egg sac of fe-
umenoides formosipes (Walckenaer 1837). males that reproduced during summer 1996.
Their study revealed that 90% ofthe variation The high variability in clutch mass and fecun-
in adult size was explained by the variation in dity is positively correlated with the size of
hnal weight of the subadult stage. As all the the female. Studies conducted by Kessler
HENDRICKX & MAELFAIT—LIFE HISTORY OF PIRATA PIRATICUS 337
— —
Figure 6, The relationship between log female Figure 7. The relationship between log female
carapace width^ and log reproductive outputinJune carapace width^ and egg size in June 1998 andJune
1998 and June 1999. 1999.
(1973), Fritz & Morse (1985) and Kreiter & acteristics like fecundity and clutch volume.
Wise (2001) revealed that foraging success This is probably due to the fact that that car-
might be responsible for the additional varia- apace width^ is a better estimate of female
tion in reproductive succes. As egg size is in- size than female mass is. Carapace width is
dependent of female size, the observed in- independent offemale condition after egg sac
crease in clutch volume and clutch mass with production and therefore a more reliable in-
maternal size is due to an increased fecundity dicator for female size compared to female
rather then due to an increase in egg size. This mass. Indeed, after the production of the egg
is in agreement with the results of other stud- sac, females can increase in weight due to
ies conducted on intrapopulation variation in feeding; leading to an underestimation of the
reproductive traits in spiders (Fritz & Morse relative amount of resources devoted to re-
1985; .Simpson 1993). production. Our results also demonstrate that
When the effect of female size on fecun- almost all reproductive traits tended to show
dity was ruled out, a significant negative cor- negative allometry with measurements of re-
relation between fecundity and egg size was productive traits. It is important to take this
observed. Therefore, this additional variation allometry into consideration if a comparison
in fecundity due to egg size variation con- in reproductive traits is made between sam-
firms a trade-offbetween egg size and fecun- ples of a different female size (Reist 1986;
dity (Stearns 1992). Because maternal fitness Roff 1992).
is the product of offspring number and off- ACKNOWLEDGMENTS
spring fitness, maternal fitness is determined D. De Bakker, M. Speelmans, E Laegee-
by the curve relatin&g offspring fitness to off- bick and S. Gurdebeke provided useful help
&sprBineggosnize19(8S6m;itLhloydFr1e98t7w)e,llwh1i97c4h;iPm.aprlkieers in collecting the spiders. This work was sup-
ported by the Special Research Foundation of
that the optimal offspring size a female pro-
the University of Ghent.
duces in a particular environment is at the
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