Journal of
Threatened Taxa | www.threatenedtaxa.org | 26 December 2018 | 10(15):
12916–12932
Postembryonic development
of the Tri-spine Horseshoe Crab Tachypleus
tridentatus (Merostomata: Xiphosura)
in a nursery habitat in the Philippines
Dorkas Kaiser 1 & Sabine Schoppe
2
1 Institute of Biosciences, University
of Rostock, Rostock 18051, Germany
2 Western Philippines University, Puerto Princesa
City, Aborlan, Palawan 5302, Philippines
1,2, Katala Foundation Inc., Puerto Princesa City, Santa Monica, 5300 Palawan, Philippines
1 dorkas.kaiser@gmx.de (corresponding author), 2 sabine_schoppe@web.de
Abstract Populations of the Tri-spine
Horseshoe Crab Tachypleus tridentatus have dramatically decreased over their
distribution range and conservation efforts are now crucial. The implementation of appropriate management
strategies and stock assessment rely on accurate growth-rate estimates. The postembryonic development of the species
in the tropics, however, is not elucidated.
To provide the information needed to assess the demographics of juvenile
populations and to judge the status of T. tridentatus
in the Philippines, we conducted a mark-recapture experiment in a nursery habitat on
Palawan Island. The results obtained
during the 10-month period provide the first consecutive data on the stepwise
growth of the species in the Philippines and the first near
comprehensive dataset collected within a single population of juveniles in the
tropics. By analyzing size-frequency (prosomal width) distributions of 853 individuals and by
using 94 juveniles that molted during the study, 13 molt stages were
differentiated. Based on the intermolt periods of six instars, we estimated the growth
curve of T. tridentatus following two models
(non-linear and power function). The data support the assumption that
growth continues year-round in the tropics and also indicate that the average
age of mature male and female T. tridentatus
in the Philippines ranges from three to four years. The agreement
with a field study in Japan suggests that 14 postembryonic stages may be
characteristic for the development of natural populations throughout the range
of the species. Though more data are
needed to validate these results, the study provides a sound baseline for
future studies in the tropics.
Keywords: Tri-spine Horseshoe Crab,
juveniles, development stages, intertidal zone, morphometry,
allometry, size-age relationship, growth curve.
Abbreviations: AA - Distance between the anal angles; BL - Body length
(PL+OL+TL); BM beach - Bernardo Marcelo Beach (study site); CL - Carapace
length (PL+OL); DS - Development stage; Eyes - Distance between the compound
eyes; IMP - Intermolt period; OL - Opisthosomal length; OW1–3 - Opisthosomal
width 1–3 ; PES - Postembryonic stage; PL - Prosomal length; PW - Prosomal
width; SPSS - Statistical Package for the Social Sciences; TL - Telson length; Statistics: CI - 95% confidence
interval; df - Degrees of freedom ; M - Mean value;
Min / Max - Minimum and maximum values; n - Sample number ; p - Significance
level; r - Pearson’s correlation coefficient; SD - Standard deviation; SE -
Standard error of the mean; T - Statistics of the one-sample t test; U -
Statistics of the Mann-Whitney U test; W - Statistics of the Shapiro-Wilk Test ; Z - Statistics of the Kolmogorov-Smirnov test.
doi: https://doi.org/10.11609/jott.4125.10.15.12916-12932
Editor: Ruth H. Carmichael, University of
South Alabama, Dauphin Island, USA. Date
of publication: 26 December 2018 (online & print)
Manuscript details: Ms
# 4125 | Received 14 March 2018 | Final received 03 August 2018 | Finally
accepted 02 December 2018
Citation: Kaiser, D. & S. Schoppe (2018). Postembryonic development of the Tri-spine Horseshoe Crab Tachypleus tridentatus
(Merostomata: Xiphosura) in
a nursery habitat in the Philippines.
Journal of Threatened Taxa 10(15): 12916–12932; https://doi.org/10.11609/jott.4125.10.15.12916-12932
Copyright: © Kaiser & Schoppe 2018.
Creative Commons Attribution 4.0 International License.
JoTT allows unrestricted use of this article in any
medium, reproduction and distribution by providing adequate credit to the
authors and the source of publication.
Funding: Mainly self-funded. The ‘Studienstiftung des Deutschen Volkes’ provided a
scholarship for the first author in 2001.
Competing interests: The authors declare no competing interests.
Author Details: Dorkas Kaiser specialized in tropical arthropod
ecology. After her MSc on horseshoe
crabs at the University of Rostock and in collaboration with the Western
Philippines University, she completed her PhD on termite ecology in Burkina
Faso at the Department of Animal Ecology and Tropical Biology of the University of Würzburg. Sabine Schoppe completed her MSc and PhD in Biology at the Justus-Liebig
University in Giessen after field research on tropical marine ecology at the
INVEMAR in Santa Marta, Colombia. For
the past 20 years, she has been working in the field of tropical ecology and
natural resource management in the Philippines.
Author Contribution: SS and DK designed the study. DK acquired and analyzed the data and prepared the manuscript.
SS was the
supervisor of DK in 2001
and contributed to a
similar extent to the development of the manuscript.
Acknowledgements:
We are especially grateful for the cooperation of the Western
Philippine University in Puerto Princesa City on
Palawan, Philippines. We extend our
special thanks to Prof. Günter Arlt from the
University of Rostock, Germany. We
further thank Sharah Barredo
for her contribution to the collection of juveniles in 2017. The authors are much obliged to the two
anonymous reviewers for their constructive comments that considerably improved
the manuscript.
INTRODUCTION
The
Tri-spine Horseshoe Crab Tachypleus tridentatus (Leach, 1819) (Xiphosura:
Chelicerata) is the largest of four extant species of
ancient marine arthropods, the origin of which can be traced back to
445 million years (Shuster et al. 2003; Rudkin et al.
2008). The information about horseshoe
crabs in the Philippines is especially scarce.
Waterman (1958) recorded T. tridentatus
and Carcinoscorpius rotundicauda
(Latreille, 1802) for western and southern
Philippines, respectively; based on a picture, both species were listed as
occurring in the Province of Palawan (Sekiguchi
1988). The first survey conducted from
northern to central Palawan confirmed the presence of T. tridentatus
in the area (Schoppe 2002).
Recent
harvest pressures and habitat loss prompted the need for management actions to
protect horseshoe crabs (Berkson et al. 2009). Although still
listed in the Data Deficient category by the IUCN (2017), observations in
Taiwan, Japan, Hong Kong, Singapore, Malaysia, Borneo, and Thailand indicate
that populations of T. tridentatus have
dramatically decreased (Itow 1993; Hsieh & Chen
2009, 2015; Chen et al. 2015; Kwan et al. 2016; Lee & Morton 2016; Wada et
al. 2016; Manca et al. 2017).
Unlike other regions in Asia, in the Philippines, there is no commercial
exploitation of T. tridentatus for the
production of amebocyte lysate or food, but habitat
loss due to land reclamation, sea sand mining, and coastal development are
destroying its natural breeding beaches and nursery habitats. As a result, T. tridentatus
faces a high risk of extinction without management efforts to
conserve its viable populations and habitats (Schoppe
2002).
The
implementation of effective management strategies and
stock assessment relies on accurate estimations of growth rates (size at age,
size at maturity, and age at first capture), size distributions, and population
size (Froese et al. 2008; Chang et al. 2012;
Cunningham & Darnell 2015; Cao et al. 2016). A lot of effort was made during the last few decades to understand the growth
biology of horseshoe crabs, including laboratory studies on the influence of
environment factors such as water temperature and salinity (Jegla
& Costlow 1982; Chen et al. 2010; Zaleha et al. 2011), pH (Tanacredi
& Portilla 2015), sediment type (Hong et al.
2009; Hieb et al. 2015), tank size (Hieb et al. 2015; Chen et al. 2016), and food quantity and
quality (Carmichael et al. 2009; Schreibman & Zarnoch 2009; Hu et al. 2013). Due to the lack of
calcified structures persisting through the molt, however, the age of the
different instars was not unambiguously determined for any horseshoe crab
species (Carmichael et al. 2003; Chen et al. 2010).
Specifications for T. tridentatus vary strongly in the literature, although
there is a consensus that the females molt once more than the males to reach
maturity. Based on laboratory studies, Sekiguchi et al. (1988) calculated that the females spent
14 years to molt 16 times before attaining maturity, while Chen et al. (2010)
estimated that the females mature in stage 15 after four years when reared in
warm water. Goto
& Hattori (1929) identified 14 postembryonic stages by measuring
individuals in their natural habitats in Japan.
Kawahara (1984), on the other hand, suggested that the females in Japan
molt 15 times in 10 years to reach maturity, while Asano (1942, cited in Lee
& Morton 2005) estimated that they molt 18 times in 16 years. To our knowledge, the postembryonic
development of T. tridentatus in their
tropical environment is not yet determined, but laboratory studies indicate
that the growth of juveniles could continue throughout the year when the
temperature is greater than 28°C (Lee & Morton 2005; Chen et al.
2010). Most studies concerning the life
history of T. tridentatus are from Japan,
China, or Hong Kong — countries where ecdysis and
spawning appear to stop during colder seasons (for instance, Sekiguchi et al. 1988; Chiu & Morton 2004; Zhou &
Morton 2004; Lee & Morton 2005; Hu et al. 2009, 2015; Kwan 2015; Kwan et
al. 2015) — while fewer studies were conducted in the tropics (Robert et al.
2014; Mohamad et al. 2016; Manca
et al. 2017; Mashar et al. 2017).
In this study, we characterized the
postembryonic development of T. tridentatus in
a nursery habitat on Palawan to provide sound baseline data needed for
conservation, particularly in southeastern Asia (Berkson et al.
2009; Shuster & Sekiguchi 2009). In 2001, the population
in the study site was estimated to comprise of 298 individuals with a
male-to-female ratio of 1.2:1 (Kaiser 2002).
The main objectives of this study were 1) to identify the number of
instars until maturity, 2) to describe the post-embryonic growth patterns, and
3) to estimate the size-age relationship of T. tridentatus
in the Philippines. These data represent
the first dataset of this type for the tropics.
MATERIALS AND METHODS
Study site
The growth of juvenile T. tridentatus was studied in a nursery habitat located
close to the Puerto Princesa City on the eastern
coast of Palawan in the Philippines (9.7620N & 118.7720E)
(Fig. 1). The study region is
characterized by a rainy season that lasts from June to November and a dry
season from December to May (PAGASA 2017).
The annual rainfall averages at 1,684mm, while the mean annual
temperature is 27.2°C. The area has a
mixed semidiurnal tidal cycle (National Ocean Service 2008); the tidal range
during spring tide may rise up to 1.9m.
The mean water temperature in the study area is 29°C and 31°C during
high and low tides, respectively (Table 1).
The average pH ranges between 7.6 and 8 and the mean salinity between 30
and 31‰ (Table 1). The study site reported
upon herein is the Bernardo Marcelo Beach (hereinafter BM beach) (Image 1), a
130m-wide and 600m-long seagrass meadow. About 65% of the sampling area (50,700m²) is
covered with seagrass, while the rest comprises sandy
patches. The mean grain size is
0.103±0.02 mm (n = 33). It
comprises a zone between the mean low and mean high water levels and the upper
sub-tidal range. The local residents use
the northern part of the beach for recreation in the weekends. Shellfish are gathered and fishermen position
bottom-set gillnets seaward of the sampling area in a seagrass
meadow without larger sand patches and an adjacent riff (about 600m wide).
Sampling strategy
(capture-mark-recapture)
The juvenile population of T. tridentatus at BM beach was assessed during daytime at
low tide from May to December 2001 for 84 days and between April and May 2017
for 23 days. Following Rudloe (1983), the assessment started about two to three
hours before the lowest water level was reached and lasted for four to five
hours. By walking barefoot in a zigzag
course, the surface of the entire sampling area was systematically searched for
juveniles and exuviae. The majority of juveniles were found with the
help of their feeding trails and mostly in the sandy-muddy substrate; some larger individuals were sensed by foot. Additionally, several adult horseshoe crabs
were handed over by fishermen and found at the market; they were measured as
well.
Morphometric parameters (prosomal length, opisthosomal length,
telson length, prosomal
width, opisthosomal widths 1–3, the distance between the anal angles, the distance between the
compound eyes) were measured to the nearest 0.1mm using a Vernier
caliper (Fig. 2). Sex was identified
based on the size and shape of the genital papillae (Bonaventura et al.
1982). Superglue was used to fix a
number on the prosoma of each individual. For identification after ecdysis,
the lateral mobile spines of the opisthosoma were
shortened (Kawahara 1982) following a coding system (Appendix 1). The identification mark of the adults was
modified following Sokoloff (1978). All juveniles were released where they were
found. The individuals found during or
shortly after ecdysis were kept under an ambient
temperature of 25–32 °C in a tank with air supply and the water of BM beach;
they were measured and marked when the carapace had sufficiently hardened. A few individuals supplied by fishermen or
shellfish gatherers were marked and released at BM beach.
Data analyses
Unless
otherwise stated, the significance threshold was set to α = 0.05 and the
significance was tested at a two-tailed level. Statistical Package for the Social Sciences
(SPSS), version 15.0 (SPSS, Inc., Chicago) and Microsoft Excel 2010 were used
to carry out the statistical tests.
Cohort demarcation
The prosomal
width (PW) was used to mark the instar stages that characterize the
postembryonic development of T. tridentatus on
Palawan. The PW of all individuals and exuviae were grouped into specified size intervals and size-frequency histograms were plotted. The resulting histograms
were multimodal and indicated different cohorts or development stages (DS) that
differed in abundance. The visual cohort
demarcations were adjusted by comparing the data with 94 individuals that
molted during the study period or whose individual exuviae
was measured (hereinafter referred to as molting individuals). The measured adults allowed the defining of the size limits for
the cohort of sexually mature males and sub-adult females, as well as the stage
of sexually mature females.
The resulting size ranges of the DS
were ascertained by conducting a modal progression analysis of the size-frequency distributions with the
Fish Stock Assessment Tool, FiSAT II (FAO
2006–2017, http://www.fao.org/). FiSAT II applies the maximum likelihood concept to separate
the normally distributed components of the data, thus allowing the
demarcation of the cohorts from the multimodal distributions. Thereafter, a one-sample t test was
used to compare the means calculated in FiSAT with
the DS defined by the visual demarcation and the molting individuals.
Body measures
The
Shapiro-Wilk test (n ≤ 50) and the Kolmogorov-Smirnov
test (with Lilliefors correction) (n > 50) were
used to examine whether the established size ranges and the body parameters
were normally distributed. The
homogeneity of variances was tested using the Levene’s
test (Field 2009). Because some data
showed a non-normal distribution when differentiating the juveniles’ sex, the
non-parametric Kruskal-Wallis test was used to
compare the body measures of females and males.
Subsequently, pair-wise comparisons for each DS were performed using the
Mann-Whitney procedure. A Bonferroni correction for multiple comparisons was applied
to the significance level (differences were regarded as significant at α
< 0.006). The Mann-Whitney
procedure was further used to compare various morphometric ratios between
juvenile and adult T. tridentatus (males,
females, and individuals with unknown sex were pooled). A one-sample t test was used to
compare the carapace lengths per DS with the mean values reported by Goto & Hattori (1929).
Hiatt growth model
According to Hiatt (1948), a linear
growth model can be used to fit the post- and pre-molt size of crustaceans and
this model was applied to assess horseshoe crab growth (Carmichael et al. 2003;
Hu et al. 2013). The model describes the
growth under natural conditions as PWn+1 = a + b PWn,
where PWn is the
pre-molt PW at instar n and PWn+1 is the post-molt PW at instar n+1. The y-intercept indicates whether the
size increment increases (a < 0) or decreases (a > 0) with an increase in
the size of the animals or whether it stays constant during the development (a
= 0). Slope b represents the growth
coefficient, allowing conclusions on the variations between the size increments
of the consecutive molts. The Hiatt
growth model was applied first to illustrate the size increment observed for
the 94 molting individuals.
Additionally, the model was applied based on the data of the cohort
demarcation by using the average PW of the instars. A Student’s t test was used to compare the
slopes of the regression lines in the two Hiatt diagrams to allow conclusions
to be drawn regarding the demarcation of the DS identified at BM beach.
Allometric growth
The allometric
growth of each morphometric parameter (y) was expressed as a power function of
the PW (x) with the equation y = a xb
(Fuiman 1983).
The relative growth coefficient was estimated by linear regression of
the log-transformed allometric growth curve of the
type log y = log a + b log x, where a is the intercept
and b the growth coefficient (Gould 1966).
The growth coefficient identifies whether the growth pattern indicates
isometric growth (b = 1 for the lengths or widths and b = 3 for weight),
positive allometry (b > 1 for lengths or widths
and b > 3 for weight), or negative allometry (b
< 1 for lengths or widths and b < 3 for weight). If the value 1.0 (or 3.0 in the case of the
weight) is outside the 95% confidence interval of b, the difference is regarded
as significant. A Student’s t test was
used to compare the slopes of the regression lines to assess whether males and
females differed significantly. To
illustrate a specific characteristic of the growth of the horseshoe crabs’ telson, a linear regression for the relation between the PW
and the telson length (TL) was again carried out
graphically.
Growth curve
The first postembryonic stage
(PES), the trilobite larvae, was not found during the study. The PW of the trilobite larvae was estimated
following the Dyar’s rule (Ln PW = a PES –
b), which was already applied by Waterman (1954). A cubic regression was used to describe the
growth curve of PES 1–9. Chang et al.
(2012) compared the fit of different models ranging from simple equations to
models describing continuous and discontinuous growth for two lobster species
and two crab species. They concluded
that the non-linear model applied by Castro (1992) was the best model to
quantify and predict the relationship between the pre-molt length and the intermolt period (IMP) for the selected crustaceans,
although they suggested that different models should be used to reduce the
uncertainty in model selection. Based on
the IMPs observed at BM beach, two methods were applied to predict the growth
curve and estimate the age of T. tridentatus
at sexual maturity. The non-linear model
describes the IMP as a function of the average carapace length (CL) per instar
stage, IMP (days) = a + b CLc (see
Castro 1992; Chang et al. 2012). The
second method describes the age as a power function of the PES, age (months) =
a PESb.
RESULTS
Cohort demarcation
The PWs of the juveniles at BM beach
ranged from 0.9cm to 21.1cm. The largest
mature female had a PW of 36.9cm.
Excluding the animals exhibiting physical injuries on the prosoma, 853 PWs were used in demarcating the growth stages
(Table 2). Owing to the natural growth
variability of individuals, the size ranges of the DS increase with increasing
age. The cohorts were, therefore,
classified with frequency distributions of increasing interval widths. Five DS were identified in the histogram,
presenting the smallest animals at 0.05cm intervals (Fig. 3a). The cohort with a PW ≤ 1.1cm was termed DS A,
with subsequent cohorts named in alphabetical order. It has to be stressed that DS A does not
represent PES 1, the trilobite larvae.
Trilobite larvae were not found during the study.
The DS F–I were depicted most
clearly with an interval size of 0.2cm (Fig. 3b). For larger juveniles and adults, an interval
size of 0.5cm was the best, but the number of measurements did not allow a
clear demarcation between the biggest juvenile stage (DS K) and the adult
stages (DS L–M) (Fig. 3c). The
difference between DS J and the cohort of sexually mature males and sub-adult
females (DS L) then revealed the DS K.
Because the size limits of this cohort could not be unequivocally
validated with our data, DS K was not included in the following statistical
analyses. The mean PWs identified in FiSAT did not differ significantly from the PWs defined by
visual demarcation (one-sample t test, p > 0.05), thereby confirming
the cohort demarcation (Table 2, Fig. 4).
Morphology of T. tridentatus on Palawan
Before reaching
sexual maturity, almost no morphometric differences could be observed between
the sexes (Kruskal-Wallis test, p > 0.05). An exception was DS H, where males and
females differed in all body parameters (Mann-Whitney U test with Bonferroni correction, p < 0.006). Individuals whose sex was not determined, 330
of 804 juveniles and exuviae, were not included in
the analyses. The DS A–B were not considered as they were too small for sex determination.
The morphometric
parameters of the instars A–M in Tables 3–4 were pooled for both sexes,
including the exuviae but excluding individuals with
injuries in the relevant body parts.
Most of the body parameters in Table 3 showed a normal
distribution. The comparison of various
morphometric ratios between juveniles and adults (pooled male and female data)
with the Mann-Whitney procedure revealed highly significant differences for the
ratios PW/OW2, PL/OL, OW2/OL, CL/PW, and CL/TL (Appendix 2). The Eyes/PW and PW/PL ratios were
statistically insignificant at the 95% probability level (Appendix 2).
Growth analyses
Hiatt growth model
The regressions of the relation
between the pre- and post-molt PWs of T. tridentatus
on Palawan support the linear correlation predicted by Hiatt (1948). The slopes of the Hiatt growth equations
describing the relation between the pre- and post-molt PWs of 94 molting
individuals (PWn+1 = 0.23 + 1.27 PWn,
r² = 0.9955; Appendix Fig. A1) and those of the DS resulting from the cohort
demarcation (PWn+1 = 0.26 + 1.27 PWn,
r² = 0.9993; Appendix Fig. A2) were statistically indistinguishable (t102 =
1.927, p > 0.05), thereby confirming the identified DS. The positive constant in the Hiatt equations
indicated that the percentage of growth decreased with increasing body
size. The mean increase in the size of
the PW of the molting individuals was between 34.7% and 33.0% in the initial
stages of their development (DS A–G), while those from DS H and I showed
average increases of 29.2% and 27.9%, respectively. A strongly reduced growth was observed in
animals exhibiting serious physical injuries; less serious damages usually had
no or little effect on the growth increment during ecdysis
(results not shown).
Allometric growth
The growth of the PL, the OL, and
the TL (hence the CL and the BL) throughout the 11 instar stages (DS C–M) was positively allometric
with the PW in both sexes, except for the PL of males that grew isometrically with the PW (Table 5). The growth of the OW2–3 and the AA in both
sexes and of the OW1 and the eyes of males were negatively allometric
with the PW (Table 5). Except for the
TL, however, the deviation from isometric growth was small in all cases of allometry. The
differences between the sexes were mostly small or insignificant; the greatest
difference lay in the PLs and TLs (Table 5).
The increase in wet weight was isometric with the PW (Table 5). The increase in wet weight and the growth of
the PL, however, was negatively allometric when data
were pooled for males, females, and individuals with unknown sex (Appendix
3). The telson
growth had three distinct growth curves (triphasic),
including juveniles of the instar stages DS A–C, juveniles of DS D–K, and
adults (Fig. 5).
Growth curve describing the
postembryonic development in the Philippines
Molting frequency: The time between
two molts was observed for 20 juveniles of six consecutive stages
(Table 6). The IMPs slowly increased
with an increase in the size of the animals.
The time between hatching and entering DS C was not identified during
the study. Hence, the exact age of the
cohorts at BM beach could not be determined, though we know that when juveniles
entered DS I, they were older than 436 days (Table 6).
Prosomal width of the trilobite larvae: A statistically significant match
was found for the carapace lengths of DS C, E, L and the lengths measured in
the respective PES of Goto & Hattori (1929), as
well as for the mature females without considering the instar stage (Table
6). Based on the average PWs of PES 2–11
(DS A–J) and PES 13–14 (DS L–M), the Dyar’s rule (LN PW = 0.294 PES –
0.578, r² = 0.9993) was used to estimate the width of PES 1 with
0.75cm. On an average, the carapace
length of T. tridentatus on Palawan was 1.67%
smaller than the PW (all measures).
Animals with a PW of 0.75cm would, therefore, have a carapace length of
0.74cm (Table 6).
Ages of
the postembryonic stages: Observations indicated that the
periods slowly increased with the increasing size of the animals (Table
6). With an estimated average IMP of 14
days for the trilobite larvae and of 30 days each for PES 2–3, the presumed age
of T. tridentatus in PES 9 (PW 8.23cm) would
be 17 months. The cubic equation PW =
0.622 + 0.216 t + 0.026 t² - 0.001 t³ (r² = 0.9995) described the postembryonic
period of PES 1–9 (Appendix Fig. A3).
Based on the observed IMPs of PES 4–9 (DS C–H), two methods were
used to predict the age at which T. tridentatus
attained sexual maturity in the Philippines.
The non-linear model for the relationship between the IMP and the
average carapace length (CL) per instar stage, IMP (days) = 19.47 + 23.85 CL0.64
(r² = 0.9015), and the power function, age (months) = 0.251 PES1.898
(r² = 0.9901). The resultant growth
curves, illustrating the size-age relationship, suggested that the mean ages of
mature male and female T. tridentatus in the
Philippines are 2.7–3.5 and 3.1–4.2 years, respectively (Fig. 6).
DISCUSSION
The presented results provide the
first consecutive data on the stepwise growth of T. tridentatus
in the Philippines. By covering all
stages except the increase from the trilobite stage, our study provides the
first nearly comprehensive dataset collected within a single population of
juveniles in the tropics. We found that
14 instars characterize the postembryonic development of the species in the
Philippines. The similarities between
the stages identified on Palawan and those of T. tridentatus
in a nursery habitat in Japan suggest that these 14 stages may be
characteristic of natural populations throughout the species distribution
range. Our findings further support the
assumption that growth continues year-round in the tropics and suggest that the average age of mature male and
female T. tridentatus in the Philippines
ranges from three to four years.
Postembryonic stages and development time
The number of postembryonic stages reported
for T. tridentatus in the literature varies
strongly and there is a relative paucity of consecutive growth data from wild
populations (Goto & Hattori 1929). The information on development time is
largely based on laboratory studies (Sekiguchi et al.
1988; Lee & Morton 2005; Chen et al. 2010).
To assign the stage and age of the individuals collected in nursery
habitats in Hong Kong (Chiu & Morton 2004; Kwan 2015; Kwan et al. 2016) and
southern China (Hu et al. 2009, 2015), recent studies applied the size-age
relationship established by Sekiguchi et al. (1988)
while rearing artificially fertilized eggs in the laboratory.
The demarcation of cohorts was
confirmed in FiSAT and by comparing the slopes of the
Hiatt growth equations for molting animals and the cohorts (Figs. A1–A2, Table
2). The similarity between the 13 DS
identified at BM beach and the 13 PES (PES 2–14) considered by Goto & Hattori (1929), which were comparable in size,
indicated that all expected life stages except the trilobite larvae (PES 1)
were found in the present study (Table 6).
Furthermore, the average carapace length estimated for the trilobite
larvae equaled the length measured by Goto &
Hattori (1929). As also reported by the
authors, small but mature females were present in the PES 13 (DS L) and large
males in the PES 14 (DS M). These
findings explain why the two studies differed significantly in terms of the
carapace length of PES 14, while the comparison between the adult females
(without considering the instar stage) showed insignificant results.
Differences, however, can be noted
compared to the findings reported from rearing experiments, especially those by
Sekiguchi et al. (1988) (Table 6). The last animal used in the study by Sekiguchi et al. (1988) died in the PES 10 with a PW of
4.84cm; animals with a comparable PW on Palawan were in PES 7. By calculating the subsequent PWs with a
constant rate of 1.28, the authors concluded that sexually mature females were
in PES 17 (Table 6). As already reported
for T. tridentatus (Waterman 1954; Kawahara
1982) and the Atlantic Horseshoe Crab Limulus polyphemus
(Linnaeus, 1758) (Carmichael et al. 2003), both the relative increase in size
of the molting individuals and the positive constant in the Hiatt equations
indicated that the percentage growth on Palawan decreased with an increase in
body size (Figs. A1–A2 in Appendix), a rate of 1.28 being
observed in the upper size range.
An approximate agreement, therefore, was only found between PES 10–14
(DS I–M) of Palawan and PES 13–17 calculated by Sekiguchi
et al. (1988) (Table 6). Moreover, the
IMPs differed greatly from those of the present study. Animals with a PW of 2.22cm molted only once
per year, like the subsequent instars (Table 6). On the other hand, animals with a comparable
size on Palawan molted on an average after 56 days; the subsequent stages also
showed a much faster development. The
observed inverse relationship between size and molt frequency (Table 6)
supports the findings reported for horseshoe crabs (Waterman 1954; Carmichael
et al. 2003; Chen et al. 2010) as well as many crustacean species (Kurata 1962; Caddy 1987; Chang et al. 2012).
Considering the high water
temperatures in the study region throughout the year and the increasing length
of the IMPs, we assumed that the trilobite larvae in the Philippines molt after
two weeks and the PES 2–3 after 30 days.
Following our estimations, the mean age of juveniles entering PES 10
(mean PW of 11.1cm) was 14.5 months (Fig. 6). The observed IMPs and our age estimations for
adults were similar to the four years estimated by Chen et al. (2010), although
an additional stage was observed in their experiment (Table 6). The authors reared juvenile T. tridentatus under conditions better comparable to those
prevailing at BM beach. The temperature
was 28–30 °C, seawater salinity was 30‰, water flow was maintained, more
substrate was provided for digging, and the tank was much larger than that in
Japan. The least time to hatch and
greatest survival of the Asian species were observed in water temperatures of
around 29°C (Carmichael & Brush 2012), and Chen et al. (2004) suggested
28–31 °C as the optimal seawater temperature for the year-round growth of the
juvenile T. tridentatus. These observations were in line with the
results of Yeh (1999, cited in Chen et al. 2010) and
Lee & Morton (2005), who reported that ecdysis in
T. tridentatus continues when the temperature
remains at >28°C but stops at <22°C.
Our findings support the assumption that growth continues year-round in
the tropics and suggest that T. tridentatus in
the Philippines attain sexual maturity at the age of three to four years. The similarity with the instars from the
Inland Sea in Japan (Goto & Hattori 1929), where
the water temperature drops down to 13°C in winter, suggests that 14 stages may
be characteristic of the postembryonic development of natural populations throughout
the distribution range of T. tridentatus. Temperature seems to have less influence on
the molt increment than on the IMP. To
confirm these findings, however, further investigations should be carried out
in habitats located at the limits of the distribution range of the species.
In light of the fact that the growth
increments per molt of T. tridentatus in the
two nursery habitats were comparable, it is likely that the caging operations
are responsible for the deviations observed in the laboratory studies. Significantly reduced growth increments at
each molt and longer IMPs in the laboratory were observed in horseshoe crabs
(Carmichael & Brush 2012) and various crustacean species (e.g., Hiatt 1948;
Harms et al. 1994; Bonilla-Gómez et al. 2013).
The effects of the holding time at the laboratory and the space available
in the holding tank were reported (e.g., González-Gurriarán
et al. 1998). The development of L. polyphemus in the laboratory
was considerably slower than that in their natural habitat — the
postembryonic growth time was nearly halved when the embryonic development took place in
nature (Jegla & Costlow
1982). As shown for horseshoe crabs
(Carmichael & Brush 2012; Hu et al. 2013) and several crustacean species (Hartnoll 2001; Chang et al. 2012), the IMP increased and
the rate of increase in size decreased when the quantity or quality of food in
culture was suboptimal. The diet of
horseshoe crabs in nature is broad and highly mixed and they move up the food
webs as they age and grow (Carmichael et al. 2004; Zhou & Morton 2004). The most commonly offered diet in culture was
brine shrimp (Artemia spp.) or dietary
supplements that were not part of their known natural diet (see Carmichael
& Brush 2012). Moreover, recent
studies with horseshoe crabs show the importance of the sediment type for
growth and survival (Hong et al. 2009; Hieb et al.
2015), while others found shorter hatching times and higher molting frequency
and molt increments with increased water circulation or increased dissolved
oxygen concentrations (Carmichael & Brush 2012). The relationships with environmental factors
and the potential interactions among different variables were not resolved
(Carmichael & Brush 2012). These
reports, however, illustrate that variations in the growth rate can be
considerable, depending on the abiotic and biotic parameters, which might
explain the large differences observed with the number of stages and the
size-age relationship reported by Sekiguchi (1988)
and the comparable development time, with only one additional instar stage
reported by Chen et al. (2010). These
findings may further imply the similarity between BM beach and the nursery
habitat of Goto & Hattori (1929) in terms of
environment conditions (apart from water temperature) and quality of food
supply. Further studies are needed to
characterize the habitat of different populations and to identify the key
drivers for growth and survival in nature and in rearing experiments.
Growth pattern
Our finding that
the relationship between the pre- and post-molt PWs of juvenile horseshoe crabs
could be fitted with the Hiatt growth model is consistent with the results of
previous studies (Carmichael et al. 2003; Hu et al. 2015). Moreover, the Hiatt equations were almost
identical to the equation for a juvenile population assessed at a nursery
habitat in southern China (Hu et al. 2015).
As also reported by Sekiguchi et al. (1988),
three distinct growth curves described the relative growth of the TL and the PW
throughout the 13 stages. Accordingly,
Chen et al. (2010) reported two phases for the nine juvenile stages considered
by them. Deviations from isometric
growth in the other body parts were fairly small on Palawan — the shape of the
horseshoe crab at different development stages was therefore similar. Isometric growth was reported by Chen et al.
(2010) for most of the body parts of the juveniles in their study. Lee & Morton (2005), in contrast,
reported a positive growth allometry for the weight
of juveniles collected in their natural habitat but reared in the
laboratory. On Palawan, significant
differences between juveniles and adults were indicated in most of the analysed body ratios.
Most findings agree with those of Chiu & Morton (2003), although
they reported insignificant differences for the CL/PW and CL/TL ratios. Several studies (for instance, Yamasaki et
al. 1988; Chiu & Morton 2003; Mohamad et al.
2016) revealed that morphometric parameters, body ratios, and the allometric relationships of adult and juvenile T. tridentatus differed significantly between populations
and geographic regions, a characteristic that might explain the differences
between the studies. The differences
might also be due to the different size ranges considered and whether or not
sex was differentiated.
Limitations
The absence of the trilobite larvae
might suggest that no recent spawning activity occurred at BM beach. Because the subsequent stages were present
and the smallest instar stages are difficult to detect, however, they may have
been simply overlooked. The newly
hatched larvae do not need to feed because they can subsist on the yolk of the
embryo (Botton et al. 1992). The low occurrence of PES 12 (DS K) at BM
beach supports earlier observations that larger juveniles were moving further
down in the intertidal zone, with subadults at the
seaward limit (Rudloe 1981; Kaiser 2002; Hu et al.
2009; Morton & Lee 2011). It might
also imply a greater mortality of juveniles in PES 12, though laboratory
studies revealed a decrease in mortality rates with increasing age (Carmichael
& Brush 2012).
It was recognized that
mark-recapture experiments were often the only method to validate the growth of
natural populations (González-Gurriarán et al. 1998;
Lee & Morton 2005; Hu et al. 2015), but several authors emphasized that
caution should be used when extrapolating from tagged animals to natural
populations (Kurata 1962; Caddy 1987). Although handling during measuring and
marking may have disturbed the juveniles in the present study, we doubt that
cutting the mobile spines for identification would have harmed the animals to
an extent that their growth was no longer natural — we observed that animals
with injuries showed lower rather than higher molt increments. Given a recapture rate of 57% in 2001 (Kaiser
2002), the labels glued on the prosoma also seemed to
pose no adverse impacts on health.
Conclusions
Determining when a horseshoe crab
reaches sexual maturity is of extreme importance for interpreting the population
dynamics to enable their management and conservation (Carmichael et al.
2015). By presenting the first
consecutive data on the postembryonic development of T. tridentatus
in the Philippines, the present study adds to the fragmentary background
knowledge about the species in southeastern Asia and provides the information
needed to assess the demographics of juvenile populations and judge the status
of T. tridentatus in the Philippines. With populations having decreased
dramatically over their distribution range, it is clear that these data are now
in demand. Because of the small number
of IMPs observed and the absence of the trilobite larvae, however, more data
are needed to validate the present findings.
The relative paucity of consecutive growth data from wild populations
and the lack of comparable data for the tropics made it difficult to judge the
universal applicability of the dataset.
Nevertheless, the reported results provide sound quantitative and
qualitative baseline data for future assessment and monitoring studies in the
tropics. To define a wider range of
applicability, future investigations should aim to determine the size-age
relationship of natural T. tridentatus
populations from different geographic regions, preferably in habitats located
at the limits of the species distribution range. These studies should also characterize the
abiotic and biotic parameters of habitats because the causes of variable growth
and survival rates have important implications for conservation and aquaculture
efforts that are aimed at restoring depleted populations of horseshoe crabs.
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Table 1. Mean values (M) ± standard deviation (SD) of the
hydrographic conditions prevailing in the nursery habitat of Bernardo Marcelo
Beach on Palawan, with sample size (n) and minimum and maximum values (Range)
Tide |
|
Temperature (oC) |
pH |
Salinity (‰) |
Dissolved oxygen (%) |
High |
Mean ± SD |
29.2 ± 1.2 |
7.6 ± 0.05 |
31.0 ± 0.82 |
71.5 ± 34.3 |
|
Range |
26.5–32.0 |
7.5–7.7 |
29.0–32.4 |
25.8–111.9 |
|
n |
50 |
17 |
59 |
35 |
Low |
Mean ± SD |
31.1 ± 0.9 |
8.0 ± 0.15 |
30.0 ± 1.05 |
94.2 ± 45.3 |
|
Range |
29–32.5 1 |
7.8–8.3 |
27.4–32.0 |
48.3–228.0 |
|
n |
54 |
10 |
67 |
46 |
1 On hot days, the water temperature
during low tide was up to 41°C.
Table 2. Mean (MPW in cm) with standard error (SE)
and the minimum (Min) and maximum (Max) prosomal
width of the development stages (DS) A–M found at the study site on Palawan,
along with sample number (n), test statistic of the Shapiro–Wilk Test (W for n ≤
50), the Kolmogorov–Smirnov test (Z for n > 50),
and associated p-values. Further shown
are the results of the modal progression analyses in FiSAT II: the mean prosomal width (M2PW
in cm), standard deviation (SDM2), the statistics of the one-sample
t test (Ta), and the asymptotic significance (two-tailed pa)
|
FiSAT II |
|||||||||||
DS |
n |
MPW (cm) |
SE |
Min |
Max |
W |
Z |
p |
M2PW (cm) |
SDM2 |
Ta |
pa |
A |
12 |
0.98 |
0.02 |
0.90 |
1.10 |
0.96 |
- |
0.717 |
0.99 |
0.06 |
-0.57 |
0.581 |
B |
36 |
1.30 |
0.01 |
1.18 |
1.44 |
0.95 |
- |
0.113 |
1.31 |
0.07 |
-0.65 |
0.523 |
C |
46 |
1.82 |
0.01 |
1.58 |
1.99 |
0.97 |
- |
0.221 |
1.84 |
0.09 |
-1.23 |
0.225 |
D |
54 |
2.48 |
0.02 |
2.22 |
2.84 |
- |
0.09 |
0.200 |
2.49 |
0.12 |
-0.61 |
0.544 |
E |
69 |
3.34 |
0.02 |
2.90 |
3.70 |
- |
0.08 |
0.200 |
3.35 |
0.21 |
-0.21 |
0.832 |
F |
102 |
4.40 |
0.03 |
3.90 |
5.10 |
- |
0.09 |
0.200 |
4.44 |
0.25 |
-1.58 |
0.118 |
G |
143 |
5.96 |
0.04 |
5.20 |
7.00 |
- |
0.06 |
0.066 |
5.96 |
0.45 |
0.08 |
0.939 |
H |
159 |
8.23 |
0.04 |
7.02 |
9.42 |
- |
0.04 |
0.200 |
8.28 |
0.63 |
-1.04 |
0.301 |
I |
143 |
11.11 |
0.06 |
9.60 |
12.70 |
- |
0.06 |
0.200 |
11.19 |
0.79 |
-1.18 |
0.239 |
J |
26 |
14.00 |
0.19 |
12.80 |
16.45 |
0.94 |
- |
0.100 |
14.11 |
0.87 |
-0.58 |
0.569 |
K |
13 |
18.84 |
0.36 |
17.13 |
21.12 |
0.91 |
- |
0.197 |
19.13 |
2.21 |
-0.79 |
0.443 |
L |
34 |
25.40 |
0.29 |
21.66 |
28.90 |
0.98 |
- |
0.618 |
25.61 |
1.33 |
-0.74 |
0.465 |
M |
15 |
32.38 |
0.68 |
29.20 |
36.90 |
0.90 |
- |
0.065 |
30.84 |
3.59 |
2.26 |
0.040 |
a A one-sample t test was used to
compare M2PW with the PWs measured per DS.
Table 3. Mean body parameters (M in cm) ± standard error (SE) of the
development stages (DS) A–M found at the study site on Palawan. The parameters are prosomal
length (PL), opisthosomal length (OL), telson length (TL), total body length (BL), opisthosomal width 1–3 (OW1–3), wet weight, and distance
between the compound eyes (Eyes)
|
PL (cm) |
OL (cm) |
TL (cm) |
BL (cm) |
OW1 (cm) |
OW2 (cm) |
OW3 (cm) |
Wet weight (g) |
Eyes (cm) |
DS |
M ±SE |
M ±SE |
M ±SE |
M ±SE |
M ±SE |
M ±SE |
M ±SE |
M ±SE |
M ±SE |
A |
0.58 ±0.01 |
0.43±0.02 |
0.35 ±0.02 |
1.35 ±0.04 |
0.59 ±0.02 |
0.78 ±0.01 |
0.75 ±0.01 |
0.07 ±0.00 |
0.58 ±0.02 |
B |
0.80 ±0.01 |
0.53 ±0.01 |
0.86 ±0.01 |
2.21 ±0.02 |
0.74 ±0.01 |
1.03 ±0.01 |
1.03 ±0.01 |
0.18 ±0.01 |
0.77 ±0.01 |
C |
1.03 ±0.01 |
0.76 ±0.01 |
1.45 ±0.01 |
3.24 ±0.03 |
0.99 ±0.02 |
1.41 ±0.01 |
1.41 ±0.01 |
0.52 ±0.02 |
1.03 ±0.01 |
D |
1.35 ±0.01 |
1.03 ±0.01 |
2.23 ±0.02 |
4.58 ±0.03 |
1.25 ±0.02 |
1.90 ±0.01 |
1.91 ±0.01 |
1.12 ±0.04 |
1.35 ±0.01 |
E |
1.79 ±0.02 |
1.41 ±0.01 |
3.22 ±0.03 |
6.45 ±0.05 |
1.60 ±0.01 |
2.51 ±0.02 |
2.54 ±0.02 |
2.67 ±0.08 |
1.79 ±0.01 |
F |
2.44 ±0.02 |
1.86 ±0.01 |
4.48 ±0.04 |
8.81 ±0.06 |
2.23 ±0.02 |
3.27 ±0.02 |
3.40 ±0.02 |
6.03 ±0.14 |
2.35 ±0.02 |
G |
3.40 ±0.02 |
2.56 ±0.02 |
6.16 ±0.05 |
12.09 ±0.07 |
3.03 ±0.03 |
4.28 ±0.03 |
4.54 ±0.03 |
15.45 ±0.31 |
3.22 ±0.02 |
H |
4.50 ±0.02 |
3.57 ±0.02 |
8.84 ±0.07 |
16.99 ±0.10 |
3.96 ±0.03 |
5.65 ±0.03 |
6.15 ±0.04 |
35.45 ±1.00 |
4.40 ±0.03 |
I |
6.10 ±0.04 |
4.90 ±0.03 |
12.35 ±0.11 |
23.28 ±0.18 |
5.21 ±0.03 |
7.34 ±0.04 |
8.19 ±0.05 |
100.77 ±2.18 |
5.95 ±0.04 |
J |
7.75 ±0.13 |
6.21 ±0.12 |
15.18 ±0.38 |
28.75 ±0.66 |
6.52 ±0.07 |
8.59 ±0.10 |
9.74 ±0.12 |
181.06 ±6.83 |
7.42 ±0.13 |
K |
10.34 ±0.23 |
8.71 ±0.21 |
21.76 ±0.42 |
40.69 ±0.86 |
8.72 ±0.21 |
11.60 ±0.21 |
12.96 ±0.21 |
471.10 ±33.91 |
10.04 ±0.20 |
L |
13.75 ±0.21 |
11.58 ±0.15 |
28.73 ±0.37 |
54.11 ±0.68 |
11.76 ±0.20 |
15.26 ±0.17 |
17.01 ±0.28 |
1302.7 ±76.88 |
13.54 ±0.47 |
M |
18.46 ±0.43 |
14.80 ±0.33 |
33.76 ±0.78 |
66.77 ±1.37 |
14.28 ±0.30 |
18.41 ±0.48 |
21.24 ±0.41 |
2215.1 ±96.83 |
17.44 ±0.47 |
Table 4. Sample size (n) and the average increase in size (in %) of the
mean body parameters shown in Table 3 for the development stages (DS) A–M
compared to the DS before ecdysis. The parameters are prosomal
length (PL), opisthosomal length (OL), telson length (TL), total body length (BL), opisthosomal width 1–3 (OW1–3), wet weight, and distance
between the compound eyes (Eyes)
|
PL |
OL |
TL |
BL |
OW1 |
OW2 |
OW3 |
Wet weight |
Eyes |
DS |
n (%-incr) |
n (%-incr) |
n (%-incr) |
n (%-incr) |
n (%-incr) |
n (%-incr) |
n (%-incr) |
n (%-incr) |
n (%-incr) |
A |
10 (-) |
10 (-) |
12 (-) |
9 |
8 (-) |
6 (-) |
6 (-) |
4 (-) |
7 (-) |
B |
31 (39.2) |
25 (21.8) |
32 (145.5) |
20 (64.1) |
35 (25.7) |
24 (32.0) |
24 (37. 6) |
11 (169.4) |
33 (32.9) |
C |
36 (28.1) |
45 (43.9) |
41 (69.4) |
33 (46.3) |
46 (32.4) |
42 (36.8) |
41 (36.6) |
24 (183.9) |
43 (32.9) |
D |
49 (31.7) |
47 (35.2) |
44 (53.8) |
37 (41.5) |
51 (26.4) |
49 (34.4) |
49 (35.3) |
39 (117.7) |
51 (31.0) |
E |
65 (32.7) |
64 (37.1) |
52 (44.5) |
50 (40.7) |
57 (28.2) |
60 (32.3) |
57 (32.8) |
56 (137.8) |
61 (32.9) |
F |
94 (36.2) |
95 (32.5) |
75 (39.2) |
70 (36.6) |
94 (39.6) |
91 (30.2) |
88 (33.9) |
77 (125.5) |
90 (31.3) |
G |
125 (39.1) |
132 (37.6) |
114 (37.6) |
101 (37.2) |
116 (35.7) |
115 (30.7) |
114 (33.4) |
91 (156.4) |
114 (36.9) |
H |
143 (32.3) |
147 (39.3) |
127 (43.4) |
112 (40.6) |
143 (30.9) |
138 (32.0) |
139 (35.6) |
55 (129.4) |
149 (36.7) |
I |
135 (35.7) |
141 (37.5) |
132 (39.7) |
121 (37.0) |
134 (31.4) |
139 (29.9) |
137 (33.2) |
116 (184.3) |
143 (35.1) |
J |
22 (269) |
25 (26.7) |
21 (22.9) |
17 (23.5) |
23 (25.1) |
22 (17.0) |
22 (19.0) |
17 (79.7) |
26 (24.7) |
K |
12 (33.4) |
12 (40.2) |
10 (43.3) |
10 (41.5) |
11 (33.9) |
12 (35.0) |
11 (33.1) |
10 (160.2) |
12 (35.3) |
L |
27 (33.0) |
27 (33.0) |
26 (32.0) |
25 (33.0) |
22 (34.7) |
28 (31.6) |
22 (31.2) |
15 (176.5) |
28 (34.8) |
M |
12 (34.3) |
13 (27.8) |
10 (17.5) |
9 (23.4) |
7 (21.5) |
13 (20.6) |
9 (24.9) |
7 (70.0) |
13 (28.8) |
Table 5. Allometric relationships
between the prosomal width (PW in cm) and various
morphometric parameters (y) based on the log-transformed equation with the
intercept a, the growth coefficient b with 95% confidence interval (in bold
indicates allometry), Pearson’s correlation
coefficient r, and the number of cases n.
The parameters are prosomal length (PL), opisthosomal length (OL), telson
length (TL), carapace length (CL), body length (BL), opisthosomal
width 1–3 (OW1–3), the distance between the anal angles (AA) and between the
compound eyes (Eyes), and the weight.
The statistics of the Student’s t test (T) and the asymptotic
significance (two-tailed) (p) indicate whether males (m) and females (f) differ
significantly
x = PW (cm) |
allometric growth: log y = log a + b log x |
Sex difference |
||||||
Sex |
y |
n |
a |
b |
95% CIb |
r |
T |
p |
m |
weight (g) |
224 |
0.07 |
2.99 |
2.956 / 3.016 |
0.997 |
1.899 |
0.058 |
f |
weight (g) |
190 |
0.07 |
3.02 |
2.990 / 3.055 |
0.997 |
|
|
m |
PL (cm) |
269 |
0.56 |
0.99 |
0.982 / 1.003 |
0.996 |
4.662 |
< 0.001 |
f |
PL (cm) |
211 |
0.53 |
1.02 |
1.014 / 1.035 |
0.997 |
|
|
m |
OL (cm) |
274 |
0.40 |
1.04 |
1.030 / 1.048 |
0.997 |
1.158 |
0.248 |
f |
OL (cm) |
218 |
0.40 |
1.05 |
1.037 / 1.053 |
0.998 |
|
|
m |
TL (cm) |
244 |
0.90 |
1.08 |
1.066 / 1.098 |
0.993 |
-9.447 |
< 0.001 |
f |
TL (cm) |
181 |
0.87 |
1.09 |
1.073 / 1.108 |
0.994 |
|
|
m |
CL (cm) |
264 |
0.96 |
1.01 |
1.006 / 1.022 |
0.998 |
13.751 |
< 0.001 |
f |
CL (cm) |
206 |
0.92 |
1.03 |
1.026 / 1.041 |
0.999 |
|
|
m |
BL (cm) |
225 |
1.84 |
1.05 |
1.041 / 1.062 |
0.997 |
1.709 |
0.088 |
f |
BL (cm) |
167 |
1.80 |
1.06 |
1.049 / 1.071 |
0.998 |
|
|
m |
OW1 (cm) |
268 |
0.53 |
0.96 |
0.940 / 0.972 |
0.991 |
2.684 |
0.008 |
f |
OW1 (cm) |
209 |
0.49 |
0.99 |
0.970 / 1.001 |
0.993 |
|
|
m |
OW2 (cm) |
268 |
0.89 |
0.88 |
0.873 / 0.888 |
0.998 |
1.442 |
0.150 |
f |
OW2 (cm) |
215 |
0.87 |
0.89 |
0.879 / 0.897 |
0.997 |
|
|
m |
OW3 (cm) |
260 |
0.84 |
0.94 |
0.932 / 0.948 |
0.998 |
2.090 |
0.037 |
f |
OW3 (cm) |
209 |
0.83 |
0.95 |
0.939 / 0.958 |
0.997 |
|
|
m |
AA (cm) |
266 |
0.32 |
0.95 |
0.939 / 0.966 |
0.993 |
0.452 |
0.652 |
f |
AA (cm) |
206 |
0.32 |
0.96 |
0.943 / 0.973 |
0.994 |
|
|
m |
Eyes (cm) |
282 |
0.55 |
0.99 |
0.983 / 0.998 |
0.998 |
1.333 |
0.183 |
f |
Eyes (cm) |
222 |
0.54 |
1.00 |
0.989 / 1.006 |
0.998 |
|
|
Table 6. Comparison between the mean carapace
lengths (CL in cm) of the instar stages (DS) on Palawan and the postembryonic
stages (PES) in a nursery habitat in Japan (Goto
& Hattori 1929), indicating the two-tailed significance (p) and the
statistics of the one-sample t test (T, df). Further shown are the mean prosomal widths (PW in cm) and the intermolt
periods (IMP in days) ± standard deviation (SD) on Palawan, and the laboratory
studies by Sekiguchi et al. (1988) and Chen et al.
(2010)
Goto
& Hattori |
Tachypleus tridentatus on Palawan |
|
Sekiguchi et
al.
|
Chen et al. |
||||||||
PES |
CL (cm) |
DS |
CL (cm) ± SD |
p |
T (df) |
PW (cm) |
IMP (days) ± SD (n) |
PES
|
PW (cm) |
IMP (days)
|
PW (cm) |
IMP (days) |
1 |
0.74 |
– |
0.74b |
|
|
0.75b |
|
1 |
0.6 |
hibern.
|
0.63 |
53 |
2 |
0.88 |
A |
1.01 ± 0.08 |
0.001 |
4.784 (8) |
0.98 |
|
2 |
0.78 |
21
|
0.89 |
81 |
3 |
1.26 |
B |
1.33 ± 0.06 |
< 0.001 |
5.630 (21) |
1.30 |
|
3 |
1.07 |
60
|
1.23 |
21 |
4 |
1.76 |
C |
1.78 ± 0.09 |
0.120 |
1.595 (35) |
1.82 |
40 ± 4.8 (4) |
4 |
1.33 |
hibern.
|
1.64 |
44 |
5 |
2.30 |
D |
2.36 ± 0.08 |
< 0.001 |
5.486 (44) |
2.48 |
56 ± 9.3 (5) |
5 |
1.75 |
60–80
|
2.20 |
35 |
6 |
3.15 |
E |
3.19 ± 0.18 |
0.096 |
1.691 (61) |
3.34 |
56 ± 6.8 (6) |
6 |
2.22 |
364
|
2.89 |
56 |
7 |
4.18 |
F |
4.30 ± 0.26 |
< 0.001 |
4.522 (92) |
4.40 |
60 ± 0 (3) |
7 |
2.80 |
364
|
3.83 |
46 |
8 |
5.56 |
G |
5.93 ± 0.37 |
< 0.001 |
11.089 (118) |
5.96 |
101 (1) |
8 |
3.73 |
364
|
4.98 |
50 |
9 |
7.42 |
H |
8.07 ± 0.48 |
< 0.001 |
15.865 (138) |
8.23 |
150 (1) |
9 |
4.27 |
364
|
6.57 |
78 |
10 |
9.64 |
I |
11.00 ± 0.81 |
< 0.001 |
19.444 (132) |
11.11 |
|
10 |
4.84 |
364
|
9.10d |
104 |
11 |
12.92 |
J |
13.94 ± 1.17 |
0.001 |
4.059 (21) |
14.00 |
|
11 |
7.00c |
364
|
12.20d |
111 |
12 |
17.27 |
K |
19.05 ± 1.47 |
0.002 |
4.188 (11) |
18.84 |
|
12 |
9.00c |
364
|
16.30d |
132 |
13 |
25.60 |
L |
25.33 ± 1.78 |
0.525 |
-0.793 (26) |
25.40 |
|
13 |
11.50c |
364
|
21.80d |
159 |
14 |
27.42 |
M |
33.45 ± 2.36 |
< 0.001 |
8.853 (11) |
32.38 |
|
14 |
14.70c |
364
|
29.10d |
189 |
|
|
|
|
|
|
|
|
15 |
18.80c |
364 |
38.90d |
228 |
ma |
25.47 |
ma |
25.31 ± 2.08 |
0.795 |
-0.263 (27) |
25.24 |
|
16 |
24.00c |
364 |
|
|
fa |
32.27 |
fa |
33.24 ± 2.85 |
0.264 |
1.177 (11) |
31.57 |
|
17 |
30.80c |
|
|
|
a Adult males (m) and females (f)
without allocation to the PES.
b PW
predicted following the Dyar’s rule. On an average, the CL is 1.67% smaller than
the PW; the predicted CL is hence 0.74cm.
c Sekiguchi et al.
(1988) calculated the PW of PES 11–17 with a constant rate of 1.28
(Hibernation: hibern).
d Chen et al. (2010) predicted the PW with the stepwise
growth equation, age (months) = 3.092 e0.183 PES (r² = 0.9959).