ヤマトオサガニの生活史の時間的・空間的変異

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ヤマトオサガニの生活史の時間的・空間的変異

逸見, 泰久

https://doi.org/10.11501/3063850

出版情報：Kyushu University, 1992, 博士（理学）, 論文博士 バージョン：

権利関係：

Temporal and spatial approaches of life-history variability

in the mud crab Macrophthalmus japonicus

Yasuhisa Henmi

Department of Biology, Faculty of Science, Kyushu University, Hakozaki, Fukuoka 812, Japan

Contents

Introduction ---

1. Temporal approach--- 5

2. Spatial approacn --- 6

Description of study species and study areas 1. Study species --- 10

2. Study areas --- 11

Methods ( 1 ) St. A - - - - 12

(2) St. B - - - - 13

(3)

st. c

(4) St. D (5) Wajiro --- 14

--- 14

--- 14

1. Temporal approach--- 16

2. Spatial approach --- 20

3. ield experiments --- 23

Results 1. Temporal approacl1 --- 27

2. Spatial approach --- 33

3. Field experirl1ents --- 43

Discussion 1. Temporal approach--- 47

2. Spatial approach --- 55

Summary --- 70

Acknowledgment --- 74

References --- 75

Introduction

Life-history tactics (patterns) is a set of coadapted traits designed, by natural selection, to solve particular ecolo~ical

environments (Stearns 1976). But it is not so easy to recoynize what environmental factors affect life-l1istory traits, or how those traits are coadapted among themselves. Furthermore, the life-history patterns of individuals observed in nature are influenced by developmental, physiological and behavioral

responses to environmental conditions. These responses can often obscure the underlying genetic component of life-histories. A general problem is to dissect phenotypic plasticity fro1n

genetically uased variability in reproductive patterns. However, tne life-history field had uch theory and not enough data to dissect phenotypic from genotypic factors t i l l quite recently

(Pace et al. 1984). In the past decade, many empirical papers of life-history tactics have been published in various taxonomic levels and the emerging patterns have been interpreted in relation to theory, but the theoretical interpretation is insufficient in many researches because of the above

difficulties.

Comparative approaches at inter-population level are effective for the interpretation of life-history evolution,

because the genetic differences among individuals may be small in the most cases and the environments may be more resemble than in the comparative approaches at inter-specific level. There may be two approaches in comparison of life-history traits at the lower

taxonomic level : temporal and spatial approaches. A temporal approacn is 1ade through tne comparison between age-groups

(cohorts) at the sate developmental stage during the different periods, but in the same habitat {Skadsheim 1984, Pace et al.

1984), and a spatial approach is between ~opulations in the different habitats, but during the same period (Strong 1972,

ewell et al. 1982, Jones and Simons 1983, Wyngaard 1986ab, Willow 1987b). In the present report, I interpret the

interannual (temporal) variations in a population and the spatial differences among populations in several neighboring habitats in the life-history patterns of ~· japonicus.

Food availability is one of the indispensable environmental factors affecting life-history patterns. However, most of the comparative researches of life-history patterns reported no information of food availability {Strong 1972, Pillay and Ono 1978, Nojima et al. 1981, Nussbaum 1981, Berven and Gill 1983, Willow 1987b, Kusano and Kusano 1988, McClintock et al. 1988, Paulet et al. 1988) or incomplete informations (Simons and Jones 1981, Kyomo 1986). There are few reports with a correct estimate of food availability {Ballinger 1977, Lampert 1978, Newell et al.

1982, ~orkman 1983). In the present report, the importance of food for life-history pattern is discussed through the comparison among several habitats differing in food availability.

In researches of reproductive pattern in crustaceans, few workers have carried out the assessment of ovary {Pillay and Nair 1971, Fielding and Haley 1976, Pillay and Ono 1978, 1987, Bauer 1986), though there are many researches with such assessments in

2

bivalves (Sastry 1970, Jewell et al. 1982, Tunberg 1984, Paulet et al. 1988). In addition, in crustaceans, the role of

he~atopancreas as a nutrient reservoir is i portant.

Particularly in researches of reproductive pattern, the seasonal cycle of hepatopancreas and the influx or energy into ovary

should be required as tne indispensable data (Pillay and air 1973, Ya aguchi and Takamatsu 1980, Pillay and Ono 1987, yomo 1988a). In the present report, the seasonal fluctuation of hepatic weights and the nutritional influx into ovary is studie through the comparison between populations in two habitats

differing food availability.

In most of intertidal deposit feeders, especially in the sessile species or the low mobile species such as the burrowing crabs, the valuation of habitat condition is not so difficult, because the food items are few, and most intertidal areas have habitat gradient for many environmental factors, corresponding with tide level (e.g. food abundance, temperature, exposure

period to air). On the other hand, many intertidal benthos have the planktonic larval stage with random dispersal period,

therefore the genetic difference between populations in the

neighboring habitats may be nearly negligible and the influences of environmental factors for phenotype can be interrupted

directory. From the above reasons, intertidal deposit feeders are apt for the researches of interactions between life-history patterns and environmental factors.

r1ost researches on life-history patterns of crustaceans

under natural conditions have not dealt with two important traits

3

of life istory: groo;.1th rate and wortality. 'rhis 1ay be because crustaceans :~roo;. through molts, which maJ·es i t im.t)ossible to mark an individual for a long time. Likewise, the number of broods per year per female has also been reported for only a few species in crustaceans (Knudsen 1964, Pillay anc Ono 1978, Seiple 1979, Fu 'Ui and Jada 1 986, \ illow 1 987c, Henmi nd haneto 1 989). rrhe relationships among all life-history traits, including those above, should be considered in order to interpret the evolution of a life-history pattern. The present report ai1ns at

explicating the relationships among growth, mortality nd reproductive traits and discussiny the environmental and biological factors affecting life-history patterns in ~­

japonicus.

Reproductive patterns (age-specific energy allocation) are the characteristics most often considered (Cody 1966, Snell and King 1977, Bell 1984b, Paul and Fuji 1989). As organisms

increase the proportional allocation of limited resources into reproduction, reproductive effort should lead to somatic cost in growth and maintenance (reproductive cost). The theory of

optimal reproductive effort postulates a trade-off between current fecundity and future fecundity (fecundity cost) or parental survival (survival cost). The evidence for such a trade-off was found in rotifer (Snell and King 1977), isopod

(Steele and Steele 1986), chaetognaths (r~acLaren 1976), lizards (Tinkle 1969) and fulmars (Ollason and Dunnet 1988), but not

found in rotifer (Bell 1984a), isopod (Tuomi et al. 1988), mussel (Bayne et al. 1983) and gulls (Thomas and Coulson 1988). In the

4

resent report, an attention 1as been focuse also on the existence of reproductive cost.

1 • Ternpora l approach

A life-history pattern is a set of life-1istory traits which are utually coadapted in a given environment. ~vcn in the same habitat, the life-history traits (e.c. growt rate, brooa size) may fluctuate seasonally or annually accordins to the 1abitat conditions (Ballinger 1977, Brody and Lawlor 1984, Taylor 1988).

Long-term studies that at the least cover a longer period than the lifespan of the study species are required to compare life- history patterns among populations or species, because the current life-history traits of individuals (e.g. reproductive effort) are affected by the past traits, which are ignoreJ in the

s ort-term research.

!•lost studies on population dynamics of benthic macrofauna were carried out on a short-term basis and usually include only one annual cycle, while Josefson (1982), Newell et al. (1982) and Bowman and Lewis (1986) considered long-term fluctuations of

life-history traits in a population. In brachyuran crabs as well, most studies of population dynamics were carried out on

just one annual cycle (lJakasone and Okadome 1981, Siwons and Jones 1981, Colby and Fonseca 1984, Fukui 1988). I know of no study dealing with ocypodid crabs on a long-term basis except one short report on Uca pugnax and

Q.

^{minax (Cammen et} al. 1984). In the present study, I compare life-history patterns among seven age-groups (cohorts) of

L·

japonicus in a habitat and examined the interannual fluctuation of habitat conditions.

5

2. Spatial approacn

'1e goal of life-history tneory is to predict how an

organism will distribute its progeny tnroug1out its lifespan in order to maximize its re~resentation in future generations. Life-history patterns of individuals obscrvea in nature are

genotypic, developmental, physiological, and behavioral responses to environ1ental conditions, even in the same species, various life-history patterns may be possible for each environmental condition. In order to establish the life-history theory, therefore, enough data of life- istory ~attern in various

environmental conditions should be required in the same species.

In recent years, many papers of intra-specific variability in life-history traits have been published in various taxonomic

groups~ in trepang (Costelloe 1988), in li1npets (Workman 1983, Bowman and Lewis 1986, Fletcher 1988), in gastropods (Lam and Calow 1989a), in bivalves (Sastry 1970, Seed 1977, Kraeuter ct al. 1982, Newell et al. 1982, Bayne et al. 1983, Gosling 1984, MacDonald and Thompson 1986, 1988, Paulet et al. 1988), in prochordates (Grosberg 1988), in pisces ^(Azui~a 1973, ¹ aekawa 1984, Gross 1985, Henrich 1988, Nishimura and Yamada 1988,

L'abee-lund et al. 1989), in amphipods (Berven 1982ab, Berven and Gill 1983, Crump 1984, Berven and Chadra 1988) and in reptiles

(Ballinger 1977, Nussbaum 1981 ).

In Crustaceans as well, intra-specific variations in life- history tactics have been reported in various levels~ among geographically separated populations (Strong 1972, Simons and

6

Jones 1981, Jones and Simons 1983, iashi}'o 1984, ¹ ishino 1984, Lonsdale anc Levinton 19 5, yngaard 1986b, Ienmi 1987, Okamoto and -urihara 1987, Corey 1988ab), among the neighboring

populations (Barnes and Barnes 1968, Smaldon 1972, Feller 1980, 1-'loryan 1980, Palmer 1930, lojima et al . 1981, Onori ct al. 1984, Belk et al. 1990), among ferales in the same population (Chapman and Howard 1979, La'anan and Cohen 1985, Lively 1986abc, 1illow 1987a), among clutches of the sctme female (Diaz 1980, Brody anJ Lawlor 1984, Omori and Tanaka 1984, Pace et al. 1984, Skadsheim 1984, Kusano and Kusano 1988) and even within a clutch (Lewis et al. 1982).

In most of these reports, however, i t is not clear whether genotypic or environmental factors contribute to the differences in life-history traits, or what kind of environmental factors are important: food availability, temperature, predation pressure or else. Perhaps, this may be because many factors complicatedly influence the life-history patterns in many works. In the

present report, I co pare life-history patterns of ~· japonicus, mainly between two populations inhabiting both sides of the

Tatara River. Though several environmental factors may influence life-history traits of the two populations, the present

investigation was designed with emphasis to the difference in food availability. Growth rate, mortality and several

reproductive traits (breeding season, egg size, brood size and brood number per year) are compared between the two populations.

The study of gonad state (1) provides accurate information of the breeding condition of the individual animal, (2) reveals

7

the breeding activity of the individuals, and {3) shO\JS the

se~uence of events in t e gonad between spawning seasons {Pillay and -Jair 1 971 ) • There are 1l1any studies on the gonad cycle, in

limpets { Bo\vman and Leu is 1 977, lorkman 1 983) , in bivalves (Sastry 1970, Bacnelet 1980, ayne et al. 1983, lacOonal and Thompson 1988), in sea stars (Farrranfarmaian et al. 1958, Soliman 1 986 ) and in crabs ( Pillay and Nair 1 971 , 1 97 3, Chiba and 1Ionma 1972ab, Erdman and Blake 1988). However, few studies conpared seasonal change of ovarian and hepatic states annually (13owrlan and Lev1is 1 97 7, 1 986) , between species { Farmanfarmaian et l.

1958, Pillay and lair 1971, Pillay and Ono 1987) and between populations {Costelloe 1988, this study) and so the relationship between the somatic growth and the reproductive effort is not clear in many studies.

Hepatopancreas is usually considered to be the main storage organ in decapod crustaceans {Ad~y6di 1969, Heath 1970, Pillay and Ono 1987, Kyomo 1988ab). ~he study of the seasonal changes of gonad and hepatic states is important for understanding the reproductive strategy {energy allocation into somatic growth and reproduction). However, there is insufficient infori ation on annual changes of gonad and hepatic cycle of crabs except several studies {Heath 1970, Yamaguchi and Takamatsu 1980, Pillay and Ono

1987, Henmi 1989c). In the present report, seasonal changes of

gonad and hepatopancreas are intensively followed. The seasonal trends of nutritional transfer inside the body are traced by the correlation graph between the seasonal values of gonad and

hepatopancreas {Pillay and Ono 1987) and these trends are

8

considered in relation to tne seasonal variation of reproductive effort. Discussions are r ade for the adaptive meanings of

differences in the seasonal nutritional transfer and the reproductive effort between the populations.

9

Description of study species and study areas

Study species

The ocypodid crab ~acrophthalmus japonicus (De Haan) is an intertidal mua crab and it has been recorded from Honshu,

Shikoku, Kyushu an Tanegashima Island, Japan, and the

continental coast of Yellow Sea and northern Bast China Sea ( Jada 1991 ). In Japan, this species is one of the most dominant

macrobenthic animals in most of the intertidal mud areas. It usually lives in isolated burrows which are dug in muddy regions of estuaries. ~- japonicus emerges from its burrow during

periods of daytime low tide to feed or engage in other surface activities nearby, but i t remains in its unplugged burrow duriny low tide at night. Like many other ocypodid crabs,

B·

japonicus feeds on organic matter separated from the surface mud by the mouthparts after the mud has been scooped into the buccal cavity by the chelae.

In Fukuoka, this species breeds from April to October (Henmi 1989a). The settlement of juveniles occurs from August with a peak in November. The juveniles grow little in winter, but srow rapidly after next April, and average carapace width reaches 15 mm in August ( ^~ 1 3 months old) and most crabs mature t i l l

September in the year. The longevity of this species is almost 25 to 28 months. However, life-history pattern of

fi.

japonicus was different geographically ( ~da 1986, Henmi 1987), annually and between sites (the present report).

Life history of the Ocypodidae has been mainly studied in

1 0

t1e fi dler crabs, Uca s (Pillay and air 1971, Ya1aguchi 1978, ~a·aso e and Okadome 1981, Colby and ~onseca 19bq), or a fe species inhabitln intertid l sandflats such as Uca

(Yarnasucni and Tanaka 1974, Ilenni and aneta 1989). rrhere are fe\v scudies on life history of the 1ucrol:)ht li.tine e .. cept only two species .hacropnthalmus hirtipes ( Sir~1ons and Jones 1 981 ) an

B·

japonicus (Omori et al. 1984, Henrni 1989ab, enmi and l'aneto 1989), which are mainly inhabiting intertidal mudflats. In 11.

japonicus, two forms (Form Land Form V) aiffering be1avior, morphology, habitat ( lada 1 978) and life history ( Henmi 1 989a) have been reported. Only recently, the Form L was recorded as another species

M ·

banzai, closely related to

J.

japonicus (Wada and Sakai 1989). Furthermore, the geographical variations in morphological and behavior characters are remarkable in each form

(species) (Wada 1986, Henmi 1987) and furthermore life-history patterns are different annually or between the neighboring habitats (the present report). Therefore, the comparative approach of life-history patterns between populations in j .

japonicus may be provide important informations on the selective pressures for life history in the different environmental

conditions.

Study areas

Field studies of

M·

japonicus (Form V) were carried out on two intertidal areas (Tatara and ajiro) in Fukuoka, northern Kyushu, Japan (Fig. 1 ). Four stations (St. A, B, C and D) on the Tatara-Umi River and one station ( Jajiro) on the Tonoharu River, located in the eastern side of Hakata Bay (33°36'N; 130Q24'E) ,

1 1

were use as study sites {Fie. 2). The researches of ~opulation dyna1ics of 7-yr period {1981 to 1987) were carried out in St. A.

Coil1tJarative approaches of life-history patterns between

po~ulations were carried out mainly in St. ¹ and St. D, an co plementally in t e other stations. Field experiments were conducted mainly in St. C.

(1) St. A

st. A is an intertidal mudflat ( - 2.0 ha) in the estuary of the Tatara-Umi River, and i t is ^.-v 1. 2 km up-stream froll1 the sea, whicn has been described in detail elsewhere {IIenmi 1984).

Salinity of the river water was measured with a hand salinometer at spring low tide (Atago Co., Ltd., Model S/Nill). Salinity averaged 1 0. 3 %o (range 0 to 28 %ci , n=42) for the river water and 20.8 %C {10 to 28 %0 , n=117) for the water in_:!. japonicus

burrows. Air-exposure duration of the burrow area usually ranged from 6 to 9 h in eacn tide cycle in accordance with moon phase, but the burrow area was not ex~osed at all after heavy rain.

Seasonal variations of temperatures of air, later and mud {surface and 1 0 c1 depth) in 1 985 are shovln in Fig. 3. The measurenents were made from 1 to 3 p • • during sprins low tides. Mean of air temperature, rainfall and shining hours in each ~~onth

at the Fukuoka leteorological Observatory in 1980 and during tne period 1981 to 1987 are shown in Fig. 4. Thus, the weather in 1980 was unseasonable, e.g. low air temperature, much rainfall and short shining hours in tne summer. The burrows of

M ·

japonicus occurred in muddy area within 12m from the concrete

1 2

Hakata Bay ^Tatara

5.0 km

Fig. 1 Map showing the locations of the study areas {Wajiro and Tatara).

Hakata

Bay

St.A

Totaro

; .

1.0 km

Fig. 2 Maps of the study areas. Four stations in the Tatara-Umi River.

40

~ ³⁰¹ .o.:~\

~

OJ

^20j j/}0 .

~

~

ClJ

¹⁰¹ p::/

r ~~·

\\\A

•

o~ I

J F M AM J J A S O N D

M on t h

Fig. 3 Seasonal changes of temperatures of air, water and mud (surface and 10cm depth) in the burrow area of St. A in 1985.

e:

air temperature, 0: water

temperature,~)(:

temperature on mud surface, A: temperature of mud at 10 em depth.

~

u

0

... 30

+--·~

Q;

~

~/"'-o---..o·~

:J

~ ^{0 ~ Q; 0...}

_E

²⁰ ₁₀

/ ^{J /} ~/ ""'' . ^~ at~

Q;

0+~+~ "\:0+

...

~

<{ 0 1000

~

E

...

E

500

0

'+-

c

·-

0 0::

.· 0 300

(/)

~

:J

_c 0 ₂₀₀

01

c c

¹⁰⁰

_c

tf) 0

J F M A M J J A s ^{0 N D}

Month

Fig. 4 Seasonal changes of air temperature, rainfall and day light hours at the Fukuoka Meteorological Observatory, ~7km from St. A. Open circles and shaded bars indicate the value in 1980.

Closed circles and open bars indicate the average between 1981 and 1987. Vertical bars indicate the range between 1981 and 1 987.

river ban]·. In St. , activity of most _. j aponicus vas

restricted nearby their O\n burrows an few crabs wandere away from their burro\s. Even during low tide, most burrows were

flooded and all burro\JS descended to the ater table. Some other species were found coexisting with [. japonicus, though they were in small numbers. Some Ilyoplax pusilla were distributed in the upper intertidal sand-rnud area and Hemigrapsus penicillatus and rlelice tridens were often seen under drifted materials or stones in the station. These crabs species is not predacious for M.

japonicus.

(2) St. B

st. B is also an intertidal mudflat ( - 0.5 ha), ^-v 500 m down stream from St. A. _. japonicus burrows occurred over ¹ ost of the rnudflat in this station. Air-exposure duration of the burrow area ranged from 4 to 8 h in each tidal cycle. The burrow area was drier during low tide as compared with St. A and some burrows did not descend to water table during low tides. In suumer,

specially during spring tides, many crabs abandoned their burrows and wandered between the burrow area and the water's edge (detail in Henmi 1984). Such drove formation has been observed also in other areas (Henmi 1989b) , but not been observed in the other study stations. In St. B, many Ilyoplax pusilla and sowe Helice japonica burrows occurred in the upper intertidal sand-mud areas.

Wandering Hemigrapsus penicillatus and Helice tridens were also often seen. Helice japonica is predacious for small

L·

japonicus, but predation by this species was seldom seen during the study period.

1 3

(3) t.

c

st. Cis an interti<ial sand-mud flat (..-0.8 ha) on the

opposite side of St. A with the Umi Piver between. Air-exposure duration of the burrow area ranged fro1 6 to 8 h in each tidal cycle. f1ost I. japonicus burrows occurred in the lower muddy intertidal area, but some in tne upper intertidal sand-mud area.

1\Iost burrows descended to the water table and wandering

J.

japonicus were seldom seen alike St. A. In the upper sand-.u intertidal area, many Ilyoplax pusilla ana some Scopimera ;lobosa burrows occurred. In tne upper-supratidal area, Hemigrapsus

penicillatus, 1ielice tridens and Parasesar1 a erythrodactylurn were often seen.

(4) st. D

St. D is an intertidal sand-mud flat (- 0.4 ha) on the opposite side of St. C with the Tatara River between. Air- exposure duration of the burrow area ranged from 6 to 10 h in each tidal cycle. Most of

M ·

japonicus burrows 1ere seen in

several muddy areas distributed patchily in this station. In the sand-mud areas, the populations were mainly composed of large crabs. 'lost burrov1s in the sand-mud area did not descend to the water table, but wandering

M ·

japonicus were seldom seen alike St. A. ~any Ilyoplax pusilla and few Deiratonotus cristatus burrows occurred over most of this station. Wandering Helice tridens and Helice japonica were often seen mainly in the sand-

tud areas.

1 4

(5) "ajiro

·Jaj iro is an intertidal sand flat (....,., 0.

o

ha) in the estuary

of the Tonoharu River. Air-exposure duration of the burrow area ranged from 7 to 10 h in each tidal cycle. rost of

J.

japonicus burrows occurred in the upper sand-mud area, but nost burrows were flooded and descended to the water table. Vandering

L·

japonicus \vere seldom seen alike St. A. rlany Scopirnera globosa burrows occurred in the lower sandy area and wandcrin~

Hernigrapsus penicillatus and Helice tridens were often seen.

1 5

1. Temporal approach

Particle size of substratum

f\ ethods

To analyze particle size, a sample of

-sao

g of mud was taken from the top 1 to 2 em of substratum, in each of the burrow area and the neighboring lower area of St. A in August 1987.

Bach sample was passed through a series of sieves (mesh size:

1.0, 0.5, 0.25, 0.125 and 0.063 mm} . The parts of each sai ple remaining on the respective meshes were weighed after drying at 80 C for three days, and their percentages relative to the whole dry weight were calculated.

Food availability

Sediment nitrogen content was the best indicator of food availability in deposit feeders (Levin and Creed 1986, Grernarc et al. 1988}. Surface mud (2 to 3 mm depth) o f - 100 g was,

therefore, collected at random from the substratum around the burrows. Each sample was dried at 60°C for two days and nitrogen content was ¹ easured by an elemental analyzer: Yanagimoto Co., Ltd., Ilodel L'lT-500 (details in Henmi 1984}. Nitrogen content was measured in the burrow area of St. A in June, August and r~ovember

from 1982 to 1987, in the burrow area of St. A and its

neighboring lower area in August, Septe11ber and October 1987.

Field sampling and handling

Sampling quadrats (50 em square, n=30 to 60) were evenly spaced in the burrow area of St. A, and all crabs in the quadrats

1 6

~ere ca~turea by reJoving the mua around t1e ^burro~s carefully without sieving. As this procedure caused an underesti ate of ne\l recruits, 16 sampling quadrats ere placed randonly in the area and each quadrat was excavated to a depth of 7 em. The excavated sedi ent was washed through a 1.0 mm mesh sieve, and

juvenile crabs s aller than 10.0 min carapace wiath (C ) were counted and measured. Sampling quadrats were placed over the entire burrow area, so the sample almost completely coverea the population in the habitat. Moreover, the nu ber of crabs sampled in a month was estimated to be less than ¹ % of the population size and most crabs were released at the study station after measurement, so the sampling 1ay have had little affect on size- distribution and population density. Crabs were sampled in St. A, monthly from May 1981 to March 1985, semimonthly frorn April to October 1985, and monthly again from November 1985 to October 1987.

Crabs were taken to the laboratory, where their carapace widths were easured to the nearest 0.1 illm using hand calipers or a micrometer under a binocular microscope. Sex and presence of eggs were recorded, and most crabs were then released at the study station. Crabs less than 5.0 mm CW were difficult to sex, so these crabs were classed as juveniles and divided equally into sexes assuming an equal sex ratio. Length of chela propodus and abdomen width at the fourth segment for both sexes sampled from June to August, 1985 and 1987, were measured as an index of morphological maturation (Simons 1981 ).

Recruits of M. japonicus at different tidal levels were

1 7

surveyed in St. A. Fifteen uadrat (50 c square) 1ere space at: 3m intervals fro the river ban~ to 1ater's e ge at s ri g low tiaes. Each quaurat was excavateu to a de th of 7 en and the excavated sediment was washed tlrough a 1.0 mm nesh sieve which retained captured crabs. Crabs burrowing deeper t an 7 em were captured oy removing the sediment carefully. he sanplin ere made monthly frow September 1984 to lay 1985.

Size-frequency distributions in the burrow area and its neighboring lower area of St. A were surveyed in Septe ber 1987, when crab density greatly increased and many crabs abandoned their burrows and wandered. Sampling quadrats (50 cw square) were spaced in the burrow area (n=46) and the lower area (n=70) and all crabs in the quadrats were captured in 1ay 1983 b y t e same method as stated above.

Growth and survivorship

Growth patterns were determined by graphical analysis of modal progression in successive size-frequency distributions.

odes in the size-frequency distribution for each sex were

distinguished by plotting cumulative size-frequencies on a probit scale (Cassie 1954). Density of each cohort was determined in the same manner as for the growth patterns. Survival rate was calculated from the temporal change of density of each age-group

(cohort). The total number of recruits in each cohort cannot be determined, so the maxi urn density of the 0+ year-class (O+yr) was taken as the measure of recruits.

1 8

Reproduction

Females with recently deposited eggs, sampled in June 1985 and 1987, were preserved individually in 10 % formalin solution.

Eggs were removed from the pleopods using a fine brush and

rinsing with water. A sample of ""2000 eggs was taken from each female and filtered by a vacuum pump onto a preweighed filter paper drawn with grid lines, and counted under a binocular

microscope. The remaining eggs were filtered onto another filter paper. Both the sample and the remaining egg mass were dried at 60°C for two days. The dry weight of a single egg for each

female was estimated by dividing the sample weight by the egg number. The number of eggs per brood for each female was

estimated dividing the weight of the egg mass plus its sample by the weight of a single egg. Each female was dried at 60°C for two days and the percentage of total brood weight relative to body weight was calculated. Diameter of 15 eggs from each female was measured using a micrometer under a microscope and the mean egg volume was calculated.

The effect of temperature on egg development was assessed.

From May to June 1 985, a total of ^...v 30 males and .._ 1 00 non-

ovigerous females were kept in cages established in the mud area of St.

c,

and oviposition of females was examined at 3-day

intervals. Only females with recently deposited eggs were taken to laboratory and kept individually in beakers containing sea water (salinity 24 ~) and incubated at 15, 20, 25 and 30°C. Egg stage was examined under a binocular microscope every day until hatching to determine incubation period.

1 9

The average brood number yr- 1 female- 1 (B) was estimated for each cohort using the following equation, modified from that of Fukui and Wada (1986).

b Mi Di

B =

L

_i=~

^{- - · - -}

^{Ii Df ( 1 )}

where Mi: the length of the ith month, Ii: incubation period in the ith month (Fig. 3, Table 9), Di: density of ovigerous females of each cohort in the ith month (Fig. 17), Df: density of females of each cohort in the first month of breeding season, a and b:

the first and last month of the breeding season, respectively (details in Henmi 1989a).

2. Spatial approach

Particle size of substratum

The top to 2 em of substratum was sampled in the burrow areas of St. A and St. Din July 1984 and the particle size was analyzed in the same manner as for in the temporal approach

(details in p. 16).

Food availability

In St. A and St. D, the nitrogen content was measured

monthly in 1985. Surface substratum (2 to 3 mm depth) was sampled in the burrow areas of each station and nitrogen content of each sample was measured in the same manner as for the temporal

approach.

Field sampling and handling

Sampling quadrats (50cm square, n=60 to 150) were evenly

20

spaced in st. A and St. D and all crabs in the uadrats ~ere

ca tured by t1e met1ods as for in t' e terJporal approach. Crabs vere sar.1pled in St. A and St. D, 1Ltonthly frorJ January to 1arcn, semi-monthly from April to September and monthly again frolll october to December 1985.

Growth and survivorship

Carapace width, sex, presence of eggs were recorded for each crab sampled in each station. Some of then were preserve in 10

%formalin solution and the otner crabs were released in t1e stations. The length of chela propodus and the width of abdorten at the 4th segment were easured for each sex w1ich sampled in st. A and St. D from June to August 1985 (details in p.17).

Growth pattern and survivorship were determined by the graphical analysis of the modal progression and the temporal change of density of each cohort in successive size-frequency distributions (details in p.18).

Egg and brood

Fer ales with recently deposited eggs, sampled in June and August 1985 in St. A and St. D, were preserved individually in 10

% for alin solution. Brood weight, single egg weight and egy volume and egg number per brood were estimated for each female

(details in p. 19-20). The brood nu1ber per female per year (B) was estimated by the Eq. (1) for each cohort in St. A and st. D.

Hepatic and ovarian weights

One+yr and 2+yr males (each n=30) sampled from St. A and St.

21

D \Jere dissected u der a microscope for semi-monthly sa11 ples from April to September 1985 ana for monthly sa ples fran January lo

~arcn and from October to December 1985. The percentage of

hepatic weight relative to body wei~ht (including nepatopa creas)

-v1as obtained after dryiny tissues at 60° C for two days. In the same manner, 1+yr an6 2+yr non-ovigerous females (each n=30) sampled from St. A and St. D 1ere dissected in 1985 and the

percentage of hepatic and ovarian weights relative to body weiynt (including hepatopancreas and ovary) were obtained. Furthcrr ore, in the same manner, 1+yr and 2+yr ovigerous females (each n=30) sampled from St. A and St. D were dissected in the breeding

season and the percentage of hepatic and ovarian weights relative to body weight (excluding brood weight) were obtained. The

seasonal change of gonad of males was not referred in the present study because gonad (testis) of male 1. japonicus was too small for making comparison between areas.

Comparisons among five habitats

Size-frequency distributions were compared among the five stations (St. A, St. B, St.

c,

St. D and Wajiro) . Sampling quadrats (50 em square) were evenly spaced in each station and all crabs in the quadrats were captured by the methods as for in the temporal approach. Crabs were sampled in each station in June, August and November 1984.

The top of 5 to 6 mm of substratum was sampled in the burrow area of each station in July 1984 and the particle size of

substratum was analyzed (details in p.16). Furthermore, surface substratum (2 to 3 mm depth) was sampled in the burrow area of

22

each station in June, ugust and ovember 1984 and the nitrogen content of each sa1 ple was measure {aetails in p.16).

size-frequency distributions at different tidal levels \Jere surveyed in St. A and St. C. In St. 1 , the burrow area was

divided into three parts {10 uidtn) accordinq to tide level.

sampling quadrats {50 cr1 square, n=15 to 20) were spaced in each part and all crabs in the quadrats were captured in rlay 1983 by the same method as stated above. In St. C, the burrow area was divided into five parts {20 m width) according to tide level.

Sampling quadrats {50 em square, n=30 to 100) were spaced in each part and all crabs in the quadrats were captured in July 1987 by removing mud around the burrow carefully witnout sieving.

3. Field experiments

Stationary tendency

Stationary tendency under the low and high crab densities was examined with relation to crab size {age). Quadrats {1 m square) were established in the middle-lower area of St. C {low density n=4, high density n=10 ) and all crabs in the quadrats were removed. Each quadrat was enclosed by boards {50 em aerial height, 30 em depth into soil) , so crabs could not leave the quadrats. Five and twelve adult crabs in each three different size-classes of both sexes were kept in the quadrats for five days and then the boards were removed. The small {13 to 16 mm CW), middle {18 to 23 mm C~) and large {25 to 30 mm CH) sizes correspond to the age of 1+, 2+ and 3+yr, respectively (Henmi 1989a). ithin the five days, all crabs in the quadrats made

23

tneir o n bu:cro\lS. Three ays after rellloving t e ooards, all crabs in t e ~uadrats were san led an their carapace vidths and sex were recorded. rrhis examination Jas conducted fro1 1 nea to spring tides in September 1987.

selection of burrouing sites

Selection of burrouing sites by t1e middle and large crabs for two soil textures (sand and nud) was exa11ined under t 1e

different densities. Eight plastic containers (45 em x 70 c1, 25 em depth) were established in the supratidal area of St. C. Half side of each container was filled with 20 em depth of sand

sampled from the upper area of St. C, the other half side was filled with the same depth of mud sampled from the lower area of st. C and the containers were covered with meshes. r ale and female crabs of the same number (the middle and/or large) were kept in the containers for three days in the different densities and in the several combinations of the middle and large crabs (4 to 32 crabs in a container, details in Table 18), and then the position of burrow established by each crab was recorded. This examination was conducted from September to October 1987.

Crab activity

Surface activity of crabs was observed under the low and high densities with relation to crab size (age). Four cages (1 m square), enclosed by the boards (50 em aerial height, 30 em depth into the soil) and covered with meshes, were established in the lower-middle burrow area of St.

c.

Two and twelve crabs in each of three size-classes of each sex were marked with numberea

24

plastic tapes and release, in the lo\1- and igh-density cage

respectively during neap t i e. Seven days after the crab release (during spring tide}, 1hen most crabs establis e tneir O\Jn

burrows, activity of all craos in the cage was recorded at 5-min intervals during three 100-,in observation erio s by the visual scanning: (1} i mediately after emersion of the cages, (2) aroun low tide, and (3} just before sub~ersion. Detailed notes were made of behavioral activities, but the behavior was divided into the follo\ling categories to facilitate analysis: feeding,

walking, standing still, occupying burro\ , fi hting, wavin nd other. This recording was carried out in the same period between the low- and high-density cages, i.e. spring tides from July to September in 1986 and 1987. Furtnermore, nitrogen content of tne surface substratum in tne cages vas measured at the three

observation periods in five observation days of August 1987.

Cage rearing

Mortality of large crabs was examined under the low and high densities. Nine cages (90 em x 50 em, 15 em aerial hei~ht, 30 em depth into the soil} covered with metal-meshes were established in the middle-lower burrow area of St. C. Five large crdbs in each sex were kept in the for cages (low density}, and eight

small, seven middle and five large crabs in each sex were kept in the five cages (high density}. The size co bination and density of the high-density cages resemble these in the habitat. All crabs in the cages were sampled monthly by removing mud around the burrows carefully, then their carapace widths and sex were recorded and released in the same cages again. New recruits v1ere

25

re. oved fror the cages. The rear in:.~ ~as con ucte in the lo - and 1ig· aensi ty cages fro .1 ugust to ^J. ove .. 1ber in 1 9 7 and 1 980.

In order to exarnine srowth and brood number per fe1ale unaer the different food availability, tJo small c~ses (50 ems uarc, 15 em aerial neight, 30 em deptn into the soil) covered with metal-meshes flere established in each of the upper sandy area an the lo\Ter muddy area of St. C in ray 1 986. dales ( 20 to 25 mm

c

^·J, age 2+yr) ana non-ovigerous females ( 1 3 to 1 6 mm c~:, age 1+yr) were sanpled in the lower area, mar·ed with numberea

plastic tapes and reared in the ca:.~es (males n=S, females n=10 in each cage). Carapace width of females at the start of the

examinations \vas 1 4. 5.±_1 • 5 rmn ( mean.±_SD, n=40) and 1 4. 2.±_1 . 3 ^IdiO (n=40) in the upper and lower cages respectively, and there was no significant difference in mean carapace width (p>0.2, t-test).

All crabs in the cages were sampled monthly by removing mud

around the burrows carefully. The presence of e~gs was examined for females, and furthermore their carapace widths and abdomen widths at the 4th segment were measured for no-marked (molted) females and then they were newly marked. After the treatments, male and female crabs except new recruits were releasee in the same cages again. No crab was added even if crabs decreased.

'umber of broods in a breeding season was calculated for fe ales survived to September in each year. The rearing was conducted from July to October 1986 and from I1ay to ovember 1987.

~itrogen content of the surface substratum in each cage was measured in June and August 1987.

26

Results

1. Temporal approach

(1) Particle size of substratum

The distributions of sediment particles in the burrow area and its neighboring lower area of St. A were shown in Table 1.

The median diameter of sediment particles was smaller in the burrow area of St. A than in the lower area. Particularly, in the lower area, the percentage of silt-clay (< 63 pm) was lower.

(2) Food availability

Nitrogen content of surface substratum in the burrow area of St. A in June, August and November 1982 to 1987 is shown in Table 2. It varied significantly among months and years (month:

p<0.001, year: p<0.001; 2-way ANOVA), though the differences were not especially large. Table 3 shows nitrogen content in the

burrow area and its neighboring lower area of st. A. Thus, nitrogen content was significantly lower in the lower area than in the burrow area.

(3) Annual variations in crab density

Recruitment of juveniles (< 5.0 mm CW) occurred from July onward in 1982 and 1987, from August in 1984 and 1985, from

September in 1981 and 1983, from October in 1986 (Fig. 5). Thus the start of recruitment varied from year to year, but the

density of juveniles was at its maximum during the period of November to January every year and thereafter decreased rapidly.

The number of recruits was very similar during the period 1981 to

27

1984, and maximum density of juveniles in each year was 22.0 to 44 .0 m-2 (Fig. 5-1), whereas recruits increased greatly after 1985 and their maximum density of juveniles was 98.5 to 188.6 m- 2 in the period 1985 to 1987 (Fig. 5-2).

Density of immature crabs (< 13.0 mm CW) decreased after the period of March to May every year (Fig. 6) and the decrease was owing to both their death and maturation (see Fig. 10 and Fig.

11). The density of immature crabs (0 to 211.6 m- 2 ) greatly fluctuated seasonally and annually, but their fluctuation is mostly accounted owing to that of recruits.

Density of mature crabs (~ 13.0 mm CW) did not fluctuated so greatly (11.0 to 32.6 m- 2 ) as that of immature crabs (Fig. 7).

From 1981 to 1985, the density of mature crabs increased after the period of May to July, because many immature crabs reached the mature size during this period and thereafter gradually decreased owing to their death. In 1986 and 1987, however, the fluctuation of the density of mature crabs was different, because of their delayed maturation. The sex ratio of mature crabs was nearly 1:1 during the study period.

Figure 8 indicates distributions of crabs at different tidal levels from the bank to the water's edge in St. A in each month.

During the early period of recruitment, many juveniles were found also below the burrow area of adult crabs (0 to 10 m from the bank). However, few crabs were found below the burrow area after January 1985 and the density of juveniles in the burrow area

increased, perhaps indicating that many juveniles moved from the lower area toward the burrow area.

28

Table 1. Distributions of diameters of sediment particles (%) in the burrow area and its neighboring lower area of St. A.

Particle size Burrow area Lower area

<

⁶³

pm

^{37.5 5.5}

63 ^- 125

pm

^{19.2 12.5}

125 ^- 250

pm

35.5 13.4

250 ^- 500

pm

5.5 53.5

500

pm

^- 1 mm 0.1 9.4

) 1 mm 2. 1 5.7

Median diameter <pm) ⁷¹ 225

Table 2. Nitrogen content (mg) per g dry surface mud in the burrow area of St . A (mean~SD, n=5) . ND: no data.

Year June August November Average

1982 ND 1 . 37~0.09 0.92~0.10 1. 14 1983 ^{0.90~0.16 0.80~0.09 1.19~0.09} 0.97 1984 1.23~0.54 1.11~0.27 1.24~0.10 1. 19 1985 1.24~0.21 1.00~0.13 1.25~0.16 1.16 1986 0.97~0.10 0.85~0.10 1.19~0.11 1.00 1987 1.10~0.29 0.88~0.17 1.64~0.16 1.21

Average 1.09 1.00 1.24 1. 11

Table 3. Nitrogen content (mg) per g dry surface mud in the burrow area and its neighboring lower area of St. A in 1987 (mean~SD, n=5). P shows significant differences

between the areas (Mann-Whitney U-test).

Burrow area Lower area

August

0.88~0.17 0.65~0.11

P=0.004

September

0.92~0.02 0.59~0.14

P=0.004

October

1.36~0.16 0.80~0.13

P=0.004

~

N

E

(f)

<lJ

- c

(])

>

:J

·~

~

0

50

0 50

0 50

0 -+- 50

"U1

c

<lJ

0

0

19 81

19 82

I I I I

•"'Y"'...,..•-r •

-~--~---....,.

19 83

19 84

~/t\

._...

^~

^...

I I I I . I • - , - • -r • """r • - , - -... ,-...---.

J F M A M J J A S 0 N 0

Month

Fig. 5-1 Seasonal variations in density of juveniles (< 5.0 mm CW) per m2 from May 1981 to December 1984. Vertical bars

indicate means+SD.

200

19 8 5

~

N 100

--- E

~

~

{f)

+-+-f..._,_+ ⁺

OJ

0

"

^{•-r--•r•~ / 1}

c

¹⁰⁰

19 86

OJ >

. :J

--""\

'+-

I

+~'""?•-,•-./

0

0 _I

~ ^I

-+-' 100

19 8 7

f\t

{f)

c

OJ t\

0

+"/;"-

0 ^{• I •} .:::::ra •

J F M A M J J A S 0 N 0

Mont h

Fig. 5-2 Seasonal variations in density of juveniles (< 5.0 mm CW) per m2 from January 1985 to November 1987. Vertical bars indicate means+SD.

-- E

U1 ..0

d

100

0 100

·. u 0

QJ 100

'- :J d

E E

'+-0

>. 0 U1 100

c

<lJ

0

0

19 81

1982

19 83

1984

Month

Fig. 6-1 Seasonal variations in density of immature crabs

(< 13.0 mm CW) from May 1981 to December 1984.

200

--

¹⁰⁰

-

_o U1

d

L

u

0

d ¹⁰⁰

E

E

~

0

(/) 0

c

<lJ 0 100

0

19 8 5

1986

19 87

J F M AM J J A 50 N· o Month

Fig. 6-2 Seasonal variations in density of immature crabs

(< 13.0 mm CW) from January 1985 to November 1987.

f./}

.0 d

~

u

d

E

l/)

c

(1J

0

30

0 30

0 30

0

30

0 30

0

30

0

30

0

19 81

·---·----·-·--· .. ...o.---·

---~o·~·-··-·_,~,~---

.. o-

---o·...

^'··o

o- ... -o--·--·

198 2 +

,.·--~+

o. , ' ^',,

"

^.___ .o· •.•. --o·---o ---- --o --~~ f"\J~---''o~o~•-•---+

- + ·. ··... '

'

....

.,

1984 /·~

/-'; ~~·

·--·-·

^,

^...

^{, .. ,_}

^,... .

. o... ,)

~---

...

^---~

1985 ;·~ ...

~~~-1~~~-u-o~-'-~~O---

_{, ,'} 0 . _T --·o""' '

I '

1986 . / --+-+

+--+-r:::'~.-:---- -o ----···O·---o

- - - o ------~ ---:-o·---o···

19 8 7 ·----

+..._ _- ~ ..,

.o----

^--·0. ... ___ T ^{. . . /}

+

----~L...-0...:.:...,..._. ,, ,,~ __ .... _ ; : . _ " " ' r 2 - - - -

o ... .. '...

;:o<:o:::: ,:?'" ....

·o"' .... ...

M A M J J A s ^{0 N D}

Mo- nth

60

50

40

60

50 40

60

50 40

60

50 40

60

50 40 60 50 40

60

50

40

-o ro

\ ()

ro

:::J

0

LO

ro

0 __...,

3

0

ro

Ul

Fig. 7 Solid lines indicate seasonal variations in density of mature crabs (> 13.0 mm CW) from 1981 to 1987. Vertical bars indicate the 95 % confidence limit. Broken lines indicate percentage of males in mature crabs. Significant differences from a 1:1 sex ratio at the 5% level were shown by solid circles in the broken lines (

rx-

^{2 test).}

,...

N

E

l.()

N 0

(f) (1J

c

(1J

>

:J

~

0

(f)

c

<lJ 0

1 ^o 0 j

100j 100j

SEP '84

N=16

OCT

DDCJ

I I i J _I ^r=:J _{J I} CJ _I

nDDn=Cl

i I I I I I I

NOV

I I I

CJDDClc=-Jnc=Jr=='

i I I I I I I I I

DEC

JAN '85 DDor=J

I I I I I

FEB

n O

i I I

MAR

APR

MAY

0 15 30 45

Distance from bank (m)

Fig. 8 Distributions of juveniles ^(< 5.0 mm CW) from the bank to the water's edge at low tide in St. A from September 1984 to May 1985.

In 19J7, ~hen' any juveniles recruited an crab ensity reatly increased, many larc:,e crabs rer, oved fro11 the burrow areu to the lo1er area of St. A and ~ug the te porary burro\/s ( ·i • 9) where few crabs have found from 1981 to 19

'

^{and crab ensity}

was very low even in 1967.

(4) Survivorship

'rhe fluctuations of crab density in eacn cohort were shown in Fig. 10 and Table 4. o crab of the 1980 cohort was found during the study period. ~nthly survival rate in eacn cohort was calculated as seasonal decrease of crab density (Table 5).

The survival rate of the 1+yr crabs was much higher in winter than in summer in all cohorts. Few crabs of the 1981 to 1985 cohorts survived over 2.5 years, but illany crabs of the 1979

cohort survived longer. Monthly survival rate of the 2+yr crabs between July and October was the highest (0.98) in the 1979

cohort, medium (0.45 to 0.57) in the 1981 to 1983 cohorts and 0 in the 1984 to 1985 cohorts.

(5) Growth

1ecently molted crabs were encountered from April through October and the progression of size-frequency modes implied tnat the growth season is fro April through October or ovember.

Estimated growth in carapace width of males and females in each cohort is sown in Fig. 11.

The smallest crab had a 1.3 mm CW (sa1pled in October 1984).

Mean body size of the O+yr crabs in each cohort v1as 2. 5 to 5. 3 rnm C~l in October, and i t increased little until December because of

29

Table 4. Crab density in each cohort (m- 2 ).

Age

O+yr 1+yr 2+yr 3+yr

Cohort Nov. Apr. Jul. Oct. Apr. Jul. Oct. Apr. Jul.

-

1979 ND ND ND ND ND 21.3 20.2 16.4 1.3

1981 35.8 24.0 21.5 14.8 14.8 9.1 0.8 0 0

1982 34.0 34.8 23.9 20.3 17.9 5.0 0.9 0 0

1983 52.0 54.5 39.5 20.1 16.8 8.8 1.5 0 0

1984 45.8 30.0 20.5 12.9 17.8 10.8 0 0 0

1985 119.0 71.5 43.5 28.5 26.7 6.3 0 ND ND

1986 99.5 57.3 35.2 32.1 ND ND ND ND ND

1987 80.3 ND ND ND ND ND ND ND ND

(a)

Male Female

30 _{13.9+4.2 rnrn}

13.5+3.5 rnrn

(190) (209)

lf)

~ 20

0

:::1

u

10

>

·- lJ

c

₀

0 10 20 30 0 10 20 30

~

0

(b)

'-

(lJ

Male Female

..0

_{17.5+3.1 mm 16.7+2.0}

E

⁽¹²⁸⁾ (151) ^mm

:J

³⁰

z

20

0 10 20 30 0 10 20 30

Carapace width ^(mm)

Fig. 9 Size-frequency distributions in the burrow area and its neighboring lower area of St. A in September 1987. Filled

histograms indicate ovigerous females. Unsexable juveniles .. were classified as 50% males, 50% females, assuming a sex ratio of 1:1. Densities of adult crabs per m2 were 34.5+12.1 (mean~SD, 46 quadrats) in the burrow area and 3.3+4.7 (70 quadrati) in the lower area, respectively.

-- E

(/) _Q

d

L

u

~

0

(/)

c

<lJ

0

50

0 50

0 50

0 50

0

1979

... ....

^~

"..

I ~--

/\ ,.1--....

;• '·'·

^'e~.-·

... ,.

19 81

~ \

r• 1._~·~----~---

19 82

1983

19 84

JUL JAN

0 1 2 3

Age

Fig. 10-1 Seasonal variations in crab density of the 1979-1984 cohorts.

200

19 85

150

100

•

~

f\

N

E •

50

I \ ^{,• \~} .

-- ^•

...

I ^{• \1\ . ... - .... •}

^\

• •

' tf)

I

^\

..0 0 _.~

•

d

¹⁰⁰

• '•

L.

\·

u

19 86

'+-⁰ ₅₀

.A _•

~

\

... ~

.

Ul

c

OJ 0 0 100

• 19 8 7

50

0

•:-..

r

JUL JAN JUL JAN JUL JAN JUL JAN

0 1 · 2 ₃

Age

Fig. 10-2 Seasonal variations in crab density of th~ 1985-1987 cohorts.