JOURNAL ORCHESI:llA CTAT[[ELSALL), TEMPERATE FOREST*

The Ecological Society of Japan

NII-Electronic Library Service

The EcologicalSociety ofJapan

Vol. 24, No. 2 JAPANESE JOURNAL OFECOLOGY June, 1974

POPULATION STUDY ON A 1[llRRES[[RIAIL

PLATENSIS JAPONICA CTAT[[ELSALL), INAMPHll)OD,

ORCHESI:llA

A TEMPERATE FOREST*

Hiroshi

Department of BioiQgy,TAMuRA

and Kazuo KosEKi

Fkeculty of Scienee, tharakiUle;ivasity. Mite

Synqpsis

TAMuRA, Hiroshi and Kazuo KosEKi (Dep. Biol., Fac. Sci., Ibaraki Univ., Mito) Populationstudy on a terrestrial amphipod, Orchestia platensis 1"aponiea ClrAT'iELsALL), in a temperate forest. Jap,J. Ecol, M, 123-139 (1974) The euterrestrial ainphipod population of Orchestia platetasis J'aponica, is intricately constructed of .three diffetrent phases, vlz. (i) age (instar) ; (2) generation ; and (3) subpopulation, The breeding periodoccurs twi¢ e a year, viz. during late May to mid-July and late July to late September. Reeruitmentduring these periods are called the

`spring

type' and the `autumn

type', respectively. The life cycle of

this population is one year, The spring type produces only the next spring type, whi}e the autumn type

exclusively the next autumn type.

individuals of each subpopulation shew a sigrnoid pattem in their body growth, The ariimal hatchedas spring type overwintersat the 10th or 11th in age, that is just before the adult stage, It becomes an

adult next spring and then breeds. On the other hand, the animal recruited as the auturnrt type over-

winters at the 6th or so ages, After wintering the anima1 begins to grow again and breeds in earlyauturnn.

The tendericy towards the reproductive separation of the two subpopulations is considered to be apossible ppestep to an allochronic speciationL

Introauction

The euterrestrial amphipods, land-hoppers,are almost restricted to the areas bordcring theIndo-Pacific (HuRLEy 1968), though semiter-

restrial species are distributed in other areas,

e.g., Orchestia cavimana HELLER in, England

(CuRRy et al. 1972). HuRLEy (1968) reviewed

the zoogeography, eeology, adaptation from

water to land and speciation of the terrestrialTalitridae. DuNcAN (1969) worked on the poprulation ecology of two grassland inhabitants,Orehestia hurklyi DuNcAN and O. patersoni(STEpHENsEN), in New Zealand. However, theecological information on euterrestrial arn-

phipods is sparse, perhaps due to the lack of

endemic species in Europe and North Ameri-ca, where the records can be traced only to

accidental introduction (HuRLEy 1968). Some euterrestrial species are known inJapan (UENo 1929, STiipHENsEN 1935, IwAsA1939), among which Orchestia platensis J'apo-

Received Sept. 4, 1973"Contribution No. 38 from the Itako Hydrobi-

dlogical Station, Ibaraki Uniyersity.

123

nica is distributed widely in relatively moist

f6rests in Japan. However, this "species" is

also commonly f6und on the coastal beachesaround Japan as a sand-hopper (NAGATA1966). These ]and- and sand-hoppers are mor-

pho}ogically indistinguishable from each oth-

er (WATANABE 1970). In the present study the

authors aimed at anaiysjng the ecology (withspecial reference to the popu]ation ecology) of

a terrestrial population alone, leaving the com-

parative ecology of the two populations for the

future.

Materials and Methods

1. Materials studied

Tlie species of the population studied was

identified with Orchestia platensis J'aponica

(TATTERsALL), based upon the descriptions of

IwAsA (1939) and STEpHENsEN (1944). Thisform was first recorded as a distinct species,

Tblorchestia ,foponica, by TAT'rERsALL in 1922.It was col]ected among'damp weeds at the

shore of Lake Biwa, central Japan. In 1933,IwAsA treated this form as a subspecies of

Orchestiaplatensis. According to him, this

NII-Electronic



The EcologicalSociety of Japan

Vel. 24, No. 2 H 7it ts me\k es June, 1974

mm

30

: 20.E::Z10F

o

-le

JAN FEB MAR APn MAY JUNE JULY

Fig, 1. Climatic conditions in 1971 at'

Meteological Observatory).

pm-"r=e:sw

z

g:ffEmE

100

7S

50

25

o

ISO

1co

50

o

Aus sEpT ecT Hov oEc

Mito (by the eourtesy of the Mito

subspecies is distributed throughout Japan,from Hokkaido to Kyushu ; on the beaches,among damp seaweed washed ashore, under

darnp fa11en leaves and in streams. The colo-

nization of this subspecies in euterrestrial

habitats was first reported by WATANABE

(1970). He fbund them in the evergreen forestat Sendai, Kagoshima-and at Ashu, Kyoto,and in the beech forest at

･Odai-ga-Hara,

Nara. Recently this forni wa$ also collected

from the forests in Okinawa (in Naha, taken

by Mr. E. KuNiyosHi, March 26, 1968; and inIshigaki Is]and, collected by Mr. S. YAsuMA,

Aug. 23, 1972), It appaars to be distributedwidely in damp forest fioors throughout trapanas a rnernber of the soil fauna (SAiTo and

KuDARA 1972)

2. Topography and c]imate of the sampling

station

The sampling station is in an artificial plan-tation of･the Japanese ceder (about 5 km2 inextent) northwest of Mito, situated on a ter-

race (31 m high) of the Nakagawa River whichfiows into the Pacific Ocean. The station cover-

124

ed both the surface and the s1ope ofthe terrace.

rl}e distance between the site and the river isabout O.5 km and the areas between them are

now cultivated iand without forests. The troes

in the station are 30 to 40 years old. AfterSepternber, 1971, the trees in the forest next to

the station were cut down for commercial pur-

poses. But, fbrtunate]y, the fe11ing was stopped

without reaching the border of the sampling

area. The undergrowth is dominated by Hbut-taynia cerdata and Qphiopqgen japonicus on

the terrace surface, and by Rubus sp. and

M7istaria sp. on the slope.

Mito is situated at the northeast end of the

Kanto Plain, and belongs to the typical climat-

ic zone of the Pacific side of Japan, its climate

being identical with the Karito-type. Based on

the data of the Mito Meteological Observato-

ry, the minimum monthly mean temperature inan average year is 2.20C in January and the

rnaximum 24.80C in August, the annual rnean

being 12.90C. The meari annual precipitationis 1,321 mm. The climate during the sarnpling

period (l971) is shown in Fig, 1. This yearappears to be a ]ittle higher in temperature




Vol. 24, No.2 JAPANESE

than the average year. At each sampling time,the air temperature, soil temperature (5 cm

below the surface) and water content of the'

soil (at thc depths of 2.5 cm to-10 cm) were

recorded. Trhe air arid. so" ternperatures are

given in Fig. 2. The water content of the soil

was almost constant throughout the yeair,ranging from 100 to 150% at the Ao-horizon.There was little difference between the terraoe

surfaoe and the sJope for all the factors meas-

ured.

JOURNAL OF

3. Sampling'and extraction

Sampies were taken periodically twice a

month durSng May tp December, 1971, Thesamples were drawn in a

"six by six" pattern

fr6rn the terrace surface and the slope, re-

spectively, by using tin samplers (25 cmx

25 cmx5 cm). Each samp]e, composed of

]itter and soil, was put into indiyidual poly-thene bags to be extracted later in the laborato-

ry.

The animals were extracted from the sarnple

with a modified Tu]lgren funnel 90 cm deep,With a top diarrreter of 30 crTi and a bottomdiameter of2 cm. The sample was put on a

sieve of hardware cloth of 5 mm mesh, attach-

ed 1S cm be}ow the top, A yial fi11ed with 70 %alcohol was attached to the bottom of thefunnel with viny] tape. Heat was supplied by a

60-watt nichrorne bulb housed in a metal cow]

above the funnel. The samples were kept heat-ed fbr 3 days until dry. The animals were con-

centrated in the vials, Only amphipods were

sorted out. Funnel eMciency for the samples

was estirnated several times by hand sorting

{he sample after treatment. An eMciency ef

between 95 to 1oo% was always obtained, sq

the nurnber that died in the funnel appears to

be negligib]e.

4. Data colle ¢ ted

All specimens in every sample were exam-

ined for the following items: the number of

podomere on the fiagellum of the 2nd antenna

to estimate the developmental age (Coo?ER1965), the tergum )ength of the lst thorax to

estimate the growth and the biomass, and the

sex to separate fernales, males and young. Thefemales were examined for the presence of

brood pouches and any eggs or young present

30

x: 20E:flt

]e

ECOLOGY

pt AIR TEMP,

June, 1974

VAY JUNE OUtY AUG SEPT OeT NOV DEC

Fig. 2. Air and soil temperatures at the sarnpling

site at 10: OO-11: oo am during 1971,

were counted to find the breeding period andthe fecundity. Furthermore, 103 individualsother than those from the periodical simpleswere dried at 80eC for several days indiyidual-jy in a hot-air drier after the above exami-

nation, in order to ca]culate the length-weightrelationship. Dry weights were measured to anaccuracy of O.Ol mg with a r-balance. ･

Results

1. Population dehslty

The numbers of Orchestia platenSis jmponicaextracted from the s'oil sarpples are presontedin the form ef mean number per sample, forboth terrace surface and slepq in Table 1. Thetable also gives the total mean densities as

Table1. Numberofaxnphfpodsextractedfrom

soil (Mitp, 1971).lndivs.fsample'rndivs.im2

DateTerraceSurface

SlopeTotal,

A. May6B. May 22C. June 8D. June 25E. July 13E July 23G. Aug. 9H, Aug. 21 I. Sept.3 J. Sept. 20K. Oct.6L. Oct.21M. Nov.3N. Nov. 18O. Dec.3P. Dec. 17

7915l52129J92]27,3340252337283237265s106998691557 76127l351672op268232'208･2843403642Sl2S2423260316

Mean 24' 7 2tl4

125

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Vol. 24, No. 2 E pt ts ue \k ts June, 1974

.hev2-e;2-i"

no

]o

2e

le

heAN NC. eF INDIVIDUALS PER SAMPLE CM)

Fig. 3. Seasonal change in crovvdingofindividuals +

for all ages together, in terrns of m against m,

individuals per m2. From the table it is clearthat the terrace surface always supported a

higher density than the slope. This biased dis-tribution appears to be related to.litter thick-ness, which is much greater on the surface than

on the slope, rather than the soil ternperatureand soil humidity, both of which seldom dififered among sites. The mean density fiuctuatedfrom 761m2 (early Mayi to 423fm2 (mid-November) throughout the year, with the an-

nual mean 2441m2.

DuNcAN sampled systematically along theradii from the center of the plants with core

borers (13 cm long, 10 cm in diarneter) in agrassland in New Zealand and reported that

Orchestia hzardyi ranged from 1.43 to 4.43 inmean number per core and O. patersoni 2.3 to8.0, these being equivalent to a range of froml68 to 554 and 228 to 1,OOO individuals per m2,respectively. Consequently, the popujation

density of the presenit specjes in Mito js not so

different from that in New Zealand. "

The mean crowding, m, (LLoyD 1967) wascalculated for each sampling time. The results ee

were showh as rn 'against

m (mean) in Fig. 3.

126

Ilie animals showed near]y random distri-bution in May, but after that they tended to

aggmgate gradually from June to July with an

increase of the mean densities. As descrjbedlater in Section 4, this period corrdsponds to a

breerling period of the present population.This trend rmay be atuibuted to the increase of

the young animals just reieased, which are re-

latively immobile. A maxirnum aggregation

oecurred-on July 23, when the first breeding

period had probably ended, and just after the

animals again showed ncarly random distri-

bution, Perhaps this might be caused by therandorn spread of young anirnals which be-came active. After August 21, along the pro-ceeding of the second breeding season, theanimals again gradual]y increased their aggre-

gative degree towards October 6, when thesecond breeding s,eason had come to an end.

Next the aggregate of the animals was loosen-

ed during late October and early November,

probably by the same reason as that described

just above. Further the animals showed once

again the trend of clumped distribution in thecold season (after mid-November), although

there was no breedings during the period (SeeSection 4): The reasen for it wiiJ be discussedin Discussion-3.

2. Deve]opment and populatien compositfon

H),alella azteca, a fresh water amphipod, in-

crease a podomere on the fiagellum of the 2nd

antenna with the developmental age (instar),though with some individual varjations

(WmDER 1940 ; GEisLER 1944), and this charac-ter is useful as a criterion of age (CoopER1965). DuNcAN (1969) observed that theindividuals of Orehestia with more than 13

podomeres added Qne podomere at each

moult and assumed that those with less than12 podomeres also added one podomere permoult.'In the present study, it was observed

that the individuals with less than 8 podomeresshowed the sarne development pattern inlaboratory culture, specimens having more

than 9 podomeres fiot being observed becauseof their death at this age. Connecting this

observation with DuNcAN's resu]ts, Orchestia

platensis J'aponica may be assumed to inereaseone podomere at each mou]t. The smallest

specimens obtained from the field samples




vor. 24, No. 2 JAPANESE JOURNAL

possessed three,podomeres ton he 2ndantenna. This agrees with the results fromlaboratory cultures started with eggs removed

from the brood pouches ofthe adult females.The greatest number of podomeres foundwas 17 but this was in a single specimen inthe whole msapling period. All other indi-viduals are foundwithinthe range of 3(1st age)

to 16 podomeres (14th agej. From the resultsof sample analysis, this population was

composed of overlapping ages, and al1 ageswere always found in the population, except

from late autumn (November) to ear]y spring

(May), durngi which the smallest ages (lst orlst and 2nd together) were missing from thepopulation.

3. Sex ratio

T he animals are sexed by observing whether

the 2nd gnathopods are chelated (male) or not

(female). Sex differentiation begins at the

10th age, but is not comp]eted at that stage.There is no evidence that the individuals with-

out chelated 2nd grathopods at the 10th age

are true females, some of them may possiblybecome rnales in the future. However, sex

differentiation appears to be cornpleted, at the

latest, by the 1 1th age, for the individuals with-out chelated grathopods at the llth age may

posses eggs or at least brood pou ¢hes at the

maximum of a given breeding period. The sex .ratio

was ca]culated as total females to total

adults (which are those actually more than the

l lth age) for every sampling time (Fig. 4). Asa whole the ratios exceeded O.5 excluding lateMay, late June and late August, so the popu-lation tended to be almost always dominatedby female throughout the year. The annual

mean of the sex ratio was O.59. Therefore, themale:female ratio is approximately 2:3.

4. Breeding period

NAGATA (1966) showed that the supralitto-

ral population of this species appears to breednearly the whole year but breeding at Onoura,Hiroshima Preft is probably separated into twoactive periods, viz spring to early summer, and

autumn. In the terrestrial population, the gra-vidity rato was calculated in the form of per-centage of females with eggs or young in thebrood pouch to the total females (larger than

127

:tsgEEkg

OF ECOLOGY

lm:

, June, 1974

Fig.4. Sex ratio (feinales/total adults) at each

sampling time.

10b

lmrVmt'

to

MAT AVG eCT nev

Fig. 5. Seasonal changes in gravidity ratio (gravid femalesltota1 ihature females), broodpozmh

ratio and number of females (per 3/4 m2).

10th agej fot eveTy sampling period (Fig. 5).The brood pouch ratio (females with broodpouch/total fema]es) is also giyen in Fig. 5.The gravidity ratio was zero ti11 May 6, ab-ruptly increased from May 22, attained one

peak (58 %) on June 8, gradually fell to 5 % on

July 13 and 23, gradual]y rose again fromAugust9to the other pealc (63 %) on Septem-ber 3, and steeply declined to zero on October6. During the winter, the ratio stayed at zero,

probably till the next spring (May). Conse-

quently this population breeds at two distinct

periods : mainly a{ mid-June arid at early Sep-tember, and ceased breeding in mid-July andin wmter. Here the authors call the individualsrecruited in the lst and 2nd breeding periodsthe

"sprjng type" and the

"autumn

type",

respectively.

5. Fecundity

The number of eggs' produced per brood byOrehestia platensis japonica -is variabla

NAGATA (1966) reported mostly 5 to 9 eggs perovigerous female, with 22 eggs at the rnaxi-

NII-Electronic




Vol. 24, No. 2 E Xi Ei! me \ ft :. June, 1974

INDrvs.

10

o

PER 3/4 M2 A: Mey 6

INsDol

lVS

40

20

lo

o

B: May 2230

20

10

20

10

o

C: june'e o30

, PER 3/4 M2

/t

G: Aug, 9

H: Aug. 21

INDIVS. PER 314 M2

20

10

o

40

30

20

40

30

>i 20vZw=10ewecLO

D: June 25

20

10

10

o40

I: Sept. 3

o50

40

30

20

,

30

20

10

o

E: July 13

60

F: July 23

10

o

l

5 30

20

10

40

30

so

40

3e

20

10

o

A

20

IO

o

,

J: Sept.20

"

o30[20

10

o

A

40

30

20

IO

135791113 O1357911]3

AGE {finstar)

Fig:6, Frequency distributions of individuals for each age

curves calcrulated by CAssiE's method (see Text).

40

30

20

10

o

l

Dec

P' Dec. 17

l3579111315

(per 314 m2); Curves are normal

mum. In the present study, too, the eggs per

brood were quite variable bath between and

within 'ages.

A female having passed through

the funnel held.from l to 9 eggs, within which Jrange, however, a vagoe trend was recogniza-

ble based on the mean eggs at each age, narnely128

that the fecundity seemed to increase with age :

2.6 eggs (10th age), 3 eggs (11th), 3.7 eggs

(12th), 5.2 eggs (13th) and 5.3 <1 4th). But there

must be a strong possibiljty that thc ovigerous

females released their eggs into the soi} duringextraction. Besides the regular samples, am-




Vol. 24, No. 2 JAPANESEJOURNAL OF

phipods were collected diroctiy with an aspi-

rator ftem the field on June 8, 1972, 1houghfew ovigerous females were found, the numberef eggs per brood ranged from3to8averaging4.8, 6.0 and 5.7 in the 1Oth, 11th and 12th ages,respectively, with thc totaj moan of S.5. 0vi-

gerous females of the 13th and 14th ages were

not obtained. Combining both results, it may

be shown that the eggs per brood presumablyincrease with age, but with large deviations.

6. Age and generation structures

Frequeri¢y distributions of individuals on

podomere numbers are given as histograms forevery sampling time in Figs. 6-A to 6-P. Theserepresent the age structure ofthe natural popu-lation at each time. Examination of them re

veals that there are always one or twobreaks in

the histograrns, which separates the popur'

lation into two or three different age gr6ups･During May the popu14tion was cornposed of

two age gtoups without individuals smaller

than the 3rd age. In early June the recruitment'

of small iRdividuals began, starting as the 3

podomeres stage. So at that date there were

three age groups. Recruitment was continued

till mid-July, when the production of this

youngest generation ceased and the oldest

group responsible for the reproduction of the

youngest disappeared from the.population.

Thistimingcompletelycoincidedwiththenofi-breeding time shown in Fig. 5, Therefore, it isinferred that the populatjon is composed of,

three cohorts which belong to different ganer-.ations from early June to early Ju]y but only

two in late July. In early August the next new

recruitment was begun by the cohort which by

new had become the oldest, so then the popu-lation contained three difTerent cohorts of gen-eration. The second recruitmentcontinued tillearly October, ･when the o]dest generation had

almost disappeared from the population.AfterNovernber recruitrnent was not seen till per-haps the next spring, but a little-growth ap-

peared to continue till mid-November;･So the

individuals of the lst and 2nd ages were not

seen in the population till the next breedingseason. Consequent]y,･although the popu-lation produoes offsprings during two different

periods throughout the year, the' different

grroups of offspring, the spring type (S) and the

129

ECOLOGY .June, 1su4

autumn type (A), are distinct from each othei,

as evidenced by the break in the histogram.Such breaks were seen till next early JUIy, whenthe adults of the spring type disappear frein･the population. In conclusion, for the coM-

pletion of the life cycje of this euterrestrial

amphipod it needs one year, and the spring

type, being independent ffom the autumri

typei appgars to produce oniy the next spring

type, whiIe the autumn type bears exclusively

the next autumn type. Here the following com-

ments must be added to this age structure:

Thepresent population appears to be intricate-

ly constrpcted ･from

threg different ponipo-nents, viz. (1) the age, which means the differ-ent moulting stages; (2) the generation, whieh

the direct descendants recruited in different

years; for example, a reiation,ship between aspring type and the subsequent spring typQ;'

and (3) the･subpopulation, which rneans the･

different breeding suocessions: for example, ･a

relationship between the spring type and theautumn type. Here;it is,･remarkable that/the

number of individuais of･the 2nd age･are

always fewer compared･wjth other ages. The'

duration of this stage may be extremely･Short-

compared to that of the other siages.'

7, Growth analysis

a. In terms of age

As seen in Figs. '6-A to 6-P, one unit･.of the

histogram･separated by the break.is regarded

as .showjng the age distribution within a given

generation. Provided that the unit shows a

norrnal disuibution-of ages, the･mede･caii fe.p

resent the ageof the -gerieration.eoncerned- at

any time. When the moda1 age is plQtted a- ･

gainst time, the graph obtained will demon-･

strate the gromb pattern of the･ representative'individual of a given genepatibn in natural･con-ditions. To sqparate a- bimodal distributioninto the two individual normal･distribiitions,

CAssiEs'method(1954)usingprobabilitypaperwas employed. The results of these ･analyseg-

are also given on the histograms in Flg. '6, asnorTnal curves. The age at which the,'curye'hadthe modal value was graphed against sEmpling''

times for each generatien. Tlie 1'eci 'tase

showh'

in Fig. 7, with the standard deviation within

whi ¢ h most individuals belonging'to 'the

sarne

assemblage ･were

found. These growth-durves-




Vol. 24,. No, 2 H pt tsme \E ts,, June, i974

1,gS

rs

/o

s

,

s]

-/

i

lt

J:

fINDtVS,TOo

A: in e

-t-GbpuE[-IHS -tt--i#ttO/HC

H ISie . "IE- MEH L"n.nvTua 1rvre--o ln/ .ml-T?FH)"1.wwltrc

'mr

i;

Sdtuva

tg

;iTU tnvEt'

i sEnn

incTtt

S -o}t

J osic!

Fig. 7. Growth curve of the theeretical indiviclual, b4sgd on age for eaeh generation during l971. Tlie bar indicates one standard deviation.

l:.loe

e/-ths･1/

'sII13'15ITlevas2S21

LEltrctH OF IST IHanAIL]Vt3EP

Fig. 8. Frequency distributions of individuals for

lst thorax lengthclasses per 3A m2. Curvesare norrnal curves

method (see text).calcuiated by CAssm's

are not based on the raw data but on the

theoreticai analysis. The "poal" growth is pos- sibly shown by the incorporation of a standard

deViation.

At the first sampiing time, early May, the

old generation (Si), which had overwintered

the'previous cold season, were nearly adults,

but they still did not possess brood pouches. The Si' began to bear brood pouches and eggs

in late May. [1ie･new･ generation (S2) fustoccumd in early June. During the first breed-ing period the middle generation (Ai) whichwas recruited in the previous autum as theautumn type, still did not reach the adult

stage, fOr it was smaller than the lO.5 age, eventaking into account the range of a standard

deviation. Actually, ali the individuals belong-.ing to

'Ai possessed no brood

･pouches and no

eggs at that time. The newly incorporated gen-eration, S2, grew rapidly during the spring,summer and mid-autumn seasons. Its growthbegan to decline from iate autumn and stoppedwith the･onset of wiqter at 10.5+1 (S.D.) age.

Dumhg the cojd season S2 did not grow. Afterwintering, it'probab)y became the old gener-ation mentioned above. On the other hand,since May

･the young generation, Ai, (5+1

age at early Mayi, whi ¢ h･had overwintered inyoung stages as the autumn type, grew rapidly

during spring. Some of them became mature

(le+1.5 age) in late July and began producingthe new generation (A2) in early August, wherithe oldest generation of the spring type, Si,had disappeared from,the population. The

130

generation Ai which now had become theoldest, slowed its growth in early August, andshowed little growth at the level of 12+1 age

after late September, disappearjng.from thepopulation just after the completion of itsbreeding. The newly recruited generation, A2,continued to grow till around early Novemberthen stopped its growth at the beginnihg ofwinter. After a long non-growth period fromIate November to the next May, this gener-ation probably becomes the young generation,Ai, mentioned above, in the next May. Asawhole, the population appears to be dividedinto two subpopulations of the spring type

and the autumn type. Both have one-year Iifecycles, but they experience･the hard winter at

different life stages from each other : the springtype does so at thc adult stage and the autumn

type at an immature stage. Moreover, each

subpopulation appears to belong to fairly dis-tinct breeding groups which are independentof each other, namely the spring type Si ap-pears to produce the next spring type S2, andthe autumn type Ai produces the next auturrmtypeAz as mentioned in Section 6. lhe oppor-tunity of interbreeding between the two groupsis considered to be largely absent, if existing at

all, because the adults of one subpopuiation

had almost aH died (by July 23 in Si; Novern-ber 3 in Ai) before the young of the other

were first produced (on August 9 in A2; June8 in S2).

b. In terms of the lst thoracic length

The data on the tergal length of the 2st




Vol. 24, No. 2 JAPANESEJOURNAL OFECOLOGY June, 1974

ro

$o:gEs

ID

M"-n

Sl

e

-1

st

" t"o. sptt-TtN--I lelO . -IT-- TVnpt TSII.SPittmpTTPt- lfl/.sututH tr"E

ASES {r-stthslt 10 msL21,O

-1

ul o,siEo,6s

o,4

Sp-tNs )reEan1teG AeTex bAEan1ltG- -

O.2o.o

/-!Xe--..... /,,.-xIl)L(.

pt.be--..-MlrcTvPE

---.-.T-Y mm rT?E

llo-- b. x g x - -----

/

X.-

L.ts..ix

o G ll e 2S ts t] " i) 1 tO S !1 ] :H 3 /t mv mxE anT -w6 sepT ecT mov ffc

Fig. 9. Theoretical field growth curye based on 1$t

thorax lengths for each generation during 1971. The bar indicates one standard deviation,

thoracic segrnent were also analysed using

CAssiE's method. Some typical examples of the

results of this analysis are given in Figs, g-A to

8-D. The subdivisions of the histograms, which

represent subpopulations or generations, are

clearly recognizable for this daja, too, with the

rnodes showing the same progression with

time as seen in the "age"

histograms. Ac-cordingly, the growth curve of the representa-

tive individual of each generation in terms of

the 1st thoracic length could be obtained (Fig.9). Tliese curves showed that the growth

pattern was almost the same as that for age.

The adult stage (1arger than 10.5 age) corre-

sponds to individuals larger than 20( × 34) Ftmin 1st thoracic length. The spring-type young,S2, when just hatched were 8( × 34) ttm in lst

thoracic length at June. These gradually grew :

10.5(× 34) ptm by lst July, 13.5( × 34) ptm by

late August, 16(× 34) strn by late September

and 19.5( x 34) pm by lato October. However,

the thoracic segment ceased growing when it

reached 20( × 34) "m. [[his occurred after No-

vember, and although the anirnals were adult

as to the size, breeding was left over unti) the

following spring. On the other hand, the

autumn-type young, A2, recruited duringAugust was also 8( × 34) ptn in lst thoraciclength. It became 10( × 34) pm by late Septem-

ber and 12(× 34) ym by late October, AfterNovember it Geased growth and wintered qs it

was in the young stage. After the next May,

this group will correspond to the generationAi in early spring, which grows rapidly till

131

1

Fig, 10.pmv

:de-,EtsJury de"G svTTTtT .tow T

Annual field ehange of growth rate based

on age for each generation during 1971.

, July. From late July to late September it breedsand produces offspring. After breeding this

group grows a little but soorr disappears from

the papulation. Therefbre, the analysis based

on the 1st thoracic length also shows that･there

is a one-year life cycle in both subpopulations,and that the growth patteirn was quite similar

to that based on age fpodomere number).

8. Growth rate

The growth rate of each generation was･ob-

tained directly by reading the increment in agefor every 1O days from Fjg. 7. The growth rate

per 10 days was plotted in Fig. 10 which shows

that the individuals as a whole ¢ ontinued te

grow during the whole period excluding winter

and early spring, but the growth pattern foreach generation was different. [he older spring

type, Si, had the maximum growth rate (O.5agesflO days) during late May to early June

with extinction from the population when re-

cruitment was completed. The younger spring

type, S2, grew very rapidly in its early life-time

and had the maximum rate (about O.9 ages/10days) during mid-August. After September

growth gradually slewed until late November,when the growth rate became zere. On the

other hand, the older autumn type, Ai, had an

increasing rate and attained the rnaxirnum rate

(about 1.0 agesllO days) during mid-July, after

which the rate declined to zero by late Septem-

ber. The new autumn type, A2, that hatchedduring early August also grew rapidly in theearly stages with the growth rate attaining the

maximum (O.6 agesflO days) by late Septem-ber, after which it began to drop to zero by late

November, whan winter occurred. On the

whole, the growth rate of the population was

NII-Electronic




VoL 24,- No. 2

.

'e.:e.lx::[::T,l!

moxsgtE

20o

1oo

//,:,

H aj th F.e..

Fig.It. Annual changes of density for the total population and each generation during 1971.

Thedotted line indicates the actual values,

and the continuous. Iine the values - corrected

by cye for the generations and for the tota1 population, based on thp sum of values for

eqch ofthe generations in the total population.

higher ih the surnmer seaso'n than in the springand autumn seasons,

9; Annual dynamics of thepepulation

The actdal number of individuals sampl'edmay be an over- or underestimate of the realderisity due, perhaps, to sarnpling biases. lhesebiases, if any, must be corrocted. But the ab-solute correctien for･a si'ngle sarnpling time isim'possible' here. So relative corrections were

employed, where the most likely trend line is(irawn by eye through the actual values. How-ever; it is diMcult to determine whether thettcnd-line

is the best, as far qs the ,total indi-Vidinals concermed, for' the total is a complex o £

cornponents, 'each

df which behayds independ-ently'of each oth ¢ r. So fust the tota1 must bedivided into its components. Each componentis regarded as an assembly of individua]s which

behave identically at any' giyen time.Thesecerrrponents are each generation of the popu-lation (as discussed earlier; Si, S2, Ai, and

A2). Consequently,' the annual changes in apopulation

'can be systematically analysed by

traeing separately the densitics of the'gener-ations with time and accuinulating them every.

time. The number of individuats of each･ gener-ation ats each sampling date is given in Figs.

132

\k ts June, 1974

6-A to 6-P as a quantity surrounded by a

normal curve. These numbers were plotted

against the sampling date in Fig. 11. These

plots showed large fiu¢ tuations jn each generation as ifthere were no trends. This seems to

have been caused by the sampling error. How-

ever, this can be corrected on the following basis: (1) during the breeding season a new

generation will continue to increase in nuni'-

ber unless there is unusual death ; and (2) after the breedSng season, the generation will

continue to decrease in number, due to the

mortality factors, until all members disappear

from the population. On such considerations,

a smooth trend curve was drawn by eye

through the plots of density against time for each generation. These resu]ts are also shown

in Fig. 11 as smooth curves. The spring-type

subpopulation, S, was born in early June and

then grew in nurnber till Iate July, after then it

gradually decreased. After breeding, it abrupt-

ly decreased and disappeared by the next July.

On the other hand, the autumn-type subpopu-

lation, A, occulred in early August and in-creased in number tiil early October, after thenit gradually deciined in number, and vanished

from the population by the next October.Therefore, both subpopulations showed thesame pattern of popu]ation dynarnics in spiteof the discrepancy in time.,Next, the correctedvalue of total density was estimated by cumu-iating the va]ues for each generation at a giventime. Tlie reiation of density with time is alsoshewn in Fig. 11. Examination reveals that thecorrected curve of total density fits the actualcurve well, except for November and Deoem-ber. The- poor fit,at the end of the year will bediscussed in a later section. From the correctedcurve, it was recgonized that a peak densityoccurred twice a year, namely ca. 230 indi-vidualslm2 in late July and ca. 330 individuals/m2 in late September. The two peaks occurrecl

just after the spring and the autumn breedings,respectively. The highest peak was in autumnrather than springi which seems to be the re-

sult of the larger number of adu]ts (the initialvalue) in the auturnn type than in the spring

type. The drop in density from late July tomid-august, and the drep after late Septembercorresponded to the times ef adult mortality inthe spring and autumn types, respectively.




Vol. 24, No. 2 JAPANESEJOURNAL OFECOLOGY June, 1974

ltible 2. Mortality and apparent natality (indivs.1 day) (in Mito, l971). le.omg

Generation Duration

Apparentnatality (+)Mortality(-)

197"Spring type

Mayl"une9 -O.oo

JunelO-July13 -1.165.0

1971-Spring

typeJune Muly 20 +3.anJuly21- -O.33

1970-Autumn type

Mayl-Aug.27 T-O.18

Aug.28-Oct.21 -1.12

1971-Autumn type

Aug.6-Sept.29 +3.74Sept 30- -O.26

10. Mortality and apparent natality

Mortality and natality are parameters whichmust be estimated for population dynamicstudies. But the data obtained in this study donot permit these parameters to be estimatedsatisfactorily because the estimates ef fbcundi-ty in this study are insuMcient to account forthe production of the young in each subpopu-

lation, and because laboratory culture of the

present speeies has been unsuccessful. How-ever, the data on the annual quantitatiyechange of each generation may permit at leastrough estimates for mortality and apparent

natality.

The annual change in each subpopulation

can be approximated using strajght lines if thefollowing divisions are made (cL Fig. 11); (1)Increasing period, from the start to the peakof the new generation; (2) lst decreasing

period, during the peak to the overwintering

period; (3) 2nd decreasing period, from liber-ation from overwintering to the breeding of the

generation; and (4) Extinction period, frombreeding to the disappearancg from the popu-iation of the generation. When the decrementor increment ofthe generation during the each

period are divided by･the days required, thenthe mortality rate or the apparent natality rate,

that is, true natality subtracted by mortalitywill be given, respectively. These estimations

for both subpopulations are shown in Table 2.It is seen that these two population parametersmay be nearly equal in both the spring typeand the autumn type. But the true natality can

not be calculated here.

133

F8w3LOita

O.5

O･ls

lo 2o so LENGTH OF IST THORAX

Fig. 12, Relationship between body weight

and Iength of 1st thoracic segrnent.

x34vm

(dry)

11. Annual change of biomass

The relationship between 1st thoracic length

and the dry weight of the individuals was plbtt-

ed in Fig. 12 (N = 103). The distribution of

the data suggested a linear relationship on

double logaritlmic paper. Therefore, the least

square method was employed to describe therelationship, the equation for which being logw = 3.13 log l-O.57, where w is body dry

weight (mg) and1is the lst thoracic iength

(ptrn). The estimated line is shown in Fig. 12.To calculate the biomass of a generation, theindividual weight is obtained by putting the

mean 1st thoracic length of tho generation at a

given time into the equation above;the peeanthoracic length is taken as the mode of the

normal distributions on the graphs shown in

Figs, 8-A to 8-D. The mean individual weightwas then multiplied by the number of indi-viduals in that generation. The biomass with

time is shown in Fig. 13 for each generation.The amnual change ofthe total population was




Vol. 24, No. 2

myigesoo

400

:f seoEa

2eo

Jee

H Z{ ts ge

MAV JVNE OULY AVG SEPT OCT NOV .DEC

Fig. 13. Annual field changes in biomass for the

total population and each generation during 1971.

estimated by accumulating the values of all

generations for each sampling time (Fig. 13).

The total biomass had two peaks through-out the year: ca. 410 mglm2 at early July andca. 480 mg/m2 at mid-October. The formerwas ¢aused mainly by the rapid growth of the

previously born autum tYpe rather than thatof the spring-type yearlings. The ]atter peakcan be attributed mainly to the growth of thespring-type born in this year. The annual

pattern of biomass in time for the total popu-lation is reasonably similar to that shown bydensity. Howeyer, it is remarkable that themain components determining the patternsdiffer in the two cases; S2 and A2 in densitywhile Ai and S2 in biomass (cf. Section 9). Onthe other hand, the total biomass was low inlate July and again in early November, the

latter depression presumably continuing until

the following Aprit. The two depressions

probably coTTespond to the disappearance ofthe adult individuals of the spring and the

autumn types from the population, respective-

SPR]HG TTPEAUTUMN

TTPE

JFHA tt O;ASONO

N AdultSes! pt Yeumg M Pte-adult

Fig. 14, Diagrrarn of life cycle of thc euterrestrial

population in Mite.

134

*ft V. June, 1974

ly, owing to their death fo11owing breeding.

The gradual decline after mid-November may

be caused by a decline in R;oduction and the

death of young individuals through the cold season,

Discllssion

1. Spring type and auturnn type

Adults of' the euterrestrial population of

Orchestia platensis Jiuponica occurred twice a

year; late May to mid-July and late July tomid-September. The frequency distributionsof ages for every sampling data always showed

a distinct break between the two offspring

groups produced by the seasonally separated

adult assernblages. The break existed unti1 the

older group disappeared from the populationafter one yealr had passed, though the breakbecame slightly shallow when the anirnal had

grovin. Therefore, it is considered that the

population is divided into two subpopulations

separated by the break. The spring-type sub-

pepulation, Si, is recruited during late May to

mid-July;it overwinters in the preadult stage,'breeds to produce the new generation, S2,

during the same period as it hatched the previ-ous year, and disappears from the populationby late July. On the other hand, the autumn-

type subpopulation, Ai, hatches during ]ateJuly te mid-September, overvvinters in the

young stage, breeds during the next auturnn

season and leaving the new generation, A2,

disappears from the population before winter.The life cycles of both of these two types take

one year. The seasonal life history of this

arnphipod can be diagrammatically sum-

rnarized as shown in Fig. 14. There rnay be }ittle possibility of the spring

adult producing the autumn adult because theoffspring derived from the spring adults are

still young during the autumn breeding season.The autumn adults are actually the offsprihg

produced by the previous autumn adults.

Likewise, the idea that the autumn adult pro-duces the spring adult is also almost impossi-ble. Further the chance of the spring adult

breeding with the autump adult to produce thenew autumn offspring also appears to be near-

]y irnpossible, because the spring adu]ts have

disappeared from the population before thesecond breeding season by the autumn adults.




Vol. 24, No.2 JAPANESE

Even if there were a few adults surviving till

the subsequent breeding season, their number i

is supposed to be yery small and their repro-

ductive capacity rnay be doubtful, since gravid females always tend to conoentrate on the

younger adult stages (11th and 12th ages)

rather than on the older adult stages (13th and 14th ages) in the ratio of gravjd females to

total fernales of each stage. Especially the trend

appears to be rematkable at the beginning ofa

new breoding season (KosEKi, unpubiished

data from a survey in 1972). There most of the

older adults have empty brood pouches and

their oostegites are involutgd (KAKiucHi, un-

published data from the survey in 1973).

Further this population is semelparous and the

older adults which had released young die

early or late thereafter (KAKiucHi, unpublish-

ed data from a iaboratory experiment in 1973).

So, at the beginning of a new breeding season,

the older adults may be rnostly survivors from the previous breeding season, which had finish-ed breeding, and it seems almost unlikely thatthey actually play a part in the subsequent

breeding.

The authers know of no other examples of

this phenomenon in amphipods. However, itdoes occur in field crickets, in northeastern

North America. The two field crickets, G,:ylleAs

pennsyIVanieus and G. yeletis, were formerlyconsidered as a single species (ALExANDER and

BIGELow 1960: MAsAKI 1972; YAMAsAKI1972). But the cricket population contains twogroups with separate breeding periods. For

example, the breeding periods are about Mayl to August l for the spring population ･and

about August 1 until the onset of winter forthe auturnn population in Manitoba (AfterBiGELow 1958). The two groups are morpho-

logi¢ ally almost identical with the exceptionthat there is a statistical diflerences between

them in the length of ovipositor relative tobody length (ALExANDER and BiGELow 1960).Both groups have one year life cycle, but thespring population is recruited as eggs in springand overwinter as late juyeniles. On the other

hand, the autumn population is produced inautumn as eggs which overwinter, they thenbreed in the next auturnn. Both groups do notinterbreed in ]aboratory conditions and thedifferences between these two populations are

JOURNAL

v

135

OF ECOLOGY June, 1974

more likely to beeome further consolidated

rather than breaking down through any futuregene exchange (BIGELow 1958). ALExANDERand BiGELow (l960) regarded these two popu-lations as being distinct species.

In spite of uniformity in morphological ap-

pearance, the population of amphipod studied

shows two distinct non-breeding periods (insurnmer and winteD and two groups whichoverwinter in distinctively separated develop-mental stages (cf. Fig. 7). But there is no

evidenee that the two types do not interbreed.Consequent]y, these two types will be regard-

ed, fer the time being, as subpopulations

which, although having temporally differentbreeding periods, belong to a single popu-lation,

2. Analysis of individual development

Both spring and autumn types of the ter-

restrial ainphipod populations under study

have a slightly sigrnoid growth curve of the

ndividual development (Figs. 7 and 9). Thegrowth appeared to start at May and to stop

at November. The daily mean air tempera-

ture at both of these dates was approximately

100C (cL Fig. 1), so the growing period was

during the part of the year which had a daily tmean air temperature exceeding 100C In ad-dition to that, the growth as a whole showed a

slight decrease in rate during late July to early

August, when the mean air temperature ex-

ceeded 250C. The maximum growth rate of

each generation appeared to differ from eachother in time (¢ f. Fig. 10). Excluding the olderspring generation, Si, there is some evidenee

for the optimum temperature for growth to bein mid-July and late August, when the mean

air temperatures were 23"C to 24eC. T[herefore,it is supposed that the growth of the popu-lation seems to begin at about 10"C and toincrease its rate with the rise of temperature

gradually til1 a little below 25"C in natural con-

ditions but the growth rnay be inhibited when

the mean air temperature exceeds 25eC. Intheory, the young animals, S2, which were

newJS born in early June grow gradualJyduring summer and autumn, then have a de-creasing growth rate in late autumn and no

growth at all in winter. In early winter theseyoung animals are in the 1 Oth or 1 1th age, that

NII-Electronic




Vol. 24, No, 2 H 7ts: ts

is,just before the adult stage. They then oyer-

winter at this stage. From birth to overwinter-

ing their 1st thoracic lengthincreases2.5 times.

Thjs is equivalent to the net production of

about 3.0 mg per individual in terms of drymatter (cf. figs. 9 and l2). After overwinter-ing, the specimen grows again and breecls inear]y spring; the net production dtuing the

period is about 2.0 mg per individual. Conse-quently, an individual of the sprjng type pro-duces about 5.0 mg during its life time, if thereis no death or predation until it attains themaximum stage. On the other hand, the analy-

sis shows that, in theory, the young, A2, re-cruited in early August also grows graduallyduring the cool season, but stops growth with

the onset of winter. During this first growthperiod the length of the lst thoracic segment

increases 1.5 times. This inerease is equivalentto a net production of about O.4 mg per indi-vidual. After winter, they grow rapidly and

breed in early autumn. In this second growthphase the specimen produces about 4.5 mg drymatter. So the net production during their lifetime is about 5.0 mg. Therefore, the indiyidn-als of both subpopulations have a similar

growth pattern, disregarding the differences inrecruitment time and the period of over-

wlnterlng,

However, to rnake the net production esti-mates more accurate the weight of the moulted

exoskeleton and the reproductive materials re-

leased at the adu]t stage must be added to theestimates abbve. So the values calculated a-

bove are underestimates. For future work it isessential to have laboratory cultures of theanimals in order to catculate production.

DuNcAN (1969) reported that the newly

hatched individuals of the grassland arnphi-

pods, Orchestiapatersoni and O. hitrleyi, bothhave 6 podomeres on the 2nd antennae, and

within a year they reach a maximum stage with

17 and 21 to 23 podomeres, respectively. The

present species is just between these in its in-crement of podomeres (3 at birth and 16 at the

largest stage in podomere number).

3. Mechanis'm of amual population fiuctuation

The euterrestrial population of Orchestia

platensis iaponica breeds twice a year : the firstis the spring type and the second the autumn

me \ ft ss

136

June, 1974

type. The spring type, Si, produces only thenext spring type, S2, while the autumn type,

Ai, produces only the next autum type, A2.Tlie spring type does not breed in auturrm and

the autunm type does not breed in spring. It is

considerecl that there existed four "gener-

ations" in the population througheut the year

under study : spring and autumn types recruit-

ed in l970, and other spring and autumn typesrecruited in 1971, From birth until their re-moval frorn the population by death, each

generation changes in its component number

of individuals. Such events in the different

generations effect the annual change in densityof the total population. The tbtal population

showed the bivoltine pattem over the year with

the maximum numbers in late July and lateSeptember, and the minima in iate May and

mid-August. This fiuctuation must be de-

termined by the natality and mortality process-es in each generation. The calculations of

natality and mortality are similar for the

spring and the auturnn subpopulations (cftTable 2). Consequently･the population fiuctu-

ation is the resujt of the fiuctuations in thesetwo similar subpopulations, and the ampiitude

of the fiuctuations will be determined by the

extent by which the population parameters of

each subpopulation are affected by the en-vironmental conditions. The greater size of thesecond peak compared with the first is causedby the larger initial number of autumn adults

cornpared to the number of spring adults. Theeffectsoftheenvironmentalconditionsoneach

subpopulation is, however, not kllown.

After November, the densities showed ex-

trerne fiuactuatjens at high levels, which can

not be explained theoretically. These unusualfluctuations were,probably caused by the im-

migration of animals frorn the neighbouring.forest where the trees were being cut downsince September. Unless this is so, it rnay besupposed that the amphipods are distributed

in clusters with the onset of the coo] season,

and during this time the clumps were aociden-

tally sampled (cf. Fig. 3). In any event, the highlevelafterNovemberispresurnablynotanatu-ral feature of the population, but is caused bysampling error, for then the population hadalready passed through the breeding peried ofthe year.




VoL 24, No. 2 JAPANESEJOURNAL

4. Speciation

Euterrestriai amphipods are considered to

be primarily derived directly frorn supra-

littoral amphipods (HuRLEy 1968). [he pres-ent species, Orchestia platensis japonica is aeuterrestrial amphipod or land-hopper and

colonizes relatively damp forests in Japan. It isa widespread species in Japan' and the mor-

phologically indistinguishable population..s are

commonly distributed even on beaches aroundJapan. Orchestia platensis platetzsis js also

found on beachos but not jn the euterrestrialhabitat. HuRLEy (l968) noted the possibilitythat one or more of serni-cosmopolitan supra-

littoral species, e.g., Orchestia pintensis, are

able to throw off terrestrial species whereyer

the situation is appropriate. As O. platensis

J'mpanica is distributed at almost the same habi-

tats as O. platensis (s. st.), except for euter-

restrial habitats, the former might have beenderived from the latter. The environmental

conditions of forests are markedly different to

those of the coastal beach, so it may by possi-ble that the euterrestrial population of o.

platensis japonica will become independent of

the supralittoral population as a distinct

species, in the future. There are three other

euterrestrial species in Japan, O. humieola

MARTENs (SflipHENsEN 1935), O. kokuboi

UENo, 1929 and O. solijLtga IwAsA, 1935.

From the New Zealand region alone, some 14

species are known in spite of that the fauna

has not been studied intensively (HuRLEyl959). In Japan, more species may be found inthe eute!rrestrial habitats, for intensive studies

on euterrestrial arnphipods have not been

canied out since IwAsA (1939). t

Moreover, it is ]jkely that the euterrestrial

populatien of Orchestia platensis J'oponiea will

give rise to two species. The population iscemposed of two subpopulations each with

different breeding periods. Even though bothsubpopulations experience the same environ-

mental conditions they do so at different stages

of their life cycles ; the spring type experjencessummer in the yoimg stages and winter jn the

pre-adult stages, while the auturnn type under-

goes winter as young stages and summer in theadult stages. The difference in breeding seasonmea is that there rnay be no gene exchange be-

137

OF ECOLOGY June, 1974

tweeri these two subpopulations. Rqproductiveisolation is one of the main factors in speci-ation (MAyR 1970). If mutation and recombi-

nation of genes is being independently ac-

cumulated in these two subpopulations, then

there is a possibility that they will proceed on

geneticaIly different ways. This means that onesubpopulation may be separated in type from

the other in the future. These genetically dififerent paths art supposed to be dcrveloped or

aocelerated in the process of adaptation to theeuterrestrial habitat.

ALExANDER and BIGELow (1960) reported

that the fie]d erickets, Grlylins pennsylvanicus

and G. veletis, have speciated through a

seasonal isolation of breeding populationswithout geographic separation, Both species

coexist in the same habitat, show a quite con-

sistent acoustical behaviour and are morpho- ,

logica]ly identical to each other, except for therelative length of the ovipositor to body Iength.

However, interbreeding experirnents showed

that hybrjds could not be obtained. They cal1-

ed such speciation `Allochronic

speciation'.

The studied euterrestrial population of O.

platensis 1'aponica is also considered to bespeciating sympatrically but allochronically,

because there is no eyidence that these sub-

populations have ever been allopatric, though

the two subpopulations can not be regarded as

distinct species as yet, because the possibilityof suocessful hybridization is still not tested.T[he mechanisms by which the separation of

spring and autumn types occurred, is, notknown. But the following points seem to guar-antee their separation: (1) existence of breed-ing pauses twice per year, in early summer and

in winter; and (2) disappearance of the oldest

group, which had completed breeding, fromthe population by the next period of breeding.

NAGATA (1966) noted that the supralittoral

population of'this species continues to breednearly throughout the year but it is probablethat two active spawnings are seen in the

period from spring to early summer and in theautumn season from October to Noyember.

Therefore, there is some indication that speci-ation is already to be seen even in the supra-

littoral population. If so, it is considered that

the colonization of the euterrestrial habitat orforest accelerated speciation in the populatjon.

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'

Vol, 24, No.2 H.2S ts nc

But, for the prasent, it is not known what en- .yironmental

conditions accelerated the speci-atjon process. In the case of field crickets, the

severe winter pemits the survival of only

specjal stages: lhte numph and egg, and thiscaused the separation of the spring populationfrom the autumn population (ALExANDER andBiGELow 1960; ALExANDER 1968). Finally,ALi{xANDER and BIGELow pointed out that"when

two closely related, sympatric species

are found to overwinter in stages whi ¢ h are

widely separated on the life history cycle, when

there is no evidence for earlier allopatry, and

when the two species also maintain brieL non-overlapping periods of reproductivo maturity

without evidence ofprevious interaction with

respe ¢ t to characters involved in sexual se-

lection, then it would appear that the possibi]i-tyofallochronicspeciationshouldnotberuledout without carefu1 study." The ecology of

Orchestia platensis 1'crponiea shows that it isproperly contained in these categories.

Future studies are required to clarify thespeciation processes, jnvolving comparisons

arpong populations in varjous ]ecalities, more

detail morphological examinations on both.subpopulations; and cross experiments in la-boratory culture.

Aclcnowledgement

The authors are indebted'to Profl TaljiIMAMuRA, Department of Biology, IbarakiUniversity, for his encouragement during thecourse of this study. The authors thank Dr.Sh6ichi F. SAKAGAMI, Zoological lnstitute,Hokkaido Univetsity, for his valuable adviceon the present study. The authors'sincere

thanks are also expressed to Dr. Kelvin W.DuNcAN, Zoology Department, Universityof Canterbury, who read and improved the ,manuscrrpt.'

References

1) AuxANDER, R, D.: Life cyc]e origins, speeF

ation, and related phenomena in crickets.

Quart. Rev. Biol, 43, 1-41 (1968).2) ALExANDER, R, D. and R. S. BiGEiDw: AIIo- chronic speciation in field crickets, and a new

species, Aehera veletis, Evolution 14, 334-346 (1960),

'

3) BIGELow. R. S.: Evolution in the fie]d cricket '

Acheta assimilis'Fab. Canad. J. Zool , 36, 139-

151 (1958).

138

ee ft Sb June, 1974

4) CAssm, R. M. : Some us ¢ s of probability paper

in the analysis of size frequency distributions.

Aust, J. mar. Freshwat. Res. 5, 513-522 (l954). S) CoopER, W. E. : Dynarnics and production ofa

naturalpopulation'ofafresh-waterrnaphipod,

11),ale1la ttzteca, Ecol. Monogr. 35, 377-394 (1965).

6) CvRRy, A.,R. F. Grayson and T. D. Milligan: New British records of the semi-terresuial

amphipod Orchestia cavimana. Freshwat. Biol. 2, 55-56 C1972). 7) DuNcAN, K. W.; The ecology of two species

of terresuial Amphipoda (Crastacea: Family Talitridae) living in waste grassland. Pedobiol.

9, 323-341 (1969). 8) GEisLER, F. S.: Studies on the postembryonic

development of 17),alella ueteca (SAussuRE). Biol.Bull. 86, {F22 (1944). 9) HuRLEy, D. E.: Notes on the ecology and

environmentaJ adaptatiens ef the terrestrial

Amphipoda.PaciMcSci.8,I07-129 (1959), 10) HuRLEy, D. E.: Transition from water to land

in arnphipod crustaceans. Am. Zoologist 8, 327-353 (1968).

11) IwAsA, M.: Japanese falitridae. Jour. Fac. Sci., Hokkaido Imp. Univ., ser. Zoo]. 6, 254- 296 (,1939).

12) LboyD, M.: "Mean

crowding". J. Anim. Ecol, 36, 1-30 (1967).

13> MAsAKi, S.: C]imatic speciation in 'crickets.

Biological science 23, 113-119 (l972) (In Japanese).

14) MAyR. E.: Population, species, and evolution

453, Harvard Univ. Press, Carnbridge (1970), 15) NAGATA, K.: Studjes on marine gammaridean Amphipoda of the Seto Inland Sea. III. Pubr.

Sete Mar. Biol. Lab. 13. 291-326 (1965). 16) NAGATA, K.: Studies on marine gamrr!aridean Amphipoda of the Seto Inland Sea. IV. Ibid. 13,327-348 (1966).

17) SAiTo, S. and H. KuDARA: Reproduction by

an amphipod, Orchestia platensis (KROyER) in a forest, in relation to reproductien eMciency

of an individual. Jap. J. Ecol, 22, 213-221

(1972) (In Japanese with Engl. synopsis).

18) STEpHENsEN, K.: Indo-Pacific terrestrial Tali- uidae, Bernice P, Bishop Mus., Oocas. Paper le, 1-20 (1935).

19) STE?HENsEN, K.: Some Japanese amphipods.

Vidensk. Medd. fra Dansk naturh. Fore. 108, 25-88 (1944).

20) UENo, M.: A new terrestrial amphipod Or-

chestia kokuboi. sp, nov. from Asamushi. Sci Rep. Tohoku Imp. Univ, 4, 7-9 (1929).21) WMrANABE, H.: Are sand-hoppers littoraJ



The 　Eoologioal 　Sooiety 　of 　Japan

Vol．24，　No．2 JAPANESE 　JOURNAL　OF 　ECOLOGY

　　an als　or 　forest　soil　anirnals ？Edaphologia

　　4，10 （1970）　（ln　Japanese）．22）WILDER

，」，：　The　effects　of 　popUlation　dens晦ア

　　upon 　grow血，　reproduction 　and 　survival 　in

　　正lyaiella　azteca ．　PhysioL　ZooL　13，439咽461

　　（1940）．23） YAMAsAK 【，　T．：Sib】ing　specie笋　and 　tbeir　analy ・

　　 ses　in　crickets．　Iden（The　Heredity）29，30−35

　　（1972）（ln　Japanese）．

摘要

　森林に生息する陸生端脚類ニホンヒメハマトビムシの・

　　　　　　　　　個体群生物学

　　　　　田村浩志・小関和夫（茨城大・理・生物）

　ニホンヒメハマトビムシは土壌動物の一員として，日

本中の比較的湿った森林に広く分布している．本研究は

水戸市郊外にあるスギ人工林に生息する個体群につい

て，1971年 5 月から12月まで月当たり2 回のサンプリン

グを行ない，令及び世代構成，個体成長そして個体群密

度の年開変動を調べたものである．

　この個体群は重複した令群から成っており，冬期を除

いて 1 令から14令までが常に揃っている　冬期には 1 令

June， 1974

或は 1令と2 令を欠く．この個体群は令，世代そして亜

個体群の 3相が複雑に組合わされた内容を持っている．

生殖期は年 2回で，1 回目は 5 月下旬から7 月中旬にあ

り，2 回目は 7 月下旬から9 月下旬にかけて見られる，

それぞれの生殖期に新生される群を「春型」及び「秋型」

と呼ぷ．この個体群の生活環は 1年で，春型と秋型の間

には生殖的隔離の傾向が見られるようである．すなわ

ち，春型は翌年の春型を生じ，また秋型は専ら翌年の秋

型を生じるようである．

　各亜個体群の個体成長は S字状のパターンを示した．

春型個体は春から秋にかけて成長し， 10月に成長率を低

下させて，11月に成熟直前の 10令ないし11令で越冬す

る．その個体は翌春に成体となり生殖する．一方，秋型

個体は11月以後 6令内外で越冬し，翌年成長を続け， 8

月初旬に成熟して生殖する．個体群密度は年間 2 回のピ

ークを示した．各ピークは春秋それぞれの生殖期直後に

見られる．

　この個体群の中に生殖的な分離の傾向を持つ 2 集団が

見られることは，この個体群が異時的種分化（Arlochron−

ic　speciation ）の前段階にあるものとしての可能性を暗

示すると考えられる．

139

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