Part VII. Population dynamics of tl1e caprellids in the Sargassum patens bed

九州大学学術情報リポジトリ

Kyushu University Institutional Repository

ヤツマタモク藻場にすむ葉上性ワレカラ類の生活史 特性及び繁殖生態についての比較研究

青木, 優和

九州大学理学研究科生物学専攻

https://doi.org/10.11501/3091190

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

権利関係：

Part VII. Population dynamics of tl1e caprellids in the Sargassum patens bed

Vll-1. Introduction

In general, Sargassum communities show drastic seasonal changes. Since the epiphytic animals in the Sargassum bed sustain their population in such a variable habitat, it is believed that, they should have adaptive life history characteristics adapting to the host plant dynamics. In order to know the actual plant-animal interaction in a Sargassun1 bed, anitnals should be identified up to the species level. Some studies have so far been made on the seasonal population fluctuations in relation to the plant biomass variation (Kito, 1977; Tara ram and Wakabara, 1981; Russo, 1989; Duffy, 1990). However, the studies that attempted to analyze the characteristics of the species populations on the Sargassum thalli record few (Mukai, 1976; Imada and Kikuchi, 1984).

There are two major difficulties in the studies of animal populations on macroalgae. One is the adoption of the methodology for the estimation of population density. This is because that most of the macroalgae have erect plant body and have large scale three-dimensional structure, in particular, Sargassum species have the most complex and temporally variable plant structure. The another problem is in the general life history traits of macroalgal epiphytic animals. In general, the epiphytic animals on short-lived rnacroalgae have short life span, short generation time and short reproductive cycle with continuous breeding resulting in the overlapping generations (Mukai, 1976; Seed, 1985).

This nature makes the population size structure analysis almost impossible.

In this study, I made a large permanent quadrat at the study area, and conducted a periodical and systematic sampling in order to analyze the plant- animal interaction in the Sargassum patens bed. Population density of caprellids was estimated by several methods and the results were compared. On the other hand, the seasonal variation of the Sargassum patens bed was analyzed both fron1

58

the micro-scale: microstructure of the plants and from the macro-scale: three- dimensional structure of the seaweed bed.

Sargassum bed is usually consisted of Sargassum plants and the algal undergrowth (mainly red algae). Thus, both the animals on Sargassum thalli and on algal undergrowth should be studied at the same time, in order to know the habitat utilization and life history characteristics of the epiphytic populations in a Sargassum bed. There were studies which described the population fluctuation of caprellids only on Sargassu1n thalli (Imada and Kikuchi, 1984; Aoki, 1988) and only on algal undergrowth (Takeuchi et al., 1990a). However, no study has attempted to cover the epiphytic populations on all substrata in a Sargassunz bed.

In this study, for the first time, the epiphytic caprellids both on the Sargassun1 patens and on the algal undergrowth were examined.

The comparative studies of phylogenetically-close species are fruitful (e.g.

Steele and Steele, 1975; Kolding, 1981; Dauvin, 1989; Powell and Moore, 1991).

In Part VII of this study, the population characteristics of Caprella species inhabiting a Sargassum patens bed were compared by the examination of the field populations with using the results obtained in the previous parts of this thesis.

VII-2. General field sa1npling method

A 9x9m permanent quadrat was set in the S. patens bed on 27 July 1985.

This quadrat was divided into nine 3x3m quadrats and each of the 3x3m quadrats was subdivided into thirty-six 0.5x0.5m quadrats (Aoki, 1988; Fig. VII-1).

Routine samplings were conducted in 15-20 day interval from October 1985 to November 1986, and once a month fron1 November 1986 to December 1988. All underwater work was carried out by SCUBA diving.

At each sampling time, one 0.5x0.5m quadrat was randomly selected from each of the nine 3x3m quadrats without repetitions and counted the number of Sargassum plants. Plants having only holdfast without leaves and mainstem were neglected. Then, one Sargassum patens plant was collected from the central part

59

-l

3m

_J__

I . - "

.-...

I

0.5m

9m

I I

..._ ~

~ I I

N

Fig. VII- 1. The diagram of permanent quadrat at the study area of Sargassum patens bed .

Height

Main stems

Holdfast

~

Rock

··:=:·:·:·.: ··:.=: ·:~:: ·.·: :·:::: '; ~··:·: . ·~ ^·=.· ·::-;:.=.·:::::: ·:: ... :·=.:':) :::'~ ·:

:·c______

:.:. · .· ·. · ... · · ·.: · .. · ..

· · · ·. ·. -: · ·: ·.· Sand bottom

. . ^. . ..

Fig. VII-2. The structure of Sargassum patens plant.

14

12

r:n ¹⁰

~ ~

ro

~

~

~ 0 ₆ 0

.

z

2

0

Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct

1986 1987 1988

Fig. VII-3. Seasonal variation of the mean number of Sargassum patens per quadrat (O.SmxO.Srn).

of the quadrat. The plant n1ass with its epiphytic anin1als was carefully collected by the method described in Aoki (1991 b), then the total 9 plants from each quadrat were examined for the epifauna after taking them back to the laboratory.

The remaining Sargassum plants in the nine 0.5x0.5m quadrats were cut at their base, collected and put in a cloth bag, and brought back to the laboratory. They were used to know the biomass of the plants per bottom area. For the additional data, only in the period: October 1985- September 1986, the number of main stems per plant and the height of each plant (Fig. VII-3) was recorded for all plants in each sampling quadrat.

After sampling S ar gas sum plants, algal undergrowth remained in the sampling quadrat were collected in order to examine the epiphytic animals. They were carefully put in the plastic sampling bags (see Aoki, 1991 b) and taken back to the laboratory.

VII-3. Seasonal changes of the S. patens con1munity and algal undergrowth

Materials and methods Sargassum patens

After the complete removal of epiphytic animals by the method which will be described later, the 9 substrate plants were examined for the height, volume and dry weight were. The total dry weight of the plants collected in the cloth bags was also recorded in order to know the total biomass per area. The dry weight data were obtained after drying the plants in an oven at 1 05°C for more than 48 hours.

Algal undergrowth

Algal undergrowth In the sampling quadrat consisted of red algae:

Gelidium spp., Procamium spp. and coralline algae. The quality of the red turf algae was rather different from that of the crustose coralline algae, thus the

60

weight of algae was not considered as a good measure of algal quantity in an area.

Therefore, the volume of total algal mass was measured to know the quantity of the algal undergrowth per area.

Other algae

Though Sargassum patens occupied more than 85% of the total standing crop in the S. patens bed (see Fig. VII-4 ), few atnount of Sargassum piluriferum occurred in the sampling area. They were also described in dry weight per area.

Results

General pattern of the seasonal changes of the S. patens bed

S. patens showed drastic seasonal fluctuations in its biomass. New plants

begin to grow up from the old holdfasts or newly settled germinates in late summer, then several main stems come up from the holdfasts. During the initial phase of growth in fall and early winter, the plant which was short and had broad leaves look liked a shrub. From January to March, the plants rapidly flourished with extending main stems and increasing the thin leaves. The standing crop of the plants reached a maximum in April and reproductive receptacles matured in that period. The plants began to decay in late April after the reproductive period was over. All of the overwintering plants detached from their holdfasts on rocks and flowed away until the end of June. After the disappearance of the plants, new main stems started to come out from the remaining holdfasts. There were various sizes of holdfasts, and old holctfasts seemed to be larger and produced more main stems than the younger ones.

Density of S. patens

Mean number of plant bodies per area varied from 5-10 in September-May of the first year and third year, but, in the second year, the mean number per area in September-May varied between 6-12, which was rather larger than the

61

other two years (Fig. VII -3; Appendix Table 1 ). TI1e plants which lost plant bodies (but had holdfasts) increased in number from May. Plant bodies were flowed away one by one, and finally, few plant bodies could only seen in the sampling area in July.

Standing crop of S. patens

The standing crop of S. patens showed drastic seasonal fluctuations. The plants grew rapidly in the winter and reached maximum in April 1986, In

January 1987 and in April 1988 respectively (Fig. VII-4; Appendix Table 1 ). In each year, the standing crop suddenly decreased from May and reached almost zero in July-August. Moreover, the standing crop varied among the years. In the first year, it increased rapidly and reached more than 200g DW per quadrat in April. However, it increased more gradually in the following years than the first year. The maximum biomass in the second year was 1/6 of that in the first year, and 1/2 in the third year. In July-August of each year, the biomass went down to ahnost zero.

Three-dimensional structure of S. patens bed

By using the data on the nun1ber of the main stems per plant and the heights of the plants, the seasonal change of three-dimensional structure of S. patens bed was deduced and shown in the serial graphs (Figs. VII-5 A-N). The number of the main sten1s per plant represented the thickness of the plant. During the initial phase of the growing season, most plants were short in height but had the largest number of main stems (Figs. VII-5 A-C). The growth of the plants varied widely; some of the plants grew larger in both height and thickness, though some of them stayed in small size. Thus the size variations of the plants in the S. patens community became larger as the plants grew (Figs. VII-5 D-H). In February to March the seaweed forest attained a stage which contained the most wide sized plants (Figs. VII-5 H,l). After this stage, two extreme forms of plants began to

62

225 ,..-..

c-;1 20

lO

s

C'l

.

0 15 -...

..._.,

...._;) 10

~

~

~ ~

~

• S. patens

A S. piluliferum

Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct

1986 1987 1988

Fig. VII-4. Seasonal variation of the mean dry weight of Sargassurn patens and Sargassum piluliferum per quadrat (O.SrnxO.Sm).

\

October 18

(A)

15

... en

r:: 10 cO

"a

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0

z

5

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~

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14 .... o·o

. . . . . . . ^~ . . . . ^., . . . . .

0 25 50 75 100 125 150 175 200 225

Height of plants (em)

November 18

250 ~

(C)

15

.+-J en

r:: 10 cO

0. c..-.

0 5

z

0

. . ... . . / L20

~

.. . 1/ " " . .. . ·;-:-4 !$'

. . . . . . : :. . /-86 «;;,.~

. -... . ;..:-10 ~~

;-12 ~

0 - -25

~o·

Height of plants (em)

October 31

(B)

15l' · ' . · · /: -

en - .

.+-J

~

fA1

'a :...

': 5

z

. .. /-2 ~0 c,

·;-74 ~~$"

... /-6 ";

.. rs

. . . . J:-:10 $'

. . . -u ^~

0 '/ /14 ,a·

75 100 125 150 175 200 225 250 ~ 0

(D)

_.,) en 15

r:: 10

co 'a

25 50

] 51

o ... - - -

0 25 50

Height of plants (em)

December 5

'"'5 100 125 150 175 200 225

Height of plants (em)

/~0

. / -2 ,c-.,";

/:4 ~v

. /-6 ~

_r8

·.s-

/LJ210 ~ .:$>~

.-14 .... o·o 250 ~

Fig. VII-S. Seasonal variation of the three-dimensional structure of Sargassum patens community.

December 24

(E)

15 .·

~

l . . . .... ^{u / /· ''}

-C..

~ l "

...

··

r-2L___./QdS

"- / • ~ -1 9~

_ _ _ _____ _ .. _ _

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^{::.:···::: :-:: ;44~'l.J'-}

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·. ·:· · ···:·.>il210~~

.

0 o 2'5 5o

~5

1~0

~5

~o·

~ C/J

(G)

15

c 10 ,... co

~

Height of plants (em)

January 30

~

~ 5

z

. . ~./:" 0 /-2 c,

0 .. "/!6 - 4 ~'<.1 ;$'

.. . ~

..._,-. /-18 ₀ r'.,V ~

. -;-L12 ~~

14 ..._0·

0 25 50 75 100 125 150 175 200

Height of plants (em)

225 250 ~

(F)

15

.._) 00

c 10

ro 0..

~

0 5

z

January 11

. J;:-0 -2 c,

·/4~~

... /-6 ^~ j~B ~~

····-10 ^~

0 · .L12 ~

+--~--,---,----r---.---r---r--;--r--r/ 14 .... o· 25 50 75 100 125 150 175 200 225 250 ~ 0

Height of plants (em)

February 19

(H)

"' 15l -.

.._) .- . ..· .

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l . · · · . . : /

co : .:· ~ - ^.'

c.. . . . · ·· :

c... .·· :

0 _:::>-- .· .

z

. . ./

0 ^I

0 25 50 75 100 125 150 175 200

Height of plants (em)

Fig. VII-S. Continued.

\

March 12

(I)

15

Cll

~ ^{10 1.-·· ... ····:}

Cffi ' / / /

~ ^{/ · . / . .- I / / /} ~;.::L_/~0 o..

l .· . : / ' .

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~ ^{. . .· / / L_l .} ---4 ~ o

. 5 l

/

....

"--/ ( /

. .

0 . . . . . c:::==:v ~

Z . ·· . . . ... ... . .. : ' · · .

,6 o,.<:>

' • . . ... ... .. . / 12 ⁰

0 I ·· I I i I r 14 ~ ,o·

0 25 50 75 100 125 150 175 200 225 250

Height of plants (em)

April21 (K)

~ 00.

15

s:: 10

~

0.. ~

0

z

March31

(J )

2l

15 l . ' .

] Io

l ,. s ^{lJ (~} t / ^{( ~}

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! ^{5 l / . ' · . . . . ::: . . .} ... ^{• ·.·· ··· ··· ···· .·.··· ···} . ^··· . .. ... / ^{· ··· ·.1/ · . ·· ;_} . / . 9/~l 1 · ... / ~~:~~~

0 · 14 ..;;.o·

0 25 50 75 100 125 150 175 200 225 250

Height of plants (em)

(L)

"' 15 l ··. -··· ...

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l .

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.·· . . .·. / 12 0

0 14 -"0'

' 2.5 50 75 100 125 150 175 200 2~5 250 ~ ~

0

Height of plants (em)

Fig. VII- S. Continued.

(M) -

.June

UJ.

..._:)

15

c 10 ro

0..

~ ~ 5

z

. r-0 ..... L.2 <?

. ·~4 ~'<.JJ$"

........ . ... ·/-6 <?

··.···r s ~~

. . . . ....... _.:...10 J$"

· · ·: · · · · jL12 ~

0 14

+0"

0 25 50 75 100 125 150 175 200 225 250

(N)

UJ.

..._:)

15

c 10 ro

Ci

~

~ 5

z

Height of plants (em)

June24

............................ -...... .

. ··/ :0

...... - 2 ^~

.. . . . . . . . ;-:( 4 N'<.JJ$"

. -6 ^<?

/s

. ---10 J$"

.L12 ~

0 / 14 ... a·

0 25 50 75 100 125 150 175 200 225 250 -.:::

Height of plants (em)

Fi g. VII-S. Cont inued.

appear. One is the plants which grow larger and thicker, and the other is the ones which began to lose their n1ain stems and became shorter and thinner (Figs. VII-5 J,K). From the end of April, the main stems and leaves of all plants gradually began to be cut and drifted away. The density of the plants became lower and the plants became thinner and shorter (Figs. VII-5 K,L). By the final stage of this plant decaying season, all of the large plants disappeared, and those remained plants were with only few main stems which too finally degenerated and flowed away (Fig. VII-5 M,N).

Measure1nents of the sampled S. patens

Height: The heights of the sampled plants were generally larger than the mean heights of the whole plants in the sampling area. The height of the plants reached maximum in April, then suddenly decreased in May-June. The maximum height of the plants was more than 200cm in tl e first year and about 150cm in the following 2 years (Fig. VII-6 A; Appendix table ^1).

Volume and dry weight: The mean dry weight of the sampled plants showed similar seasonal trend to that of the volume of the plants (Figs. VII-6 B,C; Appendix Table 1). Both the dry weight and volume reached maximum in March-April and rapidly decreased in April-May, about 1 month ahead of the decreasing seaweed height.

Dry weight-volume ratio: The change in the ratio of dry weight to volume shows the change of the quality of the plants (Fig. VII-7). The ratio gradually increased frotn November to March, then rapidly increased from April, and reached a maximum in June. The increase of the ratio was due to the increase of the relative proportion of main stems, which was caused by the thinning of the plants: losing leaves, branches and floats (Fig. VII-8).

Thickness of the plants: There was a discrepancy of decreasing timings between the dry weight and the volume. This means that the thinning of the plants occurred ahead of the shortening of the plants. Thus the thickness of the

63

25 ,--....

A

s

..._,

4-J

r::

,...--4

~

~ 0

,..q 4-J

•l'""t bn

(l) 50

~

0

Jun Apr Jul Od Jnn Apr Jul Ocl ^{Juu Apr Jul Oct}

1986 19B7 1988

70 ,--....

B

4-J

r::

,...--4

~

...

..._, bn

~

•l'""t (l)

~ ²⁰

~ ~ ¹⁰

0

Jan Apr Jul Oct Jun Apr Jul Ocl ^{Jan Apr Jul Ckt}

1986 1987 1988

450

,--....

c

§

~

~

...

t'J

s

..._,

s

(l)

@

100

~

~

0

Jan Apr Jul Oct Jan Apr Jul Ocl Jan Apr Jul Oct

1986 1987 1988

Fig. VII-6. Seasonal variation in the mean height (A) , mean dry weight (B) and mean volume ^(C) of Sargassum :eat ens plants sampled from the study area (refer text VII-

2) •

,.-._

Cl? .4

s s

"-"'

<l)

s

~ .3 0

>

... .25

,.-._

"-"' b.O .2

~ ~

b.O

.l~jV ~ \.

·~ <l)

~

..

~

~ ~

~ ^.05 Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct

1986 1987 1988

Fig. VII-7. Seasonal variation of the mean of dry weight/volume ratio of Sargassum patens.

----..

~ .4

s s

..._..

•

C)

s

:::3

,_...

0 .2

>

•

...

.21

•

----..

b.O

•

..._..

...._,;) .151

• ^•

,..c •

b.O _.11

• •

·~ C)

~ ^I

•

~ . 05

~ ~ 0

0 .2 .4 .6 .8 1

Ratio of main stem

Fig. VII-8. Sargassum patens. Dry weight/volume ratio in relation to the ratio of mainstem occupying in the dry weight of plant mass.

.35

A

,-..,

s

'-...-/

~

i1

·~ (l)

,..q

... .2

,-..,

'-...-/ b.O

.15

,..q ~

b.O _.1

·~ (l)

~

~ ~

~ 0

Jul Jan Apr Jul Oct Jan Apr Jul Oct Jon Apr Oct

1986 1987 1988

3.5

B

,-..,

s

'-...-/

,..q ~

bD 2.5

•M (J)

,.q 2

...

,-..,

M

s

'-...-/

s

(l) 1

s

.5

~

~

0 _{Jnn Apr Jul} Oct ^{Jan Apr Jul} Oct Jon Apr ·Jul Oct

1986 1987 1988

Fig. VII-9. Seasonal variation of the mean thickness of Sargassum patens plants. A: dry weight/height;

B: volume/height.

....-...

~ 70 lQ

s

C\1

.

...

C"J 50

s s

...._.,

<l)

ro

~

ro

~ 20 0

<l)

s

~

~

~

1986 1987 1988

Fig. VII-10. Seasonal variation in the mean volume of algal undergrowth per quadrat (O.SmxO.Sm).

plants was calculated. Thickness of the plants was expressed in volume/height ratio (Fig. VII-9 A) and in dry weight/height ratio (Fig. VII-9 B). It is clear that the thinning of the plants started about 1 n1onth ahead of the shortening of the plants.

Seasonal changes of other Sargassu1n seaweed

Sargassum pilurijerun1 occurred in the sampling area. The morphological characteristics of S. piluriferum was sin1ilar to that of S. patens. The total standing crop of S. pilurzjerun1 occupied less than 15% of the total standing crop throughout the year. The seasonal trend in biomass of S. piluriferum was similar to that of S. patens (Fig. VII-4).

Seasonal changes of algal undergrowth

Algal undergrowth in the S. patens bed consisted of the red algae: Gelidium spp. and Procamium spp., and some coralline algae. Their biomass was expressed in the volume of algal mass. The biomass of the undergrowth began to increase from January, then rapidly increased during the decreasing period of the S. patens biomass. Undergrowth biomass reached maximum in June or July when the biomass of S. patens was small, then rapidly decreased from July, and finally reached the bottom in September (Fig. VII-I 0; Appendix Table 1 ).

VII-4. Seasonal changes of the epiphytic caprellid populations Materials and methods

Nine Sargassun1 patens samples and 9 undergrowth samples containing the epiphytic animals were fixed in 10% neutralized formalin immediately after the collections. In the laboratory, the samples were washed and shaken in a bucket of seawater and then poured across a 0.1 mm sieve. The process was repeated until all epiphytic animals were completely removed. I used the O.lmm sieve for the

64

3000

~

A

§

~

~ ...

rJl 2000 '"d

•rl ~

~

Cl) 1500

~ ~

u ro

~ 1000 0

z

Jon Apr Jul Oct Jon Apr Jul Oct ,Jon

1986 1987 1988

80 70

b1)

B

...

rJl 60

'""d

~ 50

~

Cl)

~ ~ 40

ro u

~ 30 0

0 20

z

10

Jan Apr Jul Oct Jon Apr ,Jul Oct ^{Jan Apr Jul} Oct

1986 1987 1988

9

M

_s

c

s

...

rJl 6

'"d

~ 5

~

Cl)

~ ~ 4

ro u

~ 3 0

0 2

z

.Jnn Apr Jul Oct ,Jon Apr Jul Oct ^{Jon Apr Jul} Oct

1986 1987 1988

Fig. VII-11. Seasonal changes in the abundance of all caprellids on Sargassum Qatens thalli. A: mean number per plant; B: mean number per algal dry weight;

c:

Table VII-1. Regression equations for the relation between dry weight of substrate seaweed (X) and the total number of caprellids on the seaweed (Y). R: Coefficient of correlation.

Date

1985 Oct 3 Oct 18 Oct 31 Nov 18 Dec 5 Dec 24

1986 Jan 11 Jan 30 Feb 19 Mar 12 Mar 31 Apr 21 Jun 4 Jun 24 Aug 6

Aug 30 Sep 23 Oct 14 Nov 4 Dec 26

Regression line

y = y = y =

y =

y = y

=

1.04 X+

0.05 X+

1.14 X+

1.85 X 0.74 X 1.23 X+

y = 3.15 X +

0.96 1.81 0.56 5.08 1.75 8.07

7.06 Y = 3.82 X

Y = 4.97 X Y = 42.24 X

Y

=

Y = 2.35 ^X Y = 40.65 X

y = 0.58

6.01 + 122.06 -1311.70 + 50.57 + 41.39

y = 1.48 y

=

y

=

y = 2.28 y

=

X

X +

X + X + X X + Y = 2.61 X

181.97 0.21 0.47 9.40 0.55 3.57 5.20 2.05

R

0.86 0.19 0.52 0.85 0.82 0.58

0.61 0.53 0.79 0.68 0.33 0.80 0.63 0.96 0.25 0.23 0.51 0.81 0.76 0.75

Probability

p < 0.05

p > 0.1

p > 0.1

p < 0.05

p < 0.01

p > 0.1

p < 0.1

p > 0.1

p < 0.05

p < 0.1

p > 0.1

p < 0.05

p < 0.1

p < 0.001

p > 0.1

p > 0.1

p > 0.1 p < 0.01

p < 0.05

p < 0.05

Table VII-1. Continued.

---

Date Regression line R Probability

---

1987

Mar 21 y

=

=

=

=

=

=

1988

Jan 28 y

=

Mar 11 y

=

Apr 23 Y

=

May 19 Y

=

Jun 27 y

=

=

=

=

=

---

8000

N

A

s

lQ C'1

0 6000

...

5000

'"0 00.

• l""""i 4000

....-1 ....-1

<1)

J..-4 3000

~

ro

u 2000

~ 0

0 1000

z

Jon Apr Jul Oct Jan ^{Apr ~Jul Oct} Jnn Apr Jul Oct

1986 1987 1988

12000

N _lQ

s B

C'1 10000 0

...

8000

'"0 00.

• l""""i ....-1 ....-1

6000

<1) J..-4

0..

ro

u ₄₀₀₀

~ 0

0 2000

z

,Jan Apr Jul Oct Jon Apr Jul Oct ^{Jnn Apr ,Jul} Oct

1986 1987 1988

Fig. VII-12. Seasonal changes in the density of all caprellids on

Sargassum patens thalli. A: density per quadrat (O.SmxO.Sm) estimated by the plant number method; B: density per quadrat estimated by the regression method.

samples, because this prevented the passage of the smallest juveniles of the caprellids. From the animals retained on the sieve, caprellids were sorted out and counted under a binocular microscope. Furthermore, for the samples collected in October 1985-December 1986, the caprellids were identified up to species, and the number of newly-born juveniles and the number of adult females with brood pouch were counted. The juveniles which had 2-segmented flagellar articles on antenna 1 were recognized as the newly-born juveniles. TI1e body length and the number of flagellar articles were also examined for the juveniles of C. monoceros and of C. decipiens, in order to know the number of the juveniles in the maternal care colony. More than 70,000 caprellids were examined in this study.

Results

Epiphytic caprellids on S. patens thalli

In each year, similar seasonal trends were observed in the total number of caprellids per plant, though the number at the seasonal peaks was different among the years (Fig. VII-11 A). 1l1e number of total caprellids per dry weight and per volume repeated a steep increase and rapid decrease with two distinct peaks in each year (Fig. VII-11 B, C).

In order to estimate the total number of caprellids per unit area, two methods were employed. First method was by calculating the density from the number of caprellids per plant and the number of plants per unit area (Fig. VII- 12 A). The other method was by using the relationship between the dry weight per plant and the number of caprellids per plant. The regression equation for this relation was obtained for the data in each season (Table VII-I), then the density per unit area in a sampling time was calculated by using the equation and the dry weight data of plants per unit area (Fig. VII -12 B). The data obtained by two different methods showed almost the same trends, though the data obtained from the plant number per unit area looked to be overestimated than those obtained by the regression method. There was a pointed peak in the number in March in the

65

first year and in April-May in the third year. The peak appeared in March of the second year was very small.

Epiphytic caprellids on algal undergrowth

A regular seasonal trend was present both in the total number of caprellids per unit area (Fig. VII -13 A) and in the total number of caprellids per unit volume (Fig. VII-13 B). There were two distinct peaks in a year, one was in March and another was in June-July.

Total nu1nber of caprellids per unit area

The biomass of S. piluriferum in the sampling area was under 10-15% of total biomass throughout a year. Thus the epifauna and the biomass of S.

piluriferum was neglected, when the total number of caprellids per unit area was calculated. Compared to the number of caprellids on S. patens, that on the algal undergrowth was relatively few. Total number of caprellids appeared on the algal undergrowth in a unit area through the study period occupied only 8.5% in the total caprellid number (The data of the number of caprellids per unit area used here was obtained by using the regression method). The seasonal trend in the total number of caprellids per unit area hence reflected that of caprellids on S.

patens (Fig. VII-14).

Seasonal changes in caprellid species population

Seasonal trend of the species population in S. patens bed was examined for the 7 dominant species: C. decipiens, C. monoceros, C. tsugarensis, C. okadai, C.

danilevskii, C. verrucosa and C. scaura (see Appendix Table 2).

In order to estimate the total number of a specific caprellid population per unit area, two methods were employed and compared, as the methods used for obtaining the total which were the methods used for obtaining the total caprellid density. The only difference in the methodology was that, here the caprellids

66

300

A

N _lQ

s

C\1 250

0

... 200

rCj rn

•1"""'1

~ ~ ₍₁₎ 15

~ ~

ro

u

~ 0 0

.

z

0

Jon Apr Jul Oct Jnn ^Apr Jul Oct ^{Jan Apr Jul} Oct

1986 1987 1988

14

B

(")

s

s

... 10

r:/1 rCj

~ 8

~

(1)

~ ~ 6

ro

~ 0 4

z

0

,Jan Apr Jul Oct Jon ^Apr Jul Oct Jan ^Apr Jul Oct

1986 1987 1988

Fig. VII-13. Seasonal changes in the abundance of all caprellids on algal undergrowth. A: mean density per quadrat (O.SmxO.Sm); B: mean number per unit algal volume.

8000

C'l _lQ

s

C'l

.

--

0 r-1

~ 4000

~

Q)

~ ~ 3000

ro

~ 0 2000 0

.

1000

z

0

A J I Oct J A J I Ocl ^{tJll fl} AJJr Jul Oct

Jan pr u nn pr u

1986 1987 1988

Fig. VII-14. Seasonal variation in the density of all caprellids occurred per quadrat (O.SmxO.Sm). Solid area is the density on algal undergrowth. Mean density on S.

patens thalli was estimated by the regression method.

were divided into respective species while in the above section number of plants per unit area. The regression equation for the relation was obtained for each species in each season, then the density per unit area in a sampling time was calculated from the equation and the dry weight data of plants per unit area. In order to compare the species specific differences, the seasonal trends of the population density in the 8 species, obtained through the regression method, were shown in Fig. VII -4 2.

Caprella danilevskii

There were two peaks tn the number of C. danilevskii per plant of S.

patens; one peak was in February-March and another was in April (Fig. VII-15 A). The number of the animals per unit dry weight and per unit volume had the similar trend, though the two peaks were not much distinctive (Fig. VII-15 B,C).

The density data per unit area obtained by two different n1ethods showed a similar trend, i.e., the two distinct peaks appeared from January to June. As for the density data per unit area obtained from the plant number per unit area, the two peaks were: a smaller one in February-March and a larger one in April (Fig.

VII-16 A). On the other hand, in the density data obtained from the regression method, a larger pointed peak was observed in March and a smaller peak in April (Fig. VII -16 B). The regression equations used for the density calculation are in Table VII-2. C. danilevskii appeared in the study area at the beginning of December. The density of the animals rapidly increased from January peaked in the beginning of March, then suddenly decreased and increased to another peak in April. The population disappeared by the end of June. No individual could be observed in the study area from June to October.

In S. patens bed, firstly some adult females immigrated into the seaweed bed in December. The newly-born juveniles occurred from January to June, reached about 50% in March-April (Fig. VII-17 A). The number of newly-born juveniles produced by an adult female had a peak in March (Fig. VII-17 B).

67

Caprella darLilevskii

10

0 N ^J) ,J F M A M J

·'

1985 198G

.8 .7

.6

b..O

... .5

~ .4 (J)

..0

s

z

0

0 N 0 .J F M A M J J A s ^{0 N} n J

1985 1986

.05

c

.Ott

('")

s s

...

(J) ~

..0 .02

z ~

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

1985 1986

Fig. VII-15. Seasonal variation in the abundance of ~

danilevskii on Sargassum patens thalli. A: mean number per plant; B: mean number per algal dry weight; C: mean number per algal volume.