Journal of the Oceanographical Society of Japan Vol.25, No.3, pp.145 to 150, June 1969

Zooplankton Investigations in the Setonaikai

(Inland Sea of Japan),IV.*

Occurrence of Zooplankton in the Sea Regions from

Iyo-nada to Harima-nada in the Winter of 1966

Reiichiro HIROTA**

Abstract: 22 species of zooplankton, including 2 species of Tunicata, 2 species of chaetognatha,

13 species of Copepoda, 1 species of Branchiopoda and 4 species of Ciliophora, were discrimi-

nated at 18 stations in the sea regions from Iyo-nada to Harima-nada, in the winter of 1966.

From Iyo-nada to Bingo-nada, the representatives of the zooplankton communities changed

successively as follows according as the hydrographical conditions became more and more

embaymental: Oncaea media•¨ Paracalanus parvus•¨ Oikopleura dioica. But the species

composition of the zooplankton communities in Bisan-seto and Harima-nada varied irregularly

from station to station.

1. Introduction

Continued from the previous three reports

(HIROTA, '68a, b, '69), the description of the species composition and the regional variation of the zooplankton fauna in 7 sea regions of the Setonaikai in the winter, 1966, and the consideration on the zooplankton communities found in these sea regions were dealt in this

paper. The samples were collected at 18 stations

in Iyo-nada, Hiroshima Bay, Aki-nada, Hiuchi-nada, Bingo-nada, Bisan-seto and Harima-nada, from 28th of January to 3rd of February, 1966

(Fig. 1 and Table 1). The sampling and the quantitative measurement of zooplankton were made by the same methods as already employed in my previous investigations in the Setonaikai

Fig. 1. Map showing the collecting stations.

* Received February 7, 1969 Contributions from the Aitsu Marine Biological Station, No.13 Contributions from the Mukaishima Marine Biological Station, No.96 Special Project Research supported by the Ministry of Education "Studies on the dynamic status of

biosphere." This study was carried out as a part of JIBP Project.** Aitsu Marine Biological Station, Faculty of Science, Kumamoto University

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146 Jour. Oceanogr. Soc. Japan, Vol.25, No.3 (1969)

Table 1. Some hydrographical conditions observed at each collecting station.

Table 1. Continued.

Cl... Chlorinity. O2(cc/l) ... Dissolved oxygen in volume as reduced to standard state.O2(%) ... Saturation percentage of dissolved oxygen.

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Zooplankton Investigations in the Setonaikai (Inland Sea of Japan), IV. 147

(HIROTA, '68a). Some hydrographical conditions observed at

each station, during the survey, are shown in Table 1.

2. Species composition and regional variation of zooplankton fauna

The occurrence of the important zooplankton

groups, except Dinoflagellata, is shown in Fig. 2. And the percentage composition of the dominant

groups is as shown in Fig. 3. In this investiga-tion, Copepoda was the most predominant com-

ponent of zooplankton. And following 22 species of zooplankton, except Dinoflagellata, were dis-criminated: 2 species of Tunicata, 2 species of Chaetognatha, 13 species of Copepoda (Crus-tacea), 1 species of Branchiopoda (Crustacea) and 4 species of Ciliophora. The occurrence of these plankters at each station is shown in Table 2. And the general feature of the regional occurrence of them is mentioned in the following.

Tunicata: Oikopleura dioica, the littoral appendicularian, occurred abundantly or com-monly in all the surveyed regions. Doliolurn

gegenbauri var. tritonis, the oceanic thaliacean,

Fig. 2. Occurrence of the important groups of

zooplankton per m3 at each collecting station.

Fig. 3. Percentage composition of the dominant

groups of zooplankton at each collecting station.

occurred only at St. C-4 (Harima-nada) and very rarely.

Chaetognatha: In the species of Sagitta crassa, two forms have been known; one is a large form (S. crassa typica) and the other is a small form (S. crassa f. naikaiensis). And there are various intermediate forms between these two forms. In this investigation, S. crassa f. naikaiensis was collected commonly or rarely in all the surveyed regions. And a intermediate form, C form classified by HIROTA ('59), was collected very rarely in Hiuchi-nada, Bingo-nada and Harima-nada.

Malacostraca: Some undetermined species of Mysidacea were collected very rarely in Iyo-nada and Bisan-seto.

Copepoda: Of the littoral species, Paracalanus

parvus, Oithona similis, Oncaea media and Microsetella norvegica were the important constituents. The major form of Paracalanus

parvus was abundant in all the surveyed regions, but the minor form was common or rare. Oithona similis was abundant in Harima-nada, rare in Aki-nada, and common in the other regions. Oncaea media was generally abundant in most of the surveyed regions, but rare in the inner part of Hiroshima Bay. Microsetella nor-vegica was common at some stations from Hiroshima Bay to Hiuchi-nada, but abundant at the other stations. Of the other littoral species, Acartia clausi, Corycaeus affinis and C. andrewsi occurred commonly or rarely at all the surveyed stations. And Centropages abdomi-nalis and Oithona nana occurred commonly or scantly at most of the surveyed stations. Calanus helgolandicus occurred scantly at most of the surveyed stations. Pseudodiaptomus marinus occurred scantly at some stations from Bingo-nada to Harima-nada. Calanopia thompsoni occurred only at St. C-1 in Bisan-seto and very rarely. Clitemnestra rostrata, the oceanic form, occurred very rarely at Sts. C-13 (Iyo-nada), C-7 (Hiuchi-nada) and C-2 (Harima-nada).

Branchiopoda: Evadne nordmanni was col-lected only at St. C-14 in Hiroshima Bay and very rare.

Hydrozoa: Some undetermined hydromedusae were collected rarely or very rarely in this survey.

Ciliophora: Tintinnopsis beroidea, T. loh-

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148 Jour. Oceanogr. Soc. Japan, Vol.25, No.3 (1969)

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Zooplankton Investigations in the Setonaikai (Inland Sea of Japan), IV. 149

manni, T. radix and T. tubulosa were collected commonly or rarely at several stations in all the surveyed regions. Codonellopsis nipponica was collected at all the surveyed stations and generally abundant or common.

Larval form of benthos: Larval forms, in-cluding ascidian tadpole, ophiopluteus, bipinnaria, actinotrocha, cyphonautes, four stages (megalopa, mysis, zoea and nauplius) of Decapoda, cypris and nauplius stages of Cirripedia, Polychaeta larva, molluscan veliger and pilidium larva, occurred abundantly in all the surveyed regions, especially very abundantly at St. C-7 in Hitachi-nada. Among them, molluscan veliger was the most predominant (61.0-93.7%).

Dinoflagellata and Diatom: Noctiluca scin-tillans (Dinoflagellata, Mastigophora) was col-lected abundantly or very abundantly in all the surveyed regions. The results of the quantitative and qualitative examinations of dinoflagellates

(except Noctiluca) and diatoms at 10 stations (Sts. C-3, C-4, C-5, C-7, C-9, C-11, C-12, C-14, C-15 and BG-1) will be reported in the separate

publication, together with the values of the chlorophyll a content and the photosynthesis .

3. Consideration on the zooplankton communities

The most important component of the zoo-plankton communities in the sea regions from Iyo-nada to Harima-nada in the winter of 1966 was Copepoda (Fig. 3). And the nextly im -

portant components were Tunicata, Ciliophora or benthos larvae. On Copepoda, the percentage composition of the important species at each station is shown in Fig. 4. In most of the surveyed regions, except Iyo-nada and Harima- nada, Paracalanus parvus was the most domi- nant and Oncaea media was generally sub- dominant. But in Iyo-nada, Oncaea media was the most predominant and Paracalanus parvus was subdominant. In Harima-nada, Paracala- nus parvus, Oithona similis, Oncaea media and Microsetella norvegica occurred as the important components, and their composition varied from station to station. On the other groups, the important components were Oikopleura dioica

(Tunicata) and Codonellopsis nipponica (Cilio-phora). Putting together these results, the

important zooplankton organisms representing

the community in each sea region are generalized

as follows.

Iyo-nada......Oncaea media, with

Paracalanus parous

Hiroshima Bay... Paracalanus parvus, with

Oikopleura dioica

Aki-nada......Paracalanus parvus, with

Oncaea media

Hiuchi-nada......Paracalanus parvus, with

Oikopleura dioica

Bingo-nada......Oikopleura dioica, with

Paracalanus parvus

Bisan-seto......Paracalanus parvus and

Codonellopsis nipponica

Harima-nada......Oncaea media, Microsetella

norvegica and Oikopleura dioica (south-

western part), or Paracalanus parvus,

with Oithona similis (central part)

In this survey, the hydrographical conditions

(water temperature, chlorinity and transparency)

were the most oceanic in Iyo-nada and the most

embaymental in the eastern part of Bisan-seto.

From Iyo-nada to Bingo-nada, the representatives

of the zooplankton communities change succes-

sively as follows according as the hydrographical

conditions became more and more embaymental.

Oncaea media•¨ Paracalanus parvus•¨Oiko-

pleura dioica

But in Bisan-seto and Harima-nada, the species

composition of the zooplankton communities

varied considerably from station to station.

Accordingly, it was difficult to indicate the

conspicuous interrelation between the distribution

of the zooplankton communities and the environ-

mental conditions.

Fig. 4. Percentage composition of the dominant

copepods at each collecting station.

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150 Jour. Oceanogr. Soc. Japan, Vol.25, No.3 (1969)

Reference

HIROTA, R. (1959): On the morphological variation of Sagitta crassa. Jour. Oceanogr. Soc. Japan, 15, 191-202. (In Japanese with English abstract)

HIROTA, R. (1968a): Zooplankton investigations in the Setonaikai (Inland Sea of Japan), I. Occur-rence of zooplankton in the western half of the

Setonaikai in June, 1963. Ibid., 24, 203-211. HIROTA, R. (1968b): Idem, II. Occurrence of zoo-

plankton in the eastern half of the Setonaikai in September, 1963. Ibid., 24, 212-219.

HIROTA, R. (1969): Idem, III. Occurrence of zoo-

plankton in the sea regions from Iyo-nada to Harima-nada in the autumn of 1964. Ibid., 25, 137-144.

瀬 戸 内海 の動 物 プ ラ ン ク トンに 関 す る研 究,IV,

1966年 冬季,伊 予灘か ら播磨灘にいたる海域の動物プランク トン

弘 田 礼 一 郎

要旨 1966年1月28日 から2月3日 にかけて,伊 予灘

か ら播磨 灘にい たる7海 域 の18調 査点 で,22種 の動物

プラ ンク トンが同定 され た.

伊予灘 か ら備後灘 にかけての動物 プラ ンク トン群 集の

代表種 は,海 況が次第 に 内 湾化す るにつれ て,Oncaea

mediaか らParacalanus parvus,さ らにOikopleura

dioicaへ と変化 す る.し か し,備 讃瀬戸 と播磨灘で は群

集 の組成 が調査点 ごとに変動 して,海 況 との間 に明 白な

相互関係 を認 めるこ とが出来なか った.

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