海産枝角類の生態に関する研究

Studies on the Ecology of Marine Cladocerans

Onbé Takashi

広島大学水畜産学部紀要

Journal of the Faculty of Fisheries and Animal Husbandry, Hiroshima University

海産枝角類の生態を明らかにし,それらを餌料生物として利用する場合の基礎資料を得ることを目的として,1966年以降1972年まで瀬戸内海中部水域―備後灘・燧灘―において一連の研究を実施し,次のような結果を得た.

1. 本水域から出現が確認された海産枝角類は次の3属5種である.
Evadne nordmanni LOVEN,Fvadne tergestina CLAUS,Podon leuckarti G.O.SARS,Podon polyphemoides (LEUCKART),およびPenilia avirostris DANA.

2. これらの枝角類は種に固有の季節的消長を示し,その出現時期は毎年非常に規則的に繰り返されている.出現量の多いのは,E.tergestina,P.polyphemoides,およびP.avirostrisの3種である.これら各種の出現を規制する環境要因としては,塩分よりも水温が重要である.

3. 本水域の枝角類の地理的分布をみると,各季節に固有の種が,全域にわたってみられる.しかし分布密度の高い水域はP.polyphemoidesでは北部の比較的内湾性の強い場所に集中する傾向があり,逆にE.tergestinaとP.avirostrisは中央部より南西部に至る沖合性の強い場所に限られることが多かった.

4. 垂直分布に関しては,昼間は各種ともに0m乃至10m層に分布が局在することが認められ,特にEvadne属においてその傾向が顕著であった.またE.tergestinaやP.polyphemoidesでは昼間は表層に出現量が多く底層に少ないが,夜間には表層への出現個体が減少し底層に増加することがみられ,日周垂直移動を行なうことが認められた.

5. 各種の体長組成には明瞭な季節変化はみられない.単為生殖卵の平均抱卵数は出現初期に最も多く,時間の経過とともに徐々に減少することが,調査した3種,E.nordmanni,P.polyphemoidesおよびP.avirostrisの何れについても同様に認められた.平均抱卵数の最も低下した時期に有性生殖が最もさかんとなり,それとともに群集密度が急激に減少して消滅するに至る.

6. 1969年8月にみられた高い比率で有性生殖個体を伴なうP.avirostrisの群集について生殖タイプ別組成を調べた結果,場所によりその組成に顕著な差が認められた.耐久卵保有雌虫の割合は最高13.5%に達し,雄虫を含めた有性生殖個体は群集の50%を占めた.

7. Evadne属2種,E.nordmanniおよびE.tergestinaの単為生殖雌虫において,成長した胚は夜半から早期までの暗黒時にのみ,母虫の脱皮とともに放出される.育房中の単為生殖卵の成長はきわめて早く,夏期水温26-27°Cでは48時間で母虫より放出されるものと推定された.

8. P.avirostrisおよびE.tergestinaの耐久卵を室内容器中で母虫より放出させ,それを回収することができたので,それらと同一の形状,色彩をもつ卵を天然海底から探索した結果,本水域には多量の枝角類耐久卵が分布することがはじめて確認された.

9. E.tergestinaの耐久卵は球形で,平均直径204μ,卵膜はきわめて厚くかつ固く,淡褐色を呈する.P.avirostrisのそれは背面観は楕円形で平均長径250μ×短径180μであり,側面観は平たく厚さ約100μ,中央部腹面に顕著な凹みを有する.卵は灰色で卵膜は厚い.

10. 備後灘・燧灘の9定点における8,9月の耐久卵分布密度はきわめて高く,P.avirostrisのそれは平均6.7～7.9×104個/㎡,Evadne and/or Podonのそれは1.5～1.8×104個/㎡であった.徳島県橘湾の11定点における3月の耐久卵密度は,前者については平均0.14×104個/㎡,後者については0.28×104個/
㎡であって瀬戸内海中部における密度に比しきわめて低かった.

11. 室内採取および天然採集耐久卵はいずれも,室内条件下での孵化が認められた.P.avirostrisの耐久卵は,天然で本種の出現する時期の水温に近い17～20°Cにおいて孵化率が高く,Cl4‰の低塩分海水中でも孵化する個体があった.Evadne and/or Podonの耐久卵は12～17°Cにおいて孵化率が高く,前者同様Cl4‰の海水中でも孵化が認められた.

12. 室温に13ケ月間放置した海底泥中より得られたP.avirostrisの耐久卵の一部が孵化したことから,本種の耐久卵の長期保存の可能性が認められた.

13. P.avirostrisおよびE.tergestinaの耐久卵の初期発生とその孵化過程ならびに孵化直後の仔虫の形態などをはじめて記載した.P.avirostrisの耐久卵は発生の徴候を示す最初の形態的変化が認められてから孵化までに,平均水温19.4°Cで約100時間を要した.

14. 海産枝角類の飼育,培養に際しての問題点について,その繁殖の特性,食性,棲息密度その他の点から若干の論議を行なった.

This article is a summarization of a series of studies concerning the ecology of marine cladocerans in Bingo- and Hiuchi-Nada in the central part of the Inland Sea of Japan (the Seto Inland Sea) during a period from 1966 through 1972. The purpose of these studies is to clarify some aspects of the ecology of these animals, their importance in the coastal zooplankton having been overlooked rather largely, as well as to obtain certain fundamental biological data needed for their future utilization as food of fish larvae in marine fish farming.

The results obtained in these studies can be summarized as follows:
1) The following 5 species of marine cladocerans belonging to 3 genera have been found to occur in this sea area: Evadne nordmanni LOVÉN, Evadne tergestina CLAUS, Podon leuckarti G.O. SARS, Podon polyphemoides (LEUCKART), and Penilia avirostris DANA.
2) All these cladocerans appear during the time from spring through autumn. Each species has its own season of occurrence which repeats itself regularly every year. Temperature is the leading environmental factor limiting the seasonal distribution of each species.
3) All the species are found to be distributed throughout the area investigated. Podon polyphemoides is most densely distributed in the northern and north-eastern regions where waters are of a most embaymental character. On the contrary, Evadne tergestina and Penilia avirostris are observed to be most abundant in the central and south-western regions where waters are of a most offshore character within this sea area.
4) Restriction to surface layers between 0 and 10 m deep is a characteristic feature of the vertical distribution of marine cladocerans in the area during the daytime. This tendency is especially marked in the 2 species of Evadne: E. nordmanni and E. tergestina. Diurnal changes in the vertical distribution have been observed in E. tergestina and P. polyphemoides, which appear most abundantly in surface layers during the day and migrate downward at night.
5) No marked seasonal change in size frequency distribution is observed in each species. The mean number of parthenogenetic eggs or embryos per batch is found to be highest when the population first appears in the plankton and gradually decreases thereafter, in the following 3 species examined: E. nordmanni, P. polyphemoides and P. avirostris. Bisexual reproduction becomes most intense when the mean number of parthenogenetic eggs or embryos per batch reaches its minimum. With the onset of sexual reproduction, population decreases drastically until complete disappearance from the plankton.
6) An analysis of the population of P. avirostris containing a high-rate of sexual individuals (females bearing resting eggs, and males) has revealed obvious differences in the composition of sexual types among the samples taken at different stations on almost the same day within this area. The maximum number of females carrying resting eggs reaches as many as 13.5% of the total population.
7) Paedogenesis or neoteny has been observed in all the podonids of this study area. The parthenogenetic females of E. nordmanni and E. tergestina liberate their young (strictly speaking, miniature adults) only during the dark period of the 24-hour day: from midnight to early morning before dawn. The rate of growth of the parthenogenetic embryos within the brood pouch seems to be very rapid in summer. At temperatures of 26-27℃, the young are liberated from the mother animal within 48 hours.
8) Many resting eggs of E. tergestina and P. avirostris have been obtained in laboratory tanks. The resting egg of E. tergestina is spherical in shape with a mean diameter of 204µ. The egg membrane is very thick and stout. In reflex light, it is brownish in color. The resting egg of P. avirostris is ovoid, 250µ long and 180µ wide, and is compressed dorso-ventrally with a mean thickness of 100µ. It is concaved characteristically on its ventral surface. In transmitted light, it looks dark, but greyish in reflex light.
9) A search through the bottom mud of Bingo- and Hiuchi-Nada has revealed an affluent presence of resting eggs of marine cladocerans. The eggs of P. avirostris attain 6.7-7.9 ×104/m2 and those of Evadne and/or Podon, 1.5-1.8×104/m2, for the samples taken at 9 stations. On the contrary, much smaller number of eggs has been recovered from the bottom of Tachibana Bay, Tokushima Prefecture.
10) The resting eggs of marine cladocerans obtained in the laboratory tanks and those from the sea bottom mud have been both able to hatch under laboratory conditions. In the eggs of P. avirostris, high hatching rates have been obtained at temperatures of 17-20℃, which coincide with the sea temperatures at the time of the first appearance of this species in the plankton samples. They also hatch under a wide range of low salinities down to 4‰ chlorinity. The resting eggs of Evadne and/or Podon show higher rates of hatching at 12-17℃. As in the former species, hatching is observed in salinities down to 4‰ chlorinity.
11) Some resting eggs of P. avirostris, isolated from the sea bottom mud which had been placed in the laboratory for 13 months, hatched normally.
12) Developmental and hatching processes of the resting eggs of P. avirostris and E. tergestina are described and figured, with morphological characteristics of the newly hatched young (including that of P. polyphemoides). The incubation period of the resting eggs of P. avirostris is about 100 hours at 19.4℃.
13) Some aspects of the problems relating to the cultivation of marine cladocerans are discussed from the viewpoint of reproductive biology, feeding habits, population density, etc., of each species.

Fishing industry. Fisheries [ 660 ]

jpn

広島大学水畜産学部

[ISSN] 0440-8756

[NCID] AN00213563

[NAID] 40018463843