Takashi Onbé

Reproductive Biology of Marine Cladocerans

Publisher Summary The chapter discusses the reproductive biology of marine cladocerans. Marine cladocerans are seasonally abundant and widely distributed in continental shelf waters, estuaries, the open ocean, the Baltic, the Mediterranean, and inland seas. Marine cladocerans, like most of their freshwater counterparts, exhibit direct development, alternation of gamogenesis (amphimixis), and diploid parthenogenesis (apomixis) and padogenesis, or the initiation of reproduction by young individuals during their own early development. The chapter summarizes the life cycle of a podonid cladoceran, using Evadne nordmanni as an example. Physiology and nutrition of marine cladocerans is discussed. Ovaries in cladocerans are elongated, spindle-shaped organs located on each side of the intestine. In Evadne nordmanni, the ovary extends nearly the length of the thorax at maturity. Oogenesis occurs in this species at the narrow anterior end of the ovary, while mature eggs accumulate at the broader and more rounded posterior end. The sex of a marine cladoceran may be recognized on the basis of body size, body shape and other secondary sexual characteristics. The paired testes of marine podonids are located in the hemocoel on both sides of the intestine. Fertilization in marine cladocerans probably occurs before the gamogenic egg moves from the ovary into the brood chamber. Annual cycles of abundance have been recorded for many populations of marine cladocerans. Marine cladocerans exhibit distinct patterns of abundance in relation to temperature and salinity.

The Association of Vibrio parahaemolyticus Serotypes with Zooplankton and its Relationship with Bacterial Indicators of Pollution

Summary The role of Zooplankton in the seasonal distribution of Vibrio parahaemolyticus and the significance of bacterial indicators of pollution in the incidence of this halophile were examined. Most probable number procedures were employed for the enumeration of both bacterial indicators of pollution and V. parahaemolyticus in surface waters and plankton samples from the Fukuyama coastal area over a period of one year. In the Fukuyama port samples the highest counts of this bacterium occurred in the autumn although it was found in these waters throughout the period of study. In general, plankton samples were found to yield higher populations of V. parahaemolyticus than did water bodies from the same area. Serological studies revealed that the same serovar, first detected in plankton samples, appeared in the water column in either the same month or in the succeeding month. Statistical analysis revealed that the presence of this autochthonous pathogen was significantly correlated with total coliforms and faecal coliforms. Analyses of variance demonstrated that V. parahaemolyticus was distributed homogeneously in the Fukuyama port area and was found to be isolated with plankton samples. All 444 strains isolated were Kanagawa phenomenon negative.

Structure and species diversity of the zooplankton community of the Inland Sea of Japan

Abstract Seasonal fluctuations in composition and abundance of zooplankton of the Inland Sea of Japan were investigated in 1979–1980. Some of the areas in the environment were found to be eutrophic and sharp temperature variations occurred with seasons. In this scenario, zooplankton populations peaked in autumn. Copepods were the dominant component and six species of this group formed about 88% of the total zooplankton counts. Species groups are categorized based on distributional characteristics. Zooplankton composition was more or less homogeneous in each season within the area studied and a few oceanic species which occurred had only limited influence on community structure. Species diversity was low and competition among species was weak. The results indicated a poor quality biotope probably resulting from a deterioration of water quality and an environment which favours r -selection.

A new species of Pontella (Copepoda: Calanoida) from the inland sea of Japan with notes on its feeding habits and related species

A new species of calanoid copepod Pontella rostraticauda (family Pontellidae) is described based on specimens collected in surface waters of the Inland Sea of Japan. Due to its neustonic character, this species may have been overlooked by previous investigators. The gut content analysis revealed that this species feeds on a wide variety of foods, including both zooplankters and phytoplankters, and can be classified as an omnivorous copepod. The new species is closely allied to P. surrecta Wilson and P. alata Scott and is distinguished from them by the shape of the female urosome and the fifth legs of both sexes. Several derived characters shared by the three species and their geographical distributions provide the basis for proposing the Pontella alata species group as a unique radiation within the genus. Species of this lineage tend to occur in coastal waters around those oceanic islands comprising the western rim of the tropical Pacific. The group occurs north to southern Japan and south to the Great Barrier Reef. Until now, only four species of the family Pontellidae have been recorded from the central part of the Inland Sea of Japan (Setonaikai): Calanopia thompsoni Scott, Labidocera rotunda Mori, Pontellopsis tenuicauda (Giesbrecht), and Pontellopsis yamadae Mori (Hirota, 1961, 1964, 1968a, b, 1979; Kado, 1954, 1957; Ohtsuka, unpublished). In contrast to such a low species diversity in the Inland Sea, 21 species of the family Pontellidae have been collected in summer in the Kuroshio Current region, south of Shikoku, Japan (Matsuo and Marumo, 1982). During the course of our investigation on the neuston of the Inland Sea of Japan in the summer of 1985, a new species of the genus Pontella (family Pontellidae) was found. As far as we know, the present species appears to be the first new calanoid copepod from the Inland Sea of Japan. The new species feeds on a variety of organisms, and is apparently omnivorous. The present paper deals with the description of the new species of Pontella, its feeding habits, and its phylogenetic as well as biogeographic relationships. MATERIALS AND METHODS Neuston samples were collected at Station A-1 and Stations B-1 to B-5, respectively, on 21 June and 8, 9 August 1985, in Hiuchi-Nada, the central part of the Inland Sea of Japan (Fig. 1). The Ocean Research Institute (ORI) neuston net with a mesh size of 0.33 mm (Matsuo et al., 1976) was towed for 10 min at a speed of about 2 knots on board T/RV Toyoshio-maru, Hiroshima University. The samples were fixed with 10% neutralized Formalin-sea water immediately after capture. Prosome length was measured for intact specimens collected in August. Gut contents dissected out were examined with a scanning electron microscope (JSM T-20 SEM). Twenty intact specimens of each sex were randomly selected from the samples collected at Stations B-1, B-2, and B-3 to examine the frequency of occurrence of food items in the guts. The gut was removed from the body and immersed in gum-chloral medium on a thin glass slide, dissected, and, after adding a cover slip, examined under a differential interference microscope. Intact food organisms in the guts of the pontellids were measured with an ocular micrometer. We used the criterion of morphological similarity in derived sexual characters (synapomorphies) in both sexes to shed light on the phylogenetic relationships of the new species. Zooplankton sampling outside of the Inland Sea of Japan that yielded new geographic records of species closely related to the new species was based mostly on surface tows with standard conical nets

Observations on the biology of the marine cladoceran Podon schmackeri

ABSTRACT Observations on the morphology and reproduction of a marine cladoceran, Podon schmackeri, were carried out on specimens sorted from the C.S.K. International Zooplankton Samples. Parthenogenetic females of Podon schmackeri ranged from 0.30-0.65 mm in standard length (SL) and from 0.34-0.87 mm in gross length (GL). The number of parthenogenetic embryos varied from 1-9 per batch, with a mean of 4.6. Some individuals exhibited paedogenesis, in that advanced embryos in the brood pouch contained their own embryos in the embryonic brood space. Two males of Podon schmackeri, measuring 0.42 and 0.43 mm in SL and 0.43 and 0.46 mm in GL, respectively, were detected, and described and illustrated for the first time.

The Developmental Stages of the Marine Calanoid Copepod Labidocera rotunda Mori

瀬戸内海において暖期に出現する浮遊性カイアシ類Labidocera rotunda Moriのノウプリウス及び成体を除くコペボディド幼生の全発生段階の形態を室内飼育と野外採集から得られた標本により記載した。ノウプリークス、コペボディド期の発生段階は何れも6期であり、L. bengalensis Krishnaswamyで報告されているようなノウプリウス期の5期への短縮は見られなかった。また、口器の形態と消化管内省物の観察から、本種のノウプリウス期の栄養摂取様式はL. euchasta Giesbrechtの様な卵黄依存型ではなく、2期から小型の鞭毛虫等を摂食しているものと推測された。本種のコペボディド期は同じ種群に属するL. pectinata Thompson&Scottの同期に極めてよく類似するが、付属肢の刺毛数等に相違が見られた。性的二型が発現するのはコペボディド4期からで、右第1触角、第5脚に相違点が見られた。体長の雌雄差はL. pecinataではコペボディド4期から認められるが、本種では5期以降にみられた。