Kazutsugu Hirayama

Morphological differences between two types of the rotifer Brachionus plicatilis O.F. Müller

Abstract The rotifer Brachionus plicatilis O.F. Muller has been used extensively for aquaculture in Japan; two morphotypes, S (small) and L (large), are distinguished, based on morphological and physiological differences. Morphological differences between these two types were studied biometrically. 67 strains of the rotifer collected from all over the world were classified by eye into 37 S and 30 L type strains by differences in the shape of anterior spines of lorica. For each of the 67 strains, seven characters of the lorica were measured on each of 20 individuals derived parthenogenetically from one individual. Classifications based on both cluster and stepwise discriminant analyses yielded equivalent results. Based on cluster analysis using the seven measurements, the 67 strains were classified into two groups containing the predesignated 37 S and 30 L type strains. Lorica length, lorica shape and shape of anterior spine were considered as the distinguishing morphological characters between S and L types. Two linear discriminant functions associated with S-L types were also derived, which can be used to classify new strains that may be observed in the future into S or L type. Accordingly, the rotifer was clearly divided morphologically into the two types, S and L.

Increased sexual reproduction in brachionus plicatilis (rotifera) with the addition of bacteria and rotifer extracts

Abstract Previous works revealed that sexual reproduction of brachionid rotifers increases with increasing population density. It is also known that the renewal of environmental water suppresses sexual reproduction in Brachionus plicatilis O.F. Muller populations. The cause of these actions, however, remains unknown. In this study, we investigated the effects of bacterial additions to axenic rotifer B. plicatilis cultures as well as the the effect of rotifer extracts on sexual and asexual reproduction. Out of 17 bacterial strains tested, rotifer sexual reproducion (mixis) rates were 4–10 times higher than controls with addition of three Pseudomonas strains, a Moraxella strain and a Micrococcus strain. Rotifer mixis rate also increased with the addition of a water soluble extract from 1000 rotifers to 5 ml axenic rotifer cultures. Similar results were obtained for resting egg formation.

Genetic divergence between S and L type strains of the rotifer Brachionus plicatilis O.F. Müller

Abstract Allozyme variation was determined for 67 strains of the rotifer Brachionus plicatilis O.F. Muller including 37 strains of S type and 30 strains of L type collected from all over the world, using horizontal starch gel electrophoresis. Six of the 18 enzymes assayed were variable, viz., lactate dehydrogenase (LDH), malate dehydrogenase (MDH), 6-phosphogluconate dehydrogenase (6PGD), superoxide dismutase (SOD), phosphoglucomutase (PGM) and glucose phosphate isomerase (GPI). 33 of the 46 alleles detected for the six loci were specific for either S or L type strains. On the basis of genetic distances calculated from allele frequencies of the 46 alleles, a dendrogram of the similarity among 67 strains was drawn. The strains were divided into two groups at a large genetic distance. One group consisted of 37 strains which coincided with the strains classified as S type in morphometric analysis. The other group consisted of 30 strains which coincided with ones classified as L type in morphometric analysis. The result indicates that the two groups of the rotifer, S and L, are genetically divergent.

Morphology, reproduction, genetics, and mating behavior of small, tropical marine Brachionus strains (Rotifera)

Small, tropical marine Brachionus (SS strains) are distinguished by their lorica size and shape. Their relation to other marine Brachionus species, however, remains unknown. We examined morphometry, reproductive mode, allozyme patterns and mating behavior among marine Brachionus species in an attempt to classify SS strains. Based on morphological features, reproductive mode, and allozyme patterns, SS strains were most similar to those of B. rotundiformis Tschugunoff. Observations on mating behavior between different groups indicated that there is reproductive isolation between B. plicatilis O.F. Muller and B. rotundiformis and between B. plicatilis and SS strains. In contrast, copulation was observed between B. rotundiformis and SS strains. Thus, there is no evidence that SS strains are a new marine Brachionus species, and most likely should be included in B. rotundiformis.

Application of a growth-promoting bacteria for stable mass culture of three marine microalgae

The practical mass culture of marine microalgae, faces occasionally unexpected problems or collapse. The effect of a marine bacterium, Flavobacterium sp., which was found to promote growth of a marine diatom Chaetoceros gracilis in the axenic culture condition, was examined on the mass cultures of three marine microalgae.

Resting eggs of the marine rotifer Brachionus plicatilis Muller: development, and effect of irradiation on hatching

Diapause and hatching of Brachionus plicatilis Muller resting eggs were examined through histological and optical approaches. Compound microscope observations on 1% toluidine blue-stained embryo sections suggests that the total number of nuclei in an embryo during the internal diapause period increased from 22 on Day 2 to 39 (each n = 1) on Day 6. The outer layer of embryo membrane gradually thickens from 1.2 (Day 0) to 4.0 μm (Day 8) (each n = 10).

A consideration of why mass culture of the rotifer Brachionus plicatilis with baker's yeast is unstable

Since the introduction of the rotifer Brachionus plicatilis as a food source for larval fish [Ito, 1960], fingerling production in an average Japanese commercial marine hatchery has increased to more than one million fingerlings of red sea bream, Pagrus major, per year. However, to make the release of artificially produced fingerlings more effective as a coastal management project, it is necessary to increase our current production of B. plicatilis.

Biocontrol method in aquaculture for rearing the swimming crab larvae Portunus trituberculatus

The aim of this work is to control the waterenvironment for culturing larvae of the swimmingcrab, Portunus trituberculatus, using microorganisms.The bacterial strain PM-4 (Thalassobacter utilis)improved the survival rate of crab larvae andrepressed the growth of Vibrio anguillarum (bacterium)and Haliphthoros sp.(fungus) in seawater. PM-4 wascultured and added daily to seawater during the firstto third zoean growth stage of the crab with diatomsand rotifers. Numbers of PM-4 decreased in culturewater during the first 3 days, because of feeding bythe first zoean stage of larvae. The finalconcentration of PM-4 was 105 to 106 cellsml−1according to the plate count method in larval rearingwater.During 1989 to 1993, we tried seed productions of aswimming crab in 200 m3 containers at TamanoStation, Japan Sea-Farming Association. In 33trials of the biocontrol methods, average survivalrate of crab larvae was 28.3% when the bacterialstrain PM-4 was added. In 42 trials in which the strainPM-4 was not added, average survival rate of crab larvae was15.6%. We conclude that thebacterial strain PM-4 is effective as a biocontrolagent.

Application of unicellular algae Chlorella vulgaris for the mass-culture of marine rotifer Brachionus

Condensed suspension of Chlorellavulgaris was used for the food of the rotifer Brachionus plicatilis and B. rotundiformis inplace of Nannochloropsis oculata. Thisreport describes the characteristics of C. vulgaris as arotifer food in comparison with N. oculata and thepresent status of this field.The cell components of C. vulgarissuch as protein content, amino acids, minerals andvitamins are generally similar to those of N. oculata. However, the taxonomic status of thesealgal species are different. Based on thesimilarity of cell components, the dietary value ofC. vulgaris is equal in value to that of N. oculata for rotifer growth. Dietary value ofC. vulgaris can be improved by addition ofvitamin B12. This improved C. vulgaris is currently widely used as an indispensable food organism for rotifer culture. Recent investigationshave shown that the use of the condensed suspensionof C. vulgaris makes it possible tosignificantly increase the rotifer density atharvest. Application of condensed C. vulgaris has made rotifer culture quite easy because theculture of N. oculata is no longer required,and intensive rotifer production in aquaculture cannow be realized.

Water control by filtration in closed culture systems

Abstract This paper describes the chemical changes in culture water in closed culture systems with sand filter. Stabilization of inorganic nitrogen after sequential changes of ammonium-N and nitrite-N in a culture system with new filters takes from 40 to 60 days. The characteristic changes in chemical composition of aged culture water, namely an accumulation of nitrate and phosphate and lowering of pH and alkalinity, are discussed. The carrying capacity in a closed culture system is determined by using oxygen consumption during filtration as an index to indicate the degree of purification by filtration.