Mayumi Yoshimura

Comparison of stream benthic invertebrate assemblages among forest types in the temperate region of Japan

We compared benthic invertebrate assemblages among headwater streams in several forest types in Japan. Forests were divided into three clusters based on vegetation composition: old-growth broad-leaved forest, planted coniferous forest, and mixed forest. The numbers of individuals and families and the diversity (Shannon-Wiener) of benthic invertebrate assemblages did not differ significantly among the three forest clusters. However, principal components analysis of family abundance showed differences in the benthic invertebrate assemblages among the three forest clusters. No environmental factors were correlated with these differences. Benthic invertebrate assemblages differed depending on forest composition. The abundances of Taeniopterygidae and Athericidae in old-growth broad-leaved forest were significantly greater than in planted coniferous forest. The abundances of Heptageniidae, Baetiidae, Stenopsychidae, Uenoidae, Chironomidae, and Potamidae in planted coniferous forest were significantly greater than in old-growth broad-leaved forest. If the remaining old-growth broad-leaved forest were to be converted to coniferous plantation, species that inhabit old-growth forest may become extinct.

Longitudinal patterns of benthic invertebrates along a stream in the temperate forest in Japan: in relation to humans and tributaries

Abstract.  1 The relationship between benthic invertebrate assemblage composition and surrounding land use has been examined in various ways, but most studies are undertaken at a coarse scale, or they compare obviously different landscapes. In Japan, these obvious differences in landscape are rare, and humans reside even in remote montane areas. 2 In order to clarify the influence of human residence on benthic invertebrate assemblages, seasonal and site differences among benthic samples collected from a 15‐km stretch of Kuroson Stream and their relation with riparian land use were examined. 3 The number of individuals and genera differed significantly both spatially and temporally. Multivariate assemblage structure also differed significantly across space and time along the stream. 4 Increase of residential areas affected the in‐stream benthic invertebrate assemblage. Site differences along the stream and the effect of human residence were not masked by the potentially homogenizing influence of tributary streams. 5 The composition of surrounding vegetation was the most important factor influencing benthic community structure. The presence or absence of human residential areas and seasonal change were also important variables. 6 Benthic invertebrates may be carried and migrate to main streams from tributaries; however, this phenomenon was not observed. Assemblages of benthic invertebrates that inhabit a particular site do not change and are considered to vary seasonally across a certain range.

The Relation among Emergence Date, Days until First Effective Mating, Fecundity and Adult Longevity in Isoperla aizuana and Sweltsa sp. (Plecoptera)

Abstract The relation between emergence date and the factors concerned in reproduction were studied in the stoneflies Isoperla aizuana (Perlodidae) and Sweltsa sp. (Chloroperlidae) in the laboratory. Individuals emerging earlier had larger adult size and greater adult longevity. Fecundity was also related to emergence date, adult size, and adult longevity. Furthermore, individuals emerging earlier had more days until first effective mating and tended to mate more effectively than later-emerging individuals. The maturity of the reproductive tissue may differ with emergence date. While most of the later-emerging individuals had a lower rate of effective mating, longer-lived and larger size adults mated more effectively in females of Sweltsa sp. The differences in mating character might affect the effectiveness of mating.

Relations of intraspecific variations in fecundity, clutch size, and oviposition frequency to the body size in three species of stoneflies, Sweltsa sp., Isoperla aizuana, and Stavsolus japonicus

Most stoneflies oviposit several times during their adult stage. In this study, the relations among oviposition frequency, the number of eggs per egg mass, and body size were examined in the chloroperlid stonefly Sweltsa sp. and two perlodid stoneflies, Isoperla aizuana and Stavsolus japonicus. It was found that larger individuals tended to oviposit more frequently than smaller ones, but the relation was significant only in Isoperla aizuana. In Sweltsa sp. and Isoperla aizuana, the number of eggs per egg mass was higher for the less-frequently ovipositing individuals than for those ovipositing more frequently, and the number of eggs per egg mass decreased with successive oviposition events. Stavsolus japonicus showed the same tendency but it was not statistically significant.

Comparison of egg period, hatching rate, and first-instar nymphs among three species of stonefly

Egg period was compared among several temperature conditions (11°C, 16°C, 20°C, 23°C) in Sweltsa sp., Stavsolus japonicus, and Isoperla aizuana (Plecoptera). The shortest mean egg incubation period was 27.8 days at 20°C in Sweltsa sp., 118.1 days at 16°C in Stavsolus japonicus, and 162.0 days at 20°C in Isoperla aizuana on average. Egg hatching rate was also the highest at the water temperature that provided the shortest egg incubation period. Based on laboratory data, eggs of Sweltsa sp. were considered to be deposited in May and hatched in June in the field. Thus, they must have spent the summer as nymphs in the field. Eggs of Stavsolus japonicus and Isoperla aizuana were considered to be deposited in April to May and hatched in September to October in the field. Visible eyes of Stavsolus japonicus and Isoperla aizuana appeared in August. It is likely that the long egg period of Stavsolus japonicus and Isoperla aizuana reflects that these two species spend the summer as dormant eggs in the field.

Variations in stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) in different body parts and eggs of adult stoneflies (Plecoptera)

Variations in δ13C and δ15N might arise from differences in nutrient allocation. Residence times of δ13C and δ15N vary among tissues depending on metabolic turnover rates. However, because of their small size, entire individual insects are generally used as single samples in isotope analyses. The present study aimed to determine the degree of isotope similarity among regions of the adult body and eggs in four species of Plecoptera (Amphinemura sp., Sweltsa sp., Kamimuria tibialis Pictet and Ostrovus sp.). Levels of δ13C and δ15N differ between the four species, being lowest in Amphinemura sp., and with δ15N being highest in Sweltsa sp. Egg masses contain consistently the lowest values of δ13C in the four species, with the δ15N value of eggs being highest in K. tibialis and Ostrovus sp., and lower in Amphinemura and Sweltsa spp. In Sweltsa sp., the δ15N levels of the dermal layers and cuticle are lowest, whereas the δ13C values of the dermal layers and cuticle are almost equal to those in other regions of the body, except egg masses. Oviposited individuals of Amphinemura and Sweltsa spp. have lower δ15N levels than individuals that have not oviposited. The rates of metabolism and incorporation of dietary metabolites will differ depending on the body regions and species. Differences in egg ecology such as egg developmental period and egg buoyancy among species are considered to impact on the values of δ13C and δ15N. These results will be useful for understanding the nutritional status of aquatic insects and their energy allocation.

Seasonal and Daily Flight Timing of Oviposition in Several Stonefly Species (Plecoptera) in the Field

Peak preovipositional flight season and peak preovipositional flight time of day were compared among 12 species of Nemouridae, Chloroperlidae, Perlodidae and Perlidae. Species with a later peak date of preovipositional flight were found to have a later peak preovipositional flight time of day than species with an earlier peak in preovipositional flight season. A later peak preovipositional flight season correlated with a lower light intensity. Similarly, a later peak preovipositional flight time of day correlated with a lower light intensity and a later sunset. Individuals of one species (Sweltsa sp.), whose preovipositional flight date was later, flew over the stream at a later time of day. Species differences in peak preovi-positional flight season and peak preovipositional flight time of day may be driven by species specific sensitivity for different light intensities.

Age-related changes in the development of oocytes and testicular follicles after emergence in Stavsolus japonicus (Plecoptera)

Abstract Female stoneflies oviposit several times during the adult stage of their life cycle. The number of eggs within the deposited egg masses decreases at successive ovipositions. To clarify the reason for this decrease and to determine the conditions of testicular follicles, the patterns of development of oocytes and testicular follicles on different days after emergence are investigated in the systellognathan species Stavsolus japonicus (Okamoto) (Perlodidae). The size of the mature oocytes in the ovariole peaks a few weeks after emergence but decreases to the lowest level by 35 days after emergence. Several maturing oocytes can be observed in the ovarioles of individuals a few weeks after emergence but only one mature oocyte is observed at 35 days after emergence. The decreased number of eggs laid per mass by older individuals may therefore be due to the lower maturation of all the ovarioles.

Comparison of ion transportation before and after egg hatching in Amphinemura sp. (Plecoptera)

Abstract An increase in egg size with embryonic development in stoneflies is believed to result from the uptake of water by osmosis. The present study aims to investigate whether a selective ion transport through egg membranes exists before hatching, and whether ions are released after hatching. Viable and nonviable egg masses are incubated in Petri dishes filled with water, and the concentrations of the ions F−, Cl−, SO42−, NO3−, Na+, K−, Mg2+ and Ca2+ in the water are determined. The ion transport of an egg mass before and after hatching and a nonviable egg mass is then calculated. Before hatching, Cl−, SO42−, NO3−, Na+, Mg2+ and Ca2+ are taken up from the surrounding water into the inner egg. These ions are selectively taken into the egg. After hatching, Cl−, SO42−, Na+, Mg2+ and Ca2+ are released into the surrounding water. The amount of these ions released after hatching is lower than the amount taken up before hatching. Ions that are not released after hatching are considered to be used in embryonic development.

Comparison of stream benthic invertebrate particularly stonefly assemblages in the temperate forest in Japan in relation to forest types

There have been recent investigations on the effects of different Japanese forest types on benthic invertebrate assemblages. In this study, the difference of stonefly assemblage between the two forest types, i.e. natural broad-leaved forest and planted coniferous forest, was clarified at the genus level for both larvae and adults. Differences were manifested in both larval and adult stonefly assemblages. The abundance of some larval stonefly genera varies by forest composition, and this tendency was the same with that of adult stonefly genera. There are few natural forests left in western Japan. If remaining natural broad-leaved forest were to be converted to coniferous plantations, one of the species that inhabit natural forest streams may become extinct.