Kwang-Hyeon Chang

Diel vertical migrations of invertebrate predators (Leptodora kindtii ,Thermocyclops taihokuensis, andMesocyclops sp .) in a shallow, eutrophic lake

We studied vertical distribution patterns of three invertebrate predators – Leptodora kindtii, Mesocyclopssp., and Thermocyclops taihokuensis– in a shallow eutrophic lake, Lake Suwa , Japan. From June to October in 2000 and 2001, we collected samples in the lake center in order to examine the vertical distribution patterns and the densities of the predators in the water column during the day (0900) and at night (2330). We also examined phototactic behavior of Leptodora in the laboratory. The three invertebrate predators showed clear migration patterns. Leptodora and Thermocyclops displayed a typical migration, avoiding the surface and maintaining a high abundance in deeper water during the day, and being distributed uniformly during the night. Mesocyclops, on the other hand, showed no clear vertical distribution pattern in the water column. However, Mesocyclops showed higher densities in the water column during the night than during the day. It suggests that they stayed just above the bottom during the day and migrated upward during the night. Leptodora also showed such a density difference between day and night. In the laboratory, Leptodora showed strong negative phototactic behavior. The observed density changes between day and night in Leptodora and Mesocyclops suggests the possible underestimation of their population density by usual sampling methods, and thus the impact of predation on populations of prey zooplankton species may also be underestimated in shallow water bodies.

Direct and indirect impacts of predation by fish on the zooplankton community: an experimental analysis using tanks

The impact of Pseudorasbora parva, a common zooplanktivorous fish species in Japan, on a zooplankton community was analyzed in experimental tanks, half of which were stocked with the fish. Different zooplankton species showed different responses to the introduction of the fish. In the presence of the fish, the populations of the large cladoceran Ceriodaphnia and the predatory copepod Mesocyclops were reduced, but the population of the herbivorous copepod Eodiaptomus and the small cladocerans Bosmina fatalis and Bosminopsis deitersi increased relative to the controls. The increase of Mesocyclops seen in the control tanks might have suppressed the populations of the small cladocerans, which are vulnerable to invertebrate predation. The results suggest that the population densities of the large prey items preferred by the fish, Ceriodaphnia and Mesocyclops, were controlled directly by fish predation, but the population densities of the smaller and less preferred zooplankton were controlled indirectly through the food-web cascade.

Prey handling time and ingestion probability for Mesocyclops sp. predation on small cladoceran species Bosmina longirostris, Bosminopsis deitersi, and Scapholeberis mucronata

We compared the vulnerability of small cladoceran species (Bosminopsis deitersi, Bosmina longirostris, and Scapholeberis mucronata) to predation by Mesocyclops sp. in the laboratory based on prey handling time and ingestion probability. We also estimated the effects of ontogenetic changes on handling time by testing prey of various body sizes. All tested prey species showed an increasing pattern of handling time along with growth (increase of body size). For juveniles smaller than 0.3u2009mm, the prey handling time was similar for B. deitersi and B. longirostris, but markedly longer for S. mucronata. Mesocyclops sp. rejected large (>0.35u2009mm) B. longirostris and S. mucronata soon after capture and consequently showed a low probability of successful ingestion, whereas most large individuals of B. deitersi were consumed. The shorter handling time and higher ingestion probability of small juveniles indicate that the copepods ingest small juveniles more efficiently than larger individuals. Thus, it seems that the vulnerability of small cladocerans to copepod predation differs ontogenetically, with the greatest vulnerability being during the juvenile period. The ontogenetic reduction in the vulnerability of B. longirostris and S. mucronata was shown in the rapidly reduced ingestion probability as well as by the increase in prey handling time for large individuals. The results show that the vulnerability to predation by copepods of B. longirostris and S. mucronata reduces more quickly with increasing size than does that of B. deitersi, and it seems that the former two species are more resistant to copepod predation.

Morphological defense of Bosmina fatalis against invertebrate predators in Lake Suwa

Cyclomorphosis in cladoeeran speeies has been reeognized as an effeetive defense meehanism against predators (DODSON 197 4, OBRIEN & VINYARD 1978), and is indueed by ehemicals released from potential predators (KRUEGER & DODSON 1981, DonsoN 1989, HANAZATO 1990). In partieular, seasonal changes in the morphology of Bosmina species have been well doeumented as a defense meehanism against predation by eopepods (KERFOOT 1977, 1978, BLACK 1980, WONG 1981, SPRULES et ai. 1984) and the cladoeeran Leptodora kindtii (HELLSTEN et ai. 1999, l..AGERGREN & STENSON 2000). Bosmina fotalis (Sinobosmina fotalis) is one of the dominant cladoeeran species in some eutrophic lakes in Japan (HANAZATO & YASUNO 1985). However, its distribution is limited to east and south-east Asia (LIEDER 1983) and little attention has been paid to cyclomorphosis of this speeies compared with some eosmopolitan Bosmina speeies. Two Bosmina species in Lake Suwa, Japan, B. longirostris and B. fotalis, showed different morphologieal traits and different seasons of dominanee (CHANG & HANAZATO 2002). B. fotalis, with extreme morphological features, such as long mucrones and raised and widened antennules, dominated the cladoeeran eommunity during summer, while B. longirostris, with a less protruding morphology, was abundant in spring and fali. The period of dominanee of B. fotalis with the extreme morphologies eoincided with the period of the abundant oeeurrence of invertebrate predators, Leptodora kindtii and cyclopoid eopepods. Thus, it was assumed that cyclomorphosis of B. fotalis is indueed by factors released from either, or both, of these predators.

Feeding Habit of Pond Smelt (Hypomesus transpacificus nipponensis) and Its Impact on the Zooplankton Community in Lake Suwa, Japan

ABSTRACT We examined the feeding habit of pond smelt (Hypomesus transpacificus nipponesis) and its impact on the zooplankton community in Lake Suwa, Japan, from May to December 2001. Stomach contents were analyzed, and prey importance and the prey selectivity index were estimated from the stomach contents of fish and prey density in the lake. Results showed that herbivorous zooplankters the cladoceran Bosmina fatalis and the calanoid copepod Eodiaptomus jaonicus were the most important prey items, accounting for about 80% of the total prey biomass in the diet of the pond smelt. However, the selectivity index suggested that pond smelt preferred the predacious cladoceran Leptodora kindtii and the cyclopoid copepod Mesocyclops sp. In the lake, predation impact by pond smelt on the herbivorous zooplankton was not apparent, but the population dynamics of the large invertebrate predators L. kindtii and Mesocyclops sp. seemed to be affected by pond smelt by selective predation.

Seasonal and spatial distribution of two Bosmina species (B. longirostris and B. fatalis) in Lake Suwa, Japan: its relation to the predator Leptodora

Abstractu2002In Lake Suwa, Japan, two Bosmina species, B. fatalis and B. longirostris, coexist and show different seasonal succession patterns. To determine the relative importance of factors affecting their succession, the seasonal and spatial distribution of the two Bosmina species and their relation to environmental variables were studied at six near-shore sites on Lake Suwa from May to October 2000, with measurements at each site made at 20 and 80u2009m from the shore. We analyzed the seasonal and spatial distribution patterns of Bosmina, rotifers, and the major invertebrate predators, cyclopoid copepods and Leptodora kindti. A range of variables (water temperature, dissolved oxygen concentration, and chlorophyll a concentration) was also measured. Multiple stepwise regression analysis was performed between Bosmina and the measured biotic and abiotic variables. Two species of Bosmina showed a marked pattern of seasonal succession, B. longirostris being dominant in May and October and B. fatalis exceeding B. longirostris in summer. However, the densities of both species and their density ratio differed between sites. The ratio of B. fatalis density to total Bosmina density, which indicates the dominance of B. fatalis, was correlated positively with water temperature and Leptodora density and negatively correlated with dissolved oxygen and cyclopoid copepod density. The results suggest that invertebrate predators play an important role in regulating the succession of Bosmina species in Lake Suwa.

The Predacious Cladoceran Leptodora kindtii as a Prey for the Cyclopoid Copepod Mesocyclops sp.: Laboratory Observations of Predator-Prey Interaction

ABSTRACT We observed the behavior of the cyclopoid copepod Mesocyclops sp. as it fed on the cladoceran Leptodora kindtii in the laboratory. The feeding efficiency (capture and ingestion successes) of Mesocyclops sp. was high for smaller L. kindtii (<2.5 mm in body size) but low for larger ones. Mesocyclops sp. ingested small L. kindtii whole, but often consumed parts of larger L. kindtii. The handling time increased as the L. kindtii body size increased, except when Mesocyclops sp. consumed only the second antennae. These laboratory observations suggest that Mesocyclops sp. may be a natural predator of small L. kindtii.