Seasonal and diel changes in the vertical distribution of oncaeid copepods in the epipelagic zone of the Kuroshio Extension region

Abstract

Species composition and vertical distribution of oncaeid copepods, which are potentially important prey for juvenile fish, were investigated in the Kuroshio Extension region, the NW Pacific, in April, August, November 1998 and February 2001. Samples were collected from 8 discrete layers in the epipelagic zone (0-200 m depth) using MOCNESS (0.064 mm mesh) during both day and night. Thirty-five oncaeid species were identified. Oncaea (s.l.) zernovi and Spinoncaea ivlevi were numerically the dominant species comprising 20.0-48.2% and 15.2-26.8%, respectively, of adult oncaeid copepods in the epipelagic zone. Cluster analysis on all samples revealed that these were separated into three groups with discrete vertical ranges; the first one appearing in the 0-50 m depth surface layer in April and August and consisting mainly of Oncaea (s. str.), the second one located in the deepest layer and composed mostly of O. zernovi and S. ivlevi with some mesopelagic species, and the third one located above the second one and having intermediate species composition. Species-specific vertical distributions indicate that most oncaeid populations shifted downward from August to November, when the thermocline remarkably descended. However, most Oncaea spp. did not show a downward shift with the thermocline, and were positively correlated to appendicularian abundances, suggesting that appendicularian houses, known to be oncaeid habitats and to provide food, were a possible factor affecting their vertical distribution. Niche partitioning, allowing coexistence of congeners, might be explained by differences in body size and distribution layers in Oncaea and by differences in distribution layer in Triconia.

Analyzing the Impacts of Elevated-CO2 Levels on the Development of a Subtropical Zooplankton Community During Oligotrophic Conditions and Simulated Upwelling

“…During this study, abundances of Oncaea spp. and O. dioica were inversely correlated, as previously observed at other study sites (Itoh et al, 2014). Oncaea was positively affected by pCO 2 , recording higher abundances under medium-and high-pCO 2 treatments from (approximately) the beginning of the experiment until the end of the phytoplankton bloom, on t35 ( Figure 4D).…”

Degradation of discarded appendicularian houses by oncaeid copepods

“…When feeding of oncaeids is encounter‐limited, the community ingestion decreases to F/F max of the maximum. In this calculation, we assumed that the feeding activity and vertical distribution of oncaeid copepods are invariant regardless of the time of day, as they generally show little diel variation in either (Ohtsuka et al ; Itoh et al ). The community ingestion rate ( C : mg C m −2 d −1 ) was thus computed for each 10‐m depth layer as: where I i is the maximum individual ingestion rate (mg C ind.…”

“…Oncaeid copepods in warm waters commonly show fine‐scale vertical segregation in the upper water column (Böttger‐Schnack ; Itoh et al ), and generally the maximal abundance of Oncaea s. str. corresponds with the depth where appendicularians occur in high numbers (Nishibe et al ; Itoh et al ). In contrast, as observed here, other oncaeids such as O. zernovi , Spinoncaea spp., and Triconia spp.…”

“…In contrast, as observed here, other oncaeids such as O. zernovi , Spinoncaea spp., and Triconia spp. became dominant in the deeper layers (Nishibe et al ; Itoh et al ). VPR observations showed that the association of oncaeids with discarded houses or other large particles was infrequent in the deeper layers, and thus they might utilize much smaller organic particles, including shrunken or fragmented discarded houses which are too small to detect in VPR images.…”

“…Our laboratory behavioral observations revealed that most associated with houses for relatively short periods (< 11 min). Thus, oncaeid copepods pay multiple brief visits to houses in the field; this behavior would contribute to maintaining their fine vertical segregation (Itoh et al ). This consideration supports the hypothesis that zooplankton colonizers reside for only a short time on aggregates, presumably to reduce the risk of removal from their optimal depth as aggregates sink, or to avoid predation by fish and other macrophagous feeders (Kiørboe ).…”