Predator-induced diet vertical migration in a planktonic copepod

Predator evasion is the most commonly hypothesized reason fordiel vertical migrations undertaken by a wide variety of planktonicorganisms in lakes and seas, yet direct evidence remains elusive.We tested the predation hypothesis by exposing enclosed populationsof a marine copepod Acartia hudsonica to caged or free-rangingindividuals of their natural predator, the planktivorous fishGasterosteus aculeatus. After little more than a week, adultcopepods changed their vertical distribution and migration behaviordepending on the presence or absence of predation. Only free-rangingfish induced vertical migration in the copepod population. Cagedfish had no effect, indicating that vertical migration was nota simple chemically mediated response of copepods to the predator.Rather, copepods seemed to react to the presence of predatorsby other means, perhaps visual or mechanical stimuli, and toexhibit a downward escape response which, because encounterswith visually orienting fish occur chiefly in the daytime, effectivelylimited the copepods' occurrence in the upper water column tothe night-time hours. Alternatively, because fish imposed heavymortality on copepods, it is possible that selective predationaltered the proportions of individuals with fixed, geneticallydetermined migration behaviors. We suggest experiments to distinguishthese alternatives.     

Ovigerity,selective predation,and variable diel vertical migration in Euchaeta elongata (Copepoda: Calanoida)

Summary We present a statistical analysis of a previously published (Yen, 1983) but heretofore unanalyzed data set on the vertical distributions and diel vertical migration (DVM) of adult females of the marine planktonic copepod Euchaeta elongata in Dabob Bay, Washington, USA. Non-ovigerous females were strongly migratory on all four dates sampled, residing between 75–175 m during the day and at shallower depths during the night, commonly entering the upper 50 m of the water column. In contrast, ovigerous females were non-migratory or weakly migratory, largely remaining between 100–175 m both day and night, and entering the upper 50 m of the water column only rarely. Thus non-ovigerous females always migrated much more strongly, as measured by both amplitude of migration and the proportion of animals migrating, than did ovigerous females. These results led us to hypothesize that differential susceptibility to visually orienting predators was the cause of these differences in DVM behavior in female E. elongata, and we subsequently undertook an experimental study of the feeding selectivity of the copepod's natural predator, Pacific herring (Clupea harengus pallasi). Pacific herring exhibited a highly significant preference for ovigerous over nonovigerous adult female E. elongata. The demographic consequences of variable DVM in adult female E. elongata were investigated by way of life table analyses. Results indicated that under conditions of thermal stratification of the water column there is a distinct demographic disadvantage (reduced rate of realized population growth) incurred by non-migratory or weakly migratory ovigerous females due to delayed egg development at cooler subsurface temperatures. We conclude that ovigerous female E. elongata remain at depth both day and night to avoid visually orienting predators, and that such behavior must afford the copepod a demographic advantage of no less than a 26% reduction in adult mortality to offset the demographic cost of delayed egg development.     

A genetic reconstruction of the invasion of the calanoid copepod <Emphasis Type="Italic">Pseudodiaptomus inopinus</Emphasis> across the North American Pacific Coast

The rate of aquatic invasions by planktonic organisms has increased considerably in recent decades. In order to effectively direct funding and resources to control the spread of such invasions, a methodological framework for identifying high-risk transport vectors, as well as ruling out vectors of lesser concern will be necessary. A number of estuarine ecosystems on the North American Pacific Northwest coast have experienced a series of high impact planktonic invasions that have slowly unfolded across the region in recent decades, most notably, that of the planktonic copepod crustacean Pseudodiaptomus inopinus. Although introduction of P. inopinus to the United States almost certainly occurred through the discharge of ballast water from commercial vessels originating in Asia (the species’ native range), the mechanisms and patterns of subsequent spread remain unknown. In order to elucidate the migration events shaping this invasion, we sampled the genomes of copepods from seven invasive and two native populations using restriction-site associated DNA sequencing. This genetic data was evaluated against spatially-explicit genetic simulation models to evaluate competing scenarios of invasion spread. Our results indicate that invasive populations of P. inopinus exhibit a geographically unstructured genetic composition, likely arising from infrequent and large migration events. This pattern of genetic patchiness was unexpected given the linear geographic structure of the sampled populations, and strongly contrasts with the clear invasion corridors observed in many aquatic systems.     

Fish assemblages across a vegetation gradient in a restoring tidal freshwater wetland: diets and potential for resource competition

Marsh habitats have been the focus of recent worldwide restoration efforts due to their degradation and destruction as a result of human development. We assessed fish resource use at a naturally restoring marsh (Liberty Island, California, USA) by comparing diet composition, stomach fullness, normalized stomach weight, and diet overlap across a vegetation gradient. Fish were collected using gill nets and fyke nets at six sites during spring 2010, summer 2011 and winter 2012. We analyzed 392 stomachs from the eleven most abundant species collected. Prey composition and biomass varied seasonally for all fish species, but there were no notable differences across sites or seasons for stomach fullness or normalized stomach weight for most fish species. Results from non-metric multidimensional scaling (NMDS) and two-way analysis of similarities (ANOSIM) indicated minimal diet overlap between species (R?=?0.633, p?=?0.001) and seasons (R?=?0.413, p?=?0.001). Seasonal habitat and resource use across the vegetation gradient was species-specific. Small but significant spatial diet differences were detected for inland silverside, striped bass and bigscale logperch. Delta smelt exhibited seasonal diet differences by shifting from chironomids and zooplankton during spring, to amphipods and zooplankton during winter. More generally, fish maintained stomach fullness across all sites and seasons while maintaining minimal dietary overlap. Our study emphasizes the importance of tidal marshes as feeding habitat for several fish species, including the endangered delta smelt.     

Zooplankton invasions: a brief review,plus two case studies from the northeast Pacific Ocean

Invasions of aquatic habitats by non-indigenous species (NIS), including zooplankton, are occurring at an alarming rate and are causing global concern. Although hundreds of such invasions have now been documented, surprisingly little is known about the basic biology and ecology of these invaders in their new habitats. Here we provide an overview of the published literature on NIS zooplankton, separated by life history (holoplankton vs. meroplankton), habitat (marine, estuarine, freshwater), and biological level of organization or topic (e.g. distribution and range extension, physiology, behavior, feeding, community impacts, ecosystem dynamics, etc). Amongst the many findings generated by our literature search, perhaps the most striking is the paucity of studies on community and ecosystem level impacts of NIS zooplankton, especially in marine and estuarine systems. We also present some results from two ongoing studies of invasive zooplankton in the northeast Pacific Ocean – Pseudodiaptomus inopinus in Washington and Oregon coastal estuaries, and Tortanus dextrilobatus in San Francisco Bay. Both of these Asian copepods have recently expanded their range and can at times be extremely abundant (10³ m^–3). We also examine some aspects of the trophic (predator–prey) ecology of these two invasive copepods, and find that they are likely to be important in the flow of material and energy in the systems in which they now pervade, although their impacts at the ecosystem level remain to be quantified. Finally, the findings of both our literature search and our two case studies of invasive zooplankton lead us to make several recommendations for future research.     

The effects of thin layers on the vertical distribution of larval Pacific herring, Clupea pallasi

Temporal and spatial heterogeneity of resources is likely to have dramatic effects on the behaviors of zooplankton and ichthyoplankton, which in turn are likely to have strong effects on ecological dynamics such as predation, growth, and mating. The objective of this study was to determine whether vertically thin layers of extreme prey concentration affect the vertical distribution of larval Pacific herring (Clupea pallasi). We employed 2-m tall experimental tanks equipped with video cameras that scanned the vertical extent of the tanks to investigate the effects of thin layers on the vertical distribution of 5- and 10-day-old herring larvae. Three treatments were established: (1) a thin layer of prey (rotifers, Brachionus plicatilis) through density (salinity) stratification, (2) homogeneous vertical distribution of both prey and density, and (3) density (salinity) stratification, but with a homogeneous distribution of prey. We found that in all treatments the majority of larval herring were at the surface, near the light, despite the absence of a peak in rotifer abundance at this depth in some instances. However, there were also clear effects of the thin layers—secondary subsurface peaks in herring abundance occurred at the mid-depths in the stratified tanks, in and around the thin layers. In addition, our results provide some evidence that thin layers specifically, rather than prey patches generally, influence the vertical distribution of larval herring, i.e., larvae may use the physical properties of thin layers to locate and distribute themselves, instead of reacting solely to the prey patches. Thus thin layers can affect directly the vertical distribution of larval herring, and perhaps indirectly their horizontal distribution, as herring larvae live in environments (e.g., estuaries) where advective transport is also often vertically heterogeneous.     

Fish assemblages of interior tidal marsh channels in relation to environmental variables in the upper San Francisco Estuary

Tidal marsh wetlands represent critical habitat for many estuarine fishes and are particularly important to conserving and restoring native and at-risk species. We describe the seasonal and regional variation in the composition and abundance of fishes in interior tidal marsh channels in the upper San Francisco Estuary (SFE), and relate these to variation in environmental conditions. Fish were sampled quarterly using modified fyke nets from October 2003 to June 2005 in 18 interior tidal marsh channels spanning 3 distinct river systems: Petaluma River, Napa River and West Delta. We collected 116 samples and 9452 individuals of 30 fish species. Four non-native species dominated—Mississippi silverside, western mosquitofish, yellowfin goby, and rainwater killifish—with an additional 13 species occurring commonly (represented equally by natives and non-natives, residents and transients). Large seasonal differences in composition and abundance of fishes occurred, with the lowest abundances in winter and spring and highest abundances in summer and fall. Correlation of ordination scores and environmental variables further supported the importance of season, as well as fish species’ status (native vs. non-native), feeding preference (pelagic vs. demersal), and marsh utilization (resident vs. transient), as factors influencing fish assemblage composition. The proximity of the marsh systems to freshwater and marine influences, which largely control salinity and temperature variation, explained 26% of the variation in fish composition, while channel geomorphology explained 22%. We recommend that both edge habitat (which may be beneficial to fish foraging success) and the extent of tidal connectivity (which allows access for fishes), in addition to location along the estuarine gradient, be considered in designing and managing tidal marsh restoration.     

Seasonal plankton cycles in a temperate fjord and comments on the match-mismatch hypothesis

As a field test of the ‘match-mismatch hypothesis’(Cushing, 1972, 1975, 1990), we sampled the ichthyoplanktonand different size fractions of zooplankton at two stationsin a temperate fjord (Dabob Bay, Washington, USA) between May1985 and October 1987. We present the results of this studytogether with historical data on seasonal abundances of phytoplanktonand invertebrate predators. The peak abundance of fish larvaein late winter/early spring preceded the peak abundance of preyorganisms by an average of 3 months. Correlation analysis resultedin one negatively significant and three non-significant correlationsbetween ichthyoplankton and appropriately sized prey organisms.These field observations, showing a distinct temporal lag betweenabundances of larval fish and their zooplankton prey, do notsupport the match-mismatch hypothesis as originally formulated.We use these field data and a simulation model of the growthof young fish to discuss the role of two alternative processes—foodlimitation during late larval/juvenile stages and predation—thatmay constrain the timing of spawning of marine fish and whichmay have implications for recruitment variability as well. Thisleads us to propose that the match-mismatch hypothesis be extendedto include a ‘critical period’ that lasts longerand occurs later in the life history of marine fish than haspreviously been supposed.     

The effect of ultraviolet radiation on the vertical distribution and mortality of estuarine zooplankton

The effect of UV-B radiation on the vertical distribution ofthree calanoid copepod species (Tortanus dextrilobatus, Acartiuraspp. and Acanthacartia spp.) and three larval stages of Pacificherring, Clupea pallasi (1-, 7-, and 14-day-old larvae) wasinvestigated. A series of 2 m high columnar tanks equipped withinfra-red light and video-microscopy was used to investigatethe vertical distribution of zooplankton in the presence andabsence of UV-B radiation. In the presence of UV-B radiation,T.dextrilobatus and 1-day-old C.pallasi resided about 50 cmdeeper than in the absence of UV-B, while Acartiura spp. andAcanthacartia spp. showed no (or only minimal) change in verticaldistribution. Mortality experiments were also conducted outdoorsin which each copepod species was exposed to full or reducednatural radiation levels. Only T.dextrilobatus showed an increasein mortality when exposed to full radiation. Our results showedthat T.dextrilobatus and 1-day-old C.pallasi larvae were sensitiveto UV radiation (UVR), and to reduce or eliminate UV-inducedstress, they avoided the surface of the water column when UV-Bradiation was present. Copepod species were chosen to span arange of pigmentation: T.dextrilobatus (heavily pigmented),Acartiura spp. (moderately pigmented) and Acanthacartia spp.(not pigmented). The pigmentation did not appear to play a rolein UVR tolerance of the copepods, but may be a factor determiningUV tolerance of C.pallasi.