Zooplankton are commonly frozen at sea in ecological studies when it is impractical to extract lipids immediately from live animals. Analysis of lipids extracted from the copepod Calanus pacificus established that freezing has negligible effects on copepod dry mass, total lipid content, or lipid composition, resulting only in slight changes in free fatty acids. However, storage for 1 yr at -15°C resulted in substantial loss of polar lipids and accumulation of free fatty acids when compared with animals stored at - 80°C. Rapid freezing in liquid nitrogen, followed by maintenance of animals at temperatures below -70°C, is recommended where immediate lipid extraction is not feasible.

This article is in Free Access Publication and may be downloaded using the “Download Full Text PDF” link at right. © 1987, by the Association for the Sciences of Limnology and Oceanography, Inc.

Wax esters extracted and purified from the copepod Calanus pacificus elicit a different response in quantitative thin-layer chromatography-flame ionization detection (TLC-FID) than simpler, saturated wax esters available from commercial courses and commonly used for instrument calibration. Failure to use native copepod wax esters in calibration procedures results in underestimates of 18-32% in wax ester content, depending on the mass of lipid loaded on chromarods. In contrast to wax esters, tricylglcerols from the copepod Eucalanus californicus, as well as phospholipids from E. californicus and C. californicus and C. pacificus, elicited no significant difference in detector response in comparisons with commercially available standard compounds. Extraction of copepod lipids in solutions of chloroform:methanol results int he release of non-lipid material. Therefore, indirect gravimetric measured of lipid content that rely on calculation of lipid mass by difference after 'de-fatting' copepods greatly overestimate the true lipid content. Lipid-specific assays with appropriate calibration standards are essential for the accurate quantification of the lipid content of planktonic animals. The water content differs appreciably among co-occurring species of copepods, averaging 82-84% in Rhin-calanus nasutus, C. pacificus and Metridia pacifica, but 93% of body mass in E. californicus. The occurrence of gelatinous tissues in some planktonic copepod species underscores the need to take account of structural and life history diversity among taxa. Care must be taken in the selection of the independent variable used to scale zooplankton lipid content and other structural and metabolic relationships.

Three types of diel vertical migration behavior were expressed within a fjord population of the calanoid copepod Pseudocalanus newmani: (1) reverse migration (nocturnal descent), (2) normal migration (nocturnal ascent), and (3) no detectable migration. Reverse migration by P. newmani occurred in all three study years, but only at a deep (185 m) station at times of year when normally migrating predatory Zooplankton (the copepod Euchaeta elongata, the chaetognath Sagitta elegans, and the omnivorous euphausiid Euphausia pacified) were abundant. A predator-exclusion "natural experiment" tested the hypothesis that the reverse migration is a predator avoidance mechanism: at a shallow (55 m) station within the same fjord, large predatory Zooplankton were absent or reduced markedly in abundance. In conditions of reduced nocturnal predation by predatory zooplankton, no reverse migration by P. newmani was detected on any of 11 cruises, consistent with the hypothesis. Instead, at the shallow station, where diurnal predation by planktivorous fish was of increased importance, normal diel vertical migration by P. newmani was observed on several occasions. No migration was detectable at either the deep station or the shallow station at times of year of low feeding activity of predators in surface waters. The predominant migration behavior (reverse, normal, or no migration) varied depending whether the primary source of mortality was predation by nocturnally feeding Zooplankton or visually hunting planktivorous fish, and appeared unrelated to resource distributions or temperature gradients. The relative advantage of the three migration behaviors is explored with theoretical life table analyses. Re-examination of the demographic advantage hypothesis proposed by McLaren (1974) illustrates that vertical migration across a thermocline carries a fitness cost, not a benefit. The realized rate of increase of a migrant population of Pseudocalanus is found to be lower than a nonmigrant population for all combinations of surface temperatures, positive thermal stratification, and ratios of mortality for early and late developmental stages. However, avoidance of predators by vertical migrants reverses the outcome of this analysis. Predator avoidance by Pseudocalanus, accompanied by reduced mortality rates, confers a fitness advantage upon a population that migrates across a thermocline. Remarkably small reductions in mortality rates (as little as 12%) confer this advantage, whether migrants avoid nonvisually hunting predatory Zooplankton by night or visually hunting planktivorous fish by day. The behavioral variation documented here confirms that diel vertical migration is dynamic rather than a fixed, invariant behavioral trait within a population. In environments where predation pressure and other factors vary through time, studies of limited scope and short duration are unlikely to illuminate the ultimate causes for diel vertical migration. The heritability of the vertical migration trait is unclear. The different phenotypic responses reflected within this single population of P. newmani may reflect altered behavior of individuals. Alternatively, this behavioral variation may represent a balanced polymorphism maintained by differential mortality of genotypes experiencing selection pressures that vary through time.

Avoidance behavior (through spatial refuges, diel activity cycles, seasonal diapause, locomotory behavior) minimizes encounter rates with predators. Escape responses (through active motility, passive evasion, aggregation, bioluminescence) diminish rates of attack or successful capture. Defense responses (through chemical means, induced morphology) decrease the probability of successful ingestion by predators. Behavioral responses of individuals also alter the dynamics of populations. -from Author

A protocol is presented for the application of immunochemical methods to the identification of planktonic ciliates, exemplified by the oligotrich Strombidium sp. Probes were produced in late 1989-early 1990 to Strombidium sp. in two forms: homogenates of (1) intact cells and (2) unassimilated Strombidium sp. antigens recovered from copepod fecal pellets. The antigen-antibody reaction was detected by dot blot assays using alkaline-phosphatase conjugated secondary antibodies as a reporter, and the reaction intensity was quantified by reflection densitometry. The immunogenic fraction of the antigen solutions was characterized by Western blots. Analysis of cross-reactions of anti-Strombidium sp. serum with 28 other species revealed a single strong cross-reaction, which was effectively eliminated by immunoadsorption. The antisera are suitable for detection of predation on ciliates in situ, free from the artifacts of laboratory incubations, and potentially for automated procedures for recognition of ciliates. © 1992 Springer-Verlag.

The impact of predators on population growth of Pseudocalanus sp. was investigated in Dabob Bay, Washington. Mortality of Pseudocalanus sp. was determined from stage-specific survivorship, from seasonal changes in mortality rates of adult males and females and from incidence of injuries to adult copepods. The principal predators of adult Pseudocalanus were identified as the predatory copepod Euchaeta elongata, the omnivorous euphausiid Euphausia pacifica and the chaetognath Sagirta elegans. Predator attack rates - and prey mortality rates - are highly density-dependent and thus sensitive to prey dispersion in the water column, particularly to layering in the vertical plane. Predation rates by the three principal predators exceeded 100% of the recruitment rate to adult Pseudocalanus sp. beginning in early summer, thus restricting population growth. Planktivorous fish predation (by adult three-spine stickleback, Gasterosteus aculeatus, and juvenile chum salmon, Oncorhynchus keta) on Pseudocalanus sp. adults was estimated to be two orders of magnitude lower than consumption rates by predatory zooplankton, at a deep water station in July. Analysis of seasonal changes in prey ingested by Sagitta elegans revealed that Pseudocalanus sp. was the major prey item of S. elegans in April (61.0% of prey) and in June (67.0% of prey), thereafter declining seasonally in importance. Predation by S. elegans varied seasonally with changes in chaetognath stage structure, vertical distribution and diapause, not size structure alone. Although chaetognath recruitment and population growth appear to be directly coupled to the abundance of Pseudocalanus sp., predation by S. elegans has little reciprocal impact on Pseudocalanus sp. population growth; hence asymmetries may occur in the in teraction of planktonic prey and predators. © 1986 IRL Press Limited.

We tested the sensitivity of the vertical distributions and shell dissolution patterns of thecosome pteropods to spatial gradients associated with an eddy-associated front in the southern California Current System. The aragonite saturation horizon (Ωarag = 1.0) shoaled from >200 to <75 m depth across the front. The vertical distribution of thecosome pteropods tracked these changes, with all 5 species showing reduced occurrence at depths below 100 m where waters were less saturated with respect to aragonite. Shell dissolution patterns of the numerically dominant thecosome Limacina helicina corresponded to the cross-frontal changes in Ωarag saturation state. Severe shell dissolution (categorized here as Type II and Type III) was low in near-surface waters where Ωarag > 1.4, while peak dissolution occurred in depths where Ωarag = 1.0 to 1.4. Vertical habitat compression and increased shell dissolution may be expected to accompany future shoaling of waters that are undersaturated with respect to aragonite.