Copepod Size Research Articles

Seasonal variations in copepod size: effects of temperature, food abundance, and vertebrate predation

Seasonal dry weights of female and male Cyclops bicuspidatus thomasi, Diaptomus ashlandi and Diaptomus minutus were studied in southeastern Lake Michigan during 1975-1981. Smallest animals oc- curred during summer and early fall, and largest animals in winter and spring, a pattern reported for other copepods. The range of weights for the species and sexes decreased from winter to summer, and converged in summer to a value of approximately 2 /tg/animal. Surface water temperature and abundances ofyoung- of-the-year (YOY) fish were inversely correlated with weights of males and females of the three copepod species. Standing stocks of important phytoplankton groups, especially pennate diatoms and flagellates, were positively correlated with copepod weight. Seasonal change of copepod body size appears to be more than simply a function of temperature; seasonal predation by YOY fish may be a factor influencing sizes of adult copepods.

Chlorophyll production, degradation, and sedimentation: Implications for paleolimnology1

Chlorophyll a production, degradation, and sedimentation were studied simultaneously during summer stratification in three lakes with contrasting plankton communities. Pigment budgets showed that chlorophyll production and pigment resuspension were both major sources of water column pigments. Photodegradation rates were rapid and indicated that detritus particles that remained in the epilimnion for periods longer than about 3 days lost nearly all detectable pigments. Therefore, only rapidly sinking detrital particles or those produced in deep layers at low light intensity could make appreciable contributions to sedimentary chlorophyll degradation products. Pheophorbide a, a grazing indicator, was the dominant chlorophyll a degradation product found in sediment traps. Pigment sedimentation increased significantly with mean size of cladocerans and omnivorous copepods. In contrast, sedimentation rates of chlorophyll degradation products did not increase with primary production. In these lakes, the deposition of chlorophyll degradation products in sediments depended primarily on the size and biomass of grazers.

Food ingestion by the marine planktonic copepod Paracalanus in relation to abundance and size distribution of food

Juveniles and adult females were presented a food spectrum of three algae of different sizes (4.5, 12 and 20 μm cell width). The increase in rate of ingestion of the medium-sized alga with an increase in copepod size was significantly greater than the increase in rate of ingestion of the small alga. It is hypothesized that the perception of chemical signals from the small alga by a copepod decreases as the copepod moults from stage to stage. The rate of ingestion of the large alga by copepod stage V (CV) and adult females was lower than the rate of ingestion of the medium-sized alga at mid- and high phytoplankton concentrations. The amount of nitrogen ingested when the medium-sized alga alone was offered was either higher than (≦stage C II) or not significantly different from that when the three algae were offered together (≧stage C IV). Ingestion rates are reduced when there is a multialgal food source. This implies that there is increased stability in the ocean because multiparticle food sources are more slowly depleted than unialgal foods. Weight-specific ingestion rates of copepods fed the three algae simultaneously increased from nauplius to stage C III and then decreased as adulthood approached. The contribution of the small alga to the total amount of nitrogen ingesied was greatest for naupliar stages while the contribution of the medium-sized cells was greater for later stages. The largest alga was readily ingested by stage C V and adult females but never contributed more than 25% of the nitrogen ingested. Eight to 12% of the nitrogen ingested by adult females was from the small alga. It is hypothesized that the algal cell size for maximum nitrogen ingestion in upwelled waters is relatively small, round or square and close to the size threshold below which adult females do not sense individual cells.

Size separation of copepods by sieving

A multiple sieve system for size separation of preserved copepod specimens was devised and its performance tested. The system provides a useful tool for determining the size composition of the copepod population, since the copepod size distribution (measured as log-prosome length) differed significantly between different sieves and the sieve-specific copepod size frequency distribution could be practically substituted by a normal distribution function. However, the mean copepod size of the same sieve differed between sampling stations due to clogging by materials other than crustacean zooplankton.

Effects of prey concentration, prey size, predator life stage, predator starvation, and season on predation rates of the carnivorous copepod Euchaeta elongata

Adult females of the large carnivorous copepod Euchaeta elongata Esterly were collected from 1977 to 1980 in Port Susan, Washington, USA. Predation rates of the adult females increased with increasing prey abundance when fed the following 4 sizes of copepods: adult females of Calanus pacificus (average prosome length [PL] of 2 650 μm), adults of Aetideus divergens (PL of 1 560 μm), adult females of Pseudocalanus spp. (PL of 1 060 μm), and nauplii of C. pacificus (PL of 410 μm). Saturation feeding levels were reached when adult females of the predator were fed the small adult copepod, Pseudocalanus spp. Maximum biomass ingested of this small copepod was more than the maximum amount ingested of the larger copepods. Predation rates of the predatory copepodids at Stages IV and V also increased with increasing concentration of the 1 060 μm (PL) prey. High feeding rates exhibited by both adults and copepodids at Stage V of the predator indicate their importance as sources of mortality on populations of small copepods. Ingestion efficiency E i (prey wholly consumed [prey attacked]-1) varied as follows: adults of E. elongata were more efficient than copepodids of E. elongata; adults were more efficient than copepodids when ingesting smaller prey; starved adults were more efficient than fed ones; and both adults and copepodids were more efficient at low food concentrations. For adults of E. elongata, there were no marked seasonal variations in predation or respiratory rates that would represent acclimatory responses; however, small adults obtained during winter were more efficient at ingesting prey than were the larger adults gathered in summer. This seasonal variation in the efficiency of ingestion may be a useful indicator of physiological state: high E i values could indicate that predators are starving in winter, and low E i values could indicate that predators are satiated in summer.

Adult Body Mass and Annual Production/Biomass Relationships of Field Populations

We investigate specific production rates (per unit biomass) of populations using published data on the relation of annual production/mean biomass (P/B). Aquatic and terrestrial invertebrates between the sizes of copepods and clams are emphasized, ranging about 105—fold in body mass upon reaching maturity (Ms, in kcal to compare with respiratory energy expense) and about 102—fold in P/B. Fishes and mammals are briefly treated; phytoplankton is mentioned. For 33 invertebrates living at annual mean temperatures between about 5° and 20°C, Ms is shown to be an efficient and precise estimator, or scaling factor, of the annual P/B. The rate declines markedly with Ms according to P/B = 0.65Ms—0.37. The exponent differs significantly from the —0.25 power of comparative physiology. Most of the measured values of P/B fall within 50 to 200% of predicted values. Much of this variability is associated with the ratio of annual production/annual respiration (P/R): for a given Ms, species achieving about half the predicted P/B have P/R ratios of about 0.1; those achieving twice the predicted P/B have P/R ratios of about 1.0. Age upon reaching maturity contributes some variability, with late—maturing (>1 yr) species tending towards a higher P/B. The variability is not significantly correlated with phylogenetic relationships (excepting insects for which P/B might not be mass—dependent), trophic type, major habitat, production rate, or biomass of the populations. The values of P/B of invertebrates living at annual mean temperatures >25° may be elevated over those of temperate species of the same Ms, while those of polar forms are depressed. The reasons for a single power function governing the mass dependence of P/B of temperate invertebrates, and for the particular exponent, are unclear; an ecological cause, i.e., mortality, combining with the general size dependence of life processes, is implicated. On the average, the annual specific mortality rate equals P/B and hence also declines by 0.65Ms—0.37. Very small metazoans (pelagic rotifers, benthic meiofauna) tend to have an appreciably lower P/B than indicated by the relationship for larger invertebrates. A refuge from predation by being small is postulated which may also apply for phytoplankton. For meiofauna, a power function of mass dependence of P/B with average rates 3—5 times below those of the larger invertebrates is suggested. Annual P/B values of fishes and mammals likewise decline by a power function of Ms; the few available data yield exponents of —0.26 and —0.33, respectively. Ecological reasons are again invoked. Values of P/B and the specific mortality rates of temperate fishes seem to be 4—5 times, and those of mammals 20—25 times, higher than those of temperate invertebrates of the same mass.

A Rapid Method for the Estimation of, and Some Factors Affecting, Copepod Production Rates in the Burlington Canal

A method is described for estimating in situ biomass production rates of copepods. The youngest naupliar stages are isolated by filtration through two different size sieves, and growth is measured after laboratory incubation by filtering through a series of eight filters. Since the filters appear to be relatively efficient at separating copepods of different sizes, copepod size can be estimated from the pore size of the filters alone, resulting in a very rapid procedure for calculating biomass. This method allows experimental manipulation of environmental conditions, permitting for example, an estimation of the relative importance of temperature and food abundance in the seasonal growth cycles of these organisms. It was concluded that, in the Burlington Canal, temperature exerts a greater influence on growth rate than does food concentration, although adaptation to food levels may be expressed indirectly, due to a similarity in the seasonal cycles for temperature and food. The method is also useful in predicting the potential impact of toxic substances on copepod production rates under natural conditions. Key words: copepods, zooplankton, biomass production, temperature, food concentration

Seasonal density and production estimates of the commoner planktonic copepods of northumberland coastal waters

The seasonal density of five copepods, Acartia clausi, A. liongiremis, Pseudocalanus elongatus, Oithona similis and Temora longicornis in the Northumberland plankton for a five year period is presented. Size of adult copepods is shown to be closely correlated with sea temperature during their period of development. Adult copepods do not grow. So during a period of temperature change alteration in mean size of copepods indicates a turnover of population, while unaltered mean size indicates an unchanging population. To obtain a production figure separate generations of adult females throughout a year are distinguished statistically on a size criterion. The average weights per unit volume of sea water of these separate generations is summed to give an annual production figure for adult females. Earlier work suggests that the statistically separate generations may generally be successive, each separate generation arising directly from the preceding one. Then the weights of all stages, not only the adults, may be summed to give a higher and more accurate production figure for each species.

Particle‐size modification by two size classes of the estuarine copepod Acartia clausi1,2

Two distinct size classes of Acartia clausi were allowed to graze on two different‐sized species of the chain‐forming diatom Thalassiosira. Both copepod size classes dismembered the larger T. gravida chains, producing a high frequency of smaller particles when T. gravida was in high concentrations. In low particle concentrations of T. gravida, the production of small particles was obscured by ingestion. Some particle dismembering might have occurred with the smaller T. nordenskioldii, but the effect also was masked by ingestion. Such particle modification might require a re‐examination of past grazing data.Particle volume ingestion was plotted as a function of particle volume concentration. At high particle densities, both large and small A. clausi could ingest greater volumes of the large T. gravida particles than the smaller T. nordenskioldii particles. At low particle volume densities, both size classes of copepod ingested similar volumes of both diatoms. Large A. clausi ingested greater food volume than small A. clausi, except at the very lowest food volume density (where ingestion was similar). Great variability in the ingestion data occurred as a result of copepod collections in different seasons of different years, and the use of single grazing coefficients is discussed in the light of this variability.

Body Size of Free-Living Copepods

Two genetically similar populations of Cyclops strenuus strenuus Fischer in two small ponds in southern Norway developed a considerable difference in body size under approximately the same temperature and other environmental conditions. The largest individuals were observed in the pond with the lowest population density. The relatively low densities observed and a supposed superabundance of food indicates that influence from crowding is doubtful. Laboratory experiments indicated that the differences in density in nature are too small to produce a significant size difference. Rate of development was not influenced by density differences in nature, but inversely related to density in the experiments. Fecundity showed a clear inverse relationship to density both in nature and in the laboratory experiments.

The biology of bathypelagic organisms, especially Crustacea

An examination is made of the body sizes of species of euphasiidss, mysids, copepods and chaetognaths living in different bathymetric zones. The maximum body size of each species is used as a measure of its potential for growth in size. Bathypelagic euphausiids and mysids are shown to possess a much greater potential for attaining greater size than corresponding epipelagic species. Body size of copepods shows a potential increase down to depths of 1000–2000 m but, thereafter, there is a decrease in the potential size. Mesopelagic and bathypelagic chaetognaths are potentially larger than most epipelagic species. A comparison of the rates of growth of species in different latitudes, and so in different temperature regimes, suggests that in high latitudes the rates are lower or comparable and that increased body size is attained through increased longevity. The growth rates of epipelagic species living in latitudes higher than 40° are extrapolated to estimate longevity of corresponding bathypelagic species. Bathypelagic species probably live 2–7 times as long as epipelagic species; individual estimates are 3–7 times for euphausiids and mysids, 5 times for copepods and 2–5 times for chaetognaths. The effects of the low temperature regime in the bathypelagic environment can be defined as possibly decreasing the rates of growth, increasing longevity, and so producing organisms of larger size, and, in some cases, preventing the attainment of sexual maturity. The extreme expression of these effects coupled with aberrations of behaviour is probably ‘giantism’ of individuals. This increased potential of bathypelagic organisms for the attainment of greater body size probably confers several advantages. They produce fewer but much larger eggs and young stages than epipelagic species; these large young stages can probably use a wider range of material as food and through increased mobility over smaller animals, can locate such material. Young stages may adopt the diet of the adults fairly quickly. Larger adults are also probably more mobile and so are able to travel farther and faster in search of food and mates. Increased longevity may mean, in many bathypelagic species, that there is an increase in the proportion of their lives, relative to that of epipelagic species, when they can become sexually mature. This is important in low density populations to allow mates to be located. Behavioural mechanisms involved in breeding are discussed. The presence, or absence, of seasonal cycles in the lives of bathypelagic organisms is considered. The presence of seasonal cycles of breeding in epipelagic populations usually results in periods when no males are sexually mature; species with less clearly defined breeding seasons usually have all males above a certain body size sexually mature and the sex ratio is in favour of females. More attention should be paid to the state of sexual maturity of males of bathypelagic species this should provide evidence, or otherwise, of seasonal breeding. Increased longevity coupled with lower fecundity in bathypelagic species depresses potential levels of production in this environment.

Food selection by laboratory-reared larvae of the scaled sardine Harengula pensacolae (Pisces, Clupeidae) and the bay anchovy Anchoa mitchilli (Pisces, Engraulidae)

Food selection by laboratory-reared larvae of scaled sardines Harengula pensacolae, and bay anchovies Anchoa mitchilli, was compared. Natural plankton was fed to the larvae during the 22 days following hatching. Food levels in the rearing tanks were maintained at an average of 1,600 to 1,800 potential food organisms per liter. Larvae of both species selected as food copepod nauplii, copepodites, and copepods; initial feeding was on organisms of 50 to 75 μ body width. Larvae of H. pensacolae averaged 4.2 mm in total length at hatching and those of A. mitchilli about 2.0 mm. H. pensacolae larvae grew about 1.0 mm per day and A. mitchilli 0.70 mm per day. The mean number of food organisms in each digestive tract was greater in H. pensacolae than in A. mitchilli, and the difference in number increased as the larvae grew. Average size of food organisms eaten increased for both species with growth, because of selection by the larvae; the average size of copepodites and copepods in digestive tracts increased at a faste rate in H. pensacolae than A. mitchilli. A. mitchilli longer than 8 mm did not eat copepod nauplii.

Influence of Temperature on Size of Freshwater Copepods (Cyclops) by R. E. Coker

Influence of Temperature on Size of Freshwater Copepods (Cyclops) by R. E. Coker