Measuring the precision of estimates of total egg production based on plankton surveys

Methods are given to measure the effects of spatial and temporaldifferences in egg production on the precision of estimatesof total seasonal egg production derived from ichythyoplanktonsurveys. The techniques are applied to the results of largescale plankton surveys conducted in 1977 and 1979 off the northeasternUSA. For the three species analyzed (Atlantic mackerel, Scomberscombrus; silver hake, Merluccius bilinearis and yellowtailflounder, Limanda ferruginea). the surveys produced estimatesof total egg production having an average coefficient of variationequal to 31%. Estimates of spawning stock size based on theegg production estimates compared favorably with other independentassessments of stock size. ^*MARMAP Contribution MED/NEFC 83–39.     

THE RELATIONSHIP OF BODY SIZE AND GROWTH TO EGG PRODUCTION IN THE HERMAPHRODITIC FRESHWATER SNAIL, HELISOMA TRIVOLVIS

Among several other factors, body size has been found to influenceegg production in several species of hermaphroditic snail. Wetested whether this relationship between body size and egg productionexists in Helisoma trivolvis, a freshwater hermaphroditic species.We isolated 50 H. trivolvis from a laboratory population, measuredshell diameter, and monitored egg production for seven weeks.We found a positive relationship between body size and totalnumber of eggs produced, as well as body size and number ofeggs per egg mass. When body size and egg production are linked,it should be adaptive for larger individuals to act as femalesand smaller individuals as males. Since body size is relatedto female fecundity in this species, the relative size of snailsshould determine, at least in part, which individual acts asmale and which as female during copulation. However, the relationshipbetween body size and egg production is not nearly as strongas it is in other snail genera. Other factors such as age, genotypeand previous experience may be important in determining egg-layingcapacity and therefore gender choice in this species. In addition,we found a negative relationship between growth during thisperiod and egg production. This relationship has been foundin other pulmonates, and is evidence of resource allocationtradeoffs. (Received 3 August 2004; accepted 2 August 2005)     

The influence of changing temperature on the life history of Daphniopsis ephemeralis

We present evidence which suggests that for Daphniopsis ephemeralischanges in water temperature may directly cue changes in sexratio, ephippial and parthenogenetic egg production and indirectlydetermine female body size. Field data suggested that over anannual cycle, a population of D. ephemeralis comprised animalswith three distinct life history phases. The first was madeup of exephippial females which hatched in the fall. They produceda second group made up of large parthenogenetic females whichcould produce more than one overwintering generation. In thespring these parthenogenetic females gave rise to a third lifehistory phase which comprised males and small ephippial females.Field enclosure experiments suggested that the small size ofspring females was not related to predation. Laboratory experimentssuggested that ephippial egg production at small body sizeswas associated with ‘increasing’ temperature. Theseexperiments also suggested that the fall reproductive pattern(parthenogenetic eggs and large body sizes) was associated with‘decreasing’ temperatures. We conclude that directionalityof seasonal temperature change may be the prime factor responsiblefor D. ephemeralis life history changes observed in the field.     

In situ egg production and hatching success of the marine copepod Pseudocalanus newmani in Funka Bay and adjacent waters off southwestern Hokkaido, Japan: associated to diatom bloom

. Clutch size and egg viability were examined for wild femalesof Pseudocalanus newmani collected from four different stationsinside and outside Funka Bay, southwestern Hokkaido, Japan,during pre-phytoplankton-bloom conditions in 1997, and froma single station outside the bay during the period from pre-to post-blooming seasons in 1998, to examine spatio-temporalvariability of the reproductive parameters and to detect deleteriouseffects of diatom blooms on in situ copepod reproduction. Clutchsize and hatching success in P.newmani were relatively similarat the four stations in 1997. The eggs almost always hatchedcompletely, but deformed nauplii occurred in 20–40% ofthe cases. The proportion of deformed nauplii gradually decreasedwhen the females were fed on non-diatom Pavlova sp. in the laboratory,suggesting that the occurrence of deformed nauplii was relatedto the quality of in situ food particles, including diatoms.Clutch size varied from 12 to 30 eggs female^–1 duringthe study period in 1998; more than 90% of the variation couldbe explained by the body size of the females which dependedon ambient water temperature. Hatching success also varied,from 15 to 80%, but was not related to either clutch size, femalebody size, water temperature or diatom biomass. These resultssuggest that not only egg production but also hatchability shouldbe measured routinely when estimating recruitment of the copepodinto the planktonic population, and show that neither clutchsize nor egg viability are directly affected by diatom biomass.Although we could not obtain clear evidence of in situ deleteriouseffects of diatoms on abnormal embryos and nauplii, this mightbe related to methodological problems and properties or characteristicsof prey–predator interactions. Also, deformities of copepodnauplii have not yet been observed for reasons other than feedingon diatoms.     

Summer reproduction of the planktonic copepod Calanus sinicus in the Yellow Sea: influences of high surface temperature and cold bottom water

We have observed that Calanus sinicus retreated from neriticareas in the Yellow Sea and concentrated in the Yellow Sea ColdBottom Water (YSCBW) area in summer. To investigate the summerreproductive strategy of C. sinicus in this situation, effectsof high temperature on reproduction and hatching, as well asgeographical variation of in situ egg production rate, werestudied by onboard incubation in August 2001. Diel verticalmigration (DVM) of females was investigated within and outsidethe YSCBW, respectively. Onboard incubation at 27°C (i.e.surface temperature) resulted in lower fecundities than thatat 9.8 and 12°C (i.e. bottom temperature inside and outsidethe YSCBW) together with decreased hatching rates and increasednaupliar malformation. Egg production was more active at stationsoutside the YSCBW than inside, where chlorophyll-a concentrationwas also relatively low. Females inside the YSCBW underwentDVM although they rarely entered the surface layer, but DVMwas not observed outside the YSCBW. We conclude that surfacetemperature in summer has deleterious effects on C. sinicusegg production and hatching, and that it cannot reproduce successfullyover the whole area. Inside the YSCBW, egg production is depressedby low food availability, while females outside suffer fromhigh temperatures because of strong vertical mixing.     

Distribution and species-specific egg production of Pseudocalanus in the Gulf of Alaska

Pseudocalanus species are important contributors to the secondaryproduction of the northern hemisphere mid- to high-latitudeoceans. In the coastal Gulf of Alaska, Pseudocalanus are presentyear round and are represented by three species. In 2001, Pseudocalanusmimus was the dominant Pseudocalanus species on the shelf duringspring and summer, comprising 30–100% of the total, whilePseudocalanus newmani dominated in Prince William Sound (10–90%).Pseudocalanus minutus were only abundant in Prince William Soundduring early spring. Egg production (by number and volume) wasa function of female prosome length and decreased from springto summer; however, significant variability was attributableto regional influences that were independent of size. For thesame sized female, P. newmani produced more eggs per clutchthan P. mimus. Pseudocalanus mimus, however, tended to havea larger mean egg size than P. newmani. Consequently, clutchvolumes of the two species were indistinguishable. Pseudocalanusegg production rates (EPRs) (eggs female^–1 day^–1)were lower in July and August (ca. 2–4) than April andMay (ca. 1–9), but total egg production by the population(eggs day^–1) was nearly equivalent for the two time periodsdue to higher female concentrations in summer.     

Egg size and reproductive allocation in eusocial thrips

Reproductive allocation, in terms of fecundity and egg size,has been given little consideration in eusocial societies. Tobegin to address this, absolute and body size–adjustedegg volumes were compared, along with fecundity, between thefoundress and her subfertile soldier offspring in the eusocial,gall-inducing thrips, Kladothrips hamiltoni, Kladothrips waterhousei,and Kladothrips habrus, and a congeneric, Kladothrips morrisi,with fully fecund soldiers. Soldiers produced significantlylarger eggs than the foundress in all species except K. morrisi,where egg volumes did not differ. After accounting for bodysize, soldiers produced significantly smaller eggs than thefoundress in K. morrisi and marginally so in K. waterhousei,but egg sizes did not differ in K. hamiltoni and K. habrus.When egg size and fecundity data are combined, K. morrisi soldiersinvest less in reproduction than the foundress, and in conjunctionwith other life-history features the species can be consideredeusocial. Maximum likelihood analyses reveal relatively lowreproductive allocation skew in the ancestral lineages and highskew in the derived lineages, but the trend is not significantwhen fecundity and egg size are considered separately. Gallsize covaried negatively with soldier-to-foundress relativebody size–adjusted egg size and reproductive allocationand marginally so with fecundity, suggesting that gall sizeis a determinant of egg size and fecundity trade-offs in eusocialthrips and providing the strongest support to date that gallsize has featured in the social evolution of this clade. Thisstudy highlights that data on fecundity alone may be insufficientfor assessing reproductive division of labor.     

Laboratory studies on egg and faecal pellet production of Centropages typicus: effect of age, effect of temperature, individual variability

The rates of egg production of individually reared females ofCentropager typicus have been observed from the moulting C5-adultto death under constant conditions of food (10 000 cells ml^–1of Hymenomonas elongara) and temperature (15 and 20°C).Preliminaiy experiments showed that virgin females could produceunfecunded eggs, but that the distinctive spines of the eggsof C.typicus were only obtained after fecundation occurred.High and continuous egg production needs several matings, andthe following experiments were run with a couple. In all experiments,both males and females fed and produced faecal pellets. Thefemales show a high variability in spawning life periods, inspawning rhythms, in daily rates and, consequently, in cumulatednumbers of eggs spawned during their life. The highest dailyrates of egg production were obtained in the first half of thefemale's life, and were as high at 20°C as at 15°C.Nevertheless, the females survived longer at 15°C than at20°C, and finally produced 47.6% more eggs during theirwhole life at 15°C than at 20°C, which can be partiallydue to the difference in size (almost 10% bigger at 15°Cthan at 20°C). The production of faecal pellets occurredduring the whole of life and was more regular than egg production,with a maximum daily rate in the first part of life, and showeda decrease until death. The decline in daily egg productionwith age was parallel to the decrease in daily faecal pelletproduction. The daily production of faecal pellets was higherat 15°C than at 20°C. At 20°C, a good correlationwas observed between the total egg production per female andthe total faecal pellet production per female. The individual-basedegg production rates of our experiments have been compared withrates obtained in different conditions by several authors, andwere lower than in other published studies, probably due tothe food offered. We emphasize the difficulties of comparingresults which have been obtained for the entire life of thefemales with those based on daily egg production. The totalegg production of a female is an important parameter in regardto the population d because it fixes the maximal rate of losses(mortality plus dispersion) for which a population can continueto develop.     

Development, growth and egg production of the two copepod species Centropages hamatus and Centropages typicus in the laboratory

Laboratory experiments were set up in order to determine andcompare developmental rates, growth rates, generation timesand egg production rates for the two calanoid copepod speciesCentropages typicus and Centropages hamatus. The nauplii showeda higher developmental rate than the copepodites for both specieswith quite different individual stage durations, which gaveno indication of isochronal development. For C typicus equiproportionaldevelopment was found. The growth rates were exponential andhighest for the largest species C typicus, and for both speciesthe juvenile growth rates were very similar to the egg productionrates of the adults.     

Pigment ingestion and egg production rates of the calanoid copepod Temora longicornisr. implications for gut pigment loss and omnivorous feeding

We examined the relationship between egg production rate (E)and pigment ingestion rate (I, from gut content corrected for33% loss) for adult female Temora longicornis in Long IslandSound on 47 occasions. Linear regression of E on I [both variablesexpressed in mass of nitrogen (N) female^–1 day^–1]was: E_N = 0.0016 + 0.770 x I_N. The slope, 0.77, is the apparentgross efficiency of egg production, equivalent to the grossgrowth efficiency (GGE) assuming that females partition allnitrogen for growth into egg production. Published work suggeststhat a GGE of 0.37 would be expected for herbivorous copepods.The discrepancy between the expected value of 0.37 and observedvalue of 0.77 could result from unquantified losses of gut pigmentor because T.longicomis ingested a significant amount of nitrogenby feeding as a carnivore. We suggest that if T.longicomis femalesderive all of their nitrogen for growth by feeding on phytoplankton,and if no correction for pigment loss is employed, then thegut pigment method underestimates pigment ingestion by no morethan a factor of two.     

Zooplankton growth rates: diel egg production in the copepods Oithona, Euterpina and Corycaeus from tropical waters

Clear diel periodicity in the timing of egg laying was observedin the tropical cyclopoid cope-pods Oithona plumifera, Oithonanana, Oithona simplex and Corycaeus amazomcus, and the harpacti-coidEuterpina acutifrons in waters surrounding Jamaica, West Indies.Individual females incubated under ambient food conditions formultiple clutches displayed remarkable constancy in their clutchdurations (= egg or embryonic development time). All speciesexhibited clutch durations of 20–23 h and cycle durations(= sum of clutch and inter-clutch durations) of 24 h, exceptfor the offshore species O.plumifera where the clutch durationwas 36 h and the cycle duration 48 or 72 h. Egg laying appearedto be synchronized by photoperiod, occurring around dawn formost species. In order to avoid biases, diel periodicity mustbe accounted for in estimates of egg production rates, and futureefforts should concentrate on the responses of clutch size andinterclutch durations to natural food climates.     

The Effect of Standard Metabolic Rate on Egg Production in the Acridid Grasshopper, Xanthippus corallipes

SYNOPSIS. Models of energy budgets often allocate energy betweenthe broad categories of maintenance, growth, and reproduction.An important component of maintenance costs is standard metabolicrate, which is partially determined by body temperature. Therelationship between thermoregulation, standard metabolism,and egg production was investigated in adult females of theacridid grasshopper Xanthippus corallipes from six populationsalong an elevation gradient. Individuals thermoregulate andmaintain stable afternoon body temperatures which correlatenegatively with elevation. Standard metabolic rates were calculatedat each population's afternoon body temperature. These rateswere then used to investigate the effect of variation in standardmetabolic rate on egg production. Because of the pervasive effectof mass on standard metabolic rate and egg number, the variableswere log-transformed. The effect of mass was removed by regressionanalyses, and residuals were calculated. An analysis of theresiduals revealed that intermediate values of field standardmetabolic rate correlate with high egg production, suggestingstabilizing selection on standard metabolic rate.     

Feeding, excretion and egg production by individuals and populations of the marine, planktonic copepods, Acartia spp. and Centropages furcatus

Diel variations in vertical distribution, gut pigment content,ammonium excretion and egg production were investigated foradult females of Acartia erythraea and A.pacifica in the verticallymixed Inland Sea of Japan and Centropages furcatus in the stratified,neritic Gulf of Mexico. Gut pigment content and egg productionrate were maximal at night and ammonium excretion was maximalduring the daytime. Neither A.erythraea nor A.pacifica adultfemales showed an apparent diel migration, but the former werehighly concentrated in the surface layer during the afternoon.In contrast, C.furcatus adult females showed a clear diel migration,residing immediately above the bottom during the daytime andbeing concentrated between 10 and 25 m depth during the nighttime.Individual-based data on gut content and excretion and egg productionrates were combined with vertical-distribution data to calculatepopulation values. In the Inland Sea of Japan, the resultantpattern for Acartia spp. reflected the diel variation in physiologicalrates and even distribution of adult females, except for theafternoon, surface aggregation of A.erythraea. In the Gulf ofMexico, the pattern for C.furcatus reflected largely the dielvariation in each rate process and the heterogeneous distributionof adult females in the water column. Elevated nocturnal feedingactivity of these copepods may be due to an endogenous rhythm.The daytime maximum in ammonium excretion and night-time maximumin egg production rate indicated approximate half-day and daytime lags, respectively, after the intake of food until itsconversion into dissolved excreta and released eggs.     

The evolution of egg size in the brood parasitic cuckoos

We compared genera of nonparasitic cuckoos and two groups ofparasitic cuckoos: those raised together with host young ("nonejectors")and those in which the newly hatched cuckoo either ejects thehost eggs or chicks, or kills the host young ("ejectors"). Nonejectorsare similar to their hosts in body size and parasitize largerhosts than do ejectors, which parasitize hosts much smallerthan themselves. In both types of parasite, the cuckoo's eggtends to match the host eggs in size. To achieve this, nonejectorshave evolved a smaller egg for their body size than have nonparasiticcuckoos, and ejectors have evolved an even smaller egg. Amongejector cuckoo genera, larger cuckoos have larger eggs relativeto the eggs of their hosts, and the relationship between cuckooegg volume (mass of the newly-hatched cuckoo) and host egg volume(mass to be ejected) did not differ from that predicted by weight-liftingallometry. However, comparing among Cuculus cuckoo species,the allometric slope differed from the predicted, so it is notclear that egg size is related to the need to give the cuckoochick sufficient strength for ejection. Comparing the two mostspeciose ejector genera, Chrysococcyx cuckoos (smaller and parasitizedome-nesting hosts) lay eggs more similar in size to their host'seggs than do Cuculus cuckoos (larger and parasitize open cup–nestinghosts). Closer size-matching of host eggs in Chrysococcyx mayreflect the following: (1) selection to reduce adult body massto facilitate entry through small domed nest holes to lay, and(2) less need for a large egg, because longer incubation periodsin dome-nesting hosts allow the young cuckoo more time to growbefore it need eject host eggs.     

Temporal variability in copepod fecundity during two different spring bloom periods in coastal waters off Plymouth (SW England)

The temporal variation in egg production of the planktonic copepodsCalanus helgolandicus, Temora longicornis and Pseudocalanuselongatus was studied during two different spring bloom periodsin 1989 and 1990 by weekly sampling at two permanent stationsin coastal waters off Plymouth (SW England). Copepod egg productionwas estimated in situ by incubating individual adult femalesin filtered seawater for 24 h (72 h until hatching for P.elongatus)at the field surface temperature. The relationship between copepodegg production rates and: (i) chlorophyll a concentration (totaland >10 µm size fraction), (ii) temperature and (iii)initial copepod gut pigment content was investigated. The springbloom periods were very different in both years, with the occurrenceof a Phaeocystis sp. bloom in 1990, which negatively affectedthe feeding and fecundity of copepods. Egg production ratesin spring 1989 were significantly correlated with chlorophylla concentration (particularly with the >10 µm fraction),field temperature and copepod gut pigment contents. In spring1990, egg production rates were also correlated with copepodgut pigment contents, but no significant correlations were obtainedwith temperature or with chlorophyll a concentration, as a consequenceof the lower egg production rates obtained during the Phaeocystissp. dominance period. These results show that food availabilityis the factor which mainly affects the fecundity of neriticcopepods in short time periods.     

Zooplankton community structure and copepod egg production in coastal waters of the central Great Barrier Reef lagoon

We describe zooplankton community structure and copepod eggproduction in the vicinity of the coastal boundary zone of theGreat Barrier Reef lagoon, Australia. The abundance and eggproduction rate of constituents of the zooplankton assemblagecharacteristic of the coastal zone rapidly increase subsequentto events such as flooding and upwelling. Our sampling spannedtwo summer monsoonal seasons, the first of which (1990-91) wasvery wet. The second monsoonal season (1991-92) was very dryand was characterized by intrusive upwelling events from theCoral Sea. Chlorophyll a concentrations did not rise in thewet year, probably because of light limitation, but did riseas a result of upwelling. Terrestrial run-off in the wet yearhad a greater apparent effect on zooplankton abundance patternsthan did upwelling in the dry year, except where coastal trappingallowed sufficient time for increases in zooplankton abundanceto occur. Egg production rates by the copepods Acrocalanus gibberand Acrocalanus gracilis showed haphazard spatial differences.Nitrogen-specific egg production ranged between 0.03 and 0.21day^–1 for A.gibber, and between 0.13 and 0.41 day^–1for A.gracilis. The egg production rate by A.gibber was foodlimited for most of the year and showed a poor correlation withtemperature. ³Present address: Department of Biological Sciences, FloridaTech, 150 W University Boulevard, Melbourne, FL 32901, USA     

Resource quality effects on Daphnia longispina offspring fitness

The present study addressed the question: is the fitness ofDaphnia longispina neonates related to the quality (species)of the resource their mothers consumed? A 2 2 factorial designwith two maternal and neonate resource types was employed, withRhodomonas and Microcystis used as high- and low-quality resources,respectively. Neonate fitness was assessed using demographicparameters obtained from a life-table study. The resource typeneonates consumed always had a stronger effect on the neonatesthan did the maternal type. Neonate consumption of Rhodomonas.relative to Microcystis. increased fitness, as estimated bythe instantaneous population growth rate, by 25–28%. whilematernal consumption of Rhodomonas increased fitness by 6-8%.Neonate Rhodomonas consumption also reduced the age at reachingthe primiparous instar; increased brood size at the primiparousinstar and body length immediately after the primiparous instar;increases average dutch size, number of clutches produced, totalegg production. and mean body length up to and on age 30 days.Regardless of resource type, maternal Rhodomonas consumptionresulted in neonates which reached maturity at an earlier ageand had a larger individual size immediately after reachingmaturity. The mean clutch size, total egg production, and lengthup to and at age 30 days were all significantly affected bymaternal diet when neonates consumed Microcystis; however, thesevariables were not influenced by maternal type when neonatesconsumed Rhodomonas. This is the first study to document maternallymediated effects of resource quality on the fitness of crustaceanzooplankton neonates.     

Comparative study of the reproduction of Calanus helgolandicus in well-mixed and seasonally stratified coastal waters of the western English Channel

Field estimates of the production and viability of eggs of thecopepod Calanus helgolandicus were conducted weekly during 1993and 1994 at two fixed stations in the western English Channel.The stations were located in the coastal zone off Roscoff (NWFrance) and Plymouth (SW England). These areas are characterizedby homogeneous and seasonally stratified waters, respectively.In these two different environments, the egg production ratevaried seasonally and was significantly correlated with chlorophylla concentration, but not with surface temperature. Significantcorrelations between fecundity and particulate carbon and nitrogenwere found only off Plymouth. The seasonal pattern of fecunditydiffered between the two sites. Off Plymouth, spawning occurredearlier, lasted longer and was significantly higher than inRoscoff waters. In both localities, fecundity was generallybelow the maximum specific egg production rate (     

Estimation and interpretation of egg provisioning in marine invertebrates

Per-offspring maternal investment is an integral part of life-historytheory. To understand the evolution of per-offspring maternalinvestment in marine invertebrates, a number of mathematicalmodels have been developed. These models examine how selectionaffects the proportion of maternally derived egg energy usedto produce a newly metamorphosed juvenile (s) and make predictionsabout the distribution of s in nature. However, there are veryfew published values of s and therefore it is difficult to evaluatehow well these models match nature. We present several equationsto empirically estimate values of s for any group of marineinvertebrate, and use data from echinoderms to compare the differentequations. The calculations that directly estimate s requireinformation on the amount of egg energy, juvenile energy, andenergy metabolized during development. Currently, there arefew data available for directly estimating s, and thus generatingdistributions of s derived from direct estimates is not possible.Furthermore, the direct estimations of s are informative forplanktotrophy but not for lecithotrophy. We have developed anequation that can be used to directly estimate s for lecithotrophs.The calculations to indirectly estimate s only require egg energyor egg size for the species in question and the value of s andegg energy or size for a reference species. This reference speciesreplaces the need to measure juvenile energy and energy metabolizedduring larval development. Because egg energy or size is currentlyavailable for many species, the indirect estimates will be usefulfor generating distributions of s, and will allow comparisonswith models. Although these indirect methods are good for generatingdistributions of s, they do not provide reliable estimates ofs for any particular species. Estimating values of s to comparemodels is a critical gap in our current evaluations of marineinvertebrate life-history models.     

Distribution, abundance and production of the copepod Calanus agulhensis on the Agulhas Bank in relation to spatial variations in hydrography and chlorophyll concentration

Measurements were made of chlorophyll concentration, abundanceand biomass of cope-pods, moulting rates of Calanus agulhensisjuveniles and egg production rate of adult females, during twosurveys of the Agulhas Bank in spring 1988 and 1989. From thesemeasurements, growth, recruitment and production rates werecalculated. Biomass ranged from