Morphological responses by Bosmina longirostris and Eubosmina tubicen to changes in copepod predator populations during a whole-lake acidification experiment

Changes in zooplankton populations during the experimental acidilicationof Little Rock Lake provided an opportunity to examine specificmechanisms underlying the morphological responses of bosminidsto changing predation pressure. Two large copepods, Epischuralacustris and Mesocyclops edax, disappeared from the lake'sacidified basin in 1986 and 1989, respectively, while a smallercopepod predator, Tropocyclops extensus, increased during laterstages of acidification. The two bosminid species showed distinctlydifferent responses coinciding with the changes in copepod predation.Bosmina longirostris exhibited a significant decrease in mucrolength with the decline of M.edax and E.lacustris. Its meanbody and antennule length, however, did not change. We suggestthat the decoupling of B.longirostris mucro length and antennulelength may have been related to the persistence of the smallercopepod predator, T.extensus. Eubosmina tubicen showed no apparentresponse to declines in M.edax and E.lacustris abundance ineither mean mucro, antennule or body length. Allometric analysesindicated, however, that mucro length was related to size-dependentcopepod predation for both B.longirostris and E.tubicen.     

Predation of freshwater jellyfish on Bosmina : the consequences for population dynamics,body size,and morphology

Invertebrate predators may cause strong changes in behaviour, life-history, and morphology of prey species. However, little is known about the influence of jellyfish on such characteristics of their prey. This study analyses the impacts of the freshwater jellyfish Craspedacusta sowerbii on life history and morphological defenses in a population of the cladoceran Bosmina longirostris . Length of mucro and antennule, size-dependent number of eggs, size at maturity, and size of juveniles, adults, and egg-carrying females were investigated during a 23 days experiment using medusae-enriched and control enclosures filled with natural plankton populations. Significant differences in parameters investigated were found not only between treatments, but also within treatments over time. Changes in Bosmina life-history parameters and morphology in controls were probably due to predation by cyclopoid copepods. The significant increase in the size of adults and egg-carrying females as well as the increase in mucro and antennule length in medusae-enriched enclosures are discussed as defense strategies against the freshwater jellyfish.     

Competitive exclusion in Cladocera through elevated mortality of adults

The population dynamics of two cladocerans, Ceriodaphnia pulchellaand Diaphanosoma brachyurum competing under laboratory conditionsin lake water was analysed using cross-correlations. Both mixedand isolated populations of the two cladocerans showed delayeddensity-dependence in the death rates of juveniles and adultsas well as in fecundity rate. The regressions for each of thethree rates on total density of competitors were compared betweenthe two species. There were no significant differences in theslopes of regressions for fecundity rates and the death ratesof juveniles. However, in the inferior competitor (Diaphanosoma)which went extinct in all treatments, the death rate of adultsincreased with total density much more quickly than in the superiorcompetitor (Ceriodaphnia). The intraspecific comparisons indicatedthat while Ceriodaphnia adults survived better than juvenilesunder conditions of crowding, in Diaphanosoma, juveniles werebetter survivors than adults. These data suggest that the contentionof higher vulnerability of cladoceran juveniles than adultsto starvation and crowding may prove to be not a universal phenomenon.     

Clutch size variation and morphology in a cyclomorphic Bosmina population

Clutch size, cyclomorphosis and allometric growth were analysedin a population of the humpbacked Bosmina (E.) coregoni gibbera.This species shows cyclomorphosis in antennule length and bodyheight, traits that may reduce predation risk from Leptodorakindtii. Individuals with long antennule and extreme body heightmay pay a cost in terms of decreased reproductive capacity.On the other hand, increasing the body height may be a way toincrease the brood chamber volume during periods of good foodconditions. We tested these hypotheses by comparing the seasonalvariation in clutch size with the seasonal variation in morphology.Antennule length and body height increased from mid-May to Augustand showed usually positive allometry at times with high populationdensities of the predatory cladoceran Leptodora kindtii. Clutchsize decreased from spring to late summer contrary to the hypothesisthat cyclomorphosis in height is caused by a seasonal variationin reproductive demand. However, within-dates antennule lengthwas negatively related and body height was positively relatedto clutch size. We conclude that the long antennule may imposea cost that reduces the reproductive capacity. The hypothesisthat carapace cyclomorphosis is driven by seasonally varyingclutch sizes was rejected.     

Vulnerability of Daphnia middendorffiana to Parabroteas sarsi predation: the role of the tail spine

The predaceous calanoid copepod Parabroteas sarsi and Daphniamiddendorffiana co-exist in South Andes ponds. Daphnia middendorffianajuveniles have a tail spine with negative allometric development.A series ofexperiments was carried out with tailed and tail-removedjuveniles of three different instars. In all cases tested, feedingrates were significantly higher on tail-removed prey. Directobservations showed a higher frequency of unsuccessful attackson spined juveniles when compared with tail-removed juveniles.The proportion of dorsal attacks also increased in spined juveniles.Prey total length was a much better predictor of feeding ratethan prey body size. Three groups of juveniles with equal totallength, but differentage, body size and biomass, showed no significantdifferences in their vulnerability to predation.     

OTHER PAPERS: INTERFERENCE AND RESOURCE COMPETITION IN TWO LAND SNAILS: ADULTS INHIBIT CONSPECIFIC JUVENILE GROWTH IN FIELD AND LABORATORY

Interference and resource competition by adults inhibited growthrates of conspecific juveniles of the land snail species Mesodonthyroidus and Neohelix albolabris in separate field and laboratoryexperiments, but not in laboratory experiments on Anguispiraalternata. In 1 m² field cages at near-natural densities underambient food and water conditions, juvenile M. thyroidus apparentlycompeted with adults for food or water or both resources, growingmore slowly when living with two conspecific adults, but beingunaffected by adult presence when food and water were augmented.Neohelix albolabris juveniles were similarly unaffected in fieldcages by presence of two adults when food and water were augmented.In contrast, interference, not resource competition, apparentlyexplained growth inhibition in laboratory cages at densitiesconsiderably greater than natural densities, with non-limitingfood and moisture; both M. thyroidus and N. albolabris juvenilesgrew more slowly as conspecific adult number increased fromzero to three. (Received 17 July 1995; accepted 11 November 1996)     

Interference from a game theoretical perspective: shore crabs suffer most from equal competitors

In virtually all natural systems, interference competition amongindividuals is asymmetric. Here, we used game theoretical modelson contest behavior to predict how time lost in agonistic interactionscould affect strength of interference under asymmetric competition.We hypothesized that interference through time lost in agonisticinteractions would result in a greater reduction in availableforaging time and overall feeding rate under symmetric competitionthan under asymmetric competition. We tested this hypothesisfor male shore crabs (Carcinus maenas) that foraged on mussels(Mytilus edulis) in an experiment where prey levels were keptconstant. We varied absolute size of crabs (juveniles, smalladults, large adults), mussel density (4, 16, 32 per 0.25 m²),and competitor size (smaller, equal, larger). Large adults spentmore time in aggressive behaviors than juveniles or small adults,possibly because large adults were more persistent in interferingor because large adults were intrinsically more aggressive,as the experiment was conducted in the mating season. When handlingprey, crabs mostly avoided competitors, but juveniles and smalladults did so more than large adults. When searching for prey,crabs mostly displaced smaller competitors but threatened oravoided size-matched or larger competitors. By avoiding a competitor,the focal crab lost time but the competitor often did not, andthis asymmetry in agonistic behavior is not yet incorporatedin models on contest behavior. However, overall, negative effectsof others were strongest with size-matched competitors, in linewith our hypothesis.     

Kin discrimination in the social lizard Egernia saxatilis (Scincidae)

The ability to discriminate kin from nonkin is critical forthe evolution of kin-based sociality. Black rock skinks, Egerniasaxatilis, are viviparous lizards that typically live in "nuclearfamilies" consisting of an adult male, adult female, and oneor more cohorts of juveniles. Laboratory trials showed thatjuvenile lizards can discriminate between the scent of adultsfrom their own social group versus that of unfamiliar adults.Experiments in which we translocated individuals among familygroups revealed that this discriminatory ability was based onfamiliarity with other individuals rather than genetic relatedness.For example, neither "fostered" juveniles nor their mothersdisplayed any scent-based kin discrimination when brought togetherafter 2 months' separation. Thus, unlike the closely related(and also social) Egernia striolata, black rock skinks basekin discrimination on familiarity rather than genotypic similarity.     

Can extreme morphology in Bosmina reduce predation risk from Leptodora? An experimental test

Bosmina (Cladocera) populations, especially within the subgenus Eubosmina, show a variety of phenotypes that exhibit large differences in body size and shape and antennule length. In some populations, the morphological traits also vary during the season, with the most extreme forms occurring in periods with high densities of certain invertebrate predators. However, while temporal phenotypic variation in other cladocerans, as in the family Daphnidae, has been shown to be an adaptation to reduce the risk of predation by invertebrate predators, the reason for such changes in Bosmina is much less clear. We examined whether certain morphological traits in Bosmina species could act as a defence against invertebrate predators. We tested three Bosmina forms (subgenus Eubosmina), differing in morphology from each other, which are found in lakes together with the predator Leptodora kindtii (Cladocera). Bosmina (E.) longispina has a relatively low and elongated carapace with a caudal mucro, and short antennule, B. (E.) coregoni gibbera has a higher and more protruding carapace without caudal mucro, and a much longer antennule. Finally, B. (E.) coregoni retro extensa has a carapace like that of B. longispina but with no caudal mucro and a much longer antennule. In one experiment, B. longispina and B. gibbera were exposed for 12 h to Leptodora in Petri dishes. In a second experiment, we observed directly the escape efficiency of B. longispina, B. gibbera and B. retro extensa, and the handling time of Leptodora. The two Bosmina forms with more extreme morphological features had a lower death rate and higher escape efficiency than B. longispina. Prey that escaped did so, in most cases, within 5 min. Predator handling time was correlated to predator body length and antennule length of the prey. The results suggests that Bosmina species with extreme morphological traits may be less vulnerable to invertebrate predators. Received: 8 June 1998 / Accepted: 5 October 1998     

Statolith microstructure and age of early life stages of planktonic squids Galiteuthis phyllura and Belonella borealis (Oegopsida, Cranchiidae) from the northern North Pacific

Statolith morphology and microstructure were studied in twocommon species of panktonic cranchiid squids, Belonella borealis[four juveniles with mantle length (ML) 375–450 mm] andGaliteuthis phyllura (13 paralarvae and juveniles, ML 9–235mm),caught near the bottom and in pelagic layers over the continentalslope of Siberia in the northwest Bering Sea. The total numberof growth increments within the statoliths ranged from 277 to294 in B.borealis and from 10 to 209 in G.phyllura. Assumingthat these increments were produced daily, both species growrapidly in length (daily growth rate = 1.13mm day^–1 duringthe first 8–10 months of their juvenile phase in the mesopelagiclayers, prior to migration into deeper waters for maturation.     

The effect of temperature on the post-embryonic growth of Neomysis intermedia Czernlawsky (Crustacea, Mysidacea) under laboratory conditions

The effect of temperature on post-embryonic growth of Neomysisintermedia was investigated under unlimited food conditionsin the laboratory. The effect of temperature on the size ofnewly released animals was negligibly small, but body size wasinversely related to temperature in adults. This was mainlycaused by the difference in the number of molts before maturation.The specific growth rate of N. intermedia increased exponentiallywith a temperature coefficient, Q₁₀ of 4.6 from 0.018 d^–13C to 0.21 d^–1 at 20C in juveniles, and with a temperaturecoefficient of 2.7 from 0.006 d^–1 at 3C to 0.05 d^–1at 25C in adults. The rate in juveniles levelled off above20C, and dropped at 29C. Brood size and brood interval decreasedwith temperature increase, while the daily specific reproductionrate increased. The specific growth rate of gravid females,including production of egg matter, increased exponentiallywith a temperature coefficient of 3.3 from 0.015 d^–1 at10C to 0.093 d^–1 at 25C. The present laboratory experiments confirmed the temperaturecontrol on the growth of N. intermedia suggested in a hyper-eutrophiclake.     

Physiological epistasis, ontogenetic conflict and natural selection on physiology and life history

Ontogenetic conflict arises when optima for alleles governingfitness variation differ between juveniles and adults or betweenadult sexes. Loci that govern development of alternative phenotypesin the sexes, hereafter termed morph-determining loci, mediatedevelopment through the endocrine system. Morphotypic selectionis defined to be multivariate selection favoring discrete alternativemorphotypes (e.g., optima). When the optimal combinations ofalleles for alternative morphs differ between the sexes, itgenerates conflicting selection pressure and thus ontogeneticconflict. Selection on morph alleles promotes ontogenetic conflictbecause it perturbs physiological epistasis that governs theexpression of male versus female traits. Expression of physiologicaltraits arises from homeostasis that maintains trait expressionwithin a normal range. The genetic basis of homeostasis is likelyto arise from interactions among several genes (e.g., geneticepistasis) or protein products (e.g., physiological epistasis).For example, endocrine regulation arises from interactions betweengondatropins, which are protein hormones produced by the hypothalamic-pituitaryglands, and steroid hormones, which are produced by the gonads(e.g., HPG axis). The side-blotched lizard system is discussedwith respect to physiological bases of ontogenetic conflict.We also describe a novel molecular marker strategy for uncoveringgenome-wide physiological epistasis in nature. Finally, ontogeneticconflict exerts selection on females to evolve mate selectionor cryptic choice that is reflected in different sires beingchosen for son versus daughter production. We describe how side-blotchedlizard females ameliorate ontogenetic conflict by cryptic choiceof male genotypes to produce sons versus daughters.     

Interannual Variation in Timing of Parturition and Growth of Collared Pikas (Ochotona collaris) in the Southwest Yukon

The length of the snow-free season has a significant influenceon reproduction and growth in northern alpine environments,and these life history traits may provide sensitive indicatorsof the responses of organisms to climate change. We examinedgrowth rates and timing of parturition of collared pikas (Ochotonacollaris) from 1995–2002 in the Ruby Range, Yukon Territory,Canada. Growth rates were best described using a Gompertz model,in which the asymptotic mass, determined from the average maleand female weights, was 157 g, the growth rate constant (K)was 0.0557, and the age at inflection (I) was 18.12 days, fora birth weight of 10 g. The maximum growth rate for North Americanpikas (O. collaris and O. princeps) increased with latitude,with maximum growth rates being approximately one-third greaterin northern populations where the snow-free season is less thanthree months long. The mean parturition date varied significantlyamong years from 3 June to 3 July, and delayed parturition wascorrelated with indices of high snow accumulation and, to alesser extent, late spring snowmelt. However, parturition datedid not significantly affect the subsequent over-winter survivalof juveniles in this population, suggesting that pikas are ableto adjust to seasonal uncertainty associated with highly variablespring conditions.     

Availability,uptake and regeneration of phosphate in mesocosms with varied levels of P deficiency

A population of cyclomorphic Bosmina coregoni was studied in Lake Östersjön, southwestern Sweden and results from field samples collected in 1988, 1989, 1990 and 1991 are presented. Animals collected in summer have remarkably higher carapace and prolonged antennule compared to what we call the normal morph. In 1991 the extreme morph reach its maximum body length, body height and antennule length in July to September. The occurrence of the extreme morph coincide with the hatching of the predaceous cladoceran Leptodora kindtii.The two morphs fluctuate in abundance and in relation to each other. In early spring only the normal morph occurred in the samples followed by a period of about two months when the two morphs were found together, in July only the extreme morph was found. In September the two morphs were again present in the lake. As has been shown for other cladoceran, the conspicuous carapace and antennule could be an adaptive response that decreases mortality due to invertebrate predation. Spectacular features like these are likely also accompanied by some sort of costs.     

Juvenile morphology of the xanthid crab Leptodius exaratus (H. Milne-Edwards, 1834) (Decapoda: Brachyura), with notes on the appearance of sexual dimorphism

Summary

The morphometrical and meristic features of the carapace, cephalic appendages (antenna, antennule), mouthparts (maxillule, maxilla, first-third maxillipeds), sternum, pereiopods, abdomen, and pleopods of juveniles and the onset of morphological sexual dimorphism were described for the xanthid crab Leptodius exaratus (H. Milne Edwards, 1834), based on laboratory-reared and wild adult specimens collected from Tateyama Bay, Japan. First instar juveniles shared some of the features of adults (e.g. gross appearance of the carapace and cheliped propodus proportions), but differed from adults on almost all other morphological parameter examined. Morphological development was still not complete at the ninth instar; extrapolation from the rate of morphological changes between instars 1–9 suggests that L. exaratus requires about 13 ecdyses to transform into adults, including development of reproductive structures. Differences in the number and morphology of pleopods and abdomen width allowed early distinction of the sexes. Thus, males formed gonopods in the first abdominal somite and lost the paired vestigial pleopods in somites 3–5 from the fourth instar; females retained the pleopods in somites 2–5, but these became biramous and had increased setation. The abdomen grew wider in females than in males from the fifth instar. Several morphological features of juveniles have phylogenetic and taxonomic implications: carapace motifs clearly place L. exaratus in the superfamily Xanthoidea, whereas the patterns of setation in the scaphognathite and first maxilliped epipod allow separation of this (xanthid) species from crabs of other Xanthoidean families.     

Cyclomorphosis of Bosmina (Eubosmina) tubicen in the Okefenokee Swamp

Cyclomorphosis of Bosmina (Eubosmina) tubicen, expressed asallometric growth of antennules and mucrones, was followed infive populations in the Okefenokee Swamp. Allometry of antennuleswas strongly cyclical, being positive in fall, winter and spring,but negative in early summer. Antennule allometry and othermorphological variables (egg size, body size, antennule andmucro length) were all negatively correlated with temperature,but mucro allometry had a radically different pattern than theantennule - body size cyclomorphosis. Populations could be separatedbetween lakes according to allometry of the mucrones, and mucroallometry was also negatively correlated with zooplankton populationdensity. Morphological data from this study do not support serialreplacement of morphotypes within lakes, and correlation withecological parameters does not support the hypothesis that cyclomorphosisis caused by the presence of invertebrate predators.     

INTERPOPULATION VARIATION IN JUVENILE SURVIVAL AND GROWTH RATES OF LYMNAEA PEREGRA (GASTROPODA: PULMONATA); TEMPERATURE AT RECRUITMENT AS A SELECTION PRESSURE?

Three neighbouring populations of Lymnaea peregra had recruitmentin the summer (June and July), but one population (Sheaf) hada second recruitment in September and October. We hypothesizedthat juveniles of the Sheaf population would be subject to selectionunder both ‘summer’ and ‘winter’ conditions,and thus should be more resistant to low-temperature stressthan juveniles of the other populations. The hypothesis wassupported by the findings that Sheaf juveniles survived andgrew better over a wider range of temperatures (2, 10, 15 and20°C )while juveniles of the other two populations wereadapted only to higher temperatures (15 and 20°C). Therewas evidence that some of these traits were genetically fixed. ^*Present address: Department of Applied Science, City Polytechnicof Hong Kong, Mongkok, Kowloon, Hong Kong. (Received 15 September 1988; accepted 16 December 1988)     

Cephalopods in the Diet of Albacore, Thunnus alalunga, from the Adriatic Sea

The stomachs of 56 albacore (Thunnus alalunga) caught in thesouthern Adriatic Sea contained 222 cephalopods, or 29% of prey.Most cephalopod remains were from small specimens. Juvenilesand adults of Heteroteuthis dispar (61.5% of cephalopods) were morefrequently preyed upon in the `inshore' fishing area (bottomdepth: 100-700 m); early juveniles of Todarodes sagittatus (21.6%)were more abundant ` offshore' (750- 1,150 m); early juvenilesof Histioteuthis bonnellii (16.4%) were exclusive of the latterarea. The estimated overall number of cephalopods dying of predationby albacore is 6.3 x 10⁶ specimens/fishing season (two months). (Received 6 May 1998; accepted 13 July 1998)     

The role of pelagic water mites in the control of cladoceran population in a temperate lake of the southern Andes

In a small temperate lake of the southern Andes, Bosmina longirostrisand Ceriodaphnia dubia coexist with the predaceous water miteLimnesia patagonica. Sampling of natural populations and laboratoryexperiments were carried out. The field population of Limnesiadid not show a numerical response to the density or biomassof its prey. Laboratory experiments showed that the water miterejected C.dubia adults and juveniles as prey, but consumedBosmina. The maximum predation rate was 40 prey predator^–1day^–1 and a linear relationship between predation rateand prey density was obtained (R² = 66%). The contribution ofmortality due to predation and the predation risk are too lowto provoke a prey suppression. By means of computer modelling,the densities of predator necessary to explain half of the totalprey mortality were calculated. These densities were one ortwo orders of magnitude higher than those in the field. It isconcluded that Limnesia could really be a suppressor, but thepotential depends greatly on its density.     

Diet,intestinal morphology,and nitrogen assimilation efficiency in the damselfish,Stegastes lividus,in Guam

Synopsis Analysis of stomach contents of Stegastes lividus revealed a shift from omnivory in juveniles to herbivory in adults. Red algae, primarily Polysiphonia spp., Gelidiopsis intricata and Ceramium spp., formed the bulk of the diet in all size classes. Foraminifera and small crustaceans were of particular importance in the diet of juveniles. Net nitrogen assimilation efficiency of field-fed S. lividus was approximately 61%, and did not vary with fish size. The net assimilation efficiencies of fish fed on Enteromorpha in the laboratory ranged from size-class means of 36–79% for nitrogen and from 29–72% for total organic material, and increased from juveniles to adults. Apparently, juveniles compensate for a lower efficiency in assimilating plant food by including a higher percentage of animal material in their natural diets. The ratios of intestine length to standard length (IL/SL) and to intestine diameter (IL/ID) increased rapidly in juveniles and leveled off in adults. The retention time for ingested food items may only be about one-half as long in juveniles as in adults. The combination of observed IL/ID ratios and estimated retention times proved most valuable in the interpretation of ontogenetic changes in feeding habits and assimilation efficiency.