摘要:

SUMMARY.1. The biomass distribution of submerged vascular vegetation and benthic fauna were investigated by diving in Lake Kariba. The vegetation was well correlated with transparency of the water. Maximum biomass (1400 g dryweight m−2) and a depth penetration of 6 m were found in areas little influenced by river inflow, while these were only 110 g m−2and 2 m, respectively, in the basin receiving water from the Zambezi river.2. The lake is mesotrophic‐oligotrophic. The total biomass for the lake was 101,000 tons dryweight of rooted vegetation composed ofLagarosiphon ilicifoliusOberm. (52%),Najas pectinata(Parl) Magnus (33%),Vatlisneria aethiopicaFrenzl (11%),Ceratophyllum demersumL. (3%) andPotamogeton octandrusL. (0.5%). Average plant biomass for the potentially colonizable depth zone of 0‐12 m and for the total lake amounted to 79.9 g m−2and 18.8 g m−2, respectively.3. The distribution of the benthic fauna generally followed that of the vegetation. The total animal biomass of 118,840 tons dryweight. including shells, consisted of mussels (95.8%), snails (4.1%) and insect larvae (0.1%). Four species of mussels were found:Caelatura mossambicensis(von Martens) (81% of mussel biomass),Corbicula africana(Krauss) (9%).Mutela dubia(Gmelin) (5%) andAspatharia wahlbergii(Krauss) (5%). Among the snailsMelnoides tuberculuta(Müller),Cleopatraspp. andBellamya capillata(Frauenfeld) dominated.4. The average animal biomass was high compared to most other lakes perhaps due to lack of predators. For the colonizable 0‐12 m depth interval and the total lake it was 96.2 g m−2including shells (15.0 g m−2shell‐free dryweight) and 22.6 g m−2including shells (3.4 gm−2shell‐free dryweight). respectively. Biomass of plants and animals was even higher prior to the recent lowering of the water level by 7 m, which was estimated to have stranded 84,000 to

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1988.tb00322.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

SUMMARY.1. In an investigation of perch use by two species of larval damselfies,Coenagrion puella(Linn.) andIschnura elegans(van der Linden), larvae spaced out on a grid of vertical cocktail sticks, in the absence of food, to a greater extent than expected from a random model. Spacing was the result of direct interaction between larvae, and was not related to the level of hunger of the larvae over a 10 day starvation period.2. Prior ownership of a perch was not a predictor of success in conflicts between larvae of the same instar. In contests between different instars larger larvae were more likely to win conflicts. Some small larvae appear to remain undetected in the presence of larger conspecifies.3. At one level of food availability, perch ownership did not influence growth rate or mortality ofC. puellaorI. etegans.However, it did allow greater prey capture success rate.4.Coenagrion puellalarvae showed more movement in the absence of perches both in terms of actual distance moved and amount of swimming activity. The amount of swimming activity was reduced by addition of a predator (sticklebacks.Gasterosteus aculeatusorPungitius pungitius)to the tank both in the absence and presence of an alternative food supply for the fish. Duration of individual swimming bouts was unaffected.5. Sticklebacks attackedC. puellalarvae significantly more often in the absence of perches, with a strong bias towards swimming larvae.6. It is proposed that the major advantage of perch possession toC. puellalarvae is in the reduction of predation as a consequence of the reduction in larval movement.

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1988.tb00323.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

SUMMARY.1. The predation‐balanced polymorphism ofCeriodaphnia cornuta(a small‐eyed horned form and a large‐eyed unhorned form) in Lake Gatun, Panama, is re‐examined approximately 12 years after the original study (Zaret, 1972a). We consider morphological variability, distribution and fecundity of the population inhabiting Laboratory Bay.2. The two morphological characters emphasized by Zaret, eye size and horn length, are studied in detail. TheC. cornutapopulation exhibits wide, yet continuous variation; coefficients of variation are 28% or eye diameter and range from 114% to 136% for horn and spine lengths. Phenotypic variation in the population is characterized by: (1) negative correlations between spine lengths and eye size; (2) positive correlations among spines from different body regions (anterior horn, lateral spines, and tail spine); and (3) weak correlations between body length and spine lengths.3. While the horned and unhorned forms continue to coexist in Laboratory Bay. the frequencies of horned forms have increased in the open water areas since the time of the original study. The increase in horned form frequencies may be due to changes in fish predation intensity. Did vertical distribution patterns of the forms remain very similar to each other.4. In contrast to previous laboratory findings, egg ratio calculations from field samples revealed similar clutch sizes for the horned and unhorned forms. The egg development time of the horned form, however, was found to be approximately 30% longer than that of the unhorned form, perhaps because of increased moulting difficulties produced by the presence of

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1988.tb00324.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

SUMMARY.1. The literature contains a number of curves relating time (days) required for median hatch (=D2) to water temperature (T,°C) for the eggs of several salmonid fishes. There are relatively few data on the relationships between time to median eyed (= D1), days) or time to median swim‐up (= D3, days) and temperature.2. From published data, over most of the range 0‐9.5°C, approximate relationships are D,=0.5D2for Atlantic salmon (Salmo salar L.) and D3 = 1.7D2for eight species of Salmo and Oncorhynchus.3. Field and hatchery tests suggest that these are useful empirical models for approximate prediction of D1and D3from D2for most salm

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1988.tb00325.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

SUMMARY.1. Many examples of predator‐prey interactions from freshwaters suggest that apparent predator preferences result more from the relative vulnerabilities of prey than any positive choice by the predator.2. The rank order of vulnerability of seven invertebrate prey to nine invertebrate predators was measured in the laboratory. The ranks were used as a measure of relative vulnerability.3. The vulnerability of individuals ofSimocephalus vetulus(Muller) (Crustacea: Cladocera) to predation byEnallagma cyathigerum(Char‐pentier) (Insecta: Odonata) andNotonecta glaucaL, (Insecta: Hemiptera) was investigated in the presence of alternative prey of differing relative vulnerabilities, based on the rank order measures.4. WithE. cyathigerum, vulnerability of individualS. vetulusdepended greatly on the relative vulnerability of other prey types available. WithN. glaucathe intrinsic defences ofS. vetuluswere very effective.5. The importance of relative prey vulnerability is discussed with particular reference to small habitats with marked annual species turno

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1988.tb00326.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

SUMMARY.1. The effects of nutrient addition upon algal periphyton standing crop and benthic community metabolism were investigated in Lake Lacawac, Pennsylvania. Nutrient‐diffusing flower pot substrates were filled with combinations of P (0.015 mol Na2HPO4), N (0.15molNaNO3) and carbon as glucose (0.015mol (LG) or 0.15mol (HG)) or 0.58 mol NaHCO3(C). Fifty‐two pots representing ten treatments were placed in the lake on 5 May 1985 and sampled after 32‐38 days.2. Maximum chlorophyll‐a values of 23.82μgcm−2and 29.72μgcm−2were obtained on NPC and NPHG pots respectively, compared to 0.82μgcm−2on control pots. Algal growth was not enhanced unless N and either organic or inorganic carbon were supplied. Ash Free Dry Mass was especially high on NPHG pots.3. Gross production, net production and community respiration were assessed with light‐dark chambers affixed to the pots using microwinkler measurements of dissolved oxygen. Highest production rates were obtained on NC and NPC pots. Pots with NPLG and NPHG also showed significant increases in gross production compared to control levels (P<0.01). Specific production (=gross production/chlorophyll‐a) was significantly related to standing crop as chlorophylls (P<0.001), alkalinity in the chambers (P<0.00l) and light (P<0. 05), but not to N or P supplied by the pots.4. The periphyton community was dominated by Chlorophyta, which averaged 86.1% of total algal biovolume. Enrichment with NC and NPC significantly enhanced the growth of three Chlorophyceae(Stigeoclonium, Scenedesmusand an unidentified coccoid) and two Cryptophyceae(Cryptomonas. Rhodomonas)(allP<0.05). In contrast, the filamentous green algaMougeotiadominated other treatments, but declined significantly with NPC enrichment (P<0.05). Addition of NPHG stimulated an 8‐fold increase in total biovolume and significant increases in growth ofCryptomonas, Rhodomona

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1988.tb00327.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

SUMMARY.1. Mature crayfish, collected from an Irish lake before breeding had started, were held in breeding combinations and their mating and brooding activities observed.2. All mating attempts were initiated by the male. A single mating led to spawning within 6 days but a subsequent mating cancelled the effects of the first. Males mated more often when there were more females present. Males lacking a major cheliped mated less often than did normal males.3. Larger males mated more often than did smaller males, and although males showed no female size preference, matings were less frequent and generally unsuccessful when males were much larger than females; the female was usually killed. Large females mated successfully with smaller males.4. Females held at high densities with a larger male mated earlier than at low densities. However, aggression also increased with density; at high densities males fought and killed females.5. Males held in pairs without females fought; in occasional mating attempts spermatophores were not positioned correctly. Paired females rarely fought; all spawned normally although unmated. Although their eggs soon died and were removed during grooming, brooding behaviour continued for at least 2 months.6. Brooding females held in pairs shed pleopodal eggs during aggressive encounters. Females held singly showed a lower initial rate of egg loss.

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1988.tb00328.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

SUMMARY.1. Mature crayfish were collected in 1984 from an alkaline Irish lake before and after the mating season.2. There was some correlation between frequency of mating and decrease in vasa deferentia weight in captive males, but in the field vasa deferentia weight losses were highly variable. The data indicate that one third of adult males may not have mated in the wild.3. All females appear to have spawned, whether or not mated, with an average 85% reduction in ovary weights over the season.4. Pleopodal egg counts directly after laboratory spawning were on average 81% of ovarian counts; in the field, an estimated 50 days after spawning, they were only 61% of estimated ovarian egg numbers.5. Average ovarian egg size was positively correlated with female size. Also, larger crayfish had a wider range of egg sizes than smaller crayfish.6. It is suggested that aggression and competition for shelter among brooding females may result in a progressive loss of eggs.

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1988.tb00329.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

SUMMARY.1. The effect of several factors on feeding selectivity by three size classes of larvae ofAmbystoma tigrinum(Green) was examined. Five types of invertebrate prey were used:Chaoborus, Daphnia, damselflies,Hyalellaand water boatmen.2. Five variables were examined in relation to the electivity indices associated with each prey type: ease of prey consumption, ease of prey capture, relative prey size, prey activity, and microhabitat overlap. No single variable explained the observed electivity.3. For small larvae (snout‐vent length, SVL,3.5 and<5.5cm

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1988.tb00330.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

SUMMARY.1. We examined the seasonal distributions and mating behaviour ofEpischura lacustris, Diaptomus minutusandDiaptomus oregonensisto evaluate several proposed mechanisms for reproductive isolation. There was no divergence in onset of maturity or timing of breeding periods between sympatric diaptomid populations; size displacement between co‐occurring diaptomid species did not result in concomitant reduction in mating frequency between heterospecific pairs in the laboratory.2. Divergence in mating behaviour was supported by observations that, in conspecific mating experiments, males of both diaptomid species discriminated between females bearing ripened ovaries (gravid) and those that did not (non‐gravid), and that, in heterospecific trials, males either mated indiscriminantly with all females, or engaged in mating very infrequently.3. In mating experiments withEpischura.males did not discriminate between gravid and non‐gravid females, but mated exclusively with virgins of a specific relative size. Divergence in timing of breeding seasons betweenEpischuraand the two diaptomid species suggests that even if mating between genera is possible, it does not occur often in nature.4. We discuss the adaptive significance of different reproductive strategies of these three copepods and speculate on mechanisms that allow for coexistence of the closely related taxa in n

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1988.tb00331.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY